Important Role of Strontium Atom on the Surface of Sr2KTa5O15 with a Tetragonal Tungsten Bronze Structure to Improve Adsorption of CO2 for Photocatalytic Conversion of CO2 by H2O.

Sr1.6K0.37Na1.43Ta5O15, which belongs to the Na-substituted Sr2KTa5O15 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-1), better selectivity for CO evolution (85.5%), and better stability of the photocatalytic activity than those of bare Sr2KTa5O15 and other Na-substituted Sr2KTa5O15 samples synthesized from flux mixtures with different KCl/NaCl ratios. X-ray photoelectron spectroscopic studies revealed that the surface atomic Sr/Ta ratio of Sr1.6K0.37Na1.43Ta5O15 was larger than that of Sr2KTa5O15. 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 (HCO3-) adsorbed on the active Sr sites of Sr1.6K0.37Na1.43Ta5O15 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 CO2 and eventually on the photocatalytic activity, excess Sr on the surface that exists as SrCO3 or Sr2Ta2O7 suppresses the photocatalytic activity. Thus, Sr1.6K0.37Na1.43Ta5O15 showed higher CO formation rate than Sr2KTa5O15 did.

Sodium Cation Substitution in Sr2KTa5O15 toward Enhancement of Photocatalytic Conversion of CO2 Using H2O as an Electron Donor.

The K and Sr cations (K+ and Sr2+) in a Sr2KTa5O15 photocatalyst were found to be easily substituted by Na cations (Na+) to form Sr x K y Na z Ta5O15 by a facile one-pot flux method using a mixture of potassium chloride (KCl) and sodium chloride (NaCl). Sr x K y Na z Ta5O15 fabricated using a mixture of KCl and NaCl with a Ag cocatalyst showed enhanced photocatalytic activity without apparent change in selectivity toward CO for the photocatalytic conversion of CO2 using H2O. The present study demonstrates that the flux treatment significantly affected the phase, morphology, band gap, and surface Sr composition of the catalyst owing to the substitution of K+ and Sr2+ for Na+. The stability and durability of the catalyst were also enhanced as compared to those of the photocatalyst fabricated using only KCl flux due to more stable Ag on the surface of Sr x K y Na z Ta5O15.