J. Phys. Chem. 1985, 89, 4261-4264
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An Examlnation of a Rh/MgO Catalyst Uslng X-ray Absorption Spectroscopy R. J. Emrich,s A. N. Mansour,*+$D. E. Sayers: S. T. McMillan, and J. R. Katzerl Center f o r Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 1971 1 , and Department of Physics, North Carolina State University, Raleigh, North Carolina 27650 (Received: December 27, 1983; In Final Form: May 24, 1985)
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Extended X-ray absorption fine structure (EXAFS) spectroscopy is used to structurally characterize a Rh/MgO catalyst prepared by using a nonaqueous ion-exchange method, as a function of hydrogen reduction conditions. After reduction at 473 and 673 K, EXAFS results show that the first coordination shell contains two types of interactions, viz., Rh-Rh and Rh-0. The Rh-Rh interaction has a bond length of 2.70 A and a coordination number which increased from 2.8 to 7.9 as the reduction temperature increased from 473 to 673 K. The Rh-0 interaction has a bond length of 1.95 A and a coordination number which decreased from 1.7 to 0.9 as the reduction temperature is increased from 473 to 673 K. Reduction of the catalyst at 673 K resulted in an increase in the average size of the rhodium crystallites from 6 to 16 A. The rhodium crystallites are interacting with the support through a Rh-0 type ligand.
Introduction The role of transition metals in catalysis has become one of great importance in the last half century. The largest application of transition metals to catalysis has been as supported metal catalysts. In supported metal catalysts the transition metal is dispersed on the surface of an “inert” carrier such as an inorganic oxide. The dispersion of the metal as small crystallites allows a high percentage of the metal atoms to reside on the surface of the crystallites where they can take an active role in the catalytic process. The rising costs of active transition metals such as Pt, Pd, and Rh has generated great interest in producing catalysts in which the dispersion approaches unity. In this state every transition-metal atom is on the surface available to catalyze reactions. Highly dispersed catalysts have shown some unusual catalytic behavior. In some reactions both the selectivity and activity of the catalyst have been found to be dependent upon the dispersion of the metal.’ Since varying the crystallite size changes the types of surface sites on the metal, it was first suggested that these changes in activity and selectivity with dispersion were geometric in nature. Recent work has shown it is also possible that the support might be responsible for the changes in selectivity and activity which occur with increasing dispersion.2 At high dispersions, a large fraction of the metal atoms in a supported metal catalyst are in contact with the support. Thus, it is possible that the changes in the activity and selectivity of a catalyst with crystallite size may be due to electronic as well as structural effects induced by the support. If there are no more than a few metal atoms in a cluster, the support surface can play the role of a ligand similar to that of the ligands in homogeneous catalysis. It should then be possible to control the selectivity and activity of a catalyst as is done with soluble homogeneous catalysts but without the problems of catalyst separation and recovery which are associated with soluble catalysts. Rhodium has shown some very unique catalytic behavior both as a homogeneous c a t a l y ~ t ~and - ~ as a supported Previous work7-l’ has shown that the selectivity and activity of supported rhodium catalysts in CO hydrogenation depends markedly on the nature of the support. For example, there is a 1Wfold increase in hydrogenation activity in going from Rh/Si02 to Rh/AI2O3.l0 Similarly, the selectivity to alcohol formation of this reaction also varied, increasing with the basicity of the support (e.g., Rh/MgO exhibits -90% selectivity to methanol at 473 K).” Such unique catalytic behavior is important to investigate because it can lead to a better understanding of the nature of North Carolina State University. ‘Current address: Code R-34, White Oak Laboratory, Naval Surface Weapons Center Detachment, 10901 New Hampshire Avenue, Silver Spring, Md 20903-5000. 4 Current address: BF Goodrich Co., Research and Development Center, 9921 Brecksville Road, Brecksville, OH 44141. Current address: Central Research Department, Mobil Research and Development Corp., Princeton, NJ 08540.
