Catalytic Air Pollution Control (Heck/Catalytic) || Catalyst Characterization

3 Catalyst Characterization

3.1 INTRODUCTION

The characterization of a heterogeneous catalyst is the quantitative measure of its physical and chemical properties assumed to be responsible for its per-formance in a given reaction. These measurements have value in the prepara-tion and optimization of a catalyst and, even more importantly, in elucidating mechanisms of deactivation and subsequent catalyst design to minimize such deactivation. Physical properties such as the pore size, surface area, and mor-phology of the carrier, as well as the geometry and strength of the monolithic catalyst support, must be well defined for the given end-use application. Simi-larly, determining the composition, structure, and nature of the carrier and the active catalytic components and their changes during the catalysis process is a critical goal in characterization. Several reference sources can provide addi-tional detail regarding the equipment and procedures (ASTM Committee D-32 on Catalysis 1988; Anderson and Dawson 1976; Delannay 1984; Deviney and Gland 1985; Farrauto and Hobson 1992; Bartholomew and Farrauto 2006; Knozinger 2003).

The importance of the physical and chemical properties can be appreciated when we restate the fundamental steps involved in a heterogeneous catalytic process. (These were stated in more detail in Chapter 1.) The example used refers to a catalyzed carrier supported on a monolith.

1. Bulk mass transport of the reactants from the bulk fluid to the outside surface of the catalyst. This is strongly influenced by the hydraulic diam-eter of the honeycomb channel, the geometric surface area of the cata-lyst, and the flow turbulence.

2. Diffusion or transport of reactant(s) to active sites through the pore structure of the catalyst. This depends on the pore size and washcoat thickness of the catalyst. A well-optimized catalyst will have a sufficiently large pore size and proper washcoat thickness to permit easy access of reactant and product molecules to and from the active sites, respectively.

Catalytic Air Pollution Control: Commercial Technology, by Ronald M. Heck and Robert J. Farrauto, with Suresh T. Gulati. Copyright © 2009 John Wiley & Sons, Inc.

41

42 CATALYST CHARACTERIZATION

3. Chemisorption ofreactants(s) onto the catalytic active sites. This is depen-dent on the number and nature of the active site(s) and on the chemistry of the adsorbing molecule(s).

4. Chemical conversion of the chemisorbed species to products. This step depends on the number and nature of the activated complexes formed by the chemisorbed molecule and the catalytic site.

5. Desorption of products(s) from active sites. This step depends on the number of adsorbed species and on the strength of their bonds to the active surface.

6. Diffusion or transport of product(s) through the pore structure. This step is affected mostly by the size and shape of the diffusing molecule and by the pore and thickness of the washcoat.

7. Bulk diffusion of the products from the outside surface of the washcoat to the bulk fluid. The process is influenced by the hydraulic diameter of the monolith channel and by the turbulence of the flow.

During the conversion process, several physical and chemical properties have to be well defined to produce an optimized catalytic system. The most impor-tant properties for environmental monolithic catalysts are discussed below.

3.2 PHYSICAL PROPERTIES OF CATALYSTS

3.2.1 Surface Area and Pore Size

Surface area, pore size, pore size distribution, pore structure, and pore volume of the carrier are among the most fundamentally important properties in catalysis because the active sites are present or dispersed throughout the internal surface through which reactants and products are transported. The size and number of pores determine the internal surface area. It is usually advantageous to have a high surface area (large number of small pores) to maximize the dispersion of catalytic components. However, if the pore size is too small, diffusional resistance becomes a problem.

3.2.2 Surface Area and Pore Size Measurements

A standardized procedure for determining the internal surface area of a porous material (ASTM D4641-87 1988) with surface areas greater than 1 or 2m2/g is based on the adsorption of N2 at liquid N2 temperature onto the internal surfaces of the carrier.

Each adsorbed molecule occupies an area of the surface comparable with its cross-sectional area (16.2 A2). By measuring the number of N2 molecules adsorbed at monolayer coverage, one can calculate the internal surface area. This plot is shown in Figure 3.1a). The adsorption of N2 first rapidly rises with pressure and then flattens in the general region where monolayer coverage is

PHYSICAL PROPERTIES OF CATALYSTS 43

Volume adsorbed (ml/g catalyst) P/V(P0 - P)

0.0 1.0 P/P0 p/p (a) (b)

Volume of N2 adsorbed/desorbed (%) 100 | - ^ y

80 - ^ ^ ^ ^

60 - ^ ■

40 - /

20 I

r j ! , 1 . 1 , 1 , 1 . 1

0 0.2 0.4 0.6 0.8 1 Relative pressure, P/P0

(C)

FIGURE 3.1. a) Adsorption isotherm for nitrogen for BET surface area measurement. b) Linear plot of BET equation for surface area measurement, c) Nitrogen adsorp-tion/desorption isotherm for pore size measurement.

