Preparation And Hydrotreating Performances Of Ni₅P₄ And Ni₂P Catalysts

In recent years,the metal-rich nickel phosphide exhibits excellent catalytic/promoting properties in terms of hydrotreating,electrocatalysis,photocatalysis,etc.due to its noble metal-like properties and strong thermodynamic stability.The metal-rich nickel phosphides have a variety of methods for synthesis,including phosphate reduction,hypophosphite/phosphite decomposition,organic phosphorus?triphenylphosphine/trioctylphosphine/trioctylphosphine oxide?,water/solvothermal synthesis,etc.The synthesis methods will affect the crystal phase,morphology,and properties of nickel phosphide.Currently,the phase of metal-rich nickel phosphide includes Ni₃P,Ni₁₂P₅,Ni₂P,Ni₅P₄ and other crystal phases,but the Ni₅P₄ crystal phase are produced with organic phosphorus as a phosphorus source.For the first time,inorganic phosphorus?red phosphorus?was used as the phosphorus source,Ni₅P₄ crystal phase was successfully prepared,and the transformation relationship between the above four crystal phases was preliminarily discussed.Ni₃P-Ni,Ni₃P was selected as the nickel source reacted with red phosphorus,respectively.It was found that the best condition for the synthesis of Ni₅P₄ phase with Ni₃P-Ni and red phosphorus as the precursor was 500°C,heat treatment for 2 h,and the ratio of phosphorus to nickel was equal to 2.Increasing the temperature of the prepared Ni₅P₄ to 600°C will change to the Ni₂P phase.The structure and morphology of the catalysts were confirmed by XRD and TEM.The ratio of phosphorus to nickel was equal to 1 with Ni₃P and red phosphorus as precursors.The best conditions for the synthesis of Ni₅P₄ phase were 400°C,heat treatment for4 h and 500°C,and heat treatment for 1 h.The temperature of Ni₅P₄ was further increased to600°C to change to the Ni₂P phase state;NaH₂PO₂·H₂O were used as the phosphorus source to react with Ni₃P-Ni and Ni₃P respectively,and different nickel phosphides were prepared by changing the nickel-phosphorus ratio and heat treatment temperature.The prepared catalysts was characterized by XRD,and the conditions for the transition between phases of the phosphide and the preparation method were determined.The following conclusions can be drawn:using NaH₂PO₂·H₂O as the phosphorus source,Ni₃P can be converted into Ni₂P via Ni₁₂P₅ when the temperature rises from 300°C to 700°C;using red phosphorus as the phosphorus source,Ni₃P and Ni₂P can be converted into Ni₅P₄ at the temperature not exceeding500°C.The prepared Ni₅P₄ can be transformed into Ni₂P phase by high temperature?>600°C?treatment.The optimal condition for the synthsis of Ni₅P₄ is treatment at 500°C for 2 h with the ratio of phosphorus to nickel equals to 2,when using NiCl₂·6H₂O and red phosphorus as the source of nickel and phosphorus,respectively.Continuing to heat up above 600°C,the Ni₅P₄ crystal phase is converted into a Ni₂P crystal phase.The as-prepared Ni₅P₄ and Ni₂P are subjected to hydrodesulfurization?HDS?,hydrodenitrogenation?HDN?,and hydrodeoxygenation?HDO?.The main product in HDS reaction is biphenyl?BP?,and BP production was increased with the increasing of temperature.Among all the three samples,the dibenzothiophene?DBT?conversion rate is highest on Ni₅P₄,indicating that Ni₅P₄ has good desulfurization ability;In the HDN reaction,the production of denitrification products propylbenzene?PB?and propylcyclohexan?PCH?was very low,but the partial pressure of py-tetrahydroquinoline?THQ1?was high.As the temperature increased,the content of THQ1 decreased,while the content of quinoline,bz-tetrahydroquinoline?THQ5?,decahydroquinoline?DHQ?and o-propylaniline?OPA?increased.It shows that the denitrification ability of the as-prepared catalyst is not strong,but it has a certain ability of hydrogenation;In the HDO reaction,the selectivity of cyclohexane on Ni₅P₄ catalyst was high under high temperature,whereas,the selectivity of cyclohexane changes little with temperature on Ni₂P and Ni₂P-ref,indicating that Ni₅P₄ has good hydrodeoxygenation ability.