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Langmuir 1999, 15, 3506-3520
Chemical Bond Activation on Surface Sites Generated Photochemically from RhI(CO)2 Species Edward A. Wovchko and John T. Yates, Jr.* Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Received August 11, 1998. In Final Form: December 18, 1998 The activation of chemical bonds in molecules on photochemically produced RhI sites is reviewed in this article. The focus is primarily on the RhI(CO) species generated during the ultraviolet photolysis of the well-known RhI(CO)2 species supported on an aluminum oxide surface (designated RhI(CO)2/Al2O3). The RhI(CO)2 surface species is the heterogeneous analogue of the homogeneous phase CpRh(CO)2 and Cp*Rh(CO)2 (Cp* ) η5-C5(CH3)5; Cp ) η5-C5H5) known for its ability to activate the C-H bond in alkanes during ultraviolet irradiation. Here we review some of the key studies that have led to the molecular understanding of the activation of the RhI(CO)2/Al2O3 species by photochemical methods. Relevant examples of the homogeneous-phase bond activation studies utilizing a rhodium center will be discussed as well as theoretical studies that have provided valuable information regarding the active nature of the photoproduced Rh site. This will be followed by a review of recent photochemical studies of surface-bound RhI(CO)2. The coordinatively unsaturated RhI(CO) species produced photochemically can activate the C-H bond in C6H12 and CH4, the H-H bond in H2, the OdO bond in O2, and the CdO bond in CO2.
1. Introduction It was recently discovered that interesting chemical processes can occur by exposing the RhI(CO)2/Al2O3 species to ultraviolet light at low temperatures (99.5%) cyclohexane, c-C6H12. The infrared spectra measured in the C-H stretching region after 5 and 15 h of irradiation with ultraviolet light at 325 ( 50 nm at 297 K are presented in Figure 8. A strongly bound hydrocarbon species is observed to increase in coverage for increasing irradiation time. The C-H stretching mode frequencies for the bound alkyl species are very similar to those of c-C6H12(g) and are attributed to adsorbed cyclohexyl species. As shown in the inset of Figure 8, the cyclohexyl species were thermally stable up to 600 K, contrary to the thermal stability of the alkyl adducts observed in the homogeneous phase, which decomposed upon warming to room temperature. The stability observed for the cyclohexyl species on the surface (81) Basu, P.; Ballinger, T. H.; Yates, J. T., Jr. Rev. Sci. Instrum. 1988, 59, 1321. (82) Yates, J. T., Jr. Experimental Innovations in Surface Science; Springer Verlag/AIP: New York, 1998; pp 800-805. (83) Buckbee MearssSt. Paul, 278 East Seventh Street, St. Paul, MN 55101; 612-228-6400. (84) Wong, J. C. S.; Linsebigler, A.; Lu, G.; Fan, J.; Yates, J. T., Jr. J. Phys. Chem. 1995, 99, 335.
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Figure 7. Infrared cell design for studying high-area powdered materials and optical alignment for simultaneous photochemistry and infrared spectroscopy. Reproduced from refs 81 and 84.
indicates that the cyclohexyl species migrates from its formation site on the RhI center and becomes anchored on the Al2O3 support. The proposed reaction scheme is shown below. hν
c-C6H12(g)
Rh(CO)2(a) 98 Rh(CO)(a) + CO(g) 98 Rh(CO)(H)(C6H11)(a) f C6H11(a) (1) To avoid confusion between possible thermal effects and photoeffects in the decomposition of the RhI(CO)2 species to form the coordinatively unsaturated RhI(CO) species, electrical control of the sample support grid temperature was employed to maintain a constant substrate temperature (297 K) before and during UV irradiation in the above cyclohexane experiment. In addition, a control experiment was conducted in which the UV lamp was replaced by a tungsten-halogen incandescent lamp. The visible lamp power was adjusted to supply the same thermal energy into the substrate as that from the UV lamp (producing the same temperature increase of ∼10 K without electronic control). The control experiment is shown in Figure 9. Here the mass spectrometer response to various types of heating is presented along with the sample thermocouple response. It can be seen that a significant loss of CO is only observed during the ultraviolet photolysis of RhI(CO)2/Al2O3. It is the ultraviolet light that is causing the photodesorption of CO to produce
Figure 8. Photochemical production of chemisorbed cyclohexyl species from cylcohexane. The inset shows the cyclohexyl species thermal stability in a vacuum. Reproduced from refs 1 and 2.
