Unraveling the Structure and Surface Chemistry of the Phosphosulfide Phase Formed on Ni₂P under Hydrodesulfurization Reaction Conditions: A DFT Study

Abstract

There is experimental evidence that the actual active phase of the Ni2P catalyst under hydrodesulfurization (HDS) reaction conditions is a nickel phosphosulfide phase formed on the catalyst surface. Combining DFT calculations and an atomistic thermodynamic approach, we investigated the possibility of sulfur adsorption as well as the replacement of surface phosphorus by sulfur on the (0001) and (100) surfaces of the Ni2P catalyst. Our DFT calculations showed that sulfur could replace up to 100% of the surface phosphorus under hydrodesulfurization (HDS) reaction conditions. We identify that possible sulfur adsorption sites on both surface terminations are arrangements of Ni trimers (Ni3 sites). We found that only the phosphosulfide phase formed on the (100)-AB_Ni2P surface termination has coordinatively unsaturated Ni atoms available for the adsorption of organic sulfur-containing compounds. Our results suggest that the phosphosulfide phase formed on the (100)-AB_Ni2P termination and not on the (0001)-Ni3P2 termination could be responsible for the high HDS activity of the Ni2P catalyst. Upon the replacement of P by S, the Ni3 sites undergo tensile strain, and their reactivity toward S adsorption is modified by the imposed strain. We found that the Ni–Ni bond distance is an important parameter in describing the HDS activity of the Ni2P catalyst. By analyzing the relationship among the ensemble, ligand, and strain effects, we were able to provide a better understanding of the promoting effect introduced by the formation of the nickel phosphosulfide phase.

The Role of Hydrogen Adsorption Site Diversity in Catalysis on Transition Metal Phosphide Surfaces

“…100 Similar results were found with Ni2P/SiO2, 11 which is more active than Ni/SiO2 by 20 times. 8 Theoretical work postulates that phosphorus plays a key role in TMP-catalyzed HDS. DFT studies by Liu et al argue that sulfur atoms bound to Ni3-hollow sites can react with adsorbed-H on neighboring P-sites and avoid deactivation from S-poisoning that usually occurs on pure metal surfaces.…”

“…More recent work on Ni2P-catalyzed HDS has proposed that a phosphosulfide phase on the (101̅ 0)-Ni2P terminated surfaces may be responsible for the high activity of Ni2P for HDS. 8 This facet has reduced S-poisoning due to surface phosphorus and sulfur substitution and optimized Ni3-site geometry that alter binding energetics.…”

“…Transition metal phosphides (TMPs), such as nickel phosphide, cobalt phosphide, and molybdenum phosphide, have been historically used as hydro-processing catalysts in the petroleum industry, enabling critical hydrodenitrogenation (HDN) and hydrodesulfurization (HDS) reactions. [3][4][5][6][7][8][9][10][11] Over the past two decades, TMPs have been attracting extensive attention for applications in energy conversion 12,13 due to their exceptional activity towards the hydrogen evolution reaction (HER). [14][15][16][17] The exceptional HER activity of certain TMPs has been attributed to their nearly ideal H binding energy (HBE), similar to Pt, as deduced by first-principles calculations.…”

The Role of Hydrogen Adsorption Site Diversity in Catalysis on Transition Metal Phosphide Surfaces

“…This facet has reduced Spoisoning due to surface phosphorus and sulfur substitution and optimized Ni3-site geometry that alter binding energetics. 8 DFT studies on the Ni2P and MoP-catalyzed reverse water gas shift (RWGS) reaction have attributed the ability of these catalysts to favorable dissociation of H2 on these surfaces. 110 Similar to HDS, we believe it is necessary to think about phosphorus's role and how it impacts adsorption of other intermediates (i.e., CO2).…”

“…Transition metal phosphides (TMPs), such as nickel phosphide, cobalt phosphide, and molybdenum phosphide, have been historically used as hydro-processing catalysts in the petroleum industry, enabling critical hydrodenitrogenation (HDN) and hydrodesulfurization (HDS) reactions. [3][4][5][6][7][8][9][10][11] Over the past two decades, TMPs have been attracting extensive attention for applications in energy conversion 12,13 due to their exceptional activity towards the hydrogen evolution reaction (HER). [14][15][16][17] The exceptional HER activity of certain TMPs has been attributed to their nearly ideal H binding energy (HBE), similar to Pt, as deduced by first-principles calculations.…”