First principles investigation of dissociative adsorption of H2 during CO2 hydrogenation over cubic and hexagonal In2O3 catalysts

Dissociative adsorption of molecular hydrogen (H₂) and migration of the adsorbed H adatom as a hydride or a proton on two of the most stable facets of the In₂O₃ catalyst in the cubic (c-In₂O₃) and hexagonal (h-In₂O₃) phases for CO₂ hydrogenation to methanol were investigated by extensive density functional theory (DFT) calculations. Due to the relatively small variation in the oxygen vacancy (O_v) formation energy with respect to surface O_v coverage of these In₂O₃ surfaces, especially c-In₂O₃(110) and h-In₂O₃(012), no limit on the O_v coverage is expected to exist under the typical reaction conditions. Thus, we consider three scenarios for the dissociative adsorption of H₂ and the migration of the H adsorbate, namely on the perfect (stoichiometric) In₂O₃ surfaces, on the partially reduced In₂O₃ surfaces with a single O_v site, and on the fully reduced In₂O₃ surfaces. Our DFT calculations show that the oppositely charged In and O pair sites on the perfect and partially reduced In₂O₃ surfaces facilitate the heterolytic dissociation of H₂, leading to the formation of the anionic hydride at the In site crucial for CO₂ hydrogenation to methanol. Our comparative studies on the four In₂O₃ surfaces suggest that the h-In₂O₃(104) surface is superior to the other three surfaces due to the facile formation of the O_v sites at low coverage and the favorable formation of the hydride adsorbate at the In site from H₂ dissociation. These results indicate that this surface might be preferred for CO₂ hydrogenation to methanol.