CATALYTIC PROPERTIES OF NANOSTRUCTURED METAL-OXIDES SYNTHESIZED BY INERT-GAS CONDENSATION

The correlation between defects in metal oxides and activity in hetero geneous catalytic oxidation was investigated. Processing of nanocrysta lline metal oxides by inert gas condensation was employed to synthesiz e oxygen-deficient nonstoichiometric cerium oxide. Comparsion with sto ichiometric nanocrystalline ceria revealed the effect of nonstoichiome try on surface chemical reactivity. Stoichiometric nanocrystalline cer ium dioxide was investigated for catalytic activity in two reactions; oxidation of CO by (i) SO2 and (ii) O-2. Catalytic activity was found at significantly different temperatures of 560 degrees C in the former and 360 degrees C in the latter reaction Complementary temperature pr ogrammed reduction (TPR) studies were performed and two distinct signa ls were found, which correlated with the light-off temperatures for ea ch of the catalytic reactions. This demonstrated that the oxygen inter mediates, participating in the redox mechanisms were different for bot h reactions. Nonstoichiometric nanocrystalline CeO2-x exhibited cataly tic activity in CO oxidation by O-2 and a corresponding TPR-signal at 200 degrees C, which suggested the presence of an even more reactive o xygen species. Fourier-transform infrared spectroscopy (FT-IR) and ele ctrical conductivity were used to study the interaction of chemisorbed CO and O-2 with the metal oxides. Surface defects and chemisorbed oxy gen were discussed as possible origins of enhanced catalytic activity due to their effects on the nature of surface oxygen species. The comb ination of these studies yielded detailed insight into the different r eaction mechanisms and the chemical interaction between solid surface and gaseous species that are critical towards the engineering of catal ytic and gas sensor materials. (C) 1997 Acta Metallurgica Inc.