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Langmuir 1986, 2, 2-11
The Larzgmuir Lectures Supported Platinum, Palladium, and Rhodium Catalystst Robert L. Burwell, Jr. Zpatieff Laboratory, Department of Chemistry, Northwestern University, Euanston, Illinois 60201 Received October 28, 1985 This paper surveys the results of an investigation of the hydrogenolysis of methylcyclopentane, the hydrogenation of propylene, and isotopic exchange between deuterium and 2,2-dimethylbutane and cyclopentane on Pt/SiO,, Pt/A1203,Pd/Si02, and Rh/Si02 catalysts. It also summarizes the results of the characterization of the catalysts by X-ray line profile analysis and EXAFS, by chemisorption techniques, and by study of the removal of oxide from the metal particles of the catalysts by hydrogen. Pretreatment conditions are as important as particle size in determining the catalytic properties of these materials. The effects of differing conditions of pretreatment probably result largely from influence upon surface morphology, but exposure to hydrogen and cooling in hydrogen from higher temperatures seem to result in special effects. The metal particles do not appear in general to be bound by smooth (111) and (100) planes.
Introduction Supported-metal catalysts are among the most widely used catalysts both in the laboratory and in industry. Those of platinum, palladium, and rhodium lead to the hydrogenation of a wide variety of C-C, C-N, and C-0 multiple bonds such as C=C, C=C, C=C=C, C=CC=C, and those in benzene, ketones, and nitriles. They catalyze the hydrogenolyses of cyclopropanes and alkanes, the skeletal isomerization of alkanes, and the dehydrocyclization of heptane to toluene. They also catalyze the oxidation of carbon monoxide to carbon dioxide and of hydrocarbons to carbon dioxide and water, for example, in the catalytic muffler of the automobile. Figure 1 is a schematic representation of Pt, Pd, or Rh on silica or alumina, the two most common supports. The primary particles of silica or of alumina are roughly 15 nm in diameter and are agglomerated into a structure that has the texture of a cemented, loose gravel bed. Specific areas of the supports are usually in the range 100-500 m2 g-' and the average pore diameters 2.5-15 nm. The tiny particles of metal are distributed in the pore space of the support. During a catalytic reaction with such catalysts the reactants must diffuse into the catalyst granules and the concentration gradients must be small if the metal particles in the center of a granule are to be catalytically as effective as ones near the surface. Furthermore, temperature gradients must exist if heat is to flow in or out of the granules accordingly as the catalyzed reaction is endothermic or exothermic. If reactions are too rapid, serious mass and temperature gradients will appear. Why then does one want to use such a material rather than the metal particles without the support? First, even at room temperature, the clean metal particles as a powder would undergo some sintering with loss of area. Exposure to higher temperatures would aggravate the loss of metal particle area in the powder. However, in the supported catalyst, the location This paper surveys work in the Ipatieff Laboratory of Northwestern University on supported platinum, palladium, and rhodium catalysts. Most of the work to be described resulted from a collaborative investigation directed by J. B. Butt, Chemical Engineering, J. B. Cohen, Materials Science, and R. L. Burwell, Jr., Chemistry.
0743-7463/86/2402-0002$01.50/0
of the metal particles greatly inhibits their getting together and fusing with consequent loss in area even at temperatures of -500 OC or greater. Thus, carbonaceous deposits which accumulate on the catalyst during reactions involving hydrocarbons can be removed by oxidation. Further, metal can be used efficiently, since it is easy to make supported catalysts in which over half of the metal atoms are on the surfaces of the metal particles, i.e., particles smaller than 2 nm. Finally, if one used the metal powder in a fixed bed reactor (for example, a vertical tube containing a bed of catalyst through which the reactant gases flow), attainment of any reasonable flow rate through a bed of such tiny particles would result in an enormous pressure drop, whereas, in the case of catalyst granules of some millimeters in diameter, the pressure drop would be reasonable. In scientific work, one usually employs small catalyst granules in the range 60-120 mesh so as to reduce diffusion distances and one choses a temperature at which reaction rates are sufficiently slow that concentration and temperature gradients can be ignored. In flow reactors, the ratio of moles of reactants flowing over the catalyst per hour to moles of M, (where M, represents a surface metal atom) will ordinarily be very large, and, therefore, reactants and carrier gases must be so pure that the ratio (strong poisons per hour)/M, is negligible. For example, in catalytic runs like those to be described, one might use 0.2 g of a 1%Pt/support catalyst in which one-half of the platinum atoms are at the surface. Such a sample would contain only 5 pmol of Pt,, but typically 2 x lo5 pmol of hydrocarbon plus hydrogen would flow through the catalyst per hour. Elimination of poisons often entails rather exacting work, but if it is not undertaken, uninterpretable data result. For both scientific and practical reasons one would like to know how catalytic activities and selectivities depend upon the support and upon the size and morphology of the metal particles. Largely through the efforts of Boudart' the following classification of the effect of size upon catalytic activity has become widespreada2 If the rate per (1) Schlatter, J. C.; Boudart, M. J. Catal. 1972, 24, 482.
