Graphene-Supported Tin Single-Atom Catalysts for CO₂ Hydrogenation to HCOOH: A Theoretical Investigation of Performance under Different N Coordination Numbers

The effects of the adjustment of the N coordination number in Sn single-atom catalysts toward the activity and selectivity of CO2 hydrogenation to HCOOH are systematically explored via density functional theory calculations. The stability of the studied catalysts was evaluated by formation energy calculations, and the calculated results indicated that Sn-NxC4–x-G (x = 1–4) are structurally stable. Through the discussion of the reaction mechanism, the optimal path of CO2 hydrogenation to HCOOH on all the studied catalysts is via CO2* + H2* → HCOO* + H* → HCOOH*. In addition, they have different speed limit steps. For Sn-N1C3-G and Sn-N2C2-G, the rate-determining step of CO2 to HCOOH is CO2* + H2* → HCOO* + H*, while the rate-determining step of the other two catalysts is HCOO* + H* → HCOOH*. Meanwhile, the order of catalytic activities of Sn-NxC4–x-G is determined to be Sn-N1C3-G > Sn-N2C2-G > Sn-N3C1-G > Sn-N4-G. Furthermore, the origin of the catalytic activities for HCOOH synthesis on Sn-NxC4–x-G is revealed through the calculated p-band center. It demonstrated that the p-band center of the Sn atom is a good descriptor to evaluate the catalytic activity for HCOOH synthesis in the Sn-NxC4–x-G system.