Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 177
Advancing Materials Research Materials Research in Catalysis JOHN H.SINFELT A catalyst is a substance that accelerates a chemical reaction without being consumed in the process. The association of catalysis with rates of chemical reactions was made by the German chemist Wilhelm Ostwald around the year 1900. Ostwald’s insight provided a basis for scientific inquiry into catalysis and paved the way for the widespread investigation and application of catalytic phenomena. In the years since 1900 the science of catalysis has progressed steadily, accompanied by enormous technological advances that profoundly affect the lives of all of us.1 Catalytic processes now provide the basic technology for the manufacture of a host of vitally important materials, ranging from fertilizers to synthetic fibers and petroleum products such as gasoline and heating oil. Catalysts are used in the manufacture of an estimated $750 billion worth of products annually in the United States alone.2 Catalytic processes are commonly divided into two categories: homogeneous and heterogeneous.3 The former refer to processes in which the catalyst and the reactants are present in a single phase, as in a solution. In heterogeneous catalysis, by contrast, the reactants and the catalyst are present in separate phases—for example, reactants in a vapor phase in contact with a solid catalyst. The reaction is frequently conducted in a flow system in which the vapor is passed through a vessel containing a bed of catalyst granules or pellets.4 The composition of the vapor changes as it is depleted of molecules of reactant and enriched in molecules of product in its passage through the catalyst bed. A catalytic process involves a sequence of steps in which the active catalytic entities participating in the steps are continually regenerated, so that the catalyst is used over and over in the formation of product molecules from reactants.5 Such a sequence of steps is referred to as a closed sequence.6 In the case of heterogeneous catalysis at the surface of a solid, the active
OCR for page 178
Advancing Materials Research catalytic entity is a site on the surface or a complex of the site with a reactant molecule. For example, a chemical reaction, A+B→C, might proceed by the following sequence of steps: A+S→ A-S A-S+B→C+S. The molecule A is adsorbed on a site S to form a surface complex A-S, which then reacts with a molecule of reactant B to form the product molecule C and regenerate the site S. This simple sequence illustrates features common to all catalytic processes, namely, the generation of a reactive intermediate from the reactant, the transformation of the intermediate to a product, and the regeneration of the active catalyst site. An intriguing aspect of catalysis is the specificity observed. For example, silver catalysts are unique in their ability to catalyze the partial oxidation of ethylene to ethylene oxide:7 2C2H4+O2→2C2H4O. On other solid catalysts, the ethylene undergoes predominantly complete oxidation to carbon dioxide and water. In this example a change in the catalyst actually leads to a pronounced change in the distribution of reaction products, because the different catalysts have markedly different effects on alternative reaction paths. In some cases the observed reaction product is the same for a number of catalysts, but the specific activity (reaction rate per unit surface area or per surface site) varies widely. A good example is the catalytic hydrogenolysis of ethane to methane, C2H6+H2→2CH4, on metals,8,9 where the specific activity of osmium is almost 8 orders of magnitude higher than that of platinum (Figure 1). These examples demonstrate clearly that the chemical nature of the surface is highly important in heterogeneous catalysis. Heterogeneous catalysis involves the participation of species chemisorbed on the surface. Maximum catalytic activity is achieved when chemisorption of the reactant is fast but not very strong.6,10 If the adsorption bond is too strong, the catalyst will tend to be highly covered by reactant species that are not readily transformed or by product species that do not desorb readily from the surface. At the other extreme, if the adsorption bond is very weak, the catalytic activity may be severely limited by a low rate of chemisorption, since the activation energy for chemisorption commonly increases as the heat of adsorption decreases. Optimum catalytic activity corresponds in general to some intermediate strength of adsorption between these two extremes. Many different types of materials have been used as catalysts. In heterogeneous catalysis they are commonly separated into two broad categories— metals and nonmetals. In the first category the most commonly used metals are those in Group VIII and Group IB of the periodic table. In the second
OCR for page 179
Advancing Materials Research FIGURE 1 Catalytic activities of metals for the hydrogenolysis of ethane to methane. The activities were determined at a temperature of 478 K and at ethane and hydrogen partial pressures of 0.030 and 0.20 atm, respectively. From Sinfelt.8,9
OCR for page 180
Advancing Materials Research category, oxides are the most common catalysts. Although there are major applications of materials in both of these categories in industrial catalysis, only metal catalysts are considered in any detail in this chapter. The area embodies a number of issues that are likely to be of broad interest in materials science. Furthermore, metal catalysts illustrate well the approaches that have been used in developing the science of catalytic materials. METAL CATALYSTS In one form of metal catalyst that is widely used commercially,4 the catalyst particles consist of a porous refractory material and small metal crystallites or clusters dispersed throughout the particles (Figure 2). The term carrier, or support, is commonly used in referring to the refractory material, and the catalyst is known as a supported metal catalyst. In a small laboratory reactor the particles could be granules approximately 0.5 mm in diameter. In a commercial reactor the particle size would be somewhat larger, perhaps 2 or 3 mm, to avoid an excessive drop in pressure as the gas passes through the catalyst bed. The refractory material constituting the bulk of the particles is frequently an oxide such as alumina (Al2O3) or silica (SiO2) with a structure consisting of a network of pores with an average diameter of about 100 angstroms. The metal clusters or crystallites reside on the walls of the pores and must therefore be smaller than the pores. In some cases the metal clusters are as small as 10 angstroms. FIGURE 2 Schematic drawing of a catalyst particle with a structure consisting of a network of pores with an average diameter of perhaps 100 Å. The particle consists of a refractory material such as alumina or silica, with small metal clusters residing on the walls of the pores. In some commonly used catalysts the metal clusters are as small as 10 Å.