0 1986 American Chemical Society
Langmuir, Vol. 2, No. 1. 1986 3
The Langmuir Lectures
Figure I. Schematic representation of M/SiO: or M/AIZ03 catalysts.
unit metal area or per surface metal atom is independent of particle size, the reaction is said to be structure-insensitive on that catalyst. If the rate varies, the reaction is said to be structure-sensitive. A number of groups have investigated the question of structure sensitivity for various reactions on various supported-metal catalysts by examining the rate of a particular reaction on several samples of a particular M/support in which the average metal particle size is different. In such work, it has been generally assumed that the metal crystallites are bounded by perfect, densely packed planes, [lo01and I1111in the case of facecentered cubic crystals such as Pt, Pd, and Rh. On such an assumption, the ratio of M. atoms in edges and vertices to those in faces would steadily increase as the crystal size decreased; that is, the fraction of M, atoms that have lower coordination numbers would increase with decreasing metal particle size? The effect is shown by the two cubooctahedra in Figure 2. A t least at 0 K and for crystals with the number of atoms corresponding to a cubooctahedron, cubooctahedra are probably the thermodynamically favored crystal form. A reaction which was structure-sensitive in the sense that activity increased or decreased monotonically with metal particle size could then be explained in terms of edge atoms having an activity different from that of face atoms. However, a number of structure-insensitive reactions have been reported. The absence of an entirely satisfactory theory as to why M. atoms of such different coordination numbers as those at edges and faces should have nearly identical catalytic activities has constituted a serious gap in our general understanding of structure sensitivity. A t the time we started our work 10years ago,it appeared that the question of structure sensitivity was an important one and one that had not been completely worked out. Our plan was to prepare more extensive sets of M/support catalysts than had usually been employed in the past and ones of a wide range of average particle diameters, to characterize them as well as we could, and to examine as many different catalytic reactions on these catalysts as possible. We recognized that we would need help from other groups in the execution of this task. Catalyst Preparation In a number of previous sets of M/support catalysts, catalysts containing larger metal particles had been prepared by heating catalysts with smaller particles a t temperatures above 500 "C. However, in general, our catalysta were not exposed to temperatures greater than 450 OC during preparation. Thus, following pretreatments which terminated at 450 'C, the degree of dehydroxylationof the (2) Boudart. M.Pmc. Int. Congr. Cotol.. 6th. 1976 1971, 1. (3) (a) van Hardcvcld. R.: Harm. F. Sur/. Sri. lYf9, 15, 169. (b) Anderson. J. R. 'Structure of Metellie Catalvsta': Academic Press: London, 1975; Chapter 5.
