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

CO 2 photoconversion is a promising method to reduce atmospheric CO 2 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 CO 2 photoconversion into CO and/or methanol. In this study, various LDHs of the formula [M II 3 Ga III (OH) 8 ] 2 A⋅ m  H 2 O (M II =Zn II , Cu II ; A 2− =CO 3 2− , [Cu(OH) 4 ] 2− ) were synthesized and used for CO 2 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, [Zn 3 Ga(OH) 8 ] 2 CO 3 ⋅ m  H 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 μmol h −1  g cat −1 after it was preheated at 423 K and protected by an Ar atmosphere. Conversely, the methanol photogeneration rates of tetrahydroxycuprate‐type LDHs were suppressed to less than 0.1 μmol h −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.