Photocatalytic Conversion of Carbon Dioxide Using Zn–Cu–Ga Layered Double Hydroxides Assembled with Cu Phthalocyanine: Cu in Contact with Gaseous Reactant is Needed for Methanol Generation

Abstract

Photocatalytic conversion of CO<sub>2<sub/> into fuels is an attractive option in terms of both reducing the increased concentration of atmospheric CO<sub>2<sub/> as well as generating renewable hydrocarbon fuels. It is necessary to investigate good catalysts for CO<sub>2<sub/> conversion and to clarify the mechanism irradiated by natural light. Layered Double Hydroxides (LDH) have been attracting attention for CO<sub>2<sub/> photoreduction with the expectation of sorption capacity for CO<sub>2<sub/> in the layered space and tunable semiconductor properties as a result of the choice of metal cations. This study first clarifies the effects of Cu doping to LDH comprising Zn and Al or Ga. Cu could be incorporated in the cationic layers of LDH as divalent metal cations and/or interlayer anions as Cu(OH)<sub>4<sub/><sup>2−<sup/>. The formation rates of methanol and CO were optimized for [Zn<sub>1.5<sub/>Cu<sub>1.5<sub/>Ga(OH)<sub>8<sub/>]<sup>+<sup/><sub>2<sub/>Cu(OH)<sub>4<sub/><sup>2−<sup/>·mH<sub>2<sub/>O at a total rate of 560 nmol h<sup>−1<sup/> g<sub>cat<sub/><sup>−1<sup/> irradiated by UV–visible light. Cu phthalocyanine tetrasulfonate hydrate (CuPcTs<sup>4−<sup/>) and silver were effective as promoters of LDH for CO<sub>2<sub/> photoreduction. Especially, the total formation rate using CuPcTs-[Zn<sub>3<sub/>Ga(OH)<sub>8<sub/>]<sup>+<sup/><sub>2<sub/>CO<sub>3<sub/><sup>2−<sup/>·mH<sub>2<sub/>O irradiated by visible light was 73% of that irradiated by UV–visible light. The promotion was based on HOMO–LUMO excitation of CuPcTs<sup>4−<sup/> by visible light. The LUMO was distributed on N atoms of pyrrole rings bound to central Cu<sup>2+<sup/> ions. The photogenerated electrons diffused to the Cu site would photoreduce CO<sub>2<sub/> progressively in a similar way to inlayer and interlayer Cu sites in the LDH in this study.