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 CO 2 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-N x C 4– x -G ( x = 1–4) are structurally stable. Through the discussion of the reaction mechanism, the optimal path of CO 2 hydrogenation to HCOOH on all the studied catalysts is via CO 2 * + H 2 * → HCOO* + H* → HCOOH*. In addition, they have different speed limit steps. For Sn-N 1 C 3 -G and Sn-N 2 C 2 -G, the rate-determining step of CO 2 to HCOOH is CO 2 * + H 2 * → HCOO* + H*, while the rate-determining step of the other two catalysts is HCOO* + H* → HCOOH*. Meanwhile, the order of catalytic activities of Sn-N x C 4– x -G is determined to be Sn-N 1 C 3 -G > Sn-N 2 C 2 -G > Sn-N 3 C 1 -G > Sn-N 4 -G. Furthermore, the origin of the catalytic activities for HCOOH synthesis on Sn-N x C 4– 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-N x C 4– x -G system.