The mechanism of photocatalytic splitting of H2O into H 2 and O2 on Pt/KTa(Zr)O3 modified with various porphyrinoids was investigated. The photocatalytic activity of KTaO3 catalysts is improved by dye modification. Cyanocobalamin (vitamin B 12) is the most effective for improving watersplitting activity, and the formation rates of H2 and O2 achieved values of 575 and 280 μmolgcat -1h-1, respectively. X-ray photoelectron spectroscopy spectra of KTa(Zr)O3 photocatalysts showed that Pt loaded onto dye-modi-fied KTaO3 was slightly oxidized and had low catalytic activity for the H2 oxidation reaction. Photoluminescence (PL) spectra of KTaO3 catalysts suggested that excitation energy was transferred between KTaO3, tetraphenylporphyrinatochromium(III) (Cr-TPP), and the Pt cocatalyst. The wavelength dependence of the activity of dye-modified KTa(Zr)O3 photocatalysts indicated that excitation of both KTa(Zr)O3 and the dye was essential for achieving increased photocatalytic activity. This result suggests that twostep excitation occurred in the dyemodified KTa(Zr)O 3 photocatalysts. Because the lifetime of the charge-separated state increased, this study reveals that modification with porphyrinoids is effective for increasing watersplitting activity.
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