occurring. At high relative N2 partial pressures, coverage beyond a monolayer occurs, as does condensation of liquid N2 in the pores, giving rise to the large increase in volume adsorbed. The Brunauer, Emmett, and Teller (BET) equa-tion (3.1) describes the relationship between the volume adsorbed at a given partial pressure and the volume adsorbed at monolayer coverage:

P/V(P0 - P) = l/VmC + (C -1) P/CP0Vm (3.1)

where

P = partial pressure of N2

P0 = saturation pressure at the experimental temperature V = volume adsorbed at P Vm = volume adsorbed at monolayer coverage C = constant

This equation can be stated in the linear form Y = sx + b, where P/P0 is x and the term on the left is Y as plotted in Figure 3.1b). The intercept b is equal


to 1/Vm C, and the slope s is (C-1)/Vm C.The most reliable results are obtained at relative pressures (P/P0) between 0.05 and 0.3.

The same equipment can be used to determine the pore size distribution of porous materials with diameters less than 100Ä (lOnm), except that high relative pressures are used to condense N2 in the catalyst pores. The procedure involves measuring the volume adsorbed in either the ascending or the descending branch of the BET plot at relative pressures close to 1.0. Figure 3.1c) shows the volume of N2 adsorbed and desorbed as the pressure is increased and decreased, respectively. Capillary condensation occurs in the pores in accordance with the Kelvin equation [Eq. (3.2)].

ln(P/Po) = -2oVcos0/rRT (3.2)

where

σ = surface tension of liquid nitrogen Θ = contact angle V = molar volume of liquid nitrogen r = radius of the pore R = gas constant T = absolute temperature P = measured pressure P0 = saturation pressure

Hysteresis between the adsorption and desorption isotherms in Figure 3.1c) at relative pressures 0.6-0.9 is observed with carriers having a significant volume of mesopores (diameters between 20 (2nm) and 500Ä (50nm)). It results because pores, which fill at a given pressure during adsorption, require a lower pressure to empty during desorption. The form of the Kelvin equation given above describes the desorption isotherm, and it is the preferred one for calculations of the pore size distribution.

3.2.3 Pore Size by Mercury Intrusion

For materials with pore diameters greater than about 30 A (3nm), the mercury intrusion method is preferred (ASTM D4284-83 1988). The penetration of mercury into the pores of a material is a function of applied pressure. At low pressures, mercury penetrates the large pores, whereas at higher pressures, the smaller pores are progressively filled. Due to the nonwetting nature of mercury on oxide carriers, penetration is met with resistance. The Washburn equation [see Eq. (3.3)] relates the pore diameter d with the applied pressure P:

d = -4ycose/P (3.3)


Total pore volume (ml/g) Incremental pore volume (ml/g)

1 L \ _

1,000,000 100,000 10,000 1,000 100 10

Pore radius (A)

(a)

1 10,000,000 1,000,000 100,000 10,000 1,000 100

Pore radius (A)

(b)

FIGURE 3.2. a) Mercury penetration as a function of pore size or catalyst, b) Differ-ential porosimetry for a porous catalyst.

The wetting or contact angle Θ between mercury and the solid is usually 130°, and the surface tension of the mercury γ is 0.48 N/m. Pressure is expressed in atmospheres and d in nanometers (10 A). This technique is satisfactory for pores down to a 30 A (3 nm) diameter; however, this is a function of the instru-ment capability. The maximum diameters measured are usually 106 A. Typical pore size distribution data are shown in Figure 3.2a), where the integral pen-etration of mercury into the pores is plotted as a function of applied pressure. The differential curve [Figure 3.2b)] clearly shows a bimodal pore distribution with mean pore diameters at 10,000 (lOOOnm) and 1000Ä (lOOnm). A nitro-gen desorption isotherm is required to obtain an accurate measure in the region below 100Ä (lOnm) (see Section 3.2.2).

3.2.4 Particle Size Distribution of the Carrier

The size of the agglomerated particles that make up the carrier must be compatible with the surface roughness and macropore structure of the honeycomb surface. Slightly acidified aqueous mixtures of the carrier powders are ball milled to produce the slurry from which the honeycomb will be coated. The rheology of the slurry depends on the solids content, the particle size and surface properties of the carrier material, and the acidity of the slurry.

Sieves of various mesh sizes have been standardized, and thus, one can determine particle size ranges by noting the percentage of material, usually based on weight, that passes through one mesh size but is retained on the next finer screen (ASTM D4513-85 1988). Sieves are stacked with the coarsest on top and the underlying screens progressively finer. A precise weight of catalyst material is added to the top screen. The stack of sieves is vibrated, allowing


the finer particles to pass through coarser screens until retained by those screens finer in opening than the particle size of the material of interest. Each fraction is then weighed, and a distribution is determined.

