A study of CO oxidation on Pt-group model catalysis systems utilizing ambient-pressure X-ray photoelectron spectroscopy

Abstract

The Pt (110) and Pd (100) single crystals have been intensively investigated as model system for CO oxidation reaction under elevated pressure condition. While people agreed on the presence of structural changes during the reaction, a correlation between surface structures and its chemical properties was not made in general sense. Among various in-situ operando experimental tools, an ambient pressure X-ray photoelectron spectroscopy (AP-XPS) is an ideal choice for examining the surface chemical states under the reaction conditions, which can eventually deliver the critical information for deciphering the nature of surface catalytic reaction. In this thesis, utilizing AP-XPS, the CO oxidation reaction on Pt (110) and Pd (100) surfaces are investigated under elevated pressure conditions. First, the surface chemical composition at different gas composition is in- vestigated on Pt (110) surface. Under the oxygen-rich condition (ratio of CO to O2 ~ 0.2), the CO oxidation reaction on Pt (110) was enhanced as the temperature reached at the 553 K. At the onset of the reaction, Pt (110) surface showed both oxidic and metallic surfaces, simultaneously. As the CO/O2 ratio increased, the produc tion of CO2 increased continuously. At the diffusion limited regions, the amount of surface oxide change d little while the chemisorbed oxygen was being reduced. Second, the surface chemical states of Pd (100) were investigated under oxygen-rich conditions. The formation of the surface oxide was clearly observed at the onset of CO oxidation reaction at T = 525 K. As the reactant ratio (CO/O2) increased from 0.1 to 1.0, a majority (~ 90 %) of surface oxides remained on surface during the reaction. Upon the formation of surface oxides, the shift of oxygen gas phase peak was observed, indicating that modification of work function on surface. Interestingly, as surfaces making a transition from CO covered surface to the oxidic surface at the onset of the reaction, the work functions of surface oxides on Pd (100) and Pt (110) showed the opposite behavior. Lastly, in order to apprehend the nature of work function on surface oxides, the surface work function on Pt (110) during the reaction condition was measured and compared to the result of DFT calculation. In experiment, both the cut off position of low energy electron of XPS and the binding energy position of the non-reactant gas phase in AP-XPS were utilized to estimate surface work function. Both experimental and theoretical results showed that surface oxides of Pt (110) under elevated pressure condition displayed significantly higher values of surface work function than the previously reported values which were measured under low pressure condi- tion, e.g., photoemission electron microscopy.