Study On A High-concentration Formic Acid-ammonium Formate System For Hydrogen Generation Over Pd/C Catalyst

Owing to the rapid advancement of economics,the energy demand of China continuously grows in the past decades.At present,the energy supply of our country is still mainly relying on the traditional unrenewable energy such as coal and oil.The reservation of these fossil fuels is running exhausted,while serious environmental problems are aroused due to the combustion of these traditional fuels.It is imminent to develop new green renewable energy source to support our future economic progress.With the very high combustion value and no environmental effect of the combustion product,hydrogen has long been deemed as the most promising fuel for future application.Particularly,when constituting fuel cell with oxygen,hydrogen can give much higher energy conversion efficiency than ordinary combustion.Unfortunately,with so many advantages,hydrogen is still far from practical application due to some fatal shortcomings:difficult preparation,expensive and dangerous storage and transportation,and low volume power density.In order to surpass these problems,some organic liquids and aqueous solutions serving as hydrogen storage media for in-situ controllable generation of hydrogen have been proposed in the past two decades.Among these systems,formic acid and its solution have attracted much attention due to its high hydrogen content,high decomposition activity and no toxicity.For catalytic decomposition of formic acid,Pd-based heterogeneous catalysts were found to be most effective.Among the researches on formic acid-based hydrogen storage systems,besides few works concerning formate or formic acid-triethylamine adduct systems,most researches set their interests in single formic acid or formic acid-sodium formate mixture system.Although the combination of Pd catalyst and formic acid-based system shows good performance in hydrogen production,the hydrogen storage density,dehydrogenation activity of the fuel systems,and the activity and stability of the catalyst still require further improvement.In this paper,the research progresses of Pd-based heterogeneous catalysts and formic acid-based dehydrogenation systems were summarized.Based on the comments on the previous researches,the Pd/C catalyst which can be easily prepared via impregnation-reduction method was chosen as our catalyst.The conditions for preparing Pd/C catalyst with good performance were optimized.According to the literature,we postulated the possibility of applying formic acid-ammonium formate mixture as a more promising system for catalytic dehydrogenation.Based on the optimized Pd/C catalyst,we found that the formic acid-ammonium formate mixture is indeed a much better system than other formic acid-based systems ever reported.The formic acid-ammonium formate mixture system with high concentration can be easily decomposed at room temperature,showing much higher electrochemical power density.In order to give explanation to the especial activity of formic acid-ammonium formate system,the effect of HN4+ and NH3 on the dehydrogenation activity of formic acid-sodium formate system was investigated using amine with different basicity and structure.The main works done in this thesis are:?1?The impregnation-NaBH4 reduction method for preparing common Pd/C catalyst was optimized.The Pd/C catalyst was prepared with sodium borohydride serving as reducing agent and sodium citrate as stabilizing agent.The effect of the molar ratio of palladium precursor and sodium citrate,N2 protection during catalyst preparation,and Pd loading on the activity and stability of the Pd/C catalyst were explored.The morphology,composition,crystal structure and element valence state were characterized by HRTEM,EDS,XRD,and XPS.It was found that N2 protection had little effect on catalyst performance.When the molar ratio of palladium chloride and sodium citrate is 5:3 and the Pd loading is 5 wt.%,the catalyst shows the best performance.The optimized catalyst possesses lower activation energy for dehydrogenation of formic acid.During the reaction progress,the pivotal reason for the decreased catalyst activity is the change of catalyst itself.?2?The hydrogen production performance of formic acid-ammonium formate?FA-AF?with high concentration was investigated.The results showed that the FA-AF mixture system exhibited higher hydrogen production activity with the promoted catalyst performance.The FA-AF mixture system with the molar ratio of 1:4 of FA and AF showed the best dehydrogenation ability.The electrochemical capacity of 10 mol·dm-3 FA-AF mixture can reach 565.87 mAh mL^-1.The initial turn-over frequency of FA-AF mixture can reach 7959 h"1 at 50.The higher dehydrogenation activity of FA-AF mixture system can be partially ascribed to the effect of AF serving as a co-catalyst.The hydrolysis of AF and the adsorption of NH3 on Pd catalyst surface may also accelerate the dehydrogenation of formic acid.?3?The dehydrogenation activity of formic acid-sodium formate/methylamine system was also studied.The results suggested that the addition of methylanine into formic acid-sodium formate could also improve the dehydrogenation activity of formic acid-sodium formate system.Formic acid can be more easily adsorbed on the catalyst surface with the addition of methylamine,and the donation of electron from methylamine to Pd catalyst also changes the electron density of Pd catalyst.The addition of methylamine can also lower the activation energy of formic acid-sodium formate dehydrogenation.