Unravelling the Role of Oxygen Vacancies in the Mechanism of the Reverse Water–Gas Shift Reaction by <i>Operando</i> DRIFTS and Ultraviolet–Visible Spectroscopy

The reaction mechanism of the reverse water–gas shift (RWGS) reaction was investigated using two commercial gold-based catalysts supported on Al₂O₃ and TiO₂. The surface species formed during the reaction and reaction mechanisms were elucidated by transient and steady-state <i>operando</i> DRIFTS studies. It was revealed that RWGS reaction over Au/Al₂O₃ proceeds through the formation of formate intermediates that are reduced to CO. In the case of the Au/TiO₂ catalyst, the reaction goes through a redox mechanism with the suggested formation of hydroxycarbonyl intermediates, which further decompose to CO and water. The Ti³⁺ species, the surface hydroxyls, and oxygen vacancies jointly participate. The absence of carbonyl species adsorbed on gold particles during the reaction for both catalysts indicates that the reaction pathway involving dissociative adsorption of CO₂ on Au particles can be discarded. To complete the study, <i>operando</i> ultraviolet–visible spectroscopy was successfully applied to confirm the presence of Ti³⁺ and to understand the role of the oxygen vacancies of TiO₂ support in activating CO₂ and thus the subsequent RWGS reaction