OCR for page 181
Advancing Materials Research FIGURE 3 The ratio of surface atoms S to total atoms M in a metal crystal, as a function of M. The metal crystals are assumed to be cubes with the face-centered cubic structure. In a typical application of such a catalyst, reactant molecules diffuse into pores of the catalyst and are adsorbed on the active metal clusters. The adsorbed molecules then undergo chemical transformations on the clusters to yield molecules of a different chemical species. These molecules are subsequently desorbed to yield molecules of product in the pores. The product molecules must then diffuse through the network of pores into the gas stream flowing through the space between the particles. The rate of reaction obtained with a given mass of catalyst depends on the number of catalytic sites that are present at the surface. For a reaction occurring on the metal clusters in a supported metal catalyst, the rate per metal atom can be determined readily. However, it is the rate per surface metal atom that is of fundamental interest. To determine this quantity, we need to know the ratio of surface metal atoms S to total metal atoms M in the clusters. The ratio, S/M, will in general depend on the value of M. If we make the simplifying assumption that the clusters are present as cubes and that the atoms are arranged in the clusters in the same manner as in a large metal crystal, we can calculate the relation between S/M and M for a catalytically important metal such as platinum or nickel (Figure 3). The value of S/M, which is commonly called the metal dispersion, increases as M decreases, approaching unity when M becomes sufficiently small. Metal dispersions close to unity are commonly realized in precious-metal catalysts used for the production of antiknock components for gasoline. Such high dispersions imply that the metal clusters are extremely small, of the order
OCR for page 182
Advancing Materials Research of 10 angstroms. Alternatively, metal dispersions equal to unity would be obtained if the metal clusters were raftlike or platelike structures consisting of single layers of metal atoms on the carrier. This alternative would require a strong interaction between the metal and the carrier to impart stability to clusters with such shapes. Chemisorption Measurements of Metal Dispersion An experimental estimate of S/M can be made from a measurement of the amount of gas chemisorbed on the metal clusters. The chemisorption must be selective, readily saturating the surfaces of the metal clusters with a monolayer but not occurring on the metal-free surface of the carrier. The chemisorption of a gas such as hydrogen, carbon monoxide, or oxygen at room temperature has been used effectively for this purpose. Use of the method requires knowledge of the stoichiometry of the chemisorption process, that is, the number of molecules chemisorbed per surface metal atom. The ratio of the number of chemisorbed molecules to the total number of metal atoms present in the catalyst, coupled with knowledge of the chemisorption stoichiometry, makes it possible to determine the metal dispersion S/M. In hydrogen chemisorption on the Group VIII metals, it is generally accepted that the hydrogen molecule dissociates, so that hydrogen atoms are adsorbed on the surface. Typical data on the chemisorption of hydrogen at room temperature on a platinum-on-alumina catalyst11 are shown in Figure 4. The isotherm labeled A is the original isotherm determined on the “bare” catalyst surface. The bare surface was prepared by evacuation of the adsorption cell at high temperature (725 K) subsequent to the reduction of the catalyst in flowing hydrogen at 775 K. The catalyst was cooled to room temperature in a vacuum, hydrogen was passed over it, and isotherm A was measured. The adsorption cell was again evacuated at room temperature for 10 min (to approximately 10–6 torr), and a second isotherm, labeled B, was measured. Isotherm A represents the total chemisorption, and isotherm B represents the weakly chemisorbed fraction, since it is removed by simple evacuation at room temperature. Isotherm B includes adsorption on the alumina carrier. The difference isotherm, labeled A-B, is obtained by subtracting isotherm B from isotherm A and is independent of pressure over the range of pressures used in obtaining the isotherm. It represents the strongly chemisorbed fraction, that is, the amount that cannot be removed by evacuation at room temperature. The quantity H/M in the right-hand ordinate of Figure 4 represents the ratio of the number H of hydrogen atoms adsorbed to the number M of platinum atoms in the catalyst. If we assume a stoichiometry of one hydrogen atom per surface platinum atom in the case of the strongly chemisorbed fraction, the value of H/M determined from the dif-
OCR for page 183
Advancing Materials Research FIGURE 4 Typical hydrogen chemisorption at room temperature on a platinum-on-alumina catalyst containing 1 wt % platinum. Isotherm A is the original isotherm, and isotherm B is a second isotherm determined after evacuation of the adsorption cell for 10 min to a pressure of approximately 10–6 torr subsequent to the completion of isotherm A. The difference isotherm A-B, obtained by subtracting isotherm B from isotherm A, represents the strongly chemisorbed fraction. The quantity H/M is the ratio of the number of hydrogen atoms absorbed to the number of platinum atoms in the catalyst. From Via, Sinfelt, and Lytle.11 ference isotherm A-B corresponds to the ratio of surface platinum atoms to total platinum atoms in the catalyst. This ratio is about 0.9 for the catalyst in Figure 4. High-resolution electron microscopy provides independent evidence of the highly dispersed nature of platinum in platinum-alumina reforming catalysts.12 Such studies have shown that the platinum exists as very small crystallites or clusters of the order of 10 angstroms in diameter. Platinum clusters of this size necessarily have a large proportion of their atoms present in the surface. The fraction would be very close to the value of 0.9 derived from the chemisorption data in Figure 4.
