Optimized photoreduction of CO2 exclusively into methanol utilizing liberated reaction space in layered double hydroxides comprising zinc, copper, and gallium

Abstract

Layered double hydroxides (LDHs), typically comprising Zn, Cu, and Ga, photoreduce CO2 into methanol and CO, however, selective methanol synthesis using CO2 and heterogeneous photocatalyst is very rare. In this study, the amount of interlayer water molecules is reduced to 31% of that for as-synthesized LDHs by preheating the LDHs at 423 K in vacuum, and the performance for CO2 photoreduction using 0–0.28 MPa of CO2 and 0–0.56 MPa of H2 was investigated. If the LDHs are preheated in vacuum and never in contact with air prior to the photoreduction tests, methanol was produced exclusively in all experiments of this study. LDHs comprising inlayer Cu sites were more active compared to [Zn3Ga(OH)8]2CO3·mH2O, [Zn3Ga(OH)8]2Cu(OH)4·mH2O, and [Zn3Ga(OH)8]2Pd(OH)4·mH2O LDHs. A contour plot for methanol formation rates was drawn for the most active [Zn1.5Cu1.5Ga(OH)8]2CO3·mH2O and the volcano top positioned at 0.12 MPa of CO2 and 0.28 MPa of H2; 2.8 μmol-methanol h−1 gcat−1 and the selectivity was >97 mol%-methanol. 13CH3OH formation in the presence of 13CO2 and [Zn1.5Cu1.5Ga(OH)8]2CO3·mH2O confirmed photocatalytic methanol synthesis. Under 0.12 MPa of CO2 and 0.28 MPa of H2, the intensity of the Cu K preedge peak progressively decreased at the rate of 170 μmol-Cu h−1 gcat−1 upon the UV–visible light irradiation for the [Zn1.5Cu1.5Ga(OH)8]2CO3·mH2O LDH, demonstrating photogenerated electron accumulation at the CuII/I sites for subsequent CO2 reduction.