In Situ Polarization Modulation Infrared Reflection Absorption Spectroscopic and Kinetic Investigations of Heterogeneous Catalytic Reactions

Abstract

A molecular-level understanding of a heterogeneous catalytic reaction is the key goal of heterogeneous catalysis. A surface science approach enables the realization of this goal. However, the working conditions (ultrahigh vacuum (UHV) conditions) of traditional surface science techniques restrict the investigations of heterogeneous catalysis system under industrial working conditions (atmospheric pressures). Polarization Modulation Infrared Reflection-Absorption Spectroscopy (PM-IRAS) can be operated in both UHV and atmospheric pressure conditions with a wide temperature span while providing high resolution (4 cm-1 is used in this dissertation) spectra. In this dissertation, PM-IRAS has been employed as a major technique to: 1) obtain both electronic and chemical information of catalysts from UHV to elevated pressure conditions; 2) explore reaction mechanisms by in situ monitoring surface species with concurrent kinetic measurements. In this dissertation, NO adsorption and dissociation on Rh(111) have been studied. Our PM-IRAS spectra show a transition of NO adsorption on three-fold hollow sites to atop sites occurs at low temperatures (<275 K). NO dissociation is found to account for this transition. The results indicated the dissociation of NO occurs well below the temperature previously reported. Characterizations of highly catalytically active Au films have also been carried out. Electronic and chemical properties of (1 x 1)- and (1 x 3)-Au/TiOx/Mo(112) films are investigated by PM-IRAS using CO as a probe molecule. The Au overlayers are found to be electron-rich and to have significantly different electronic properties compared with bulk Au. The exceptionally high catalytic activity of the Au bilayer structure is related to its unique electronic properties. CO oxidation reactions on Rh, Pd, and Pt single crystals are explored from low CO pressures under steady-state conditions (less than 1 x 10-4 Torr) to high pressures (0.01-10 Torr) at various gaseous reactant compositions. Surface CO species are probed with in situ PM-IRAS to elucidate the surface phases under reaction conditions. These experimental results are used to correlate reaction kinetics and surface reactant species. It is evident that there is a continuum over the pressure range studied with respect to the reaction mechanism. The most active phase has been shown to be an oxygen-dominant surface. The formation of a subsurface oxygen layer is found to deactivate the reaction.