An in situ Mössbauer spectroscopic investigation of titania-supported iron–ruthenium catalysts

Abstract

The changes induced in titania-supported iron and iron–ruthenium catalysts following treatment in reducing and anaerobic atmospheres have been studied in situ by ⁵⁷Fe Mössbauer spectroscopy. The results show that the surface area of the titania and the ruthenium concentration in the bimetallic materials influence the metal–support interactions and consequently the nature of the solids formed in air and their reduction products.

The materials containing ⩽ 1% iron with varied ruthenium concentrations formed by calcination in air contained large and/or small particles of α-Fe₂O₃. These generally underwent progressive reduction when treated in hydrogen at increasing temperatures. The behaviour of small-particle α-Fe₂O₃ in samples composed of 0.2 % Fe–1 % Ru when treated in hydrogen at temperatures > ca. 450 °C was distinctly different in that the ⁵⁷Fe Mössbauer spectra showed that partial oxidation of Fe^II to Fe^III occurred under the reducing conditions. Similar effects were observed when these pre-reduced materials were treated in argon. A mechanism for the anaerobic oxidation involving electron transfer from iron to ruthenium and to the titania support is proposed.

Titania-supported small-particle α-Fe₂O₃ in the presence of ruthenium was partially reduced at 235 °C to Fe^II by hydrogen and by treatment in hydrogen and carbon monoxide. Mixtures of large- and small-particle α-Fe₂O₃ in the presence of ruthenium were incompletely reduced in hydrogen at 235 °C to metallic iron which, when treated in carbon monoxide and hydrogen at similar temperatures, was converted into the iron carbide of composition Fe_2.2C. The iron component in ruthenium-rich Fe–Ru catalysts supported on titania which had been pre-reduced in hydrogen at 235 °C underwent anaerobic oxidation when treated in carbon monoxide and hydrogen.

The Fe^II formed in hydrogen-prereduced titania-supported high-iron-content Fe–Ru materials was also oxidized when treated in carbon monoxide and hydrogen. The results contrasted with those recorded from alumina-supported analogues, which underwent disproportionation, and from similar silica-supported materials which suffered reduction.