Synthesis And Application Of Supported Nano-gold Catalyst

Since 1987,when Japanese professor Haruta discovered that the nano-sized gold particles exhibited extremely high catalytic activity in CO low-temperature oxidation,nano-gold catalysts have always been a hot area of academic research.Compared with Pt and Pd,gold catalyst is distinguished by its low-temperature reactivity and high chemo selectivity.Thus,it is widely used in many reactions,such as H₂O₂ synthesis,amine oxidation,epoxidation of alkene and nitro reduction.A large number of studies show that the activity of supported nano-gold catalyst mainly depends on the gold particle size,the type of support as well as preparation method.In this case,the key to get high-performance nano-gold catalyst is to obtain the Au nanoparticles?NPs?with appropriate size by using suitable support in an efficient and convenient way.In this paper,a new kind of dendritic mesoporous silica nanoparticle?DMSN?is employed as support.Through organic complex grafting,the highly dispersed Au NPs with narrow particle size distribution were obtained easily by the confinement effect of mesopores about 3 nm.This supported nano-gold catalyst was applied to oxidation and hydrogenation reactions respectively.In the first part of this paper,the photocatalytic liquid-phase reduction of 4-nitrophenol?4-NP?to 4-aminophenol?4-AP?was chosen as probe reaction to investigate effects of support and relative content,variety,chemical state of metal to catalytic performance.Au/Ag bimetallic catalyst was found to manifest the best catalytic activity compared with other monometallic catalysts when the loading amount was 2 wt%.And it showed the optimal photocatalytic reaction performance when Ag existed in the form of Ag_xO semiconductor nanoparticles.Combining with the theories of semiconductor energy band,work function and electronegativity of metal atom,we have proposed the photoinduced excitation enhanced electron transfer model on bimetallic nanoparticle interface for the first time,and understood the nature of the 4-NP photocatalytic conversion at the molecular level consequently.In the second part of this paper,the prepared nano-gold catalyst was used in the selective oxidation of benzyl alcohol in fix-bed reactor.After modifying the catalyst by introducing Co₃O₄ NPs,the catalytic activity of a series of catalysts with different synthetic method,calcination temperature and loading amount of metal and oxide was examined.It was found that the activity and life of the catalyst have been greatly improved.The yield of benzaldehyde can be kept over 90%for 650 h at 250?when the loading amounts of Au and Co₃O₄ were fixed at 2 wt%and 3 wt%respectively.To our best knowledges,this is the longest life time catalyst for the selective oxidation of benzyl alcohol in fix-bed reactor.Finally,the synergistic effect of different active components,including reactive metal oxide,Au NPs and amino functional groups is proposed to understand the activation mechanism of molecular oxygen at the confined nanointerface.In the third part of this report,DMSNs loaded with Au NPs were surface modified by post-grafting and also applied to the selective oxidation of benzyl alcohol.With the increase of surface hydrophobicity,the chemical activity and life span are significantly improved.In the near future,more intensive investigations need to be carried out to understand the origin of the improvement in chemical reactivity and life times.