Hydrothermal Preparation And The Hydrodechlorination Performance Of Nickel Phosphides

Transition metal phosphides,as a new type of catalyst,have attracted particular interest because of their outstanding performances in various fields,including photonics,electronics,magnetism,catalysis and so on.Nickel phosphides,as one of important transition metal phosphides,exhibit excellent catalytic activity and stability for hydrogenation reactions.Nickel phosphides show a variety of different phases,such as Ni2P,Ni3P,Ni5P2,and Ni12P5 etc.It is a big challenge to obtain single crystal phase with exposed active facets for nickel phosphides.Trichloroethylene(TCE),as extraction agents,degreasing detergents and refrigerants,is widely used in industrial production.It is one of the most widely distributed pollutants in the environment.In all kinds of methods of the degradation of trichloroethylene,hydrodechlorination is proved to be the most safe and efficient method.In this thesis,the synthesis of Ni-P compounds was accomplished by a facile hydrothermal process.A series of different nickel phosphides particles were prepared by adjusting the parameters of the reaction.Surfactants(polyethylene glycol or polyvinyl pyrrolidone)were added into the preparation process of nickel phosphides,and the relationship between surfactants and crystal phase of nickel phosphides was investigated.Meanwhile,the samples were characterized by X-ray diffraction(XRD),Scanning electron microscope(SEM)and Transmission electron microscopy(TEM)to explore the transformation mechanism.And the catalytic performances of samples were determined by the decomposition rates of C-Cl bonds.The detailed results are as follows:(1)The reaction parameters including the ratios of reactants,the reaction temperature,the reaction time and the solvent influence the composition of the products directly.The experiment results show that:when Ni/P?1/2?T?180? or t?48h,it is beneficial for the generation of Ni12P5,otherwise,it is beneficial for the generation of Ni2P.(2)The crystal structures of Ni-P compounds were adjusted by PEG or PVP,but the mechanisms are totally different.When the concentration of PEG in solution reaches the critical micelle concentration,the hydrophilic side will adsorb preferentially on some facets of Ni2P,and then these facets stop to grow.This preferred orientation changes the growth rate of each crystal facet,and finally leads to the transformation of the whole crystal structure.As a result,hexagonal Ni2P turns into tetragonal Ni12P5 crystal.However,PVP is reductive,the reduction ability strengthens as the concentration increases,as a result,Ni12P5 with weaker oxidizability generates.(3)PVP has regulating effect on the particle size of nickel phosphides.In the influence of PVP,nickel phosphides transform into small particles,and these small particles connect together forming three-dimensional porous structure.(4)The catalytic performance of samples,which were modified by surfactants,was 2 to 3 times superior to that of non-modified catalysts.Meanwhile,after modifidation,the optimum reaction temperature was decreased and the active temperature range became wider.