Figure 2. 55 (a)and 562 atom (b)cubooctahedra. On the smaller cubooetahedron there are 42 M. atoms of which 24 are edge atoms and 12 are vertex atoms and on the larger cuktahedron there are 272 M,atoms of which 72 are edge atoms and 24 are vertex a t o m The numbers on the atoms are their coordination numbers. (b) is reprinted with permission from ref 3a. Copyright 1975, Academic Press.
support should have been the same for the entire set, unlike the situation for most previous sets of catalysts. All of the Pd/SiOp and some of the Pt/SiO, and Rh/SiO, catalysts were prepared by ion exchange' of the weakly acidic surface S O H groups with Pd(NH,),*+," Pt(NH,)pP and Rh(NH3)5(H20)3+ ions.' Half of the Pt/SiO, catalysts were prepared by impregnation of the catalyst pore space with aqueous H2PtCb8and many of the Rh/SiOp catalysts were prepared by impregnation with solutions of polynuclear rhodium carbonyls. The Pt/AI20, catalysts were prepared by impregnation with aqueous solutions of Pt(NH3)2(N02)2.9 We avoided the usual preparation, impregnation with aqueous H,PtCI,, which results in the introduction of Cl- into the alumina surface, a process that converts the alumina into a strong acid. The silica gel support was a wide pore one, Davison 62, which has an average pore diameter of 14 nm, and the alumina was Cyanamid PHF, a 99.99% pure y-alumina with an average pore diameter of 12 nm. The PHF alumina was particularly low in sulfate, a contaminant that leads to artifacts when Pt/AL2O3is exposed to hydrogen a t 450-500 OC.O ' Catalyst Characterization
We shall express rates of catalytic reactions as turnover frequencies, N,,molecules converted per surface metal
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J . Catol. 1962, 1. 336. (9) Kobsyashi. M.; h u e . Y.;Takahashi, N.; Bunuell, R. L..Jr.; Butt, J. 9.:Cohen, J. 9. J . Cotol. 1980.64, 74. (10) Kunimori, K.; Ikeds. Y.; Soma. M.; Uehijima. T. J. Catol. 1983, 79, 185. Kunimori, K.: Uehijima, T. J . Carol. 1985.92. 196. Mador, 1. L.; Rosa". A. M.; Crieney, R. K. J . Cotal. 1984.87, 276.
4 Langmuir, Vol. 2, No. 1, 1986
atom per second. Rates and moles of metal in a sample of catalyst can be determined in standard fashions, but one must also measure D , the percentage exposed, which :is the fraction of the total metal atoms which are on the surfaces of the metal particles. This done,
The Langmuir Lectures 27- SiO, -Ion 2
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Nt = (mol of hydrocarbon converted per s)/(mol of Pt,) = (mol converted per s ) / ( D X total mol of Pt) Measurement of Percentage Exposed by Chemisorption of Hydrogen. A standard method for determining D involves the assumption that the moles of M, is equal to one-half of the moles of H2 chemisorbed a t about 25 "C. Then, Dh = H/M = M,/M, where Dh designates D measured by hydrogen chemisorption. Two methods for measuring Dh have been used in literature, volumetric and pulse procedures. We have used a pulse method,6 but several groups have checked our Pt/Si02 catalysts by the volumetric method with good agreement.11r12 Determination of D by measurement O f Dh is a purely empirical concept. It is unlikely that there are conditions under which each Pt, atom holds one atom of H and not two or none. However, the evidence is good that the average H/Pt, is near unity under the standard measurement conditions as judged by comparison with D measured by absolute methods, X-ray and transmission electron microscopy (TEM) (for an early example, see ref 8). In the pulse method, a thermal conductivity cell is used to measure the adsorption of hydrogen from pulses of hydrogen injected into a flowing inert gas, in our case argon. Any oxygen in the argon will be adsorbed on the surface of the metal particles during pretreatment and then react with hydrogen to give a value of Dh that is too large. In our work we kept the oxygen content below 0.02 ppm, both here and in catalytic runs. Chemisorption of Hydrogen. The ion exchanged and impregnated catalysts after initial preparation were reduced in hydrogen a t T 5 450 "C and stored