Characterization of cobalt Fischer–Tropsch catalysts: 2. Rare earth-promoted cobalt-silica gel catalysts prepared by wet impregnation

Abstract

Rare earth (RE) promoted cobalt-silica gel catalysts for Fischer–Tropsch (F–T) synthesis were investigated systematically by various analytical techniques. The addition of small amounts of praseodymium to cobalt-silica gel catalysts greatly enhances their activity and selectivity in the F–T synthesis. The optimal atomic ratio of Pr/Co for 6% cobalt-silica gel catalysts was found to be between 0.26 and 0.35. Spectroscopic and other experimental results show that impregnated praseodymium readily substitutes the surface silicon atoms to form negatively charged centres on the surface of the support. Due to the presence of praseodymium, the deposited cobalt species can partially react with the support. Consequently, significant amounts of surface cobalt silicates or hydrosilicates can be formed. Most of these species undergo reduction during activation and can be converted to a metallic cobalt phase that remains incorporated at the silica surface.