Fe2(MoO4)3/MoO3 nano-structured catalysts for the oxidation of methanol to formaldehyde

Nano-structured iron molybdate materials, comprising Fe2(MoO4)3 nano-particles anchored onto MoO3 nano-rods, have been synthesised and evaluated as catalysts for the selective oxidation of methanol to formaldehyde. The catalysts were benchmarked against a standard iron molybdate catalyst prepared by co-precipitation and were found to have comparable performance under the test reaction conditions. The materials have been characterised using a combination of electron microscopy, powder X-ray diffraction and Raman spectroscopy. The synthesised MoO3 nano-rods were found to have a uniform structure, 8–10 μm in length with a rectangular cross-section of 50–100 nm × 100–200 nm. Fe was impregnated onto the MoO3 nano-rods via incipient wetness with an aqueous solution of Fe(NO3)3·9H2O. Calcination of these precursors formed 20–200 nm Fe2(MoO4)3 islands on the surface of the nano-rods via a solid-state diffusion mechanism, and the size of the Fe2(MoO4)3 islands could be controlled by varying the Fe loading. The effect of the temperature and duration of calcination were investigated, and it was found that the optimum conditions were 450–500 °C for 2 h. At 350 °C, the temperature was too low for the solid-state reaction between the MoO3 nano-rod and the surface Fe to occur, and no Fe2(MoO4)3 was formed. At higher temperatures, the nano-rod morphology was compromised, and irregular, partially coalesced particles of Fe2(MoO4)3 were generated, which had a lower catalytic performance than the nano-structured Fe2(MoO4)3/MoO3 materials. At 400 °C, the solid-state reaction occurred sufficiently slowly for mechanistic aspects of the Fe2(MoO4)3 island formation to be elucidated by transmission electron microscopy