Nano-gold Catalyst And Its Liquid Phase Selective Oxidation Catalytic Properties

Gold catalysts have been often used to catalyze gas-solid phase reactions. Recently liquid phase selective oxidations catalyzed by gold are extremely interesting. In this paper, colloidal gold particles were prepared by chemical reduction method. Gold catalyst supported on active carbon was prepared by immobilization of colloidal gold and Al2O3-,CeO2-,TiO2-supported gold catalysts and Al2O3-supported Au-Pd catalysts were prepared by anionic impregnation. The catalytic performances in liquid selective oxidation of glucose to gluconic acid and benzalcohol to benzaldehyde over these gold catalysts were accessed systematically using air as oxidant under mild conditions. The influences of the support, preparation method, activation procedure and gold loadings of the catalyst were also evaluated. The techniques such as N2-physical adsorption, XRD, TEM, H2-TPR and XPS characterization techniques were employed to characterize the composition and the structure of gold catalyst and then the relation between the structures with the activity was studied. Some novel results are obtained as follows:1. Colloidal gold particles with the size of 5nm below can be prepared by controlling the amount of PVP stabilizer and HAuCl4 concentration. The gold particles show very high activity for selective oxidation glucose to glucose acid using air as oxidant under mild conditions.2. The influences of glucose concentration, air velocity, reaction temperature and gold loadings on the activity for glucose oxidation over Au/C catalyst were assessed and the best reaction conditions were obtained. It is found that the activity increases with air velocity, reaction temperature and gold loadings. However, glucose concentration impacts slightly on the activity.3. The activity of supported gold catalyst for glucose selective oxidation is significantly affected by the support employed. The order of Au/C > Au/CeO2 > Au/Al2O3 > Au/TiO2 has been found because of the interaction between gold species and support. Strong interaction of gold species and active carbon results in the presence of partly oxidized gold species. Treating active carbon with dilute hydrochloric acid solution can remove effectively the inorganic impurity adsorbed on the surface and then enhance the surface oxygen concentration. That is in favor of gold enrichment and good for Au/C activity.4. The activity of calcined Au-Pd/Al2O3 bimetallic catalyst for glucose selective oxidation to gluconic acid was lower than monometallic Au catalysts because of the presence of PdO phase. For benzalcohol selective oxidation to benzaldehyde, the reduced catalyst gives higher activity than corresponding monometallic catalysts. Some synergetic effect of gold and palladium is responsible for it.