NEAR-AMBIENT-PRESSURE X-RAY PHOTOELECTRON SPECTROSCOPY INVESTIGATION OF SMALL MOLECULES ACTIVATION MECHANISM ON ZnO SURFACES

Abstract

Metal oxides used in heterogeneous catalysis are applied widely in industrial chemical processes. However, the role of metal oxide catalysts is still controversial and needs further detailed elaboration. In this thesis, information regarding electronic configuration evolution of model oxide catalysts was investigated carefully by NAP-XPS. The activation of CO2 or O2 gas molecules has been characterized by considering the pressure band bending effect on ZnO polar surfaces in the presence of these gases. On the other hand, the activation of H2 was also studied, which is the key step in hydrogenation processes. The polarity- and pressure-dependent hydrogen dynamics on ZnO polar surfaces at room temperature not only reveal the distinct behaviours of the two polar surfaces of ZnO but also provide direct evidence of the catalytic activity of ZnO surfaces toward H2 molecules under realistic reaction conditions. Moreover, by the deposition of Cr on ZnO surfaces, the Cr-ZnO interfacial contacts were suggested to be the active sites for the activation of CO, which is in contrast to the pristine ZnO system, where no apparent activation was observed. The findings in this thesis will enrich the understanding of the activation mechanism of small gas molecules on oxide surfaces.