GENERATION OF HYDROGEN FROM SODIUMBOROHYDRIDE USING VARIOUS CATALYSTS AND ADDITIVES FOR FUEL CELL APPLICATIONS

Abstract

The cobalt boride is used as catalyst for various important chemical reactions such as hydrogenation of alkenes, citral, aldehyde, and reduction of nitrogen oxide, water splitting reaction, ODH of propane, Oxygen Evolution Reaction, Na-O2 batteries, and hydrogen generation. The unsupported and supported cobalt-boride catalysts have been prepared by considering various method of synthesis. The supports used for the synthesis of supported catalysts are ceria, carbon nanotubes, activated carbon, silica, titania, and alumina. The active component of these catalysts is cobalt-boride and the species dispersed over the support differently. The objectives of the present thesis work were to study the generation of hydrogen from sodium-borohydride using various catalysts/additives for fuel cell applications. The generation of hydrogen was considered by a) hydrolysis of sodium borohydride solution using supported/bulk metal-boride catalysts, and b) thermolysis of sodium borohydride using metal halides additives. Metal-boride catalysts were active for the hydrolysis of sodium borohydride at room temperature. The bulk metal-boride catalysts (CoB, NiB, and FeB) were synthesized by simple reduction precipitation method. The supported metal boride catalysts (CoB/SiO2) were synthesized by two-step reduction precipitation followed by impregnation method. The surface area of the metal-boride catalysts was also increased by using various support materials. Since the metal-boride catalysts are not active for thermolysis of sodium borohydride for the generation of hydrogen. The metal halide additive/catalysts composite mixtures were active for the thermolysis of sodium borohydride at low temperature. The metal-halides additives/catalysts were used for the thermolysis study are MnCl2, CaCl2, and ZnCl2. The sodium-borohydride/additive composite mixtures were prepared by facile solution method. The ii bulk (CoB) or supported metal-boride (CoB/SiO2) catalysts or the composite mixture (xMnCl2/NaBH4) were characterized using various characterization techniques to improve our understanding of hydrogen generation from sodium borohydride by considering various factors such as metal loading, effect of calcination temperature, effect of supports of catalysts, and the effect of thermolysis temperature to find the most suitable additive. Moreover, the synthesized catalysts and composite materials were characterized by BET, XRD, FE-SEM, in situ UV-vis spectrophotometer, FTIR and Raman spectroscopy. A series of CoB, FeB, and NiB catalysts were prepared by the chemical reduction method using base stabilized sodium borohydride solution as a reducing agent. The CoB catalyst was most stable and highly dispersed even at high calcination temperature. The hydrolysis study suggested that CoB catalyst was most effective and suitable for the generation of hydrogen from hydrolysis of sodium borohydride. The generation of hydrogen using base stabilized CoB-BS catalysts was most active using both the base/without base stabilized sodium-borohydride solution. The generation of hydrogen using base stabilized sodium borohydride solution using various catalysts was as follows: CoB-BS > NiB-BS > FeB-BS. The effect of support on the CoB catalysts were examined and studied. The supported cobalt boride catalysts (xCoB/SiO2, xCoB/Al2O3, xCoB/MgO) were prepared by two-step impregnation-reduction method. The synthesized catalysts were studied for the hydrolysis of based stabilized sodium-borohydride solution for the generation of hydrogen. The synthesized catalysts were characterized by using BET, XRD, and Raman spectroscopy techniques. Various parameters such as catalysts loading, effects of calcination temperature, effect of supports were considered. The synthesized catalysts were calcined at various calcination temperature from 373 K to 773 K. The study suggested that the support plays a significant role on enhancing the generation of hydrogen from base stabilized sodium borohydride solution. Moreover, the calcination temperature also played a significant role in enhancing the catalytic iii performance. It was necessary to calcined the CoB impregnated support at moderate temperature before reduction of the cobalt so that an active CoB is dispersed and anchored with the support properly. The study suggested that the surface area gradually increased with increasing calcination temperature up to 573 K and furthers increasing calcination temperature the surface area decreased for the catalysts xCoB/(support). However, all the calcined samples were highly amorphous in nature even at 673 K and started formation of crystalline phase at 773 K in 50CoB/Al2O3. It was also observed that a Co3O4 species formed with the CoB in all catalysts during the second step of catalyst synthesis (reduction step). The most active catalyst was found to be 50CoB/SiO2 calcined at 573 K. The order of catalytic activity for the generation of hydrogen for all catalysts: 50CoB/SiO2 > 50CoB/Al2O3 > CoB > 50CoB/MgO. The effect of additives/catalysts for the thermolysis of sodium borohydride is also important for the generation of hydrogen. A series of MnCl2 impregnated sodium borohydride composite mixture was prepared by facile solution method at room temperature. The additive loading was varied from 10 wt% to 50 wt% during the synthesis of composite materials. Other additives were also used such as CaCl2 and ZnCl2. However, the 20 wt% of additive was an optimum loading for the synthesis of 20MnCl2/NaBH4 composite mixture. The generation of hydrogen was obtained from the material 20MnCl2/NaBH4 at 373 K. The generation of hydrogen increased with increasing thermolysis temperature (373 K to 823 K). However, the study suggested that the generation of hydrogen was incomplete at 373 K from the material 20MnCl2/NaBH4. The most effective additive was found to be CaCl2. The addition of additive assists in lowering the thermolysis temperature of NaBH4 for the generation of hydrogen. The effect of additive considering various additives as as follows: 20CaCl2/NaBH4 > 20MnCl2/NaBH4 > 20ZnCl2/NaBH4. The FTIR analysis and thermolysis study suggested that the generation of hydrogen was incomplete at low temperature (373 K). iv Thus, the generation of hydrogen from sodium borohydride (hydrolysis/thermolysis) using various catalysts (supported/bulk) and metal-chloride additives with the information obtained from various characterization studies of BET, XRD, FTIR, FE-SEM, and Raman spectroscopy, the effect of various parameters could be established. The parameters included the effect of metal in metal boride catalysts, effect of calcination temperature, effect of supports, and effect of various additives.