TiO2-anatase-supported oxorhenate catalysts prepared by oxidative redispersion of metal Re0 for methanol conversion to methylal: A multi-technique in situ/operando study

TiO2-anatase-supported rhenium catalysts were prepared by oxidative redispersion of metallic rhenium. The structure and the activity of the as-prepared catalysts were found to be similar to those prepared by the incipient wetness impregnation technique. Interestingly, our results suggest that the active phase of those catalysts retain oxorhenate species in which the rhenium could be involved as Re6+ in small clusters, as suggested by UV–visible absorption and XPS experiments. Such catalysts were found to be highly active in methanol conversion to methylal. However, we have evidenced that the low rhenium surface coverage catalyst is prone to surface poisoning because of the formation of rhenium carbonyl species and coke in the absence of oxygen. A comprehensive multi-techniques in situ and operando study made it possible to show that the redox couple(s) implied in the target reaction involve Re7+, Re6+and Re4+ species and to make a rational link between the pieces of the puzzle.