This method is reliable only for particles larger than about 40μπι. Below this size, sieving is slow and charging effects influence measured values. Sophis-ticated instrumentation is available for measuring the distribution of finer particles. Methods include electronic counting, light scattering, image analysis, sedimentation, centrifugation, and volume exclusion. An example of electronic counting is described in ASTM D4438-85 (1988). In this method, the dried powder from the slurry is suspended in an electrolyte and is pumped through a tube containing a small orifice. An electric current passes through this tube, and as individual particles pass through the orifice, a fraction of the current is interrupted. This fractional change in current is proportional to the particle size. The magnitude of the change in current flow is subdivided over the range of sizes limited by the size of the orifice. The particle diameters are calculated on the equivalent sphere of the excluded volume, and assuming constant density for the particles, the results are commonly recorded as weight percent-age as a function of incremental particle size.

More recently, laser techniques have been found effective (ASTM D4464-85 1988). A He-Ne laser beam is passed through an aqueous suspension of particles, and it is diffracted in proportion to the radius of the particle. A distribution for an A1203 carrier is illustrated in Figure 3.3. The values on the left axis indicate the cumulative % of particles, the size of which is displayed

Cumulative (%) Distribution (%)

90-

80-

70-

60-

50-

40-

30-

20-

10-

0 -0.1 1 10 100 1000 10000

Particle size (μηι)

FIGURE 3.3. Particle size measurement using laser light scattering analysis.

Cumulative particle size ■14

+ 12

•10

8

- 6

-4

■2


on the x-axis. The right axis shows the % distribution for a given particle size.

3.2.5 Washcoat Thickness

Optical microscopy is the method used most frequently to obtain washcoat thickness directly. A portion of washcoated monolith is mounted in epoxy and sliced to obtain a cross section. The contrast between washcoat and monolith is sufficient to permit optical thickness measurements. A typical cross section of a washcoat on a ceramic auto exhaust monolith is shown in Figure 2.3 (see Chapter 2).

3.2.6 Mechanical Strength of a Monolith

Monoliths, particularly when used in a stacked mode (e.g., in stationary pol-lution abatement), must resist crushing axially. For automobile and truck appli-cations, and ozone abatement in aircraft, resistance to vibration and radial strength is important during the process of housing the catalytic system in a steel container.

The strength measurement for monoliths is simple. A single unit representa-tive of the lot is placed between parallel plates of a device capable of exerting compressive stress, and the force necessary to crush the material is noted. The monolith can be placed within the plates so as to measure axial or radial crush strength. A sufficient number of units must be tested to obtain proper statistics.

3.2.7 Adhesion

Washcoated monolithic catalysts are subjected to high gaseous flow rates and to rapid temperature fluctuations. As a result, adhesion loss is a likely cause for concern. The most common technique is to subject the washcoated mono-lith to a jet of gas that simulates the linear velocities anticipated in use and to note the weight change of the catalyst due to erosion. One can also collect the attrited particles and correlate their weight with the weight of washcoat ini-tially bound to the monolith.

Monolithic materials are frequently subjected to substantial thermal gradi-ents (thermal shocks) in start-up and shutdown, as in auto exhaust emission control. Frequent thermal shocks cause the washcoat to delaminate due to the expansion difference between it and the monolith. This delamination is most pronounced when the monolith is metallic because of its greater thermal expansion compared with an oxide washcoat.

One can evaluate this phenomenon by cycling the catalyzed monolith between temperature extremes anticipated in service and by noting weight losses. Catalyzed monolith can be mounted on a rotating "carousel" that moves in and out of streams of heated gas.


3.3 CHEMICAL AND PHYSICAL MORPHOLOGY STRUCTURES OF CATALYTIC MATERIALS

3.3.1 Elemental Analysis

The proper combination of chemical elements is essential in catalysts for optimum performance. More often than not, small amounts of promoter oxides intentionally added (often less than 0.1%) can influence activity, selectivity, and life.

Catalyst suppliers must meet many manufacturing specifications that apply to the proper chemical analysis of a catalyst. Carriers are derived from raw materials, which contain various impurities. Some impurities can be detrimen-tal to catalytic performance and, therefore, must be removed. Impurities such as alkali and alkaline earth compounds, if used in excess, act as fluxes, causing sintering or loss of surface area in A1203. When added in the proper amount, the same impurities can enhance stability against sintering or, in some cases, improve selectivity. Hence, it is not just the presence of impurities but also the manner in which they have been introduced that is important. Obviously, one has greater control when starting with a relatively pure material to which predetermined amounts of promoters can be added.

The quantitative procedures used to analyze catalysts are no different than those for any other chemical material. Special procedures are often needed to dissolve catalysts in preparation for analysis—particularly refractory materials such as certain noble metals and ceramics. Refer to ASTM D4642-86 for pro-cedures for analyzing Pt supported on A1203.