an active site and not a thermal process due to a localized heating of the Al2O3 particles during irradiation. 2.3. RhI(CO)2 Photoexcitation. We have studied the photoexcitation of RhI(CO)2/Al2O3 in more detail by directly monitoring the kinetics of the loss of the RhI(CO)2 infrared
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Figure 9. Mass spectrometer (A) and thermocouple (B) responses to various forms of heating of the RhI(CO)2/Al2O3 catalyst. Electrical heating (a) and incandescent lamp heating (b) of the grid show that, although there is a temperature increase, only small amounts of CO are desorbed. In contrast, UV irradiation (c) of the powder shows a similar temperature change but a large rise in CO pressure. Reproduced from ref 2.
Figure 10. Energy threshold determination for RhI-CO photodissociation. The cross section for RhI(CO)2 photodepletion is plotted as a function of the average photolysis energy. The horizontal bars show the range of energies for each filter used. Reproduced from ref 9a.
bands as a function of photon energy9a on a 0.5% Rh/Al2O3 sample. By working at 205 K, possible thermal effects were minimized. Due to the low energies employed (500 K).96-98 However, in the work presented here, the oxidation of CO takes place at cryogenic temperatures (∼174 K) and on isolated RhI(CO) sites produced photochemically. No substantial CO2 formation was observed by thermally excited processes at 174-181 K. (95) Wey, J. P.; Neely, W. C.; Worley, S. D. J. Catal. 1992, 134, 378. (96) Peden, C. H. F.; Goodman, D. W.; Blair, D. S.; Berlowitz, P. J.; Fisher, G. B.; Oh, S. H. J. Phys. Chem. 1988, 92, 1563. (97) Goodman, D. W.; Peden, C. H. F. J. Phys. Chem. 1986, 90, 4839.
2.7.3. CO2 Activation on Isotopically Labeled RhI(*C*O)2. The thermodynamic stability of CO2 contributes to the general lack of commercial processes that utilize CO2 as a feedstock. One method of overcoming the stability is to utilize ultraviolet light as an alternative source of energy to conventional thermal energy sources. The photochemical production of surface sites, capable of activating the CdO bond of CO2, provides an interesting and potentially useful route to the utilization of CO2 as a feedstock for the photochemical production of other useful products. (98) Oh, S. H.; Fisher, G. B.; Carpenter, J. E.; Goodman, D. W. J. Catal. 1986, 100, 360.
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Figure 17. Proposed mechanism for the activation of CO2 on photochemically generated RhI(13C18O) sites. Figure reproduced from ref 7.
Several recent reviews discuss the chemistry of CO2 on metal and metal oxide surfaces99,100 as well as the coordination and chemistry of CO2 with transition metal complexes.101-103 Dissociation of CO2 occurs on several metal surfaces, including Fe, Ni, Re, Al, and Mg, but to little or no extent on Pt and pure Rh.99,100 There are numerous examples of organometallic compounds that involve transition metals containing CO2 ligands.101-103 However, only a few fully characterized examples of CO2 directly coordinated to a single Rh center are reported.101,105-106 The best example of a rhodium η1-CO2 complex is Rh(diars)2(Cl)(CO2) [diars ) o-phenylenebis(dimethylarsine)], reported by Herskovitz and co-workers.104 There have been few reports of the photochemistry of transition metal carbon dioxide complexes.101 One study, involving the photolysis of Cp2Mo(CO2) in the presence of CO2, resulted in the formation of Cp2Mo(CO3), Cp2Mo(CO), and CO.107 We have found that the CdO bond of carbon dioxide can be activated on the photochemically produced rhodium monocarbonyl species to produce CO and O at 256 K.7 Isotopically labeled RhI(13C18O)2 species were employed in order to spectroscopically distinguish the various rhodium gem-dicarbonyl isotopomers produced during the course of photoactivation. The two major products display infrared bands at 2077 and 1958 cm-1 assigned to the (99) Freund, H.-J.; Roberts, M. W. Surf. Sci. Rep. 1996, 25, 225. (100) Solymosi, F. J. Mol. Catal. 1991, 65, 337. (101) Gibson, D. H. Chem. Rev. 1996, 96, 2063. (102) Leitner, W. Coord. Chem. Rev. 1996, 153, 257. (103) Behr, A. Carbon Dioxide Activation by Metal Complexes; VCH Verlagsgesellschaft mbH: Weinheim, Germany, 1988. (104) Calabrese, J. C.; Herskovitz, T.; Kinney, J. B. J. Am. Chem. Soc. 1983, 105, 5914. (105) Vigalok, A.; Ben-David, Y.; Milstein, D. Organometallics 1996, 15, 1839. (106) Aresta, M.; Nobile, C. F. Inorg. Chim. Acta 1977, 24, L49. (107) Belmore, K. A.; Vanderpool, R. A.; Tsai, J.-C.; Khan, M. A.; Nicholas, K. M. J. Am. Chem. Soc. 1988, 110, 2004.
