Study On Metal-Oxide Interface Promoting Hydrolysis Of Ammonia Borane And Synchrotron Radiation

Energy shortage is one of the most serious problems in the world today.As a renewable and clean energy carrier,hydrogen has been recognized as one of the most ideal alternatives to traditional fossil fuels in the future.However,due to the low boiling point and bulk density of hydrogen in air conditions,the large-scale application of hydrogen has encountered obstacles,including hydrogen storage,transportation and release.Therefore,how to solve the problem of hydrogen storage and transportation safely and efficiently is the key to its industrial application.Ammonia borane（NH₃BH₃,AB）is considered to be an excellent material for chemical hydrogen storage due to its low molecular weight but high hydrogen content（19.6 wt%）,long-term stability at room temperature and non-toxicity.Noble metal catalysts have the best catalytic activity for the hydrolysis of ammonia borane.However,precious metals are expensive and scarce,which greatly limits their wide application.Therefore,developing low-cost non-precious metal catalysts or introducing non-precious metals into precious metal catalysts to reduce the use of precious metals while maintaining their high activity and stability has become a new challenge.Multiple studies have shown that the presence of transition metal oxides（TMOs）can accelerate the activation of water molecules.Therefore,designing and regulating the heterogeneous structure of metal-oxide is an effective way to catalyze the hydrolysis of ammonia borane.In this thesis,a series of Cu_x-（CoO）_1-x/TiO₂ samples with different Cu/Co ratios were designed.The strong interaction between Cu and CoO regulated the distribution of surface electrons,and the high catalytic activity of non-noble metal catalyst for NH₃BH₃ hydrolysis was successfully achieved.On this basis,Co was further used to adjust the composition of Pt catalyst,not only forming PtCo alloy,but also constructing the interface between PtCo alloy and oxide,realizing the regulation of the size and electronic structure of Pt and optimizing its catalytic activity.The structural characteristics of the catalysts were revealed by TEM,XPS,XAFS and other characterization techniques,and the reaction mechanism of AB hydrolysis catalyzed was explored by a series of kinetics and isotope effect experiments.The specific contents of this paper are as follows:（1）A series of Cu_x-（CoO）_1-x/TiO₂ samples with different Cu/Co molar ratio were prepared（x=0.1～0.9）.All catalysts can significantly increase the dehydrogenation rate of ammonia borane,and with the change of Cu/Co molar ratio,the TOF value presents a volcanic curve.The TOF of Cu_0.5-（CoO）_0.5/TiO₂ catalyst is 40.8 molH2 molmetal^-1 min^-1 at room temperature,much better than the activity of Cu/TiO₂ catalyst.The electron transfer between Cu and CoO in Cu_0.5-（CoO）_0.5/TiO₂ has been demonstrated by XPS and XAFS characterization methods.Electron-rich Cu can accelerate and activate NH₃BH₃ molecules,and CoO can accelerate the adsorption and activation of H2O molecules on the surface,speeding up the rate-determining step of the reaction.The Cu-CoO interface provides a bifunctional synergistic site for H2 generation and reduces the reaction barrier,thus greatly increasing the rate of hydrolyzing AB to hydrogen.（2）Further combining the metal-bonding interaction of metal alloying with the synergistic metal-oxide interface to construct the alloy-oxide interface is another effective method to enhance the activity of the catalyst.We synthesized Pt/TiO₂ catalysts with different molar ratios of Pt/Co（x=1.7,3.4,5.1）by impregnating TiO₂ with CoO and dispersing deposition of Pt nanoparticles by urea precipitation method.Compared with Pt/TiO₂ nanoparticles,the catalytic activity of Pt₁COx/TiO₂ on the hydrolysis of ammonia borane is remarkably enhanced.Among them,Pt₁CO3.4/TiO₂ catalyst with Pt:Co=1:3.4 has the best performance,and the TOF value of AB hydrolysis reaction can reach 2162.6 molH2 molPt^-1 min^-1 at room temperature,which is better than most reported Pt-based catalysts.TEM shows that the addition of Co can reduce the size of Pt nanoparticles and expose more active sites.XAFS test shows that Pt-Co metal bond is formed in Pt₁Co_3.4/TiO₂,and the oxidation state of Pt increases obviously compared with that in Pt/TiO₂,indicating that electron transfer occurs on the surface of Pt,which is also indicated by XPS.There is a strong interaction between Pt and Co,and CoO in the remaining oxidation states could accelerate the O-H bond breaking of H₂O molecules in the rate-determining step,thus jointly improving the activity of the catalyst.