| Preparation of nano-scale, highly dispersed gold particles provides their high catalytic activity as gold catalysts. In order to prepare highly dispersed and stable catalysts, surfactant or stabilizing agent can be added; and there is also another option to select meterials with functional groups such as thiol, amino or carboxyl as the supports, which have some interaction with gold to prevent agglomeration of gold nanoparticles and loss of their activities.Supports have a significant effect on the supported catalysts and it is so important to select proper supports to prepare good catalysts. Not only can supports be used as carriers of active components, dispersing active components, provide suitable pore structure, mechanical supporting and thermal stability for the catalysts, but also they can help active sites to enhance the activity and selectivity of the catalyts, so as to save precious metals and so on.In this paper we select chitosan (CS) and branched polyethylene imine (PEI) that are rich in amino group as the appropriate carrier and stabilizer, and prepare gold nanoparticles by reducing the gold solution with various reductants in the presence of chitosan or polyethylene imine.These obtained Au/CS nanoparticles with low gold loading and high activity are about 30～60nm in diameter and Au/PEI nanoparticles are about 2～7nm in diameter, which both show relatively strong interaction between gold and amino group to avoid agglomeration of gold nanoparticles effectively. Eeven though the catalysts are positioned for several months, their catalytic activities are still relatively good, which achieves the desired objectives.We study the general morphology and activity of nano-gold catalyst with Uv-vis, TEM, HPLC characterization. The addition of PEI can promote the rate of nucleation and stabilize the gold nanoparticles, thereby slow down the rate of its growth further, and ultimately form smaller particle size, narrow particle size distribution of nanoparticles. In this paper we select the catalytic reduction of p-nitrophenol to p-aminophenol as a model reaction to investigate and evaluate the catalytic performance of Au/CS and Au/PEI nanoparticles, which are prepared with different preparation conditions, such as:reducing agents, carriers, loading amounts, reduction temperatures, reduction time, the ratios of Au to reducing agent, reaction time and proportions of different solvents. It was found that the types of reducing agents during the process of preparing gold nanoparticles has the greatest impact on catalytic activity, followed by the reduction time, the ratios of Au to PEI, reduction temperature, yet the ratio of reducing agent to Au has a minimal impact. When maintaining the same total amount of gold, catalysts with different loading amounts were used in the catalytic reacition, and it was found that the catalyst with higher loading amount has a better activity. And when the catalysts are positioned for a few months under the condition of low temperature,the catalyt still show good catalytic activity.In addition, it was detected the original reaction rate will be significantly reduced when ethanol is added into the reaction system, which is probably because that ethanol has lower polarity than water, and the higher the ethanol content is, the slower the reaction is, or even incomplete. The addition of ethanol might reduce the rate of hydrolysis of NaBH4, thus hinder the overall reaction rate. The catalytic reaction mechanism needs to be further studied. |
PDF Full Text Request