The catalytic oxidation of HCHO on metal single atoms supported by defective graphene: essential roles of the d electrons and radius of metal atoms

Abstract

In this work, HCHO oxidation on metal single atom (from Sc to Zn) catalysts loaded on single carbon vacancy graphene (M-SG) was comprehensively studied through density functional theory calculations. Results show that the lowest dissociation barrier (E_{bar_O2}) for the O₂ molecule is 0.91 eV on Cr-SG, which is expected to endow Cr-SG with the best performance towards HCHO oxidation. The Cr atom in Cr-SG can both accept and donate electrons using its 3d orbital with “acceptance and feedback” mode, which forms chemical bonds between Cr-SG and gas molecules with high adsorption energies for HCHO (−1.39 eV) and O₂ (−2.53 eV) molecules. Both ER and LH mechanisms are considered for the initial step, which indicates that HCHO oxidation more prefers to occur along the LH path on Cr-SG. The rate-limiting step is step 4 for the decomposition of HCO₂ species with an energy barrier (E_{bar_step4}) of 0.92 eV, which is much lower than that of a commercial Ag catalyst. More importantly, E_{bar_step4} on other M-SG (M represents 4d-, 5d-, lanthanide, and actinide metal atoms) is further studied, where the lowest value is observed on Cr-SG. A machine learning model was constructed to reveal the relationships between E_{bar_step4} and the intrinsic properties (the number of d electrons, radius, group number and electronegativity) of metal atoms, where the number of d electrons and radius of metal atoms have a more significant impact on E_{bar_step4}, where the synergistic effect of the half-filled d orbitals and relatively small radius (r_M) makes Cr-SG have the smallest E_{bar_step4} value among our studied M-SG systems. This work not only proposes Cr-SG as an efficient HCHO oxidation catalyst, but also discovers essential factors for single metal atom catalysts for HCHO oxidation, thus greatly facilitating the development of corresponding catalysts.