J. Phys. Chem. B 2003, 107, 5519-5528
5519
MgO-Supported Rh6 and Ir6: Structural Characterization during the Catalysis of Ethene Hydrogenation Andrew M. Argo and Bruce C. Gates* Department of Chemical Engineering and Materials Science, UniVersity of California, DaVis, California 95616 ReceiVed: August 8, 2002; In Final Form: February 1, 2003
We report a comparison of MgO-supported Rh6 and MgO-supported Ir6 as catalysts for ethene hydrogenation. The catalysts were prepared by decarbonylation of hexanuclear metal carbonyl precursors on the support. Extended X-ray absorption fine structure and infrared spectroscopies were used to characterize the structures of the working catalysts and the adsorbates formed on them during catalysis. The data identify the octahedral clusters as the catalytically active species. Supported Rh6 is 1-2 orders of magnitude more active than supported Ir6 for ethene hydrogenation; this difference in activity is consistent with that observed for conventional rhodium and iridium catalysts consisting of bulklike particles of metal on SiO2. The results demonstrate that the influence of metal composition on catalytic activity extends from quasi-molecular metal clusters to metallic particles.
Introduction The adsorption and catalytic properties of noble metals vary significantly from one metal to another. For example, the reactivity and bonding of ethene on metal single crystals depend strongly on the metal composition.1 At 100 K, ethene is adsorbed on the close-packed faces of Rh, Ir, and Pt in a di-σ-bonded form, whereas on the closed-packed faces of Pd, Cu, or Au it is π-bonded. Catalytic activities of metals (e.g., for formic acid dehydrogenation2 and ethene hydrogenation) depend on the metal composition, those in the latter case being strongly influenced by the strength of adsorption of ethene and hydrogen.3-5 Strongly adsorbed ethene reacts slowly because the energy barrier for surface reaction is high, and weakly adsorbed ethene reacts slowly because it is present in low concentrations; an intermediate strength of adsorption results in the highest rate of catalytic reaction.6-8 These patterns apply with quite some generality in catalysis by metals, but when metal particles become smaller and smaller, crossing into the nanosize range and below, the patterns must be altered, and the properties of a metal catalyst must approach those of molecular species. Our goal was to investigate some of the smallest available supported metal clusters and provide the first comparison of two of them with the same structure (Rh6 and Ir6) as catalysts. Each cluster has an octahedral structure, and each was supported on MgO. Because catalyst structures depend on reaction environments, we determined them during catalysis using extended X-ray absorption fine structure (EXAFS) spectroscopy to characterize the cluster structures and infrared (IR) spectroscopy to characterize the adsorbates (ligands) formed on the clusters from the reactants. Experimental Section Materials, Sample Preparation, and Handling. The syntheses of Ir6/MgO and [Rh6(CO)15]2-/MgO9-13 were carried out as before on a vacuum line or in a glovebox (Vacuum Atmospheres HE-63-P) purged with N2 that was recirculated * Corresponding author. E-mail:
[email protected].
through O2- and moisture-scavenging traps (supported Cu particles and zeolite 4A, respectively). The glovebox was equipped with O2 and moisture detectors, indicating concentrations of Ir. The results presented here show that the order of catalytic activities is the same for the two hexanuclear clusters as for the particles of metalsthe rate of ethene hydrogenation on MgO-supported Rh6 is 1-2 orders of magnitude greater than that on Ir6, and correspondingly, the activity of SiO2-supported rhodium particles for this reaction is 2 orders of magnitude greater than that of SiO2-supported iridium particles.3 The relatively high rate of ethene hydrogenation on the rhodium particles was suggested to be an indication of the nearly optimal strength of adsorption of ethene on that metal.4,66 Mohsin et al.66 concluded that ethene hydrogenation proceeds at a relatively low rate on iridium because ethene is adsorbed so strongly