JOHNSON MATTHEY TECHNOLOGY REVIEW · 217 40 Years of Cleaner Air: The Evolution of the Autocatalyst...

JOHNSON MATTHEY TECHNOLOGY REVIEW

Johnson Matthey’s international journal of research exploring science and technology in industrial applications

www.technology.matthey.com

Volume 58, Issue 4, October 2014 Published by Johnson Matthey

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Contents Volume 58, Issue 4, October 2014

JOHNSON MATTHEY TECHNOLOGY REVIEW

Johnson Matthey’s international journal of research exploring science and technology in industrial applications


180 Recent Developments in the Study of the Surface-Stability of Platinum and Platinum-Iridium Mass StandardsBy Peter J. Cumpson

189 The 28th Santa Fe Symposium on Jewelry Manufacturing TechnologyA conference review by Christopher W. Corti

195 Determining Temperature Boundary of the A1(A1+B2) Phase Transformation in the Copper-55 at% Palladium Alloy Subjected to Severe Plastic Deformation By Oksana S. Novikova and Alexey Yu. Volkov

202 In the Lab: Towards a Molecular Level Understanding of Electrochemical Interfaces and Electrocatalytic ReactionsFeaturing Angel Cuesta

205 247th American Chemical Society National Meeting and Exposition: Part IIA conference review by Ian Casely

212 Measuring Water Solubility of Platinum Group Metal Containing SubstancesBy Matthew Gregory

217 40 Years of Cleaner Air: The Evolution of the Autocatalyst

By Chris Morgan

221 “Nanofabrication and its Application in Renewable Energy”

A book review by Greg Agar

224 Johnson Matthey Highlights

www.technology.matthey.comJOHNSON MATTHEY TECHNOLOGY REVIEW

180 © 2014 Johnson Matthey

http://dx.doi.org/10.1595/147106714X684551 Johnson Matthey Technol. Rev., 2014, 58, (4), 180–188

Recent Developments in the Study of the Surface-Stability of Platinum and Platinum-Iridium Mass StandardsQuantifying mercury and carbon contamination on platinum-iridium alloy surfaces using XPS

By Peter J. CumpsonAdvanced Metrology Laboratory (AML) and School of Mechanical and Systems Engineering, Newcastle University, Newcastle-upon-Tyne NE1 7RU, UK

Email: [email protected]

We review developments in the study of the stability of platinum-iridium standard weights, in particular the kilogram prototypes manufactured from alloy supplied by Johnson Matthey in the 1880s that still stand at the heart of the International System of Units (abbreviated SI from the French: Système international d’unités). The SI has long since moved on from length standards based on physical artefacts fabricated from this alloy, but the SI unit of mass is still defi ned in this way, as the mass of a real physical object. The stability of these reference masses has been a concern since the 1930s, with mass loss or gain at the surface being the principal concern. In recent years X-ray photoelectron spectroscopy (XPS) has been particularly valuable in elucidating the types of contamination present and the mechanism by which contamination takes place. While direct studies on the International Prototype Kilogram are understandably diffi cult, at Newcastle University we have examined the surfaces of six Pt mass standards also manufactured in the mid-19th century, using XPS to identify contamination chemically. XPS shows a signifi cant quantity of mercury on the surfaces of all six. The most likely source of Hg vapour is the accidental breakage of thermometers and barometers, and the mechanism of contamination may be similar to

the poisoning of platinum group metal (pgm) catalysts by Hg, an effect known for almost a century.

Introduction and History

The history and technology of mass and length standards fabricated from pgms have been described previously in Platinum Metals Review (1, 2). One of the earliest contracts in the history of Johnson Matthey Plc was the supply of high-purity Pt-Ir alloy that formed the basis of artefact standards for the metre and kilogram. Johnson Matthey supplied material (3) for the fabrication of 30 standard metres and 40 standard kilograms following an agreement with the French government in 1882 and it is these artefacts that defi ned the SI units of length and mass through most of the 20th century. Many more have been supplied since (see for example the prototype kilograms illustrated in Figure 1).

In the second half of the 20th century there was a concerted move to replace artefact standards with defi nitions in terms of fundamental quantities, so that artefact length standards are now of purely historical interest. Of all the SI units only the kilogram remains as an artefact standard (4) and even though this may not be the case for much longer, the stability of transfer standards will be crucial for many years to come. The unit of length, the metre, is relatively inexpensive to realise by interferometry, whereas alternative methods (5–7) for defi ning a unit of mass are extremely expensive to realise and will likely only ever occur at one site (or a very small number of sites) internationally. It therefore seems certain that even when the kilogram is redefi ned in terms of fundamental constants by these


http://dx.doi.org/10.1595/147106714X684551 Johnson Matthey Technol. Rev., 2014, 58, (4)

means there will be a continuing need to disseminate the standard of mass via physical standard weights and even the existing Pt-Ir kilogram standards may have a key role as secondary or transfer standards for many years to come.

Research on Surface Stability

It had been suspected since around 1939 and known since 1992 that the mass of prototype kilogram standards increases slightly over time (8–11) and surface effects have long been suspected as being responsible (12). The most obvious source of potential mass variation at the surface is water adsorption and this has been the focus of a number of studies (13). Kochsiek (14) measured water adsorption onto thin Pt-Ir foils having higher surface area per unit mass than kilogram prototypes. For the relative humidity range 30% to 70% this resulted in only the equivalent of a 10 g mass change of a Pt-Ir kilogram with respect to a stainless-steel reference weight. Adsorption was also reversible, indicating that the water is physisorbed and therefore eliminating water as a possible cause of any long-term mass gain.

It is known that solvent cleaning can remove some or all of the added mass. Bigg (15) examined weighing data on the UK national prototype, number 18 and concluded that within a year of an ethanol/ether/ammonia wash it regained ‘signifi cant’ mass. Bigg concluded that cleaning should be repeated prior to the most

accurate comparisons. Calcatelli et al. (16) studied the adsorption of contaminants gravimetrically and reported a gain in mass over the fi rst twelve months after cleaning of 9.8 g for a Pt-Ir kilogram prototype. On the basis of published measurements by the International Bureau of Weights and Measures (BIPM) and National Physical Laboratory (NPL) for the UK prototype kilogram 18, Martin Seah and myself at NPL developed a ‘diffusion limited’ model of carbonaceous growth (17). This gave a quantitative prediction of mass increase of carbonaceous contamination, mC, of the form (Equation (i)):

mC(T) = √2D (T + c)1/2 (i)

where D is an effective diffusion constant for small molecules through the existing contamination of density on a prototype of surface area , T is the time elapsed since the last cleaning procedure and c is a constant describing quantity of contamination that builds up in the fi rst minutes or hours after cleaning, before the process becomes diffusion-limited. This is consistent with historical weighing data over more than 60 years. Some of the measurements giving rise to Equation (i) were for material removed during cleaning, so Equation (i) models only the reversible part of the contamination of the surface (i.e. which can be cleaned using solvent washing or other cleaning). There remained the possibility of an additional irreversible contribution from other species (which, as we shall see, can come from Hg vapour).

Meanwhile, in the period 1960–1990 a suite of surface characterisation methods and instruments had been developed by the surface science community and progressively applied to practical problems such as the control of contamination in semiconductor manufacture, for example. Notable amongst these techniques are X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS) and atomic force microscopy (AFM). The XPS technique is an excellent tool to address many types of contamination problem and is a key tool in the characterisation of practical heterogeneous catalysts (18). Briefl y, monochromatic X-rays are directed on a specimen surface and the energies of emitted photoelectrons are analysed. The spectrum of emitted photoelectron intensity as a function of kinetic energy gives information on the composition of the top few nanometres of the surface, a depth limit set by the short distance through which electrons at those energies can pass through matter without energy loss. XPS must be performed

Fig. 1. A group of three platinum-iridium kilogram weights fabricated by Johnson Matthey Noble Metals for the Bureau International des Poids et Mesures. Each is 39.2 mm in diameter and 39.5 mm high



in ultra-high vacuum to prevent contamination of the surfaces under analysis and this makes XPS a relatively expensive analytical technique, but it can be extremely powerful in unambiguous identifi cation of all chemical elements except hydrogen and helium, together with some information about their oxidation state. The key benefi t of XPS is that when applied to the mass stability problem it gives an unambiguous identifi cation of the chemical nature of the contamination, which weighing alone cannot do.

Surface analytical techniques such as XPS were fi rst applied to the analysis of mass standard surfaces by Ikeda et al. (19). Later Martin Seah and myself at NPL, UK, found XPS particularly valuable in showing for the fi rst time the unexpected presence of Hg on samples of Pt-Ir alloy foil stored in the same laboratory environments as kilogram weights. Using XPS we were able to demonstrate that: (a) atmospheric Hg contamination (20) is a problem in causing part of the monotonic increase (21) in Pt-Ir prototype masses; and (b) the remaining mass increase is largely due to the growth of a carbonaceous layer, consistent with Equation (i). Adsorption of Hg onto the surfaces of pgms, with consequent inhibition of their catalytic activity, has been known for almost a century (22, 23) but was a surprise to those working in the fi eld of mass metrology.

Recent Work at Newcastle University

It is twenty years since Hg was fi rst identifi ed on samples of Pr-Ir alloy exposed to laboratory air in a weighing laboratory by XPS analysis of small pieces of metal foil (20). More recently XPS instrumentation has increased in capability in several respects. In particular the sample size that can be accepted into the analytical chambers of modern instruments is much larger than previously possible. This has been largely driven by the needs of the semiconductor community to analyse whole silicon wafers of four inches or more in diameter. Therefore when we came to commission a new state-of-the-art XPS instrument at Newcastle University in 2012 it became clear that we could revisit the problem of Hg contamination on these standard weights directly. We could now analyse the surface of an intact standard weight inside our new XPS instruments.

Of course the Pt-Ir prototype kilograms that form part of the SI are far too valuable to study by these methods. Neither can new ones be fabricated, since it would be diffi cult to replicate with confi dence the polishing

techniques or the exposure of these surfaces over the 130 years since manufacture. Instead, we have analysed imperial weights made of Pt in the Victorian era, but which are now museum pieces:• Reference weight RS1: a Pt troy pound held by the

Royal Society in London since the 1840s• Reference weight RS2: a cylindrical Pt avoirdupois

pound held by the Royal Society since at least 1853 (the container made for it bears this date, though the weight itself has 1844 stamped on it; there is no written record that the Royal Society owned it earlier than 1853, although an earlier date cannot be ruled out)

• Reference weights SM1, SM2, SM3 and SM4: small Pt weights of 1, 2, 3 and 6 grains which have been kept in a circular threaded ivory box, probably since 1830, now held by the Science Museum in London.

No records appear to exist on the origin of the metal for these weights. Consideration of the fabrication dates alone mean that it is possible that the metal for SM1 to SM4 was refi ned using Thomas Cock’s powder metallurgical route, possibly at the then Johnson company that later became Johnson Matthey (24).

XPS spectra were acquired using a Thermo Theta Probe spectrometer (Thermo Scientifi c, East Grinstead, UK). Survey spectra were at a pass energy of 200 eV and narrow scans at 80 eV. Argon cluster ion sputtering was performed using the Thermo Scientifi c™ MAGCIS™ gun at 6000 eV cluster ion energy and using settings that the manufacturer has observed to lead to clusters of approximately 1000 atoms with a wide size distribution (25).

XPS spectra from both RS1 and RS2 show large C 1s peak intensities originating in a thick layer of carbonaceous contamination. RS1 and RS2 show a number of surface species such as zinc, sodium, tin and others and approximately one atomic layer of Hg. A survey spectrum from RS1 is shown in Figure 2.

The steeply rising inelastic background (26, 27) under the Hg and Pt peaks indicates that they are below a thick carbon contamination layer. The spectrum in Figure 3 is from SM4, which shows by far the greatest level of Hg contamination in our study. Remarkably, the Hg 4f peaks in Figure 3 are much stronger than the Pt 4f peaks, showing (given that the intrinsic XPS sensitivity to both elements is similar) that within the XPS accessible depth there is more Hg than Pt. Indeed, if the same Hg accumulates on a Pt-Ir prototype kilogram per unit surface area then that prototype would increase in mass by just



1200 900 600 300 0Binding energy, eV

Cou

nts,

s–1

× 1

04

16

14

12

10

8

6

4

2

0

C K

LL

N

a 1s

Zn

2p

1/2

Zn

2p

3/2

O

KLL

Cu

2p 1

/2 C

u 2p

3/2

Sn

3p 3

/2

Hg

4pO

1s

P

t 4p

Zn

LMM

Sn

5d 5

/2

N

1s

H

g 4d

3/2

H

g 4d

5/2

Pt 4

d 3/

2P

t 4d

5/2

C 1

sC

l 2s

Cl 2

pB

i 4f

Hg

4fP

t 4f

Pt 5

pO

2s

Fig. 2. XPS survey spectrum from reference weight RS1. Mercury and carbonaceous contamination are clearly present. Auger and photoemission features are both present (though the binding energy axis properly applies only to the photoemission peaks)

110 105 100 95 90 85 80 75 70 65Binding energy, eV

Cou

nts,

s–1

× 1

02

100

90

80

70

60

50

40

30

20

10

Fig. 3. Narrow scan spectrum from the surface of SM4. Hg dominates the XPS spectrum here, with the Pt 4f peaks between 70 eV and 76 eV in binding energy having a much lower intensity. Therefore, remarkably, most of the metal within the XPS sampling depth is Hg contamination, not Pt. The region of the Pt peaks between 70 eV and 78 eV is somewhat complex, and probably comprises Pt metal, Pt surface oxide(s) and some intensity from an Al 2p peak originating in alumina inclusions, possibly from polishing during manufacture



under one quarter of a milligram or 0.25 parts per million (ppm).

Argon cluster sputtering of RS1 was continued and after a total of 1320 seconds the Hg and carbonaceous contamination had been very effectively removed, leaving a clean Pt surface and a small amount of inorganic tin contamination. Argon clusters are a relatively new tool in XPS, with the ability to remove carbonaceous material while having a low sputter rate in any underlying inorganic material. Compared to Hg, Pt has some unusual potential electron scattering properties at kinetic energies of around 200 eV (28), an effect which is not relevant however at the specifi c kinetic energies used in this work.

Recently the presence of Hg at the surface of a 19th century Pt kilogram prototype in Switzerland has been confi rmed (29), so that all seven 19th century Pt weights so far directly analysed by XPS show signifi cant Hg contamination.

Carbonaceous Contamination

Figure 4 shows measurements from the present work added to the diffusion-limited growth model of Equation (i). The thickness of carbonaceous contamination, dC, can be calculated from Equation (ii):

dC = ln cos (ii)IPt, GCIB cleaned

IPt, as rec’d( )We used an attenuation length λ = 3.0 nm in this equation for consistency with earlier published work on this problem, though more accurate estimates of the attenuation length are possible in principle (30). The intensity ratio of Pt 4f peaks before (IPt, as rec’d) and after (IPt, GCIB cleaned) argon gas cluster ion beam (GCIB) removal (31) of the carbonaceous contamination therefore gives the thickness of carbonaceous contamination. The emission angle for photoelectrons analysed is . The mass of carbonaceous contamination is then obtained by multiplying the thickness by the surface area of a prototype and the estimated density of the contamination (Equation (iii)).

mC = AdC (iii)

The estimate of density is the main contribution to the uncertainties in this entire calculation, which leads to relatively large error bar in Figure 3. We estimate a density of 1300 kg m–3. This still seems a reasonable value for a dense cross-linked layer,

but in this work we place a large uncertainty on this value, of around 40% following recent work by Davidson where a much lower density was found (32).

Ultraviolet (UV)/ozone treatment (often used in cleaning carbonaceous contamination from silicon wafers in the electronics industry) is also effective in removing organic contamination from Pt and Pt-Ir surfaces (33), at the expense of a monolayer of oxidation of platinum. Figure 5 shows weighing results for RS2 as a methanol, water and UV/ozone (UVOPS) treatment (33) is applied fi ve times in succession. The weight lost at the nth cleaning step approaches zero and XPS confi rms the surface to be as clean as air-exposed sputtered samples after fi ve cleaning steps.

The surface analytical community will welcome the widespread availability of GCIB sources because they are expected to have two key properties:(a) Removing organic material with the minimum of

damage to underlying organic material (34) and(b) Leaving inorganic surfaces (such as metals)

undamaged.