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metalsupport interactions. The effects of these interactions are especially significant in supported metal systems with high degrees of dispersion and with metals dispersed on supports with unique electronic or structural properties. Unfortunately, such systems are not amenable to investigation by many experimental methods. Extended X-ray absorption fine structure (EXAFS) spectroscopy can provide information about the atomic structure in the immediate vicinity of an absorbing element without the requirement of long-range structural order, and it has been used to provide unique structural information on highly dispersed supported We present here the results of the EXAFS studies of a highly dispersed Rh/MgO catalyst prepared by using a nonaqueous preparation method and reduced at two different temperatures in flowing H2 immediately preceding the X-ray absorption measurements. The objective of the study is to characterize any physical, chemical, or structural changes which may occur in the vicinity of the Rh atoms as a function of increasing reduction temperature. Experimental Section The Rh/MgO catalysts were prepared by using a modification of the so-called “ion-exchange” procedure. Typically, one takes an aqueous solution containing a salt of the metal of interest and adjusts the pH of the solution to the appropriate level before addition of the support. When the support is added the surface hydroxyl groups react with the metal ions. The difficulty of preparing highly dispersed metals on a basic oxide support such as MgO is related to hydrolysis of the support by the aqueous (1) Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. “Chemistry of Catalytic Processes”; McGraw-Hill: New York, 1979; Chapters 2 and 3. (2) Tauster, S.; Fung, S.; Garten, R. J . Am. Chem. SOC.1978, 100, 170. (3) Evans, D.; Osborn, J. A.; Wilkinson, G. J. Chem. SOC.A 1968,3133. (4) Brown, C. K.;Wilkinson, G. J . Chem. SOC.A 1970, 2753. ( 5 ) Roth, J. F.; Craddock, J. H.; Hershman, A,; Paulik, F. E. CHEMTECH 1971, 600. (6) Bhasin, M. M.; Bartley, W. J.; Ellgen, P. C.; Wilson, T. P. J . Cural. 1978, 54, 120. (7) Ichikawa, M. Bull. Chem. SOC.Jpn. 1978, 51, 2268. (8) Ichikawa, M. Bull. Chem. SOC.Jpn. 1978, 51, 2273. (9) Ichikawa, M.; Shikakura, K. “Proceedings of the 7th International Congress on Catalysis, Tokyo, 1980”;Elsevier: New York, 1981; Paper B-17. (10) Gleason, E. F. M. Chem. Eng. Thesis, University of Delaware, 1981. (11) McMillan, S. T. B.S. Thesis, University of Delaware, 1981. (12) Sayers, D. E.; Stern, E. A.; Lytle, F. W. Phy. Rev. Left. 1971, 27, 1204. ._.
(13) Sinfelt, J. H.; Via, G. H.; Lytle, F. W. J . Chem. Phys. 1977,67, 3831. (14) Sinfelt, J. H.; Via, G. H.; Lytle, F. W. J. Chem. Phys. 1978,68,2009. (15) Sinfelt, J. H.; Via, G. H.; Lytle, F. W. J. Chem. Phys. 1979, 71, 690. (16) Via, G. H.; Meitzner, G.; Lytle, F. W.; Sinfelt, J. H. J. Chem. Phys. 1983, 79, 1527. (!7) van’t Blik, H. F. J.; von Zon, J. B. A,; Huizinga, T.; Vis, J. C.; Koningsberger, D. C.; Prins, R. J. Phys. Chem. 1983,87, 2264. (18) Short, D. R.; Mansour, A. N.; Cook, Jr., J. W.; Sayers, D. E.; Katzer, J. R. J . Catal. 1983, 82, 299. (19) von Zon, J. B. A. D.; Koningsberger,D. C.; van’t Blik, H. F. J..; Prins, R. Sayers, D. E. J . Chem. Phys. 1984,80, 3914.
