Palladium cobalt binary doping of molecular sieving silica membranes

Benjamin Ballinger, Julius Motuzas, Simon Smart, João C. Diniz da Costa n

The University of Queensland, FIMLab – Films and Inorganic Membrane Laboratory, School of Chemical Engineering, Brisbane, QLD 4072, Australia

a r t i c l e i n f o

Article history:Received 22 July 2013Received in revised form19 September 2013Accepted 28 September 2013Available online 7 October 2013

Keywords:SilicaGas separationPalladiumCobaltBinary dopingMolecular gap

a b s t r a c t

This work investigates the preparation and performance of palladium cobalt binary doped silica (PdCoSi)membranes. In view that palladium nitrates precipitate under typical silica sol–gel synthesis conditions,membranes were prepared via the acid catalysis of tetraethylorthosilane (TEOS) with ethanol, palladiumand cobalt chlorides. The largest H2 fluxes reached 0.6�10�6 mol m�2 s�1 Pa�1 at 500 1C for H2 permea-tion for the reduced membranes, whilst the best He/N2 permselectivies of 70 were observed for theoxidised membranes. The redox effect resulted in membranes with different performance as the reductionincreased the total pore volumes of xerogels. However, XPS analysis revealed that palladium monoxidepreferentially reduced to metal palladium, whilst reduction was not significant for cobalt oxide. Hence, thereduction to metallic palladium tailored molecular sieving domains through the loss of oxygen. As a result,the permeance of the larger molecular gases CO2 and N2 increased, whilst only a small increasewas observed for the smaller molecular gas He. Interestingly, the membranes essentially remained asmolecular sieves as evidenced by temperature dependent activated transport and high permselectivities.The molecular sieving domains created preferential pathways for CO2 and N2 permeance, though the silicamatrix integrity remained generally intact. Hence, binary and multi doping of metal oxides in silicamatrices can be used as a strategy to attain further functionalities not previously attained by pure silica orsingle metal oxide doped silica membranes using TEOS as a precursor.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Inorganic membranes derived from silica precursors are attractivefor gas separation applications due to their ability to produce porousnetworks capable of molecular sieving [1,2]. The development ofthese materials centres on the highly versatile sol–gel synthesis ofthe silica precursors, pre-eminently tetraethyl orthosilicate (TEOS).For instance, acid catalysed systems with a low water to silica ratiofavour the growth and interlinking of polymeric silica chains, thusforming a structure with a high proportion of uncondensed silanolbonds (Si–OH) in comparison to siloxane bridges (Si–O–Si) [3,4].Consequently, the stresses imparted during the solvent extractionprocess induce micropore formation, with typical pore sizes ofaround 3 Å [5]. In fact, TEOS derived microporous silica displays atrimodal pore size distribution as determined by positron annihila-tion spectroscopy (PALS) which reflects its amorphous nature [6]. Thesmallest pore diameters act as bottle necks and control the diffusionof gases percolating through the porous network [7].

Pure silica membranes can deliver molecular sieving propertiesby separating gas molecules based on their kinetic diameters ormolecular sizes. Further tailoring of the silica matrix has been

accomplished by the addition of organic templates with the addedbenefit of conferring further functionalities to the silica membrane.For example, incorporation of organically branched silica precursors,such as methyltriethyl orthosilicate (MTES) can decrease the hydro-philicity of the silica while modifying the pore size [8]. In additionto this, hybrid organically linked silica, such as BTESE, can tailor theporosity of the silica while increasing stability under aggressiveworking conditions [9,10]. Another example is the carbonisation ofnon-ligand surfactants, thus forming carbon domains embedded insilica films to improve hydrostability of silica membranes [11,12].

