Transition metal phosphides, particularly nickel phosphide (Ni2P) and molybdenum phosphide (MoP), have emerged as highly effective nanostructured catalysts for hydrotreating processes, including hydrodesulfurization (HDS) and hydrodenitrogenation (HDN). Their unique electronic and structural properties, combined with high thermal stability, make them superior to traditional sulfided catalysts in certain applications. The synthesis, control of phosphidation degree, and identification of active sites are critical factors influencing their catalytic performance.

The synthesis of transition metal phosphides typically involves the reduction of metal phosphate precursors. A common method is the temperature-programmed reduction (TPR) of metal oxides or salts with a phosphorus source, such as ammonium dihydrogen phosphate or hypophosphite. For Ni2P, the reduction of nickel phosphate precursors occurs in a hydrogen atmosphere at temperatures between 500 and 700°C. The reaction proceeds through intermediate phases, such as Ni12P5, before forming the final Ni2P phase. Similarly, MoP is synthesized by reducing molybdenum phosphate precursors at slightly higher temperatures, often above 700°C, to ensure complete phosphidation. The choice of phosphorus precursor and reduction conditions significantly impacts the final phosphide structure, crystallinity, and surface area.

Control of the phosphidation degree is essential for optimizing catalytic activity. Incomplete phosphidation leads to residual oxide phases, while excessive phosphorus can form inactive metal-rich phosphides or phosphorus deposits. For Ni2P, the ideal P/Ni molar ratio is close to 0.5, corresponding to the stoichiometry of Ni2P. Deviations from this ratio can alter the electronic properties and surface morphology, affecting the density of active sites. In MoP, a P/Mo ratio near 1 is optimal, though slight excess phosphorus can enhance stability without sacrificing activity. Advanced characterization techniques, such as X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), are used to monitor the phosphidation degree and ensure phase purity.

Active sites in transition metal phosphides are primarily associated with phosphorus vacancies. These vacancies create coordinatively unsaturated metal sites that act as Lewis acids, facilitating the adsorption and activation of reactant molecules. In Ni2P, the Ni sites in the (001) and (101) planes are particularly active, with the (001) plane exhibiting higher HDS activity due to its higher density of P vacancies. For MoP, the Mo sites adjacent to P vacancies are responsible for hydrogenation and C–S bond cleavage. The concentration of P vacancies can be tuned by adjusting the reduction temperature and phosphorus content during synthesis. Higher reduction temperatures generally increase vacancy density but may also lead to sintering and reduced surface area.

In hydrodesulfurization, transition metal phosphides demonstrate remarkable activity for removing sulfur from refractory organosulfur compounds, such as dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT). Ni2P exhibits a dual functionality, promoting both direct desulfurization and hydrogenation pathways. For DBT, the direct desulfurization route dominates, with biphenyl as the main product. In contrast, 4,6-DMDBT undergoes hydrogenation followed by desulfurization due to steric hindrance from the methyl groups. The turnover frequency (TOF) for DBT HDS over Ni2P ranges from 0.5 to 2.0 s−1 at 300°C and 3.0 MPa H2 pressure, depending on the catalyst preparation method and support. MoP shows slightly lower activity but excels in hydrogenation, making it suitable for feedstocks with high aromatic content.

Hydrodenitrogenation over phosphide catalysts involves the cleavage of C–N bonds in nitrogen-containing compounds, such as quinoline and indole. Ni2P is highly effective for HDN, with quinoline conversion rates exceeding 90% under typical hydrotreating conditions. The reaction proceeds through partial hydrogenation of the aromatic ring, followed by C–N bond scission. The presence of P vacancies enhances the adsorption of nitrogen-containing molecules and facilitates hydrogen transfer. MoP, while less active than Ni2P for HDN, shows selectivity toward ring-opening reactions, which are critical for deep denitrogenation.

The stability of phosphide catalysts under hydrotreating conditions is a key consideration. Unlike sulfided catalysts, phosphides do not require a sulfur-containing feed to maintain activity. However, prolonged exposure to high temperatures and hydrogen pressures can lead to phosphorus loss through the formation of volatile phosphines or surface oxidation. Strategies to mitigate deactivation include the use of supports with high phosphorus affinity, such as silica or alumina, and doping with promoters like cobalt or tungsten. These additives can stabilize the phosphide phase and enhance resistance to sintering.

Comparative studies between Ni2P and MoP reveal trade-offs in activity and selectivity. Ni2P generally outperforms MoP in both HDS and HDN due to its higher hydrogenation capability and stronger metal-sulfur interactions. However, MoP exhibits better tolerance to poisons like nitrogen and polyaromatic hydrocarbons, making it suitable for processing heavy feeds. The choice between these catalysts depends on the specific feedstock and desired product slate.

In summary, transition metal phosphides represent a promising class of nanostructured hydrotreating catalysts with distinct advantages over conventional materials. Precise control of synthesis parameters, phosphidation degree, and active site density is crucial for optimizing their performance in HDS and HDN applications. Future research should focus on improving their stability and scalability for industrial deployment.