0 5 10 15

( + c)/years

mC(

), g

150

100

50

0

Fig. 4. Growth in carbonaceous contamination, ΔmC, on a Pt-Ir prototype kilogram as a function of the time since last cleaning, . Here c = 0.387 years, representing a small initial contamination layer acquired rapidly before this diffusion-limited model becomes valid. The fi lled square is calculated from XPS measurements on standard weight RS1, scaled to the larger surface area of a kilogram prototype. The straight line is a fi t to out diffusion-limited accretion model (though the fi tted parameters are determined only using the weighing data, not the XPS measurement)

√



The second property is particularly interesting from the point of view of cleaning mass standards in vacuum, where UV/ozone treatment cannot be applied. However our measurements using GCIB and XPS suggest that argon clusters are not suffi ciently selective in this application. Exposure suffi cient to remove the carbonaceous contamination also removes an unacceptable mass of substrate metal (35). GCIB does not therefore offer a viable cleaning method for Pt-Ir prototype kilograms, at least under presently available source conditions, though it has been very useful to us in analysis, helping to quantify the carbonaceous contamination on RS2.

Quantifying Mercury Contamination

Hg is present in signifi cant quantities on all six of the 19th century standard weights we examined (36). The most intense peaks in the XPS spectra of Pt and Hg lie very close to each other in kinetic energy. Therefore the photoelectrons from these peaks have almost the same kinetic energy for Hg and Pt (differing by only around 2%). This means that the inelastic mean free path (IMFP) of photoelectrons from Hg and Pt will be almost the same when passing through any particular material. This allows us to apply a relatively simple but precise equation for estimating the thickness of the Hg contamination (Equation (iv)), a method originally proposed by Hill et al. (37) and later applied by Iwai et al. (38) and Cumpson (39):

dHg = ln 1 + cos (iv)(IHg/sHg)

(IPt/sPt)

where dHg is the effective thickness of the Hg layer, IHg is the intensity of the Hg photoelectron peak, IPt

the intensity of the Pt photoelectron peak and sHg

and sPt are sensitivity factors due to Schofi eld (40). Table I shows that all six standard weights contained signifi cant Hg at their surfaces.

However most of the Hg contamination may already be ‘buried’ in defects and grain boundaries and be invisible to XPS. Normally one would perform a sputter depth-profi le, either by XPS or SIMS, gradually removing layers of metal using an ion beam, to measure

1 2 3 4 5

Number of steps, n

Mas

s lo

ss o

n nt

h U

VO

PS

cl

eain

ing,

g

1500

1000

500

0

–500

Fig. 5. Weighing results for fi ve successive cleanings of reference weight RS2 by the UV/ozone method. Points represent the mass lost during cleaning

Table I XPS-Based Measurements of Mercury at the Surface of Six Platinum Weights Made in the 19th Century (34)a

Mercury determined by XPS

Platinum reference weight

Peak intensity ratioIHg / IPt

Equivalent mercury thicknessdHg, nm

Equivalent mass of mercury if on a Pt-Ir kilogram prototype mHg, g

RS1 0.17 0.23 23

RS2 0.15 0.21 20

SM1 0.77 0.90 87

SM2 0.41 0.53 52

SM3 0.93 1.04 100

SM4 3.75 2.57 249aWe estimate uncertainty in the equivalent mass on a kilogram prototype to be around ±20% at one standard deviation



the concentration of Hg as a function of depth into the surface. This, however, would be destructive. It is very diffi cult to envisage any possible experiments that are both non-destructive and suffi ciently sensitive to detect all the Hg present. Neutron activation analysis (NAA) would have the sensitivity and selectivity to identify Hg within the top few micrometres of the surface, but due to the long-lived activated Ir species created (41) this may be straightforward only for Hg within pure Pt weights, not Pt-Ir alloys. NAA facilities are also quite rare. The best approach may be a skilled application of transmission electron microscopy (TEM), wavelength-dispersive X-ray analysis in a scanning electron microscope (SEM) or electron-probe microanalysis (EPMA) system and dynamic SIMS depth profi ling applied to sample Pt-Ir surfaces.

Hg vapour from accidental spillages from barometers and thermometers is suffi cient to explain the Hg accumulation. Hg has been used in a wide range of scientifi c instruments over the last 200 years. Davis (42) describes the current storage location of the international prototype kilogram. Figure 3 of that paper is a photograph of the safe where the international prototype and its six offi cial copies have been stored since the late 1980s. Beside the bell jars enclosing the international prototype is a small white thermometer containing alcohol and Hg (43). The Hg is fully enclosed and therefore no immediate danger to the prototypes around it. Nevertheless the presence, today, of a glass tube containing liquid Hg even within the storage location of all the prototypes at the centre of the SI shows how pervasive the use of Hg within scientifi c instruments has been. Other sources of Hg exist. It has been suggested that compact fl uorescent light fi ttings may release between 1 mg and 4 mg of Hg when broken (44). Hg vapour has been measured in the breath of some people who have Hg amalgam dental fi llings (45). Measurements of Hg released range from around 0.7 ng (46) to 1.4 ng (47) per breath. Therefore, even when Hg thermometers and barometers are discarded, there remain a few sources of Hg that are almost impossible to eliminate completely.

In industrial catalysis, where Hg vapour can poison pgm catalyst surfaces, Hg can be removed from feedstock chemicals by surface adsorption on metal sulfi de pellets. Four pure metals have been used previously as Hg ‘getters’ in vacuum systems (48): sodium, gold, cadmium and indium. Oxidation of the metal surface can prevent Hg uptake, so for operation in air, of these four candidates only gold is suitable. We

have therefore proposed (36) that it would be sensible to store kilogram prototypes under a thin, clean, gold foil or mesh to reduce Hg reaching the prototype kilogram. This gold ‘mercury getter’ could be inserted inside the existing glass bell jars in which kilogram prototypes are generally stored. It may be that this simple and inexpensive measure alone would halt the divergence of the prototype kilograms (42) suspected since 1939 and confi rmed since 1992.

Conclusions

Kilogram weights made from Pt-Ir alloy have formed the basis of the SI unit of mass for over a century. The choice of material from which to make these weights was an excellent one, given what was known at the time. Barely a few decades later, increasing understanding about the catalytic properties of pgms (as well as the poisoning of such catalytic behaviour by traces of Hg, for example) presaged some modest problems of surface mass-stability that would be increasingly clear in weighing data in the second half of the twentieth century. From around 1990 modern techniques of surface chemical analysis shed light on the mechanisms of this mass instability. Recent measurements on six 19th century Pt artefact mass standards confi rm the presence of Hg. It now seems almost certain that the prototype kilograms that form the basis of the SI have accumulated signifi cant quantities of mercury contamination and that this has caused some (or perhaps most) of the divergence between kilogram prototypes observed in recent years.

Carbonaceous contamination built up over more than a century has been modelled and removed by well-documented solvent and UV/ozone cleaning protocols. Though there is a strong trend to remove all artefact standards from the SI (in the way that, for example, the unit of length has been redefi ned in terms of fundamental quantities instead of the length of a physical bar of metal) the kilogram is still defi ned as the mass of the international prototype of Pt-Ir alloy. Comparisons of mass using state-of-the-art balances are still more precise than the accuracy of other methods of realising a mass standard from fundamental constants or atomic properties. Therefore, once the stability issues arising from Hg and organic contamination are controlled, Pt-Ir prototype kilograms still make extremely good mass standards. It is just possible that they will continue to defi ne the unit of mass in the SI for rather longer than expected.



Acknowledgements

Thanks are due to Dr Keith Moore and Professor Martyn Poliakoff of the Royal Society and Dr Derek Brain and Dr Luke Pomeroy of the London Science Museum for facilitating the artefact loans. The author wishes to thank Dr Jose Portoles, Dr Anders Barlow and Dr Naoko Sano for useful comments, and Mr Mike Foster for useful advice. XPS was performed at the National Engineering and Physical Sciences Research Council (EPSRC) XPS User’s Service (NEXUS) at Newcastle University, an EPSRC Mid-Range Facility.

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48 R. D. Heyding and E. A. Flood, Can. J. Chem., 1954, 32, (6), 591

The Author

Professor Peter Cumpson is Director of NEXUS in the Mechanical Engineering School at Newcastle University. After studying Physics at Cambridge he spent 20 years at the NPL, the largest of the UK’s four national measurement institutes. He moved to Newcastle in 2008 and has built up NEXUS “just down the hall” from the smallest such national measurement institute: the UK Gear Metrology Laboratory. Professor Cumpson has research interests in surface chemical analysis, microelectromechanical system (MEMS) sensors, nanoparticle characterisation and helium ion microscopy.




The 28th Santa Fe Symposium on Jewelry Manufacturing TechnologyBetter understanding of lost wax casting of platinum leading to improved casting alloys

Reviewed by Christopher W. CortiCOReGOLD Technology Consultancy, Reading, UK


The 28th annual Santa Fe Symposium® was held from 18th–21st May 2014 in Albuquerque, New Mexico, USA, and attracted another large attendance of delegates from 15 countries worldwide, representing a good cross-section of those involved in jewellery manufacturing from mass manufacture to specialised craft operations. In general, many were fi nding the market is tougher now than a few years ago although there is a good optimistic outlook.

The programme of presentations was wide ranging from straight technical to other aspects of manufacturing – training, continuous improvement, market trend analysis, the new social ways of conducting business and product liability. On the technical side, platinum and silver featured strongly as did e-manufacturing. As noted in previous reports, the major sponsors of the conference were given the opportunity to have a display table in the lobby area and thus Johnson Matthey New York, Platinum Guild International and Palladium Alliance International had a strong presence and their technical brochures and publications were quickly collected by delegates.

As has become customary, the Symposium began with another in the series: ‘Basic Metallurgy – Part IV: Deformation Processing, Joining and Corrosion’ by Chris Corti (COReGOLD Technology Consultancy, UK) in which the difference between hot and cold working was discussed, along with a review of the various

joining technologies used in jewellery manufacture – soldering, welding and diffusion bonding. Corrosion was discussed in terms of tarnishing of Ag and the carat golds and stress corrosion cracking, which can occur particularly in the low carat golds.

Platinum Casting

The investment (lost wax) casting of Pt jewellery is not an easy process and casting porosity is a feature that casters fi nd diffi cult to avoid. In 2013, Teresa Fryé (TechForm Advanced Casting Technology, USA) gave an excellent presentation on the latest research she had conducted on the benefi ts of hot isostatic pressing (HIP) of 950 Pt castings to remove porosity and the improved properties that resulted, titled ‘The Effect of Hot Isostatic Pressing of Platinum Alloy Casting on Mechanical Properties and Microstructures’. Unfortunately this presentation was not published in the 2013 Symposium Proceedings but has appeared in this year’s 2014 Proceedings. This work has demonstrated that HIP of castings in four 950 Pt alloys results in the porosity, due to shrinkage on solidifi cation, being substantially reduced, if not eliminated. This led to a marked increase in ductility without sacrifi cing tensile strength or hardness. For castings in the 950 platinum-ruthenium alloy, grain size was fi ner and more uniform and this alloy benefi ted the most from the HIP treatment since it tends to have larger amounts of porosity in the as-cast condition, Figure 1. In contrast, the 950 platinum-cobalt alloy, which tends to have the least porosity in the as-cast condition, showed the least benefi t of HIP of all alloys tested. HIP is an additional process stage post casting



(a) (b)

Fig. 1. Test cast ring in Pt-5Ru (centre) showing microsections of castings: (a) as-cast with double sprue, showing substantial shrinkage porosity; (b) after HIP showing removal of porosity (Courtesy of Teresa Fryé (TechForm Advanced Casting Technology, USA))

1 mm 1 mm

and adds to the costs but is fi nding increasing uptake by jewellery manufacturers since it better guarantees product quality.

This year, Ulrich Klotz (Research Institute for Precious Metals & Metals Chemistry (FEM), Germany) gave what for me was the highlight presentation of the Symposium. This was a late entry and consequently, for commercial reasons, cannot be published for a year (so should appear in the 2015 Proceedings.). His presentation, ‘Platinum Investment Casting: Material Properties, Casting Simulation and Optimum Process Parameters’ was a report on a collaborative European research project that has recently been concluded. A major feature was the use of computer modelling of the casting process, both by centrifugal casting and tilt casting and this has aided process optimisation. The effect of casting parameters on porosity is shown in Figure 2. In order to model the casting process, it is necessary to input various physical properties of the materials involved and Klotz reported on the work undertaken to obtain meaningful property data that were used in the simulations. Experimental casting trials were used to validate the modelling results. Both 950 Pt-Ru and 950 Pt-Co alloys, traditionally used in jewellery casting, were tested. Ru raises the melting point of Pt whilst Co lowers the melting point and has a narrower melting range. One feature of the project was some preliminary work to examine whether a tertiary alloy of Pt-Co-Ru would lead to improved castability;

the early results suggest that it did and so opens an opportunity to develop improved Pt casting alloys.

Silver Alloys Containing Platinum Group Metals

There were two interesting presentations on Ag alloys for jewellery application, both of which involved platinum group metals (pgms) as alloying metals. In the fi rst, Shankar Aithal (Stuller Inc, USA) reported on the ‘Development of a Harder Sterling Silver Alloy’. The composition of this new alloy was not revealed except to say it contained fi ve alloying metals including a reduced copper content, elements to confer tarnish resistance and that it is pgm-based. This suggests the main alloying metal is a pgm such as palladium or possibly Pt. Indeed, a 2013 US patent application by Stuller indicates that it is a Ag-Cu-Pd-Sn-Zn composition with 2.75% Pd (1). The new alloy is much harder than conventional sterling silver with an as-cast hardness of 90 HV–110 HV (compared to 65 HV) and can be age-hardened to 140 HV–170 HV, Table I. It also has a superior tarnish resistance. This looks to be a promising commercial alloy as conventional sterling silver (92.5% Ag–7.5% Cu) is considered soft for jewellery applications.

Many new sterling silver (92.5% Ag) alloys have appeared on the market in recent years, most claiming improved tarnish and/or fi restain resistance. Grigory Raykhtsaum (LeachGarner, USA) reviewed these in



an attempt to bring some order and structure to these developments in his presentation, ‘Sterling Silver – U.S. Patent Review’. He reviewed the US patent literature

from 1926 and identifi ed 43 patents which he classifi ed in terms of the objectives: tarnish resistance, improved strength or ‘other’. He noted that there were two periods of strong patent activity: 1926–1940 and 1989–2012. In the latter period, he noted use of pgms (either Pd or Pt) as alloying metals in fi ve out of 19 patents. In one, Pd is claimed to improve tarnish resistance and, in others, Pt is claimed to improve both tarnish resistance and strength.

Powder Metallurgy and Three-Dimensional Printing of Jewellery

Currently in the jewellery industry, rapid prototyping technology is used to produce plastic models direct from digital fi les by 3D printing, using photocurable polymers. These are, in turn, used as models to directly investment cast the fi nal part in precious metals. The next step forward in the digital manufacturing age is the manufacture of jewellery using metal powders by 3D printing (several acronyms are in use, for example, direct metal laser sintering (DMLS), direct metal laser melting (DMLM) or selective laser melting (SLM)) direct from digital designs (computer fi les). This is an additive manufacturing process where a thin layer

Porosity

Fig. 2. Simulation of shrinkage porosity in casting of platinum. Flask temperature of 850ºC and melt temperature of: (a) 1850ºC; and (b) 1980ºC (Courtesy of Ulrich Klotz (FEM, Germany))

4.90% 4.41% 3.93% 3.44% 2.95% 2.46% 1.98% 1.49% 1.00%

(a) (b)

Table I Properties of SS941 Sterling Silver Containing Palladiuma

Colour a* = 0.05, b* = 4.68, L* = 95.08

Density, g cc–1 10.40

Melting range, ºF 1680–1730

Melting range, ºC 916–943

Hardness (as-cast), HV 90–110

Hardness (annealed), HV 68–83

Hardness (age-hardened), HV 140–170

Hardness (60% cold work), HV 150

Ultimate tensile strength (UTS), psi 45,000

Yield strength, psi 20,000

Elongation (annealed), % 35

Elongation (age-hardened), % 30aCourtesy of Shankar Aithal (Stuller Inc, USA)



of metal powder is spread on a build platform and then selectively melted to build up the part layer by layer. It is a relatively new technology developed in the engineering sector and is now being adapted to jewellery and dental markets where items are smaller and require a superior surface fi nish. Several companies have developed suitable equipment for DMLS and some alloy companies are collaborating with them to develop the technology, using suitable precious metal powders for the process, including Au and Pt. Three presentations were focused on this new manufacturing route.