Further functionalities of the silica matrix have been exploredsuch as the doping of transition metals/metal oxides which hasresulted in a wide range of beneficial effects [13–17]. For instance,it has been shown that the incorporation of metal oxides into silicamembranes can increase their chemical stability [7,18–20] andlong term operability in excess of 2000 h [7]. Furthermore, dopedmetals can have a significant effect on the permeation character-istics of the membrane. Iwamoto et al. [21] reported that dopingnickel into silica improved the hydrogen permeation through themembrane above what would be expected by a pure molecularsieving transport mechanism. Following studies showed that thenickel nanoparticles within the silica displayed reversible hydro-gen adsorption properties [22,23]. This increased the number ofhydrogen solubility sites throughout the porous network whichenhanced hydrogen permeation.

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Journal of Membrane Science

0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.memsci.2013.09.057

n Corresponding author. Tel.: þ61 7 3365 6960; fax: þ61 7 3365 4199.E-mail address: j.dacosta@uq.edu.au (J.C. Diniz da Costa).

Journal of Membrane Science 451 (2014) 185–191

Although hydrogen can undoubtedly access metal nanoparti-cles, this is not the case for larger gas molecules. Uhlmann et al.[24] exposed cobalt oxide doped silica membranes to H2S andfound no interaction to occur between the sulphur-philic cobaltoxide and the H2S. It was therefore proposed that the pore sizesaround the cobalt oxide nanoparticles must be smaller than thekinetic diameter of the H2S molecule, in the range of 3 Å, similar tothe non-doped silica porosity. It has also been shown that theporous network of silica membranes can be flexible, expandingand contracting to the dynamic change in nanoparticle size. Milleret al. [25] produced a cobalt oxide doped silica membrane with apartially condensed silica precursor (ES40) which exhibited achange in permeation characteristics based on its reduction/oxidation (redox) state. They proposed that upon exposure ofthe membrane to hydrogen, Co3O4 embedded in the silica reactedto form Co(OH)2 and CoO. The consequential expansion in particlesize was accommodated by the silica matrix which resulted in theopening of bottlenecks in the pore structure. As a result thereduced membrane had a lower permselectivity than the oxidisedmembrane.

This work shows for the first time the investigation of a binarymetal doped microporous silica membrane. Of particular interest,we chose cobalt and palladium as the primary materials for thebinary metal doping. Palladium has many advantages as it hasbeen extensively used for the preparation of permeable hydrogenmembranes [26], whilst cobalt has been proven to deliver highperformance silica derived membranes. However, palladium isdifficult to solubilise in silica sol–gel processes, generally leadingto precipitation and separation from the colloidal silica derivedmembranes. Our strategy in this work involved the use ofpalladium chloride precursors which was stabilised in a silicasol–gel containing highly concentrated hydrochloric acid. Further-more, cobalt chloride was implemented instead of cobalt nitratewhich is generally employed by the research community tohomogenise the counter ions in solution. The dual phase palla-dium cobalt doped silica (PdCoSi) membranes delivered unusualstructural, permeation and gas separation features which aregenerally not found in pure silica, organic templated silica orsingle metal doped silica membranes. The membrane was char-acterised by X-ray photoelectron spectroscopy (XPS) and scanningelectron microscopy (SEM) to determine the thickness and inter-facial transition of the silica top layer. PdCoSi xerogels werestudied by nitrogen gas adsorption and XPS to determine theinfluence of reduction on the chemical and structural changes inthe membrane. The PdCoSi membranes were tested at tempera-tures up to 500 1C for single gas permeation.

2. Experimental

2.1. Xerogel preparation and characterisation

The palladium cobalt doped silica (PdCoSi) xerogels weresynthesised via the acid catalysed sol–gel processing of tetraethylorthosilicate (TEOS). Our initial work showed that palladiumprecipitated during the sol–gel synthesis, and it was thus notincorporated within the silica matrix. Therefore, palladium chlor-ide was selected and dissolved in a concentrated hydrochloric acidsolution. The solution was subsequently diluted with ethanol,followed by the addition of cobalt chloride. The solution wascooled to 0 1C and stirred until all metal chlorides were fullydissolved. Finally TEOS was added drop wise to solution, whichwas reacted for 3 h. The final molar ratio of all reagents was 4.0TEOS:0.36 PdCl2:0.64 CoCl2:41.5 H2O:11 HCl: 256 EtOH.

