Toward a deeper understanding of H-Ni₃C interactions: rule-based insights

Ni₃C exhibits high catalytic activity for hydrogen evolution and hydrogenation reactions; however, the electronic structure of the Ni₃C surface has not been sufficiently investigated to clarify its interaction mechanism with H. In this study, we theoretically elucidate the fundamental rules governing the interaction between H atoms and Ni₃C surfaces: (1) the formal charge and valence of C are -2 and 2, respectively, (2) bonding Ni to C has a formal charge of +2, and (3) the formal valence of Ni²⁺ is 2. Resonance structures based on these rules enable an a priori discussion of the geometries, electronic structures, and interactions of Ni₃C surfaces with H atoms. Specifically, reconstruction of the Ni₃C(113) surface, correlation between Ni-H bonding strength and the number of Ni-C interactions, and heterolytic dissociation of hydrogen have been derived, which are important for understanding Ni₃C catalysis. These insights cannot be obtained from the d-band centre theory, nor can they be completely predicted using state-of-the-art machine learning interatomic potentials. This study clearly demonstrates the importance of understanding the degree of metal-carbon covalency for accurately interpreting the catalytic activity of nickel carbides.