Paper SS-TuP20
Mechanisms of CO Oxidation on Well-defined Pd Oxide Films on Pd(111)

Tuesday, November 11, 2014, 6:30 pm, Room Hall D

The surface chemistry of late transition-metal oxides has attracted significant attention due largely to observations that the formation of metal oxide layers can drastically alter the catalytic performance of parent metals in applications of oxidation catalysis. For example, previous in situ studies have shown that CO oxidation over palladium undergoes a significant rate increase when the palladium surface becomes oxygen rich. This observation has been attributed to the formation of various types of palladium oxide layers or surface oxygen phases, though the reaction mechanisms have not been determined in detail. In this presentation, I will show results of recent experiments and density functional theory (DFT) calculations in which we studied CO oxidation on well-defined Pd oxide surfaces. In the experiments, we utilized mass spectrometry to investigate the CO oxidation kinetics on both surface and bulk Pd oxides during TPRS and under isothermal conditions and used reflection absorption infrared spectroscopy (RAIRS) to monitor the evolution of CO binding states and hence the formation of new surface phases as the oxides undergo reduction.

In direct rate measurements under isothermal condition, we find that the initial reaction rates are nearly ten times larger on PdO(101) compared with the Pd₅O₄ surface oxide, demonstrating that PdO(101) has a much higher intrinsic activity toward CO oxidation compared with the surface oxide. The measurements also show that the reactions occur on both oxide surfaces in an autocatalytic fashion, where the CO₂ generation rate increases as oxygen on the surface is consumed. The RAIRS data shows that the reduction of PdO(101) by CO initially creates oxygen vacancies and that CO preferentially binds to atop-Pd sites located next to the O-vacancies. This atop CO-species yields a characteristic IR peak between 2090 cm^-1 and 2060 cm^-1. DFT results agree very well with the measured C-O stretching frequencies, and further show that CO achieves significantly stronger binding on an atop-Pd site located next to an oxygen vacancy vs. on the pristine PdO(101) surface. In addition to oxygen vacancies, we also find that surface metal domains develop during the early stages of isothermal reaction of CO on PdO(101) and the Pd₅O₄ surface oxide, except that on the Pd₅O₄ surface CO reaction only leads to the creation of metallic domains without producing oxygen vacancies. Thus, the autocatalytic kinetic behavior observed for both oxides arises from a similar mechanism wherein surface reduction during CO oxidation continually creates sites that bind CO strongly and thus facilitate the adsorption and subsequent reaction of CO.