How N-Doping Promotes Hydrogen Dissociation at Graphene-Based Single-Atom Catalysts

Nitrogen-doped graphene single-atom catalysts (SACs) have shown remarkable promise in selective hydrogenation and hydrogen storage. However, the rationalization of hydrogen evolution in these systems is still challenging. In this paper, by systematically calculating hydrogen adsorption and dissociation energies on active 3-fold sites M-C_3-<i>x</i>N_<i>x</i>, with <i>x</i> ranging from 0 to 3 and with M = Co, Ni, Pd, we show that hydrogen dissociation is endothermic─except for Pd-N₃ and Pd-CN₂─and proceeds via two distinct mechanisms, depending on the nitrogen content of the site. For nitrogen-poor sites, dissociation follows a heterolytic pathway with a relatively high activation energy (0.6-1.1 eV), with a notable exception being the Pd-C₂N site with a low barrier (0.37 eV). In contrast, nitrogen-rich sites favor homolytic dissociation, with a significantly lower activation barrier (below 0.4 eV). However, for Ni-N₃, the electronic confinement of hydrogen imposed by nitrogen neighbors prevents true homolytic dissociation, with the two dissociated H atoms on the metal spontaneously recombining. Across all N-doped graphene SAC models considered, the calculated activation barriers exhibit a Brønsted-Evans-Polanyi scaling. This study provides a detailed understanding of hydrogen dissociation on graphene SACs, paving the way for the design of catalysts tailored to specific applications.