Effective hydrogenation of TiO₂ photocatalysts with CH₃OH for enhanced water splitting: A computational and X-ray study

We have investigated computationally and experimentally the efficacy of CH₃OH treatment of anatase TiO₂ nanoparticles (NPs) and compared the results obtained with H₂ as the hydrogenation source. The results indicate that TiO₂ NPs heated at 300 °C in 120 Torr of CH₃OH vapor is more effective than the samples heated under 800 Torr of H₂ at the same temperature. Ultraviolet-visible spectroscopy (UV-VIS), Photoluminescence spectroscopy (PL) and X-ray diffraction spectroscopy (XRD) were employed to study the optical property. The effect of temperature of hydrogenation has been systematically studied for NPs in the 200–450 °C range for both H-sources based on H₂ evolution, employing Xe lamp as the light source. The hydrogenated TiO₂ NPs using H₂ and CH₃OH have been analyzed by soft X-ray absorption near-edge structure analyses with and without Xe lamp irradiation in situ. The result indicates that CH₃OH hydrogenation can induce a much stronger Ti–O interaction along direction d_z ² and increase the unoccupied DOS of Ti t_2g states. This is why the CH₃OH treated TiO₂ exhibits a greater photo-activity. The crucial role of the hydrogenation using CH₃OH was also investigated systematically and confirmed by quantum-chemical calculations with complementary synchrotron-based X-ray techniques. The results are consistent with the predicted barriers for the formation of H atoms on the TiO₂ surface, 4–9 kcal/mol, considerably lower than that in the H₂ case, 48 kcal/mol.