THERMAL EXCITATION OF OXYGEN SPECIES AS A TRIGGER FOR THE CO OXIDATION ON PT(111)

Thermal excitation of adsorbed oxygen species is found to initiate the CO oxidation on Pt(111). We have prepared three different coadsorptio n systems to study the reactivity of different oxygen species; (1) CO on the O-2 preadsorbed Pt(111) surface, (2) CO on the nearly perfect P t(111) p(2x2)-O surface, and (3) CO on the disordered atomic oxygen-pr eadsorbed Pt(111) surface. Four CO2 desorption peaks (alpha-CO2 at 125 K, beta(3)-CO2 at similar to 225 K, beta(2)-CO2 at similar to 260 K, and beta(1)-CO2 at 320 K) are observed. The desorption temperatures of CO2 strongly depend on the adsorbed states of oxygen species. We have shown that the alpha-CO2 state, beta(2,3)-CO2 states, and beta(1)-CO2 State are correlated with adsorbed O-2, disordered oxygen atoms, and p(2 X 2) oxygen atoms, respectively. The difference in CO2 desorption temperature is related to thermal excitation of each oxygen species, w hich is derived from the structural information of coadsorbed states d uring thermal evolution by means of low-energy electron diffraction an d infrared reflection absorption spectroscopy. (C) 1995 American Insti tute of Physics.