Synthesis And Mechanism Study Of High-performance Borane-ammonia-hydrogen Alloy Catalysts

Hydrogen energy is a kind of clean energy with great development potential.It is a convenient,inexpensive and effective method to produce hydrogen from chemical hydrogen storage materials in the catalytic process.Among all kinds of chemical hydrogen storage materials,ammonia borane（AB）has become a promising hydrogen storage carrier material due to its high hydrogen content,good stability,non-toxic and other characteristics,which has attracted extensive interest and attention of researchers.However,on existing catalysts,the rapid dehydrogenation of ammonia borane is still limited by slow catalytic kinetics.Therefore,exploring efficient nano-catalysts for the hydrolysis of ammonia borane to hydrogen production at room temperature and further revealing their catalytic mechanism is crucial to renewable energy conversion technology,but it remains a huge challenge.In this paper,the catalyst was designed mainly from two aspects of component advantage and carrier effect,so as to obtain a high efficient catalyst with both electronic effect,carrier effect,size effect and functional effect.The research work of this thesis mainly includes the following two aspects:1.Highly uniform and ultrafine bimetallic Rh Ni alloy nanoclusters encapsulated within nitrogen-functionalized hollow mesoporous carbons（defined as Rh Ni@NHMCs）are developed as the highly active,durable,and selective nanocatalysts for fast hydrolysis of AB under mild conditions.Through a series of kinetic experiments,the results show that the turnover frequency（TOF）of the catalyst is 1294 mol _H2 mol _Rh^-1 min^-1,which exceeds the previously reported Rh-based catalysts,and has a lower activation energy（Ea）of 18.6 k J mol^-1.Detailed mechanism studies reveal that,when catalyzed by the Rh Ni@NHMCs,a covalently stable O-H bond by H₂O first cleaves in electropositive H^*and further attacks B-H bond of AB to stoichometrically produce 3 equivalent of H₂,whose catalytic kinetics is restricted by the oxidation cleavage of O-H bond.Compositional and structural features of the Rh Ni@NHMCs result in synergic electronic,functional and support add-in advantages,kinetically accelerating the cleavage of attacked H₂O（O-H bond）and remarkably promoting catalytic hydrolysis of AB accordingly.2.Ultrafine binary Ru P alloy nanoclusters homogeneously encapsulated onto nitrogen-functionalized hollow mesoporous carbon supports（Ru P@NHMCs）are reported as a high-performance platinum（Pt）-free nanocatalyst for catalytic hydrolysis of AB at room temperature.Remarkable catalytic activity with a very high turnover frequency of 1774 mol _H2 mol _Ru^-1 min^-1 and a low activation energy of 36.3 k J mol^-1is observed based on compositional and structural synergies of Ru P@NHMCs.Results of control experiments and catalytic kinetics studies reveal that the rate-determining step of catalytic hydrolysis of AB is the oxidation cleavage of a covalently stable H-OH bond,while Ru P@NHMCs result in multiple electronic,functional,size and support effects that kinetically accelerate the clearage of attacked H-OH.Furthermore,Ru P@NHMCs disclose a good catalytic activity with a high yield of>99%for tandem hydrogenation of nitroarenes coupled with the hydrolysis of AB.In summary,through the synergy of components and structures,this article designed a highly active platinum-free catalyst with multiple effects.It is hoped that the catalyst design principles in this article can provide new opportunities for the synthesis of other platinum-free high-performance nanocatalysts.