On the technology aspect, Damiano Zito (ProGold SpA, Italy) spoke on the ‘Optimization of SLM Technology Main Parameters in the Production of Gold and Platinum Jewelry’. This presentation was a further development on a project reported at the 2013 Symposium, where surface quality equivalent to that produced by investment (lost wax) casting was the objective. Laser scanning parameters were shown to be important. This year’s work focused on use of selected alloying elements to improve laser absorption by the powders, thus favouring melting, and to examine the structure and morphology of the support structures to optimise their density and maintain adequate thermal dissipation of the laser energy. Both an 18 carat red gold and a 950 fi neness Pt alloy were used in the study as atomised powders of 0–53 microns size range. A lamellar design parallelepiped model was used to study the process parameters on a Realizer SLMTM 50 machine. Two types of support structures with two spacing sizes were evaluated and applied to the model with three different slope angles with respect

to the horizontal build platform. The study showed that Pt required less laser power than 18 carat gold to produce good uniform walls to the model, Figure 3. It was also shown that thin supports are more easily detached from the item and were advisable for high-slope walls, whereas more massive support structures are advisable for horizontal walls.

The addition of semiconductor elements such as germanium and silicon at low concentrations to increase electrical resistivity (i.e. lower thermal conductivity) was found to improve laser absorption of Au and led to a lower contact angle of the molten particle as well as reduced surface roughness. For Pt, which has a much lower thermal conductivity, such additions are not needed. As a consequence, laser power levels needed for melting Pt powders were much less than required for 18 carat gold. It was also found that the Pt alloy produced less porosity than the Au alloy. The work also showed that surface quality can be optimised and processing loss reduced by correct selection of supports.

The support structures used in 3D printing technologies are important and were the focus of a presentation by Frank Cooper (Jewellery Industry Innovation Centre (JIIC), Birmingham City University, UK) in his review, ‘DMLM Supports: Are they the Jewelry Industry’s New Sprue, Riser and Gate Feed?’. He explained the need for support structures and the complex role they play during 3D printing and the types of structures used. As he pointed out, they are sacrifi cial structures that need to be removed after completion of the build and so represent a waste of material and energy.

Fig. 3. Cross-section of walls of model part produced by selective laser melting (3D printing) using 62.5 W laser power and 0.33 m s–1 scanning speed in: (a) platinum and (b) 18 carat gold (Courtesy of Damiano Zito (ProGold SpA, Italy))

200 μm200 μm

(a) (b)



The software used to create the support structures was discussed. Cooper noted that all the jewellery DMLM technologies on the market are supplied with an integral support generation package as part of a full suite of software. He gave examples of their use and how the operator has to select the appropriate orientation and placement of supports for their piece from several options. Looking to future developments, Cooper suggested software would be developed that would automatically analyse a part and determine the optimum build orientation and support structure consistent with good surface fi nish quality. Such developments are already in train.

A more general look at the business model for the commercialisation of 3D printing of jewellery was presented by David Fletcher (Cooksongold, Heimerle + Meule Group, UK) titled, ‘Use of eManufacturing Design Software and DMLS in the Jewelry Industry’. He noted that DMLS technology had now improved to the point where surface fi nish quality matched that achievable by investment casting. Also, that DMLS would not replace existing mature production processes such as lost wax casting as they are cost-effective for many types of design. He considered that e-manufacturing (DMLS) will focus on six areas: removal of tooling costs, light-weighting of products, customisation of design – individualised and incorporating security features, fast lead times (within hours from design to part), cost-effectiveness for designs that take advantage of the strengths of DMLS process and very high design fl exibility. He then went on to discuss nine areas essential to its success, including the user interface, automated build structures and automated light-weighting, customisation and automated costing and quotation models.

Other Manufacturing Processes

Several presentations examined developments in traditional manufacturing and in new technologies. James Binnion (James Binnion Metal Arts, USA) discussed his use of ‘DC Arc Melting for Jewelry Casting’ and Sessin Durgham (Rio Grande Inc, USA) discussed the bench use of welding in his presentation, ‘The Good Weld: Pulse Arc Welding for the Metal Artist, Jeweler and Manufacturer’. The fusing of Au to iron metal work was described by a goldsmith, Chris Nelson (Chris Nelson, Goldsmith, USA) in his presentation, ‘Iron Mused/Gold Fused... “the New Iron Age”’ and the

use of an old technology was described by metalsmith Anthony Lent (Anthony Lent Studios, USA) in his presentation, ‘The Drop Hammer and 19th-Century Die-Forming Processes for Contemporary Artisanal Manufacturing’. Researcher, Bruno Henriques (University of Minho, Portugal) described the ‘Impact of Hot Pressing Processing Parameters in the Production of Powder Metallurgy Jewelry Parts’ in which Ag and coloured Au were combined for decorative effect. The use of diffusion bonding of mixed metals was discussed by Chris Ploof (Chris Ploof Designs, USA) in his presentation, ‘Mokume Gane Bonds: the Effect of Quenching on Bond Strengths’.

Another old technology was described by Jurgen Maerz (Jurgen Maerz, Jewelry Industry Consulting LLC, USA) in his presentation, ‘Before Lost Wax casting: A Look at Traditional Jewelry Making in Germany Using Sand Casting’ whilst Boonrat Lohwongwatana (Chulalongkorn University, Thailand) discussed new developments in thixocasting in his presentation, ‘Semi-solid Casting of Silver and Titanium: From Theory to Practice’. Ag becomes more fl uid in casting. In conventional lost wax/investment casting, rubber moulds are used to make replicas in wax of the master model. Ilaria Forno (Politecnico di Torino, Italy) described her research on silicone rubber for moulds to improve its thermal behaviour and hence model quality and mould life in her presentation, ‘Deepening into Silicone: Is it a Rubber Wall?’. On the traditional craft jewellery front, silversmith Ann Cahoon (North Bennet Street School, USA) investigated several ‘Bench Myths’ with respect to sawing, fi ling and polishing to see if they withstood rational analysis.

The use of photocurable resins for producing jewellery models by rapid prototyping technology (aka stereolithography) has been mentioned earlier. However, Tsuneo Hagiwara (Digital Wax Systems (DWS) Srl, Italy) gave an interesting presentation on ‘New Jewelry Products Produced in Photocurable Ceramic-fi lled Resins’. They can be made more attractive by adding colour and allow complex 3D shapes to be produced including ‘gemstones’. They are hard and wear-resistant, take a good polish and are colour-fast.

Market, Training and Education

Three presentations were focused on education and training needs of the jewellery industry. Elizabeth



Brehmer (LeachGarner) spoke on ‘Is the Jewelry Industry Addressing Training and Educational Needs?’, whilst Lisa Johnson (Rio Grande Inc) spoke on ‘Process Development for Continuous Improvement’ which described her company’s progress. Kageeporn (Kate) Wongpreedee (Srinakharinwirot University, Thailand) discussed jewellery education initiatives in Thailand in her presentation, ‘Jewelry Education Evolution and the Promise of Future Jewelry Technologies’. She has been actively involved in new, innovative university courses to train future supervisors and managers and in research of new technologies to assist the Thai industry, such as lead-free nielloware, mokume gane and gem treatment by ion implantation.

On the marketing front, Juliet Hutton-Squire (Adorn Insight, UK) gave an insight into product development in her presentation, ‘Using Essential Trend Analysis to Successfully Align Jewelry Production with Evolving Consumer Demand’. In contrast, Vashti Jattansingh (Raymond James Financial Inc, USA) gave an insightful look into ‘Product Liability for the Jewelry Manufacturing Industry’ and Anne Miller (IBM, USA) spoke about how the jewellery retail environment is changing in the new digital world and how to respond

in her presentation, ‘Social Business and Jewelry Manufacturing Adoption’.

Concluding Remarks

This was another excellent symposium in the annual series for those involved in jewellery manufacture, be it by machine in mass production or by handcraft in a workshop. All participants acknowledge that a good part of the value in attending is the opportunity for talking with the experts, networking with fellow professionals and for exchanging ideas and samples. The Santa Fe Symposium® proceedings book of the papers and the PowerPoint® presentations can be obtained from the organisers (2). The archive of presentations are being made available for download free, it is understood.

References1 J. R. Butler, Stuller Inc, ‘Sterling Silver Alloy and Articles

Made From Same’, US Patent Appl., 2013/0,112,322

2 The Santa Fe Symposium: http://www.santafesymposium.org/ (Accessed on 13th August 2014)

The Reviewer

Christopher Corti holds a PhD in Metallurgy from the University of Surrey, UK, and has recently retired from the World Gold Council after thirteen years, the last fi ve as a consultant. During this period, he served as Editor of Gold Technology magazine, Gold Bulletin journal and the Goldsmith’s Company Technical Bulletin. He continues to consult in the fi eld of jewellery technology and as a recipient of the Santa Fe Symposium® Research, Technology and Ambassador Awards, he is a frequent presenter at the Santa Fe Symposium. From 1978–1988 he was a Research Manager at the Johnson Matthey Technology Centre, Sonning Common, UK, and from 1988–1992 he was Technical Director at Johnson Matthey’s Colour and Print Division.




Determining Temperature Boundary of the A1(A1+B2) Phase Transformation in the Copper-55 at% Palladium Alloy Subjected to Severe Plastic DeformationHigher temperature of phase transformation found for Cu-55 at% Pd alloy

By Oksana S. Novikova* and Alexey Yu. Volkov Institute of Metal Physics, Ural Division, Russian Academy of Sciences (RAS), 18 Sofi a Kovalevskaya Street, Ekaterinburg 620990, Russia

*Email: [email protected]

The changes in phase state, electrical properties and microhardness of copper-55 at% palladium alloy samples with different initial states (as-quenched and deformed via severe plastic deformation (SPD)) were studied during isothermal annealing. Ordered B2-phase formation in the disordered (A1) matrix was found to occur at a signifi cantly higher temperature than is indicated in the generally accepted phase diagram of the Cu-Pd system. Corresponding electrical resistivity is also lower than reported elsewhere for alloys of similar compositions, at = (27.67 0.04) × 10–8 m, making this the lowest resistivity yet reported for a Cu-Pd alloy with 55 at% Pd.

Introduction

The physico-mechanical properties of Cu-Pd alloys vary signifi cantly during atomic ordering (for example, resistivity is reduced, mechanical properties and corrosion resistance are improved). These alloys have long been used in dentistry, instrument making and jewellery.

There have been few studies on the kinetics of phase transformation in Cu-Pd alloys with Pd content higher than 50 at%. For instance, only three articles concerning alloys with a Pd content higher than 50 at% were mentioned in the most recent review on the Cu-Pd system (1). One of the reasons for this is the low transformation rate: it is mentioned (2) that only a 20% volume fraction of the ordered B2 phase was observed in the Cu-55 at% Pd alloy after four months of continuous annealing with very slow cooling from 500ºC to 250ºC. For the same reason, the temperature of the start of the formation of the two-phase state in a disordered matrix, i.e., the boundary of A1(A1+B2) transformation in the phase diagram (Figure 1), remains undetermined in this alloy. To the knowledge of the present authors, no more relevant literature data is available at the time of writing.

However, a study of Cu-Pd alloys with enhanced Pd content is both of scientifi c and practical interest: new perspectives in the use of these alloys in the capacity of exhaust gas catalytic converters and membrane materials for hydrogen separation from gas mixtures have recently been disclosed (3).

Earlier, it has been established that preliminary SPD leads to substantially accelerated kinetics of the A1B2 phase transformation in an equiatomic Cu-Pd alloy, which signifi cantly reduced the formation time of an equilibrium state in this alloy (4). The aim of the present work is to study the phase composition of Cu-55 at% Pd (hereinafter Cu-55Pd) alloy samples in the course of isothermal annealing after preliminary SPD.



Experimental Procedure

The initial components taken for the alloy preparation were of 99.98% purity. Melting was performed in a vacuum of at least 10–2 Pa and the alloy was poured into a graphite crucible.

An alloy ingot 8 mm in diameter was homogenised at 850ºC for 3 h and quenched in water. Deformation was carried out at room temperature in two stages without intermediate annealing. First, the ingot was drawn from its initial diameter of 8 mm up to a rod with 3 mm diameter. Part of the rod was then drawn to a wire 0.22 mm in diameter (for resistivity measurements). The rest of the rod was cold rolled to plate samples 0.2 mm thick (for X-ray diffraction (XRD), transmission electron microscopy (TEM) and microhardness measurements). After SPD, the wire samples have a degree of true deformation of ε ≈ 7.1 and plate samples were deformed up to ε ≈ 3.8. Some experiments were carried out on as-quenched alloy samples in the disordered state fi xed by quenching from 750ºC in water. This was necessary to compare rates of phase transformation depending on initial alloy state.

The chemical composition of all samples studied in this paper was analysed using a JEOL JCXA-733 microprobe instrument (accelerating voltage of 25 kV, probe current of 50 nA). The analysis showed that the sample composition was slightly lower in Pd than expected, at 66.9 wt% Pd, with the balance 33.1 wt% Cu. The measurement error did not exceed 0.4 wt%. The impurity content in the alloy was less than the lower limit of instrument sensitivity at 0.05 wt%. The major impurity detected was platinum. Thus the exact composition for the alloy studied was 45.3 at% Cu-54.7 at% Pd, which is referred to in this paper as Cu-55Pd.

The electrical resistivity of the samples was measured by the standard four-probe technique, at a direct current of I = 10 mA. Samples were fi rst subjected to isothermal annealing of different durations followed by quenching in water. All of the heat treatments were carried out in evacuated quartz or glass ampoules. Resistometric investigations were carried out at room temperature. The absolute deviation of the electrical resistivity measurements was found to be Δ = 0.04 × 10–8 m (for experiment details, see (5)).

(Cu,Pd)

Palladium, wt%

30 35 40 45 50 55 60 65 70

Tem

pera

ture

, ºC

650

600

550

500

450

400

350

300

250

200

2DLPS

1DLPS

20 25 30 35 40 45 50 55 60

Palladium, at%Fig. 1. Portion of the phase diagram of the Cu-Pd system (1). The vertical line denotes the alloy composition studied. The sign “” marks the temperature boundary of the A1→(A1+B2) transformation of the Cu-55Pd alloy that was determined in this study



Standard -2 XRD scans were collected using a Rigaku D/MAX-2200/PC diffractometer. The CuK radiation was monochromatised by a graphite single crystal. An estimation of lattice parameters of disordered and ordered phase was carried out by the precise lattice parameter determination software of the diffractometer. Uncertainty of the estimation did not exceed 0.00007 nm.

Measurements of samples’ microhardness were carried out by the standard method on the PMT-3 device, with a 50 g load.

The microstructure was investigated by TEM using a JEM 200 CX microscope with accelerating voltage of 160 kV. The foils for TEM investigations were made out of the plate samples by electroetching.

Results and Discussion

Electrical resistivity as a function of holding time at 350ºС was measured for different initial states of the Cu-55Pd alloy (Figure 2). Samples were subjected to preliminary SPD (ε ≈ 7.1) (curve 1) or quenching (750ºС) (curve 2). The maximum duration of heat treatment was 306 h.

The change in electrical resistivity is clearly shown in Figure 2 to be fi xed only in the strongly deformed sample (curve 1). After an incubation period, the resistivity of the sample after SPD starts to decrease, which indicates the beginning of the phase transition. After holding for 306 h the resistivity of this alloy sample changed to = 27.67 × 10–8 m. Note that this is the fi rst time such a low resistivity has been recorded for the Cu-55Pd alloy. The minimum electrical resistivity value previously reported for the Cu-55.31Pd alloy was = 43.5 × 10–8 m (6).

From the shape of curve 1 in Figure 2 it can be concluded that holding at a temperature of 350ºС for 306 h is obviously insuffi cient for reaction completion. Indeed, the rate of resistivity decrease was still signifi cant in the later stages of this experiment and, consequently, the formation of the structure had not yet been terminated by the end of the experiment. Thus, the electrical resistivity of the studied alloy in the equilibrium state at 350ºС is expected to be considerably lower than that which has already been attained.

As was shown earlier (7–9), a change in physico-mechanical properties is closely connected with structure related phase transformations (A1B2) that take place in Cu-Pd alloys. For instance, in the

course of atomic ordering of the Cu-50Pd alloy its resistivity virtually falls by an order of magnitude (10). In terms of these experimental results (Figure 2) it can be concluded that after preliminary SPD the atomic ordering process in the investigated alloy occurs quite actively at 350ºС. However, this conclusion contradicts the phase diagram (Figure 1). In order to check the resistometry data, XRD analysis was employed (Figures 3(a) and 3(b)).