on iridium. As in the catalysis of formic acid dehydrogenation on metals to the right of rhodium in the periodic table, the reactant is supposedly adsorbed too weakly, whereas on metals to the left of rhodium it is supposedly adsorbed too strongly. This interpretation is commonly associated with “volcano plots” showing the catalytic activities of metals as a function of the strength of bonding of the reactant to the metal. This point was illustrated for formic acid dehydrogenation in a plot of catalytic activity as a function of the enthalpy of formation of the bulk metal formate, suggested to be representative of a surface formate reaction intermediate on the catalyst.2 Similarities in Ethene Hydrogenation Catalyzed by MgOSupported Rh6 and MgO-Supported Ir6. Both Rh6 on MgO and Ir6 on MgO are active for ethene hydrogenation catalysis
MgO-Supported Rh6 and Ir6 under mild conditions, and these two samples are characterized by similar kinetics of the catalytic reaction. EXAFS data show that the metal-metal bond distances characterizing Ir6 and Rh6, respectively, were consistently longer (even in view of the experimental errors37) during catalysis than those characterizing the samples in He prior to catalysis. Earlier,50,51 we showed that such changes in the metal-metal bond distance are associated with the presence of reactive intermediates on tetrairidium clusters. Candidate intermediates are π-bonded ethene, di-σ-bonded ethene, and ethyl, and there is IR evidence for each on Rh6 and on Ir6 during catalysis at high PC2H4 (>200 Torr), consistent with the EXAFS observation of structural changes in the clusters during catalysis. Differences between Ethene Hydrogenation Catalyzed by Rh6 and by Ir6. Although ethene hydrogenation on Ir6/MgO is qualitatively similar in several respects to that on Rh6/MgO, these samples display significant differences, as follows. Kinetics. In addition to large differences in the rates of reaction, differences in the reaction orders were observed, as summarized above. The reaction orders observed for ethene hydrogenation catalyzed by Ir6/MgO, close to the values of 0.5 in H2 and zero in ethene, suggest a catalyst surface largely covered with adsorbed ethene, with hydrogen possibly adsorbed on sites unavailable to ethene. In contrast, the data characterizing Rh6/MgO, with a reaction order in H2 >1 and a negative order in ethene, suggest incomplete coverage of the clusters with ethene and competitive adsorption of hydrogen and ethene.49,67,68 Thus, the kinetics data suggest that the Ir6 clusters were nearly saturated with adsorbed hydrocarbon during catalysis, whereas the Rh6 clusters, under the same conditions, were not. Spectroscopic Evidence. The low signal-to-noise ratio of the IR spectra characterizing Ir6/MgO during catalysis limits the value of the information obtained in these experiments. The data are too noisy to allow reliable correlations of the intensities of adsorbate bands with the catalytic activity. In contrast, the IR data characterizing Rh6/MgO are of high quality, indicating the presence of various reactant-derived ligands inferred to be diσ-bonded ethene, π-bonded ethene, ethyl, and ethylidyne on the clusters during catalysis at the higher ethene partial pressures. In contrast, we were able to identify only di-σ-bonded ethene, π-bonded ethene, and ethyl on Ir6/MgO during ethene hydrogenation at elevated ethene partial pressures. These observations are consistent with mechanisms suggested for the hydrogenation of ethene on Pt(111),69 Pd (111),70 Ir4/ γ-Al2O3,50 and Ir4/MgO50 and with the role of di-σ-bonded ethene, π-bonded ethene, and ethyl as reaction intermediates. Pradier and Berthier71 showed that the rates of hydrogenation of butadiene and of isoprene on the low-index faces ((111), (110), and (100)) of platinum were in large measure determined by the strength of adsorption of the hydrocarbon on the face. Reasoning similarly, we might suggest that ethene is adsorbed more strongly on Ir6 than on Rh6, resulting in a higher energy barrier for reaction; similar inferences have been drawn3 for ethene hydrogenation on supported particles of metal. However, this suggestion is inconsistent with the IR peak positions for π-bonded ethene on Rh6 or on Ir6 during catalysissthe band characterizing π-bonded ethene occurred at a higher frequency for the species on Ir6 than for that on Rh6. We are left to conclude that there is insufficient information to justify an interpretation of the activities of the clusters of the two metals. Differences between Supported Clusters and Bulk Metal Catalysts. It also remains to be resolved why the clusters are so much less active than the respective bulk metals for the hydrogenation reaction, but we can offer the following sugges-
