The synthesis of hydrocarbons from CO and H 2 over well-characterized supported PtFe catalysts

Abstract

A combination of Mössbauer spectroscopy, chemisorption measurements and kinetic studies of the synthesis of hydrocarbons from CO and H 2 has been employed to investigate alumina-supported Fe, Pt and PtFe catalysts. Three different chemical states of Fe have been identified in the reduced catalysts: ferrous ions, ferromagnetic Fe metal and PtFe bimetallic clusters. The relative amounts of these species depend upon the Pt:Fe ratio and the total metal loading. At low concentrations (0.1%) iron was not reduced by hydrogen beyond the ferrous state at 773 K, and the ferrous ions had no catalytic activity in the synthesis reaction. However, catalysts containing 1.75 wt.% Pt and varying amounts of Fe showed marked differences. The addition of this amount of platinum to iron on alumina led to the formation of PtFe clusters on reduction. At high Pt:Fe ratios (∼5) all of the iron combined with platinum to form clusters. The iron in the PtFe clusters was catalytically inert in the CO/H 2 synthesis reaction and the clusters exhibited essentially the catalytic behaviour of platinum. At low Pt:Fe ratios (0.1) ferromagnetic iron metal, in addition to Fe 2+ ions and PtFe clusters, was produced. The ferromagnetic iron dominated the activity and selectivity pattern of the catalyst because of the higher specific activity of the iron compared to Pt or PtFe clusters. The inert behavior of the iron in PtFe clusters is attributed to a decrease in the electron density on the iron as indicated by the Mössbauer isomer shift. The direction of electron transfer is opposite to that proposed for alkalimetal-promoted Fe catalysts on which the production of higher molecular weight species and the activity was enhanced. Finally, this investigation shows that bimetallic alloys with stoichio-metries identical to the metal loading are not necessarily formed in reduced supported catalysts.