Dispersion of ruthenium oxide on barium titanates (Ba6Ti17O40,Ba4Ti13O30,BaTi4O9and Ba2Ti9O20)and photocatalytic activity for water decomposition

Abstract

Ruthenium oxide supported on barium titanates (Ba₆Ti₁₇O₄₀, Ba₄Ti₁₃O₃₀, BaTi₄O₉ and Ba₂Ti₉O₂₀) was employed as a photocatalyst for water decomposition. The RuCl₃-impregnated titanates were subjected to either reduction or reduction–oxidation. High-resolution electron microscopic images demonstrated that ruthenium metal and ruthenium oxides were uniformly dispersed on BaTi₄O₉ with an average particle size of 2.6 nm. Similar uniform ruthenium oxide dispersions were observed for the other barium titanates; the average particle sizes were 4.7 nm for Ba₆Ti₁₇O₄₀, 2.3 nm for Ba₄Ti₁₃O₃₀, and 4.4 nm for Ba₂Ti₉O₂₀. Particle size distributions were narrower for BaTi₄O₉ and Ba₄Ti₁₃O₃₀, and slightly larger for Ba₆Ti₁₇O₄₀ and Ba₂Ti₉O₂₀. Stoichiometric production of oxygen and hydrogen occurred for a RuO₂/BaTi₄O₉ photocatalyst. A small amount of hydrogen and no oxygen were produced from the other barium titanates (Ba₆Ti₁₇O₄₀, Ba₄Ti₁₃O₃₀ and Ba₂Ti₉O₂₀) combined with ruthenium oxides. EPR spectra at 77 K in He or O₂ with UV irradiation demonstrated that a strong signal, assigned to a surface O^- radical, appeared for BaTi₄O₉ but not for the other barium titanates. These produced small complicated signals, indicating that only BaTi₄O₉ has a high efficiency for photoexcited charge formation. Raman spectra showed that a strong single peak at a high wavenumber of 860 cm^-1, characteristic of BaTi₄O₉, was absent in the rest of the barium titanates. The different photocatalytic properties among these titanates are discussed on the basis of structure differences of the barium titanates, and the presence of internal fields; a long Ti–O bond in the distorted TiO₆ octahedra is proposed to be important in photocatalysis.