Platinum Ion-Doped TiO₂: High Catalytic Activity of Pt²⁺ with Oxide Ion Vacancy in Ti⁴⁺_1–xPt²⁺_xO_2–x Compared to Pt⁴⁺ without Oxide Ion Vacancy in Ti⁴⁺_1–xPt⁴⁺_xO₂

Ti 0.97 Pt 2+ 0.03 O 1.97 and Ti 0.97 Pt 4+ 0.03 O 2 have been synthesized by a solution combustion method using alanine and glycine as the fuels, respectively. Both crystallize in anatase TiO 2 structure with 15 nm average crystallite size. X-ray photoelectron spectroscopy (XPS) confirmed Pt ions are in the 2+ state in Ti 0.97 Pt 0.03 O 1.97 (alanine) and 4+ state in Ti 0.97 Pt 0.03 O 2 (glycine). The rate of CO oxidation occurring over Ti 0.97 Pt 2+ 0.03 O 1.97 (0.76 μmol·g –1 ·s –1 ) is ∼10 times more than that over Ti 0.97 Pt 4+ 0.03 O 2 at 60 °C (0.08 μmol·g –1 ·s –1 ). A large shift in 100% hydrocarbons conversion to lower temperature was observed for Pt 2+ ion-substituted TiO 2 relative to that for Pt 4+ ion-substituted TiO 2 . After reoxidation of the reduced compound by H 2 as well as CO, Pt ions are stabilized in mixed valences, 2+ and 4+ states. The role of oxide ion vacancy has been demonstrated by CO oxidation and H 2 + O 2 recombination reactions in the presence and absence of O 2 . We analyze the activated lattice oxygens upon substitution of Pt 2+ and Pt 4+ ions in TiO 2, using first-principles density functional theory (DFT) calculations with supercells of Ti 31 Pt 1 O 63, Ti 30 Pt 2 O 62, and Ti 29 Pt 3 O 61 for Pt 2+ ion substitution and Ti 31 Pt 1 O 64, Ti 30 Pt 2 O 62, and Ti 29 Pt 3 O 61 for Pt 4+ ion substitution in TiO 2 . We find that the local structure of Pt 2+ ion has a distorted square planar geometry and that of Pt 4+ ion has an octahedral geometry similar to that of Ti 4+ ion in pure TiO 2 . The change in coordination of Pt 2+ ion gives rise to weakly bonded oxygens, and these oxygens are involved in high rates of catalytic reaction. Thus, the high catalytic activity results from synergistic roles of Pt 2+ ion and oxide ion vacancy and weakly bonded lattice oxygen.