Disclosing the Reaction Mechanism of CO₂ Hydrogenation to Methanol over CuCeO_x/TiO₂: A Combined Kinetic, Spectroscopic, and Isotopic Study

Abstract

Recent studies have focused on the promotion of methanol synthesis by CeO2 and the inhibition of the reverse water gas shift reaction (r-WGSR) by TiO2 support from the perspective of the nature of active sites and morphology. Nevertheless, the rational design of catalysts needs to account additionally for the reaction mechanism, a topic that has not yet been addressed for CuCeO x /TiO2. Herein, methanol and CO formation pathways over this catalytic system were unraveled. Kinetic, isotopic, and spectroscopic techniques were combined to obtain mechanistic insights from both reactions. Particularly, H2/D2 kinetic isotopic effect (KIE), gas hourly space velocity (GHSV) effect, cofeeding of products, operando-diffuse reflectance infrared Fourier transform spectroscopy (operando-DRIFTS) under steady-state, transient, and H2/D2 exchange conditions, and 12CO2/13CO2 steady-state isotopic-transient kinetic analysis-DRIFT/mass spectrometry (SSITKA-DRIFTS/MS) were performed. The results unequivocally ruled out an r-WGSR + CO hydrogenation pathway for methanol synthesis and, furthermore, demonstrated that this species is formed through the stepwise hydrogenation of formates over Cu–Ce interfacial sites. Moreover, it was concluded that the formation of CO occurs assisted by hydrogen on a site different from that involved in the methanol synthesis. Likewise, the kinetic relevance of surface-detected carbonate species was discarded. From these mechanistic insights, a formal kinetic model was derived, which accurately fitted the experimental data and correctly predicted the properties of the active sites and the behavior of CuCeO x /TiO2 catalysts with different CeO2 concentrations. This study highlights the importance of combining kinetic and spectroscopic analyses with isotopic labeling in order to elucidate reaction mechanisms over catalysts in which the interface between a metal and metal-oxide plays a relevant role.