Investigation of the Reducibility of Supported Oxomolybdate Species for Mapping of Active Centers of Partial Oxidation Reaction: In Situ Mo K-Edge XAS and DFT Study

摘要

We have investigated the molecular and electronic structure of a TiO2-supported oxomolybdate phase upon reduction under methanol or dihydrogen atmosphere. In situ Mo K-edge X-ray absorption spectroscopy, supported by density functional theory (DFT) calculations and ab initio X-ray absorption spectroscopy (XAS) spectra simulation showed that the reducibility of supported molybdate species is closely related to the geometric environment of Mo atoms. Indeed, Mo atoms not involved in terminal oxo sites are easily reducible whereas Mo dioxo species are not under the conditions we have applied. Between those extreme cases, Mo atoms involved in monooxo terminal sites exhibit intermediate reducibility, which varies according to their local environment and their interaction with the support. Spin density calculations showed that under CH3OH flow, the surface of the catalyst is partially reduced. The structure of the reduced phase after reaction with methanol was compared to that afforded after reduction by dihydrogen. Our results show that the reduction under H-2 flow does not increase the number of reduced Mo atoms but causes a higher spin density per Mo atom. On the basis of the individual mapping of the electronic structure of Mo species as drawn by DFT and XAS spectra simulation, a model that details the structure of the Mo active sites in the selective oxidation of methanol is proposed.