OCR for page 184
Advancing Materials Research The successful application of chemisorption methods in the characterization of platinum-on-alumina reforming catalysts led to their use with other supported metals, including most of the Group VIII metals.10 This represents a major advance in the characterization of supported metal catalysts. With this capability, the activity of such a catalyst can be referred to the amount of metal in the surface rather than to the metal content of the catalyst as a whole. Data on the activities of different metal catalysts for a given reaction can therefore be compared in a more fundamental manner. Characterization of Metal Catalysts by X-Ray Absorption Spectroscopy Extended x-ray absorption fine structure (EXAFS) refers to the fluctuations in absorption coefficient commonly observed on the high-energy side of an x-ray absorption edge. The fluctuations of interest in EXAFS begin at approximately 30 eV beyond an absorption edge and extend over an additional range of 1,000–1,500 eV. The fine structure is observed in the absorption of x-rays by all forms of matter other than monatomic gases and was first considered theoretically by Kronig.13–15 The possibilities of EXAFS as a tool for investigating the structures of noncrystalline materials, however, have been realized only recently. They have emerged as a result of advances in methods of data analysis16,17 and experimental techniques, the latter primarily in the application of high-intensity synchrotron radiation as an x-ray source.18 EXAFS is concerned with ejection of an inner-core electron from an atom as a result of x-ray absorption. The ejected electron (photoelectron) is characterized by a wave vector K, which is proportional to the square root of its kinetic energy. The kinetic energy of the photoelectron is the difference between the energy of the x-ray photons and a threshold energy associated with the ejection of the electron. A typical spectrum19 for bulk platinum at 100 K is shown in Figure 5. The data cover a wide enough range of energy to include all three of the characteristic L absorption edges, LIII, LII, and LI, corresponding, respectively, to ejection of photoelectrons from 2p3/2, 2p1/2, and 2s states. At energies higher than the threshold value corresponding to a particular absorption edge, fluctuations occur in the absorption coefficient, which constitute the extended fine structure. In the treatment of EXAFS data, the absorption coefficient in the region of the EXAFS is divided into two parts. One part is identical to the absorption coefficient for the free atom. The other part, which depends on the environment around the absorber atom, is the oscillating part constituting EXAFS. Division of the latter part by the former normalizes the EXAFS oscillations. The normalized oscillations are represented by the EXAFS function X(K). Details concerning the determination of X(K) from experimental EXAFS data are given in the literature.11,17
OCR for page 185
Advancing Materials Research FIGURE 5 X-ray absorption spectrum of bulk platinum at 100 K in the region of the L-absorption edges. LIII, LII, and LI correspond, respectively, to ejection of photoelectrons from 2p3/2, 2p1/2, and 2s states. From Sinfelt, Via, and Lytle.19 Plots of the function K3·X(K) as a function of K are shown in the left-hand sections of Figure 6 for bulk platinum and for two platinum catalysts containing 1 weight percent platinum.11 In one catalyst the platinum was dispersed on silica, and in the other it was dispersed on alumina. Chemisorption measurements on the catalysts indicated platinum dispersions in the range of 0.7 to 0.9. The data in Figure 6, which were obtained at a temperature of 100 K, are for EXAFS associated with the LIII absorption edge. Fourier transforms of K3·X(K) are shown in the right-hand sections of the figure. The Fourier transform yields a function ϕ(R), where R is the distance from the absorber atom.20 Peaks corresponding to neighboring atoms are displaced
OCR for page 186
Advancing Materials Research FIGURE 6 Normalized EXAFS data at 100 K and associated Fourier transforms for bulk platinum and for dispersed platinum catalysts containing 1 wt % platinum. From Via, Sinfelt, and Lytle.11 from true interatomic distances because of phase shifts. The feature in the transforms for the catalysts near R=0 is an artifact introduced by asymmetry in the EXAFS function, which in turn is due to a limitation in ability to extract the background absorption from the total absorption. Improvements in the characterization of background absorption have largely eliminated this artifact in more recent work. The EXAFS fluctuations for the dispersed platinum catalysts are substantially smaller than those for bulk platinum. Correspondingly, the magnitudes of the peaks in the Fourier transforms are also smaller (note that the scales
OCR for page 187
