Interest in the surface chemistry of late transition-metal oxides has been stimulated by observations that the formation of metal oxide layers tends to dramatically alter the catalytic performance of transition metals in applications of oxidation catalysis. For example, in situ investigations of CO oxidation over single crystal surfaces of the Pt-group metals have revealed significant changes in surface reactivity as a function of the oxidizing conditions, with these changes attributed to the formation of distinct types of metal oxide layers or surface oxygen phases. In this talk, I will discuss results of recent experiments in which we used reflection absorption infrared spectroscopy (RAIRS) and mass spectrometry to investigate the oxidation of CO on well-defined �surface� and �bulk� Pd oxide surfaces that were grown on Pd(111) in ultrahigh vacuum using oxygen atom beams. We find that both the Pd5O4 surface oxide and a multilayer PdO(101) film are reactive toward CO, but the PdO(101) surface is significantly more reactive than the surface oxide.We investigated the evolution of CO binding states on Pd oxide surfaces during both temperature programmed reaction (TPR) and isothermal reaction experiments. We find that RAIRS can distinguish CO bound on distinct types of surface sites that develop as the Pd oxide surfaces undergo reduction during reaction with CO, including oxygen vacancies and sites associated with metallic domains as well as the surface vs. bulk oxides. During TPR experiments, reaction of a single monolayer of CO on PdO(101) initially produces oxygen vacancies which agglomerate to form metallic domains over a narrow temperature range near 400 K. Further heating above about 450 K causes the metallic domains to be oxidized by oxygen from the subsurface, thereby regenerating PdO(101) surface sites before CO reaction or desorption is complete. We observe a different evolution of surface sites and phases when Pd oxide surfaces react isothermally in a constant CO background. At 400 K, CO oxidation on the PdO(101) film initially produces surface oxygen vacancies and the CO2 production rate increases toward a maximum. After the rate maximum, RAIRS reveals the loss of PdO(101) domains and the formation of coexisting metallic and Pd5O4-like domains. The Pd5O4 structure remains on the surface once the CO oxidation rate drops to zero, thus demonstrating that the surface oxide is less reactive toward CO than PdO(101) for the conditions studied. Lastly, I will also discuss results of DFT calculations that have identified many of the IR features for CO bound on specific sites of the Pd5O4 and PdO(101) surfaces. Overall, our results demonstrate the dynamic nature of Pd oxide surfaces during reaction with CO, and provide new information for understanding elementary processes which occur during CO oxidation on these surfaces, including IR signatures for CO bound on specific Pd oxide structures.

IR investigations of CO oxidation on well-defined Pd oxide surfaces

247th National Meeting (2014)

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