The sol was then dried at 60 1C for 96 h to form xerogels underatmospheric conditions. Xerogel samples were crushed by mortar

and pestle and calcined at 630 1C for 2.5 h with a 1 1C min�1

ramping rate under ambient air pressure. Xerogels were alsoreduced by heat treatment under a H2 flowrate of 40 ml min�1.Under reducing conditions, the xerogels were heated to 500 1C at100 1C intervals, holding the temperature for 24 h at each point.This procedure was subsequently repeated in the cooling cyclefrom 500–100 1C.

Structural properties related to surface area and pore volumeswere determined from N2 adsorption isotherms performed with aMicromeritics Tristar3000 apparatus. Samples were degassed in aMicromeritics VacPrep061 at 200 1C under a vacuum of 2 Pa for12 h. XPS measurements were obtained with a Kratos Axis ULTRAX-ray Photoelectron (XP) Spectrometer incorporating a 165 mmhemispherical electron energy analyser. An Al Kα (1486.6 eV)monochromatic X-ray source was used at a 901 takeoff angle with0.5 eV step size to obtain XP spectra. To allow for charge correc-tion, the Si2p peak was set to a binding energy of 103.6 eV. Highresolution transmission electron microscopy (HR-TEM) measure-ment of xerogel powders was performed using a JEOL-JEM 2100transmission electron microscope operating at 200 keV.

2.2. Membrane preparation, characterisation and testing

Membrane tube α-alumina substrates with γ-alumina inter-layers were purchased from the Energy Research Centre of theNetherlands. The membranes were prepared by dip coating thetubes into the silica sol. The tube was left in solution for 1 min toensure the sol had sufficiently soaked into the pores and waswithdrawn at a rate of 10 cmmin�1. The dip coating took place in alaminar flow cabinet with a high quality particulate filter tominimise the chance of defects in the film caused by airborne dust.The calcination methodology was identical to that of the xerogels.This process was repeated 4 times to ensure a defect free mem-brane was produced. The reduction process was performed dis-cretely during gas permeation testing by exposing the membrane toH2 at a range of temperatures. Initially, a 2 bar H2 feed was allowedto permeate through the membrane at 100 1C for 24 h. Thepermeance was then recorded having reached an equilibrium state.

Fig. 1. Membrane permeation rig arrangement.

B. Ballinger et al. / Journal of Membrane Science 451 (2014) 185–191186

This procedure was repeated along an identical temperature profileas was performed for the xerogel reduction sequence.

Single gas permeation testing was performed using a custommade permeation rig, schematically shown in Fig. 1. The tempera-ture of the furnace was controlled with an external PID tempera-ture controller while the pressures in the permeate and retentatestreams were logged on a computer using MKS pressure transdu-cers. Initially a feed gas was introduced into the system throughvalves 1–4. The initial system configuration has V5 closed and V6open, such that pressure builds on the retentate side of themembrane. To initiate testing, the permeate valve (V6) is closedand the pressure in the permeate volume is logged. The membranepermeation test followed a dead-end volumetric standard proce-dure, where the permeance can be obtained by plotting thenatural logarithm of the pressures against time, as shown in thefollowing equation [2].

lnpr�p0pr�pðtÞ

� �¼ P

ℓARTV

t ð1Þ

In this equation, pr is the retentate pressure which is kept constant,p0 and p(t) are the pressures on the permeate side (i.e. within thepermeate volume) at time 0 (initial time) and t, respectively. A, R, Tand V are all constant and represent the membrane area (m2), idealgas constant (J mol�1 K�1), operation temperature (K) and perme-ate volume (m3), respectively. Typical pressure values werep0�5�10�2 Pa and p(t) from �5�10�2 Pa to 1 kPa.

