Unveiling the pathways and site requirements of methanol oxidative dehydrogenation on MoO3/TiO2 catalysts: An operando-FTIR study

Abstract

Methanol oxidative dehydrogenation was studied on sub-monolayer and crystalline MoO₃/TiO₂-supported catalysts using operando-FTIR spectroscopy. Results revealed two distinct methyl formate (MF) formation pathways, determined by the molybdenum oxide structure. Quantitative and qualitative evidence indicated that MF and dimethoxymethane (DMM) formation occur via distinct reaction intermediates. MF formation is linked to surface formate consumption, supported by the similarity between steady-state MF formation rate measured in a fixed-bed reactor and transient initial formate consumption rate determined by operando-FTIR. Apparent activation energies for HCOO* consumption (90 and 88 kJ mol⁻¹) and MF formation (83 and 51 kJ mol⁻¹) for 2.5 and 15 at. Mo nm⁻² samples, respectively, indicate that the formation pathway depends on the molybdenum oxide structure. Oligomeric, octahedral molybdenum oxide catalysts produce MF via adsorbed formate consumption, while crystalline MoO₃ catalysts enable a parallel pathway, likely involving hemiacetal intermediates. This change in reaction pathway correlates with the structural transition from oligomeric to crystalline molybdenum oxide, as characterized by XRD, in situ Raman spectroscopy, FTIR of low-temperature CO adsorption, and XPS, among other techniques. The increase of surface formate consumption is related to the enhancement of the redox properties of the catalyst, attributed to interactions of molybdenum oxide with titania support and the presence of readily reducible Mo⁶⁺ sites that influence adsorbed formaldehyde reaction pathways. The observed activity and selectivity are explained by a three-active-site mechanism: molybdenum oxide redox sites for methanol dehydrogenation, molybdenum oxide acid sites for hemiacetal and DMM formation, and molybdenum-titania interfacial sites for HCOO* and MF formation.