Advancing Materials Research in the figures are not the same for the dispersed platinum catalysts and bulk platinum). These features are a consequence of a lower average coordination number and a higher degree of disorder of the platinum atoms in the dispersed catalysts. The degree of disorder is characterized by a parameter a, which is the root mean square deviation of the interatomic distance from the equilibrium value. From the EXAFS data in Figure 6, values of average coordination number, interatomic distance, and disorder parameter σ can be obtained for the platinum clusters in the catalysts.11 The average number of nearest-neighbor atoms around a platinum atom in a cluster is 7 for the Pt/Al2O3 catalyst and 8 for the Pt/SiO2 catalyst. The values are significantly lower than the value of 12 for bulk platinum. This result is expected, since most of the platinum atoms in the clusters are surface atoms with lower coordination numbers than the atoms in the interior of a crystal. Also, atoms at corners and edges have lower coordination numbers than the interior atoms in surface planes of crystals and become increasingly important as the size of a metal crystal decreases. Nearest-neighbor interatomic distances in the platinum clusters differ from the value for bulk platinum by less than 0.02 angstrom, which is within the estimated uncertainty in the determination of distances. Although differences in distances are small, the value of the disorder parameter σ for the platinum clusters is greater by a factor of 1.4 to 1.7 than the value for bulk platinum. Application of Nuclear Magnetic Resonance to Metal Catalysts In recent years the author has been collaborating with Professor Charles Slichter and his students at the University of Illinois in the application of nuclear magnetic resonance (NMR) for the characterization of platinum catalysts and molecules chemisorbed on the catalysts. Following is a brief discussion of some experimental results on 195Pt NMR line shapes for a series of air-exposed platinum-on-alumina catalysts of widely different platinum dispersions.21 The results were obtained using the spin echo technique.22 Data are shown in Figure 7 for catalysts in which the percentage of surface atoms in the platinum clusters or crystallites—that is, the platinum dispersion-varies by an order of magnitude from 4 to 58. In the figure, each catalyst has a designation Pt-X-R, in which X is the platinum dispersion and “R” signifies “as received,” that is, air exposed. The values of platinum dispersion were determined from hydrogen chemisorption isotherms. In Figure 7 the ordinate is the NMR absorption, and the abscissa is the ratio of the static field to the characteristic NMR frequency.21 The NMR lines are broad, in marked contrast with the narrow NMR lines observed with liquids. For the catalysts with low platinum dispersion (4 to 11 percent), there is a pronounced peak at H0/v0=1.138 kG/MHz. The resonance for bulk
OCR for page 192
Advancing Materials Research FIGURE 9 Effect of Cu composition on catalytic activity of Ni during the hydrogenolysis of ethane to methane and the dehydrogenation of cyclohexane to benzene.8,9,24 The activities are reaction rates at 589 K. or not it forms solid solutions with the latter in the bulk. The effect of the copper is associated with its presence in, or on top of, the surface layer of the active Group VIII metal. Selective inhibition of the hydrogenolysis activity of a Group VIII metal has also been observed when gold or silver is substituted for copper. In general, it has been observed that a Group IB metal suppresses the hydrogenolysis activity of a Group VIII metal and improves its selectivity for catalyzing such reactions as the dehydrogenation and isomerization of hydrocarbons. In accounting for the differences between hydro-
OCR for page 193
Advancing Materials Research genolysis and these other reactions, it has frequently been suggested that hydrogenolysis requires surface sites consisting of arrays of active metal atoms that are larger than the arrays required for the other reactions. The availability of large arrays of active metal atoms relative to small arrays decreases sharply when an inactive component such as copper is dispersed in, or on top of, the surface layer of the active metal. The possible role of an electronic interaction between the Group VIII and Group IB metal has also been considered. Bimetallic Clusters For industrial application of bimetallic catalysts, high metal surface areas are desirable. Highly dispersed bimetallic clusters can be prepared by impregnating a carrier with an aqueous solution of salts of the two metals of interest. The material is dried and then brought in contact with a stream of H2 at elevated temperature to reduce the metal salts. This procedure results in the formation of bimetallic clusters even where individual metal components exhibit very low miscibility in the bulk.23,32,33 Examples of such metal clusters that have been investigated are ruthenium-copper and osmium-copper supported on silica, in which the metal clusters cover about 1 percent of the surface of the silica. Size of the clusters ranges from about 10 to 30 angstroms in these systems. As copper is incorporated with ruthenium or osmium in bimetallic clusters, the selectivity for conversion of cyclohexane to benzene is improved greatly (Figure 10); hydrogenolysis to alkanes is inhibited markedly, whereas dehydrogenation to benzene is relatively unaffected.5,33 The behavior is similar to that described for unsupported Ru-Cu aggregates and therefore provides clear evidence for the interaction between Cu and the Group VIII metal on the carrier. As in the case of the unsupported materials, the copper in the bimetallic clusters is present at the surface. When the initial research on bimetallic clusters such as ruthenium-copper and osmium-copper was conducted, the characterization of the clusters was limited to methods involving chemical probes because of the difficulty of obtaining information with physical probes. However, the situation changed markedly when it became evident that x-ray absorption spectroscopy was effective for investigating the structures of catalysts. Results of EXAFS studies on Ru-Cu and Os-Cu bimetallic clusters have provided strong evidence in support of the conclusions about structure derived from the studies with chemical probes. The quantitative analysis of EXAFS data on bimetallic cluster catalysts has been limited to consideration of contributions of nearest-neighbor atoms to EXAFS.34–39 In Figure 11, the EXAFS fluctuations represented by the solid line in all three fields of the figure are due to contributions from nearest-