The surface morphological features of the membrane wereexamined using a JEOL JSM-7001F field emission scanning electronmicroscope (SEM) with a hot (Schottky) electron gun at anaccelerating voltage of 10 kV. Depth profiling was performed witha differentially pumped Kratos minibeam III ion gun using 4 keVargon ions at an ion source extractor current of 750 nA. The order

of sputter rates for silicon was in the region of 2.9 nm min�1. Theanalysed surface area of 0.8 mm�0.3 mmwas centred in a sputterarea of 2 mm�2 mm. XPS spectra were analysed over the bindingenergy range 850–50 eV after each 5 min etch cycle.

3. Results

Fig. 2a shows a SEM micrograph of a representative crosssection view of the PdCoSi membrane. A thin PdCoSi layer ofapproximately 50–70 nm coated on an interlayer consisting oflayers of γ-alumina is observed in the inset in Fig. 2a. The XPSdepth profile displayed in Fig. 2b corresponds to the SEM image,with the elemental concentrations of silicon and aluminiumintersecting around 60–70 nm, marking the interface betweenthe top layer and interlayer. It can also be seen that the concen-tration profiles of the doped metals have different characteristics.For instance, the concentration of palladium and cobalt follows thesame trend as that of aluminium by increasing from very lowvalues at the membrane interface to �30 nm depth, whereconcentrations remained almost constant into the interlayer. Thisis contrary to the trend observed for silica, thus giving a clearindication of palladium and cobalt partial affinity to aluminium.A similar finding was also reported for cobalt oxide silica mem-branes prepared on alumina substrates [27]. Another point ofinterest is that the ratio of cobalt to palladium is generally �4,2-fold higher than the synthesis ratio. This may reflect thatpalladium has limited solubility in silica as observed with thesynthesis process. Therefore, this limitation results in palladiumhaving a lower concentration in the thin film than what wasoriginally present in the dipping sol.

Fig. 2. (a) SEM and (b) XPS depth profile of the membrane and the α- and γ-alumina interlayers.

1x10-6

1x10-7

1x10-8

1x10-90 100 200 300 400 500 600

Temperature (˚C)

Per

mea

nce

(mol

.m-2

.s-1

.Pa-1 1x10-6

1x10-7

1x10-8

1x10-90 100 200 300 400 500 600

Temperature (˚C)

N2

HeH2

CO2

Fig. 3. Single gas permeance of the (a) oxidised and (b) reduced membrane.

B. Ballinger et al. / Journal of Membrane Science 451 (2014) 185–191 187

Fig. 3 shows the permeation results from the PdCoSi membranein (a) its oxidised and (b) reduced state after reduction at 500 1C.The oxidised membrane exhibits significant temperature activatedpermeation for all gases. He (dk¼2.7 Å), which has a smallerkinetic diameter than 3 Å, showed a positive activation, whilethe larger gases, CO2 (dk¼3.3 Å), N2 (dk¼3.6 Å), showed negativetemperature activated permeance. Furthermore as the kineticdiameter of the gas increases the permeation decreases, whichindicates a molecular sieving permeation regime. These resultsstrongly suggest that high quality membranes have been producedwith a diffusion-controlling pore size of around 3 Å.

Although the membrane operates under a molecular sievingregime both before and after reduction, it can be seen that there isa significant increase in the permeation of the larger gases after

reduction has occurred. The increase is between 3 and 6 times forboth N2 and CO2, a larger increase occurring for CO2. This results ina much lower selectivity for the reduced membrane, as shown inFig. 4, due to the permeance of He only increasing by 15–20%.A drop in the selectivity between the oxidised and reducedmembrane can be seen with a decrease from 70 to 30 recordedfor He/N2 and 40 to 15 for He/CO2. The selectivities for He/N2 andHe/CO2 all increase with temperature due to the molecular sievingregime of the membrane.

The significant changes in the gas permeation of the PdCoSimembrane after exposure to hydrogen gas is a novel finding forTEOS derived silica membranes. Only cobalt doped ES40 derivedsilica membranes have shown similar properties [25]. To elucidatefurther the change in permeation characteristics, the permeationof H2 was monitored during the reduction process as shown inFig. 5. The H2 permeance shows dissimilar behaviour between theoxidised state (100 to 500 1C ascending profile) and the fullyreduced state (500 to 100 1C descending profile). For instance, theH2 permeance along the descending temperature profile did notreturn to the original values obtained in the ascending profile.These results strongly suggest that the redox effect is creatingstructures with different molecular sieving domains. This isexemplified by the apparent energy activation for H2 permeation,which was 4.2 kJ mol�1 along the descending temperature profile,lower than 8.5 kJ mol�1 for the ascending temperature profile.This results in the formation of a hysteresis whereby the permea-tion of H2 is dependent on the temperature in which it haspreviously been exposed.