3.3.2 Thermal Gravimetric Analysis (TGA) and Differential Thermal Analysis (DTA)

Thermal gravimetric analysis measures the weight change of a material upon heating in a highly sensitive balance. The energy change (heat adsorbed or liberated) associated with any transformation is measured with the differential thermal analysis mode. It is common practice for both to be measured simul-taneously in a combined TGA/DTA unit. A few milligrams of catalyst are loaded into a quartz pan suspended in the microbalance. A controlled gas flow and temperature ramp is initiated, and a profile of weight and energy change versus temperature is recorded. Frequently, TGA units are equipped with a mass spectrometer so the off-gases from the catalyst can be measured as a function of temperature. Catalyst salt components are impregnated, usually from aqueous solution, into a porous carrier. After deposition onto the carrier, the combination is calcined in air to a temperature sufficient to decompose the salt:

Ba(CH3COO)2 + 402 -> BaO + 4CQ2 + 3H2Q (3.4)

CHEMICAL AND PHYSICAL MORPHOLOGY STRUCTURES 49

0.3 - 307 °C T G A

80 160 240 320 400 480 560

Temp (°C)

FIGURE 3.4. Thermal gravimetric analysis and differential thermal analysis of the decomposition of barium acetate on ceria.

This is shown in Figure 3.4 where barium acetate is decomposed after it has been impregnated into a high-surface-area carrier such as cerium oxide.

The solid line shows the weight change associated with heating the ceria after impregnation with the acetate salt of barium. A small weight loss (mea-sured on the right axis) is observed between 80 °C and 240 °C due to the evaporation of H20. The .dotted line (measured on the left axis) shows the DTA profile with a negative slope consistent with the endothermic evapora-tion of H20. Decomposition of the acetate begins around 250 °C where the weight change (solid line) is observed to decrease sharply until about 400 °C where it plateaus. Associated with this exothermic decomposition event (posi-tive slope) is the heat liberated in the DTA profile showing a maximum at 307 °C. The ceria was preconditioned to be stable during this procedure. This simple test allows production to establish the optimized conditions for drying and calcining the catalyst during its preparation. This particular material, BaO/ Ce02, is being considered for adsorbing or trapping NOx in a lean-burn engine exhaust, which will be discussed in the section on controlling emissions from diesel engines.

This technique is also used to establish the temperature and environmental conditions under which a metal will oxidize (temperature-programmed oxida-tion or TPO) or a metal oxide is reduced in an environment such as H2 or CO (temperature-programmed reduction or TPR).

In Chapter 5 on catalyst deactivation, other examples of how these useful tools can be used to understand catalyst deactivation and regeneration modes will be described.


3.3.3 The Morphology of Catalytic Materials by Scanning Electron Microscopy (SEM)

In Chapter 2, the reader was introduced to a variety of carrier materials upon which the catalytic components are dispersed to maximize the number of sites available for reactants to chemisorb. Clearly, for environmental applications, γ-Α1203 is the most common material. It has a surface area of over 200m2/g with a highly porous structure as shown in the SEM of Figure 2.1a). Its pores range from about 20 to 200 A (2-20 nm). As it experiences elevated tempera-tures, it slowly transforms into other lower porosity and more crystalline intermediary structures terminating with alpha oc-Al203, which has the lowest surface area and is the most crystalline as shown in Figure 2.1b). The structural transformations of γ-Α1203 will be discussed in more detail in Chapter 5 "Cata-lyst Deactivation" because they represent one of the most commonly occur-ring deactivation modes for supported catalysts.

3.3.4 Location and Analysis of Species within the Catalyst by Electron Microscopy

Environmental monolithic catalysts contain a variety of catalytic metal and metal oxide promoters in addition to the A1203 carrier. Each has a specific function and must have its proper place within the washcoat. Some compo-nents must be in intimate contact with each other to contribute to electronic promoting effects, but they could poison other components and cause deacti-vation. Such is the case with certain metals that form undesirable alloys or compounds. For this reason, some catalyst manufacturers use multiple wash-coat layers of specific compositions to maximize effectiveness while minimiz-ing the poisoning effects. This process can be observed when the electron microscope is used in the backscatter mode (wavelength-dispersive analysis or WDS) as shown in Figures 3.5a-d). In the scanning mode, the electron beam focused on the sample is scanned by a set of deflection coils. Backscattered electrons or secondary electrons emitted from the sample are detected. As the electron beam passes over the surface of the sample, variations in composition and topology produce variations in the intensity of the secondary electrons. The raster of the electron beam is synchronized with that of a cathode ray tube, and the detected signal then produces an image on the tube. Spot or area analysis is also possible when an electron microscope is equipped with an energy dispersive analyzer (EDX) or WDS. The bombardment of a sample with electrons generates X rays characteristic of the elements present. Thus, the EDX can determine the composition of any portion of the sample, whereas the WDS permits the mapping of the location of species present. This is par-ticularly important when foreign matter is present either from a contamination problem in manufacturing or by poisoning during a catalytic process. The electron microprobe is another form of electron microscopy that is extremely important for metal location studies requiring high resolution (Delannay 1984;


(a) (b)

FIGURE 3.5. a) Wavelength dispersive analysis of Pt in a double-layered washcoat. b) Wavelength dispersive analysis of Si02 in a double-layered washcoat. c) Wavelength dispersive analysis of Pd in a double-layered washcoat. d) Wavelength dispersive analy-sis of A1203 in a double-layered washcoat.

