In Situ Mössbauer Study of Mixed-Valency Catalysts for Methanol Oxidation

Abstract

Mixed-valency catalysts (the iron oxyborates ferroferriborate, Fe²⁺ ₂Fe³⁺BO₅, and the magnesium ferroferriborate, Mg_xFe²⁺ _2-xFe³⁺BO₅) have been investigated in situ by Mössbauer spectroscopy during the oxidation of methanol. The ferroferriborate exhibits two kinds of thermally activated electron exchange: direct (3d⁶Fe²⁺↔3d⁵Fe³⁺) and indirect (3d⁶Fe²⁺↔O↔3d⁵Fe³⁺) In Mg_xFe²⁺ _2-xFe³⁺BO₅ the direct exchange is blocked by Mg²⁺ ions and only indirect exchange (3d⁶Fe²⁺↔O↔3d⁵Fe³⁺) occurs.

The principal investigations have been performed in a special cell allowing the registration of catalyst spectra during the catalytic reaction. Spectra have been obtained at T = 423−673 K in the presence in air-methanol gas mixture. The investigations have shown that no phase transformations of the catalyst occur during the catalytic reaction. The Mossbauer parameters of the spectra components have displayed some changes. It has been shown that the reaction initiates catalytically stimulated electron exchange between iron ions of different valencies which is accompanied by thermally-activated electron exchange. The sample with which both kinds of electron exchange are observed, has a larger amount of mixed valency ion pairs and leads to a higher oxidation degree of methanol. However, its selectivity in the oxidation of methanol to formaldehyde is lower.