Effect of Particle Size on the Deep HDS Properties of Ni2P Catalysts

Nickel phosphide nanoparticles encapsulated in mesoporous silica (Ni2P@mSiO(2)) were used to probe particle size effects in the deep hydrodesulfurization (HDS) of 4,6-dimethyldibenzothiophene, (4,6-DMDBT). The HDS properties of the well-defined nanoparticle catalysts were compared to those of Ni2P/SiO2 catalysts prepared by different methods and having different particle sizes. The Ni-2 PpmSiO(2) nanocatalysts had Ni2P particle sizes of 6.3, 11.4, and 16.0 nm, while the Ni2P/SiO2 catalysts had particle sizes of 3.2 and 5.7 nm. Linear correlations of CO chemisorption capacity and 4,6-DMDBT HDS activity with calculated Ni2P surface area were observed for the Ni2P@mSiO(2) nanocatalysts. The CO chemisorption measurements yield a value of 0.28 CO molecules per surface Ni atom, with some Ni sites likely blocked by the mesoporous silica shell that encapsulates Ni2P nanoparticles, as well as possibly by excess P at the particle surfaces. HDS turnover frequencies (TOFs), normalized on the basis of surface Ni sites and CO chemisorption, yield values of (1.1-2.1) x 10(-5) s(-1) (TOFNi) and (4.7-8.4) x 10(-5) s(-1) (TOFCO) for the Ni2P@mSiO(2) nanocatalysts at a reaction temperature of 553 K. The TOFNi values are similar to or higher than those measured for the Ni2P/SiO2 catalysts. With respect to sites titrated by CO (TOFCO), theNi(2)P/SiO2 catalyst prepared from a hypophosphite-based precursor was over two times more active than the Ni2P@mSiO(2) nanocatalysts and the Ni2P/SiO2 catalyst prepared from a phosphate-based precursor. The Ni2P@mSiO(2) nanocatalyst having 6.3 nm Ni2P nanoparticles and the Ni2P/SiO2-hypo catalyst had higher HDS activities than a commercial sulfided Ni-Mo/Al2O3 catalyst; the results indicate that a catalyst composed of 3-5 nm Ni2P particles would have a 4,6-DMDBT HDS activity competitive with commercial Co-Mo/Al2O3 and Ni-Mo/Al2O3 catalysts. The Ni2P@mSiO(2) and Ni2P/SiO2 catalysts strongly favored products of the hydrogenation pathway for sulfur removal as did the sulfided Ni-Mo/Al2O3 catalyst.