Bartholomew and Farrauto 2006). It is similar to the scanning electron micro-scope; however, its primary function is to detect characteristic X rays produced by the electron beam interaction with the specimen. The X-ray emissions can be used to determine the elemental composition of the specimen quantita-tively and to detect the location of a particular element within the morphology or topological structure of the specimen.

Examples of the use of wavelength dispersive analysis is shown in a series of micrographs shown as Figures 3.5a)-d) for a double-coated layered washcoat. The top coat is composed of Pt dispersed on Si02 [Figures 3.5a) and 3.5b)], and the Pd is dispersed on A1203 as the bottom coat [Figures 3.5c) and d)].


AI

50 75 Distance in μηι

■■■is- Si — T - Rh Pt

FIGURE 3.6. Electron microprobe showing a two-washcoat-layer monolith catalyst. The top layer is Rh on A1203, and the bottom layer is Pt on A1203.

Figure 3.6 shows a microprobe line profile of a ceramic monolith upon which has been deposited two different washcoat layers. The top layer from roughly 25 to 50 microns contains Rh on A1203, whereas the second or bottom layer from 50 to 100 microns contains Pt on A1203. The total thickness of the two layers is 100 microns as evidenced by the decrease and the absence of Pt at depths greater than 100 microns and by the appearance of Si, which is a component of the monolith substance.

3.3.5 Structural Analysis by X-Ray Diffraction

Provided that a material is sufficiently crystalline to diffract X rays and is present in an amount greater than 1%, X-ray diffraction (XRD) can be used for qualitative and quantitative analysis. Crystal structures possess planes made by repetitive arrangements of atoms, which are capable of diffracting X rays. The angles of diffraction differ for the various planes within the crystal. Thus, every compound or element has its own somewhat unique diffraction pattern. Comparing the patterns allows differentiation of various structures.

3.3.6 Structural Analysis of A1203 Carriers

Figure 3.7 shows the XRD patterns of two A1203 structures: amorphous γ-A1203 and a-A1203. The gamma (γ) is the high-surface-area, lower temperature structure, whereas the alpha (a) is produced at higher temperatures and has low surface area (Farrauto and Hobson 1992; Anderson and Dawson 1976; Bartholomew and Farrauto 2006). Below crystallite sizes of 50 A, a well-defined X-ray pattern will not be obtained. Materials with crystallites smaller than this


Intensity (counts) Intensity (counts) 20,000

15,000

t . uuu-

3,000-

2,000-

1,000-

n-V^*%u. /

Alpha

Λ U

Gamma

--

JHhrtiW 0 20 40 60 80 100

10,000

5,000

0

2-Theta (degrees)

FIGURE 3.7. X-ray diffraction patterns of γ- and α-Α1203

are more precisely called amorphous since they possess no long-range order to diffract X rays. Structures in this class, which are common for freshly pre-pared catalysts, must be characterized by other techniques such as those listed below.

3.3.7 Dispersion or Crystallite Size of Catalytic Species

3.3.7.1 Chemisorption. One of the most frustrating facts facing the catalytic scientist is that often when a structure has a definite XRD pattern and can be well characterized, it usually has less-than-optimum activity. This is because most catalytic reactions are favored by either amorphous materials or extremely small crystallites. Small crystals can agglomerate or grow and produce large crystallites that diffract X rays and thus generate easily read patterns; however, the atoms of the small crystals are buried within the larger crystal, making them inaccessible to reactant molecules. Frequently, the purpose of the preparation technique is to disperse the catalytic components in such a way as to maximize their availability to reactants.

% Dispersion = Number of catalytc sites on the surface Theoretical number of sites present as if atoms

-xlOO

(3.5)

When this is done effectively, only small crystals are present and the diffraction of X rays is minimized, because little long-range structure exists. As the crystals


get smaller and smaller, the XRD peaks get broader and broader and eventually are undetectable above the background. However, it is these "X-ray-amorphous" species that are often the most active for a given catalytic reaction.

Standardized techniques exist (ASTM D3908-82 1988; Anderson and Dawson 1976; Bartholomew and Farrauto 2006) for obtaining information regarding the distribution and number of catalytic components dispersed within or on the carrier. Selective chemisorption can be used to measure the accessible catalytic component on the surface by noting the amount of gas adsorbed per unit weight of catalyst. The stoichiometry of the chemisorption process must be known to estimate the available catalytic surface area. One assumes that the catalytic surface area is proportional to the number of active sites. A gas that will selectively chemisorb only on to the metal and not the support is used under predetermined conditions. Hydrogen and carbon mon-oxide are most commonly used as selective adsorbates for many supported metals. There are reports in the literature of instances in which gases such as NO and 0 2 have been used to measure catalytic areas of metal oxides; however, due to difficulty in interpretation, they are of limited use.

