Application of Mössbauer spectroscopy in industrial heterogeneous catalysis: effect of oxidant on FePO₄ material phase transformations in direct methanol synthesis from methane

Abstract

The effect of the FePO₄ material phase transformation in the direct selective oxidation of methane to methanol was studied using various oxidants, i.e. O₂, H₂O and N₂O. The phases of the heterogeneous catalyst applied, before and after the reactions, were characterized by M¨ossbauer spectroscopy. The main reaction products were methanol, carbon monoxide and carbon dioxide, whereas formaldehyde was produced in rather minute amounts. The Mössbauer spectra showed the change of the initial catalyst material, FePO₄ (tridymite-like phase (tdm)), to the reduced metal form, iron(II) pyrophosphate, Fe₂P₂O₇, and thereafter, the material phase change was governed by the oxidation with individual oxidizing species.Mössbauer spectroscopy measurements applied along with X-ray diffraction (XRD) studies on fresh, reduced and spent catalytic materials demonstrated a transformation of the catalyst to a mixture of phases which depended on operating process conditions. Generally, activity was low and should be a subject of further material optimization and engineering, while the selectivity towards methanol at low temperatures applied was adequate. The proceeding redox mechanism should thus play a key role in catalytic material design, while the advantage of iron-based heterogeneous catalysts primarily lies in them being comparably inexpensive and comprising non-critical raw materials only.