Preparation And Catalytic Performance Of Nano Ruthenium Catalysts Supported On Chitosan-based Materials For Ammonia Borane Hydrolysis

The significant increase in energy demands has led to rapid consumption of fossil fuels and serious environmental problems.Therefore,the development of environmentally friendly renewable energy has become a consensus of the peoples.Hydrogen is considered to be the most potential renewable clean energy because of its abundant resources,environmentally friendly and high energy density.However,the current use of hydrogen energy has the problems of inconvenient storage and transportation,which hinder its large-scale application.Hydrogen production from the hydrolysis of solid chemical hydrogen storage materials is expected to solve the above problems to a certain extent.Particularly,ammonia borane(NH3BH3,AB)has attracted significant attention owing to its high hydrogen content(19.6 wt%),high solubility in polar solvents,and high stability.However,the hydrolytic dehydrogenation of AB cannot take place kinetically at room temperature.Therefore,developing a suitable catalyst is of great significance for this hydrolysis reaction.As the active component for catalyzing AB hydrolysis,ruthenium(Ru)has been widespread concerned because of its lower cost in comparison with other precious metals and excellent performance.In addition,pure Ru nanoparticles(NPs)tend to agglomerate during the reaction procedure,leading to decreasing catalytic activity and high cost.Therefore,the relevant researchers focus on exploring suitable materials for supporting Ru NPs.Chitosan(CS)is a kind of natural polymer material second only to cellulose,which contains a large number of amino groups and hydroxyl groups.The lone electrons on these groups can form chelation with Ru3+,which can better disperse and fix the corresponding Ru NPs,thereby improving their catalytic performance.In this thesis,chitosan(high viscosity,>400 mPa·s)is used as the base support and composites with different amounts of face-centered cubic ferrosoferric oxide(Fe3O4),hexagonal boron nitride(h-BN),and graphitic carbon nitride(g-C3N4),respectively.Then,CS and the obtained composite materials are used to prepare supported nano Ru catalysts with an adsorption-chemical reduction method at room temperature by using NaBH4 as the reducing agent.The materials are analyzed by XRD,FT-IR,SEM,TEM,XPS,and N2 adsorption-desorption techniques.The most suitable catalysts are optimized based on the corresponding catalytic activity for AB hydrolysis,respectively.Moreover,the corresponding reaction kinetics,cycling stability and the influence of different NaOH dosages on reaction rate are studied.The relevant contents are as follows:1.A series of nano-Ru catalysts supported on CS are successfully prepared.Ru NPs show good dispersion in the optimal Ru/CS catalyst with a Ru loading of 2.0 wt%.Catalyzed by Ru/CS,the reaction orders of AB hydrolysis to the catalyst and AB concentrations are 0.99 and 0.31,respectively,and the apparent activation energy(Ea)is 41.3 kJ mol-1.Appropriate amount of OH-ions can increase the charge density on the surface of Ru NPs and further promote the oxidative addition of O-H bonds in water,which is beneficial to increase the reaction rate of AB hydrolysis.When the amount of NaOH is 1.2 g,the corresponding turnover frequency(TOF)reaches 331.8 min-1 over Ru/CS.In addition,the Ru/CS catalyst displays good stability.Under the present conditions,it still maintains high catalytic activity even after five runs in the cycling experiments.2.A series of magnetic composite materials are prepared via dispersing different amounts of CS into the precursor solution for preparing face-centered cubic Fe3O4 through the co-precipitation method by using FeCl2·4H2O and FeCl3·6H2O.Then,the obtained composite materials are used to prepare the corresponding supported nano-Ru catalysts.When the mass ratio of CS to Fe3O4 is 2:1,the corresponding Ru/CS-Fe3O4 exhibits the best catalytic activity in AB hydrolysis.The reaction orders for AB hydrolysis to the catalyst and AB concentrations are 1.14 and 0.40 over the optimal Ru/CS-Fe3O4,respectively,and the Ea value is 50.0 kJ mol-1.When the NaOH dosage is 0.4 g,the corresponding TOF value is 219.6 min-1.3.A series of composite materials consisting of CS and h-BN are prepared with an ultrasonic-assistance combined with pH value control strategy.Then,the corresponding supported nano-Ru catalysts are prepared.When CS content in the composites is 20%,the corresponding Ru/CS-h-BN catalyst shows the highest activity.Electron microscope analyses reveal that the Ru NPs are uniformly dispersed on the CS-h-BN supports.Catalyzed by Ru/CS-h-BN,the reaction orders for AB hydrolysis to the concentrations of the catalyst and AB are 1.01 and 0.33,respectively and the Ea value is 35.2 kJ mol-1.When the NaOH dosage is 1.6 g,the corresponding TOF value is 429.6 min-1 over Ru/CS-h-BN.The catalyst still has high catalytic activity even after five runs.4.g-C3N4 is prepared via the thermal polycondensation of melamine and composited with different amounts of CS with an ultrasonic-assistance combined with pH value control strategy.Then,the obtained composite materials are used to prepare supported nano-Ru catalysts.When the mass ratio of CS to g-C3N4 is 1:6,the corresponding Ru/CS-g-C3N4 displays the best catalytic activity.Catalyzed by Ru/CSg-C3N4,the hydrolysis of AB is 0.94 order concerning the catalyst concentration,and 0.33 order for AB concentration.The corresponding Ea value is calculated to be 34.8 kJ mol-1.When the dosage of NaOH is 1.2 g,the corresponding TOF value reaches up to 504.7 min-1.The cycle experiment shows that Ru/CS-g-C3N4 still maintains high catalytic activity even after four runs.In summary,the as-prepared nano-Ru catalysts supported on CS-based supports display superior catalytic performance for AB hydrolysis to many previous results.Benefiting from the suitable porous structure,close interactions among different components,high heteroatom content(N 28.00 at%,O 11.39 at%)and high dispersion of Ru NPs,the Ru/CS-g-C3N4 catalyst exhibits the best catalytic performance under the present conditions in this thesis.This work expands the research scope of solid chemical hydrogen storage materials to a certain extent,provides an available catalyst,as well as the corresponding strategy parameters.