Preparation of Rh/N-SBC Nanocatalyst and Its Catalytic Performance for Hydrolytic Dehydrogenation of Ammonia Borane
With the continuous consumption of fossil fuel and releasing of greenhouse gas carbon dioxide,increasing efforts have been made to develop new approach to meet the urgent need to recyclable and pollution-free energy.In the development of various new energy sources,hydrogen energy,producing water as the only byproduct,is green and sustainable with high energy density and it has been a globally accepted clean energy source,is an ideal energy carrier.As a liquid hydrogen storage material,ammonia borane has a hydrogen storage content of 19.6％,excellent stability at room temperature and low storage cost.In addition,the reaction by-product NH4BO2 after the hydrolysis and dehydrogenation of ammonia borane can be recovered by irreversible chemical reaction without pollu-tion to the environment.Under mild conditions,the development of an efficient and highly selective catalyst is the key to realize the ap-plication of ammonia borane hydrolytic dehydrogenation.In this paper,nitrogen-doped sucrose-based carbon material was used as the support,the active component Rh was loaded,and Rh/N-SBC catalyst was prepared by low-temperature reduction,which was used to catalyze the hydrolysis and dehydrogenation of ammonia borane.Then,the microstructure was analyzed by a series of characterization methods.To study the microscopic morphology and structure of the catalyst,Rh/N-SBC catalysts were characterized by transmission electron microscopy(TEM).X-ray diffraction(XRD)and X-ray photoelectron spectroscopy(XPS)were used to investigate the effects of different calcination temperatures on the phase composition and crystal structure,and analyze the chemical element valence state and bond type of Rh/N-SBC catalysts.In addition,the single-variable method was used in this experiment to explore its effect on the performance of catalyzing the hydrolysis and dehydrogenation of ammonia borane.By changing the quality of sucrose during the prepa-ration of the support,a series of Rh/N-SBC catalysts with different N-doping amounts in the support were synthesized and applied to the hydrolysis and dehydrogenation of ammonia borane to explore the relationship between the variables.By changing the calcination temperature(800,900,1000 and 1100 ℃),the effect of different calcination temperature of the support on the catalyst activity was investigated.To further explore the effect of Rh loading on catalyst activity,catalysts(0.2％Rh/N-SBC,0.3％Rh/N-SBC,0.4％Rh/N-SBC and 0.5％Rh/N-SBC)were prepared to catalyze AB dehydrogenation by changing the loading of active component Rh.The effect of temperature on the dehydrogenation of AB catalyzed by Rh/N-SBC was explored and the activation energy of the catalytic reaction was calculated via changing the reaction temperature(-5,0,5 and 10 ℃).Furthermore,the same batch of 0.4％Rh/N-SBC catalyst was usedto catalyze the hydrolysis and dehydrogenation of ammonia borane to explore the cycle stability of the catalyst.The investiga-tion results indicated that N was doped on the support and the metal Rh was successfully loaded on N-SBC.The microscopic morpholo-gy of Rh/N-SBC catalyst presented a sheet-like structure,which provided sufficient specific surface area for the attachment of the met-al active component Rh and greatly improved the catalytic activity of the catalyst.Additionally,the incorporation of N element into the carbon material effectively changed the electronic structure of the raw material,so the catalyst had outstanding catalytic activity for the dehydrogenation of ammonia borane.As the mass of sucrose in the carrier increased from 0.05 to 0.3 g,the corresponding Rh/N-SBC catalyst catalyzed AB dehydrogenation rate increased first and then decreased.When the sucrose content was 0.10 g,turnover frequen-cy(TOF)value of the catalytic reaction was up to 4821.1 min-1.The dehydrogenation rate of ammonia borane increased with the rise of the calcination temperature of the support.The catalytic activity reached the optimum when the calcination temperature reached 1000 ℃,and then the catalytic activity began to decrease with the increase of the calcination temperature of the support.When Rh loading reached 0.4％,the catalytic activity of the catalyst was the largest and the catalytic activity decreases when Rh loading contin-ued to increase.The temperature had a significant effect on the decomposition rate of ammonia borane.With the increasing reaction temperature,the time required for the complete decomposition of AB was shortened from 2.25 to 0.23 min.The calculated activation energy of Rh/N-SBC catalyzed AB decomposition and dehydrogenation was 55.69 kJ·mol-1.After 5 cycles,the complete hydrolysis of AB to produce hydrogen could still be achieved,indicating its outstanding stability.This excellent catalytic performance could be at-tributed to the fact that the incorporation of N into the carbon material effectively changed the electronic structure of the support and significantly improved the catalytic performance of the catalyst for AB hydrolysis to produce hydrogen.
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