Effect Of Reductive Oxides And Their Composites On The Gold Catalysts For CO Oxidation

Gold catalysts have potential to be widely used in many fileds such as hydrogen fuel cells, CO gas masks, enclosed CO2 lasers and automobile exhaust processors due to their high activities at low-temperature for CO oxidation. However, the industrial application would be limited for the unsatisfied synthesis repeatability and catalytic stability of active gold catalysts. Besides the morphology and size of the gold nanoparticles, the support is another key factor that influences the catalytic activity. Therefore, the work in this text would concentrate on the effect of support, the active supports of CeO2, MnO2 and silica with high stability were selected to construct composite supports. The gold catalysts with high activity and good stability would be obtained.Firstly, the influences of CeO2 and MnO2 structures on the gold catalysis were evaluated for CO oxidation. Then the composite Au/MxNy/SiO2 catalysts were synthesized with the deposition-precipitation method. The effects of supports were investigated from characterizations of the structures and catalytic abilities of catalysts.In the first part, ceria was introduced with the incipient wetness impreagnation method to modify the silica surface; Au/CeO2/SiO2 catalysts were obtained by a traditional deposition-precipitation method with the hydrochloroauric acid as the precursor. The dispersion of active gold and ceira sites were characterized by XRD, N2 physi-adsorption, TEM et al, and the possible mechanism insights were obtained from TPR, HR-TEM and in situ DRIFTS. HMS with hierachical pores ensured the homogeneous dispersion and the interaction betweem gold and ceria nanoparticles, so it achieved the best catalytic activity and good stability. The silica supports with high ceria content guarantee the small gold nanoparticles with a high dispersion, and the composite catalysts worked much like Au/CeO2.In the second part,α,βandγ-MnO2 were synthesized with a hydrothermal method. A good crystalline and small transverse size was benefical for the CO oxidation from XRD and SEM characterizations. The catalytic activity orders of MnO2 obtained from (NH4)2S2O8was:γ>α>β-MnO2, the order changed after the deposition of gold colloids, at the low temperature range, Au-a-MnO2 and Au-y-MnO2 showed the better activity while Au-β-MnO2 fristly completely converted CO to CO2. The Au/a-MnO2 with the support synthesized from KMnO4 has higher activity than that with (NH4)2S2O8.In the third part, Au/MnO2/SiO2 were synthesized with different methods, MnO2 was introduced by the incipient wetness impregnation or in situ method. Au/Mn2O3/SiO2 had a better ability than Au/MnO2/SiO2 on CO conversion. Nano-rod MnO2 were introduced to the surface of SBA-15 by the in situ method, the composite gold catalysts obtained an improved catalytic activity for CO oxidation. Branchlike nano-MnO2 with the small size were dispersed uniformLy on the HMS with the in situ method, after gold nanoparticles were deposited on MnO2/HMS, the enhanced catalytic activity was obtained on Au/MnO2/HMS.Inconclusion, active gold catalysts with the reductive oxides as the support were deposited on silica. The composite catalyst Au/MxNy/SiO2 showed an improved activity compared to Au/SiO2, and achieved an enhanced stability compared to Au/MxNy.