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Abstract

An in situ study of the adsorption of CO on atomically smooth and defect Pd(111) surfaces was performed over wide ranges of temperatures (200-400 K) and pressures (10(-6)-1 mbar) by X-ray photoelectron spectroscopy and sum frequency generation. Both of the techniques indicated that CO was adsorbed as threefold hollow, bridging, and terminal species to form well-known ordered structures on the surface. In the course of the in situ experiments, no signs of CO dissociation or of the formation of carbonyl structures (Pd(CO)(n), n > 1) were detected. The mechanism of C-O bond activation in the course of methanol decomposition on the surface of palladium was considered. It was found that the adsorption of methanol on the surface of palladium essentially depends on pressure. Along with the well-known reaction path of methanol dehydrogenation to CO and hydrogen, a slow process of methanol decomposition with C-O bond cleavage was observed at elevated pressures. In this case, the formation of carbon deposits finally resulted in the carbonization and complete deactivation of the surface. A mechanism for C-O bond activation on the surface of palladium was proposed; the geometry of adsorption complexes plays an important role in this mechanism.