Shoji Iguchi

A ZnTa2O6 photocatalyst synthesized via solid state reaction for conversion of CO2 into CO in water

Because of the environmental problems and the resulting exigent demand for CO2 recycling processes, great attention is being paid to the photocatalytic conversion of CO2 into useful chemicals such as CO, HCOOH, HCHO, CH3OH, and CH4. We have previously reported that the Ag-loaded, Zn-modified Ga2O3 photocatalyst exhibits excellent photocatalytic activity required for the conversion of CO2 into CO by using H2O as a reductant and that the Ag particles that exist together with the Zn species act as good cocatalysts for the selective formation of CO. In this study, we demonstrated the photocatalytic activity of ZnTa2O6 under UV light irradiation, which was prepared via solid-state reaction, for the conversion of CO2 in an aqueous NaHCO3 solution. A Ag cocatalyst-loaded ZnTa2O6 photocatalyst evolved CO as a reduction product of CO2 with 46% selectivity toward CO evolution among the reduction products. In contrast, when Pt and Au were introduced as cocatalysts, the ZnTa2O6 photocatalyst evolved H2 with high selectivity (>99.9%).

Drastic improvement in the photocatalytic activity of Ga2O3 modified with Mg–Al layered double hydroxide for the conversion of CO2 in water

Photocatalytic conversion of CO2 to useful chemicals is a promising countermeasure against increasing concentrations of CO2 in the atmosphere as photocatalytic reactions can drive a thermodynamically uphill reaction (so-called “artificial photosynthesis”). Our group has earlier reported that Ga2O3 photocatalysts loaded with a Ag cocatalyst exhibit high activity for the conversion of CO2 to CO in water, and further modification of the catalysts with ZnGa2O4 improves the selectivity for CO over the other reduction products by suppressing H2. In this study, Ga2O3 modified with a Mg–Al layered double hydroxide (LDH) considerably enhanced not only the amount of CO evolved but also the selectivity toward CO evolution. By using 1.0 g of a Mg–Al LDH/Ga2O3 composite photocatalyst loaded with 0.25 wt% of a Ag cocatalyst, the CO formation rate and selectivity for CO were 211.7 μmol h−1 and 61.7%, respectively. Moreover, the turnover number (TON) of electrons utilized to reduce CO2 to CO was 75.4 per Ag atom during 5 h of photoirradiation.

A nanoLDH catalyst with high CO2 adsorption capability for photo-catalytic reduction

A benign catalyst with a considerable activity towards CO2 reduction is in demand to explore green processes. Layered double hydroxides (LDHs) are a promising candidate material for this purpose, because they exhibit a high catalytic activity even in aqueous solvents and are free from poisoning by water molecules. Herein, we demonstrate that NiAl LDH nanocrystals (∼20 nm) exhibit a remarkably high photocatalytic activity toward CO2 reduction in aqueous media, thanks to their capability of adsorbing CO2 at high concentration. The present LDH photocatalyst with a high catalytic activity was obtained through a nanocrystallization induced by a homogeneous and rapid pH increase from an aqueous solution of concentrated metal salts. The rate of photocatalytic CO2 reduction over the nanoLDH catalyst (50 μmol h−1) is 7 times higher than that over a highly-crystalline standard LDH catalyst (7.2 μmol h−1) prepared through a conventional method. Systematic investigation revealed that the excellent catalytic properties of the present nanoLDH originate from its high affinity towards CO2 introduced as the gaseous state. This specific nature of the surface could be related to the metastable surface which was quenched by rapid hydroxide formation from concentrated solution of metals salts. The nanoLDH catalysts demonstrated here can be synthesized in a simple one-pot reaction in an aqueous solvent at a mild temperature. Further exploration of the material design by complexation with co-catalysts would give rise to catalysts for artificial photosynthesis based on nanoLDH materials.

Highly Concentrated CO Evolution for Photocatalytic Conversion of CO2 by H2O as an Electron Donor

Kentaro TERAMURA1,2,*, Kazutaka HORI1, Yosuke TERAO1, Hiroyuki TATSUMI1, Zeai HUANG1, Shoji IGUCHI1, Zheng WANG1, Hiroyuki ASAKURA1,2, Saburo HOSOKAWA1,2, Tsunehiro TANAKA1,2,* 1Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyotodaigaku Katsura, Nishikyoku, Kyoto 615–8510, Japan 2Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, Kyotodaigaku Katsura, Nishikyo-ku, Kyoto 615–8520, Japan *E-mail: teramura@moleng.kyoto-u.ac.jp

Application of Layered Double Hydroxides (LDHs) in Photocatalysis

Layered double hydroxides (LDHs, [M2+1−xM3+x(OH)2]x+(An−x/n)·mH2O), also known as hydrotalcite-like compounds, are natural and/or synthetic clays consisting of highly ordered two-dimensional hydroxide sheets, where M2+, M3+, and An− are divalent and trivalent cations, and the interlayer anions of valence n. Recently, LDHs have attracted great attention in the field of photocatalysis because of their characteristic layer structures, remarkable adsorption properties, and large specific surface areas. Recent applications of LDHs to the photocatalytic reactions such as the degradation of organic compounds, the water splitting (H2 and O2 evolution in the presence of sacrificial reagents), and the conversion of CO2 are reviewed in this chapter. Moreover, advances in synthesis techniques and characterization methods are also summarized. The variety of metal components in LDHs (M2+ and M3+) caused significant changes to the photocatalytic activities; in particular, the use of Ni−Al LDH enabled us to achieve the selective formation of CO in the photocatalytic conversion of CO2 in an aqueous solution, whereas the reduction of proton (H+) to H2 was suppressed.