Preparation Of Ultrafine Pt-based Catalysts For H₂ Production From Ammonia Borane Hydrolysis

As is known to all,hydrogen energy is a kind of renewable and clean energy,the safe and effective storage and transportation are the biggest obstacles for its practical application.Among many possible hydrogen storage systems,chemical hydrides with high hydrogen storage content are considered as the most prospective technology.Among many chemical hydrides,ammonia borane（NH₃BH₃,AB）,which has high hydrogen content（19.6wt%）and is stable solid at room temperature and pressure and soluble in water,has become a focus for researchers.AB per mole releases 3 equivalent of H₂ when completely decomposed.Hydrogengeneration efficiency of AB hydrolysis depends largely on the selection of catalysts.The noble metal catalysts are extensively employed in the hydrolysis of AB due to their excellent catalytic of activity.However,the low production and high price of noble metals seriously limit their large-scale applications.Moreover,the stability of metal nanoparticles（NPs）is reduced due to their agglomeration during reuse.Hence,improving the efficiency and the stability of noble metal in catalyst has been a hot area.In order to reduce the usage of noble metals without sacrificing their high catalytic activity and stability at the same time,currently the following several main approaches can be adopted,namely,minimizing size of nanocatalysts,fabricating alloy-catalyst of noble and non-noble metals,and confining the metal NPs in porous supports.Based on this,the main results of this paper are divided into two parts as follows:Firstly,natural mineral halloysite（HNTs）has been widely used in various supported nanocatalysts due to its special nanotube structure and stable physical and chemical properties.The traditional method of preparing HNTs supported metal NPs usually involves in modifying the HNTs surface with organics to absorb more metal ions,followed by in situ reducing the metal ions into metal NPs.However,this method has disadvantages of complicated operation,larger particles and residual organics,and the residual organics can affect the activity of the catalyst.Here,we successfully prepared the SEA-Pt/HNT nanocatalyst（～1.5 nm）using the strong electrostatic adsorption method without any organics.The results showed that the hydrogen selectivity of SEA-Pt/HNTs catalyzed hydrolysis of AB is 100%at room temperature.The total conversion frequency（TOF）value was 321 mol_H2molmin^-1,whereas the TOF of IMP-Pt/HNTs prepared by traditional impregnation method was only 148 mol_H2molmin^-1 under the same conditions.Secondly,in order to further improve the atomic utilization rate of the noble metal Pt and the activity of the catalyst,nitrogen doped porous carbon（N-MCSs）with large specific surface area was used as the support,and the Pt_xCo_1-x/N-MCSs nanocomposites（～1.6 nm）were prepared via introducing non-noble metal Co into metal Pt system by co-SEA.Electron transfer existed between Pt and Co,which facilitated the dissociation and adsorption of reactants at the active sites.Therefore,compared with the supported monometal catalysts,bimetallic catalysts exhibited better performance in the catalytic hydrolysis of AB.Among them,the Pt_0.33Co_0.67/N-MCSs showed the best catalytic performance.Its TOF value based on Pt content was 3617 mol_H2molmin^-1,which was about 13 times that of monometallic Pt/N-MCSs catalyst.After adding 0.4 M Na OH into the reaction system,OH^-coordinate with the surface of PtCoNPs,which effectively regulated the electronic structure of the metal and enhances the synergistic effect of Pt-Co.At this time,the TOF value of Pt_0.33Co_0.67/N-MCSs with the best catalytic activity reached4902 mol_H2molmin^-1,further improving the atomic utilization rate of Pt.On the other hand,the addition of OH^-changed the charge property of the N-MCSs surface,making the by-products with strong electron-absorbing ability prefer to adsorb on the negatively charged N-MCSs surface,thus improving the cyclic usability of the catalyst.