ESR studies of surface adsorption and catalysis under ultra-high vacuum conditions. Progress report, March 1, 1980-February 29, 1981

Abstract

This project is directed to developing ESR for the study of surface adsorption and catalysis on clean well-characterized metallic and oxide surfaces under ultra-high vacuum (UHV) conditions and to compare the results with real surfaces, which are important in actual catalysis. Results with ESR-UHV system on the NO/sub 2//Cu system showed that only in the presence of H/sub 2/O can the surface process(es) proceed that lead to stable Cu/sup 2 +/ complexes which are observable by ESR. By monitoring the CREMSEE (cyclotron-resonance from microwave-induced secondary electron emissions) threshold, definite evidence for the oxidation of the Cu surface by the clean NO/sub 2/ has been obtained. In real systems, as opposed to the clean surfaces prepared under UHV conditions, small amounts of H/sub 2/O are usually present. The UHV studies permitted a demonstration of the reactivity requirements. Detailed studies of the oxidation process of NO/sub 2/ on Cu have been performed. A newer ESR-UHV system with better expected performance has been built. An extensive study of motional dynamics of free radicals on supported surfaces has been completed. These include the O/sub 2//sup -/ radical absorbed on Ti supported on crushed vycor and NO/sub 2/ adsorbed on crushed vycor. The O/sub 2//sup -/ is found to exhibit planar rotation about an axis perpendicular to the internuclear axis of the O/sub 2//sup -/ and parallel to the normal to the plane. The higher temperature motion may be fit with a small activation energy of 0.5Kcal/mole, while the lower temperature motion shows anomalies. The NO/sub 2/ exhibits planar-rotation at the lower temperature, but above 77/sup 0/K it becomes more nearly isotropic presumably due to translational diffusion. Preliminary experiments imply that addition of acetylene increases the rate of rotational motion of the O/sub 2//sup -/. Preliminary studies leading to time-resolved UHV-ESR experiments of photo-induced catalysis have been performed with the new excimer laser.