Synthesis And Performance Of Catalysts For Hydrogen Generation From B-N Hydrides

Hydrogen has been considered as a secondary clean energy resource in the next generation.However,the storage and transportation of hydrogen are still the major obstacles for its large-scale application.B-N hydrides ammonia borane?NH₃BH₃?and hydrazine borane?N₂H₄BH₃?,having hydrogen content as high as 19.6 wt% and 15.4 wt%,respectively,have received extensive attention as promising hydrogen storage materials.Moreover,ammonia borane and hydrazine borane are stable at room temperature for safe storage and transportation,showing their high potential application as portable hydrogen source.Hydrogen stored in ammonia borane and hydrazine borane can be released via pyrolysis,methanolysis and hydrolysis approaches.In comparison with the thermal dehydrogenation,where high temperature and energy consumption are required,the hydrolytic dehydrogenation of ammonia borane and hydrazine borane would take place under mild condition in the presence of suitable catalysts,and consider as a low energy consumption,high safety and high practicability hydrogen production technology.In the presence of suitable catalysts,hydrogen can be released from ammonia borane via the hydrolysis of the BH3 group,and hydrazine borane could be completely converted to H2 via the hydrolysis of the BH3 group and decomposition of the N2H4 group.The development of cost-effective and high stable catalyst is the key point to promote the practical application of ammonia borane and hydrazine as hydrogen storage materials.In the thesis,we have designed and prepared a series of nanocatalysts for catalytic dehydrogenation of B-N hydrides.The main research contents are given as follows:Metal nanoparticles?NPs?in nano-size are easily aggregated during the preparation and application processes,leading to the loss of activity or even deactivation.SBA-15 with high specific surface area and uniform pore structure was selected as host matrix for metal NPs.In this work,ultrafine Ru NPs within mesopores of SBA-15?Ru@SBA-15?have been prepared by using the double solvents method combined with the overwhelming reduction approach.Compared with SBA-15 supported Ru NPs and free Ru NPs,the synthesized Ru@SBA-15 catalyst shows a superior catalytic activity for the hydrolysis of ammonia borane.The excellent catalytic activity of Ru@SBA-15 catalyst could be attributed to the small size of Ru NPs,and the reason that the reactant molecules can readily access the Ru NPs through the mesopores of SBA-15.Metal@SiO₂ core-shell nanomaterials are well-known for better stability because the silica shells can effectively protect the metal NPs cores from aggregation.For the synthesis of metal@SiO₂,the traditional method is firstly to prepare metal NPs,and then to coat with SiO₂,which existing many problems such as structural defects,uneven sizes,complex synthetic route and so on.Ru@SiO₂ core-shell nanospheres have been synthesized via a simple one-pot synthetic route in reverse micelle system at room temperature.The method can avoid the modification of the core surface and simplify the experimental operation.The characterized results show that the synthesized catalysts are quite uniform and the cores/shells are readily distinguishable.The number of Ru NPs inside the SiO₂ can be tuned by changing the concentrations of Ru precursor.The characterization results show that Ru NPs of around 2 nm were effectively embedded in the center of SiO₂?25 nm?,and the number of Ru NPs increases inside the spherical particles of SiO₂ with the increase of Ru loading.Ru@SiO₂ with a Ru loading of 6 wt% displays the best catalytic performance?200 min-1?amongst all the as-synthesized catalysts.In order to reduce the cost of the catalyst,we use the non-noble metal Cu instead of noble metal Ru.Core-shell structured Cu@SiO₂ nanospheres have been synthesized via a simple one-pot synthetic route in reverse micelle system and used as catalyst for hydrogen generation from the hydrolysis of ammonia borane.The Cu@SiO₂ nanospheres exhibit superior catalytic performance for the hydrolysis of ammonia borane,which is 6-fold higher than that of Cu/SiO₂ and 23-fold higher than that of the free Cu NPs.The excellent catalytic performance could be attributed to the fact that Cu has larger reactive surfaces of Cu in the core-shell Cu@SiO₂ catalyst,which has more catalytic active sites.Moreover,the Cu@SiO₂ catalyst preserves 90% of its initial catalytic activity after ten runs,indicating that the catalyst has a good reusability.Compared with the monometallic catalyst,the bimetallic catalyst often exhibits higher catalytic performance due to their synergistic effect.A series of Cu_xCo_1-x@SiO₂ core-shell structured nanospheres with different molar ratios have been synthesized via a simple one-pot synthetic route in reverse micelle system.The results show that the as-synthesized bimetallic Cu-Co@SiO₂ nanospheres show a higher catalytic performance for the hydrolysis of ammonia borane in comparison to their monometallic counterparts.Especially,the Cu_0.5Co_0.5@SiO₂ nanospheres show the best activities among all the synthesized Cu_xCo_1-x@SiO₂ catalysts,the activation energy of the reaction is estimated to be 24 kJ mol^-1.The above synthesized catalysts are efficient in hydrolyzing BH3 moiety in ammonia borane,however,hydrogen in NH3 moiety does not participate.While in the presence of suitable catalysts,hydrazine borane could be completely converted to H2 via the hydrolysis of the BH3 group and decomposition of the N2H4 group.Up to now,only Ni combined with noble metal nonocatalysts can be used for complete dehydrogenation of hydrazine borane.Moreover,the reported effective catalysts are numbered and the catalytic activities are generally low.In this work,a series of Rh_1-xMo_x NPs without any support or surfactant have been synthesized via a facile one-step synthetic route.The results show that the as-synthesized Rh0.5Mo0.5 NPs have high dispersion and a small particle size without the assistance of a support/surfactant.The resultant Rh0.5Mo0.5 catalyst exhibits excellent catalytic performance for the dehydrogenation of hydrazine borane.This is the first report that a catalyst without Ni can catalyze the complete dehydrogenation of hydrazine borane.In order to reduce the cost of catalyst,noble-metal-free CuNiMo catalysts have been prepared through a facile one-step synthetic route.The characterization results show that the addition of Mo not only can induce the reduction of the metal NPs size,but also can modify the electronic structure of catalyst surface.Moreover,the basic sites of the catalyst can also be strengthened by the addition of Mo,which is beneficial to the selective cracking of the N–H bond in the dehydrogenation of N2H4 moiety.For the above reasons,the hydrogen selectivity and activity can be significantly improved.As a result,the noble-metal-free Cu0.4Ni0.6Mo catalyst can achieve a complete dehydrogenation of hydrazine borane to H2.Up to now,there is no report on noble-metal-free catalyst that can achieve a complete conversion of hydrazine borane to H2.The results of this study are expected to accelerate the practical process of hydrazine borane as a hydrogen storage material.