The -2 XRD scan for the sample after quenching from 750ºС only contains lines of the A1 (face-centred cubic (fcc)) phase (Figure 3(a), XRD pattern 1). The lattice parameter of the alloy in the disordered state was measured to be a = 0.3779 nm. The phase state after annealing of the quenched alloy for 306 h at 350ºC does not appear to change (XRD pattern 2). In comparing XRD scans in Figure 3(a), a redistribution of peak intensities should be noted. Moreover, the lattice parameter of the alloy after annealing for 306 h remains unchanged at a = 0.3779 nm. This is close to the corresponding literature value а = 0.3781 nm for an alloy of similar composition (1).

The XRD scan of the alloy subjected to SPD (ε ≈ 3.8), as well as of the alloy after quenching from 750ºС, contains lines of only the A1 (fcc) phase (Figure 3(b), XRD pattern 1). The lattice parameter of the alloy after SPD is a = 0.3783 nm, which is a little larger in comparison with the lattice parameter of this alloy in the quenched state. There is no reported data on the lattice parameter for Cu-55Pd alloy subjected to SPD. A distinction between the lattice parameters of the fcc-phase of Cu-Pd alloys after quenching and SPD has been observed earlier (10) and had the same order of magnitude. The increase in the lattice parameter of

Fig. 2. The dependence of resistivity on the holding time at 350ºС of Cu-55Pd alloy in different initial states: after SPD (curve 1) and after quenching from 750°С (curve 2)

45

40

35

30

25

, 1

0–8

m

3 4 5 6log t, s

2

1



Cu-Pd alloys after SPD can be assumed to result from the formation of a nanocrystalline state. In this state, the crystal lattice parameter is determined by the proximity of boundaries and the large value of their relative volume. The size of the regions of coherent scattering, d, of the Cu-55Pd alloy after SPD has been estimated by the Williamson-Hall method (11) and is determined as d = 56 nm.

After annealing of the Cu-55Pd alloy preliminarily subjected to SPD (ε ≈ 3.8) for 306 h at 350ºC, the two-phase state (A1+B2) is observed (Figure 3(b), XRD pattern 2). In the XRD pattern 2 (Figure 3(b)), well-pronounced lines from an ordered B2 phase (а = 0.2980 nm) are revealed in the background of reflections from a disordered A1 phase (а = 0.3781 nm). The value of the lattice parameter a of the Cu-55.3Pd alloy in the ordered state is reported to have been equal to 0.2978 nm (1). It can be noted that the values of the lattice parameters а = 0.3781 nm (A1-phase) and а = 0.2978 nm (B2-phase) (1) were not of experimental origin but were calculated on the basis of earlier data (2).

As follows from the phase diagram (Figure 1), the annealing temperature chosen in the present experiment belongs to the boundary of the phase transition A1(A1+B2) for the Cu-55Pd alloy. However, proceeding from all the results obtained (Figures 2 and 3(b)), a true boundary of the existence of a single-phase disordered state that is characteristic of the alloy under investigation exists at a higher temperature. As was already mentioned above, a determination of the boundary temperature was earlier performed on

the quenched alloy, where the rate of ordered phase formation is very low. A preliminary SPD considerably accelerates the phase transformation in the studied alloy in the course of subsequent heat treatments. This can be seen in Figure 2.

On the basis of the results obtained, an attempt was made to determine a temperature interval for the boundary between the two-phase (A1+B2) and completely disordered (A1) states characteristic of the alloy under investigation. All further experiments were carried out on the samples subjected to SPD. Annealings were performed in the temperature interval from 350ºС to 550ºС with a step of 50 K.

A set of XRD patterns taken from the alloy samples after annealing in the specifi ed temperature interval for 336 h is presented in Figure 4(a). In the spectra distinct refl ections from the B2 phase are observed in the background of refl ections from the fcc matrix after annealing between 400ºС and 450ºС. Moreover, in the sample annealed at 500ºС, traces of an ordered structure are also present. In this XRD pattern, elevations above background intensity in the regions of 2 ≈ 43º and 2 ≈ 79º angles exhibit clear distinction and are in correspondence with the peaks from the B2 phase (marked with a circle).

Figure 4(b) shows a set of XRD patterns taken from the alloy samples after annealing at 500ºC, 520ºC, 540ºC and 550ºС for 336 h (2 ranged from 42º to 44º). After annealing the sample at 500ºС, the peak from the B2 phase at 2 ≈ 43º is seen to be the most intense (lowest XRD pattern, Figure 4(b)). With increasing annealing temperature the intensity of the refl ection decreases.

200A1

331A1

400A1

222A1

311A1

220A1

111A1 420A1

2

1

Inte

nsity

20 40 60 80 100 120 1402, º

2

1

001B2

011B2

111B2

012B2

002B2 112B2

013B2

111A1 200A1 220A1 311A1 222A1 331A1 420A1

Inte

nsity

20 40 60 80 100 120 1402, º

Fig. 3. XRD patterns from the alloy Cu-55Pd in the initial state (pattern 1) and after annealing (pattern 2) at a temperature of 350ºС for 306 h (2): (a) quenched; (b) subjected to SPD

(a) (b)



No ordered phase was detected in the alloy subjected only to annealing at a temperature of 550ºС (upper XRD pattern, Figure 4(b)). The revealed temperature boundary of the A1(A1+B2) transformation of the Cu-55Pd alloy is located signifi cantly higher than is expected from the conventional phase diagram (Figure 1).

As has been demonstrated elsewhere (5, 8, 9), it is possible to study the kinetics of phase transformation in Cu-Pd alloys with the help of resistivity measurements. In Figure 5 it can be seen that during the course of the experiments the electrical resistivity of virtually all the alloy samples decreased. This unambiguously testifi es to the occurrence of the processes of atomic ordering in the material. And it is noteworthy that after annealing at temperatures below 450ºС for 336 h the electrical resistivity of the alloys studied exhibited a notable decrease. A heat treatment at 500ºС for 96 h led to an insignifi cant change in ; further increases in the annealing duration hardly affected the resistivity of the alloy. In the course of holding the alloy at a temperature of 550ºС, some increase in its resistivity can be observed. In this case, the growth of the electrical resistivity of an initially deformed alloy is caused by recrystallisation that develops in the course of annealing and by the formation of the short-range order under subsequent quenching. The obtained resistometric data are in full agreement with the XRD analysis.

The phase transformation in a previously deformed alloy is accompanied by one additional solid-state reaction, namely, by recrystallisation (12). Thus, in the course of heat treatment, in the bulk of material there simultaneously coexist three structural components, namely: an initially deformed matrix, recrystallised grains of a disordered phase and nuclei of a new, ordered phase. Based on the data presented above, the temperature interval of the phase transformation (Figure 4) and its kinetics (Figure 5) can be assessed.

200A1

331A1

400A1

222A1311A1

220A1111A1

420A1

Inte

nsity

20 40 60 80 100 120 1402, º

011B2

Inte

nsity

42.0 42.5 43.0 43.5 44.02, º

001B2 011B2 111B2 012B2 222B2112B2

002B2

550ºC

500ºC

450ºC

400ºC

550ºC

540ºC

520ºC

500ºC

Fig. 4. XRD patterns from the studied alloy subjected to SPD and subsequent holding at various temperatures for 336 h: (a) 400ºС –550ºС; (b) 500ºС –550ºС (2 = 42º –44º)

(a) (b)

Fig. 5. The dependence of the electrical resistivity of Cu-55Pd on the holding time at different temperatures

, 1

0–8

m

45

40

35

30

25 3 4 5 6

log t, s

350ºC

400ºC

450ºC

500ºC

550ºC



The plot of microhardness vs. treatment temperature allows the origin of the recrystallisation processes in the alloy under consideration to be assessed (Figure 6).

After annealing at temperatures from 250ºС to 350ºС, the microhardness of the studied alloy increases. As shown in Figures 2 and 3(b), the process of atomic ordering develops in this temperature interval. Thus, the increase in strength is governed by the appearance of a large quantity of nuclei of the ordered phase in

a highly deformed matrix. An analogous phenomenon was observed earlier after low-temperature annealing of the SPD-treated alloy Cu-40Pd (7). It has been established that in this case a fi ne-grain structure (grain size of 2 μm–3 μm) is formed in the material with approximately equal volume amounts of the ordered and disordered phases.

Annealing in the temperature interval from 400ºС to 450ºС causes a considerable reduction in the hardness of the alloy (Figure 6), which can be explained as a result of the development of recrystallisation processes. Increasing the temperature of heat treatment to 550ºС is accompanied by decreasing microhardness to very low values typical of a perfectly recrystallised, disordered state of the alloy. Figure 4 shows that at this temperature the recrystallised grains of only one, disordered, phase grew in the severely deformed matrix of the alloy studied; the formation of an ordered phase does not take place.

Figure 7(a) shows a typical TEM image of the microstructure of the Cu-55Pd alloy after SPD and annealing at 500ºС for 336 h. After such treatment the alloy forms a two-phase (А1+В2) state: precipitations of the ordered body-centred cubic (bcc)-phase (B2) are observed in the fcc-matrix (A1). Precipitations of the B2 phase generally have a rounded shape and their most probable size is less than 0.2 μm. Bright-fi eld image of the microstructure containing the B2-phase coarse precipitation and the selected area diffraction pattern

Hv 5

0

1800

1600

1400

1200

1000

800

0 100 200 300 400 500 600Temperature, ºC

Fig. 6. The dependence of microhardness on the holding temperature for 336 h of samples of Cu-55Pd alloy subjected to SPD

Fig. 7. The bright-fi eld images of Cu-55Pd alloy after SPD to ε ≈ 3.8 (the plate 0.2 μm thickness) and annealing at 500ºC for 336 h: (a) a typical microstructure; (b) the microstructure containing B2-phase precipitation (the inset shows a selected area diffraction pattern of the precipitation)

0.2 m 0.2 m

(a) (b)

222

112

002

211012

110



of the precipitation are shown in Figure 7(b). The selected area diffraction pattern is a superposition of two cross-sections of the reciprocal lattice of the B2 phase and does not have additional refl ections. The general picture of the microstructure in Figure 7 is in good agreement with the results of XRD analysis (see lowest XRD pattern, Figure 4(b)).

It can also be noted that, despite the high temperature and the long annealing time, a residual dislocation density remains (Figure 7(a)) which is caused by the incomplete recrystallisation process. More detailed TEM investigation would be beyond the scope of this paper.

Conclusions

In summary, the temperature boundary of the A1(A1+B2) transformation of the Cu-55Pd alloy takes place at around 550ºC, approximately 200ºC higher than expected from the generally accepted Cu-Pd phase diagram. Preliminary SPD has been shown to greatly accelerate phase transformation in the Cu-55Pd alloy. However, after annealing for 336 h at temperatures below 450ºС, the equilibrium phase state had not been reached. The room temperature electrical resistivity of the Cu-55Pd alloy in the annealed state obtained while holding at 350ºС for 306 h after preliminary SPD is = (27.67 0.04) × 10–8 m. This resistivity is much lower than that indicated elsewhere as the minimum value for alloys with similar compositions.

Acknowledgments

The work was performed within the framework of the theme 'Deformation' No 01201463331 (projects No 12-U-2-1004 and No 14-2-NP-118).

References 1 P. R. Subramanian and D. E. Laughlin, J. Phase

Equilib., 1991, 12, (2), 2312 F. W. Jones and C. Sykes, J. Inst. Met., 1939, 65,

(2), 4193 G. S. Burkhanov, N. B. Gorina, N. B. Kolchugina,

N. R. Roshan, D. I. Slovetsky and E. M. Chistov, Platinum Metals Rev., 2011, 55, (1), 3

4 A. Yu. Volkov, O. S. Novikova and B. D. Antonov, Inorg. Mater., 2013, 49, (1), 43

5 O. S. Novikova and A. Yu. Volkov, Phys. Met. Metallogr., 2013, 114, (2), 162

6 R. Taylor, J. Inst. Met., 1934, 54, (1), 2557 A. B. Telegin, N. N. Syutkin and O. D. Shashkov, Fiz.

Met. Metalloved., 1981, 52, (3), 627, (in Russian)8 A. Yu. Volkov, Platinum Metals. Rev., 2004, 48, (1), 39 T. Shiraishi, J. Japan Inst. Metals, 1982, 46, (3), 245 10 A. Yu. Volkov, O. S. Novikova and B. D. Antonov, J.

Alloys Compd., 2013, 581, 62511 G. K. Williamson and W. H. Hall, Acta Metall., 1953, 1,

(1), 2212 B. A. Greenberg, N. A. Kruglikov, L. A. Rodionova, A. Yu.

Volkov, L. G. Grokhovskaya, G. M. Gushchin and I. N. Sakhanskaya, Platinum Metals Rev., 2003, 47, (2), 46

The Authors

Oksana S. Novikova is Junior Researcher at the Institute of Metal Physics, Ural Branch, RAS. She studies disorder-order phase transformations of palladium and noble metal alloys and their microstructure, physical and mechanical properties. She is a co-author of 6 scientifi c papers.

Dr Alexey Yu. Volkov is Head of the Laboratory, Institute of Metal Physics, Ural Branch, RAS. His scientifi c interests focus on a wide range of problems in material science, including evolution of microstructure, kinetics of order-disorder transformations, and the physical and mechanical properties of ordered alloys based on gold and noble metals. He is a co-author of 8 patents and 91 scientifi c papers.




Angel Cuesta is a Senior Lecturer at the University of Aberdeen, UK. His research is of interest in the field of materials for electrochemical applications and focuses on combining classical electrochemical techniques, in situ vibrational and optical spectroscopy and in situ scanning probe microscopy to obtain as detailed a description as possible, at the molecular level, of the electrode-electrolyte interface and of electrocatalytic reactions.

About the Research

The kinetic activation of the electrochemical reactions, particularly that of the oxygen reduction reaction (ORR) taking place at the cathode, is probably the major contribution to the loss of efficiency of fuel cells and, consequently, is responsible for the high platinum loadings required in the membrane electrode assemblies (MEAs). This is a problem of a fundamental nature, associated with the sluggishness of the heterogeneous electron transfer. The design of more active and/or cheaper, durable electrocatalysts can only be achieved through a detailed understanding, at the molecular level, of the structure of the electrode-electrolyte interface and of the mechanism of electrocatalytic reactions.

The interest of the group has focused recently on the study of atomic ensemble effects in electrocatalysis and on trying to elucidate the mechanism of fuel cell relevant electrocatalytic reactions with time-resolved spectrokinetic experiments. The ‘site-knockout strategy’, by which only one kind of atomic ensemble is removed from the surface of a metal, leaving the electronic properties of the rest of the surface

unaffected (or affected only to a negligible degree) showed that, in the absence of trigonal sites, the oxidation of formic acid and methanol on Pt(111) electrodes proceeds exclusively through the direct pathway, the path leading to the formation of adsorbed carbon monoxide, a catalytic poison, being completely blocked (see Scheme I). Similarly, in sulfuric acid or phosphoric acid solutions, removing trigonal sites was shown to multiply the rate at which the ORR proceeds on Pt(111) electrodes at 0.85 V vs. reversible hydrogen electrode (RHE) by 25 and 10, respectively.

