Studies On Preparation And Properties Of Au Nanocatalyst By Adsorption Phase Reactor Technology

Au Nano-catalyst which is found to be very potentially useful in contaminant elimination, fine chemicals synthesis and etc. has been developing very rapidity since 1990's. The study on synthesis and properties of Au catalyst is essentially significant in the field of synthesis of nanomaterial. As most of those common preparation methods need not only to cut down the cost but also to improve the properties of the catalysts, further study on synthesis and properties of Au catalysts are still required. In view of the common methods in which Au catalyst is prepared, the way that Ag/SiO₂ is prepared by adsorption phase nanoreactor technique enlightened the author on how to synthesis Au catalyst. In this dissertation, the adsorption layer formed in ethanol-water-SiO₂ system was used as a nanoreactor, and by covering the SiO₂ surface with Ni(OH)₂, the Au/Ni(OH)₂-SiO₂ catalyst was successfully prepared and studied in catalysis of cyclohexane oxidation, which contains certain singnificance in both theoretics and application.Utilizing TEM and XRD, author designed a series experiments including Ni(OH)₂ loading amounts, HAuCl₄ adsorption time, NaBH₄ addition amounts, aging temperature and Au loading amount experiments. Then, by comparing the appearance and grain size of Au particles on the catalyst prepared in these experiments, the results were summarized as followers: (1) While the amount of Ni(OH)2 loading is increased, the Au particles become well-distributed, and the size of Au grain size is getting smaller. (2) The grain size of Au is decreased firstly and then increased with the increase of adsorption time of HAuCl₄. (3) The effect of NaBH₄ addition amount is not so prominent in the range of the experiments. The smallest average size of Au particles is achieved at 0.3mL NaBH₄ (0.0121 g/mL) addition. (4) Au particles formed at the surface of support are unstable firstly, but after being aged at 70℃, as the combination between Au particles and support become stronger, the growth and self-agglomeration of Au particles is avoided. (5) The amount of Au adsorbed on the support is limited. With the increase of the mount of Au loading, the mount of Au particles is increased, but once the percentage of Au/Catalyst is as high as 3.85%, big agglomerated Au particles will appear on the support.Based on the study of the effects of different preparation conditions on the appearance and size of Au particles of the catalysts, author compared of the results of cyclohexane catalytic oxidation using these catalysts, and the following conclusion were obtained: (1) The properties of Au catalysts are decided by both the Au grain size and the combining force between Au and support. The smaller the size of Au, the better the activity of catalyst. The stronger the combining force the better the stability. (2) The selectivity of catalysts is basically decreased with increased conversion which in turn is a function of time. (3) The activity and stability of Au catalyst will be improved after being aged at 70℃.