Mössbauer and X-ray photoelectron spectroscopic evidence for the structure of supported bimetallic catalysts : FeRu, FeRh, FePd, FeIr, and FePt on SiO2

Silica-supported bimetallic catalysts, consisting of iron and a more noble Group VIII metal M (Ru, Rh, Pd, Ir, Pt) with metal loading 5 wt% and molar ratio Fe:M = 1:1, have been investigated with in situ Mössbauer spectroscopy and X-ray photoelectron spectroscopy. Reduced FeRu, FeRh, FeIr, and FePt on SiO2 contain the noble metal M in the zero-valent state, whereas iron is only partially reduced to Fe0, the latter being present in an FeM alloy. Between 50 and 80% of the iron is present as Fe3+ in iron(III) oxide, which is resistant to reduction by H2 up to at least 875 K. The Mössbauer parameters of the ferric iron change upon chemisorption of CO at 295 K, indicating that the iron(III) oxide is highly dispersed. In contrast to the other FeM/SiO2 catalysts, reduced FePd/SiO2 contains all Pd and almost all Fe in the zero-valent state. The presence of both bcc FePd alloy and a-Fe metal indicates that phase segregation has occurred. Passivation of the FeM/SiO2 catalysts in air at 295 K results in oxidation of Fe0 to Fe3+, while the metal M remains reduced. An exception is FeRu/SiO2, in which about half of the Ru is oxidized by air at 295 K. All passivated FeM/SiO2 catalysts show reduction of Fe3+ to Fe2+ or Fe0 by H2 and by CO at 295 K, which is promoted by the noble metal. Implications of the results on models for the structure of a supported bimetallic catalyst are discussed.