In the Lab

Towards a Molecular Level Understanding of Electrochemical Interfaces and Electrocatalytic ReactionsJohnson Matthey Technology Review features new laboratory research

About the Researcher

• Name: Angel Cuesta• Position: Senior Lecturer• Department: Department of Chemistry• University: University of Aberdeen• Street: Meston Building, Meston Walk• City: Aberdeen• Post Code: AB24 3UE• Country: UK• Email Address: [email protected]• Website: http://www.abdn.ac.uk/staffnet/profiles/

angel.cuestaciscar/



CH3OH

H2O

CO + 4H+ + 4e–

non-CO pathway

H2O 2H+ + 2e–

CO2

6H+ + 6e–

CH3OH

CO + 4H+ + 4e–

non-CO pathway

H2O

2H+ + 2e–

CO2

6H+ + 6e–

H2O

Scheme I. The oxidation of methanol on platinum (111) electrodes: (a) with trigonal sites and (b) without trigonal sites

(a)

(b)

3

2

1

00 200 400

1.5

1

0.5

0

Inte

grat

ed a

bsor

banc

e, c

m–1

time, seconds

0 50 100

× 35

10 (dICO /dt), cm

–1 s–1

Inte

grat

ed a

bsor

banc

e, c

m–1 3

2

1

00 200 400

6

4

2

0

time, seconds

10 (dICO /dt), cm

–1 s–1× 20

(a) (b)

1.5

0.1

0.5

0

10 (d

I CO/d

t), c

m–1

s–1

0 2 4 6 8

102 IHCOOad, cm–1

10 (d

I CO/d

t), c

m–1

s–1

0 2 4 6

102 IHCOOad, cm–1

6

4

2

0

(c)(d)

Fig. 1. The dehydration of formic acid on platinum to adsorbed carbon monoxide: time dependence of the integrated absorbance of HCOOad (IHCOOad, black), of the integrated absorbance of on-top COad (ICO, red), and of dICO/dIHCOOad (blue) obtained from a series of time-resolved ATR-SEIRA spectra recorded during the dehydration of HCOOH on Pt at (a) 0.405 V vs. RHE; (b) 0.355 V vs. RHE; and the dependence of dICO/dIHCOOad on IHCOOad during the initial stages of the reaction at (c) 0.405 V; (d) 0.355 V. The inset in (a) shows the first 100 s in an expanded scale



In situ infrared spectrokinetic studies of the electrooxidation of formic acid with a time resolution of ca. 100 ms were possible thanks to the high sensitivity of surface-enhanced infrared absorption spectroscopy (SEIRAS) in the attenuated total reflection mode (ATR-SEIRAS). As shown in Figure 1, the dehydration of formic acid on Pt to adsorbed CO (COad) could be monitored. A clear correlation was found between the coverage by bridge-bonded bidentate adsorbed formate (HCOOad) and the rate at which COad forms at low total coverage. After reaching a maximum, the rate of COad formation gradually decreases to zero with increasing COad coverage, in agreement with the site demanding nature of this reaction unveiled by a previous study of atomic ensemble effects (Scheme I). The results also showed a linear decrease of the logarithm of the rate constant of the reaction of formation of COad from HCOOad with increasing potential. This is consistent with previous observations and suggests an electrochemical mechanism for the dehydration of formic acid to COad on Pt, in which the first step is the oxidative electroadsorption of formate (HCOOH → HCOOad + H+ + e–) and the second and rate-determining step is the reduction of adsorbed formate (HCOOad + H+ + e– → COad + H2O).

Current work in the group is focused on testing the role of HCOOad in the direct path of the electrooxidation of

formic acid (in collaboration with Professor Masatoshi Osawa, Catalysis Research Center, Hokkaido University, Japan), on performing spectrokinetic studies of the electroreduction of CO2 and on the application of ATR-SEIRAS to the study of the nafion/catalyst interface in MEAs under working conditions.

Selected Publications

G. Cabello, E. P. M. Leiva, C. Gutiérrez and A. Cuesta, Phys. Chem. Chem. Phys., 2014, 16, (27), 14281

J. Joo, T. Uchida, A. Cuesta, M. T. M. Koper and M. Osawa, J. Am. Chem. Soc., 2013, 135, (27), 9991

A. Cuesta, G. Cabello, M. Osawa and C. Gutiérrez, ACS Catal., 2012, 2, (5), 728

M. Osawa, K.-i. Komatsu, G. Samjeské, T. Uchida, T. Ikeshoji, A. Cuesta and C. Gutiérrez, Angew. Chem. Int. Ed., 2011, 50, (5), 1159

A. Cuesta, ChemPhysChem, 2011, 12, (13), 2375

A. Cuesta, G. Cabello, C. Gutiérrez and M. Osawa, Phys. Chem. Chem. Phys., 2011, 13, (45), 20091

D. Strmcnik, M. Escudero-Escribano, K. Kodama, V. R. Stamenkovic, A. Cuesta and N. M. Marković, Nature Chem., 2010, 2, (10), 880

A. Cuesta, J. Am. Chem. Soc., 2006, 128, (41), 13332

For more information, please visit: http://www.alfa.com/




247th American Chemical Society National Meeting and Exposition: Part IIMetal organic framework coverage from the ACS spring conference on ‘Chemistry and Materials for Energy’

Reviewed by Ian CaselyJohnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading, RG4 9NH, UK


Introduction

American Chemical Society National Meetings and Expositions are held twice a year in spring and autumn and constitute the largest gathering of chemical scientists at any point in the conference calendar. This year the 247th meeting (1) was held from 16th–20th March 2014, hosted at the Dallas Convention Centre, Texas, USA. The overarching theme of the conference was Chemistry and Materials for Energy which was refl ected in the focus of the Plenary and Kavli Foundation sponsored Lectures.

This selective review will focus on metal organic frameworks (MOFs), covering a range of synthetic and applied work. Part I of this review covered a range of biomass related work and was published previously (2).

Stability and Purifi cation

MOFs are a relatively new class of highly ordered crystalline solids consisting of metal ions or metal-oxo clusters linked together by organic linkers (3). The resulting highly porous structures contain numerous accessible channels and pores which make them interesting candidates for myriad applications including gas separation and storage, catalysis and sensing.

Jared DeCoste (Leidos Inc, USA) presented an overview of the use of MOFs in air purifi cation of toxic chemicals and in particular focused on efforts to overcome the common issue of water stability applied to real situations (4). Current focus on respiratory protection masks is concerned not just with chemical weapons protection in military applications but also toxic industrial chemicals in a much broader context, including ammonia, carbon monoxide and volatile sulfur and phosphorous gases. Typically activated carbon has been widely used and modern formulations involve impregnating the carbon with a range of different metal salts and amines to optimise the range of protection. Although the performance of any sorbent for the removal of toxic chemicals is critical, they also need to withstand high humidity and temperature during storage as well as in use (with no decrease in their effi cacy) and be robust to rough handling without creating dust. MOFs have garnered much interest for respiratory applications recently as they typically have highly porous structures which makes them ideal for gas sorption, they are highly tuneable in terms of metals, linkers and incorporated functional groups and can show signifi cant thermal stability.

The water stability of MOFs has been studied extensively, but great focus has been placed on copper-1,3,5-benzenetricarboxylate (Cu3(BTC)2) referred to as Cu-BTC probably due to its superior chemical removal capabilities and that its water degradation is on a slow timescale making it conducive to systematic study. The adsorption capacity of Cu-BTC has been experimentally found to be between 27–32 mmol g–1 at 298 K and to occur



via two energetically distinct processes, with water molecules firstly coordinating to the metal site and subsequently filling the pores. Water coordinates to the metal centres along the Cu-Cu axis in the bimetallic paddlewheel structure and results in a colour change from dark purple to light blue as the dehydrated material is exposed to moisture. At 40% relative humidity (RH) and 313 K very little degradation occurs over 28 days, whereas at 90% RH and 298 K the structure is almost completely destroyed within two weeks. This process was tracked by X-ray diffraction (XRD) which showed a new structure had been formed, as well as infrared (IR) showing the formation of protonated carboxylic acid groups and 13C magic angle spinning (MAS) nuclear magnetic resonance (NMR) which showed the presence of numerous aromatic and carboxylic acid resonances remote from the Cu centres.

In Cu-BTC, ligand functionalisation is not as simple as just changing the functional groups on the linker due to increased steric hindrance and unfavourable reaction conditions during synthesis. Therefore post-synthesis modifi cation needs to be used and one approach has used plasma-enhanced chemical vapour deposition (PECVD) using perfl uoroalkanes to coat the surface and pores of Cu-BTC, thereby enhancing the water stability (5). Material treated in this way displays superhydrophobicity by fl oating on water and also shows an increased contact angle on a pressed pellet from around 60º to 120º, Figure 1.

Incorporation of these hydrophobic fl uorinated groups renders the material stable in liquid water over 24 h compared to the complete degradation of untreated Cu-BTC and the ammonia removal capacity was not affected. After ageing at 100% RH and 313 K for three days this material showed an ammonia removal capacity almost fi ve times greater than the untreated material, due to the structure being more robust over time.

Post Synthetic Modifi cations

One of the big challenges in current MOF research is chemists having at their disposal enough synthetic routes to access any desired new materials and structures. Common problems with traditional techniques such as solvothermal synthesis are a limit on the type of functional ligands that can be used. Incorporating coordinatively unsaturated metal centres can be diffi cult due to reaction with many multitopic

linkers and in some cases diffi culty in controlling the arrangement of metals and linkers as the low-energy structure is not always the desired product.

A number of different procedures have been developed to mitigate these issues and recently post-synthetic modifi cation of the linker or metal has proven successful. Joseph Hupp (Northwestern University, USA) presented work on solvent assisted linker exchange (SALE), a straightforward technique whereby a parent MOF crystal is exposed to the desired linker in a carefully chosen solvent where the aim is to produce a new MOF material possessing the topology of the parent structure but incorporating the linkers from the reaction solution (6). A challenging target in this area is the incorporation of a variety of longer linkers into a structure and pillared-paddlewheel MOFs have proven to be a facile means by which to study this. Scheme I shows the range of linkers examined and structures formed, starting from solvent-assisted linker exchanged material-5 (SALEM-5) which was itself formed by SALE from a previously isolated MOF via exchange with linker L1, creating pillars approximately 9 Å long. In particular, this work aimed to investigate the incorporation of longer linkers and the effect of pKa of the conjugate acid of the pillar.

Reaction of SALEM-5 with a four-fold excess of the 11 Å long pillar L3 was conducted in dimethylformamide (DMF) and after 24 h at 100ºC, 1H NMR spectroscopy

(a) (b)

(c) (d)

Fig. 1. Pictures of Cu-BTC: (a) dispersed in water; (b) treated with C2F6-plasma, repelling and fl oating on top of water. Contact angle images of: (c) Cu-BTC; and (d) C2F6-plasma-treated Cu-BTC with 2 μl droplet of water (Reprinted with permission from (4). Copyright (2014) American Chemical Society)



showed that quantitative exchange of the parent linker had occurred. Powder XRD analysis of the product showed it to still be highly crystalline and that the fi rst refl ection peak had shifted from 2θ = 5.6º to 4.76º, suggesting a larger unit cell which would be consistent with incorporation of the larger linker. Following this, similar reactions were run but using the longer linkers L4 (14 Å) and L5 (17 Å) to probe the limits of the SALE process. By using L4 a steady increase in the amount of product SALEM-7 was seen by 1H NMR spectroscopy and a maximum ligand exchange of 90% could be achieved over the course of four days by regularly changing the reaction solution with fresh linker. Following this trend, the reaction with the longest linker L5 proceeded very slowly, likely due to the large degree of strain associated with incorporation of such a large pillar and only approached >90% exchange and formation of SALEM-8 after two weeks. Consistent with the previous XRD data, the fi rst refl ection in SALEM-7 and SALEM-8 lies at decreasingly lower angles of 2θ = 4.20º and 3.72º, respectively, consistent with an increase in unit cell as pillar length is increased.

Following this, the effect of linker pKa was examined as a factor affecting the success of a SALE reaction. The

higher the conjugate acid pKa of a linker, the stronger the metal (in this case zinc) linker bond which leads to more energetically favourable structures. Therefore, mixed linker experiments were conducted whereby SALEM-5 was exposed to mixtures of pairs of different pillars, such that the pairs had different lengths and pKa values. The fi rst pair comprised L6 and L7 in equimolar amounts, L6 is shorter than L7 (7 Å vs. 9 Å) but has a higher conjugate acid pKa (4.44 vs. 3.62) and over the course of the reaction L6 was able to replace almost all of the L1 pillars in the starting SALEM-5. In line with expectations, L7 was only minimally incorporated and this is probably due to its larger steric demands and lower affi nity for the metal centre. In contrast, exposure of SALEM-5 to a mixture of L3 (11 Å, pKa = 4.86) and L6 (7 Å, pKa = 4.44) showed rapid exchange with L3 and a maximum incorporation of L6 of 11%. This is despite L6 being signifi cantly shorter than L3 (as such it should face a lower kinetic diffusion barrier into the SALEM-5 structure) and seems to indicate that the stronger L3-Zn interaction is the main driving force of the reaction. This result, coupled with other studies starting from different SALEM compounds with longer pillars and larger pores, show that although linker

Scheme I. (a) Summary of the SALE reactions performed on the SALEM-5 system; (b) representation of structures of linkers used in the experiments (Reprinted with permission from (6). Copyright (2013) American Chemical Society)

L1L0–L2 MOF

L3 L5

SALEM-5L4

SALEM-6 SALEM-8

SALEM-7

(a) (b)

N

N

OHHO

O

O

Br

Br

O

O

–O O–

O––O

N

N

N

N

N

N

N

N

N

N

N N

N

N

N

N

O

OO

O

L0 L1 L2

L3 L4 L5 L6 L7



length is an important consideration to the outcome of a SALE reaction, it is equally important to consider the relative pKa values of the linkers involved to gain an understanding of the eventual direction or success of a SALE reaction.

An alternative and much less developed technique is the post-synthetic metathesis of metal nodes within the MOF structure so that robust MOFs which would otherwise be diffi cult to synthesise can be accessed. A common problem of MOFs is their limited chemical stability and much work is ongoing trying to make robust MOFs containing synthetically useful hard Lewis base carboxylate functionalities in conjunction with hard Lewis acids such as trivalent iron or chromium or tetravalent titanium and zirconium ions. However, incorporation of these higher valent metals is less predictable or controllable and few examples have been obtained, particularly as single crystals.

Tian-Fu Liu (Texas A&M University, USA) presented work on MOF postsynthetic metathesis and oxidation (PSMO) in single crystal to single crystal transformations to obtain otherwise inaccessible chemically stable materials by following two design rules: (a) start from a template MOF containing labile M-ligand bonds; and (b) exchange with metal ions which can access high oxidation states whilst maintaining the same metal coordination environment. Towards this end, the authors synthesised a Mg-MOF, porous coordination network (PCN)-426-Mg, by reaction of magnesium nitrate and the 2,3,6-trimethyl-1,4-benzoquinone perylene tetracarboxylate (TMQPTC) ligand (depicted

in Scheme II) under solvothermal conditions at 100ºC for 24 h (7). The Mg-O bond in these type of structures is more labile than other similar common coordination bonds and single crystal XRD studies reveal the Mg ion is octahedrally coordinated as part of an oxo-trinuclear cluster more commonly found in iron and chromium compounds.

With the PCN-426-Mg template material in hand, direct metal metathesis reactions with trivalent Fe and Cr salts were attempted, using both hydrated and anhydrous precursors. Energy-dispersive X-ray spectroscopy (EDS) showed that only partial metal exchange had occurred, 87% for Fe and only a trace amount for Cr. This is largely due to Fe3+ and Cr3+ having very low ligand exchange rates and in the case of Cr3+ this renders it almost kinetically inert. These harder Lewis acidic metal ions also possess larger hydrolysis equilibrium constants which results in a more acidic environment being produced during the course of the reaction which damages the skeleton of the Mg-MOF template.

Consequently the analogous exchange reactions and oxidation using anhydrous divalent metal salts iron(II) chloride (FeCl2) or chromium(II) chloride (CrCl2) was investigated due to their much higher ligand exchange rates and small hydrolysis equilibrium constants. Reaction of the PCN-426-Mg crystals with the Fe or Cr reagent rapidly produced a pronounced colour change and in the case of Fe2+ for example, resulted in a change from colourless to purple within 20 min and reached complete conversion in 3 h. Following this

COOHHOOC

COOHHOOC

TMQPRC

MgIIPCN-426-Mg

MOF

FeII/CrII

Air oxidation

PCN-426-FeIII

orPCN-426-CrIII

Scheme II. Synthetic route to synthesise PCN-426-Mg MOF template and subsequent PSMO reactions to access stable Fe(III) and Cr(III) analogues (Adapted from (7))



fi rst exchange step, the product crystals were isolated, washed, suspended in DMF and underwent a very mild oxidation step by bubbling a stream of air through the suspension for 15 min. The colour changes associated with each of these exchange and oxidation steps are shown in Figure 2.

Single crystal XRD analysis of the product crystals show that they are isostructural with the starting Mg-template and that the PSMO method has allowed access to otherwise unobtainable Fe3+ and Cr3+ based MOFs. The PSMO technique is very effective here using the divalent metal ions as it overcomes the problems of slow ligand exchange rates, partial exchange and kinetic inertness encountered with the trivalent ions. This also eliminates the issue of decomposition of the template MOF from acid produced by the higher hydrolysis equilibria of the trivalent ions and additionally benefi ts from a mild oxidation step which produces very robust MOF materials. This was exemplifi ed by the stability of PCN-426-Cr(III) which remains intact from pH 12 to 4 M hydrochloric acid for at least 12 h.

