Effect of Oxygen Vacancies on Adsorption of Small Molecules on Anatase and Rutile TiO₂ Surfaces: A Frontier Orbital Approach

Adsorption is a critical step in the initial stage of a catalytic reaction. Oxygen vacancies often become a central topic of discussion with respect to the reaction mechanism on metal oxide surfaces because oxygen vacancies change the surface electronic properties and affect the adsorption process. In this study, we use first-principles calculations and regression analyses to investigate the effect of oxygen vacancies on a metal oxide surface on the adsorption of small molecules. We adopt the anatase (101) and rutile (110) TiO₂ surfaces as research targets. The calculation results show that the removal of an oxygen atom causes a large difference in the electronic structure due to the presence of two unpaired electrons. Our previous study showed that the adsorption energy of a defect-free surface is linearly correlated with the energy of the highest occupied molecular orbital for small adsorbed molecules. In the case of a defective anatase surface, we found that the energy of the lowest unoccupied molecular orbital also plays an important role in enhancing the adsorption of small molecules, especially those with an unpaired electron. Notably, our results demonstrate that hardness is a prime descriptor to explain the adsorption of various molecules by TiO₂ surfaces.