Theoretical Study on the Catalytic CO2 Hydrogenation over the MOF-808-Encapsulated Single-Atom Metal Catalysts

The search for new catalytic agents for reducing excess CO2 in the atmosphere is a challenging but essential task. Due to the well-defined porous structures and unique physicochemical properties, metal-organic frameworks (MOFs) have been regarded as one of the promising materials in the catalytic conversion of CO2 into valuable platform chemicals. In particular, introducing the second metal (M) atom to form the M-II-O-Zr4+ single-atom metal sites on the Zr nodes of MOF-808 would further greatly improve the catalytic performance. Herein, CO2 hydrogenation reaction mechanisms and kinetics over a series of MOF-808-encapsulated single-atom metal catalysts, i.e., M-II-MOF-808 (M-II = Cu-II, Fe-II, Pt-II, Ni-II, and Pd-II), were systematically studied using density functional theory calculations. First, it has been found that the stability for the encapsulation of a divalent metal ion follows the trend of Pt-II > Ni-II > Pd-II > Cu-II > Fe-II, while they all possess moderate anchoring stability on the MOF-808 with the Gibbs replacement energies ranging from -233.7 to -310.3 kcal/mol. Two plausible CO2 hydrogenation pathways on Cu-II-MOF-808 catalysts, i.e., formate and carboxyl routes, were studied. The formate route is more favorable, in which the H2COOH*-to-H2CO* step is kinetically the most relevant step over Cu-II-MOF-808. Using the energetic span model, the relative turnover frequencies of CO2 hydrogenation to various C1 products over M-II-MOF-808 were calculated. The Cu-II-MOF-808 catalyst is found to be the most active catalyst among five M-II-MOF-808 catalysts.