Chapter 9
Control of the Methanol Reaction Pathway by Oxygen Adsorbed on Mo(112) Tetsuya Aruga, Ken-ichi Fukui, and Yasuhiro Iwasawa
Downloaded by UNIV LAVAL on June 18, 2014 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch009
Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan
The effect of oxygen adatoms on the reaction path of methanol on the Mo(112) surface has been examined in relation to the genesis of solid catalysis as well as the creation of new active surfaces. It has been found that the formation of a p(1x2)-O layer results in a new methanol dehydrogenation path, which differs from the oxidative dehydrogenation usually observed on molybdenum oxides. The CO adsorption experiment indicated that half the first-layer Mo atoms on the Mo(112)-p(1x2) surface are completely blocked while the rest are almost free from the electronic effect of oxygen modifiers, suggesting that the new dehydrogenation path is due to the selective blocking of the second-layer Mo atoms, leaving one-dimensional rows of bare Mo atoms. It has been one of the long-sought goals of the surface chemistry to optimize the structure and electronic properties of catalyst surfaces for particular catalytic reactions. To this end, considerable efforts have been devoted to achieve a full understanding of microscopic principles of the catalysis. Practically, a complete set of techniques for surface modification should be established to modify the catalyst surfaces and control the reaction paths. In order to establish reliable means to modify the electronic properties and steric confinement of the surface, we have examined the modification of the Mo(112) surface (Figure 1) by atomic oxygen. Molybdenum, both in metallic and oxide forms, is used in many industrial catalysts. This is partly because molybdenum exhibits a wide range of chemical reactivity according to its various oxidation states. MoO and iron/molybdenum oxides are used as industrial catalysts for methanol oxidation to form formaldehyde selectively. The iron/molybdenum oxide catalyst consists of Fe (MoO ) and MoO , and shows kinetics and selectivity similar to that of MoO (1), suggesting that Mo-0 sites play a dominant role in the methanol oxidation. MoO has a layered structure along the (010) plane. The (010) surface is not chemically active because there are no dangling bonds and unsaturated Mo atoms. Actually, Sleight et al. (2) studied methanol adsorption on the (010) surface of a MoO single crystal and found that no methanol chemisorbs on the MoO (010) surface. On the other hand, methanol is decomposed completely to CO and H on metallic Mo (3,4), suggesting oxidative dehydrogenation of methanol occurs on partially oxidized Mo sites or defect sites. It would be interesting if the active site for the selective dehydrogenation of alcohols can be prepared on well-defined crystal surfaces. 3
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0097-6156/93/0523-0110S06.00/0 © 1993 American Chemical Society In Catalytic Selective Oxidation; Oyama, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
9. ARUGAETAL.
111
Control of Methanol Reaction Pathway
Molybdenum has a body-centered cubic lattice, and its (112)-(lxl) surface is composed of densely packed W _ = 2.73 Â) atomic rows separated by 4.45 Â from each other. Oxygen atoms are expected to occupy trough sites and hence the first-layer atoms are accessible for gas-phase molecules, allowing directly probing the electronic effect of oxygen adatoms on the first-layer Mo atoms by the adsorption of simple molecules. The oxygen adsorption on Mo(112) results in the successive formation of ordered structures as a function of oxygen coverage. Upon annealing the Mo(l 12) surface with a very high coverage of oxygen atoms, Mo0 grows epitaxially (5). The modification of Mo(l 12) by oxygen adatoms will change the methanol chemistry drastically, which provides a model for the industrial catalysts for methanol dehydrogenation. In the present chapter, we will survey the experimental findings on the modification of Mo(112) by submonolayer-coverage oxygen adatoms and its effects on the reaction of methanol. The results presented here indicate that the selective blocking of the second-layer Mo atoms results in a novel dehydrogenation reaction of methanol on this surface. This dehydrogenation reaction (CH OH —> H CO + PL) differs from the oxidative dehydrogenation reaction (CH OH + Ο -» HLCO + H 0) observed for molybdenum oxides. We also discuss the reaction scheme for methanol dehydrogenation on oxygen-modified Mo(l 12) surfaces. M o
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Downloaded by UNIV LAVAL on June 18, 2014 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch009
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Ο adatom Figure 1. Oxygen on Mo(l 12)
Preparation of Ordered Oxygen Overlayers on Mo(112) The clean Mo(112) surface exhibits a sharp p(lxl) pattern in low-energy electron diffraction (LEED), indicating that the surface preserves the bulk structure as shown in Figure 1. The exposure of the clean surface to oxygen at room temperature, followed by annealing to 600 K, results in a series of ordered structures as observed by LEED (3) and sometimes the surface was facetted, while the Q
In Catalytic Selective Oxidation; Oyama, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
112
CATALYTIC SELECTIVE OXIDATION
p(lx2)-0 surface of 0 =1.O remained unchanged even after annealing to 1000 K. The oxygen modified surfaces of 0
