Photocatalysts with tunnel structures for decomposition of water. Part 1.—BaTi4O9, a pentagonal prism tunnel structure, and its combination with various promoters

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Abstract
Photocatalysts have been prepared by impregnating barium tetratitanate, BaTi4O9, which has a pentagonalprism tunnel structure, with aqueous solutions of CoCl2, Ni(NO3)2, In(NO3)3, RuCl3, H2IrCl6 or H2PtCl6, and then activating with either reduction or oxidation. For decomposition of water under irradiation with light from a Xe lamp, reduction caused low photocatalytic activities. Oxidation of RuCl3-impregnated BaTi4O9 at 848 K led to active photocatalysts which produce H2 and O2 in the correct stoichiometric ratio. The photocatalytic activity increased with an increase in the amount of loaded Ru until it reached 1 wt. %, and remained nearly constant up to 3 wt. %. The X-ray photoelectron spectra showed that the active Ru was in a tetravalent state, forming RuO2 on the BaTi4O9 surface. UV diffuse reflectance spectra showed that BaTi4O9 had a threshold of absorption of light at around 410 nm which reached a maximum at 320 nm. From high-resolution transmission electron microscopy and microanalysis the spherical RuO2 particles of 1.4–3.0 nm in diameter were found to be dispersed uniformly on the regular lattice of BaTi4O9. It is concluded that the pentagonal-prism tunnel structure of BaTi4O9 has a ‘nest’ effect and is responsible for the high dispersion of RuO2 particles, which leads to the high photocatalytic activity.