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

Abstract

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 MII–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., MII–MOF-808 (MII = CuII, FeII, PtII, NiII, and PdII), 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 PtII > NiII > PdII > CuII > FeII, 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 CuII–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 CuII–MOF-808. Using the energetic span model, the relative turnover frequencies of CO2 hydrogenation to various C1 products over MII–MOF-808 were calculated. The CuII–MOF-808 catalyst is found to be the most active catalyst among five MII–MOF-808 catalysts