| Borohydride has been regared as a kind of ideal hydrogen source material in the research fields of hydrogen production due to its high hydrogen content and the mild reaction conditions of hydrolysis.Among the borohydrides,NH3·BH3 and Na BH4 are the two of most widely studied materials of which the H2 storage densities are 19.6 wt.%and10.6 wt.%,respectively.Both NH3·BH3 and Na BH4 can release H2 at the room temperature through hydrolysis reaction in the presence of catalyst.But the poor dynamic performance and low reaction rate is a difficult problem for the hydrolysis of borohydride.Hence,designing and developing a highly active and long-life catalyst becomes the major research direction for the hydrogen generation from the hydrolysis of borohydrides.Here,several silica and carbon nanospheres with high stability and special construction have been designed and prepared in this work which are used as carriers for the novel nano platinum-cobalt bimetallic catalysts.The influence of the structure and composition of the catalysts on the hydrolysis of borohydrides is systematically studied.In addition,the hydrolysis mechanisms and dynamical properties of NH3·BH3 and Na BH4 have also been disscussed through a series of tests.Supported Pt-Co bimetallic nano-catalyst with Snake berry shape has been prepared by in-situ reduction method at low temperature with the amino-modified SiO2 nanosphere as carrier.The Pt-Co nanoparticles have shown good dispersity on the surface of SiO2 which preform a uniform particle size.When the composing proportion of Pt/Co is 0.1/0.9,the prepared SiO2@Pt0.1Co0.9 preforms a better catalytic activity which the TOF value is 256mol H2·min-1·mol Pt-1 and the Ea valus is 37.0 k J·mol–1.The kinetic studies of NH3·BH3hydrolysis indicate that the hydrolysis of NH3·BH3 was a zero-level reaction which the hydrolysis rate has no effection by the concentration of NH3·BH3 with excessive water.Meanwhile,the participation of SiO2 has enhanced the stability of the Pt-Co bimetallic nano-catalyst which shows good catalytic activity in the hydrolysis of NH3·BH3 even after5 cycles of reactions.The H-SiO2 nanosphere with a hollow structure has been prepared by template method with the organic nanosphere as the template.The H-SiO2 nanosphere with higher specific surface area can also make the carrier be more fully exploited which can provide more attachment sites for the growth of metal nanoparticles to effectively increase the dispersion and decrease the grain size of Pt-Co nanoparticles during the preparation of the catalyst.The research has found that the grain size of obtained Pt-Co nanoparticles on the surface of H-SiO2 can be further reduced to 3 nm through regulating the solubility of the by-product,Na BO2,further enhancing the dispersion and catalytic activity of catalyst.The catalytic activity of H-SiO2@Pt0.1Co0.9 in the hydrolysis of NH3·BH3 has shown a notable improvement,of which the TOF value reaches to 496 mol H2·min–1·mol Pt–1 and the Ea is 36.8k J·mol–1.The H-SiO2@Pt0.1Co0.9 also performs a good stability in the hydrolytic cycle test which maintains 89.6% of the initial catalytic activity after 5 cycles'hydrolytic reactions.P-SiO2 nanosphere with porous structure has been prepared by selective corrosion of SiO2 nanosphere in alkaline solution.The SiO2 nanosphere shows strong chelation with the Pt-Co nanoparticles and the pores of P-SiO2 nanosphere have reduced the oxidation of Pt-Co nanoparticles in the pores after the catalyst was synthesized and during the hydrolytic reaction which have significantly improved the catalytic activity and stability of the Pt0.1Co0.9/P-SiO2 catalyst.The TOF value the hydrolysis of NH3·BH3 with Pt0.1Co0.9/P-SiO2increases to 674 mol H2·min-1·mol Pt-1 and the Ea reduces to 35.4 k J·mol–1.The active stability of Pt0.1Co0.9/P-SiO2 has improved significantly which remains 92.6%of the initial catalytic activity after 5 cycles'hydrolytic reactions.Meanwhile,the hydrolysis mechanism of NH3·BH3 hydrolysis is deeply researched with H-isotopic tracer method by the reaction of NH3·BH3 and D2O.The best preparing condition of supported metal catalyst prepared with the reduction of Na BH4 has been researched through the synthesis of CNSs@Co with carbon nanospheres(CNSs)as carrier.The study found that CNSs@Co prepared at 5°C,10 mmol Co·g C–1 and solvent ratio of Et OH/H2O:0.6/0.4 performs the optimal structure and catalytic activity of which the mean grain size of Co is about 7 nm.The high hydrogen generation rate(HGR)of Na BH4 hydrolysis with CNSs@Co is 7447 m LH2·min-1·g Co-1 at 30°C.TOF and Ea are19.6 molH2·min-1·mol-1cat and 40.8 k J·mol–1,respectively.After mixing with Pt,the grain size of Pt-Co nanoparticles in the bimetal catalyst-CNSs@Pt0.1Co0.9 is 5 nm.The catalytic activity has a significant enhancement compared with CNSs@Co whose HGR reaches to 8943m LH2·min-1·g M-1 and the TOF value is 280 mol H2·min-1·mol Pt-1.The catalytic activity of CNSs@Pt0.1Co0.9 after 5 times'hydrolysis of Na BH4 maintains 85.1% of the initial catalytic activity.Porous CNSs(P-CNSs)with abundant nanoporous has been prepared through the modified of CNSs by carbothermal method of metal oxide.P-CNSs appear a stable structure which would not collapse during chelating the metal nanoparticles in the prepartion.Porous structure effectively segregates and chelates the generated Pt-Co nanoparticles in the pores,inhibiting the agglomeration during the preparation and hydrolysis.The gain size of Pt-Co nanoparticles on P-CNSs is only about 3 nm and the Pt0.1Co0.9/P-CNSs performs excellent catalytic activity in promoting the hydrolysis of Na BH4 whose HGR reaches 18743m LH2·min-1·g M-1 and the TOF value is 439 mol H2·min-1·mol Pt-1.The porous structure of P-CNSs plays good protective effect for the Pt-Co nanoparticles to avoid exposuring in the air to be oxidized and deactivate.Hence,the Pt0.1Co0.9/P-CNSs shows good stability in structure and activity.The catalytic activity of CNSs@Pt0.1Co0.9 after 5 times'hydrolysis of Na BH4 maintains 91.4% of initial activity. |
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