in air. Measurement of hydrogen chemisorption requires that the surfaces of the metal particles be devoid of both oxygen and hydrogen. Such clean metal particles were prepared by exposing catalysts to what we shall call the standard pretreatment, 02,3000,0.5,H2,3000,1;Ar,4500,1, cool in Ar. Here, O2,3OO0,0.5 indicates that the catalyst was heated in flowing oxygen for 0.5 h a t 300 "C. All pretreatments started with this step. A short argon purge separated this step from H2,300",1. The purpose of the oxygen treatment is to remove any carbonaceous contaminant which the metal may have acquiredl although we made considerable effort to avoid this eventuality. The same pretreatment preceded many of the catalytic runs, but we also studied pretreatments in which the catalyst was pretreated in flowing hydrogen to some temperature usually for 1h and then cooled in hydrogen to give a catalyst covered with adsorbed hydrogen. Such pretreatments are designated H2,Tact("C), for example, H2,45O0. We found that H2,4500 led to uptakes of hydrogen on clean Pt about 20% greater than those resulting from adsorption of hydrogen at 25 "C (see footnote 23 in ref 13). The hydrogen in excess of that adsorbed at 25 "C did not react with oxygen at 25 "C6This seems to be a phenomenon confined to supported catalysts since sorption of (11) Aika, K.; Ban, L. L.; Okura, I.; Namba, S.; Turkevich, J. J. Res. Inst. Catal., Hokkaido Univ. 1976, 24, 54. (12) Cant, N. J. Catal. 1980, 62, 173. (13) Wong, S. S.; Otero-Schipper, P. H.; Wachter, W. A.; Inoue, Y.; Kobayashi, M.; Butt, J. B.; Burwell, R. L., Jr.; Cohen, J. B. J . Catal. 1980, 64, 84.
0
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40
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Figure 3. Distribution of diameters in the (100) direction for 27-Pt/Si02-IonX. The y axis represents relative probabilities.
COLUMN LENGTH L IRNGSTROMI
Figure 4. Distribution of diameters in the (111)direction for 40-Pt/SiOpafter four sequential pretreatments crosses, standard; circles, H2,25'; triangles, H2,200'; crosses, H2,45O0. 02,3000,0.5 was the first step of all pretreatments.
hydrogen in bulk platinum at 450 "C is tiny.14 Others have reported a similar phenomenon with supported platinum.15J6 Perhaps the solubility of hydrogen in small particles is much larger than that in bulk platinum, or, as various authors have suggested, perhaps the hydrogen is located immediately subsurface or at the metal/support interface. Measurement of Metal Particle Size by X-ray Techniques. D was also measured by X-ray diffraction using Fourier line-shape analysis (D,).17 In this, the following information is extracted from the shape of an X-ray diffraction peak, say the (111) line: the lattice constant, distribution of particle diameters normal to the (111)planes and thence the area weighted average diameter in that direction ( L ) ,the strain density (something like the dislocation density), and the thermal amplitudes. With Pt/Si02, for example, four diffraction peaks could be analyzed, ( l l l ) , (110), (loo), and (311). The four values of (L)define the general shape of the crystals. With the exception of the Pt/Si02 of lowest Dh, 7 % , the metal crystallites in M/Si02 were equiaxed, that is all values of ( L ) were nearly the same. Thus, the crystallites could be roughly inscribed in a sphere. Further, the Pt crystals had zero strain density (again excepting Dh of 7 % ) and were perfect single crystals to within the precision of the measurements. The Fourier line-shape analysis is considerably (14) Ebisuzaki, Y.; Kass, W. J.; O'Keeffe, M. J . Chem. Phys. 1968,49, 3329. (15) Pail, Z.; Menon, P. G. Catal. Rev.-Sci. Eng. 1983, 25, 229. (16) Renouprez, A. J.; Tejero, J. M.; Candy, J. P. Proc. Int. Congr. Catal., 8th, 1984,3, 47. (17) Sashital, S. R.; Cohen, J. B.; Burwell, R. L., Jr.; Butt, J. B. J . Catal. 1977,50, 479.
Langmuir, Vol. 2, No. 1, 1986 5
The Langmuir Lectures .
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Figure 6. Diaoeiative adsorption of H, and O2 on the surface of a transition metal after Langmuir.
.
