The Study Of UV-Visible Spectroscopy Characterization And Catalytic Activity Of Gold Nanoparticles

Particle size and surface structure of gold catalyst have a vital link with catalytic activity and selectivity, therefore how to prepare gold nanoparticles of smaller size and of narrow size distribution is always the key to the preparation of a highly active nano-gold catalyst.Polyethylene imine (PEI) as a stablizer and a reducing agent, plays an important role in nucleation and growth of gold nanoparticles. In this paper, in the presence of PEI, a series of water-soluble gold nanoparticles of small particle size were synthesized by in situ reduction. Containing enormous amounts of amino groups on polymer chains, PEI was employed to serve as a ligand to Au(Ⅲ) and a proper reducing agent to produce Au(O) clusters. PEI also plays an important role in promotion of nucleation of Au(O) species and in restraining growth of gold nanoparticles. PEI displays a higher affinity with the surface of nanoparticles than the solvent molecules (water) and subsequently acts as a stabilizer preventing the aggregation of the particles. In brief, PEI can promote the nucleation rate, and prevent aggregation of nanoparticles, leading to the production of gold nanoparticles of smaller particle size and narrow size distribution. TEM characterization showed that gold nanoparticles prepared in the presence of PEI are mainly spherical particles with the average particle size of5-13nm in diameter and with crystal faces (220),(222),(311),(400) exposed.UV-Vis spectra of solution of chloroauric acid and the complexes formed by Au (Ⅲ) with PEI were studied, and the effect of pH value and temperature on the formation of Au (Ⅲ) complexes and gold nanoparticles were also investigated. On UV-visible spectra, an absorption peak around350nm appeared attributed to ligand to metal charge transfer (LMCT) within the complexes formed by Au (Ⅲ) with PEI. And near netrual pH values are preferred for the formation of Au (Ⅲ) complexes and gold nanoparticles. The suitable reduction temperature range is60℃～75℃and proper ratio of PEI:Au should be controlled between160～1000g/mol. In order to obtain supported catalysts with superiorities of easy separation from reaction mixture and possible recycling, amino-group richly containing SiO2-g-PEI supports and carboxylic-group richly containing SiO2-g-PAA supports were synthesized via coupling effect and surface-initiating graft polymerization. IR spectra show that:CPTMO, PEI, AMPS and PAA were grafted onto the surface of SiO2consequently. Elementary analysis was used to find out the grafting amount of polyethyleneimine. Acid base titration was used to determine the degree of grafting of PAA. XRD characterization showed that gold nanoparticles supported on SiO-g-PEI and SiO2-g-PAA possess the average particle size of10～18nm and15-40nm in diameter, respectively.The catalytic performances of prepared nanoparticles were evaluated by a model reaction of catalytic reduction of4-nitrophenol to4-aminophenol. It was found that the prepared gold nanoparticles show good catalytic activities including those supported ones. Remarkable performance was observed especially for the gold nanoparticle prepared when PEI:Au=240g/mol, pH value=6.7and after being reduced for1hour. The apparent superiority of these gold nanoparticle prepared in the presence of PEI is that they are quite stable as catalysts. After being restored at4℃in darkness for several months, they can behave effectively as freshly produced.