| Summary: | 碩士 === 國立臺灣師範大學 === 化學系 === 102 === Steam reforming of ethanol is considered as an important reaction for producing hydrogen. In Chapter 3, we computational examined the high oxidative capability and acetaldehyde selectivity for coinage metals in the reforming reaction. The energetic results show that coinage metals have much lower activation energies and higher exothermicities for the oxidative dehydrogenation steps. In the electronic structure analysis, coinage metals with saturated d bands can efficiently donate electrons to O* and OH* and better promote their p bands to higher energy levels. The negatively charged O* and OH* with high-lying p bands are responsible for lowering the energies in oxidative steps. In Chapter 4, we investigated the catalytic mechanism and oxygen effect in the oxidative steam reforming (OSR) of ethanol on Rh(111). The mechanistic results show that both surface acetaldehyde (CH3CHO*) and oxametallacycle (CH2CH2O*) species are key intermediates. The formation of major products CO(g) and CO2(g) through high-barrier oxidation steps are considered the rate-determining steps in the overall catalytic process. Surface oxygen (O*) species can lower the barriers for the rate-limiting oxidation steps. The kinetic calculations also well-predict the experimental observations. These results concluded that the catalytic performance of Rh-based catalysts can be improved by using promoters with better oxidative capability.
In Chapter 5, we examined Fischer-Tropsch synthesis (F-T synthesis) reaction mechanism on Ru(0001) and Co(0001) surfaces, which contains investigations of CO activation, hydrogenation of CHx (x = 0~3) species, C-C coupling processes, and termination reaction and found the difference of the mechanism. We also discussed the effect of promoter, Na, and found its effect in the mechanism. According to our calculations, CO prefers to direct dissociate or form the HCO intermediate before C-O bond scission rather than forming COH on both Ru(0001) and Co(0001) surfaces. In addition, the CH will be the most abundant adsorbing species on both two surfaces and CH3 is also trapped on Co(0001) surface. The favorable coupling reactions on both surfaces are CH2 + CH2. CH + CH and CH+CHO are also possible coupling reaction on Co(0001) surface. The hydrogenation reaction (termination reaction) of CHx and C2Hy are kinetically and thermodynamically favored on the Co(0001) surface. Finally, Na enhances the adsorption of O-containing species, such as CO, HCO and HCHO, and decreasing the barrier for C-O bond cleavage. On the other hand, Na has no effect on CHx, keeping the advantage of the faster C-C coupling reaction rate on Co(0001). In summary, adding metal which attracts O better than C to Co or covering Co on oxide supports may improve the selectivity of long-chain hydrocarbon to heavier molecular weight and decrease the production of oxygenate.
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