0022-365418512089-4261SO1.SO10 0 1985 American Chemical Societv
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phase. The addition of a basic support, such as MgO, to an aqueous solution, such as Rh(N03)3,increases the pH of the solution rather sharply. At a pH of 6, the solubility product of rhodium oxide is exceeded and the rhodium precipitates out of solution, making further "ion exchange" impossible. This problem was overcome by substituting a nonaqueous solvent-methanol (Fischer, Research Grade)-as the exchange medium.20 A volumetric amount (5 mL) of a 10 wt % solution of Rh(N03)3(Engelhard Industries, Lot No. Rh-211) was added to 600 mL of methanol and allowed to equilibrate for several hours while stirring continuously (pH -1.5). Approximately 10 g of magnesium oxide (Fischer Scientific, ACS Certified, Lot No. 761935) was then added to the solution and allowed to react for a period of approximately 6 h. During this period the exchange process was monitored by noting the rise of the pH of the solution to a value of 6 and by withdrawing small (- 3-5 mL) aliquots of the slurry. The aliquots were centrifuged and the amount of rhodium in solution was measured by atomic absorption. The AA standard was an aqueous solution of rhodium chloride (Alfa Division, Lot, No. 122377). On the basis of AA analysis, the rhodium loading of the catalyst was found to be 3% by weight. Following the ion-exchange process, the catalyst was filtered and washed with methanol and allowed to dry overnight at room temperature. The catalyst was subsequently calcined in ultrahigh-purity oxygen (Matheson) at 623 K for 3 h. The BET surface area of the magnesia was 49 m2/g and the rhodium dispersion was 0.68 as measured by hydrogen chemisorption." The chemisorption experiment consisted of first reducing the sample in flowing hydrogen at 473 K for 12 h, outgassing in flowing argon at 473 K for 2 h, and then cooling the sample to room temperature (298 K) in flowing argon. Next, a 1% hydrogen in argon gas was pulsed through a gas sample valve over the catalyst and the hydrogen uptake was monitored by a thermal conductivity detector a t the exit of the reactor. In calculating the dispersion, the Rh-H stoichiometry was assumed to be equal to 1 . There was no H2chemisorption by the unimpregnated MgO support under the above conditions and hence no correction for H2 chemisorption by the MgO was necessary. A portion of the catalyst was then ground to a fine powder and pressed into a thin (0.2-0.3 mm) wafer suitable for the X-ray absorption measurements. The wafer thickness was chosen to yield a ratio of the incident to transmitted X-ray intensity of three to five. The wafer was placed into a sample holder which was inserted into a controlled-atmosphere cell for X-ray absorption spectroscopy measurements. The sample cell was helium-leak tested prior to and immediately after the experiments. N o measurable leaks were observed. The cell has also been designed to function as a "Dewar" in order to perform X-ray absorption measurements at low temperatures. The dried sample was calcined in situ by slow (3 K/min) heating to 523 K, maintained at this temperature for 1 h, and cooled to room temperature, all in flowing oxygen (- 100 cm3/min). Subsequently, reductions were carried out at 473 and 673 K. Each reduction involved a slow (2-3 K/min) rise to the desired temperature, maintenance of this temperature for 1 h, and cooling of the sample to room temperature, all under flowing hydrogen. The X-ray absorption measurements were performed on X-ray beam line 1-5 at the Stanford Synchrotron Radiation Laboratory during the dedicated run of February 1981.2' The SPEAR energy was 3.0 GeV and the beam currents were in the range of 40-80 mA. Each absorption spectrum required a collection time of approximately 25 min. After each reduction, the K edge absorption spectrum of d h was recorded for each sample at liquid nitrogen temperature and under a static pressure of slightly less than 1 atm of hydrogen. A 3-rm-thick rhodium foil and a powdered sample of rhodium sesquioxide were used as a reference in subsequent data analysis procedures. The normalized EXAFS, (20) Murrell, L. L.; Yata, D. C. J. 'Reparation of Catalysts 11"; Delmon, B., Grange, P., Jacobs, P., Poncelet, G., Eds.; Elsevier: Amsterdam, 1979; V O ~111, . pp 307-319. (21) Winick, H.; Bienenstock, A. Annu. Rev. Nucl. Parr. Sci. 1978, 28, 33
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kcfl-l) Figure 1. Normalized EXAFS data of Rh foil (a), rhodium sesquioxide, R h 2 0 3 (b), 3.0 wt 7% Rh/MgO reduced at 473 K (c), and 3.0 wt 76 Rh/MgO reduced at 673 K (d). All data were taken near liquid nitrogen temperature.
x(k), was extracted from the total absorption spectrum by procedures described elsewhere using cubic spline fitting.22 All of the subtracted background functions determined from the spline routines were substantially the same, indicating the quality of the radiation produced by SPEAR was uniform throughout the experiments.