OCR for page 194
Advancing Materials Research FIGURE 10 Selectivity of conversion of cyclohexane over silica-supported bimetallic clusters of ruthenium-copper and osmium-copper at 589 K, as represented by the ratio D/H,5,33 where D is the rate of dehydrogenation of cyclohexane to benzene, and H is the rate of hydrogenolysis to alkanes. neighbor backscattering atoms for a silica-supported osmium-copper catalyst.35 The solid line was derived experimentally by inverting a Fourier transform of EXAFS data associated with the LIII absorption edge of osmium over a range of distances chosen to include only backscattering contributions from nearest-neighbor atoms. The points in the upper field (labeled A) of the figure represent values of an EXAFS function calculated using parameters for the osmium-copper clusters obtained from the data by an iterative least-squares fitting procedure. They provide a good illustration of the quality of fit achieved in the analysis. In the lower two fields of Figure 11, the points represent the separate contributions of nearest-neighbor copper and osmium backscattering atoms (fields B and C, respectively) to the osmium EXAFS for the osmium-copper catalyst. In addition to the information that can be obtained from the EXAFS associated with an x-ray absorption edge, valuable information can be obtained from an analysis of the structure of the edge itself. From a study of LIII or LII absorption threshold resonances, one can obtain information on
OCR for page 195
Advancing Materials Research FIGURE 11 Contributions of nearest-neighbor copper and osmium backscattering atoms (points in fields B and C, respectively) to the EXAFS associated with the osmium LIII absorption edge of a silica-supported osmium-copper catalyst containing 2 wt % Os and 0.66 wt % Cu. (The points in field A show how the individual contributions combine to describe the experimental EXAFS represented by the solid line.) From Sinfelt et al.35
OCR for page 196
Advancing Materials Research FIGURE 12 The effect of copper on the threshold x-ray absorption resonance associated with the LIII absorption edge of the osmium in a silica-supported catalyst. Upper left section compares the resonance for a silica-supported osmium catalyst containing 1 wt % osmium with that for pure metallic osmium, the extent of increase indicated in the lower left. A similar comparison is made on the right between osmium-copper clusters and pure metallic osmium. From Sinfelt et al.35 electronic transitions from a core level, 2p3/2 or 2/p1/2, respectively, to vacant d states of the absorbing atom.40,41 The electronic transitions are sensitive to the chemical environment of the absorbing atom.42 In the case of silica-supported osmium-copper catalysts, the magnitude of the absorption threshold resonance associated with the osmium atom is decreased by the presence of the copper. This effect is illustrated in Figure 12 for the LIII absorption edge of osmium.35 The absorption coefficient µN is a normalized coeffi-
OCR for page 197
Advancing Materials Research cient.42 In the upper half of the figure, the left-hand section compares the resonance for a silica-supported osmium catalyst containing 1 weight percent osmium with that for pure metallic osmium. The magnitude of the resonance is higher for the osmium clusters dispersed on the support, the extent of increase being indicated by the difference spectrum in the lower left-hand section of the figure. This effect is similar to that observed for iridium and platinum dispersed on alumina.42 In the upper right-hand section of Figure 12, the magnitude of the resonance for silica-supported osmium-copper clusters is again higher than that for pure metallic osmium, the extent of increase being indicated again by the difference spectrum in the lower right-hand section of the figure. However, the increase in this case is about 30 percent lower than is observed when the supported osmium alone is compared with pure metallic osmium—that is, the area under the difference spectral line in the lower right-hand section is about 30 percent lower than the area under the corresponding spectral line in the lower left-hand section of the figure. In the case of the catalyst containing osmium alone on silica, the osmium clusters behave as if they are more electron deficient than pure metallic osmium, that is, there appear to be more unfilled d states to accommodate the electron transitions from the 2p3/2 core level of the absorbing atom. In the silica-supported osmium-copper clusters, however, the osmium atoms appear to be less electron deficient than they are in the pure osmium clusters dispersed on silica. The presence of the copper thus appears to decrease the number of unfilled d states associated with the osmium atoms. Up to this point the discussion of bimetallic clusters has been concerned with combinations of a Group VIII metal and a Group IB metal. Another type of bimetallic cluster of interest is a combination of atoms of two Group VIII metals, for example, platinum-iridium.43–45 Dispersed platinum-iridium clusters can be prepared by bringing a carrier such as silica or alumina into contact with an aqueous solution of chloroplatinic and chloroiridic acids. After the