formation of RhI(12C16O)(13C18O) species and bands at 2036 cm-1 and near 1958 cm-1 assigned to the formation of RhI(13C16O)(13C18O). The infrared band assignments for the rhodium gem-dicarbonyl isotopomers were supported by separate studies made of a thermally driven isotopic exchange experiment with 13C16O(g) and by comparison to the frequencies predicted using frequency calculations based on known force constants. The formation of 12C16Ocontaining gem-dicarbonyl species indicates unequivocally the activation of CO2, since no 12C-containing species were present on the surface prior to the introduction of CO2. The two unique isotopomers of the rhodium gem-dicarbonyl species containing 12C16O and 13C16O are indicative of two distinct pathways for CO2 activation, as shown in Figure 17 (the isotopic labels are indicated by *). Path I, the dominant channel, involves direct dissociation of the CdO bond in bound CO2 ligands to form RhI(12C16O)(13C18O) species. Path II involves oxygen atom exchange between CO2 and CO ligands followed by C-O bond dissociation in the exchanged CO2 ligand. A similar oxygentransfer process was observed by Cooper and co-workers108 with Fe(η-C5H5)(CO)2(CO2)- complexes. The surface intermediate involving CO2 is postulated to have η1 bonding between the Rh center and CO2. The η1 bonding is favored by electron-rich transition metals (RhI, d8) that can donate electron density into the π* orbital of CO2, thereby reducing the CdO bond strength.101-103 Reversible η1 coordination of CO2 with a d8 RhI center leads to a high partial negative charge on the CO2 fragment. The electron transfer causes CO2 instability, and the CO2 anionic fragment adopts a more energetically favorable bent geometry.99,101 Dissociation of CO2 may then occur. Side-on or η2 type bonding is not observed, since no oxidized rhodium carbonyl species are observed during CO2 activation. (108) Lee, G. R.; Cooper, N. J. Organometallics 1985, 4, 794.
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Figure 18. Catalytic cycle and energy diagram for the carbonylation of methane on photochemically generated RhCl(PH3)2 sites. Constructed from refs 110 and 111.
The oxygen atoms generated during the dissociation presumably transfer to the aluminum oxide support. Rasko´ and Solymosi109 reported that the photoinduced dissociation of CO2 on Rh/TiO2 surfaces is mediated by filling surface vacancy sites with oxygen atoms from CO2. A similar process may exist here with oxygen atoms filling vacant anion sites in the Al2O3 surface. 3. Summary The studies presented here have shown clearly that the coordinatively unsaturated Rh site produced by photoexcitation of RhI(CO)2/Al2O3 species is capable at low temperatures of activating strong chemical bonds, such as C-H bonds in alkanes, the HsH bond in H2, the OdO bond in O2, and the CdO bond in CO2. These studies have provided a close linkage of concepts from homogeneous organometallic chemistry to phenomena on surfaces, particularly in the comparisons between CsH bond activation processes. While the use of photoactivation for (109) Rasko´, J.; Solymosi, F. J. Phys. Chem. 1994, 98, 7147.
achieving chemical reactions is prohibitively expensive, these studies may provide mechanistic insight into processes which occur on surfaces as a result of thermal activation. Extension of the concepts discovered in the heterogeneous photochemistry can be envisionedsnamely to activation of other types of chemical bonds and the use of other metal centers. In addition, extensions along theoretical grounds will occur, such as, for example, the recent study using DFT methods for the conversion of methane into acetaldehyde using the RhCl(CO)(PH3)2 photocatalyst shown in Figure 18.110,111 Acknowledgment. We acknowledge with thanks the support of the Department of EnergysOffice of Basic Energy Sciences. LA9810197 (110) Margl, P.; Ziegler, T.; Blo¨chl, P. E. J. Am. Chem. Soc. 1995, 117, 12625. (111) Margl, P.; Ziegler, T.; Blo¨chl, P. E. J. Am. Chem. Soc. 1996, 118, 5142.
