Dimethyl Ether Steam Reforming Of Composite Bifunctional Catalysts

Due to its low operating temperature, high efficiency, environmental benignity, and high reliability, the proton exchange membrane fuel cell (PEMFC) is perceived as an ideal power source for electric vehicles. Indeed, the technology of PEMFC for electric vehicles has progressed rapidly in recent years. In contrast, as the fuel for PEMFC, hydrogen including its source and supply is still a challenge although much work has been done. Among the available choices, hydrogen production via the on-board reforming of hydrogen-rich compounds is believed a reasonable and practical route. Remarkably, dimethyl ether (DME) as an alternative fuel is addressed a cost-effective transition from petroleum to hydrogen due to but not limited to the following merits, e.g.,1) no toxicity or harmlessness; 2) higher hydrogen content (13.0 wt.%); 3) gas-like property and liquid-storage density; 4) available power source for PEMFC. However, the related investigation on SRD is just at the very beginning stage of catalyst screening.SRD is composed of two-consecutive reactions, i.e., DME hydrolysis and the steam reforming of the methanol (SRM) produced in the first step. Therefore, bifunctional catalyst with acidic and metallic functions is required for SRD, and alumina or zeolite combined with Cu-based oxides are generally used. Based on our understanding on the series-reaction nature of SRD, in this thesis, hybrid catalyst of vatious commercial Cu/ZnO/Al2O3 and zeolites was extensively and comparatively studied for SRD. Results indicate that the time-on-stream DME conversion, hydrogen yield, and selectivity of carbon-containing products were strongly dependent on the composition and mixing method of the hybrid catalyst. Significantly, the powdery mixture of HUSY and Cu/ZnO/Al2O3 showed much higher and more stable DME conversion and hydrogen yield than the granular mixture with the same composition. In contrast, irrespective of the mixing methods of the hybrid catalyst, similar results were obtained over Cu/ZnO/Al2O3 and HZSM-5 (SiO2/Al2O3=38). These quite different results were reasonably explained based on the acidic and structural properties of the zeolites in combination with the two-step reaction mechanism of the titled reaction. The effect of the soures of Cu/ZnO/Al2O3 and zeolites, weight ratio of Cu/ZnO/Al2O3 to zeolite, and space velocity on the catalytic performance of the hybrid catalyst was quantitatively evaluated. The hybrid catalyst of HZSM-25+Cu/ZnO/Al2O3(G207) was found to be the best catalyst for SRD in considering both the DME conversion and hydrogen yield.A series of Cu/ZnO/Al2O3 catalysts were prepared via a complex-combustion method by changing the complex agent, solvent, and Cu/Zn molar ratio (nCu/nZn).Under the reaction conditions of DME/H2O/N2=1/4/5, GHSV=3000 mL·g cat.-1·h-1, T=200-300℃, the powdery mixture of Cu/ZnO/Al2O3+HZSM-5(25) was comparatively evaluated for the hydrogen production via SRD. The hybrid catalyst containing Cu/ZnO/Al2O3 (CWG), which is prepared using deionized water as solvent and glycerin as complex agent, displayed higher catalytic activity at low reaction temperatures. Moreover, the best nCu/nZn was determined to be 50/40. The structural, textural, and reductive properties of the Cu/ZnO/Al2O3 were characterized with XRD, H2-TPR and N2 adsorption/desorpton at -196℃. Combining the SRD and characterization results, the higher dispersion of Cu over CWG was revealed to be a key factor for its higher SRD performance. Irrespective of the complex ageat and solvent, Cu/ZnO/Al2O3 with nCu/nZn of 50/40 showed the highest BET surface area. It can be concluded that the activity of Cu/ZnO/Al2O3 is mainly influenced by the complex agent and solvent while the stability of the catalyst is principally determined by nCu/nZn ratio.