Results Figure 1 shows the EXAFS function x(k) for rhodium foil, rhodium sesquioxide and the two reduced catalyst samples plotted as a function of the photoelectron wave vector, k. A qualitative comparison of the frequency of the EXAFS oscillations of the reduced samples with those of the rhodium foil and rhodium sesquioxide samples shows that the spectra of the two reduced catalyst samples more closely resemble the spectrum of the foil sample rather than that of the rhodium sesquioxide sample. The frequency of the oscillations of both reduced samples in the higher (>7 k') k region is similar to that of the foil sample, indicating that the first Rh-Rh interaction distance in the reduced catalysts is similar to that observed in the bulk material and is unchanged with increasing reduction temperature. In the lower (
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1.6 A which is attributed to a Rh-0 interaction shell because of its position and the characteristic shape of the imaginary part of the Fourier transform within the peak. The dominant feature of the transform at 2.4 A is due to the first Rh-Rh coordination shell. The position of this shell is characteristic of bulk Rh, thus suggesting that the catalyst had a large fraction of metallic Rh after reduction at 473 K but still contains a sizeable amount of partially oxidized Rh. The small features at 3.4,4.4, and 5.0 8,are most likely the 2nd, 3rd, and 4th nearest-neighbor Rh-Rh coordination shells, since their locations are very close to those of these shells in the foil transform and the catalyst is assumed to be largely metallic. The Fourier transform of the catalyst reduced at 673 K (Figure 2d) indicates that the Rh crystallites are metallic. The small feature attributed to a Rh-O interaction shell is reduced and the feature at 2.4 A is increased by nearly 3-fold in peak amplitude. The features attributed to higher order Rh-Rh interaction shells have significantly increased in peak amplitudes and now resemble in intensity those of the foil sample. Also, after reduction at 673 K, a small feature appears at 6.7 8,in the Fourier transform, which corresponds to the position of the 5th nearest Rh-Rh interaction shell in the f g l sample. Qualitatively, the disproportionately large increase in the peak amplitude of the first Rh-Rh shell and the significant increase in the peak amplitudes of the higher coordination shells, which are observed with increasing reduction temperature, strongly suggest the Occurrence of sintering of the rhodium particles. This first coordination shell was then isolated and backtransformed into k space. The inverse Fourier transform range for the Rh foil and the two catalyst samples was 1.16-2.99 8, and
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Figure 2. Fourier transform of k3-weightedEXAFS data of Rh foil (a), rhodium sesquioxide, Rh203(b), 3.0 wt % Rh/MgO reduced at 473 K (c), and 3.0 wt % Rh/MgO reduced at 673 K (d). Both the magnitude (solid) and the imaginary part (dashed) of the Fourier transforms are shown.
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Figure 3. Inverse Fourier transforms (x vs. k) of the first coordination shell for the experimental data (solid) for 3.0 wt % Rh/MgO reduced at 473 K (a and b) and 673 K (c and d) compared with calculated spectra (dashed) obtained by using parameters from a single Rh-Rh fit (a and c) and a double Rh-Rh and Rh-0 fit (b and d). Parameters were obtained by minimizing residuals relative to the shell coordination number, coordination distance, relative disorder, and inner potential.
the oxide sample range was 1.16-2.70 A. Quantitative analysis of the data was made using a nonlinear least-squares fitting procedure.22 The inverse Fourier transforms, which are shown in Figure 3 as solid curves, are fitted by using the first shell of bulk rhodium and the first shell of rhodium sesquioxide as standards for the Rh-Rh and Rh-0 interactions, respectively. One- and two-shell fittings were made varying up to four parameters (coordination number Nj, relative disorder Au,*, shell distance Rj and inner potential E,) per shell. The fitting parameters are shown in Table I. The calculated data resulting from the fitting programs are compared in Figure 3 with the actual inverse Fourier transforms of the catalyst samples reduced at 473 and 673 K. The catalyst sample reduced at 473 K was best fitted with a two-shell interaction model (Figure 3b) with each rhodium atom being surrounded on the average by approximately two atoms of oxygen and three atoms of rhodium. The average Rh-Rh interaction distance is the same, within experimental error, as the The average distance of the Rh-0 bulk value of 2.69 interaction is 1.93 A, which is much shorter than the average distance of 2.05 f 0.05 8, reported for the hexagonal form of rhodium s e s q ~ i o x i d e .The ~ ~ inverse data of the catalyst reduced at 673 K was also best fitted with a two-shell interaction model (Figure 3d). In this sample, each rhodium is surrounded by ~
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(23) Wyckoff, R. W. G. ‘Crystal Structures”, 2nd ed.; Wiley: New York, 1963; VoL-1, p 10. (24) Coey, J. M. Acta Crystallogr., Sect. B 1970, B26, 1876.
