Theoretical insights into catalytic CO2 hydrogenation over single-atom (Fe or Ni) incorporated nitrogen-doped graphene

Developing highly efficient and cheap catalysts for the CO2 hydrogenation is the key to achieve CO2 conversion into clean energy. Herein, periodic density functional theory (DFT) calculations are performed to investigate possible reaction mechanisms for the hydrogenation of CO2 to formic acid (cis- or trans-HCOOH) product over a single Fe or Ni atom incorporated nitrogen-doped graphene (Fe-N3Gr or Ni-N3Gr) sheets. Our calculations found that the CO2 hydrogenation proceeds via a coadsorption mechanism to produce cis- or trans-HCOOH over FeN3Gr and Ni-N3Gr surfaces, which is classified into 2 steps: (1) the CO2 hydrogenation to form a formate (HCOO*) intermediate and (2) hydrogen abstraction to produce cis- or trans-HCOOH. The formation of transHCOOH over both Fe-N3Gr and Ni-N3Gr surfaces exhibit the obvious superiority due to the low barrier all through the whole channel. The highest energy barriers (Ea) in the case of trans-HCOOH formation on Fe-N3Gr and Ni-N3Gr surfaces are only 0.57 and 0.37 eV, respectively, which indicated that the CO2 hydrogenation to trans-HCOOH could be realized over these catalysts at low temperatures, especially the Ni-N3Gr surface. On the other hand, our findings show that the competitive reaction that produces CO and H2O is almost impossible or extremely difficult to proceeds under ambient conditions due to the large Ea for the formation of these side products. Moreover, the microkinetic modeling of the CO2 hydrogenation on both surfaces was investigated to confirm these results. Thus, the Fe-N3Gr and Ni-N3Gr catalysts reveal excellent catalytic activity and highly selective for CO2 hydrogenation to trans-HCOOH. This theoretical investigation not only provides a promising catalyst but also gives a deeper understanding of CO2 hydrogenation reaction.