Reaction mechanism for CO2 hydrogenation

In the CO2 hydrogenation, three reactions are main competitors, namely RWGS, methanol synthesis, and DME production [244,276,277]. The methanol and DME production reactions have a slower rate in reaching thermodynamic equilibrium compared to RWGS reaction. 

It has generally been assumed that most of the reaction mechanisms that occur for WGS are also applicable for RWGS. The two main mechanisms proposed for WGS reactions are (1) surface redox mechanism and (2) dissociative mechanism of water to afford OH and H species, with subsequent formation of formate species (HCO2-M, obtained by an attack of OH groups on M-CO moieties) or metalla carboxyl species (MO2H, which might result from the reaction of M-H moieties with CO2 or, even better, by protonation of metal-bound CO2 see Chaps. 2 and 4) as intermediates [200]. 

7 Carbon Dioxide Conversion in High Temperatare Reactions 

The dissociative mechanism involving formate intermediates has been widely considered as the major route for CO productitMi in the RWGS reaction [280-282]. With Cu as the catalyst active site, the formate species derived from association of H2 and CO2 is mainly proposed to be the key intermediate for CO production [281, 282]. Several studies have been carried out to improve the formate species decomposition to CO [282-284]. 

It was also postulated that CO2 adsorbed on K2O may also migrate to Cu and then associate with H atom forming formate species [282]. 

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