Effects Of Ru Crystal Phase And Support Structure On The Hydrolysis Performance Of Ammonia Borane

Ammonia borane?NH₃BH₃,AB?has excellent physical and chemical properties,such as low molecular weight(30.87 g·mol^-1),high theoretical density hydrogen storage capacity?19.6 wt%?,high stability of solid form at room temperature,etc.,all of these features largely meet the U.S.Department of Energy's goal for hydrogen-car applications.So far,the hydrolysis reaction is considered to be the most convenient,gentle and efficient way for AB to release hydrogen.In recent years,people began to focus on designing a high-efficiency catalyst with high activity,good selectivity and strong stability.Generally,the design of high-efficiency catalysts can be achieved by changing the active components,morphology,particle size,support,and additives of the catalyst.In this dissertation,starting from the effect of carrier crystal surface,acid-base effect of carrier and the effect of metal crystal phase,the influence of the catalytic hydrolysis of AB on the performance of the catalyst was studied.The characterization results were used to give reasonable explanations.1.Effect of ceria morphology on the catalytic activity of Ru/ceria for the dehydrogenation of ammonia boraneAlthough the effect of CeO₂ morphology on catalytic properties has been studied,research interests mainly focus on the oxidation reactions while the effect of CeO₂morphology on other reactions,such as dehydrogenation reaction,has rarely been reported.In this study,the dehydrogenation of AB was chosen as a model reaction to study the effect of CeO₂ morphology?“c”,“r”and“o”denote cube,rod,and octahedron,respectively?on the catalytic properties of Ru/CeO₂.Our study shows that the activity follows the order of Ru/c-CeO₂>Ru/r-CeO₂>Ru/o-CeO₂,which is different from the order observed in other catalytic oxidation reactions.X-ray photoelectron spectroscopy?XPS?,Raman and H₂-temperature program reduction?H₂-TPR?measurements reveal that there is a weak interaction between Ru and c-CeO₂,and metallic Ru is the dominant species in Ru/c-CeO₂,which are possibly responsible for the highest catalytic activities among three Ru/CeO₂catalysts.2.Effect of the LDH acid-base effect of Ru/LDH catalyst on the dehydrogenation activity of ammonia boraneLDH offers potential opportunities for the design of new catalyst precursors or catalyst supports.As a catalyst carrier,it provides not only a highly dispersed and stable morphology for metal nanoparticles,but also controls the M²⁺/M³⁺ratio to adjust its acidity and alkalinity.Researchers have used different proportions of MgAl-LDH as a carrier to study the effect of acidity and alkalinity of LDH after loading metal particles on different catalytic reactions,such as reaction of CO₂ hydrogenation to formic acid,effective tandem synthesis of iminomethyl esters.etc..However,the LDH support-based catalysts have rarely used for the hydrolysis of AB.To this end,we have prepared Ru nanoparticles loaded on different proportions of LDH?2:1,3:1,4:1?,and tested the catalytic activity of the as-prepared catalysts for AB hydrolysis.The results show that Ru/Mg₂Al-LDH shows the best catalytic performance.The influences of factors,such as the acidity and basicity of the support,the specific surface area on the catalytic activity of the dehydrogenation of AB were investigated by NH₃-TPD,BET and other characterization methods.Combined with the characterization results,it is concluded that the surface of Mg₂Al-LDH has more Bronsted acidic sites,resulting in the weakest interaction with Ru species,which leads to higher catalytic activity in AB hydrolysis.This work will help us understand the hydrolysis mechanism of AB and the acid-base effect of the carrier on the dehydrogenation reaction.3.Ru crystal effect of Ru/?-Al₂O₃ catalyst on the hydrolysis dehydrogenation activity of ammonia boraneRu is the most attractive catalyst in the hydrolytic dehygrogenation of ammonia borane.So far,the employed Ru-based catalysts are mainly focusing on hexagonal close packed?hcp?structured Ru.In this work,we study the crystal phase effect of Ru on the hydrolytic dehydrogenation of ammonia borane,using face-centered cubic?fcc?structured and hexagonal close packed structured Ru nanoparticles supported on?-Al₂O₃.The results show that the catalytic activity is different even these two kinds of Ru nanoparticles hold similar size.Impressively,with the Ru particle size change,the variation trend of activity is opposite for face-centered cubic and hexagonal close packed structured Ru/?-Al₂O₃?ie:the catalytic activity of fcc Ru increases with increasing Ru size?.Finally,some possible reasons for their difference are proposed based on density functional theory calculation and characterization results.