Photocatalytic conversion of carbon dioxide into methanol using zinc–copper–M(III) (M = aluminum, gallium) layered double hydroxides

Ordered layered double hydroxides (LDHs) consisting of zinc and/or copper hydroxides were synthesized and combined with aluminum or gallium. These LDH compounds were then applied as photocatalysts to convert gaseous CO2 (2.3 kPa) to methanol or CO under UV–visible light using hydrogen. ZnAAl LDH was the most active for CO2 photoreduction and the major product was CO formed at a rate of 620 nmol h � 1 g � 1 cat, whereas methanol was the major product formed by the inclusion of Cu in the LDH photocatalysts, e.g., at a formation rate of 170 nmol h � 1 g� 1 cat using ZnACuAGa photocatalyst. The methanol selectivity improved by the inclusion of Cu from 5.9 to 26 mol% and 39 to 68 mol%, respectively, when ZnAAl (the conversion 0.16–0.11%) and ZnAGa LDH catalysts were used (the conversion 0.02–0.03%). Specific interaction of Cu sites with CO2 was spectroscopically suggested to enable coupling with protons and photogenerated electrons to form methanol.