Resonance structure for covalent bonding in PdCx: A theoretical perspective on the geometry, composition, and surface reactivity of PdCx

Palladium carbide (PdC x ) has attracted significant attention owing to its high selectivity and activity for hydrogenation. However, PdC x is obtained only as a non-stoichiometric compound, and the theoretical framework explaining its structure, composition, and surface reactivity remains incomplete. This study provides a detailed investigation of its structure and composition using density functional theory (DFT), yielding a rational computational model for investigating surface reactions of PdC x . The DFT calculations reveal that the covalent nature of PdC x is key to understanding its structure, composition, and surface reactivity. The formal valences of Pd and C are assumed to be 2, and the properties of PdC x can be explained by constructing the resonance structures based on the valences. Specifically, resonance theory explains that carbon is exposed on the (001) plane, which will serves as the active surface for hydrogenation, and that the carbon filling into adjacent layers becomes unstable. In addition, the carbons in PdC x (i.e., non-exposed carbons on surfaces) have little influence on activation of hydrogen. Therefore, PdC x slabs with half-filled carbon atoms are rational models for investigating catalytic activity, and the covalent character of PdC x is important for elucidating its surface reactivity.