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 H-2 as the hydrogenation source. The results indicate that TiO2 NPs heated at 300 degrees C in 120 Torr of CH3OH vapor is more effective than the samples heated under 800 Torr of H-2 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 degrees C range for both H-sources based on H-2 evolution, employing Xe lamp as the light source. The hydrogenated TiO2 NPs using H-2 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 H-2 case, 48 kcal/mol.