The measurements are usually carried out in a static vacuum system similar to that used for BET surface area measurements. The pressure of gas above the sample is increased, and the amount adsorbed is measured at equilibrium as shown in Figure 3.8a). When there is no further adsorption with increasing pressure (shown in the flat portion of the figure), the catalytic surface is satu-rated with a monolayer of adsorbate. Noting the number of molecules of gas adsorbed and knowing its stoichiometry with the surface site (i.e., 1 H per metal site), one can determine the catalytic surface area by multiplying mole-cules adsorbed by cross-sectional area of the site and dividing by the weight of catalyst used in the measurement. For example, the cross-sectional area of Ptis8.9Ä2andNi6.5Ä2.

Volume adsorbed Effluent volume of adsorbate

No adsorption Saturation

Adsorption

Number of pulses (b)

FIGURE 3.8. a) Chemisorption isotherm for determining the surface area of the cata-lytic component, b) Pulse chemisorption profiles for the dynamic chemisorption method.


The static vacuum technique is time consuming, so alternative methods have been devised. For years, a dynamic pulse technique has been used in which a pulse of adsorbate such as H2 or CO is injected into a stream of inert gas and passed through a bed of catalyst. Gas adsorption is measured by comparing the amount injected with the amount passing through the bed unadsorbed.

As shown from left to right in Figure 3.8b), the first two pulses are used for calibration and bypass the catalyst sample. The second set of pulses that pass through the catalyst are first diminished due to adsorption. Once saturation or monolayer coverage is reached, no further adsorption from the gas phase occurs. The amount adsorbed is found by the difference in areas under the peaks compared with those under the calibration pulses. The major difference between dynamic and static methods is that the former measures only that which is strongly adsorbed, whereas the latter, performed under equilibrium conditions, measures strong and weakly chemisorbed species. Thus, static tech-niques usually give better dispersion results.

3.3.7.2 Transmission Electron Microscopy. In transmission electron micros-copy (TEM), a thin sample, usually prepared by a microtome, is subjected to a beam of electrons. The dark spots on the positive of the detecting film cor-respond to dense areas in the sample that inhibit electron transmission. These dark spots form the outline of metal particles or crystallites, and hence, their sizes can be determined.

Figure 3.9 shows a TEM of sintered Pt, about 500 Ä (50 nm) in size, dis-persed on Ti02. Crystallites of Pt about 100 Ä (10 nm) in size, dispersed on Ce02, are shown in the TEM of Figure 3.10. Using image analysis, a size

large Pt particle about 500 A in size

FIGURE 3.9. Transmission electron micrograph of Pt on Ti02 .


FIGURE 3.10. Transmission electron micrograph of Pt on CeO:

distribution and average crystallite size can be calculated. Assuming a spheri-cal shape for the crystallites, the percent dispersion (i.e., ratio of surface atoms to total atoms in the crystallite xlOO) can be calculated. It should be under-stood that this technique measures only a small fraction of the catalyst, so obtaining data representative of the entire sample is difficult. However, it does give a direct measure of the catalytic components.

3.3.8 X-Ray Diffraction

The larger the crystals of a given component, the sharper the peaks on the XRD pattern for each crystal plane. The Scherrer equation relates the breadth B at half-peak height of an XRD line due to a specific crystalline plane to the size of the crystallites L:

B = kX/Lcos9 (3.6)

where

λ = X-ray wavelength Θ = diffraction angle k = constant usually equal to 1

As the crystallite size increases, the line breadth B decreases (Bartholomew and Farrauto 2006). Figure 3.11 shows the sharp XRD pattern of Ce02 treated at 1,500 °C. The Ce02 treated at 800 °C has a broad profile indicative of a much


40,000-

30,000-

20,000-

10,000-

0-

[ntensity (coi

~^ ,—

mts)

Ce0 2 —* (1,500°C) >1μ

\

/ \

1

Λ \ Ce02 (800°C)

VC__ 49 46 46.5 47 47.5 48 48.5

2-Theta (degrees)

FIGURE 3.11. X-ray diffraction profile for different crystallite sizes of Ce02.

smaller crystallite size. Cerium oxide is a constituent in automobile catalytic converters, and its crystal size impacts its ability to function as an oxygen storage component (see Chapter 7, "Automotive Catalysis").

3.3.9 Surface Composition of Catalysts by X-ray Photoelectron Spectroscopy

The composition of the catalyst surface, as opposed to its bulk, is of critical importance since this is where the reactants and products interact. It is on these surfaces that the active sites exist and where chemisorption, chemical reaction, and desorption take place. Furthermore, poisons deposit in the layers of the surface of the catalyst and, thus, knowing its concentration will give valuable insight into activity and deactivation. Techniques such as XRD and electron microscopy measure the structure and/or chemical composition of catalysts extending below the catalytic surface. The composition of the surface is usually different from that of the bulk, and thus, its analysis must be carried out by techniques specific to the surface.

