Another is association mechanism: CO2 adsorbs on the surface of catalysts and reacts with dissociated H to form an intermediate species, such as formate (*HCOO), carboxyl (*COOH), carbonate (CO32−), and bicarbonate (HCO3−), which is decomposed to CO and H2O [27, 37]. This mechanism was originally proposed for metal/reducible support catalysts [27, 38–40], such as Pt/CeO2. For this mechanism, the key intermediate proposed for the RWGS reaction may vary among different catalysts or the same catalyst operating under different conditions [41–44]. However, redox and association mechanisms have been debated. The mechanism of RWGS reaction is strongly dependent on catalyst types and reaction conditions; as such, no consensus has been reached.