Synthesis And Properties Of Supported Nano Silver Catalysts With High Activity

With the rapid development of the global economy and the continuous expansion of industry and agriculture,environmental pollution problems,especially water pollution become more and more serious.4-nitrophenol?4-NP?as one of the most common non-biodegradable persistent toxic pollutants has attracted people's attention.Silver nanoparticles have the advantages of high surface energy,high catalytic activity and good selectivity,etc.The preparation of supported nano silver catalysts with high activity for the catalytic reduction of 4-nitrophenol is of great significance and research value.In this dissertation,silver nanoparticles with high catalytic activity supported on non-active SiO₂and active Ni?OH?₂ were prepared respectively.The catalytic properties of the as synthesized nanocomposites for reduction of 4-nitrophenol were also studied under the condition of room light.The contents were summarized as follows:Nano silica powder was prepared by neutralizing sodium silicate with hydrochloric acid.The silica-supported nano-silver composites?Ag/SiO₂?were prepared by using the powder as carrier in alkaline ethanol solution,and characterized with XRD,UV-Vis,SEM,TEM,XPS and BET.The silver nanoparticles are uniformly dispersed on the surface of the silica sphere.When the theoretical content of silver is 30%,the specific surface area of the composite is 204.3 m2/g.As the silver contents increase,the size of nanosilver shows a tendency to increase.The prepared Ag/SiO₂ composites with different silver contents were studied for catalytic reduction of 4-NP,the results showed that there is no catalytic reduction ability for pure silica.When the silver contents increased from 5%to 30%,the catalytic degradation rate of Ag/SiO₂ composites increased,but the rate of catalytic degradation was slowed down when the silver content was increased to 50%.The reduction reaction belongs to the zero-order reaction model,it was found that Ag?30?/SiO₂ composites showed highest catalytic activity(K^'=85 min^-1g^-1)and nearly 96.5%of 4-NP has been reducted at 6 mins.The catalyst has good recyclability.The electrochemical impedance spectroscopy?EIS?indirectly confirmed that Ag NPs promoted the reduction of 4-NP by accelerating electron transfer.Ultrathin nickel hydroxide nanosheets were prepared by simple chemical precipitation method.A highly dispersed nickel hydroxide supported nano silver catalyst with a particle size of only about 4.7 nm was prepared by simply mixing ethanol solution of AgNO₃ with Ni?OH?₂ at room temperature.The catalytic performance of catalysts with different silver contents was studied.The results showed that Ag?10?/Ni?OH?₂ composites exhibited the highest catalytic activity and the catalytic activity was worse with the silver content increased to 15%.Reaction temperature and the amount of catalyst both have a great impact on the catalytic rate.When the amount of reducing agent is much larger than the 4-NP content,the amount of reducing agent has little effect on the catalytic rate.The nickel hydroxide supported nano silver catalyst prepared by the experiment has high stability and reusability.It was found that Ni?OH?₂ as a substrate played an important role in the formation of silver nanoparticles,which not only acted as a superior adsorbent of silver ion but also a source of OH^-that was able to accelerate the reaction,which were confirmed by several groups of control experiments.EIS and fluorescence?FL?spectra were employed to indirectly elucidate the mechanism for reduction of 4-NP.The results showed that the presence of Ag NPs can promote the transfer of interfacial electron,which is beneficial for the reduction of 4-NP to 4-AP.