| 其他摘要 | Hydrogen is an ideal clean source of energy. The development of efficient on-board hydrogen storage materials has been generally recognized as a key technical challenge in commercialization of hydrogen energy. Compared to pressurized tanks and cryogenic liquid hydrogen storage means, material-based solid-state hydrogen storage holds greater promise to provide safe, efficient and high capacity on-board hydrogen sources. Among the solid state hydrogen materials, irreversible chemical hydrides have been demonstrated for practical uses. Recently, sodium borohydride (NaBH4) has received the most extensive attention owing to its combined advantages of high hydrogen capacity, good storability in alkaline solution, easy control of hydrogen generation (HG) rate. Currently, the dominant researches involves three aspects: firstly, a practical hydrogen system should have short startup time and good control of HG rate, thus, the study of efficient catalysts constitutes the most important aspect in developing on-demand HG systems; secondly, catalytic hydrolysis of NaBH4 solution is a complicated process, in particular, the reaction order with respect to NaBH4 concentration is a subject of great interest and controversy; lastly, a comprehensive mechanism understanding of the catalytic hydrolysis process of NaBH4 is still unclear. In the present study, we focus on the three problems mentioned above. Systematic investigations have resulted in positive progresses in preparation of low cost and catalytically effective catalysts, exploring the reaction order with respect to NaBH4 concentration and the mechanism of catalytic hydrolysis of NaBH4 solution.
(1) Modified eletroless plating method - a new catalyst preparation technology for hydrolysis of NaBH4 solution was developed by increasing the concentrations of metal salts and reducing agent, as well as decreasing the use of some strong complex agents and elimination of stabilizers. By using the modified electroless plating method, we have prepared porous Co-B, Co-W-B and Fe-Co-B amorphous catalysts supported on Ni foam that are highly effective for catalyzing hydrogen generation from alkaline NaBH4 solution. The effects of different preparation methods on their microstructures and catalytic activities were experimentally examined. The function of hydrogen bubbles as dynamic template in the formation of catalyst with a porous structure was analyzed. By optimizing these preparation and reaction conditions, hydrogen generation rate of 12 L min-1 g-1 (Co-B), 15 L min-1 g-1 (Co-W-B) and 22 L min-1 g-1 (Fe-Co-B) have been achieved, which are comparable to the high level of noble metal catalyst.
(2) The effects of reaction conditions such as NaBH4 concentration, NaOH concentration and temperature on hydrolysis performances - HG rate, hydrogen storage density and conversion were examined and analyzed with the presence of Co-B, Co-W-B and Fe-Co-B/Ni foam catalysts. Combined with the experimental results, the intrinsic correlation between prepararion methods, structures and catalytic activities of the catalysts were established.
(3) The effects of NaBH4 concentration and Co-B catalyst amount on the hydrolysis kinetics of NaBH4 were experimentally studied over a wide NaBH4 concentration range with the presence of Co-B amorphous catalyst. Moreover, the catalytic hydrolysis kinetics was analyzed in terms of the Michaelis-Menten (M-M) model. Our results showed that the Co-B catalyst amount exerts no influence on the reaction order with respect to NaBH4 concentration and the HG rate increases linearly with increasing catalyst amount; while the hydrolysis reaction order of NaBH4 depends on the NaBH4 concentration, that is, zero-order kinetics at high NaBH4 concentrations, and first-order kinetics at low NaBH4 concentrations. Under the conditions applied in the present study, a critical NaBH4 concentration of 0.4 M has been identified to distinguish zero- and first-order kinetics. The M-M model studies provide valuable insights into the catalytic hydrolysis kinetics of NaBH4.
(4) Calcination treatment is an important step in preparation of supported metal catalysts that are effective for promoting the hydrolysis reaction of NaBH4. We found that the calcination treatment of supported non-noble transition metal catalysts results in the appearance of an induction period in their first time use, and the duration of the induction period depends on the calcination temperature, atmosphere and pressure. But in all cases, the induction period completely disappears upon reusing the catalysts. Through X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and synchronous thermal analyses, we explored the mechanism underlying the “induction period” phenomenon. Our results show that the appearance/disappearance of the induction period should be associated with the ‘trapped hydrogen’ in the catalysts. This finding provides valuable insight into the mechanistic understanding of catalytic hydrolysis reaction of NaBH4. |
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