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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. [ABSTRACT FROM AUTHOR]

The photocatalytic conversion of CO2 into useful chemical compounds in water without using organic sacrificial reagents is a promising method to overcome environmental and energy problems. Various synthesized layered double hydroxides (LDHs) are capable of reducing CO2 to CO in an aqueous solution under UV light irradiation. However, it is difficult to oxidize H2O to O2 in a photocatalytic system using LDHs as photocatalysts. In this study, we investigated the photocatalytic conversion of CO2 using a Ni–Al LDH in an aqueous solution of NaCl. Hypochlorous acid (HClO) was produced as an oxidation product of Cl− with the formation of reduction products such as CO and H2 under photoirradiation. We propose the inclusion of Cl− in the reaction solution to be one of the most promising ways for obtaining a hole scavenger, an approach that would enable the construction of an artificial photosynthesis system for the conversion of CO2. [ABSTRACT FROM AUTHOR]

Layered double hydroxide (LDH) photocatalysts, including Ni-Al LDH, are active for the photocatalytic conversion of CO2 in water under UV light irradiation. In this study, we found that a series of LDHs exhibited anodic photocurrent which is a characteristic feature corresponding to n-type materials. Also, we estimated the potentials of photogenerated electrons and holes for LDHs, which are responsible for the photocatalytic reactions, using electrochemical techniques. The flat band potential of the Ni-Al LDH photocatalyst was estimated to be -0.40 V vs. NHE (pH = 0), indicating that the potential of the photogenerated electron is sufficient to reduce CO2 to CO. Moreover, we revealed that the flat band potentials of M(2+)-M(3+) LDH are clearly influenced by the type of trivalent metal (M(3+)) components.

The photocatalytic conversion of CO2 into useful chemical compounds in water without using organic sacrificial reagents is a promising method to overcome environmental and energy problems. Various synthesized layered double hydroxides (LDHs) are capable of reducing CO2 to CO in an aqueous solution under UV light irradiation. However, it is difficult to oxidize H2O to O2 in a photocatalytic system using LDHs as photocatalysts. In this study, we investigated the photocatalytic conversion of CO2 using a Ni-Al LDH in an aqueous solution of NaCl. Hypochlorous acid (HClO) was produced as an oxidation product of Cl(-) with the formation of reduction products such as CO and H2 under photoirradiation. We propose the inclusion of Cl(-) in the reaction solution to be one of the most promising ways for obtaining a hole scavenger, an approach that would enable the construction of an artificial photosynthesis system for the conversion of CO2.