impregnated carrier is dried and possibly heated to 525–575 K, it is exposed to flowing hydrogen at a temperature of 575–775 K. The resulting material contains platinum-iridium clusters dispersed on the carrier. An x-ray absorption spectrum at 100 K showing the L absorption edges of iridium and platinum36 is given in Figure 13 for a catalyst containing bimetallic clusters of platinum and iridium. The data were obtained over a wide enough range of energies of the x-ray photons to include all of the L absorption edges of iridium and platinum. Since the extended fine structure associated with the LIII edge of iridium is observable to energies of 1,200– 1,300 eV beyond the edge, there is overlap of the EXAFS associated with the LIII edges of iridium and platinum in the case of a catalyst containing both of these elements. Separating the iridium EXAFS from the platinum EXAFS in the region of overlap is therefore necessary in the analysis of the data.36 Briefly, the results of the analysis on interatomic distances indicate
OCR for page 198
Advancing Materials Research FIGURE 13 X-ray absorption spectrum at 100 K in the region of the L absorption edges of iridium and platinum for a catalyst containing platinum-iridium clusters. From Sinfelt, Via, and Lytle.36 that the average composition of the first coordination shell of atoms (nearest neighbors) surrounding a platinum atom is different from that surrounding an iridium atom. The catalyst appears to exhibit platinum-rich and iridium-rich regions. One might visualize a distribution of metal clusters with different compositions, some of which are platinum-rich and others of which are iridium-rich. Both the platinum-rich and iridium-rich clusters would contain substantial amounts of the minor component on the basis of the distances derived from the EXAFS data. Alternatively, one can visualize platinum-rich and iridium-rich regions within a given metal cluster. This possibility seems reasonable on the basis of surface energy considerations. According to this view, the platinum-rich region would be present at the surface, since platinum would be expected to have a lower surface energy than iridium. In support of this expectation, recent work on platinum-iridium films indicates that platinum concentrates in the surface.46 When the ratio of surface atoms to total atoms is equal to 0.5 for clusters containing 50 percent each of platinum and iridium, one can visualize a situation in which essentially all of the platinum is present in the surface and all the iridium in the interior. There would then be a close resemblance to the ruthenium-copper clusters considered earlier. When the ratio of surface atoms to total atoms approaches unity, the notion of complete or nearly
OCR for page 199
Advancing Materials Research complete segregation of the platinum in a surface layer and of iridium in a central core cannot be accommodated if the clusters are spherically symmetrical. The notion can, however, be accommodated without difficulty if the clusters have a two-dimensional, raftlike shape rather than a spherical shape. One can then visualize a central iridium or iridium-rich raft with platinum atoms around the perimeter. In highly dispersed catalysts of the type visualized here, the effect of the platinum on the catalytic properties of the iridium, and vice versa, would presumably be a consequence of the interaction between the two components at the boundary. Catalysts containing platinum-iridium clusters dispersed on alumina are of interest in the reforming of petroleum fractions for production of high-octane-number gasoline components. They are more active and exhibit much better activity maintenance than the platinum-alumina catalysts originally used in reforming.23,43 In parallel with the development of platinum-iridium catalysts in the Exxon laboratories, another reforming catalyst containing platinum and rhenium (a Group VIIA metal) was under development in the laboratories of the Chevron Corporation. During the 1970s, platinum-iridium and platinum-rhenium catalysts were introduced widely in catalytic reformers. The use of these catalysts was a key factor in making unleaded gasoline feasible. OTHER CATALYTIC MATERIALS Metals are key components of catalysts for a number of well-known and important chemical processes. The same can also be said for various nonmetallic materials. A particularly impressive example is the class of materials known as aluminosilicates, which are widely used in the catalytic cracking of petroleum fractions. In the cracking process, large hydrocarbon molecules are converted into smaller molecules. The resulting hydrocarbons provide components for products such as gasoline and heating oil. The cracking activity of aluminosilicates is due to the presence of acidic sites in the surface.47 The existence of these sites is readily demonstrated by the affinity of basic molecules such as ammonia, pyridine, or quinoline for the surface of aluminosilicates. In typical cracking catalysts, the structure may be viewed as consisting of tetrahedrally coordinated silicon and aluminum atoms linked through the sharing of oxygen atoms at the corners of SiO4 and AlO4 tetrahedra. For aluminum-containing tetrahedra bonded at all four corners to silicon atoms in tetrahedral coordination, there is an excess of one unit of negative charge. This arises because a trivalent aluminum atom has been substituted for tetravalent silicon in a silica structure. Consequently, there must be present a compensating positive charge to provide electroneutrality. A proton co-
OCR for page 200