J. Phys. Chem. B, Vol. 107, No. 23, 2003 5527 tions: (1) The smallness of the clusters may limit the structures formed from ethene and H2 that can bond to them in ways to facilitate reaction; evidence showing how the smallness of Ir4 clusters on supports limits the reactions of propene and H2 has been presented.72 (2) The strong interactions of the clusters with the supports can influence the reactivity and catalytic properties of the clusters; recent theoretical work73 shows that the Rh atoms in Rh6 clusters at the interface with the support (a zeolite) bear significant positive charges (consistent with the Rh-O and Ir-O bonding distances shown by the EXAFS data), whereas the Rh atoms not at the interface are nearly uncharged. In this respect, the supported clusters take on some of the character of (zeolite-) supported cationic metal complexes such as Rh(I)(CO)2, in which the Rh atom bears a significant positive charge74 (approximately the same as that on the Rh atoms at the interface in the zeolite-supported Rh6). The charges in supported metals are expected to decrease as the clusters or particles become larger, as the catalytic properties approach those of the bulk. Conclusions IR and EXAFS spectroscopies were used to investigate ethene hydrogenation catalyzed by Rh6/MgO and by Ir6/MgO. EXAFS spectra identify the Rh6 and Ir6 octahedra as the catalytically active species. IR spectra indicate π-bonded ethene, di-σ-bonded ethene, and ethyl on Rh6 and on Ir6 during the catalysis of ethene hydrogenation; these are all plausible reaction intermediates. Ethylidyne was also observed on Rh6/MgO during catalysis (but it has been suggested to be a spectator50,51). Rh6 on MgO is 1-2 orders of magnitude more active for ethene hydrogenation than Ir6 on MgO, and this ranking of activities approximately matches that reported earlier for particles of rhodium and iridium supported on SiO2.3 Thus, the influence of metal composition on ethene hydrogenation catalysis is similar for small quasimolecular metal clusters and bulk metal for our metals and reaction. Acknowledgment. This research was supported by the National Science Foundation (grant CTS-9617257). We acknowledge beam time and the support of the U.S. Department of Energy, Division of Materials Sciences, under contract no. DE-FG05-89ER45384, for its role in the operation and development of beamline X-11A at the National Synchrotron Light Source. The NSLS is supported by the Department of Energy, Division of Materials Sciences and Division of Chemical Sciences, under contract no. DE-AC02-76CH00016. We are grateful to the staff of beamline X-11A for their assistance. We acknowledge the Stanford Synchrotron Radiation Laboratory, which is operated by Stanford University for the Department of Energy, Office of Basic Energy Sciences, for access to beam time on beamline 2-3. The EXAFS data were analyzed with the XDAP software.33 References and Notes (1) Yagasaki, E.; Masel, R. I. In Catalysis; Spivey, J. L., Ed.; Royal Society of Chemistry: Cambridge, U.K., 1994; Vol. 11, p 163. (2) Rootsaert, W. J. M.; Sachtler, W. M. H. Z. Phys. Chem. N. F. 1960, 26, 16. (3) Schuit, G. C. A.; van Reijen, L. L. AdV. Catal. 1958, 10, 242. (4) Beeck, O. Discuss. Faraday Soc. 1950, 118. (5) Kemball, C. J. Chem. Soc. 1956, 735. (6) Gates, B. C. Catalytic Chemistry; Wiley: New York, 1992. (7) Horiuti, J.; Miyahara, K. Hydrogenation of Ethylene on Metallic Catalysts; NBS-NSRDS 13; U.S. Government Printing Office: Washington, DC, 1968. (8) Bond, G. C. Catalysis by Metals; Academic Press: New York, 1962. (9) Kawi, S.; Gates, B. C. Inorg. Chem. 1992, 31, 2939.