Molecular Container

An exciting area of materials chemistry is functional materials, particularly those that incorporate photoactive groups acting as valves that can be optically remote controlled, potentially leading to controlled molecule storage/release materials or photon-driven pumps. An elegant example in this fi eld was presented by Christof Wöll (Karlsruhe Institute of Technology, Germany) using a surface mounted MOF (SURMOF) hybrid material, where the MOF component can be grown on

a surface in a very controlled fashion before surface functionalisation enables the MOF channels to be capped with photosensitive molecules (8). This leads to a ‘molecular container’ in which substrate molecules can be stored and released on demand via irradiation, Figure 3.

The material consists of two discreet domains, comprising an initial layer of Cu2(BPDC)2(BiPy) (BPDC denotes biphenyl-4,4-dicarboxylic acid and BiPy denotes 4,4-bipyridine), onto which a second layer of Cu2(AB-BPDC)2(BiPy) (AB-BPDC denotes 2-azobenzene biphenyl-4,4-dicarboxylic acid) where the bipyridine linker used is now functionalised with an azobenzene group. This was achieved by the controlled layer-by-layer growth of each MOF on a suitable substrate. In this case a gold substrate functionalised with a long chain mercapto alcohol was used to help nucleate surface growth and control the SURMOF crystal orientation. This was sequentially immersed in a 0.5 mM solution of copper acetate, washed with ethanol and immersed in a 0.1 mM solution of the appropriate ligand before the process was repeated. Under optimised conditions SURMOF growth was very uniform, giving a fi lm roughness as low as the height of a unit cell, and took 50 cycles to complete for the base layer and 45 cycles for the second covering layer, Figure 4.

XRD analysis of the fi nished material showed that both layers grow almost exclusively in the [001] direction as desired. The bottom layer contains pores of 1.5 nm diameter and can act as a storage medium for gases or small molecules. A quartz crystal microbalance (QCM) was used to determine that the loading of a substrate,

PCN-426-Mg

A Fe(II)

Cr(II)

B

D

Air oxidation

C

PCN-426-Fe(III)

PCN-426-Cr(III)

Air oxidation

E

Fig. 2. Optical microscope photographs of: A as-synthesised PCN-426-Mg; B PCN-426-Mg after metathesis with FeCl2 for 3 h; C PCN-426-Fe(III) after metal node oxidation; D PCN-426-Mg after metathesis with CrCl2 for 3 h; and E PCN-426-Cr(III) after metal node oxidation (Reprinted with permission from (7). Copyright (2014) American Chemical Society)

V1 V2

Fig. 3. Optically triggered release from two-component SURMOFs. Porous fi lms composed of two different layers are synthesised on solid surfaces. The bottom layer serves as a reservoir and can be loaded with different molecules, whereas the top layer serves as a valve that can be opened and closed (V1 = UV light, V2 = visible light) (Reprinted with permission from (8). Copyright (2014) American Chemical Society)



in this case butanediol, in the material was 1.9 μg cm–2 which corresponds to an absorption capacity of fi ve butanediol molecules per pore. The upper layer contains an azobenzene strut on the MOF linker group which is planar in the resting ground state and allows substrate molecules to be absorbed by the material. Irradiation with ultraviolet (UV) light (365 nm) causes the azobenzene to switch to the cis conformation, effectively closing the ‘lid’ and confi ning the substrate. Once closed, monitoring by QCM revealed there is very little leakage of the substrate . The subsequent release of the substrate can be controlled by a transition back to the trans conformation through thermal treatment or irradiation with visible light (560 nm) and results in complete desorption of the butanediol from the MOF.

Conclusions

This review has taken only a small snapshot of the numerous talks and posters presented on the area of MOFs but the sheer number of presentations demonstrates the breadth of interest in the fi eld. A signifi cant amount of work was directed at fundamental material and method understanding but there were

several examples looking into fi nding potential applications for these materials and how to overcome some of the associated challenges. For the area of MOFs to transition from academically interesting curiosities to commercially viable materials more work needs to be done to identify and demonstrate their applicability in a number of different areas. Particular challenges to realising this are stability to water and trace impurities under real world operating conditions, as well as cost and performance versus the current materials used. Despite this, it is surely only a matter of time until their full potential is realised.

References 1 247th ACS National Meeting & Exposition:

http://www.acs.org/content/acs/en/meetings/spring-2014.html (Accessed on 5th June 2014)

2 I. J. Casely, Johnson Matthey Technol. Rev., 2014, 58, (3), 162

3 H.-C. Zhou, J. R. Long and O. M. Yaghi, Chem. Rev., 2012, 112, (2), 673

4 J. B. DeCoste and G. W. Peterson, Chem. Rev., 2014, 114, (11), 5695

NN

N=N(a) (b)

trans cis

(100)(001)(010)

(001)(010)(100)

V1

V2

Fig. 4. Sketch of a layered photoswitchable SURMOF: (a) the bottom, passive SURMOF is grown on a gold substrate and an active SURMOF containing photoswitchable azobenzene is grown on top; (b) view of the pore window in the [001] direction. The azobenzene embedded in the MOF structure as well as molecular azobenzene can be switched from the trans to the cis state by UV light (V1) and vice versa by visible light (V2) (Reprinted with permission from (8). Copyright (2014) American Chemical Society)



5 J. B. DeCoste, G. W. Peterson, M. W. Smith, C. A. Stone and C. R. Willis, J. Am. Chem. Soc., 2012, 134, (3), 1486

6 O. Karagiaridi, W. Bury, E. Tylianakis, A. A. Sarjeant, J. T. Hupp and O. K. Farha, Chem. Mater., 2013, 25, (17), 3499

7 T.-F. Liu, L. Zou, D. Feng, Y.-P. Chen, S. Fordham, X. Wang, Y. Liu and H.-C. Zhou, J. Am. Chem. Soc, 2014, 136, (22), 7813

8 L. Heinke, M. Cakici, M. Dommaschk, S. Grosjean, R. Herges, S. Bräse and C. Wöll, ACS Nano, 2014, 8, (2), 1463

The Reviewer

Ian Casely graduated in Chemistry from Imperial College London, UK, in 2005 with subsequent PhD studies at the University of Edinburgh, UK, focused on rare-earth metal N-heterocyclic carbene organometallic complex synthesis and reactivity. In 2009 he moved to the University of California Irvine, USA, as a postdoc to work on small molecule activation chemistry and established a project developing the organometallic chemistry of bismuth complexes. Returning to the UK in 2011 to work at the University of Oxford, UK, on Group 4 metal based heterogeneous slurry polymerisation, in July 2012 he joined the PGM Applications group at the Johnson Matthey Technology Centre, Sonning Common, UK. He is now working on the conversion of biomass to chemicals as well as in the area of metal organic framework chemistry.




Measuring Water Solubility of Platinum Group Metal Containing SubstancesSolubility data available in the literature for the fi rst time

By Matthew GregoryJohnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading, RG4 9NH, UK

*Email: [email protected]

The water solubility of 22 platinum group metal (pgm) containing substances was evaluated to provide useful data for regulatory compliance and to aid assessment of their environmental impact. The fl ask method from OECD Guideline 105 (1) for the testing of chemicals (water solubility) was used to test each material. For substances that could not be isolated as pure solids, a simplifi ed water solubility test was carried out. The results provide reliable data on solubility previously unavailable in the literature.

Introduction

There is a paucity of robust data on the water solubility of many pgm compounds. Besides being a fundamental property for each substance, this data is essential for a number of purposes: for example to comply with regulatory requirements (see further discussion below) and as a basis for modelling the environmental effects or environmental distribution of these compounds. Water solubility levels can also serve as an indication for bioavailability of a substance and are thus an important consideration when planning and interpreting ecotoxicology or mammalian toxicology studies.

It has become necessary, as part of the European Union regulation Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) (2, 3) to determine the water solubility of every substance placed on the market in quantities greater than one tonne. In order

to achieve this, the industry has organised itself into consortia representing sectors of common interest to share costs and minimise testing.

The work presented in this paper covers pgm compounds for which no reliable data on water solubility was available in the existing literature and which are being considered by the Precious Metal Consortium (PMC) under the auspices of The European Precious Metal Federation (EPMF). To our knowledge, this is the fi rst comprehensive publication of water solubility data of pgm compounds.

Water Solubility Testing

The water solubility of a substance is defi ned as the saturation mass concentration of the substance in pure water at a given temperature. Water solubility is expressed as the mass of solute per volume of water with the SI unit of kg m–3, but g l–1 is also commonly used.

OECD Guideline 105 (1) describes two methods for determining water solubility. These are the column elution method and the fl ask method. The column elution method involves the use of a micro column charged with an inert carrier material and an excess of test substance. The test material is then eluted with water. Water solubility is determined when the mass concentration (the mass of material dissolved in a given volume) is constant as a function of time. The fl ask method involves dissolving the test material in water at a higher temperature than the test temperature. The saturated mixture is then cooled and allowed to equilibrate at the test temperature. The mass concentration of the test substance is then determined by inductively coupled plasma-optical emission spectrometry (ICP-OES). According to the Guideline,



solubility below 10–2 g l–1 should be evaluated by the column elution method, whereas the fl ask method is preferred for substances with water solubility above 10–2 g l–1.

In this investigation only the fl ask method was used. The column elution method was not considered to be appropriate for tests on inorganic compounds due to the diffi culty of coating the support material with the test substance. A preliminary test was carried out on each substance to determine approximate water solubility. As well as determining which method was to be used it also established how much sample was needed to carry out the test.

A diffi culty in following this test method is that several of the substances cannot be isolated as pure solids (some are described as “damp solids”), and whilst a known mass of the material can be tested this does not relate to a known quantity of the pure substance. In such cases a ‘Simplifi ed Water Solubility Test’ was carried out. This test involved preparing a saturated solution, which was then fi ltered, and the mass concentration of the test substance determined by ICP-OES. In the simplifi ed test the experiment was conducted over 24 hours, which was deemed suitable for fulfi lling the water solubility end-point, whereas the fl ask method is repeated at 48 hours and 72 hours (1).

All of the experimental work for each substance was carried out in duplicate to give extra confi dence in the data and allow for calculated errors of the water solubility of each substance.

Some additional water solubility studies were carried out by an external laboratory. This was done for confi dentiality reasons, with some companies involved in the study feeling uncomfortable providing their materials to a competitor for analysis. For this reason there is no comparison data between laboratories.

ExperimentalInstruments

A Grant OLS200 thermostatically controlled shaking water bath was used to agitate the solutions at a controlled temperature. The Precisa 2625MA-FR analytical balance was used to accurately weigh out the required amount of each substance. A Denley BS400 centrifuge was used to separate the solid and liquid phases of the test material after specifi c time periods. The concentration of pgm in the resulting solution was determined using a Perkin Elmer Optima 3300

RL ICP-OES. The pH of the test solutions were determined with a Mettler Toledo MP225 pH meter.

Chemicals

De-ionised water with a resistivity of 18.2 MΩ cm, produced by a Milli-Q® Advantage A10 system from Millipore, was used to dissolve the test substances. For ICP-OES measurements, pgm stock standards, prepared in-house and standardised gravimetrically, were used to provide calibration standards. These were used to quantify the pgm content of the saturated solutions obtained from the test method. The pgm materials tested were sourced from EPMF member companies registering these substances under REACH. The tested materials were considered representative samples. For commercial reasons, the producers are not specifi cally mentioned for each individual material but are listed in the Acknowledgement in this paper.

Preliminary Test

In order to calculate how much sample was required for the analysis, a preliminary test was carried out to determine the approximate solubility. Increasing volumes of water were added to 0.1 g of test material with the mixture being shaken between each addition for 10 minutes. The addition of water was continued until either the substance had completely dissolved or 100 ml of water had been added. In the latter case an approximate solubility of <1 g l–1 was recorded.

OECD 105 Flask Method

From the preliminary test, the quantity of test material necessary to saturate 10 ml of water was calculated. Five times this amount was weighed into a conical fl ask. This was repeated in fi ve further fl asks to allow the study to be carried out in duplicate, for three different analysis times. To each fl ask, 10 ml of deionised water was added. The fl asks were stoppered tightly with rubber bungs and agitated at 200 rpm in a shaking water bath at 30ºC. After 24 hours two fl asks were removed from the water bath and allowed to equilibrate at room temperature for a further 24 hours. Samples from these fl asks were then transferred to centrifuge tubes and centrifuged for 30 minutes to separate the solid from the liquid. The solution was analysed by ICP-OES for the pgm content and the pH was measured. The other four fl asks were treated similarly,



and after initial equilibration at 30ºC two fl asks were removed at 48 hours and the fi nal two after 72 hours.

If the last two vessels varied by less than 15%, the test was deemed satisfactory. If the last two vessels varied by more than 15% then the analysis was repeated with longer equilibration time until the variation was less than 15%, satisfying the criteria specifi ed in the Guideline (1).

Simplifi ed Water Solubility Test

0.2 g of test material was added to 20 ml of deionised water in a conical fl ask. In cases where the substance was only available as a solution, the equivalent of 0.2 g of the initial solution (for example 2 g of a 10% solution) was used. This was done for rhodium 2-ethylhexanoate solution in 2-ethyl hexanol, dihydrogen tetrachloropalladate, platinum(0) divinyltetramethylsiloxane complex, palladium(II) nitrate and platinum(II) nitrate. The fl asks were stoppered tightly with rubber bungs and agitated at 200 rpm in a shaking water bath at 30ºC. After 24 hours the solutions were fi ltered using 0.1 μm cellulose nitrate membrane fi lters and the pgm content was determined by ICP-OES. The pH of the solution was also measured. The analysis was repeated for each material to provide duplicate data.

Additional Tests

Some additional water solubility studies were carried out by a separate laboratory (AQura GmbH, Marl and Hanau, Germany) on a further fi ve pgm containing substances. The same fl ask method from OECD Guideline 105 was used for each substance. The following apparatus were used in this additional work: Heidolph Promax 2020 shaking machine; Mettler

Toledo XS205 analytical balance; Lauda Thermostat RMS 6; HeraeusTM VarifugeTM GL centrifuge; Horiba Ultima 2 ICP-OES; Metrohm 736 GP Titrino burette; Hamilton Mikroelektrode pH electrode. The reagents used in the tests were: purifi ed water produced from a Milli-Q® system; 1.0 g l–1 standards of platinum, palladium, iridium and ruthenium traceable to standard reference materials (SRM) from the National Institute of Standards and Technology (NIST).

Results

The solubility of each of the substances was calculated using the pgm concentrations obtained from the ICP-OES analysis and the pgm assay from the certifi cate of analysis and conformance provided with each substance. Table I shows a summary of the water solubility of each of the test materials analysed by the OECD 105 Flask Method. The results for dicarbonyl(2,4-pentanedionato)rhodium(I) showed a tendency of increasing values at each equilibration time. The experiment was repeated at additional 24 hour intervals until the method criteria were met. In this particular case the samples were repeated with equilibration times up to 192 hours and 216 hours.

Table II shows a summary of the water solubility of each of the test materials analysed by the simplifi ed water solubility test. The amount of test material used was suffi cient to produce a saturated solution for substances with water solubility below 10 g l–1. For substances that were soluble at this level, >10 g l–1 was reported as the limit for the water solubility. The pH of each of the test solutions was measured and the results of this can be seen within Table I and Table II. A summary of the water solubility data obtained from AQura can be seen in Table III, along with the pH of each test solution.

