An in situ Mössbauer investigation of the influence of metal–support and metal–metal interactions on the activity and selectivity of iron–ruthenium catalysts

Abstract

The hydrogen reduction of a series of alumina- and silica-supported iron–ruthenium catalysts has been studied in situ by iron-57 Mossbauer spectroscopy. The data, complemented by results from temperature-programmed reduction experiments, show that ruthenium enhances the reducibility of iron and that the metal–support interaction in the silica-supported samples is weaker than in those supported on alumina.The performance of the iron–ruthenium catalysts for carbon monoxide hydrogenation has been evaluated in a fixed-bed microreactor, and the activity of the alumina-supported catalysts was found to be higher than those supported on silica. However, alumina-supported catalysts in which the iron content exceeded ca. 27 atom % showed a gradual decrease in activity. At ca. 65 atom % this began to approach the activities of the silica-supported materials, which were generally less dependent on the iron content of the bimetallic phase. Catalysts with low metal loadings were more active than their counterparts with high metal loadings. These variations in catalytic activity have been interpreted in terms of the stronger interactions between the metallic phase and alumina, as compared with silica, supports and the smaller metallic particle sizes which are obtained on alumina.Metal–support interactions and particle dispersions have also been found to influence selectivity. Hence ruthenium, which interacts strongly with alumina, showed high activity and tended to form higher-molecular-weight hydrocarbons, whilst silica-supported ruthenium was less active and favoured the formation of lower hydrocarbons. The addition of iron to alumina-supported ruthenium shifted the product distribution towards lower hydrocarbons but had little influence on the selectivity of catalysts supported on silica.The variations in metal–support interaction which result from changes in the iron concentration and which are reflected in the catalytic properties of the solids have been correlated with the Mossbauer data, which showed the capacity of iron to donate electrons to ruthenium.