| dc.description.abstract | This thesis is divided into two parts.
Part Ⅰ：Supported copper catalyst
Samples of copper on rice husk ash (Cu/RHA) have been prepared by the ion exchange method, with various copper loadings and have been calcined at different temperatures. Such samples were tested for dehydrogenation of ethanol to acetaldehyde. The samples were characterized by DSC, XRD, FTIR, TEM, XPS, TPR, BET, and H2-N2O titration techniques. FTIR spectra of dried and calcined samples illustrate the formation of chrysocolla in Cu/RHA. XRD and XPS results of the calcined sample show the presence of two different species of copper, as CuO and as Cu2+ ions. Upon reduction in hydrogen above 523 K, the copper species are partially reduced to Cuo and/or Cu+. TEM images show that copper crystallites are spherical in shape and are evenly distributed. TPR results reveal that various copper loadings in Cu/RHA exhibit similar metal-support interaction (MSI). Ethanol conversion for dehydrogenation of ethanol is found to be independent with calcination temperature and has little effect on Cu loading. Ethanol is selectively converted to acetaldehyde at the reaction temperature of 483~573 K. The Cu/RHA exhibit higher catalytic activity and lower deactivation rate in comparison with Cu/SiO2 catalysts. Catalytic activity of Cu/RHA is found to depend on both Cu surface area and interaction of metal species with the support.
Part Ⅱ：Supported gold catalyst
Hydrogen production by partial oxidation of methanol, using air as oxidant, has been studied over a series of Au/MnOx and Au/CuO/ZnO catalysts, prepared by co-precipitation technique. The activity of Au/CuO/ZnO catalysts has been compared with Au/ZnO and CuO/ZnO catalysts.Catalyst characterization included ICP, TGA, TPR, BET, XRD, TEM and XPS analysis.
In Au/MnOx catalysts, XRD and XPS analyses demonstrate that in uncalcined catalyst the support is present mainly as manganese carbonate and gold as both in metallic and oxidic form. After calcination at 573 K, the carbonate converted to oxides of manganese, mainly as Mn2O3 and oxidized Au phase is reduced to metallic gold. TEM analysis indicates that the manganese support is present as plate-like shape and the gold particles as spherical shape. The size of the gold particles increases from 5.2 nm to 7.5 nm when the calcination temperature increases up to 873 K. Different gold loading, calcination temperature and reaction temperature strongly influence the catalytic activity. The optimum gold content for hydrogen selectivity is 1.9 at.% Au. The uncalcined catalyst shows the highest activity and it decreases with increasing calcination temperature. The higher activity of uncalcined catalyst could be due to the presence of smaller metallic Au crystallites with partially oxidized Au and excess hydroxyl group. It appears plausible to suggest that pore size play a key role in determining the hydrogen selectivity. In the reaction temperature range of 473~563 K, both hydrogen selectivity and methanol conversion increase with increasing the temperature and reach a steady state at 523 K. The overall reaction consists of consecutive partial oxidation of methanol at low temperature and methanol steam reforming at high temperature.
In Au/CuO/ZnO catalysts, TGA and XRD analyses demonstrate that the uncalcined Au/CuO/ZnO catalyst samples contain hydrozincite (Zn5(CO3)2(OH)6) and aurichalcite (Zn3Cu2(CO3)2(OH)6) as major components. They decompose into CuO and ZnO during the calcination procedure. TEM analysis reveals that the size of the gold particles increases from 2.5 to 13 nm with the rise in calcination temperature up to 673 K. TPR studies of the CuO/ZnO and Au/CuO/ZnO samples indicate that a shift to lower temperatures for the reduction of Cu2+ to metallic copper in the gold containing sample. This phenomenon is caused by an interaction between gold, CuO and ZnO. The catalytic activity of Au/CuO/ZnO, Au/ZnO and CuO/ZnO catalysts for the POM reaction was studied at 523 K using O2/CH3OH molar ratio of 0.5. The Au/CuO/ZnO catalysts are highly active for POM reaction to produce hydrogen with lesser amount of CO. The results obtained by XRD and TPR characterization indicate that oxidized gold species and reduced copper species in catalysts are active for high activity. The effect of calcination temperature on Au/CuO/ZnO performance shows that the ideal calcination temperature is 573 K. The effect of reaction temperature on catalytic performance of the Au/CuO/ZnO catalysts was studied in the temperature range of 423~548 K. At 448 K the reaction sets on and complete consumption of O2 is observed. Methanol conversion reaches 100 % at 498 K and hydrogen selectivity reaches 100 % at 523 K. CO formed by methanol decomposition is converted to CO2 leading to very low CO throughout the temperature range studied. Water formed by methanol combustion is consumed by steam reforming reaction at high reaction temperatures, leading to 100 % selectivity towards hydrogen formation. | en_US |