It is proposed that the membrane is being reduced at eachsuccessive temperature along the ascending profile. This is sup-ported by the fact that approximately 20 h were required to recorda stable permeance. On the contrary, along the descending profile,less than an hour was required to reach the permeation equili-brium, which was similar to all other gases tested. It is thereforeproposed that along the descending temperature profile, the timetaken to reach equilibrium is not a function of membrane reduc-tion, rather the equilibrating of the pressure on the permeate sideof the membrane.

It has been shown in literature that the exposure of micro-porous silica membranes to H2 at elevated temperatures changesthe phase of the metal dopant [25,27,28]. To monitor this change,an XPS analysis was conducted on xerogels before reduction andafter reduction at 500 1C. The high resolution spectra from thePd3d and Co2p 3/2 regions are displayed in Fig. 6a and b,respectively. The metallic (0) and palladium (II) oxide species areassigned a binding energy of 335 eV and 336.3 eV [29–31] whilethe cobalt oxides (Co(II), Co(III)) and metallic cobalt species areassigned 779.9 eV, 780.3 eV and 778.5 eV [32,33]. It can be seenthat the relative proportion of Pd0, in comparison to Pd2þ in thexerogels increases after the reduction process.

80

70

60

50

40

30

20

10

00 100 200 300 400 500 600

Temperature (˚C)

Per

mse

lect

ivity

(-)

HeN2 CO2

He

OxidisedReduced

Fig. 4. Permselectivities (711%) of He/N2 and He/CO2 for the membrane in itsoxidised and reduced state.

6x10-7

5x10-7

4x10-7

3x10-7

2x10-7

1x10-7

00 100 200 300 400 500

Temperature (˚C)

Per

mea

nce

(mol

.m-2

.s-1

.Pa-

1 )

Fig. 5. Hydrogen permeation of PdCoSi membrane during reduction process.The arrows denote the sequence of hydrogen permeation tests.

Inte

nsity

(-)

Binding Energy (eV)346 342 338 334 330

Pd0Pd2+ Co3+

794 789 784 779 774Binding Energy (eV)

Co2+

Oxidised

Reduced

Fig. 6. High resolution XP spectra for the (a) Pd 3d and (b) Co 2p 3/2 regions.

B. Ballinger et al. / Journal of Membrane Science 451 (2014) 185–191188

On the other hand, Fig. 6b shows only a minimal change, withperhaps a slight enhancement of the Co(II) peak upon reduction.The presence of shake-up satellite peaks between 784 and 792 eV[34] are due to the multiple electron excitation of the cobalt (II)oxide. The relative intensities of both regions are similar, and thusa negligible change in the amount of cobalt (II) oxide has occurredover the reduction process. In addition, no metallic cobalt specieswas detected after reduction. This shows that although the cobaltis the stoichiometrically dominant metal species in the PdCoSimatrix, it accounts for only a small fraction of the total metalreduction in the xerogel samples.

To determine the effect of the reduction process on the changein porosity, the xerogel powders were analysed by N2 adsorption.Fig. 7 shows the isotherms for the oxidised xerogel and the xerogelreduced at 500 1C. Both isotherms show similar tendencies, witha almost negligible hysteresis indicating a type IV isotherm [35].The fact that the xerogels exhibit mesoporous structure when themembrane produces activated diffusion with a large permselec-tivity is an interesting finding. It is often implied that xerogelsdisplaying a narrow pore size distribution in the ultra-microporous region produce a highly selective membrane throughthe activated diffusion regime, while xerogels featuring a meso-porous component do not produce this same effect. However, thiswork shows that it is possible to produce a highly selective thinfilm with a xerogel exhibiting a mesoporous component. Thissuggests that the difference in the drying conditions between

xerogels and thin films has a far greater impact on the resultantstructure than originally thought.

