Ni₂P Catalysts:Preparation,Characterization And Catalytic Performance In Hydrodesulfurization

Ni₂P is a novel hydrodesulfurization active phase with high activity.However,the synthesis of Ni₂P is more difficult than traditional sulfide catalysts.When the commonly-used method of temperature-programmed reduction（TPR）of nickel phosphate in H₂ is adopted to synthesize Ni₂P/γ-Al₂O₃,phosphate will react with Al₂O₃ at high temperature to cause phosphorous loss;meanwhile,only Ni₂P/Al₂O₃ catalyst with poor dispersion will be obtained at such a high temperature.As a result,the synthesis of Ni₂P/Al₂O₃ is challenging.In this dissertation,Ni₂P/Al₂O₃ was prepared by isolating hypophosphites and nickel source.Hypophosphite can disproportionate at lower temperature（-300 ℃）and release PH₃ in-situ to further react with nickel salt to form Ni₂P/Al₂O₃ at a relatively low temperature.The lower temperature decreases the interaction between phosphorous species and Al₂O₃ greatly,thus makes the preparation of Ni₂P/Al₂O₃ catalyst with high dispersion feasible.The method avoids the deposit of phosphates（the co-product of the disproportionation of hypophosphite）on the surface of the catalyst,which can decrease pore blockage of the catalysts.The influence of the preparation conditions on the formation of Ni₂P was investigated,such as the types of phosphorous and nickel sources,phosphidation temperature,P/Ni ratio,phosphidation time and atmosohere.Three optimized synthetic methods and conditions for preparing bulk Ni₂P were determined as follows:（1）Ni（CH₃COO）2+ H₃PO₂ in N₂;（2）Ni₉S₈+ NH4H₂PO₂ in N₂;（3）NiO + NH₄H₂PO₂ in 10%H₂S/H₂,the precusors were all phosphided at P/Ni=3 and 300 ℃.The experimental results showed that in N₂ flow,oxidizing nickel compounds are not good nickel sources because they consume more PH₃.The use of NiO led to the formation of Ni（PO₃）₂ easily,while sulfidation of NiO to Ni9S8 made the formation of Ni₂P much easier.Ni（CH₃COO）2 decomposes to Ni upon heating,which has been proved to be the best nickel source.The X-ray diffraction（XRD）patterns of the products suggest that the conversion of Ni（CH₃COO）₂ during phosphidation process occurs in the following sequence:Ni（Ⅱ）→Ni--Ni₁₂P₅→Ni₂P→Ni₅P₄→Ni（PO3）2,while Ni₉S₈ follows the sequence Ni₉S₈→Ni₂P→Ni₅P₄→compounds made of Ni（Ⅱ）and P（Ⅴ）.Heating NiO and NH4H₂PO₂ in 10%H₂S/H₂ can also form pure Ni₂P,which indicates the promotion of H₂S to the formation of Ni₂P,and Ni₂P is stable in the existence of H₂S.The three optimized methods for preparing bulk Ni₂P were then adopted to prepare a series of Ni₂P/Al₂O₃ catalysts.Dibenzothiophene（DBT）was used as a model compound to test the activity of Ni₂P/Al₂O₃ catalysts in hydrodesulfurization（HDS）,and the catalysts were also characterized.The results showed that the Ni₂P/Al₂O₃ catalyst made by Ni₉S₈ and NH₄H₂PO₂ presursors（P/Ni=3 and 300 ℃）has the highest HDS activity,probably because the smaller particle size of Ni₂P and the small amount of S atoms on the catalysts leads to a Ni-S-P phase,which is considered to be the possible active phase in HDS reactions on Ni₂P catalysts.Furthermore,the Ni₂P-MoS₂/Al₂O₃ catalysts were prepared following two procedures:（1）by co-impregnation of Ni（NO₃）₂ and（NH₄）6Mo₇O₂4,followed by calcination,sulfidation and phosphidation;（2）by separate impregnation of Ni（CH₃COO）₂ and（NH₄）₂MoS₄,followed by direct phosphidation.The catalyst made in the first method（with Ni/Mo=3 and P/Ni=3）showed the highest HDS activity,which is much higher than that of Ni₂P/Al₂O₃ catalyst（89.6%vs 29.5%DBT conversion respectively）.The reason might be that there is synergistic effect between Ni₂P and MoS₂,and the promotion mechanism will be studied further.In the phosphidation using hypophosphite,the mechanism of the disproportionation of NaH₂PO₂ is vital to explain the formation of Ni₂P.NaH₂PO₂ and its possible intermediates were studied by thermogravimetry and differential scanning calorimetry（TG-DSC）,and the products from heating NaH₂PO₂ to different temperature were studied systemically by XRD and 31P（solid and liquid）nuclear magnetic resonance（31P NMR）.The results showed that the disproportionation of NaH₂PO₂ occurs in three steps,and the reaction equations were also proposed.The overall efficiency of PH₃ was 40%:30%for the first step,8%for the second step and 2%for the third step.The release of PH₃ is concentrated in the first step（310-330 ℃）.When the P/Ni=3,the obtained PH₃/Ni=1.2,which is comparable to the P/Ni ratio in the method of TPR of phosohate to synthesize Ni₂P.