Reaction mechanism and structure of activated complex of CO2 formation in CO oxidation on Pd(1 1 0) and Pd(1 1 1) surfaces

The infrared (IR) chemiluminescence spectra of CO2 were measured during steady-state CO oxidation over Pd(1 1 0) and Pd(1 1 1). CO2 formation rate always gave a maximum in the surface temperature dependence, and the maximum temperature was denoted as TSmax. Analyses of IR emission spectra at surface temperatures higher than TSmax showed that the bending vibrational temperature (TVB) was higher for Pd(1 1 1) than for Pd(1 1 0). These results suggest that the activated CO2 complex of CO2 formation is more bent on Pd(1 1 1), and straighter on Pd(1 1 0). On the other hand, at surface temperatures lower than TSmax, antisymmetric vibrational temperature (TVAS) was much higher than TVB on both Pd surfaces, suggesting that the activated complex is much straighter at the lower surface temperatures. Considering adsorption sites and their energy levels, the model of the activated complex of CO2 formation at the transition state is discussed and proposed.