Formic Acid Synthesis by CO2 Hydrogenation over Single-Atom Catalysts Based on Ru and Cu Embedded in Graphene

At variance with conventional heterogeneous catalysts, where only a small number of transition or noble metal atoms at surfaces play the role of active sites, in the single-atom catalysts (SAC) each metal atom is involved in the catalytic process. Starting from isolated Ru and Cu atoms embedded on defects in graphene, denoted as Ru-dG and Cu-dG, we apply density functional theory (DFT) to examine utilizing these structures to catalyze the conversion of CO2 into the formic acid (FA). Our atomistic modeling of this reaction, highly relevant for reducing the CO2 level in the atmosphere, includes three different reaction pathways. The first relies on a direct hydrogenation of CO2 with protons from the H-2 molecule. Due to energy barriers higher than 35 kcal/mol on both Ru-dG and Cu-dG, this reaction path does not represent a favorable route for FA synthesis. The other two reaction mechanisms start with the dissociative adsorption of H-2 and then proceed via completely different paths. At Ru-dG the CO2 hydrogenation occurs with the H atoms from the dissociated H-2, while the Cu-dG favors the proton transfer from an additional H-2, coadsorbed with CO2 on hydrogenated SAC. Since we find that these pathways were accompanied with the activation energies smaller than 20 kcal/mol, our DFT study indicates that the Ru adatoms embedded into the defected graphene are promising candidates for designing a SAC enabling an efficient conversion of CO2 to FA. Since adsorbed H species markedly decrease Cu binding at the vacancy sites, the Cu-dG is considerably less robust catalyst than Ru-dG.