Effects of preparation, dispersion, and extent of reduction on activity/selectivity properties of iron/alumina CO hydrogenation catalysts

Abstract

Physical, chemical, and catalytic properties of catalysts prepared by decomposing Fe(CO) 5 (carbonyl-derived catalyst, CDC) and Fe(NO 3) 3 deposited by aqueous impregnation (conventionally prepared catalyst, CPC) on alumina are compared in this paper. The physical and chemical properties of these catalysts were determined by hydrogen adsorption, oxygen titration, CO temperature-programmed desorption (TPD), and Mössbauer spectroscopy; activity/selectivity properties in CO hydrogenation were determined in a single-pass fixed bed, differential microreactor. Fe/ alumina CDCs prepared from γ-alumina dehydroxylated at 923 K are highly reduced, highly dispersed systems. COTPD spectra of a CPC and CDCs indicate that the metal crystallites formed on the CPC or on CDCs prepared on poorly dehydroxylated aluminas are probably decorated by support moieties while activity tests indicate them to be less active in CO hydrogenation than CDCs prepared on highly dehydroxylated supports. Furthermore, the activity/selectivity properties of Fe/alumina CDCs do not change with metal dispersion when the extent of reduction is held fairly constant. On the other hand, CO hydrogenation activity of Fe/alumina changes by an order of magnitude with dispersion when changes in % D are accompanied by changes in extent of reduction. These results suggest that the “apparent” structure sensitivity observed for CO hydrogenation on iron is probably due to secondary effects such as decoration by support moieties and/or the presence of unreduced metal oxide in the near vicinity of reduced metals.