The tools available for surface composition characterization (Bartholmew and Farrauto 2006; Deviney and Gland 1985) are X-ray photoelectron spec-troscopy (XPS), Auger spectroscopy (AES),ion scattering spectroscopy (ISS), and secondary ion mass spectroscopy (SIMS). XPS is used more widely than the others for studying the surface composition and oxidation states of indus-trial catalysts, and thus, its application will be discussed in limited detail.

The acronym "XPS" refers to the technique of bombarding the surface with X-ray photons to produce the emission of characteristic electrons. These are measured as a function of electron energy. Because of the low energy of the


4.00E+04

3.00E+04

I Q 2.00E+04

1.00E+04

Palladium 3d Peak Fit

Pd3d5 Pd(ll)

Pd3d3

Pd3d3

345 344 343 342 341 340 339 338 337 336 335 334 333

Binding Energy (eV)

FIGURE 3.12. An XPS spectrum of various oxidation states of Palladium on A1203 is shown.

characteristic electrons, the depth to which the analysis is made is only about 40 A (4nm). The composition of this thin layer as a function of depth can be determined by removing or sputtering away top layers and by analyzing the underlying surfaces. Several important catalytic properties have been studied by this technique, including the oxidation state of the active species, the inter-action of a metal with an oxide carrier, and the nature of chemisorbed poisons and other impurities.

If a small amount of a gas phase impurity, i.e., S, Cl, P, and so on, deposits on the surface of the catalyst, its concentration is not likely to be detected by bulk chemical analysis. XPS allows only the top monolayer to be analyzed and, thus, allows insight into its effect on the performance. Another example is the oxidation state of the surface catalytic component. Palladium can exist in three oxidation states, all of which have different activities toward specific reactions. For example, Pd metal is the active component in hydrogenation reactions, whereas higher oxidation states of Pd are active for hydrocarbon oxidations. XPS allows us to determine the respective oxidation states. In Figure 3.12, the Pd is found in three oxidation states: 25% as Pd (o), 56% as Pd (+2), and 19% as Pd (+4).

3.3.10 The Bonding Environment of Metal Oxides by Nuclear Magnetic Resonance (NMR)

Zeolites have long been of great importance in the chemical and petroleum industries for their unique pores size and acidity, both of which have a strong influence on catalytic activity. They are now finding greater use in environmen-tal applications as adsorbents for hydrocarbons during cold exhaust conditions and as carriers for metal cations that generate high activities for special reac-tions such as the selective NOx reduction.


29Si MAS NMR of Fresh Si(OAI) Y_Z e o l i t e ( F a u j a 2 i t e)

Si(1AI)

I

Si(2Alj

Si(3AI) ^ / Si(4AI) I Γ^

i Λ 7

-56 -64 -72 -80 -88 -96 -104 -112 -120 -128 -136 -144 ppm

FIGURE 3.13. NMR profile of a Y faujasite with varying amounts of Si-O-Al bridges.

The zeolites of greatest interest in the environmental catalysis are com-posed of varying ratios of Si02 and A1203. Si(+4) and Al(+3) are bound together through an oxygen bridge forming a tetrahedral structure. NMR allows one to determine the number of Si-O-Al bridges. In Figure 3.13, an NMR profile of a Y faujasite zeolite is shown.

The first major peak occurs at -106-ppm units indicating no bridges Si(OAl), whereas the second occurs at -100-ppm units indicating the number of single Si-O-Al bridges Si(lAl). The peaks at progressively lower ppm units show fewer bridges Si(2Al), Si(3Al), Si(4Al) as is evident by their low intensity. Thus, the distribution of tetrahedral Si-O-Al sites that represent the active sites can be determined. The higher the Si/Al ratio in the zeolite, the more tetrahedral bridges are expected to exist. When most zeolites undergo high-temperature exposure, they deactivate and the tetrahedral Si-O-Al bridge is broken. One can follow deactivation by the decrease in peak intensities. The impor-tance of this will become clear when deactivation of zeolites is discussed in Chapter 5.

Atomic nuclei spin in much the same ways as electrons. The movement of an electric charge generates a magnetic field. The silicon isotope (29Si) with an uneven number of protons and neutrons is present in significant amounts in all compounds, including zeolites. In such a case, a dipole (separation of charge) exists that can interact with an external magnetic field. The dipole can exist in different spin states, and at a particular frequency of the imposed magnetic field, it can be elevated to a higher spin state during "resonance."


Upon relaxation of the external field, the dipole decays to a lower energy state and emits energy that is characteristic of that material. The frequency in which "resonance" occurs is influenced by the elements to which the isotope is bonded. So the number of Si-O-Al bridges will alter the frequency of the resonant energy of 29Si, creating the profile shown in Figure 3.13.

