Catalytic properties of nanostructured metal oxides synthesized by inert gas condensation

The correlation between defects in metal oxides and activity in heterogeneous catalytic oxidation was investigated. Processing of nanocrystalline metal oxides by inert gas condensation was employed to synthesize oxygen-deficient nonstoichiometric cerium oxide. Comparsion with stoichiometric nanocrystalline ceria revealed the effect of nonstoichiometry on surface chemical reactivity. Stoichiometric nanocrystalline certum dioxide was investigated for catalytic activity in two reactions; oxidation of CO by (i) SO₂ and (ii) O₂. Catalytic activity was found at significantly different temperatures of 560°C in the former and 360°C in the latter reaction. Complementary temperature programmed reduction (TPR) studies were performed and two distinct signals were found, which correlated with the light-off temperatures for each of the catalytic reactions. This demonstrated that the oxygen intermediates, participating in the redox mechanisms were different for both reactions. Nonstoichiometric nanocrystalline CeO_2-x exhibited catalytic activity in CO oxidation by O₂ and a corresponding TPR-signal at 200°C, which suggested the presence of an even more reactive oxygen species. Fourier-transform infrared spectroscopy (FT-IR) and electrical conductivity were used to study the interaction of chemisorbed CO and O₂ with the metal oxides. Surface defects and chemisorbed oxygen were discussed as possible origins of enhanced catalytic activity due to their effects on the nature of surface oxygen species. The combination of these studies yielded detailed insight into the different reaction mechanisms and the chemical interaction between solid surface and gaseous species that are critical towards the engineering of catalytic and gas sensor materials,