Photoconversion of carbon dioxide in zinc-copper-gallium layered double hydroxides: The kinetics to hydrogen carbonate and further to CO/methanol

Photocatalytic reaction mechanism for the conversion of CO2 into methanol and CO using layered double hydroxides (LDHs) consisting of Zn, Cu, and Ga was investigated. X-ray absorption fine structure was applied to determine the LDH site structures and to monitor the diffusion of photogenerated electrons to Cu-II sites. Electron diffusion to Cu sites was an order of magnitude faster in the direction of the cationic layers (580 mu mol h(-1) g(cat)(-1)) than in the perpendicular direction. According to Fourier-transform infrared spectroscopy, CO2 was in equilibrium with hydrogen carbonate (1629 cm(-1) for (HCO3)-C-13) for the reaction with hydroxy group from the cationic layer or interlayer site and/or with interlayer water. The equilibrium reactions were faster for [Zn3Ga(OH)(8)](+)(2)[Cu(OH)(4)](2-)center dot mH(2)O (400-110 mu mol h(-1) gcat(-1)) than for [Zn1.5Cu1.5Ga(OH)(8)](+)(2)(CO3)(2-)center dot mH(2)O. Furthermore, the reductive decomposition of hydrogen carbonate was suggested in H-2 under UV-visible light, suggesting photocatalytic pathway to methanol/CO. (C) 2013 Elsevier B.V. All rights reserved.