Effect of Particle Size on the Sulfur Resistance of Nickel Phosphide Hydrodesulfurization Catalysts

The effect of particle size on sulfur resistance of nickel phosphide (Ni2P) catalysts for the hydrodesulfurization (HDS) reaction was probed using Ni2P nanoparticles encapsulated in mesoporous silica (Ni2P@mSiO2). The Ni2P@mSiO2 nanocatalysts, having well-defined particle sizes of 5.9, 11.2 and 17.1 nm, were investigated for the HDS of dibenzothiophene (DBT) in the presence of a co-feed of H2S. The Ni2P@mSiO2 nanocatalysts were characterized by a range of techniques including CO chemisorption to determine active site densities, and sulfur quantification to probe sulfur incorporation during HDS measurements or treatment in a mixture of H2S/H2. The extent of sulfur deactivation depended strongly on Ni2P particle size, with Ni2P@mSiO2-5.9 nm showing little change in DBT HDS activity or turnover frequency (TOF) before and after co-feeding H2S, while Ni2P@mSiO2-17.1 nm exhibited a significant decrease in HDS activity and TOF after co-feeding H2S. Further, the Ni2P@mSiO2-5.9 nm nanocatalyst exhibited stable HDS activity when brought on-stream, while Ni2P@mSiO2-17.1 nm experienced a 40% loss of HDS activity during the initial 3 h on-stream and then stable activity thereafter. The composition of the phosphosulfide phase (NixPySz layer) formed on the surface of the Ni2P particles under HDS reaction conditions also depended strongly on particle size, with the Ni2P@mSiO2-5.9 nm (S/Nisurf = 0.16) having a substantially lower S content than the Ni2P@mSiO2-17.1 nm (S/Nisurf = 0.53). These results indicate that the surface restructuring of Ni2P associated with formation of a phosphosulfide layer is particle size dependent and leads to active sites with significantly different intrinsic activities for the HDS reaction.