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 H₂ 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 H₂ and then proceed via completely different paths. At Ru-dG the CO₂ hydrogenation occurs with the H atoms from the dissociated H₂, while the Cu-dG favors the proton transfer from an additional H₂, 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.