Optimized photoreduction of CO 2 exclusively into methanol utilizing liberated reaction space in layered double hydroxides comprising zinc, copper, and gallium

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 00.28 MPa of CO2 and 00.56 MPa of H-2 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)](2)CO3 center dot mH(2)O, [Zn3Ga(OH)(8)](2)Cu(OH)(4)center dot mH(2)O, and [Zn3Ga(OH)(8)](2)Pd(OH)(4)center dot mH(2)O LDHs. A contour plot for methanol formation rates was drawn for the most active [Zn1.5Cu1.5Ga(OH)(8)](2)CO3 center dot mH(2)O and the volcano top positioned at 0.12 MPa of CO2 and 0.28 MPa of H-2; 2.8 mu mol-methanol h(-1) gcat(-1) and the selectivity was >97 mol%-methanol. (CH3OH)-C-13 formation in the presence of (CO2)-C-13 and [Zn1.5Cu1.5Ga(OH)(8)](2)CO3 center dot mH(2)O confirmed photocatalytic methanol synthesis. Under 0.12 MPa of CO2 and 0.28 MPa of H-2, the intensity of the Cu K preedge peak progressively decreased at the rate of 170 mu mol-Cu h(-1) gcat(-1) upon the UV-visible light irradiation for the [Zn1.5Cu1.5Ga(OH)(8)](2)CO3 center dot mH(2)O LDH, demonstrating photogenerated electron accumulation at the Cu-II/I sites for subsequent CO2 reduction. (C) 2018 Elsevier B.V. All rights reserved.