Bonding and chemistry of small molecules chemisorbed on atomically-stepped metal surfaces.

摘要

The chemistry of carbon monoxide (CO) and dinitrogen (N2) on atomically-stepped Pt and Ru single crystal surfaces was studied by reflection-absorption infrared spectroscopy, Auger electron spectroscopy, low-energy electron diffraction, and mass-spectroscopy of adsorption-desorption kinetics.; Adsorption of molecular N2 on Pt surfaces at 90 K was shown to be highly structure-sensitive. Using stepped Pt(335) and Pt(779) surfaces, it was demonstrated that N2 chemisorption occurs exclusively at the atomic steps. In these one-dimensional phases, N2 molecules are chemisorbed at approximately every other Pt step atom and are identical. The substrate-mediated chemical interaction between the molecules causes a coverage-induced downward shift of the internal vibrational frequency.; The step sites can be additionally reversibly populated under ambient flux of N2 gas. A fast exchange between adsorbed and incoming molecules occurs under these conditions. The adsorption of N2 molecules on the Pt step sites proceeds through the mobile precursor state on the atomic terraces and steps. The adsorption kinetic model is proposed.; On the atomically-stepped Ru(109) surface, the two-atom high steps energetically and/or sterically facilitate the thermal dissociation of adsorbed CO, which otherwise does not occur on the atomically-smooth Ru(001) surface. Produced at the Ru steps, the adsorbed C and O species are mobile near 520 K. This low-temperature mobile carbon species is likely to be an intermediate in the Fischer-Tropsch synthesis of hydrocarbons on Ru.; The close-packed terraces of the Ru(109) surface are 9--10 atoms wide. Compared to the infinitely-extended close-packed Ru surface, these finite size terraces accommodate CO adlayers differently: CO is adsorbed somewhat weaker, the maximum achievable coverage of CO is smaller, and the high-coverage adlayer superstructures do not form. It is proposed that the observed finite size effect is caused by the changes in the surface electronic structure due to lattice relaxations close to atomic steps.; The low-coordinated metal sites can be strongly affected by the adsorption process itself. The adsorption of CO on the supported metallic Rh clusters atomically disperses them to form RhI(CO)2 species. Using infrared and UV-visible spectroscopy, the formation of the Rh I ion during CO chemisorption on Rh/Al2O3 catalysts was directly observed, in agreement with previous postulates.