Density functional theory study of sulfur adsorption at the (001) surface of metal-rich nickel phosphides: Effect of the Ni/P ratio

A significant increase of the sulfur uptake by Ni_xP with an increasing Ni to P ratio from 2 to 3 was reported recently in ultradeep adsorption desulfurization (ADS) of diesel fuel. It corresponded to increased Ni_xP surface coverage, while no bulk Ni-P-S phases were identified. Density functional theory is applied in this communication to calculate the adsorption energies for S adatoms adsorbed on the surface of Ni₂P, Ni₁₂P₅, and Ni₃P phases. The preferable (001) surface-terminated slabs are considered to link the coverage tendency with the peculiarities of the electron density of states at the Fermi energy. The electron charge redistribution due to the adsorption of S atoms is calculated, and the nature of bonding of S with the substrates is discussed. The combination of ab initio calculations performed in the ground state for the considered systems with statistical thermodynamics renders the possibility to obtain the coverage at the temperatures beyond T = 0 K in good agreement with experimental data. The increase of surface coverage of Ni_xP phases with an increase in the x value is the result of fine interplay between the number of possible sites for S adsorption and the difference in the adsorption energies for these sites.