Titania-supported iron–ruthenium and iron–iridium catalysts. An in situ iron-57 Mössbauer spectroscopic study of the effects of pretreatment in hydrogen

Abstract

The changes induced in some titania-supported iron–ruthenium and iron–iridium catalysts following pretreatment in hydrogen have been examined in situ by ⁵⁷Fe Mössbauer spectroscopy. Fe³⁺, which is formed in the supported metallic phase by initial treatment in air, undergoes partial reduction when treated in hydrogen at low temperatures. Further treatment in hydrogen at moderate temperatures results in some of the Fe²⁺ being oxidised to Fe³⁺ which, on further exposure to hydrogen at elevated temperatures, is predominantly reduced to Fe⁰ in an iron-containing alloy. The extent of the initial reduction in hydrogen and subsequent oxidation processes, together with the nature of the reduced materials formed at elevated temperatures, is dependent on the nature of the noble metal, the ratio of iron to noble metal, the conditions under which the catalyst is initially dried in air and the surface area of the titania support. The extent of the anaerobic oxidation is optimised in materials with an iron : noble metal mole ratio of ca. 1 : 2.5 when supported on high-surface-area titania by a preparation involving slow initial drying of the catalyst in air at low temperatures and subsequent treatment in hydrogen at ca. 400 °C. The results are associated with the influence of the metallic composition, the surface area of the support and the thermal treatment on the initial dispersion of the metallic phase, the hydrogen adsorption capacities of the supported metals and the influence of these properties on metal–metal and metal–support interactions.