Having said this, the reduction process results in an enhance-ment of the N2 volume adsorbed in the porous structure. Thiscomes from both an increase in the microporosity and mesopor-osity as observed in Fig. 7. The change of porosity corresponds tothe decrease in permselectivity of the membrane, with a permea-tion increase for the larger gases occurring after the reductionprocess. This agrees with the conventional wisdomwhich suggeststhat larger xerogel pore sizes result in reduced permselectivities.

These results strongly suggest that the increase in the xerogelpore values is linked to the conversion of palladium monoxide tometallic palladium during the reduction process. It has beenwidely documented that the dissolution of metal species in thesol–gel process produces metal nanoparticles which aggregate inthe porous structure of silica [24,25,36]. Thus, if the porousstructure is rigid, a change in the state of the dopant species willchange the porosity of the silica matrix. Assuming that the densityof PdO and Pd nanoparticles are similar to their respective bulkdensities (8.3 g cm�3 and 12.02 g cm�3 [37,38]), a 40% decrease involume will occur for any reduction that takes place.

We postulate that the reduction of palladium monoxide tometallic palladium creates an additional molecular gap domain asschematically depicted in Fig. 8 for (a) oxidised and (b) reducedconditions. The porous silica structure envelopes the cobalt oxideand palladium monoxide particles embedded in the silica film.A representative TEM micrograph on oxidised PdCoSi (Fig. 9a)shows two discrete groups of small nanoparticles, generally in therange of �2 to �10 nm in size, embedded in amorphous silica.The nature of the observed particles was determined by employinglattice fringe measurements. The distances between lattice fringesfor larger particles were 0.24 and 0.28 nm which correspondaccordingly to (220) and (311) in Co3O4 [7,39]. The smaller particlefringes are better observed in Fig. 9b, where they have aninterlattice distance of 0.224 nm which is very similar to thetheoretical distance (0.2245 nm) of (111) in PdO [40]. It is alsoworth noting (in Fig. 9a) that some particles overlap, formingparticle on the particle building blocks which may open porosity ifone of the materials is reduced. The reduction process preferen-tially reduced palladium monoxide only, whilst any reductioneffect on cobalt oxide was not significant. This suggests thatpalladium and cobalt interact under reducing conditions [27]. Thisis supported by a study by Guczi et al. [41] which showed bytemperature programmed reduction (TPR) that the binary doping

0 0.2 0.4 0.6 0.8 1

Vol

ume

Ads

orbe

d (c

m3 .

g-1 S

TP)

Partial Pressure (P/P°)

25

20

15

10

5

0

Reduced

Oxidised

Fig. 7. N2 adsorption isotherms of reduced and oxidised xerogels.

Pd

CoOx

CoOx

PdO

CoOx

CoOx

β2

αα

β1

Fig. 8. A schematic representation of the change in porosity due to the interaction between cobalt oxide and palladium metal/oxide for the (a) oxidised and (b) reducedmembrane.

B. Ballinger et al. / Journal of Membrane Science 451 (2014) 185–191 189

of palladium and cobalt in silica via the sol–gel process results inthe interaction of the two metal species. Since palladium has amuch higher reduction potential than cobalt [42,43], palladiumwill preferentially reduce with exposure to hydrogen.

Hence, it is proposed the formation of molecular gap domainscreated from the loss of oxygen resulted in an increase of the totalpore volume, which almost doubled as per N2 isotherms in Fig. 7.It is known that pore volume has a direct relation with membraneperformance, as xerogels with higher pore volume tend to producemolecular sieve silica membranes with higher fluxes. However, thepalladium reduction effect on the membrane performance showsinteresting trends. In one hand, He permeance showed only asmall increase after reduction. As He has the smallest kineticdiameter, these results clearly indicate that the integrity of theoverall porosity for He permeance remained constant and possiblycontrolled by the constrictions in the amorphous silica structure.

