Preparation And Application Of Supported Gold Nanopaticles As Catalyst

In recent years,environmental protection and resource utilization have become a new concern.P-nitrophenol(4-NP)is often used as an intermediate for fine chemical products such as pesticides,pharmaceuticals and dyes.Because of its large dosage,high toxicity,strong irritation and difficulty of natural degradation,there is urgent need for the right way to achieve rapid degradation of 4-NP.At present,the feasible methods are microbial degradation method and reduction method.Due to poor timeliness of microbial degradation,common method is reduction method.Therefore,it is particularly important to select the appropriate reaction system and catalyst.Owing to the high specific surface area,active surface bonding state,electronic state,surface electron coordination and other characteristics,precious metal nanoparticles have unique catalytic activity.Gold,which has historically been considered to be an inert metal,has promising catalytic properties for the many reactions including the 4-NP decomposition reaction after the formation of Au nanoparticles(Au NPs)which is widely used in catalyst preparation.Although the Au NPS have many advantages as catalysts,but the high cost,difficulty of recovery,easiness of reunion inactivation and other shortcomings limit its practical performance.In order to improve the stability of Au nanoparticles,reduce costs and simplify recovery,scientists have covered Au nanoparticles with porous materials.Although the low cost,high stability and easy recovery catalyst can be prepared by the above method,but the loading capacity of nanoparticles is low,which seriously reduces the reaction rate.Herein,this paper prepared three mesoporous silica structure catalysts with high loading capacity of nanoparticles.1.Au NPs with different particle sizes were prepared by hydrothermal method.As a precursor of synthetic catalyst,polyvinylpyrrolidone(PVP)was added to prevent the agglomeration of Au NPs during the preparation.The prepared Au NPs have strong absorption at the characteristic absorption peak of about 520 nm,and can catalyze the reduction reaction of 4-NP efficiently,but its solvent and acid resistance are poor,at the same time it is easy to inactivation.In order to increase the mechanical strength and cyclic stability of the gold nanoparticle catalyst,the prepared Au NPs were used as precursor and the surface ions of the Au NPs was modified with CTAB(cetyltrimethylammonium bromide),then the Au NPs was coated with mesoporous silica,and the CTAB was removed by calcination to obtain the standard MCM-41 structure.The core-shell structure,with about 100 nm particle size and the relatively uniform mesoporous structure,maintain the particle size and chemical environment of the Au NPs.Moreover,the solvent and acid resistance are obviously enhanced and the catalytic performance is stable.However,the loading of the Au NPs in the catalyst prepared by this method is low,and the catalytic efficiency of the catalyst can not be improved efficiently.2.The loading catalyst of Au nanoparticles with high Au nanoparticles was obtained by one-pot method.In order to synthesize a catalyst with a higher gold nanoparticle loading,Au@MSNs were synthesized by one-pot method using formaldehyde to reduce HAu Cl4 aqueous solution.The samples were added 0.2 m L,0.4 m L,0.6 m L,0.8 m L,1.0 mL,2.0 mL,3.0 mL HAu Cl4 aqueous solution(0.05 mol/L),respectively.In the samples with additions of 0.2-0.6 m L and 0.8-3.0 mL,the structure of mesoporous Si O2 were different,due to the different sizes of gold cores.The catalyst has a very high catalytic efficiency under the condition of 2.0 m L aqueous solution of HAu Cl4 and has good reproducibility.The catalyst has obvious catalytic effect in the reduction reaction of methylene blue and Rhodamine B and the oxidation reaction of TMB.However,the size of Au NPs under the condition of 2.0 m Laqueous solution of HAu Cl4 is relatively large,which leads to the low TOF(conversion efficiency)of the catalyst.3.The size of complex Au nanoparticles was reduced by recoating to improve the catalytic activity.In order to improve the mechanical properties of the sample,the samples were subjected to recoating of Si O2 under alkaline conditions,and the color of the samples after the recoating became pink.The process minimizes the grain size of the gold core and adds a layer of Si O2 to cover the Au NPs so that the Au NPs are buried deeper.The catalyst sample after the recoating has maintained its catalytic performance,and the reaction can be accelerated after 5 cycles.The conversion efficiency of the samples subjected to recoating was more than 90%,which was higher than that of the recoating.However,the conversion efficiency of the catalyst after recoating was significantly higher and has better catalytic effect.Under the condition of heating at 100 °C,the rate of catalytic reaction has been improved obviously,which proves that the catalyst still maintains the catalytic stability under high temperature condition.