Operando X-ray Absorption Spectroscopic Study of Ultradispersed Mo/TiO2 CO2-Hydrogenation Catalysts: Why Does Rutile Promote Methanol Synthesis?

CO2 to methanol conversion is an important process for an ecologic and energetic shift, which stimulates the development of active catalysts based on noncritical elements. We have recently reported that Mo atoms anchored on rutile titania nanorods exhibit distinctive performance in methanol synthesis, that is higher activity and methanol selectivity than Mo dispersed on anatase and P25 commercial forms. In this work, for deciphering the origin of the higher performance of rutile, this support is compared to other selected oxides of interest, and Mo/TiO2 catalysts are investigated operando at high pressure by synchrotron X-ray absorption spectroscopy at the Mo K edge combined with chemometric treatments. The main structural change occurs during the reductive activation pretreatment, when MoVI species convert to more reduced ones. However, while on anatase titania the active species are mostly MoO2-like clusters, on rutile titania, single Mo atoms are not only more abundant but may also replace surface Ti atoms, which would explain the more stable single-atom dispersion and the rutile-like coordination of Mo as well as the low sensitivity of the latter to the chemical environment and the metal loading. As a result, Mo-doped rutile titania acts as a promising atomically dispersed catalyst for the hydrogenation of CO2 to methanol.