Advancing Materials Research ordinated to the structure satisfies this requirement and is strongly acidic in its behavior. The first aluminosilicates used as cracking catalysts were amorphous; that is, the primary crystallites of the materials were too small to be observed by the usual x-ray diffraction procedures for obtaining structural information. They included naturally occurring clays, which were acid treated, and synthetic aluminosilicates. Such materials were used as cracking catalysts for almost three decades, beginning in 1936 with the first commercial application of the Houdry catalytic cracking process.48 In the early 1960s crystalline aluminosilicates (zeolites) were introduced in catalytic cracking. The primary structural units of these materials are still SiO4 and AlO4 tetrahedra, but larger secondary units are now clearly distinguishable. The secondary units are composed of the primary tetrahedra, and exist in the form of regular polyhedra, rings, or chains. Various types of zeolite structures are obtained by linking these secondary units together in different ways. In the zeolites commonly used in catalytic cracking, which are known as Y-zeolites, there are internal cavities of uniform size. The cavities are interconnected through well-defined openings or windows, giving rise to an intracrystalline pore structure. The diameters of the internal cavities and interconnecting windows are approximately 12 and 8 angstroms, respectively. Catalysis occurs within the intracrystalline pore structure. Crystalline aluminosilicates have been responsible for dramatic improvements in the cracking process. The amount of gasoline produced from a gas-oil fraction is much higher than can be obtained with amorphous aluminosilicate catalysts.49 The octane number of the gasoline is also higher, because of higher contents of aromatics and isoparaffins. The much higher activities of the zeolite cracking catalysts relative to the amorphous aluminosilicates50 stimulated investigations into the acidities of these new materials. Results obtained by a variety of methods indicated that the density of Brönsted acid sites was much higher for the Y-type zeolites used in cracking than for amorphous aluminosilicates.51 The higher selectivity to aromatics and isoparaffins indicates that hydrogen transfer reactions occur more readily with zeolites as catalysts.52 The excellent performance of zeolites in catalytic cracking has intensified research on these materials. The research has led to new forms of zeolites and to new catalytic applications.49,53,54 Other examples of oxide catalysts of great industrial importance are the bismuth molybdate systems used in the production of acrylonitrile and the cobalt molybdate catalysts used in the desulfurization of petroleum fractions.55 The former generally contain additional components such as K2O and P2O5 and use silica as a support. The cobalt molybdate catalysts are sulfided in actual operation and contain alumina as a supporting material. These examples constitute only a small fraction of the oxide materials that have found application in catalysis.
OCR for page 201
Advancing Materials Research Another group of materials with potential interest as catalysts includes various carbides, nitrides, and borides of the transition metals.56 Such materials have the beneficial feature of high thermal stability and at the same time offer the possibility of having catalytic properties of a kind normally associated with precious metals.57,58 The further exploration of these materials in catalysis would appear to be worthwhile. An important aspect of research in this area is the development of methods for preparation of materials with high surface areas. CONCLUSIONS Catalysts are fascinating materials, embracing many different chemical compositions. The elucidation of their structures and surface properties provides exciting challenges for scientists. The development of new methods for probing catalytic materials has greatly extended our capabilities for investigation at a microscopic level. Such methods provide information to complement that obtained from more traditional studies of chemisorption, kinetics, and reaction mechanisms. The synthesis of new materials for application as catalysts is a continuing activity, with a great deal of opportunity for the future. On a long-term basis, the outlook for the field of catalysis is excellent. NOTES 1. J.H.Sinfelt, Perkin Medal address in Chem. Ind., No. 11, pp. 403–406 (June 4, 1984). 2. G.Bylinsky, Fortune, pp. 82–88 (May 27, 1985). 3. J.H.Sinfelt, Science 195, 641 (1977). 4. J.H.Sinfelt, Sci. Am. 253 (3), 90 (1985). 5. J.H.Sinfelt, Rev. Mod. Phys. 51 (3), 569 (1979). 6. M.Boudart, Kinetics of Chemical Processes (Prentice-Hall, Englewood Cliffs, N.J., 1968), p. 61. 7. H.Voge and C.R.Adams, Adv. Catal. 17, 151 (1967). 8. J.H.Sinfelt, Adv. Catal. 23, 91 (1973). 9. J.H.Sinfelt, Catal. Rev. Sci. Eng. 9 (1), 147 (1974). 10. J.H.Sinfelt, Prog. Solid State Chem. 10 (2), 55 (1975). 11. G.H.Via, J.H.Sinfelt, and F.W.Lytle, J.Chem. Phys. 71, 690 (1979). 12. G.R.Wilson and W.K.Hall, J. Catal. 17, 190 (1970). 13. R.de L.Kronig, Z. Phys. 70, 317 (1931). 14. R.de L.Kronig, Z. Phys. 75, 191 (1932). 15. R.de L.Kronig, Z. Phys. 75, 468 (1932). 16. D.E.Sayers, F.W.Lytle, and E.A.Stern, Phys. Rev. Lett. 27, 1204 (1971). 17. F.W.Lytle, D.Sayers, and E.Stern, Phys. Rev. B 11, 4825 (1975). 18. B.M.Kincaid and P.Eisenberger, Phys. Rev. Lett. 34, 1361 (1975). 19. J.H.Sinfelt, G.H.Via, and F.W.Lytle, Catal. Rev. Sci. Eng. 26 (1), 81 (1984). 20. D.E.Sayers, F.W.Lytle, and E.A.Stern, Adv. X-Ray Anal. 13, 248 (1970).