:. , ':I :'? .... Figure 5. Transmission electron micrograph of 63.5-Pt/SiOr lonX. The average particle diameter of the Pt is 1.9 nm. The bar is 10 nm long. Courtesy of L. L. Ban and J. Turkench. 'I
superior to the more common measurement of a peak width at half-height, both with respect to the determination of the average particle diameter and with respect to the amount of information obtained. However, with conventional X-ray sources, one is limited to measurement of particle diameters exceeding some value, in our case about 2.5 nm. Information on smaller particles was obtained by use of the much more intense synchroton source at CHESS (Comell University) but only with the most intense X-ray diffraction line, the (111).'8 As an example of the ordinary procedure, Figure 3 shows the distribution of particle diameters in the (100) direction in 27-Pt/SiOz-IonX," where 27 indicates the value of Dhand IonX indicates that the catalyst was made by ion exchange. A controlled atmosphere cell was used to measure (L) after various pretreatments. As shown in Figure 4 for 40-Pt/Si02, nearly the same value of D. resulted for Pt/SiO, stored in air clean Pt (after the standard pretreatment) and hydrogen-covered Pt (after H2,4500).'9 Unlike the case for Pt/SiOz, the 13.8- and 29.3-Pd/Si02 catalysts had significant strain density.m The X-ray data indicated that Rh/SiOz with Dh 5 37% were equiaxed. The morphology of much smaller particles of Rh on SiOz is controversial. Prestridge and YatesZ1 interpreted their TEM data as indicating that the particles were monolayers, but others have interpreted TEM data to indicate that the particles are spheroidalw van't Blik et a1.2' concluded from EXAFS (extended X-ray absorption fine StructureP examination that 15-20 atom clusters on AI,O, were not monolayers. Measurement of Metal Particle Size bv TEM. Ban and Turkevich examined our Pt/Si02 catalhts by transmission electron microscopy." One example, that of 63.5Pt/SiOTIonX containing 0.5% Pt, is shown in Figure 5. The calculated value of DTeMis 1.9 nm and the dis~~
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(18) Nandi. R K.;Cmrgopouloe. P.; Cohen. J. B.; Butt. J. B.: B m U . R. L., Jr.; Bildcrback. D. H. J. Cotol. 1Y82. 77.421. (19) Nandi. R. K.:Molinam. F.; Tanz. C.;Cohcn. J. B.: Butt. J. B.: B-d. R. L:. Jr. J..Cotol. 1982. 78, zS3. (20) Nandi. R. K.;Pitchai, R.:Wong, S.S.;Cohen. J. B.; Burwell, R. L., Jr.: Butt. J. B. J. Carol. 1981. 70, 298. (21) Prestridge. E. B.; Y a h . D. J. C. Nature (London) 1971,234,II. (22) Yaeamh, M. J.; Rome". D.: Fuentas, 5.; Dominguez, J. M.J.
Chim. Phys. Phys.-Chim. R i d . 1981, 78, 861. (23)Newcomb. S.B.; Stobbs. W. M.;Little, J. A. Phys. Status Solidi A 1983. 76, 191. (24) 'vtm't Blik. H.
tribution of particle diameters is narrow. In general, the agreement among Dh, D., and DmM (for Pt/SiOz) was good. Reaction of Metal Particles with Oxygen. Since before exposure to hydrogen in the reactor the catalysts had been oxidized during storage and during O2,30O0,the reaction of oxygen with the metals was examined. The surface oxygen acquired by clean Pt during 0z,250,0.25 corresponds to leas than a monolayer and, in the cases of Pt and Pd, it is removed by a few pulses of H, at ambient temperat~res,6~ but the removal is incomplete with Rh.' During storage, further Oz is adsorbed and oxygen removal becomes more difficult. On Pt/SiO, at 25 'C, the removal is slower for catalysts of large Dhthan for those of small Dh. O2,30O0leads to the sorption of still further oxygen by the stored catalyst and increased slowness in reaction with HP. However, in all cases oxygen is removed essentially completely by Hz,10Oo,1if it is assumed that H2,300' gives complete removal of oxygen. O/Pt could be measured by measuring the logs in H2consequent to trapping a pulse of H2 in the reactor and measuring the pulse released at 450 "C. Pd, Pt, and Rh particles with Dh > -75% were converted by Oz,30O0to materials with a composition near Pt30,, PdO, and Rh203. respectively. In general, a standard pretreatment could be repeated several times without change in Dh for materials of any of the D
