Mechanistic insight into the catalytic activities of metallic sites on nitrogen-doped graphene quantum dots for CO2 hydrogenation

The origin of selectivity and activity of the CO 2 hydrogenation reaction on single-atom catalysts composed of three adjacent 3d transition metals (Fe, Co, and Ni) supported on N-doped graphene quantum dots were systematically investigated and compared using density functional theory (DFT) calculations, natural bond orbital (NBO), and quantum theory of atoms in molecules (QTAIM) analysis. This study reveals that π-backbonding between the metal and CO 2 * does not occur and [CO 2 ] δ+ species drive the reaction. The CO 2 * reacts with H 2 via the Eley-Rideal (ER) mechanism by using the synergistic effects of the N site. The higher the partial positive charge on the C atom, the lower the Ea of the reaction. Subsequently, a tautomerization reaction, which is facilitated by hydrogen bonding, occurs and hydrogen is transferred to HCOO* resulting in the formation of CHOOH*. This study shows the selective formation of formic acid from CO 2 is accessible on these SACs and Fe-SAC is the best one between these three catalysts. Although CO 2 is more inert than formic acid the H 2 molecule reacts with the adsorbed formic acid more difficult than the adsorbed CO 2 . It is because the hydrogenation of formic acid causes C-H bond formation resulting in failure of the coordinated O atom's octet and the unstable H 2 COOH* is formed. This step is the rate-determining step of HOCH 2 OH formation from CO 2 , with Ea of 1.94, 2.03, and 2.23 eV for Fe, Co, and Ni, respectively. Finally, the system undergoes another tautomerization reaction resulting in the formation of HOCH 2 OH (formaldehyde monohydrate).