Langmuir 1998, 14, 1451-1457
1451
Selective and Nonselective Dehydrogenation in Primary Alcohols: Reactions of Ethanol and 1-Propanol on Co-covered Mo(110) D. A. Chen and C. M. Friend* Department of Chemistry, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138 Received July 3, 1997. In Final Form: January 9, 1998
The reactions of ethanol and 1-propanol on a 1.3 monolayer Co overlayer was investigated in an effort to understand the reactivity of Co thin films deposited on Mo(110). Electron energy loss data show that O-H bond scission to afford the alkoxide occurs by 275 K for both alcohols on a Co monolayer deposited on Mo(110). Carbon monoxide and H2 are the major gaseous products identified by temperature-programmed reaction, and residual carbon and oxygen are detected after heating to 760 K. In addition to CO and H2, ethanol reaction produces acetaldehyde, methane, and ethylene, while 1-propanol reaction produces propanal, ethylene, and propene. Selective deuteration at the β position precludes aldehyde production, demonstrating that β C-H bond cleavage is the rate-limiting step for aldehyde formation and that there is a delicate balance between the nonselective and selective dehydrogenation pathways. Isotopic labeling also shows that methane production from ethanol and ethylene from 1-propanol occur by selective β C-C bond scission. The reactions of ethanol and 1-propanol on the Co overlayers are compared with those of 2-propanol and methanol. The activity of Co thin films on Mo(110) for both selective and nonselective dehydrogenation allows alcohol reaction pathways to be controlled by minor changes in structure and bonding.
Introduction The reactions of alcohols has been well studied on transition metal surfaces due to the importance of oxygencontaining intermediates such as alkoxides in various industrial processes catalyzed by transition metals. For example, a Co-Mo-Al2O3 catalyst is used for commercial deoxygenation reactions, and Co itself is also an important Fischer-Tropsch catalyst in the synthesis of methanol from CO and hydrogen. Multimetallic systems are known to have superior catalytic properties over their singlemetal counterparts, but the origin of these improved properties is poorly understood. Since the commercial catalysts are extremely complicated materials, we have created a model bimetallic surface by depositing ordered Co overlayers on Mo(110) so that the structure, bonding, and reactivity of these oxygen-containing surface species can be studied on a fundamental level. We are particularly interested in investigating whether the thin Co films exhibit reactivity characteristic of bulk mid-transition metals or whether Co-Mo interactions modify the chemistry of the Co surface. The growth of Co overlayers on Mo(110) has been previously characterized.1-5 On the basis of low-energy electron diffraction (LEED) studies, the Co overlayer adopts the lattice structure of the Mo(110) substrate at coverages below 1 monolayer (ML), followed by lateral compression of the Co atoms to achieve the lattice constant of bulk Co(0001) at a coverage of 1.3 ML. Previous studies of alcohol reactions on Co overlayers as a function of Co coverage have established that alcohol chemistry is not (1) Tikhov, M.; Bauer, E. Surf. Sci. 1990, 232, 73-91. (2) He, J.-W.; Goodman, D. W. Surf. Sci. 1991, 245, 29-40. (3) Chen, D. A.; Friend, C. M.; Xu, H. Langmuir 1996, 12, 15281534. (4) Chen, D. A.; Friend, C. M. Surf. Sci. 1997, 371, 131-142. (5) Chen, D. A.; Friend, C. M. J. Phys. Chem. 1996, 100, 1764017647.
sensitive to the Co coverage and that Co-Mo interactions do not give rise to new product formation.5,6 In this study, we show that the reactions of ethanol and 1-propanol on the Co thin films are similar to those observed on bulk mid-transition-metal surfaces. The reactions of alcohols have not been investigated on bulk Co surfaces,7 but the chemistry on Co is expected to be qualitatively similar to its the neighbors in the periodic table, such as Fe,8 Rh,9-11 and Ni.12,13 In these cases, as well as for Pd14,15 and Pt,16 the major reaction pathwayssnonselective C-H bond scission to produce CO and selective C-H bond scission to produce aldehydes or ketonessinvolve C-O bond retention. Retention of the C-O bond in alcohol reactions on mid-transition-metal surfaces is attributed to the intermediate metal-O bond strength (∼90 kcal/mol); in contrast, the strong metal-O bond formed on Mo(110) (∼130 kcal/mol) results in alcohol reaction via C-O bond scission.17,18 As predicted based on the Co-O bond strength (∼90 kcal/mol), the C-O bond is retained in reaction on the 1.3 ML Co overlayer, yielding oxygen-containing products such as CO and aldehydes. The C-O bond scission products are believed to be (6) Chen, D. A.; Friend, C. M. J. Phys. Chem. B 1997, 101, 57125716. (7) Co(0001) undergoes a low-temperature phase transition from the hexagonal close-packed to the face-centered cubic structure, and therefore single-crystal Co is a difficult material to study. (8) Benziger, J. B.; Madix, R. J. J. Catal. 1980, 65, 36-48. (9) Houtman, C. J.; Barteau, M. A. J. Catal. 1991, 130, 528-546. (10) Brown, N. F.; Barteau, M. A. Langmuir 1992, 8, 862-869. (11) Houtman, C.; Barteau, M. A. Langmuir 1990, 6, 1558-1566. (12) Johnson, S. W.; Madix, R. J. Surf. Sci. 1982, 115, 61-78. (13) Gates, S. M.; Russell, J. N.; Yates, J. T. Surf. Sci. 1986, 171, 111-134. (14) Davis, J. L.; Barteau, M. A. Surf. Sci. 1987, 187, 387-406. (15) Davis, J. L.; Barteau, M. A. Surf. Sci. 1990, 235, 235-248. (16) Sexton, B. A.; Rendulic, K. D.; Hughes, A. E. Surf. Sci. 1995, 121, 181-198. (17) CRC Handbook of Chemistry and Physics, 69th ed.; CRC Press, Inc.: Boca Raton, FL, 1989. (18) Bond strengths are for diatomic molecules in the gas phase.
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1452 Langmuir, Vol. 14, No. 6, 1998
Chen and Friend
Chart 1
associated with reaction at defects in the Co overlayer which leave Mo sites exposed. The reaction selectivity of ethanol and 1-propanol can be altered by isotopic labeling since deuterium substitution at the β position prevents selective C-H bond scission to form aldehydes (Chart 1, carbon adjacent to the oxygen). Reaction by C-O bond retention was also observed for 2-propanol and methanol on the Co overlayers, which have been previously investigated.5,6 The major difference in the chemistry of methanol, 2-propanol, ethanol, and 1-propanol reported herein is the relative importance of the selective and nonselective C-H bond breaking pathways. For 2-propanol reaction, the two pathways are equally significant, but for ethanol and 1-propanol, the decomposition pathway dominates. Furthermore, β C-C bond scission occurs more rapidly than C-H bond scission in the primary alcohols, yielding methane from ethanol and ethylene from 1-propanol, whereas no C2 products are observed from the reaction of 2-propanol. Experimental Section All experiments were performed in an ultrahigh vacuum chamber, which has been described previously, with a base pressure of
