Hydrogenation of CO₂ to methanol over In-doped m-ZrO₂: a DFT investigation into the oxygen vacancy size-dependent reaction mechanism

Selective methanol synthesis via CO2 hydrogenation has been thoroughly investigated over defective In-doped m-ZrO2 using density functional theory (DFT). Three types of oxygen vacancies (Ovs) generated either at the top layer (O1_v and O4_v) or at the subsurface layer (O2_v) are chosen as surface models due to low Ov formation energy. Surface morphology reveals that O1_v has smaller oxygen vacancy size than O4_v. Compared with perfect In@m-ZrO2, indium on both O1_v and O4_v is partially reduced, whereas the Bader charge of In on O2_v remains almost the same. Our calculations show that CO2 is moderate in adsorption energy (∼-0.8 eV) for all investigated surface models, which facilitates the formate pathway for both O1_v and O4_v. O2_v is not directly involved in CO2 methanolization but could readily transform into O1_v once CO2/H2 feed gas is introduced. Based on the results, the synthesis of methanol from CO2 hydrogenation turns out to exhibit conspicuous vacancy size-dependency for both O1_v and O4_v. The reaction mechanism for small-sized O1_v is controlled by both the vacancy size effect and surface reducibility effect. Thus, H2COO* favors direct C-O bond cleavage (c-mechanism) before further hydrogenation to methanol, which is similar to the defective In2O3. The vacancy size effect is more competitive than the surface reducibility effect for large-sized O4_v. Therefore, H2COO* prefers protonation to H2COOH before C-O bond cleavage (p-mechanism) which is similar to the ZnO-ZrO2 solid solution. Furthermore, we also determined that stable-CH3O*, which is too stable to be hydrogenated, originates from the O1_v surface. In contrast, CH3O* with similar configuration is allowed to be further converted to methanol on O4_v. Overall, our findings offer a new perspective towards how reaction mechanisms are determined by the size of oxygen vacancies.