Design of intermetallic PdZn catalysts supported on ZnAl layered double oxides for enhanced CO2 thermal hydrogenation to methanol

Developing efficient PdZn/ZnO catalysts for the thermal hydrogenation of CO 2 to methanol remains challenging. In this study, the supported PdZn/ZnO catalysts were fabricated by depositing Pd metalloporphyrin ( viz. , TCPP(Pd)) precursor on Zn-Al layered double hydroxides (ZA-LDH) or the derived Zn-Al layered double oxides (ZA-LDO), whereby the effects of the preparation method, Pd loading amounts, and reduction temperatures on the nanostructure and catalytic performance of PdZn/ZnO catalysts were thoroughly examined. The optimized catalyst, 3.28 %Pd@ZA-LDO-R400, with PdZn particle size of 20 nm exhibited superior CO 2 conversion of 8.1 %, methanol space–time yield (STY MeOH-Pd ) of 15 g MeOH g Pd − 1 h −1 , and good stability during 100 h on stream at 300 °C and 3 MPa. Notably, the crystal structures of the PdZn alloy, specifically the PdZn α and PdZn β , were significantly influenced by the reduction temperature. As the reduction temperature increased, the PdZn α phase transformed into the PdZn β phase. Density functional theory (DFT) calculations confirmed that PdZn β has a lower energy barrier for H 2 dissociation compared to PdZn α , thereby enhancing catalytic activity. Furthermore, in-situ DRIFTS characterization revealed that CO 2 hydrogenation to methanol over the PdZn/ZnO catalyst predominantly follows the formate pathway, with key species including HCOO* and CH 3 O*. The hydrogenation of HCOOH* to CHO* was identified as the rate-limiting step in this reaction. Accordingly, this study offers an effective method for fabricating intermetallic PdZn β catalysts with improved performance in CO 2 hydrogenation.