|dc.description.abstract||Both the rice husk ash (RHA) and the RHA-Al2O3 composite oxides prepared by impregnation of RHA in aluminum sulfate were used as a catalyst support, respectively. Nickel catalysts supported on RHA and RHA-Al2O3 were prepared by the ion exchange technique. The catalysts were characterized by nitrogen adsorption method, inductively coupled plasma-atomic emission spectrometer(ICP-AES), scanning electron microscopy(SEM), transmission electron microscopy(TEM), X-ray diffraction(XRD), thermogravimetric analyzer(TGA), temperature-programmed desorption (TPD) of hydrogen and n-butylamine and temperature-programmed reduction (TPR). The catalytic activities of catalysts were tested by CO2 hydrogenation under normal atmospheric pressure. During the investigations of preparation and operation conditions, effects of nickel loading, calcination, reduction, alumina content of RHA-Al2O3 composite oxides and reaction temperature on catalytic performance were also examined. From the experimental results described and discussed above, models were developed to visualize the nature of the RHA-supported and RHA-Al2O3-supported nickel catalysts.
For the RHA-supported nickel catalysts (Ni/RHA) system, the experimental results show that the pH value of 8.5 is the optimum for the preparation of nickel catalysts supported on RHA. That nickel silicate with a layer structure formed after drying step. The thermal decomposition of the layered nickel silicates starts above 773K and leads to the formation of NiO. Reduction of NiO from the thermal decomposition of the layered nickel silicates is found to be unusually difficult. After reduction, the nickel crystallites appear to be spherical in shape. They are homogeneously distributed over the support and exhibit a narrow size distribution. The dispersion of nickel gradually decreases with nickel loading. The nickel surface area increases with nickel loading up to 16.7 wt.% Ni and then decreases with further increase in nickel loading. Furthermore, the mean size of nickel crystallites increases with nickel loading. On the other hand, the results show that activity of catalyst decays as a function of reaction time until 3h due to coking. The activity of catalyst was found to be independent of calcination temperature and time. The catalytic activity was increased with an increasing reaction temperature up to 773K, but decreased with a further increase in the reaction temperature. Moreover, RHA supported nickel catalysts display both higher specific nickel surface area and activity than silica gel as revealed by the H2-TPD and the hydrogenation tests.
In the RHA-Al2O3 composite oxides supported nickel catalysts (Ni/RHA-Al2O3) aspect, the results show that the BET specific surface area of support decreases with the increase in alumina content, while the acidity of support increases with the increase in alumina content. The nickel aluminate formed after the drying step. The decomposition temperature of nickel aluminate to nickel oxide started above 773K. Furthermore, increasing the metal loading decreases the metal dispersion. The crystallite size of nickel supported on RHA-Al2O3 is larger than that of nickel supported on RHA. Generally, the catalytic activity increased with the reaction temperature increases up to certain value and then remain constant. The calcination temperature of 773K and calcination time of 4h were the optimum conditions for the preparation of catalysts. The activity of catalysts was strongly influenced by the reduction temperature rather than by reduction duration. Increasing the alumina content in RHA-Al2O3 composite oxides decreases the catalytic activity of the catalyst. In comparing the catalytic activity of 4.44wt.% Ni/RHA-Al2O3-4 with that of 4.29 wt.%Ni/RHA, it is found that the former exhibits a higher selectivity when the reaction temperature is higher than 773K.||en_US|