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approximately eight atoms of Rh and one atom of oxygen with no significant changes in the coordination distance.
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The Journal of Physical Chemistry, Vol. 89, No. 20, 1985
Discussion EXAFS analysis of the Rh/MgO samples shows that over the temperature range of hydrogen reduction studied, the Rh-Rh first-shell coordination number increased by a factor of about 3 (2.8-7.9) and the disorder of Rh atoms measured relative to bulk Rh remained constant. For both reduction conditions, the Rh-Rh coordination numbers are lower than the bulk value of 12, indicating the highly dispersed nature of the Rh crystallites. After reduction at 673 K, the Rh-Rh coordination number is similar to coordination numbers reported in earlier papers for different metal-supported catalyst^'^^^^^^^ except for a 0.5 wt % Rh/AIzO3 where a coordination number of 1.5 was reported.I6 The Rh-Rh first-shell coordination distance is similar to that of bulk Rh (2.69 A) which is in agreement with other results obtained on supported metal catalysts under the same experimental condition^.'^-'^ Furthermore, higher shells (second, third, and fourth) of Rh-Rh interactions are clearly displayed in the Fourier transforms after both hydrogen reduction temperatures. Hence we conclude the Rh atoms are not present in the form of isolated atoms attached to the supportZSor in the form of a raftlike s t r ~ c t u r e . ~The ~-~~ raftlike structure for Rh atoms is excluded because of the presence of the second shell in the Fourier transform. However, since EXAFS measures the average structure near an absorbing atom, a small fraction of the Rh atoms may still be present in either form. EXAFS analysis also shows that over the temperature of hydrogen reduction studied, the Rh-0 coordination number decreased by a factor of about 2 (1.7-0.9) and the relative disorder of 0 atoms measured relative to bulk rhodium sesquioxide increased by a factor of about 2 (0.006-0.011 AZ). The Rh-0 coordination distance (1.95 A) is significantly shorter than that of Rhz03(2.05 A) but is similar to that of R h o z which has a rutile str~cture.~'On the basis of the R h o z phase diagram, this oxide would be a more stable structure under the calcination conditions used in these experiment^.^^ There are two possible explanations for the presence of Rh-0 interaction. One is the presence of an incompletely reduced catalyst and is supported by the decrease of Rh-0 first-shell coordination number after hydrogen reduction at 673 K. XPS m e a s ~ r e m e n t made s ~ ~ on the same sample indicated the presence of such Rh-0 interaction only after hydrogen reduction at 473 K. To explain the discrepancy between EXAFS and XPS results after hydrogen reduction at 673 K, we assume that the Rh-O bond represents the interaction of Rh crystallites with 0 atoms on the surface layer of the support. On the basis of this argument and with a hemispherical shape for Rh crystallites to illustrate the e f f e ~ t ,the ~ ~average . ~ ~ size of the metal crystallites was determined to be 6 and 16 8, after hydrogen reduction at 473 and 673 K, respectively. Therefore, XPS measurements of the Rh 3d spectrum would only show the presence of a Rh-0 interaction after reduction at 473 K because the Rh 3d core electrons have a mean free path35of only 10 A which is smaller than the average size (25) Yates, Jr., J. T.; Duncan, T. M.; Worley, S. D.; Vaughan, R. W. J . Chem. Phys. 1979, 70, 1219. (26) Prestidge, E. B.; Yates, D . J. C. Nature (London) 1971, 234, 345. (27) Yao, H. C.; Rothschild, W. G. J . Chem. Phys. 1978, 68, 4774. (28) Yates, D. J. C.; Murrell, L. L.; Prestidge, E. B. J . Cataf. 1979, 57, 41. (29) Yates, D. J. C.; Murrell, L. L.; Prestidge, E. B. In *Growth and Properties of Metal Clusters"; Bourdon, J., Ed.; Elsevier: New York, 1980; p 137. (30) Shannon, R. D. Solid Stare Commun. 1968, 6, 139. (31) Muller, 0.; Roy, R. J . Less-Common Met. 1968, 16, 129. (32) Onuferko, J.; McMillan, S.T.; Baltanas, M.; Katzer, J. R., paper in preparation. (33) Lytle, F. W.; Greegor, R. B. J . Caral. 1980, 63, 476. (34) von Zon, J. B. A. D.; Koningsberger, D.C.; van't Blik, H.F. J.; Sayers, D. E, submitted to J . Chem. Phys. (35) Seah, M. P.; Dench, W. A. Surf.Interface Anal. 1979, 1- 2.
