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J. Phys. Chem. B 1997, 101, 5712-5716
Competition between C-O Bond Scission and Retention in Methanol Reaction on Co-Covered Mo(110) D. A. Chen and C. M. Friend* Department of Chemistry, HarVard UniVersity, 12 Oxford Street, Cambridge, Massachusetts 02138 ReceiVed: March 13, 1997; In Final Form: May 9, 1997X
Methanol reaction on Co overlayers (1.0-1.3 ML) deposited on Mo(110) yields CO and H2 as the gaseous products. Isotopic labeling and vibrational studies show that molecular methanol desorption around 210 K competes with O-H bond scission to form methoxide. Methoxide is identified as the major surface intermediate at 250 K using electron energy loss spectroscopy. Vibrational data also indicate that methoxide decomposes to adsorbed CO by 350 K. The formation of C-O bond retention products such as CO on the Co overlayers is consistent with the intermediate Co-O bond strength and is characteristic of methanol reaction on other mid transition metal surfaces. However, C-O bond dissociation to produce atomic carbon and oxygen accounts for ∼54% of the total methanol reaction. The C-O bond dissociation pathway is attributed mainly to reaction at defects on the overlayer, based on comparison with studies of CO on Co(0001). The dependence of the CO yield on Co coverage suggests that there is no special reactivity associated with mixed Co-Mo sites that gives rise to new product formation. Defects in the Co overlayers exhibit a higher activity for C-O bond dissociation, suggesting that the reactivity of Co itself is structure-sensitive.
Introduction The reactions of alcohols on transition metal surfaces have been investigated in detail because of their importance in deoxygenation, partial oxidation, and Fischer-Tropsch catalysis. Although many of these commercial processes employ Cocontaining multimetallic catalysts, the superior properties of multimetallic over single-metal catalysts are not well understood.1-3 In this work, we have investigated the reactivity of methanol on Co overlayers deposited on Mo(110) in order to develop a fundamental understanding of oxygenate chemistry on bimetallic surfaces. We are specifically interested in determining whether C-O bond retention is favored on Co overlayers and whether there is any special reactivity associated with mixed Co-Mo sites. The Co-on-Mo(110) system is ideal for these investigations, since it has been studied in detail previously.4-7 At coverages below 1.0 monolayer (ML), the Co atoms are believed to adopt the Mo lattice structure so that a pseudomorphic layer with a Co-Co lattice spacing of 2.73 Å by 3.15 Å is formed. As the Co coverage is increased to 1.3 ML, the Co atoms compress such that the Co-Co lattice spacing is decreased to that of bulk Co (2.51 Å). Thus, the greatest number of mixed Co-Mo sites are found at low Co coverages, and a more closely packed Co overlayer with a structure similar to the (0001) plane of bulk Co exists at 1.3 ML. By the investigation of methanol reactivity as a function of Co coverage, the sensitivity of chemical reactivity to the nature of these sites is tested. Previous studies of methanol on Mo(110) have shown that the primary reaction pathway on Mo(110) is low-temperature O-H bond dissociation to form methoxide followed by C-O bond scission to yield gaseous H2 and adsorbed carbon and oxygen at ∼300 K.8 The C-O bond dissociation is driven by the strong Mo-O bond enthalpy of approximately 130 kcal/ mol.9,10 The chemistry of methanol on the continuous Co overlayers is similar to the reactivity on bulk mid transition metal surfaces. On the 1.3 ML Co overlayer approximately 46% of the methanol X
Abstract published in AdVance ACS Abstracts, June 15, 1997.
S1089-5647(97)00941-3 CCC: $14.00
reacts via C-O bond retention, yielding gaseous CO at 430 K. The evolution of CO is generally consistent with the intermediate Co-O bond strength (∼90 kcal/mol) and is similar to the reactions on other metals with intermediate metal-O strengths, such as Fe(110),11 Fe(100),12 Rh(111),13,14 Pd(111),15 Ni(100),16 Ni(110),17 Ni(111),18 and Pt(111).19 Decomposition to atomic carbon and oxygen also occurs and is partially attributed to the dissociation of adsorbed CO. Electron energy loss and temperature programmed reaction studies show that methanol desorption competes with O-H bond breaking at ∼200 K and that adsorbed CO is first formed from dehydrogenation of methoxide around 300 K. Furthermore, there is no special reactivity associated with the mixed Co-Mo sites present at low Co coverages, since no new products are formed. Experimental Section Experiments were performed in two separate ultrahigh vacuum chambers, which have been described previously,20,21 with base pressures of 85%) are reported.52 This change in selectivity was attributed to changes in Ni electronic structure due to lattice strain. We intend to use density functional calculation to investigate the possible effects of lattice strain on