Design and stabilisation of a high area iron molybdate surface for the selective oxidation of methanol to formaldehyde

Abstract

The performance of Mo-enriched, bulk ferric molybdate, employed commercially for the industrially important reaction of the selective oxidation of methanol to formaldehyde, is limited by a low surface area, typically 5–8 m² g⁻¹. Recent advances in the understanding of the iron molybdate catalyst have focused on the study of MoO_x@Fe₂O₃ (MoO_x shell, Fe₂O₃ core) systems, where only a few overlayers of Mo are present on the surface. This method of preparing MoO_x@Fe₂O₃ catalysts was shown to support an iron molybdate surface of higher surface area than the industrially-favoured bulk phase. In this research, a MoO_x@Fe₂O₃ catalyst of even higher surface area was stabilised by modifying a haematite support containing 5 wt% Al dopant. The addition of Al was an important factor for stabilising the haematite surface area and resulted in an iron molybdate surface area of ∼35 m² g⁻¹, around a 5 fold increase on the bulk catalyst. XPS confirmed Mo surface-enrichment, whilst Mo XANES resolved an amorphous MoO_x surface monolayer supported on a sublayer of Fe₂(MoO₄)₃ that became increasingly extensive with initial Mo surface loading. The high surface area MoO_x@Fe₂O₃ catalyst proved amenable to bulk characterisation techniques; contributions from Fe₂(MoO₄)₃ were detectable by Raman, XAFS, ATR-IR and XRD spectroscopies. The temperature-programmed pulsed flow reaction of methanol showed that this novel, high surface area catalyst (3ML-HSA) outperformed the undoped analogue (3ML-ISA), and a peak yield of 94% formaldehyde was obtained at ∼40 °C below that for the bulk Fe₂(MoO₄)₃ phase. This work demonstrates how core–shell, multi-component oxides offer new routes for improving catalytic performance and understanding catalytic activity.