Active Oxygen Vacancy Site for Methanol Synthesis from CO 2 Hydrogenation on In 2 O 3 (110): A DFT Study

Methanol synthesis from CO 2 hydrogenation on the defective In 2 O 3 (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 In 2 O 3 (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 CO 2 hydrogenation over both D1 and D4 surfaces were studied, and the D4 surface was found to be more favorable for CO 2 activation and hydrogenation. On the D4 surface, one of the O atoms of the CO 2 molecule fills in the O v4 site upon adsorption. Hydrogenation of CO 2 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 H 2 CO 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 H 2 CO to H 3 CO indicates that this step is the rate-limiting step in the methanol synthesis on the defective In 2 O 3 (110) surface.