Table I Water Solubility of Substances Tested Using OECD 105 Flask Method (1)a

Substance CAS number Water solubility, g l–1 pH

Potassium hexachloroplatinate(IV) 16921-30-5 11.0 ± 0.08 2.6

Palladium(II) chloride 7647-10-1 4.03 ± 0.014 2.2

Diamminedichloropalladium(II) 14323-43-4 0.63 ± 0.001 5.0

Rhodium(III) iodide 15492-38-3 <0.01 4.3

(Continued)



Table I (Continued)


Rhodium(III) nitrate hydrate 13465-43-5 1170 ± 2.5 –0.9

Ruthenium(III) chloride hydrate 14898-67-0 1140 ± 13.2 0.6

Dicarbonyl(2,4-pentanedionato)rhodium(I) 14874-82-9 0.65 ± 0.044 6.0

Ammonium hexachlororhodate(III) 15336-18-2 99.8 ± 1.80 2.5

Diammonium sodium hexakis(nitrito-N)rhodate 64164-17-6 0.76 ± 0.032 5.6

Tetraammonium decachloro-mu-oxodiruthenate(IV) 85392-65-0 31.1 ± 0.51 1.4

Iridium(IV) oxide 12030-49-8 0.0002 ± 0.00002 6.0

Ammonium hexachlororuthenate(IV) 18746-63-9 26.2 ± 0.06 1.4

a This method was applied to pure substances

Table II Water Solubility of Substances Tested Using Using the Simplifi ed Testa


Rhodium trihydroxide 21656-02-0 0.0001 ± 0.00005 7.3

Rhodium 2-ethylhexanoate solution in 2-ethyl hexanol 20845-92-5 0.0098 ± 0.00030 3.5

Ruthenium trihydroxide 12135-42-1 <0.00002 6.0

Dihydrogen tetrachloropalladate 16970-55-1 >10b 1.4

Platinum(0) divinyltetramethylsiloxane complex

68478-92-2 0.0349 ± 0.00052 5.2

Tetrakis(triphenylphosphine) palladium(0) 14221-01-3 <0.00012 6.5

Palladium(II) nitrate 10102-05-3 >10b 1.2

Platinum(II) nitrate 18496-40-7 0.0141 ± 0.00195 1.0

Potassium hexachloropalladate(IV) 16919-73-6 3.41 ± 0.039 1.7

Ammonium hexachloropalladate(IV) 19168-23-1 >10b 1.3

a See text for description. This method was used for substances in which the quantity of pure material could not be determined,

including those supplied as “damp solids”b The water solubility of these substances was at or above the level specifi ed in the test



ConclusionA range of pgm containing substances were analysed for water solubility. Each substance was measured in duplicate and an associated error has been quoted in each case. Two methods were used to determine the water solubility. The shake fl ask method according to OECD 105 guideline was used for pure solids and a simplifi ed water solubility test was used for substances that could not be isolated as pure solids. A column elution method is also described in the guideline for substances with solubility below 10–2 g l–1 but this was rejected due to diffi culties in coating the support material with the test substances. This investigation has provided reliable water solubility data for a range of pgm containing substances which were previously unavailable in the literature.

AcknowledgementsThe study was supported by the Precious Metals Consortium (PMC), c/o European Precious Metals

Federation (EPMF), with special acknowledgement to Dr Klaus Rothenbacher, scientifi c manager of EPMF, for providing additional information into the necessity of the study. Samples were provided by C. Hafner GmbH & Co KG, Heraeus Precious Metals GmbH & Co KG, Johnson Matthey Plc, Umicore NV and Vale Europe Ltd.

References

1. ‘Test No. 105: Water Solubility’, in “OECD Guidelines for the Testing of Chemicals”, Section 1, OECD Publishing, Adopted 25th July, 1995

2. Regulation on Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH), Regulation (EC) No 1907/2006, 18th December, 2006; Offi cial Journal of the European Union, L 396/1, 30th December, 2006

3. Corrigendum to Regulation (EC) No 1907/2006; Offi cial Journal of the European Union, L 136/3, 29th May, 2007

Table III Water Solubility of Substances Investigated by AQuraa


Potassium tetrachloroplatinate(II) 10025-99-7 191 ± 1 2.3

Tetraamminepalladium(II) chloride 13815-17-3 327 ± 2 7.8

Dihydrogen hexachloroiridate(IV) hydrate 16941-92-7 456 ± 5 <0.1

Ruthenium acetate 55466-76-7 660 ± 12 0.8

Palladium(II) 2,4-pentanedionate 14024-61-4 0.0111 ± 0.0005 4.2

a Tests were carried out using the OECD fl ask method (1)

The Author

Matt Gregory graduated from the University of Surrey, UK, with an MChem in Chemistry. In 2007 he joined the Analytical Department at Johnson Matthey Technology Centre, Sonning Common, UK, where he currently works as a Research Scientist in the Assay team specialising in analysis using inductively coupled plasma-mass spectrometry (ICP-MS) and inductively coupled plasma-optical emission spectrometry (ICP-OES).




40 Years of Cleaner Air: The Evolution of the Autocatalyst Autocatalysts have prevented billions of tonnes of pollution from entering the atmosphere and offer solutions to current concerns about urban vehicle pollution

By Chris MorganJohnson Matthey Emission Control Technologies, Orchard Road, Royston, Hertfordshire, SG8 5HE, UK


The 40th anniversary of the manufacture of the world’s fi rst commercial batch of autocatalysts for passenger cars at Johnson Matthey Plc’s site in Royston, UK, was marked in May 2014. Despite the enormous progress made in reducing the emission of pollutants from vehicles since the 1970s, there has also been considerable recent discussion about the levels of nitrogen oxides (NOx), especially nitrogen dioxide (NO2), and particulate matter (PM) in today’s urban environment. This article describes the evolution of catalyst technologies over the last forty years and the next generation of products which will enable further advances in air quality.

Initial Breakthroughs

From the 1940s onwards large cities, particularly in the USA and around Tokyo, were experiencing increasing levels of air pollution. In the 1950s work in California proved that photochemical smog formed from reactions between NOx and hydrocarbons (HCs) (1) and that internal combustion engine exhaust was a major source of such pollution (2). Clean Air Acts were passed in the UK and in the USA, and from 1968 all new passenger cars in the USA had to meet exhaust gas emissions standards. Initially these were achieved through better engine tuning, but an amendment requiring a further

90% reduction in emissions by 1975 forced the use of catalytic exhaust gas aftertreatment systems.

Johnson Matthey had been conducting research in the area since 1969, and in 1971 fi led a patent for a rhodium-promoted platinum catalyst (3). This was used in a ‘two-way’ device, designed to remove the carbon monoxide (CO) and HC emissions caused by incomplete combustion of the fuel. Exhaust gas recirculation was used to control NOx emissions. Proof of durability was a critical step in the acceptance of the new technology. In 1972 Johnson Matthey demonstrated to the US authorities that a catalyst system still met the 1975 emissions standards after 26,500 miles of driving on a Chrysler Avenger (Figure 1) (4), helping to maintain the timetable for introduction of the legislation. In parallel, the introductions of lead-free gasoline and ceramic honeycomb materials that could support high temperature catalytic processes were also essential.

To achieve lower NOx emissions a dual-bed catalytic converter was developed (4). The engine was run rich, i.e. with an excess of fuel to generate reducing conditions, and NOx was reduced to nitrogen over the fi rst catalyst. Secondary air was introduced into the exhaust to generate net oxidising conditions before the second catalyst, which was designed to oxidise CO and HC to CO2 and water. While this system was effective it compromised fuel economy. The development of oxygen sensors in the late 1970s allowed the design of closed loop engine management systems which could accurately control the air to fuel ratio (AFR). This allowed the exhaust gas composition to be balanced at the optimum point for ‘three-way’ conversion: simultaneous oxidation of CO and HC and reduction of NOx over a single catalyst. Such three-way catalysts



(TWCs) were essential to meet another tightening of US emission limits effective from 1981.

Developing Today’s Catalysts

The early TWC designs are recognisable as the precursors to today’s gasoline autocatalysts. There have since been major improvements in coating design, application of platinum group metals (pgms), thermal stability of raw materials and properties of the cerium-containing oxygen storage materials. Along with advances in engines and substrates, these have enabled increasingly stringent emissions limits and durability targets to be met on current gasoline vehicles around the world, whilst using substantially less pgms.

The emissions story in the US and Japan was focused primarily on gasoline vehicles, but in Europe, where passenger car emissions legislation was introduced in 1993, diesel engines had a signifi cant market share. Heavy duty applications also predominantly used diesel engines. Diesel oxidation catalysts (DOCs) were effective in removing CO and HC emissions, but soot became a signifi cant concern, especially in local low emission zones. In 1990 Johnson Matthey patented the use of NO2 to reduce the combustion temperature of diesel soot (5). This technology was launched as the continuously regenerating trap (CRT®) in 1995, comprising a DOC to oxidise CO and HC and form NO2 upstream of a cordierite particulate fi lter (6). This achieved much success as a retrofi t device to control

emissions from local bus and truck fl eets, and was later incorporated onto new vehicles. PM limits for diesel passenger cars did not require the fi tment of fi lters until the introduction of Euro 4 legislation in 2005.

A limitation of the CRT® design was that each NO2 molecule could only react once with the carbonaceous soot, requiring engines to be run at a high NOx:PM ratio. Incorporating pgm-containing washcoat into the fi lter walls, enabling ‘used’ NO molecules to be reoxidised to NO2 in situ, allowed much more effi cient fi lter regeneration at lower NOx:PM ratios. This was the basis of the catalysed soot fi lter (CSF) (7), now common in light and heavy duty diesel applications.

The Future of Cleaner Air

Compared to those from 1974, today’s vehicles have much cleaner and more effi cient combustion processes, improved fuel injection system design and sophisticated engine management systems and sensing technologies. In conjunction with advanced catalyst technologies a modern passenger car typically emits one-hundredth as much pollution as one from ca. 1960. However, the impact of air quality on human health, particularly in urban environments, is high on the political agenda. Many European cities are breaching European Union limits on NO2, leading to renewed debate about local measures to reduce pollution (8). European real world driving emissions (RDE) legislation is being prepared, with the aim of ensuring that vehicle pollution is controlled over a wide

Fig. 1. (a) The Chrysler Avenger test vehicle; and (b) exhaust emissions obtained using the 1975 control system over 26,500 miles testing. (o = emissions at 26,500 miles before servicing of the vehicle, x = emissions at 26,500 miles after normal servicing of the engine, during which process the catalyst was not touched) (4)

(a) (b)



range of driving styles and ambient conditions, not just over the specifi ed drive cycles. Meanwhile pollution is a major issue in many Asian cities, with, for example, Beijing considering the introduction of increasingly stringent emission limits.

The introduction of diesel particulate fi lters on light duty diesel (LDD) and heavy duty diesel (HDD) vehicles has been an important step towards addressing these concerns, supported by future legislation to control the number of particulates emitted (and not just their total mass) from gasoline direct injection engines in Europe and similar legislation expected for non-road mobile machinery. Selective catalytic reduction (SCR) is already a widely used technology to control NOx emissions from HDD applications in developed markets. Furthermore Euro 6 passenger car legislation, which came into effect in September 2014, more than halved the permitted NOx emissions from compression ignition engines, necessitating the introduction of specifi c NOx control technologies on almost all new European diesel passenger cars.

There are currently two competing technologies for diesel NOx control: SCR and NOx adsorber catalysts (NACs) (9), each with advantages and disadvantages. SCR systems, based on copper, iron or vanadium materials, reduce NOx to nitrogen through reactions with stored ammonia. High NOx conversion rates can be achieved and the reaction occurs under a standard diesel AFR, maintaining fuel economy. However, the ammonia is derived from the decomposition of urea solution injected into the exhaust gas upstream of the SCR catalyst. This requires an additional storage tank, urea injection and control system, the cost and space requirements of which can be prohibitive for smaller vehicles. The urea decomposition threshold temperature of ca. 180ºC, limits the effectiveness of SCR systems in extended low temperature regimes such as low speed city driving in winter. There are also concerns about the release of excess ammonia into the atmosphere, leading to the development of additional ammonia slip catalysts (ASCs).

NACs trap NOx emissions during normal operation, typically by oxidation over a pgm and storage as nitrate on an alkaline earth such as barium. As the NAC has a fi nite NOx storage capacity, it is necessary to periodically regenerate the catalyst. This is achieved by running the engine rich for a few seconds to increase the concentrations of reductants including CO, HC and hydrogen in the exhaust gas. Under the rich conditions the nitrate decomposes and the

released NOx species are converted to nitrogen through reaction with the exhaust gas reductants over a second catalytic component, typically supported rhodium. The periodic requirement to run the engine rich adds to the complexity of the powertrain design and worsens fuel economy, which is undesirable given legislative targets to reduce CO2 emissions/improve fuel economy. The additional cost of pgms can be a concern for larger vehicles. Furthermore there is a temperature window where NACs operate effectively: at higher temperatures the storage mechanism is less stable and at lower temperatures the NOx release and reduction reactions are less effective.

To address NOx control for RDE on diesel passenger cars a likely solution is a combination of NAC and SCR systems, harnessing the strengths of each technology. An upstream NAC will store NOx emissions at low temperatures when the SCR system is less effective. The NAC will also act as an oxidation catalyst to convert HC and CO emissions. A downstream SCR will provide NOx control under higher speed, higher temperature conditions, also enabling extended lean operation for improved fuel economy. Optimisation of such systems is taking place, with focus on matching the operating temperature windows of the NAC and SCR components.

As engines become increasingly fuel effi cient less waste heat enters the exhaust. This is a critical concern for the aftertreatment system as it leads to lower catalyst operating temperatures. A common diesel catalyst architecture comprises a DOC followed by CSF, urea injection and SCR. Due to the thermal mass of the CSF (required to withstand uncontrolled soot regeneration) and heat losses from the exhaust pipe in the urea mixing zone in front of the SCR, it can take many minutes of city driving before the SCR warms up suffi ciently to provide high levels of conversion effi ciency. An elegant solution is to integrate the SCR coating onto the particulate fi lter (10), thus enabling the SCR coating to heat up and become active more quickly, whilst also improving the compactness of the system (Figure 2). Such SCR coated on fi lter (SCRF®) technologies are now in series production – another world fi rst for Johnson Matthey, Royston. Design challenges include the incorporation of signifi cantly higher coating loadings onto a fi lter than were required for CSF, leading to a requirement for high porosity fi lter substrates and optimisation of the fi ltration effi ciency and pressure drop characteristics.



Conclusions

Since the development of the fi rst catalytic converters there have been many advances in powertrain, substrate and catalyst technologies over the last forty years. Emissions control systems are now required in countries across the world and they have prevented billions of tonnes of pollutants from entering the atmosphere. However, NOx and PM pollution continues to affect human health, particularly in urban environments. The introduction of new technologies to control diesel NOx emissions, alongside the more widespread fi tment of particulate fi lters, will lead to further improvements in air quality.

References

1. A. J. Haagen-Smit and M. M. Fox, Ind. Eng. Chem., 1956, 48, (9), 1484

2. L. H. Rogers, J. Chem. Educ., 1958, 35, (6), 310

3. G. J. K. Acres, Johnson, Matthey & Co, Ltd, ‘Improvements in and Relating to Catalysis’, British Patent 1,390,182; 1971

4. G. J. K. Acres and B. J. Cooper, Platinum Metals Rev., 1972, 16, (3), 74

5. B. J. Cooper, H. J. Jung and J. E. Thoss, Johnson Matthey, Inc, ‘Treatment of Diesel Exhaust Gases’, US Patent 4,902,487; 1990

6. P. N. Hawker, Platinum Metals Rev., 1995, 39, (1), 2

7. M. V. Twigg and P. R. Phillips, Platinum Metals Rev., 2009, 53, (1), 27

8. ‘Environment: Commission Takes Action Against UK for Persistent Air Pollution Problems’, European Commission, IP/14/154, Brussels, Belgium, 20th February, 2014

9. T. Johnson, Platinum Metals Rev., 2008, 52, (1), 23

10. A. P. Walker, ‘Challenges and Solutions in Diesel Emission Control’, Catalysis Club of Philadelphia 2008 Spring Symposium, USA, 22nd May, 2008

Fig. 2. Schematic demonstrating how the SCRF® improves system compactness and an example of how, over the European drive cycle, it warms up more quickly than an SCR downstream of a CSF (due to its closer proximity to the engine) enabling earlier NOx conversion

NO

x em

issi

ons,

g

2.5

2

1.5

1

0.5

0 200 400 600 800 1000 1200Time, s

Engine out NOx

Tailpipe NOx – SCR

Tailpipe NOx – SCRF®

SCR

inle

t tem

pera

ture

, ºC 400

300

200

100

0 200 400 600 800 1000 1200Time, s

DOC

DOC

CSF SCR

SCRF®

Inlet temperature – SCRF®

Inlet temperature – SCR

The Author

Chris Morgan is Technology Director at Johnson Matthey’s European Emission Control Technologies business, responsible for the development and scale-up of autocatalyst coatings for light duty gasoline and diesel applications. Chris previously managed the Gasoline Product Development team, developing new families of three-way catalysts and leading Johnson Matthey’s early work on coatings for gasoline particulate fi lters. He joined Johnson Matthey in 1997, after completing a DPhil at the University of Oxford, UK, on high temperature ceramic superconductors.