3.4 TECHNIQUES FOR FUNDAMENTAL STUDIES

This chapter has attempted to show the methodology commonly used for the characterization of real commercial catalysts. Many techniques are designed to obtain more fundamental characterizations that are beyond the scope of this book. The reader should review external sources (Bartholomew and Farrauto 2006).

All of the techniques described in this chapter are performed after the cata-lyst is removed from the reactor; that is, ex situ. It is very desirable, but also extremely difficult, to measure catalyst properties in situ during an actual process. Once removed from a catalytic environment, the nature of the adsorbed surface species will be changed, and what is measured will signifi-cantly differ from the actual catalytic surface. Nevertheless, many of its other properties (e.g., surface area, pore size, crystalline structure, crystallite size, and chemical composition) are essential data in determining which factors influ-enced its performance. Infrared DRIFT spectroscopy allows the nature of the adsorbed species to be observed before, during, and after catalytic reaction. In Figure 3.14, CO adsorption is studied on the surface of a automobile catalyst

0.040

0.035-

0.030-

„, 0.025-o a 0.020-1 .n g 0.015 < 0.010

0.005 0.000

-0.005

Pd-CO-i

2,200 2,150 2,100 2,050 2,000 Wavenumbers (cm-1)

1,950 1,900

FIGURE 3.14. DRIFT spectra of CO chemisorbed on different precious metal parti-cles of catalysts prepared in different ways. The CO chemisorption followed by FT-IR measurements was performed at room temperature after the catalysts were treated at 400 °C for 1 hr. with a 7%H2 in Ar gas.

REFERENCES 61

containing Pt, Pd, and Rh prior to the addition of 0 2 . The nature of the bonding of the CO to each element is shown. The addition of 0 2 will remove the CO from the most active sites.

Catalytic scientists have created an exciting new approach toward the study of fundamental catalytic reactions. So-called operand spectroscopy permits the characterization of the surface of a catalyst during real catalysis (Banares 2005).

REFERENCES

American Society for Testing and Materials (ASTM): Committee D-32 on Catalysis, Second Edition, Philadelphia, PA (1988).

ASTM: D3663-85, "Standard test method for surface area of catalysts," pp 3-6 (1988).

ASTM: D4641-87, "Standard test method for determination of nitrogen adsorption/ desorption isotherm for pore size measurements," pp 41-44 (1988).

ASTM: D4284-83, "Standard test method for determination of pore volume distribu-tion by mercury porosimetry," pp 26-29 (1988).

ASTM: D4513-85, "Standard test method of particle size of catalytic materials by sieving," pp 71-72 (1988).

ASTM: D4438-85, "Standard test method for distribution of catalytic materials by electronic counting," pp 63-65 (1988).

ASTM: D4464-85, "Standard test method for particle size by laser light scattering," pp 66-67 (1988).

ASTM: D4642-86, "Standard test method for chemical analysis of Pt on A1203 cata-lysts," pp 14-17 (1988).

ASTM: D3908-82, "Standard test method for hydrogen chemisorption of supported Pt on A1203 catalyst by volumetric method," pp 13-16 (1988).

Anderson, R., and Dawson, P. Experimental Methods in Catalytic Research. Academic Press, New York (1976).

Banares, M. "Operando methodology: Combination of in-situ spectroscopy and simul-taneous activity measurements under catalytic reaction conditions," Catalysis Today 100: 71 (2005).

Bartholomew, C, and Farrauto, R. J. Fundamentals of Industrial Catalytic Processes. Chapter 3. Second Edition. Wiley and Sons, New York (2006).

Delannay, E Characterization of Heterogeneous Catalysts. Dekker, New York (1984). Deviney, M. L., and Gland, J. Catalyst Characterization Science: Surface and Solid State

Chemistry, ACS Symposium Series No. 288. American Chemical Society, Washington, DC (1985).

Farrauto, R. J., and Hobson, M. C. "Catalyst characterization," in Encyclopedia of Physi-cal Science and Technology. Vol 2. Editor Meyers, R. A. pp 735-761. Academic Press, New York (1992).

Knozinger, H. "Catalyst characterization" in Encyclopedia of Catalysis. Vol. 2, pp 142-182. Editor Horvath, I, Wiley-Interscience, Hoboken, NJ (2003).


CHAPTER 3 QUESTIONS

1. Why are BET surface areas and Hg measured pore sizes important in heterogeneous catalysis?

2. What do X-ray diffraction patterns tell us about the components of the catalyst?

3. How may a TGA/DTA be used in a. Catalyst preparation? b. Activation of a catalyst? c. Burn-off of hydrocarbons oils deposited on the catalyst?

4. How is NMR used in determining the thermal history of zeolites? 5. What can we learn from CO chemisorption measurements of metal-

supported catalysts?

Catalytic Air Pollution Control (Heck/Catalytic) || Catalyst Characterization

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