Based on reaction kinetics, spectroscopy, and density functional theory (DFT) calculations, two mechanisms of RWGS have been proposed [24–29]. One is redox mechanism: CO2 is reduced to CO with the catalyst being oxidized directly, and then H2 reduces the catalyst again to complete the catalytic turnover [24]. This mechanism was originally proposed for Cu-based catalysts, where Cu is oxidized by CO2 and reduced again by H2 . This mechanism has also been proposed for catalysts containing reducible oxides, which can be readily reduced and oxidized under reaction conditions [32–36].