Tetragonal lead tungstate, PbWO 4 , consisting of a WO 4 tetrahedron showed high and stable photocatalytic activity for the overall splitting of water, producing a stoichiometric quantity of H 2 and O 2 under UV irradiation when loading RuO 2 on the metal oxide. The activity was found to strongly depend on the preparation temperature of PbWO 4 . In the temperature range 773-1273 K, a maximum appeared at around 1023 and 973 K when prepared in air and under a N 2 atmosphere, respectively. X-ray diffraction analysis and SEM observation showed that high activity was achieved with a combination of crystallized PbWO 4 with a high dispersion of RuO 2 particles. The DFT calculation for PbWO 4 with the d 10 s 2 -d 0 electronic configuration showed that the top of the valence band (HOMO) was composed of hybridized O 2p + Pb 6s orbitals, while the bottom of the conduction band (LUMO) consisted of hybridized W 5d + O 2p + Pb 6p orbitals. Considerable dispersion was observed for both the conduction and valence bands, which differed from the small dispersion observed for the photocatalytically inactive CaWO 4 with a similar crystal structure. Photoexcited electrons and holes with high mobility were considered to be responsible for the high photocatalytic performance of PbWO 4 . The useful role of a Pb 2+ ion with the d 10 s 2 electronic configuration in photocatalysis has been elucidated.

Ruthenium oxide-loaded Sr2+-doped CeO2 with f0d0 electronic configuration was found to have photocatalytic activity for overall water splitting into H2 and O2 under UV irradiation, whereas neither undoped CeO2 nor stoichiometric compounds such as SrCeO3 and Sr2CeO4 was photocatalytically active.

Lead tungstate, PbWO4, with a tetragonal structure was found to make a stable photocatalyst for the decomposition of water to produce H2 and O2 when combined with RuO2. This is the first example of tungsten oxide that is photocatalytically active for the reaction. The structural feature of PbWO4 is the presence of tetrahedral WO4. The DFT calculation showed that Pb2+ with d10s2 electronic configuration had significant influences on both valence and conduction band structures.