CHEMISORPTION OF CO AND H-2 ON CLEAN AND OXYGEN-MODIFIED MO(112)

Chemisorption of CO and H-2 on clean and oxygen-modified Mo(112) has been investigated by low-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and temperature-programmed desorption (TPD). Oxygen adatoms are expected to be adsorbed in trough sites between close-packed Mo rows and, hence, the first-layer Mo atoms remain accessible to gas-phase molecules. Oxygen adsorption on a clean Mo(112) surface at 300 K, followed by annealing to 600 K, has produced a series of LEED patterns with increasing oxygen coverage. Models for these structures are presented. These ordered oxygen adlayers are stable up to 1100 K and have been used to examine the effect of oxygen modification on the adsorption and dissociation of CO and H-2. Both CO and H-2 molecules are dissociated on clean Mo(112). The amount of dissociated species decreases linearly with oxygen coverage and is suppressed completely on a p(2 x 1)-O surface (theta(O) = 0.5). The structure of the Mo ensemble required for the dissociation of CO and H-2 is discussed. The desorption peaks of molecular CO show characteristic changes as a function of oxygen coverage. These observations made it possible to discuss the local environment of the first-layer Mo atoms bonded with CO. The spatial extent and the strength of the electronic modification effect of oxygen adatoms is also discussed. We also found that the Mo(112) surface undergoes an hydrogen-induced surface reconstruction to form a p(1 x 2) structure. The hydrogen coverage corresponding to the completion of p(1 x 2) has been determined to be 1.5.