Preparation And Performance Investigation Of Highly-efficient Catalysts Towards Formic Acid/Hydrazine Borane Dehydrogenation

With the ever-growing depletion of fossil fuel and increasing environmental contamination,exploiting and utilization of sustainable energy become the trend of future energy society.Hydrogen energy?H2?,as a highly-efficient,clean,sustainable and abundant source of secondary energy,has attracted increasing interest.However,the controlled storage and transportation of hydrogen are still the major hurdle to its practical application.Formic acid?HCOOH,FA?and hydrazine borane?N2H4BH3,HB?,as the representative of liquid chemical hydrogen storage material,have received much attention due to their high hydrogen contents and good stability.Formic acid,which has a hydrogen mass capacity of4.4 wt%,can be safely handled at room temperature because of its non-toxicity and liquid state.While HB has a higher hydrogen content?15.4 wt%?,and its theoretical gravimetric hydrogen storage capacity?GHSC?is 10.0 wt%for the system HB-3H2O.Whether FA or HB,hydrogen generation efficiency mainly relys on the development and selection of catalysts.Hence,the development of highly-efficient,low-cost and stable catalysts for the decomposition of FA and HB at mild conditions is the key to the practical application.In this thesis,the main contents are as follows:1.In recent years,scientific research workers have made some progress in the process of catalyzing the dehydrogenation of FA,but most catalysts still need high reaction temperatures and additives.Moreover,there is still a lot of room for improvement in the catalytic performance of heterogeneous catalysts.?1?The bifunctional NH2-N-rGO supported MPd?M=Ni and Pd?UPs has been prepared by a facile one-pot reduction method.The as-prepared Ni0.4Pd0.6/NH2-N-rGO hybrid has ultrafine particle size?¹.8 nm?,excellent solubility and abundant active sites.Based on the analyses,except for the strong interaction between the metal UPs and the substrate,some electrons have also transferred from Ni to Pd,forming the rich electrons Pd sites,which improved the catalytic performance.Ni0.4Pd0.6/NH2-N-rGO shows 100%H2selectivity and conversion for the FA dehydrogenation at 298 K without any additives,and the initial TOF value is calculated to be 954.3 mol H2·mol catalyst^-1·h^-1.The use of Ni could reduce the cost effectively,and thus promote the practical application of FA as a hydrogen storage material.In order to further enhance the catalytic activity,we have synthesized Au0.33Pd0.67/NH2-N-rGO composite with excellent hydrophilcity and dispersity.The as-prepared hydrophilic Au0.33Pd0.67/NH2-N-rGO composite shows the most outstanding catalytic activity,excellent stability,and 100%H2 selectivity and conversion for FA dehydrogenation,with which the initial TOF value can reach the highest value of 4639.2 mol H2·mol catalyst^-1·h^-1 ever reported without any additives at 298 K.Moreover,the initial TOF value reaches as high as 20449.9 mol H₂·mol catalyst^-1·h^-1 at 353 K,which can be comparable to most homogeneous catalysts.The supreme performance can be attributed to the ultrafine and high dispersity of the metal UPs,excellent solubility,and the strong metal-support interaction between Au0.33Pd0.67 and NH2-N-rGO.The significant improvement of the catalytic performance of Au0.33Pd0.67/NH2-N-rGO composite may provide a feasible way for practical application of FA as thehydrogen carrier.?2?AgAuPd/rGO hybrid has been successfully synthesized by co-reduction of GO and the metal precursors using ethanol as the reduction agent at 353 K.The resultant Ag0.2Au0.4Pd0.4/rGO composite shows 100%conversion,100%H2 selectivity,and excellent activity for dehydrogeneration of FA in aqueous solution without any additive at room temperature,with the TOF_initial of 92.8 mol H₂·mol catalyst^-1·h^-1.?3?Pd NPs shows better catalytic activity and lower prices than other precious metal?Au and Pt etc.?NPs towards the decomposition of FA,but the mono-metallic Pd NPs are easily deactivated by CO poisoning,and still need high reaction temperatures and/or additives.In order to increase the CO-resistance and improve the catalytic activity,we have synthesized Pd composite at room temperature.Pd/NH2-N-CNTS composite exhibits excellent catalytic activity toward FA decomposition at room temperature,with 100%conversion and H2 selectivity,and the TOFinitial is 1607.6 mol H2·mol catalyst^-1·h^-1.2.Recently,all catalysts reported for the dehydrogenation of HB are containing noble metals,but the scarcity and the high price hindering their large-scale practical applications.While noble-metal free catalysts only show catalytic activity for the hydrolysis of BH3 group,with the conversion of 50%.Additionally,it has been reported that,amorphous/low crystalline structure has more defects,dangling bonds and unsaturated coordination sites when ompared to crystalline ones,leading to the better catalytic activities of amorphous/low crystalline catalysts.Therefore,in order to reduce the cost and further improve the catalytic performance,we have synthesized a series of amorphous/low crystalline noble-metal free catalysts and used to catalyze the full decomposition of HB.?1?Firstly,MIL-101 has been chosen as the support material,because its high porosity,large surface area and anchoring effect.NiFe-CeOx/MIL-101 has been prepared by a sequential impregnation-reduction method,and the introduction of CeOx can decrease the crystalline effectively.The resultant Ni0.5Fe0.5-CeOx/MIL-101 composite shows 100%conversion for the decomposition of HB at 343 K without any additives,and the TOF value is calculated to be 351.3 h^-1.Extend MIL-101 to other MOF structures,the NiFe-CeOx/MOF hybrid prepared by the same method all show excellent catalytic activity.?2?In order to further improve the catalytic activity of noble-metal free catalysts,we have synthesized amorphous/low crystalline Ni-MoOx/BN composite.The introduction of MoOx could both reduce the particle size,and improve the dispersity.The resultant Ni-MoOx/BN composite shows 100%conversion for the decomposition of HB at 323 K without any additives,and the TOF value is calculated to be 600 h^-1.This value is the highest one among all the reported noble-metal free catalysts used for HB decomposition,and even comparable to some noble-metal containing catalysts.Moreover,Ni-MoOx/BN nanocatalyst also shows excellent recycle stability,with 100%conversion ever after the 5^th run.