Surface Hydroxyls and Chemisorbed Hydrogen on Titania and Titania Supported Cobalt

The dehydroxylation and rehydroxylation properties of titania (Degussa P-25) were investigated. FTIR and TPD-MS data indicate that hydroxyl groups are not completely removed at 500 °C in vacuo, and rehydration/rehydroxylation occur under helium (water content < 1 ppm) flow at room temperature. In addition, repeating dehydroxylation/rehydroxylation treatments for ten times does not significantly modify the dehydroxylation/rehydroxylation property of titania surface. Desorption of hydrogen from titania was observed at 535 °C after titania was reduced above 400 °C. The maximum surface density of hydrogen was determined to be 0.75 H atom/nm². TPD data show four types of hydrogen on 3% titania supported cobalt catalysts: hydrogen adsorbed on cobalt metal (desorption temperature around 100 °C), reverse spillover hydro gen (150-250 °C), hydrogen from H-TiO_2-x- Co interacting species (-330 °C), and recombined hydrogen from Ti³⁺-H on titania (-535 °C). The absence of hydrogen desorption peak at 535 °C for titania supported cobalt reduced above 400 °C can be explained in terms of the migration of H-TiO_2-x moieties onto cobalt metal surface during reduction. Removal of hydroxyl groups by thermal treatment before reduction enhanced the amount of hydrogen desorption from normal cobalt surface. This indicates that the hydroxyl groups play a role in the surface migration of H-Ti_2-x.