On the other hand, the permeance of the larger kineticdiameter gases CO2 and N2 increased considerably by reducingselectivities as shown in Fig. 4. Interestingly, the permeance trendsfor both oxidised and reduced membranes remained dependenton temperature, a characteristic of molecular sieving membranes.As there was no change of transport phenomena mechanism, thenthe total structure remained almost intact after reduction, apartfrom the creation of the molecular gap domains. Essentially thesedomains created preferential diffusional percolation pathwayswhich allowed for the increase in the permeation of CO2 and N2.

The molecular gap domains are depicted in Fig. 8 by twobottleneck regions, α as in typical amorphous silica network andβ as in the interface between silica and metal oxide particle. Thekinetic diameter of He is smaller than the pore diameter in the β1and β2 regions, though the kinetic diameter of N2 and CO2 is larger

than in the β1 region but smaller than in the β2 region. Hence, Hecan permeate through both regions, but the larger molecules N2

and CO2 can diffuse through β2 region only. As silica contains

amorphous structures, the α region can discriminate by pore size

exclusion the diffusion of N2 and CO2 if α¼β1, or not if α¼β2.In the oxidised membrane, diffusion is controlled by β1, whichresults in a highly selective membrane since He is smaller than thediffusion limiting pore size and N2 and CO2 are much larger. Uponreduction, the β region expands and diffusion is controlled by theα and/or β2 regions. The permeance of He shows a small increase,as less resistance is imparted due to the more open porosity.However, the permeances of both N2 and CO2 greatly increase withan increase in the diffusion limiting pore size. This is due to arelatively large decrease in the repulsion forces between the silica

matrix and these large gases which is a function of the Lennard–Jones potential. This corresponds to the larger permeance increasefor CO2 compared with N2.

The proposed mechanism in Fig. 8 slightly differs from thosepostulated by Miller and co-workers [25] who used a pre-hydrolised ES40 silica precursor, instead of TEOS as in this work.ES40 was robust and flexible enough to allow for the formation ofmolecular gap via the cobalt oxidation and reduction. In this work,the silica structure derived from TEOS remains rigid and moleculargap is achieved by the redox effect on the palladium only, as cobaltremains in its original oxidation state.

4. Conclusions

The binary doping of palladium and cobalt into the silica matrix(PdCoSi) was found to produce a high quality, molecular sievingmembrane, with permeance characteristics a function of theredox state. The membrane displayed a large drop in permselec-tivity when reduced at 500 1C, the result of a 3–6 fold increase inthe permeation of N2 and CO2. Analysis of the PdCoSi xerogelsshowed that upon reduction a portion of palladium monoxide wasreduced to metallic palladium, while cobalt oxide stayed in itsoxidised state. This was accompanied by an increase in the porousvolume of the xerogels, in both the micro- and mesoporousregions. It was postulated that the increase in pore volume was aresult of both the decreasing size of the palladium nanoparticles,and the rigidity of the porous network. Furthermore, it wasproposed that the inability to reduce the cobalt oxide was due toan intimate contact between the cobalt and palladium nanoparti-cles, with the palladium preferentially reducing due to its higherreduction potential. This is a novel result and shows for the firsttime that extra functionalities can be achieved by the redox effectof binary metal doping in silica membranes.

Acknowledgements

The authors acknowledge financial support provided by theAustralian Research Council through Discovery Project GrantDP110101185. The authors would like to thank the facilities andthe scientific and technical assistance of Dr. Barry Wood of theAustralian Microscopy & Microanalysis Research Facility at theCentre for Microscopy and Microanalysis at The University ofQueensland.

(220)0.28nm (311)0.24nm

5 nm

(111)0.224nm

5 nm

Fig. 9. TEM micrographs of oxidised CoPdSi where (a) cobalt and (b) palladium oxide fringes are highlighted. Note: PdO nanoparticles are encircled.

B. Ballinger et al. / Journal of Membrane Science 451 (2014) 185–191190

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