Preparation Of ZIF-derived N-doped Carbon Composite Catalysts For Hydrogen Production By Ammonia Borane

The development of more efficient and cleaner energy has been a perennial topic in the 21^stcentury with the deepening of global energy crisis.Hydrogen,which has been recognized as an abundant,nontoxic and reproducible energy carrier,is one of the most prospective candidates for a sustainable energy supply.Generally,hydrogen is stored in adsorbents by physical adsorption or conserved in adsorbents or compressed in specific cylinder for storage and transportation,which is inconvenient and also exerts a potential security risk.In comparison,the solid-state storage and transportation of hydrogen in hydrogen-containing solid materials,such as formic acid,hydrous hydrazine,hydrazine borane and ammonia borane are deemed as effective and safe platforms since H is covalently bonded into stable chemicals.Particularly,ammonia borane（NH₃BH₃,AB）has drawn increasing attention due to its fairly stable properties and environmentally friendly nature.It has a great deal of advantages including small molecular weight(30.7 g mol^-1),large hydrogen content（19.6 wt%）,non-toxicity,and excellent stability in solution,and therefore AB is proposed as one of the most promising hydrogen storage materials.With adding of suitable catalysts,one mole of AB can release three mole of H₂at room temperature.The conversion efficiency depends mainly on the material of the catalyst.Thus,exploring extremely efficient and inexpensive catalysts for hydrogen generation from AB is urgently desired for practical application.In this study,NiCo-NC and Co₂P/（0.59-Cu₃P）-NC catalysts for the hydrolysis of AB are synthesized by using the zeolite imidazolate framework（ZIFS）template strategy.The specific research contents are as follows:（1）Synthesis of NiCo-NC Catalyst and Study on Hydrogen Production from Ammonia Borane HydrolysisWith the help of bimetallic zeolitic imidazole frameworks（ZnCo-ZIF）template strategy,we successfully decorate non-noble NiCoalloy nanoparticles on ZnCo-ZIF derived porous nitrogen-doped carbon（NC）as a high-efficiency catalyst（NiCo-NC）for hydrogen generation from hydrolysis of AB.Befitting from the abundance of nitrogen in porous carbon,the powerful synergistic effect between Niand Coand the synergistic effect between the metallic alloy and the supporting substrate,the as-prepared NiCo-NC nano-catalyst displays distinguished catalytic performance with a high total turnover frequency（TOF）of 35.2 mol _H2mol^-1_metalmin^-1in 0.6 M NaOH medium at 298 K.It can be seen from the cyclic reaction that the catalyst still has good stability after 6 cycles.In addition,NiCo-NC also has magnetic separation property,with saturation magnetization（MS）of 12.5 emu g^-1and coercivity（HC）of587.6 Oe,due to its magnetic properties,NiCo-NC can be separated from the solution by an external magnet for easy reuse.（2）Synthesis of Co₂P/（0.59-Cu₃P）-NC Catalyst and Study on Hydrogen Production from Ammonia Borane HydrolysisWith the aid of a Cu doped CoZn-ZIFs（Cu CoZn-ZIF）template strategy,we successfully design three-dimensional porous N-doped carbon-based nanoframeworks（Co₂P/（0.59-Cu₃P）-NC）bimetal phosphide as non-noble-metal and efficient catalysts.Specifically,Coand Cu atoms can be geometrically separated to highly degree due to the presence of Znin the Cu CoZn-ZIF precursor,and evaporation of Znduring pyrolysis can maintain the framework.The results shows that porous Co₂P/（0.59-Cu₃P）-NC bimetal phosphide exhibits large specific surface area,hierarchical pore structure and well-exposed active site.Based on kinetics analyses and ion effects,the catalyst has achieved an unprecedentedly higher total turnover frequency（TOF）of 798 mol _H2mol^-1_catmin^-1in 0.4 M NaOH solution at 298K,which surpasses all the transition-metal phosphides catalysts for hydrogen generation from AB ever reported.Through experiments and theoretical studies,we confirm that the doped phosphorus element leads to a synergistic electronic effect between Co₂P,Cu₃P and the support in the catalyst.Then,the induced strong electron transfer can reduce the reaction energy barrier,enhance their interaction with AB,and accelerate the catalytic reaction.Among them,in the presence of NaOH,OH^-effect can change the electronic structure of the catalyst,which is conducive to the formation of reactants intermediates,and OH^-can form hydrogen bonds with H atoms in water molecules,which promotes the process of reaction rate-determination,and finally obtains excellent catalytic performanceIn conclusion,the two kinds of highly efficient catalysts prepared in this work as well as the proposed mechanisms and strategies have certain guiding significance for the design and application of non-noble metal catalysts in hydrogen energy system in the future.