Important Role of Strontium Atom on the Surface of SrKTaO with a Tetragonal Tungsten Bronze Structure to Improve Adsorption of CO for Photocatalytic Conversion of CO by HO.

SrKNaTaO, which belongs to the Na-substituted SrKTaO series of compounds with a tetragonal tungsten bronze structure, was fabricated using a flux mixture of KCl and NaCl (KCl/NaCl molar ratio = 55:45). It exhibited higher CO formation rate (94.6 μmol h), better selectivity for CO evolution (85.5%), and better stability of the photocatalytic activity than those of bare SrKTaO and other Na-substituted SrKTaO samples synthesized from flux mixtures with different KCl/NaCl ratios. X-ray photoelectron spectroscopic studies revealed that the surface atomic Sr/Ta ratio of SrKNaTaO was larger than that of SrKTaO. To clarify the factor responsible for the improvement in the photocatalytic activity facilitated by Na substitution, as well as to elucidate the reaction mechanism, the surface species were characterized by in situ Fourier transform infrared spectroscopy. It was observed that the bicarbonate species (HCO) adsorbed on the active Sr sites of SrKNaTaO was reduced to CO via the formate species during photoirradiation. The plot of the CO formation rate vs. the surface atomic Sr/Ta ratio for tetragonal tungsten bronze-type Sr-K-Ta-O complex oxides had the summit, indicating that Sr atoms on the surface enhance the photocatalytic activity, while an excessive amount of Sr on the surface leads to the decrease in the photocatalytic activity. Hence, it can be concluded that while the presence of Sr on the surface has a determining effect on the adsorption of CO and eventually on the photocatalytic activity, excess Sr on the surface that exists as SrCO or SrTaO suppresses the photocatalytic activity. Thus, SrKNaTaO showed higher CO formation rate than SrKTaO did.