5528 J. Phys. Chem. B, Vol. 107, No. 23, 2003 (10) The synthesis of [Rh6(CO)15]2-/MgO was essentially parallel to that of Dossi et al.,11 except that n-hexane was used instead of CH2Cl2 to introduce the metal carbonyl clusters onto the support. (11) Dossi, C.; Psaro, R.; Ugo, R. J. Organometallic Chem. 1988, 353, 259. (12) Dufour, P., Huang, L., Choplin, A.; Sanchez-Delgado, R.; The´olier, A.; Basset, J.-M. J. Organomet. Chem. 1988, 354, 243. (13) Smith, A. K.; Hugues, F.; The´olier, A.; Basset, J. M.; Ugo, R., Zanderighi, G. M.; Bilhou, J. L.; Bilhou-Bougnol, V.; Graydon, W. F. Inorg. Chem. 1979, 18, 3104. (14) Panjabi, G.; Argo, A. M.; Gates, B. C. Chem.sEur. J. 1999, 5, 2417. (15) Jentoft, R. E.; Deutsch, S. E.; Gates, B. C. ReV. Sci. Instrum. 1996, 67, 2111. (16) Odzak, J. F.; Argo, A. M.; Lai, F. S.; Gates, B. C.; Pandya, K.; Feraria, L. ReV. Sci. Instrum. 2001, 72, 3943. (17) Dilution of the catalyst in inert R-Al2O3 particles minimized preferential channeling of reactants through the catalyst bed in addition to minimizing heat- and mass-transfer limitations, as described previously.18 (18) Xu, Z.; Gates, B. C. J. Catal. 1995, 154, 335. (19) Goellner, J. F. Ph.D. Dissertation, University of California, Davis, CA, 2000. (20) (a) Zabinsky, S. I.; Rehr, J. J.; Ankudinov, A.; Albers, R. C.; Eller, M. J. Phys. ReV. B. 1995, 52, 2995. (b) Ankudinov, A. Ph.D. Dissertation, University of Washington, Seattle, WA, 1996. (21) Crystal Structures, 2nd ed.; Wycoff, R. W. G., Ed.; Wiley: New York, 1963; Vol. 1, p 10. (22) Coey, J. M. D. Acta Crystallogr., Sect. B 1970, 26, 1876. (23) Mason, R.; Rae, A. I. M. J. Chem. Soc. A 1968, 778. (24) Donnay, J. D. H. Crystal Data DeterminatiVe Tables, 3rd ed.; Donnay, J. D. H.; Ondik, H. M., Eds.; U.S. Department of Commerce, National Bureau of Standards, and the Joint Committee on Powder Diffraction Standards: Washington, DC, 1972; Vol. 2, p C-4. (25) Tro¨mel, M.; Lupprich, E. Z. Anorg. Allg. Chem. 1975, 414, 160. (26) Churchill, M. R.; Hutchinson, J. P. Inorg. Chem. 1978, 17, 3528. (27) van Zon, F. B. M.; Maloney, S. D.; Gates, B. C.; Koningsberger, D. C. J. Am. Chem. Soc. 1993, 115, 10317. (28) Duivenvoorden, F. B. M.; Koningsberger, D. C.; Uh, Y. S.; Gates, B. C. J. Am. Chem. Soc. 1986, 108, 6254. (29) Lu, D.; Rehr, J. J. J. Phys. (Paris) 1986, 47, 67. (30) van Zon, F. B. M.; Maloney, S. D.; Gates, B. C.; Koningsberger, D. C. J. Am. Chem. Soc. 1993, 115, 10317. (31) van Zon, J. B. A. D. Ph.D. Dissertation, Eindhoven University of Technology, The Netherlands, 1988. (32) van Zon, J. B. A. D.; Koningsberger, D. C.; van’t Blik, H. F. J.; Sayers, D. E. J. Chem. Phys. 1985, 82, 5742. (33) Vaarkamp, M.; Linders, J. C.; Koningsberger, D. C. Physica B 1995, 209, 159. (34) Kirlin, P. S.; van Zon, F. B. M.; Koningsberger, D. C.; Gates, B. C. J. Phys. Chem. 1990, 94, 8439. (35) Weber, W. A. Ph.D. Dissertation, University of California, Davis, CA, 1998. (36) Weber, W. A. Personnel communication, 1999. (37) Vaarkamp, M. Catal. Today 1998, 39, 271. (38) Corey, E. R.; Dahl, L. F.; Beck, W. J. Am. Chem. Soc. 1963, 85, 1202. (39) Martinengo, S.; Chini, P. Gazz. Chim. Ital. 1972, 102, 344. (40) Albano, V. G.; Sansoni, M.; Chini, P. Martinengo, S. J. Chem. Soc., Dalton Trans. 1973, 651. (41) Albano, V. G.; Sansoni, M.; Chini, P. Martinengo, S. J. Chem. Soc., Dalton Trans. 1973, 651. (42) Argo, A. M. Ph.D. Dissertation, University of California, Davis, CA, 2001. (43) Ferrari, A. M.; Neyman, K. M.; Mayer, M.; Staufer, M.; Gates, B. C.; Ro¨sch, N. J. Phys. Chem. B 1999, 103, 5311. (44) Goellner, J. F.; Gates, B. C.; Vayssilov, G. N.; Ro¨sch, N. J. Am. Chem. Soc. 2000, 122, 8056. (45) Goellner, J. F.; Neyman, K. M.; Mayer, M.; No¨rtemann, F.; Gates, B. C.; Ro¨sch, N. Langmuir 2000, 16, 2736.
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