First principles investigation of dissociative adsorption of H during CO hydrogenation over cubic and hexagonal InO catalysts.

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