In-situ Moessbauer and EXAFS investigations of the structural and magnetic properties of bimetallic iron-nickel/silica and iron-copper/silica catalysts

Abstract

In monometallic Fe/SiO2 and Ni/SiO2 catalysts, the transition element undergoes only partial reduction to the metallic state on treatment with hydrogen, unlike in Cu/SiO2 where the reduction is complete. In-situ Mossbauer and EXAFS investigations of Fe-Ni/SiO2 catalysts with different Fe/Ni ratios show that the reducibility of both Fe and Ni is greater in the bimetallic catalysts than in the corresponding monometallic catalysts. Reduced bimetallic catalysts show evidence for the formation of FCC or BCC alloy phases that are superparamagnetic or ferromagnetic depending on the composition as well as the heat treatment given to the precursor. In the Ni-rich composition, Fe(25)-Ni(75), a superparamagnetic alloy is formed, while both ferromagnetic and superparamagnetic alloys are formed in the Fe-rich composition, Fe(75)-Ni(25). In the Fe(50)-Ni(50) catalyst, a ferromagnetic alloy, predominantly in the FCC structure, is formed. The presence of Fe in the FCC and Ni in the BCC structure is indeed noteworthy. In the case of Fe-Cu/SiO2 catalysts, the promotion effect of Cu on the reduction of Fe is marginal and alloy formation is observed mainly in Fe-rich compositions. Alloy formation and the reducibility of Ni in Cu-Ni/SiO2 catalysts are known to increase with increasing Cu content. It therefore appears that in all these bimetallic catalysts, mutual promotion of the reducibility of the transition metals and alloy formation go hand in hand.