Hydrogen Generation From Catalytic Hydrolysis Of Sodium Borohydride Solution Using Supported Cobalt-based Alloy Catalysts

The depletion of fossil fuels and environmental pollution have stimulated much interest in the establishment of a clean and sustainable energy system. Hydrogen is considered to be one of the most attractive energy carriers due to its high energy density, ideal usage efficiency and zero emission. It has been suggested as an alternative to traditional fuels since it can be used widely in proton exchange membrane fuel cell or other types of fuel cells. The technology of hydrogen genneration from sodium borohydride using cobalt-based catalysts has the advantages of low cost, high hydrogen genneration rate and high purity hydrogen. In the thesis, Co–W–P/γ-Al2O3, Co–Ni–W–P/γ-Al2O3 and Co–Ni–Mo–P/γ-Al2O3 supported cobalt-based catalysts were prepared by electroless deposition. Their catalytic performance of sodium borohydride hydrolysis for hydrogen generation was also investigated.The main results are concluded as follows:(1) Co–W–P alloy catalysts were prepared on the γ-Al2O3 supports by electroless deposition. Hydrolysis of sodium borohydride solution to produce hydrogen was used as a probe reaction to evaluate the catalytic activity of the obtained catalysts in this chapter. The influences of catalyst preparation conditions and hydrolysis reaction conditions on the hydrogen generation rate were investigated. The catalysts electroless deposited for 360 s with 3:7 Co SO4/Na2WO4 concentration ratios in electroless bath exhibit fine catalytic property, which have a great specific surface area of 256 m2 g-1. The highest hydrogen generation rate can reach 11.82 L min-1g catalyst-1 in the hydrolysis conditions of 10 wt.% Na OH, 4 wt.% Na BH4, 1.5 g catalyst, 45 oC. The Co–W–P/γ-Al2O3 catalysts also exhibit favorable cycling performance and lower activation energy(49.58 k J mol-1) in comparison with that of other reported Co-based catalysts.(2) Co–Ni–W–P catalysts were synthesized on the γ-Al2O3 by electroless deposition.The optimal hydrolysis reaction conditions on the catalytic activity of Co–Ni–W–P/γ-Al2O3 were 10 wt.% Na OH and 7 wt.% Na BH4. The inclusion of W in the Co–Ni–W–P/γ-Al2O3 facilitates the hydrolysis of Na BH4. Co–Ni–W–P/γ-Al2O3 catalysts prepared with Na2WO4 concentration of 1.5 g L-1 and electroless deposited in 220 s exhibit the highest hydrogen generation rate. The results show that the obtained Co–Ni–W–P/γ-Al2O3 catalysts exhibit better catalytic activity than Co–Ni–P/γ-Al2O3 catalysts. Further, Co–Ni–W–P/γ-Al2O3 catalysts also exhibit lower activation energy(49.19 k J mol-1) and favorable cycling performance. After 5 cycles, the hydrogen generation rate was still slightly higher than that for the first cycle. It may be due to the decrease of catalyst particle size.(3) Co–Ni–Mo–P catalysts were synthesized on the γ-Al2O3 by electroless deposition.The optimal hydrolysis reaction conditions on the catalytic activity of Co–Ni–Mo–P/γ-Al2O3 were 10 wt.% Na OH and 7 wt.% Na BH4. It further proves that the addition of Mo in the Co–Ni–Mo–P/γ-Al2O3 facilitates the hydrolysis of Na BH4. The Co–Ni–Mo–P/γ-Al2O3 catalysts prepared with Na2 Mo O4 concentration of 0.8 g L-1 and electroless deposited in 360 s exhibit the highest hydrogen generation rate 13.842 L min-1 g catalyst-1. The catalysts have a large specific surface area, which is 259 m2 g-1 detected by nitrogen adsorption-desorption isotherm. Hydrogen generation rate increases significantly with the elevating reaction temperature. The kinetic parameters of Ea, △H# and △S# were calculated as 52.43 k J mol-1, 49.86 k J mol-1 and-14.47 J mol-1K-1, respectively. Experiment results also show that Co–Ni–Mo–P/γ-Al2O3 catalysts after 5 circles remain high hydrogen generation rate, which just decreases by 20%.