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

Abstract

Nickel phosphide nanoparticles encapsulated in mesoporous silica (Ni2P@mSiO2) 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 Ni2P@mSiO2 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@mSiO2 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) × 10–5 s–1 (TOFNi) and (4.7–8.4) × 10–5 s–1 (TOFCO) for the Ni2P@mSiO2 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), the Ni2P/SiO2 catalyst prepared from a hypophosphite-based precursor was over two times more active than the Ni2P@mSiO2 nanocatalysts and the Ni2P/SiO2 catalyst prepared from a phosphate-based precursor. The Ni2P@mSiO2 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@mSiO2 and Ni2P/SiO2 catalysts strongly favored products of the hydrogenation pathway for sulfur removal as did the sulfided Ni–Mo/Al2O3 catalyst.

Hydrodesulfurization Properties of Nickel Phosphide on Boron‐treated Alumina Supports

“…The TOFs of the optimized catalysts, 2.32 × 10 À 4 and 2.72 × 10 À 4 s À 1 for the Ni 2 P/0.8BÀ Al 2 O 3 -hypo and Ni 2 P/1.2BÀ Al 2 O 3 -phos catalysts, respectively, can be compared to silica-supported Ni 2 P prepared similarly and tested under the same reactor conditions. Drawing from data published previously, [12] 4,6-DMDBT HDS TOFs of 3.13 × 10 À 4 and 7.57 × 10 À 5 s À 1 can be calculated for boron-free Ni 2 P/SiO 2 -hypo and Ni 2 P/SiO 2 -phos catalysts at a reaction temperature of 573 K. This comparison reveals that B addition at optimal loadings to γ- ) than in hypophosphite (H 2 PO 2 À ), [2,11,[55][56] and the results of the current study on B-modified γ-Al 2 O 3 supports are consistent with these earlier findings. Thus, while weakened interactions of Ni and P with the alumina support in the Ni 2 P/xBÀ Al 2 O 3 catalysts enabled more complete reduction of the Ni 2 P, clear advantages remain for using hypophosphite-based precursors owing to the lower TPR temperatures used in the reduction of the precursors.…”

“…HDS measurements were carried out using a fixed-bed, continuous flow reactor operating at a total pressure of 3.0 MPa and temperatures in the range 513-593 K; details about the reactor system and HDS measurements have been reported previously. [12] A model feed consisting of 1000 ppm 4,6-DMDBT (C 14 H 12 S, Alfa Aesar, 97 %) and 500 ppm dodecane (C 12 H 26 , Alfa Aesar, 99 + %) in decahydronaphthalene (C 10 H 18 , Alfa Aesar, cis + trans, 98 %) was used, with the dodecane serving as an internal standard for gas chromatographic (GC) analysis of the reactor effluent. A sample (0.15 g, 16-20 mesh size) of Ni 2 P/xBÀ Al 2 O 3 catalyst was diluted with quartz flakes to a total volume of 5 mL and loaded into a reactor tube having a diameter of 1.1 cm and length of 40 cm; the internal reactor temperature was monitored with an axially mounted thermocouple in direct contact with the catalyst bed.…”

“…[1][2] The most widely employed method for preparing metal phosphide catalysts, either in bulk or oxide-supported form, is temperatureprogrammed reduction (TPR) in flowing hydrogen of phosphate-based precursors prepared from solutions of metal and phosphate salts (e. g. Ni(NO 3 ) 2 + NH 4 H 2 PO 4 ) either evaporated to dryness or impregnated onto the support material. [2] In the case of Ni 2 P, the highest hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) activities have been achieved when the phosphide phase is supported on silica (SiO 2 ), [3][4][5][6][7][8][9][10][11][12] although high activities have been achieved on Al-containing oxide supports when hypophosphite (H 2 PO 2 À ) or phosphite (H 2 PO 3 À ) is used in place of phosphate (PO 4 3À ) as the P source. [11,13] Alternatively, phosphine (PH 3 ) can be used as a gas phase phosphiding reagent, [14][15][16] but the high toxicity of PH 3 makes it unattractive for the large scale synthesis of metal phosphide catalysts.…”

“…In efforts to optimize the deep HDS properties of Ni 2 P-based catalysts, considerable research has focused on reducing particle sizes, with recent studies indicating small Ni 2 P particles (2-5 nm) stabilized on an oxide support could be competitive with current sulfide-based hydrotreating catalysts (e. g. NiÀ Mo/ Al 2 O 3 ). [12,[17][18] Less research has focused on the role of support additives in improving the HDS properties of oxide-supported Ni 2 P catalysts, although additives to alumina-supported catalysts such as boron, fluorine and phosphorus have often been shown to be beneficial for sulfide-based hydrotreating catalysts. [19][20][21][22][23] Herein we report an investigation of the hydrotreating properties of alumina-supported Ni 2 P catalysts in which the γ-Al 2 O 3 support is modified by boron addition prior to synthesis of Ni 2 P. The Ni 2 P/xBÀ Al 2 O 3 catalysts were synthesized using phosphate and hypophosphite precursors, with the hypophosphite-based precursors providing the advantage of a lower reduction temperature of 773 K (vs. 923 K for the phosphate-based precursors).…”

“…In efforts to optimize the deep HDS properties of Ni 2 P‐based catalysts, considerable research has focused on reducing particle sizes, with recent studies indicating small Ni 2 P particles (2–5 nm) stabilized on an oxide support could be competitive with current sulfide‐based hydrotreating catalysts (e. g. Ni−Mo/Al 2 O 3 ) . Less research has focused on the role of support additives in improving the HDS properties of oxide‐supported Ni 2 P catalysts, although additives to alumina‐supported catalysts such as boron, fluorine and phosphorus have often been shown to be beneficial for sulfide‐based hydrotreating catalysts .…”