Structure Sensitivity in the Kinetics and the Dynamics of CO Oxidation over Stepped Pd(335) Studied by the Molecular Beam Infrared Chemiluminescence Technique: Determination of Working Sites during the Steady-State Reaction

Abstract

Kinetics and dynamics of CO oxidation have been studied on a stepped Pd(335) surface at a steady-state condition and compared with those on flat Pd(111). The infrared (IR) chemiluminescence technique was applied to determine where the active catalytic sites are on the Pd(335) surface. Since the vibrational energy state of the product CO2 is sensitive to the structures of the reaction sites on Pd surfaces, information about the working reaction sites during the steady-state CO oxidation can be obtained from the IR emission spectra of the product CO2. The production rate of CO2 was higher on Pd(335) than on Pd(111), indicating that the steps on the surface enhance the catalytic activity for CO oxidation under the steady-state condition. However, the rate data do not necessarily show the real active sites for the CO + O recombination reaction. At a surface temperature of 850 K, the vibrational Boltzmann temperature (TV) of the product CO2 on Pd(335) was quite different from (much lower than) that on Pd(111), although the Pd(335) surface has four-atom wide (111) terraces. The lower TV value on Pd(335) was similar to that on Pd(110)(1 × 1), indicating that a relatively linear activated CO2 complex was formed. Therefore, during the steady-state CO oxidation on Pd(335), the reaction does not take place on the (111) terrace sites, but mostly on the step sites at 850 K. On the contrary, as the CO coverage increased at a lower surface temperature and at a high CO/O2 ratio, the TV values on Pd(335) tend to approach those on Pd(111), indicating that the contribution of the active sites on the steps is decreased and the working reaction sites shift to the (111) terrace sites.