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

Area of Science:

• Catalysis
• Materials Science
• Surface Chemistry

Background:

• Nitrogen-doped graphene single-atom catalysts (SACs) are promising for selective hydrogenation and hydrogen storage.
• Understanding hydrogen evolution mechanisms on these SACs is crucial but challenging.

Purpose of the Study:

• To systematically investigate hydrogen adsorption and dissociation energies on M-C3-xNx active sites (M=Co, Ni, Pd; x=0-3).
• To elucidate the distinct mechanisms and activation barriers governing hydrogen dissociation based on nitrogen content.

Main Methods:

• First-principles calculations of hydrogen adsorption and dissociation energies.
• Analysis of hydrogen dissociation pathways (heterolytic and homolytic) on various nitrogen-doped graphene SAC models.
• Investigation of Brønsted-Evans-Polanyi scaling relationships for activation barriers.

Main Results:

• Hydrogen dissociation is generally endothermic, except for Pd-N3 and Pd-CN2 sites.
• Nitrogen-poor sites favor heterolytic dissociation with high activation energy (0.6-1.1 eV), except for Pd-C2N (0.37 eV).
• Nitrogen-rich sites favor homolytic dissociation with low activation barriers (<0.4 eV), though Ni-N3 shows recombination due to electronic confinement.

Conclusions:

• Detailed understanding of hydrogen dissociation mechanisms on N-doped graphene SACs is achieved.
• Brønsted-Evans-Polanyi scaling is observed across all models.
• Findings provide insights for designing tailored SACs for specific catalytic applications, including hydrogen evolution and storage.