Enhancing the H₂ evolution from water-methanol solution using Mn²⁺-Mn⁺³-Mn⁴⁺ redox species of Mn-doped TiO₂ sol-gel photocatalysts

Mn-doped TiO₂ materials synthetized by the sol-gel method were obtained and tested in the photocatalytic hydrogen production from a methanol-water solution. The Mn amount was varied between 1.0-10.0%wt. Powder X-ray diffraction patterns and Raman spectra of the synthesized solids showed the anatase as the predominant crystalline phase. A high specific surface area was found in the Mn-doped sol-gel catalysts between 88-136 m²/g while in bare TiO₂ sample 64 m²/g only. Mn-doped TiO₂ solids evaluated in the production of H₂ showed higher photoactivities (1376 μmol h^-1g^-1 for 1.0%wt. Mn and 1736 μmol h^-1g^-1 for 5.0%wt. Mn) in comparison with the bare TiO₂ semiconductor (264 μmol h^-1g^-1). This improvement in photoactivity is suggested as a combination of charge separators Mn²⁺, Mn³⁺ and Mn⁴⁺ which can act simultaneously as electron and hole traps respectively. The synergetic effect between the manganese oxidation states and electrons transferred from methanol toward TiO₂ particles favored the H₂ production.