Identifying the active nickel sites and hydrogen species in the phosphosulfide overlayer on the Ni₂P hydrotreating...

Density functional theory (DFT) calculations and an atomistic thermodynamic approach were employed to identify the active nickel sites and determine the chemical nature of the hydrogen species in the nickel phosphosulfide overlayer formed on the (0001) and (101̄0) surfaces of the Ni₂P hydrotreating catalyst. Our results showed that Ni(1) sites are less resistant to sulfur than Ni(2) sites, in agreement with experimental observations. Under HDS conditions, the fully sulfided (0001) surface of Ni₂P was found to be structurally similar to the (111) surface of the low-activity sulfide Ni₃S₂, where closely coordinated Ni atoms are prone to sulfidation. Non-hydrogenated surfaces were always the most stable, suggesting that NiH and SH groups are unlikely to form on these surfaces. In contrast, the most stable (101̄0) surface was identified as a hydrogenated surface with coordinatively unsaturated Ni(2) atoms surrounded by SH groups. Depending on the electronegativity of their ligands (S or Ni), the hydrogen species exhibit either protonic (H^δ+) or hydridic (H^δ-) character. Our results suggest that protons within SH groups are the most likely reactive hydrogen species on the nickel phosphosulfide overlayer under reaction conditions, providing atomic-level insight into the origins of HYD activity in Ni₂P catalysts.