Selective oxidation of methanol on iron molybdate catalysts and the effects of surface reduction

The reaction and reduction of an iron molybdate catalyst (Mo:Fe = 2.2:1) with methanol was studied with a pulsed-flow reactor, temperature-programmed desorption (TPD), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The first pulse of methanol under anaerobic conditions shows similar conversion and selectivity to formaldehyde as in aerobic conditions, indicating that gaseous oxygen is not directly involved in the reaction but is used instead to reoxidize the catalyst surface. With further reduction at low temperature (under 250 °C) under anaerobic conditions, conversion of the catalyst drops to zero, due to the loss of a significant amount of surface oxygen. In contrast, at elevated temperatures (at or above 250 °C) oxygen migration occurs through the catalyst so that oxidative products are still formed, and many monolayer equivalents of lattice oxygen are used. The process of oxygen migration leads to the formation of new phases of α-FeMoO4, MoO2, and Mo4O11. The selectivity of the catalyst during reduction is initially changed toward CO, but, rather surprisingly, with further reduction above 300 °C it shifts toward CO2, which indicates the appearance of iron oxide at the catalyst surface.