of the metal crystallites (16 A) after reduction at 673 K. Assuming spherical or cubical shape for the Rh crystallites does not affect the above conclusion. However, the presence of a small fraction of unreduced Rh crystallites at the lower reduction temperature (473 K) cannot be excluded. The Rh-support contribution has been shown to exist in an earlier study for a 0.5 wt % Rh/A1203 after reduction at 773 K.I9 The authors claimed that the Rh-support contribution gives a Rh-O coordination number of 1.3 with bond length of 2.71 when the measured Rh-Rh coordination number was 5.0. In another the Rh-support contribution was measured as a function of the average size of the Rh crystallites for different Rh/AlZ0, catalysts. From the measured Rh-0 coordination numbers and with a hemispherical shape for the Rh crystallites with fcc structure, it was estimated that, on the average, each interfacial Rh atom is surrounded by 2-3 oxygen ions of the support with an average Rh-0 bond length of 2.7 A. When the same type of procedure was used,34our results on the Rh/MgO catalyst indicate the same type of Rh-support interaction but with a Rh-0 bond length of 1.95 A. The shorter bond length distance could be explained by the fact that a much stronger interaction exists between Rh atoms and MgO support than the A1203 support which was confirmed by XPS, UV spectroscopy, and adsorption calorimetry in an earlier The increase in the Rh-0 disorder as the reduction temperature increased from 473 to 673 K is probably due to the increase in the average size of the Rh crystallites from 6 to 16 A, which may indicate a less specific interaction when the crystallite size increases as expected. The large increase observed in the Rh-Rh coordination number after reduction at 673 K for Rh/MgO is surprising when compared with results on Rh/Alz03 and Rh/Si02. EXAFS data for a 2.4 wt % Rh/AlZO3catalyst had a coordination number of 6 after reduction at 473 K, which remained constant after reduction at 673 K.37 Hydrogen chemisorption data collected on a 2.4 wt % Rh/AlZO3catalyst and a 0.85 wt % Rh/SiO, catalyst indicated that the metal particle size was unchanged after reduction at 473 and 673 K." One possible explanation for the increase in particle size for the Rh/MgO catalyst is that the magnesium oxide support was sintering. IR studies of the hydroxyl bonds in magnesium oxide show extensive sintering occurs at 1173.3a TPD and surface studies of this catalyst indicate that the presence of Rh may lower this temperature s o m e ~ h a t . ' ~However, .~~ TPD measurements made on this catalyst suggest that the lowest temperature at which sintering could occur is 843 K." Furthermore, since the surface area of the support is initially rather low (49 m2/g), sintering of the support could account for at best only a small fraction of the observed change. It seems likely then that the observed increase in the Rh-Rh coordination number is due to sintering of the Rh metal itself. The cause and mechanism of this sintering is unknown, but is almost certainly related in some way to the magnesium oxide support surface. Acknowledgment. The work was supported in part by the National Science Foundation under Grant DMR80-11946. Some of the materials incorporated in this work were developed at SSRL with the financial support of the National Science Foundation under contract DMR77-27489 in cooperation with the Department of Energy. We gratefully acknowledge the assistance of the SSRL staff and Drs. D. R. Short and S. M . Khalid in the acquisition of the data presented here. Registry No. Rh, 7440-16-6; MgO, 1309-48-4. (36) Zakumbaeva, G. D.; Shpiro, E. S.;Beketaeva, L. A,; Dyusenbina, B. B.; Aitmagambetova, S. Z.; Uvaliev, T. Yu.; Khisametdinov,A. M.; Antoshin, G. V.; Minachev, Kh. M. Kinet. Katal. 1982, 23, 943. (37) Koningsberger, D. C.; Van Zon, J. B. A. D.; Van't Blik, H. F. J.; Visser, G. J.; Prins, R.; Mansour, A. N.;Sayers, D. E.; Short, D. R.; Katzer, J. R. J . Phys. Chem., in press. (38) Anderson, P. J.; Horolock, R. F.; Oliver, J. F. Trans. Faraday SOC. 1965, 61, 2754. (39) McMillan, S. T.; Gleason, E. F.; Katzer, J. R., paper in preparation.