OCR for page 202
Advancing Materials Research 21. H.E.Rhodes, P.-K.Wang, H.T.Stokes, C.P.Slichter, and J.H.Sinfelt, Phys. Rev. B 26, 3559 (1982). 22. C.P.Slichter, Principles of Magnetic Resonance, 2nd ed. (Springer, New York, 1980). 23. J.H.Sinfelt, Bimetallic Catalysts: Discoveries, Concepts, and Applications (Wiley, New York, 1983). 24. J.H.Sinfelt, J.L.Carter, and D.J.C.Yates, J. Catal. 24, 283 (1972). 25. P.van der Plank and W.M.H.Sachtler, J. Catal. 7, 300 (1967). 26. D.A.Cadenhead and N.J.Wagner, J. Phys. Chem. 72, 2775 (1968). 27. C.R.Helms, J.Catal. 36, 114 (1975). 28. F.L.Williams and M.Boudart, J. Catal. 30, 438 (1973). 29. J.H.Sinfelt, A.E.Barnett, and G.W.Dembinski, U.S. Patent 3 442 973 (1969). 30. J.H.Sinfelt, A.E.Barnett, and J.L.Carter, U.S. Patent 3 617 518 (1971). 31. J.H.Sinfelt, Y.L.Lam, J.A.Cusumano, and A.E.Barnett, J. Catal. 42, 227 (1976). 32. J.H.Sinfelt, Acc. Chem. Res. 10, 15 (1977). 33. J.H.Sinfelt, J. Catal. 29, 308 (1973). 34. J.H.Sinfelt, G.H.Via, and F.W.Lytle, J. Chem. Phys. 72, 4832 (1980). 35. J.H.Sinfelt, G.H.Via, F.W.Lytle, and R.B.Greegor, J. Chem. Phys. 75, 5527 (1981). 36. J.H.Sinfelt, G.H.Via, and F.W.Lytle, J. Chem. Phys. 76, 2779 (1982). 37. G.Meitzner, G.H.Via, F.W.Lytle, and J.H.Sinfelt, J. Chem. Phys. 78, 882 (1983). 38. G.Meitzner, G.H.Via, F.W.Lytle, and J.H.Sinfelt, J. Chem. Phys. 78, 2533 (1983). 39. G.Meitzner, G.H.Via, F.W.Lytle, and J.H.Sinfelt, J. Chem. Phys. 83, 353 (1985). 40. N.F.Mott, Proc. Phys. Soc. London 62, 416 (1949). 41. Y.Cauchois and N.F.Mott, Philos. Mag. 40, 1260 (1949). 42. F.W.Lytle, P.S.P.Wei, R.B.Greegor, G.H.Via, and J.H.Sinfelt, J. Chem. Phys. 70, 4849 (1979). 43. J.H.Sinfelt, U.S. Patent 3 953 368 (1976). 44. J.H.Sinfelt and G.H.Via, J. Catal. 56, 1 (1979). 45. R.L.Garten and J.H.Sinfelt, J. Catal. 62, 127 (1980). 46. F.J.Kuijers and V.Ponec, Appl. Surf. Sci. 2, 43 (1978). 47. G.A.Mills, E.R.Boedeker, and A.G.Oblad, J. Am. Chem. Soc. 72, 1554 (1950). 48. E.Houdry, W.F.Burt, A.E.Pew, Jr., and W.A.Peters, Petroleum Refiner, 17 (11), 574 (1938). 49. H.Heinemann, Catal. Rev. Sci. Eng. 23 (1 & 2), 315 (1981). 50. J.N.Miale, N.Y.Chen, and P.B.Weisz, J. Catal. 6, 278 (1966). 51. B.C.Gates, J.R.Katzer, and G.C.A.Schuit, Chemistry of Catalytic Processes (McGraw-Hill, New York, 1979), p. 80. 52. P.B.Weisz, Chem. Technol. 498 (1973). 53. G.T.Kerr, Catal. Rev. Sci. Eng. 23 (1 & 2), 281 (1981). 54. J.A.Rabo, Catal. Rev. Sci. Eng. 23 (1 & 2), 293 (1981). 55. Note 51, pp. 366–379, 411–422. 56. R.B.Levy, in Advanced Materials in Catalysis, edited by J.J.Burton and R.L.Garten (Academic Press, New York, 1977), pp. 101–127. 57. J.M.Muller and F.G.Gault, Bull. Soc. Chim. Fr. 2, 416 (1970). 58. R.B.Levy and M.Boudart, Science 181, 547 (1973).
Representative terms from entire chapter: