Selective Photoconversion of Carbon Dioxide into Methanol Using Layered Double Hydroxides at 0.40 MPa

CO2 photoconversion is a promising method to reduce atmospheric CO2 concentrations and mitigate energy problems simultaneously. Among the various efficient and stable semiconductor photocatalysts used for this purpose, layered double hydroxides (LDHs) have attracted attention as catalysts for CO2 photoconversion into CO and/or methanol. In this study, various LDHs of the formula [(M3GaIII)-Ga-II(OH)(8)](2)A center dot mH(2)O (M-II = Zn-II, Cu-II; A(2-) = CO32-, [Cu(OH)(4)](2-)) were synthesized and used for CO2 photoconversion at a reaction pressure of 0.40 MPa in the presence of H-2 to result in the exclusive production of methanol. Furthermore, the pretreatment of carbonate-type LDHs at 423 K boosted the reaction rates by a factor of 7.5-20. Interestingly, [Zn3Ga(OH)(8)](2)CO3 center dot mH(2)O was the only LDH that produced methane primarily by an eight-electron reduction (rather than the production of methanol by a six-electron reduction) at a total formation rate of 2.7 mmolh(-1)g(cat)(-1) after it was pre-heated at 423K and protected by an Ar atmosphere. Conversely, the methanol photogeneration rates of tetrahydroxy-cuprate-type LDHs were suppressed to less than 0.1 mmolh(-1) g(cat)(-1) at 0.40 MPa. In summary, the contribution of the interlayer reaction space created by the partial removal of water molecules and/or carbonate ions of LDHs was suggested.