Active oxygen vacancy site for methanol synthesis from CO₂ hydrogenation on In₂O₃(110): A DFT study

Methanol synthesis from CO₂ hydrogenation on the defective In₂O₃(110) surface with surface oxygen vacancies has been investigated using periodic density functional theory calculations. The relative stabilities of six possible surface oxygen vacancies numbered from O _v1 to O_v6 on the perfect In₂O₃(110) surface were examined. The calculated oxygen vacancy formation energies show that the D1 surface with the O_v1 defective site is the most thermodynamically favorable while the D4 surface with the O_v4 defective site is the least stable. Two different methanol synthesis routes from CO₂ hydrogenation over both D1 and D4 surfaces were studied, and the D4 surface was found to be more favorable for CO₂ activation and hydrogenation. On the D4 surface, one of the O atoms of the CO₂ molecule fills in the O_v4 site upon adsorption. Hydrogenation of CO₂ to HCOO on the D4 surface is both thermodynamically and kinetically favorable. Further hydrogenation of HCOO involves both forming the C-H bond and breaking the C-O bond, resulting in H₂CO and hydroxyl. The HCOO hydrogenation is slightly endothermic with an activation barrier of 0.57 eV. A high barrier of 1.14 eV for the hydrogenation of H₂CO to H₃CO indicates that this step is the rate-limiting step in the methanol synthesis on the defective In₂O₃(110) surface.