electronic structure in the Co on Mo(110) system. Conclusions Methanol reaction on 1.0-1.3 ML Co overlayers produces CO and H2, which are the products expected from reaction on bulk Co, based on the Co-O bond strength. C-O bond scission to produce atomic carbon and oxygen also accounted for ∼54% of the total methanol reaction. No new products were formed as a result of Co-Mo interactions at submonolayer Co coverages. Methoxide is identified as the intermediate at 250 K, and O-H bond scission and methanol desorption are competing processes below this temperature. Acknowledgment. We gratefully acknowledge the support of this work by the U.S. Department of Energy, Office of Basic Energy Sciences, Grant No. DE-FG02-84-ER13289. References and Notes (1) Prins, R.; de Beer, V. H. J.; Somorjai, G. A. Catal. ReV. Sci. Eng. 1989, 31, 1-41. (2) Grange, P. Catal. ReV. Sci. Eng. 1980, 21, 135-181. (3) Furimsky, E. Catal. ReV. Sci. Eng. 1983, 25, 421-458. (4) Tikhov, M.; Bauer, E. Surf. Sci. 1990, 232, 73-91. (5) He, J.-W.; Goodman, D. W. Surf. Sci. 1991, 245, 29-40. (6) Kuhn, W. K.; He, J.-W.; Goodman, D. W. J. Vac. Sci. Technol. A 1992, 10, 2477-2486. (7) Chen, D. A.; Friend, C. M.; Xu, H. Langmuir 1996, 12, 15281534. (8) Weldon, M. K.; Uvdal, P.; Friend, C. M.; Serafin, J. G. J. Chem. Phys. 1995, 103, 5075-5084. (9) The metal-oxygen bond strengths are for diatomic molecules in the gas phase. (10) CRC Handbook of Chemistry and Physics, 69th ed.; CRC Press, Inc.: Boca Raton, FL, 1989. (11) Rufael, T. S.; Batteas, J. D.; Friend, C. M., Surf. Sci., in press. (12) Albert, M. R.; Lu, J.-P.; Bernasek, S. L.; Dwyer, D. J. Surf. Sci. 1989, 221, 197-213. (13) Houtman, C.; Barteau, M. A. Langmuir 1990, 6, 1558-1566. (14) Solymosi, F.; Berko, A.; Tarnoczi, T. I. Surf. Sci. 1984, 141, 533548. (15) Davis, J. L.; Barteau, M. A. Surf. Sci. 1987, 187, 387-406. (16) Benziger, J. B.; Madix, R. J. J. Catal. 1980, 65, 36-48. (17) Bare, S. R.; Stroscio, J. A.; Ho, W. Surf. Sci. 1985, 150, 399-418. (18) Russell, J. N.; Chorkendorff, I.; Yates, J. T. Surf. Sci. 1987, 183, 316-330. (19) Sexton, B. A.; Rendulic, K. D.; Hughes, A. E. Surf. Sci. 1995, 121, 181-198. (20) Roberts, J. T.; Friend, C. M. J. Am. Chem. Soc. 1986, 108, 72047210. (21) Uvdal, P.; Wiegand, B. C.; Serafin, J. G.; Friend, C. M. J. Chem. Phys. 1992, 97, 8727-8735.
Chen and Friend (22) Chen, D. A.; Friend, C. M. Surf. Sci. 1997, 371, 131-142. (23) Chen, D. A.; Friend, C. M. J. Phys. Chem. 1996, 100, 1764017647. (24) An “(8 × 2)” notation for this Co overlayer has been previously reported by Bauer et al.4 and Goodman et al.53,54 However, a more detailed analysis of the LEED pattern shows that the overlayer actually has a 4 4h structure.33 1 1 (25) Bridge, M. E.; Comrie, C. M.; Lambert, R. M. Surf. Sci. 1995, 67, 393-404. (26) Bridge, M. E.; Comrie, C. M.; Lambert, R. M. J. Catal. 1979, 58, 28-33. (27) These experiments were performed with C18O in order to eliminate contribution from background CO. (28) Papp, H. Surf. Sci. 1985, 149, 460-470. (29) Prior, K. A.; Schwaha, K.; Lambert, R. M. Surf. Sci. 1978, 77, 193-208. (30) Jagannathan, K.; Srinivasan, A.; Hedge, M. S.; Rao, C. N. R. Surf. Sci. 1980, 99, 309-319. (31) Johnson, B. G.; Berlowitz, P. J.; Goodman, D. W. Surf. Sci. 1989, 217, 13-37. (32) These experiments were performed with 13CH3OH in order to eliminate contribution from background CO. (33) Jentz, D. W.; Clark, P. G.; Friend, C. M. In preparation. (34) McBreen, P. H.; Erley, W.; Ibach, H. Surf. Sci. 1983, 133, L469L474. (35) Rodriguez, J. A.; Goodman, D. W. Surf. Sci. Rep. 1991, 14, 1-107. (36) The modes at 553 cm-1 (Figure 3b) and 546 cm-1 (Figure 3d) for methoxide-d0 and -d4, respectively, are attributed to ν(Mo-O), since it is possible that a small amount of methoxide decomposes to atomic oxygen. However, these modes are not readily detectable in the spectrum of a higher methoxide coverage (Figure 4a), where the signal intensities of the methoxide peaks are relatively more intense. (37) For example, when a 3:1 ratio of methanol and CO is heated to 250 K on a 1.0 ML Co overlayer, the ν(CO) frequency is 1992 cm-1 compared to 1978 cm-1 for CO produced from heating methanol to 350 K. (38) The 330 K onset temperature for CO formation on the 0.5 ML Co overlayer represents a lower limit because the overtone of ν(C-O) from methoxide at ∼2000 cm-1 causes difficulty in detecting the exact onset temperature of CO production. Furthermore, less adsorbed CO is produced on the 0.5 ML Co overlayer, and therefore, the weak ν(CtO) signal may not be detectable at 300 K. (39) Gates, S. M.; Russell, J. N., Jr.; Yates, J. T., Jr. Surf . Sci. 1984, 146, 199-210. (40) Johnson, S. W.; Madix, R. J. Surf. Sci. 1981, 103, 361-396. (41) For example, CO does not dissociate on Ni(111),55 Ni(100),56 Rh(111),57 Pd(111),55 or Pt(111),55 and methanol does not react by C-O bond scission on these surfaces either.13,15,18,19,58 Furthermore, the C-O bond scission pathway accounts for only a very minor amount (