“Nanofabrication and its Application in Renewable Energy”Edited by Gang Zhang (Peking University, P.R. China) and Navin Manjooran (Siemens AG, USA), RSC Nanoscience & Nanotechnology Series, No. 32, Royal Society of Chemistry, Cambridge, UK, 2014, 290 pages, ISBN: 978-1-84973-640-4, £145.00, US$240.00




Reviewed by Greg AgarJohnson Matthey Gold and Silver Refining Inc,4601 West 2100 South, Salt Lake City, Utah, 84120, USA


Introduction “Nanofabrication and its Application in Renewable Energy” offers a broad overview of current nanoparticle technologies available for renewable energy. This particular title is the latest in a 32 title series that emphasises nanotechnology and offers details of graphene fabrication, photovoltaics, thermoelectric materials and energy storage systems. Researchers, specialists and graduate students appear to be the desired audience based on the level of technical detail in the book. The editors Gang Zhang and Navin Manjooran are both highly decorated in their respective fi elds. Zhang received both his Bachelor of Science degree and PhD in physics from the world renowned Tsinghua University, China. During his career Zhang has authored and co-authored more than 100 articles. Manjooran has 11 patents/disclosures, 5 books and 37 publications; Manjooran is also a member of the US Technology Advisory Board.

This selective review focuses on two chapters of particular interest, namely microelectromechanical systems (MEMS) and energy storage.

Microelectromechanical Systems

Energy capture and storage is paramount to many

industries with various applications such as mobile electronics and biomedical devices. The research authors Bin Yang and Jingquan Liu (Shanghai Jiao Tong University, China) discuss MEMS in detail in Chapter 3 “Micro/nano Fabrication Technologies for Vibration-Based Energy Harvester”. MEMS utilise electrodes as variable capacitors in two primary formats, i.e. comb finger electrodes and parallel-plate electrodes, which are clearly defined by the authors. These two types are then split into three sub-categories: in-plane overlap varying, in-plane gap closing and out-of-plane gap closing (Figure 1). The authors further explain the functionality and features of each type. The application of such machines to biomedical and electrical engineering is promising, medical implants and all electronic devices could benefit from applications of this technology. Prior biomedical devices often required an external power source making them difficult to implant. The advantage of the energy harvesters is continuous power from an internal power supply which can be steadily charged by simple vibration thereby eliminating the need for external batteries.

Chapter 3 stays true to its title and clearly explains several techniques in nanofabrication. Techniques such as photolithography, deep reactive-ion etching (DRIE), sol-gel, aerosol deposition and bulk bonding processes are explained, although the explanations are limited to an overview of each technique and omit the exact details of the procedures. The reader will need to look elsewhere for such details and ample references are provided.

Energy StorageChallenges in energy production and its subsequent storage threaten the overall sustainability of some energy sources such as fossil fuels, coal, solar and hydrogen. In Chapter 5 “Nanotubes for Energy Storage” research author Hui Pan (University of Macau, China) outlines concerns and explains possible solutions to diffi culties that current energy storage technologies face (Figure 2)

and how innovation in the fi eld of nanotechnology may be able to rectify the situation. The role of nanomaterials in supercapacitors and lithium battery energy storage systems is also discussed.

Storage of hydrogen by nanomaterials is explained in great detail but it is made quite apparent that a signifi cant amount of variability exists within current technologies. The chapter covers topics such as carbon nanotubes, functionalised/doped carbon nanotubes, inorganic nanotubes, boron nanotubes, silicon carbide nanotubes, tungsten nanotubes and graphite nanotubes. The chapter also discusses the functionality and design of each nanotube type as well as their applications in energy. The amount of information provided in Chapter 5 is substantial and markedly useful in selecting nanotube types for a particular research application.

Conclusion “Nanofabrication and its Application in Renewable Energy” is a useful introduction for a reader relatively new to the topic. The scope of the title gives the reader a fair idea of how new technologies can be applied and what their current state is. I would highly recommend the title to graduate students and new researchers. There is a high level of technical detail but the simple presentation makes it accessible to both researchers and students alike.



Fixed frame

(a)

Direction of

movement

Direction of

movement

Spring

Spring

Comb fi ngers Comb fi ngers

Fixed frameSpring

Spring

Direction of movement

Springs

Top cap electrode

(b) (c)

Fig. 1. Three types of electrostatic energy harvester: (a) in-plane overlap varying; (b) in-plane gap closing; (c) out-of-plane gap closing (Reproduced by permission of The Royal Society of Chemistry)

kWh L–1

Target 20259 wt%

Target 2010 45 g l–1

6 wt%

Metal hydride

Chemical hydride

MOF (298 K/10 bar) 2 wt%

MOF (77 K/80 bar)60 g l–1

6.9 wt%

Cur

rent

sta

tus

Liquefi ed H2

10,000 psi compressed H2

5000 psi compressed H2

0 0.5 1 1.5 2 2.5 3 3.5

kWh kg–1

Fig. 2. The current status of today’s hydrogen storage technologies in volumetric and gravimetric terms (Reproduced by permission of The Royal Society of Chemistry)



The Reviewer

“Nanofabrication and its Application in Renewable Energy”

Greg Agar is currently a graduate student at the Department of Physics, University of Utah, USA, and a refi nery supervisor for Johnson Matthey Gold and Silver, Salt Lake City, USA. His research interests include nanomedicine and nanosensors.

EMISSION CONTROL TECHNOLOGIESFormation of Reactive Lewis Acid Sites on Fe/WO3–ZrO2 Catalysts for Higher Temperature SCR Applications R. Foo, T. Vazhnova, D. B. Lukyanov, P. Millington, J. Collier, R. Rajaram and S. Golunski, Appl. Catal. B: Environ., 2015, 162, 174

An active ammonia-SCR catalyst can be prepared by impregnating WO3–ZrO2 with Fe. WO3–ZrO2 containing Fe (in wt%) 0, 0.5, 2.3 and 10 was investigated and the relationship between the catalytic activity and surface acidity was studied by pyridine adsorption. 3 wt% Fe/WO3–ZrO2 is the most active and reduced NOx by 10%–20% at 150ºC and between 400ºC and 550ºC, 80%–85% conversion is achieved. New Fe3+ Lewis acid sites were formed and play an essential role by activating NOx and increasing the strength of the Brønsted acidity.

Comparison of Different Kinetic Models for NOx Storage on a Lean NOx TrapT. C. Watling, P. D. Bolton and D. Swallow, Can. J. Chem. Eng., 2014, 92, (9), 1506

The NOx breakthrough curves were used to investigate the kinetics of NOx storage on a lean NOx trap (LNT). The measurements were taken using a laboratory reactor in a range of temperatures, from 125ºC to 450ºC. The breakthrough curves were performed until the saturation of LNT to: (a) allow NO oxidation to NO2 to be researched in the absence of NOx storage; (b) to allow the effective NOx capacity as a function of temperature to be analysed; and (c) to provide a more demanding test for potential models. There are similarities of the breakthrough curves at 125ºC and 450ºC; at the beginning, there is a temperature independent portion followed by a temperature dependent portion which are explained by fast sites and slow sites. The amount of NOx stored as a function of temperature was bell-shaped.

Parameter Estimation of a DOC from Engine Rig Experiments with a Discretized Catalyst Washcoat ModelB. Lundberg, J. Sjoblom, Å. Johansson, B. Westerberg and D. Creaser, SAE Int. J. Engines, 2014, 7, (2), 1093

DOCs with varying compositions of Pt loading, washcoat thickness and volume were studied. Parameters were tuned against engine rig data, using specially selected engine operating points to illustrate the interplay between kinetics and mass transport. The resulting catalyst model included discretized washcoat as tanks in radial and axial series. For a catalyst model with internal transport resistance, it was found that some internal mass transport related parameters must be tuned in addition to the kinetic parameters. A model with little to no internal transport resistance could still have a good fi t if kinetics parameters compensated for the transport limitations. It was important to use a kinetic model capable of describing exclusively intrinsic kinetics.

FINE CHEMICALS: CATALYSIS AND CHIRAL TECHNOLOGIESEnzymatic Desymmetrising Redox Reactions for the Asymmetric Synthesis of Biaryl AtropisomersS. Staniland, B. Yuan, N. Giménez-Agulló, T. Marcelli, S. C. Willies, D. M. Grainger, N. J. Turner and J. Clayden, Chem. Eur. J., 20, (41), 13084

The enantioselective synthesis of biaryl atropisomers with ortho-hydroxymethyl and formyl groups involves the oxidation of symmetrical diol substrates with a variant of galactose oxidase and the reduction of dialdehydes by a panel of ketoreductases. M or P enantiomers were produced and the absolute confi gurations were determined by time-dependent DFT calculations of circular dichroism spectra. The different biaryl structures have different selectivities which enables the active site of galactose oxidase to be analysed in detail.

Iridium-Catalyzed C-H Borylation of Heterocycles Using an Overlooked 1,10-Phenanthroline Ligand: Reinventing the Catalytic Activity by Understanding the Solvent-Assisted Neutral to Cationic SwitchC. C. C. Johansson Seechurn, V. Sivakumar, D. Satoskar and T. J. Colacot, Organometallics, 2014, 33, (13), 3514

Borylation of N-Boc-indole at the 3-position with B2pin2 (pin = pinacolato) was achieved in consistent 99% yield

Johnson Matthey HighlightsA selection of recent publications by Johnson Matthey R&D staff and collaborators





using 0.5 mol % of a preformed Ir catalyst, [Ir(Cl)(COD)(1,10-phenanthroline)] (2; COD = cyclooctadiene). This performance was substantially better than that of the corresponding in situ formed catalyst. This is thought to be due to formation of a competing inactive cationic complex 1 when carried out in a noncoordinating solvent such as octane. Using catalyst 2, the total synthesis of Meridianin G was accomplished in 87% overall isolated yield in a one-pot, three-step process.

NEW BUSINESSES: FUEL CELLSSpecifi c Adsorption of Perchlorate Anions on Pt{hkl} Single Crystal ElectrodesG. A. Attard, A. Brew, K. Hunter, J. Sharman and E. Wright, Phys. Chem. Chem. Phys., 2014, 16, (27), 13689

In this study, the specifi c adsorption of perchlorate anions on Pt{hkl} electrodes (i.e. Pt{111}, Pt{100}, Pt{110} and Pt{311}) were analysed in perchloric acid electrolyte by cyclic voltammetry. The aim was to test a hypothesis made in recent publications of the adsorption of perchlorate anions on polycrystalline platinum. As the concentration of perchloric acid increased, both the OHad and electrochemical oxide states were notably perturbed for Pt{111}. This may be due to the competition of the perchlorate anions and OHad for the adsorption sites. The hydrogen underpotential deposition region of Pt{111} was unaffected but perchlorate anion decomposition to chloride on Pt{111} was observed. For Pt{100}, there was no difference in the onset of electrochemical oxide formation or any state in the potential of the OHad state. For Pt{110} and Pt{311} there were negligible changes in the onset of electrochemical oxide formation. The specifi c adsorption of perchlorate anions on Pt{111} had a detrimental effect on the ORR. These results confi rm the previously reported fi ndings.

PRECIOUS METAL PRODUCTS: CHEMICAL PRODUCTSPulsed Electrical Discharge Synthesis of Red Photoluminescence Zinc Oxide NanoparticlesS. S. Su, I. T. H. Chang, W. C. H. Kuo, D. Price, Z. Pikramenou and J. Lead, J. Nanopart. Res., 2014, 16, (9), 2611

A pulsed electrical discharge in a liquid medium was used to synthesise ZnO nanoparticles which were then characterised using TEM, EDS and PL spectroscopy. The effects of the processing parameters such as liquid media, current, frequency of the electrical discharge and the electrode gap distance on the properties of the nanoparticles were investigated. The average size of the synthesised ZnO nanoparticles was between 10 nm and 25 nm. An increase in the arc current was found to notably increase the average particle size. The frequency of discharge and electrode gap distance can affect the average particle size and distribution due to a difference in the cooling rate. The as-synthesised ZnO nanoparticles display an ultraviolet emission of ~3.4 eV and a red visible emission of ~1.98 eV in the PL spectrum.

PROCESS TECHNOLOGIESAssessment of Different Methods of Analysis to Characterise the Mixing of Shear-thinning Fluids in a Kenics KM Static Mixer using PLIFF. Alberini, M. J. H. Simmons, A. Ingram and E. H. Stitt, Chem. Eng. Sci., 2014, 112, 152

The blending of two non-Newtonian shear thinning fl uids where a minor secondary fl ow is blended into a major primary fl ow was analysed using Kenics KM static mixers. The concentration distribution at the mixer outlet was determined using planar laser induced fl uorescence (PLIF). The total fl uid superfi cial velocity (0.1–0.6 m s–1); pipe internal diameter (0.0127–0.0254 m);


[Ir(COD)Cl]2RT

Toluene or hexaneIr

Catalytically inactive

Catalytically active

THF

1 Green

Cl–

+

CH2Cl2or

MeCNClIr

NH2

Borylation/Suzuki-Miyaura sequence

3 steps87% overall yield

Total synthesis of Meridianin G2 Purple

NN

N

N

N

N

N

N

C. C. C. Johansson Seechurn et al., Organometallics, 2014, 33, (13), 3514

NH


the volumetric fl ow ratio between the primary and secondary fl ows (10:1 and 25:1) and changing the rheology of the secondary fl ow have been investigated. The capability of the Kenics KM static mixer can be determined by the analysis of PLIF images using the traditional CoV, maximum striation area methods, the areal distribution method and the new individual striation method.

Upgrading Biomass Pyrolysis Vapors over β-zeolites: Role of Silica-to-Alumina RatioC. Mukarakate, M. Watson, J. ten Dam, X. Baucherel, S. Budhi, M. Yung, H. Ben, K. Iisa, R. M. Baldwin and M. R. Nimlos, Green Chem., 2014, doi: 10.1039/C4GC01425A

The vapour phase upgrading of pine pyrolysis products was carried out in a fl ow microreactor using a range of β-zeolites with various silica-to-alumina ratios (21, 25, 38, 75, 250) with the aim of investigating how the acid sites affect product distribution, yields and coking. During the experiment, 40 discrete 50 mg batches of biomass were pyrolysed and the vapours were upgraded over 0.5 g of catalyst on a horizontal fi xed bed semi-batch reactor. Products were measured by MBMS and py-GCMS. The authors found that aromatic hydrocarbons and olefi ns with no detectable oxygen-containing species were mainly produced when the catalyst was fresh for β-zeolites with a lower silica-to-alumina ratio (more acid sites). The fresh catalysts with a higher silica-to-alumina ratio (less acid sites) produced a range of oxygenated products i.e. furans, phenol and cresols.

Impact of Chemical Heterogeneity on the Accuracy of Pore Size Distributions in Disordered Solids

I. Hitchcock, S. Malik, E. M. Holt, R. S. Fletcher and S. P. Rigby, J. Phys. Chem. C, 2014, 118, (35), 20627

The determination of pore size distribution (PSD) is important for developing structural models and understanding the activity and selectivity of heterogeneous catalysts. A new integrated gas sorption and mercury porosimetry technique was used to get complementary experimental data to the extensive studies recently carried out by simulations. A subset of pores was prepared within a disordered network of an amorphous material (e.g. silica or alumina) and the sorption behaviour of nitrogen and argon were compared. The heavy metal surface led to higher pressure in silica but not alumina. This was not the case for nitrogen.

A New Combined Nuclear Magnetic Resonance and Raman Spectroscopic Probe Applied to In Situ Investigations of Catalysts and Catalytic Processes

J. C. J. Camp, M. D. Mantle, A. P. E. York and J. McGregor, Rev. Sci. Instrum., 2014, 85, (6), 063111

In this study, Raman and nuclear magnetic resonance (NMR) spectroscopies were used together in a new single experimental probe which enables simultaneous measurements on the same sample to be recorded. The progress of a reaction can be monitored using the probe and the authors demonstrated this with the evolution of the homogeneously catalysed metathesis of 1-hexene. Magic angle spinning (MAS) NMR was also used with a custom made MAS 7 mm rotor which is able to spin up to 1600 Hz. This was applied to the heterogeneous metathesis of 2-pentene and ethene to investigate the structure-performance relationships.


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