Zno Nanowire-Supported Transition-Metal Catalysts Toward The Catalytic Activity Of 4-Nitrophenol

As a carcinogen widely existing in industrial sewage,4-nitrophenol（4-NP）poses a huge threat to the environment and human health,therefore,it becomes an urgent problem to be solved about how to treat 4-NP in sewage in today’s society.Among many 4-NP treatment methods,the chemical reduction method of converting 4-NP to 4-aminophenol（4-AP）with Na BH₄ as reducing agent under the drive of catalyst is considered to be one of the simplest and most efficient treatment methods.This method can not only eliminate the harm of 4-NP,but also obtain 4-AP,a fine chemical raw material with industrial value.More importantly,the method is mild,controllable and environmentally friendly.Therefore,it is of great significance to develop economical,efficient,controllable,and easily recyclable catalysts for the catalytic reduction of4-NP.In this work,a series of ZnO nanowire film-supported transition metal catalysts（X-ZnO NWF@SSM,X=Ni,Cu,Co,Fe）were developed using stainless steel mesh as the matrix material for the catalytic hydrogenation of 4-NP.SEM,TEM,XRD,XPS,UV-vis and other characterization methods were used to conduct in-depth research.Specifically,a layer of nanocrystalline Zn was first deposited on the surface of the stainless steel mesh as a seed layer by electrodeposition,and then grown into a ZnO nanowire film by hydrothermal reaction.In this process,the effects of nanocrystalline Zn seed layer and hydrothermal temperature on the morphology of ZnO nanowire films were investigated.The results showed that the grain size of nanocrystalline Zn gradually decreased with the increase of current density,and the wire diameter of ZnO nanowires prepared at the same hydrothermal temperature also decreased with the increase of current density;in addition,the wire diameter of ZnO nanowires showed a trend of first increasing and then decreasing with the increase of hydrothermal temperature.Through comprehensive comparison,it was found that the ZnO nanowires prepared at the deposition current density of 3 A dm^-2 and the hydrothermal temperature of 130°C were the most uniform and dense,with an average wire diameter of 45.74 nm,which can be used as the carrier for subsequent loading of transition metal nanoparticles.Ni nanoparticles were deposited on the surface of ZnO nanowires through electrodeposition method by taking ZnO nanowire film supported Ni nanoparticles catalyst as an example.The effects of electrodeposition parameters on product morphology and catalytic performance were studied,and the electrodeposition parameters and surface morphology corresponding to the samples with the best performance were screened.The results showed that the product obtained under the conditions of deposition potential of-0.3 V cm^-2 and deposition time of 30 s has the highest catalytic activity(the rate constant was 0.101 min^-1)with a Ni loading of 0.91 mg cm^-2.SEM results showed that Ni nanoparticles with an average particle size of 55.37 nm were uniformly supported on the ZnO nanowires,forming a silver willow-like heterostructure.Based on the above results,three other catalysts,Cu-ZnO NWF@SSM,Co-ZnO NWF@SSM and Fe-ZnO NWF@SSM,were prepared by the same method.By adjusting the electrodeposition parameters,the morphology and metal loading of the four catalysts were similar,and the effect of transition metal species on the performance of the catalyst was studied.The results showed that the catalytic performance of Cu-ZnO NWF@SSM was much higher than that of the other three transition metal catalysts,and its reaction rate constant was 2.581min^-1;Meanwhile,the Cu-ZnO NWF@SSM had excellent cycling stability,and the conversion rate of 4-NP was still greater than 90%after 13 cycles.The results of electron paramagnetic resonance and density functional theory calculations showed that the catalytic reduction process of 4-NP on the surface of Cu-ZnO NWF@SSM follows the radical reaction mechanism,and its excellent catalytic performance was mainly attributed to the support and dispersion of Cu nanoparticles by ZnO nanowires,as well as the synergistic effect between them.