First-principles study of oxygen reduction reaction on defective TiO₂ surfaces

Some oxides (TiO₂, ZrO₂, …) have been proposed as promising electrocatalysts for the oxygen reduction reaction (ORR) in polymer electrolyte fuel cells. The oxides can be made active by introducing defects such as Nb or oxygen vacancies (Vos), and some of them have high activities in acid media comparable to Pt. However, little is known about the microscopic mechanisms of the ORR . To improve our understanding of the ORR at defective oxide surfaces, we have studied the ORR on Nb-doped and oxygen-deficient TiO₂ surfaces using density functional theory with PBE+U functional. Thorough calculating the free energies of the reaction intermediates on the rutile (110), anatase (101), (100), (001) and (001)-(1x4) reconstructed surfaces, we found the followings. (1) At Nb-doped surface, the intermediates adsorb too strongly on the surface due to charge transfer from Ti³⁺. (2) At oxygen-deficient surface, adsorbates bind too strongly to the Vo site due to the existence of 2Ti³⁺s. On the other hand, (3) if the excess electrons in Ti³⁺ are removed by charge transfer to adjacent OH* adsorbates, then the intermediates moderately bind to the Vo. These results indicate that the surface Vo site without any excess electron is a candidate for the catalytic sites of the ORR.