Study On Dehydrogenation Of Formic Acid-based Systems Over Pd-based Catalysts

With the increasing environmental pollution and energy crisis caused by the massive burning of fossil fuels,hydrogen energy,as the most promising energy in the 21st century,has received extensive attention.In 2018,the state approved the overall plan for the conversion of old and new kinetic energy in Shandong Province.As the pioneering area of Shandong reform,Jinan will establish a trinity hydrogen valley in China.Therefore,whether for solving the world energy crisis and environmental pollution problems,or for the national development strategy,the development and utilization of hydrogen energy will always be a major research topic in the future.Compared with the physical methods,chemical methods for the storage and release of hydrogen are usually more efficient,easier to handle and safer.Among many chemical hydrogen storage media,formic acid is considered to be an excellent one owing to its renewability,low toxicity,safe storage,easy transportation and efficient hydrogen release through direct dehydrogenation over various catalysts under near ambient conditions.At present,Pd-based heterogeneous catalysts are mainly used for the catalytic dehydrogenation of FA.Although there are many studies on improving catalyst performance,and some progresses have been made,the problems of poor stability,high cost,and complex preparation remain unsolved.Thus,the development of stable,efficient and inexpensive Pd-based heterogeneous catalysts is still a great challengeIn 2016,our group found that the formic acid-ammonium formate(FA-AF)mixture showed a much better hydrogen production performance over Pd/C catalyst than single formic acid or formic acid-sodium formate(FA-SF)system.Its initial turnover frequency(TOF)reached 7959 h-1 at 323 K,but the stability of Pd/C catalyst was unsatisfactory,and the kinetics and related mechanisms of the decomposition of FA-AF mixture are not clear.Based on a systematic overview of the research progresses of Pd-based heterogeneous catalysts and dehydrogenation of the FA-based systems,we intend to prepare PdAu/C catalysts by a simple impregnation-reduction method to improve the stability of Pd/C.The effect of the composition of PdAu/C catalyst on the dehydrogenation of FA-AF system and the decomposition mechanism and kinetics of FA-AF system were studied.Based on this work,we intend to develop new Pd-based catalysts to further improve the catalytic performance towards dehydrogenation of FA and reduce the cost.The paper focuses on the following studies:(1)The dehydrogenation of FA-AF mixture over PdAu/C catalysts was studied.A series of PdAu/C catalysts were prepared by a simple simultaneous reduction method using sodium borohydride as a reducing agent without the addition of any stabilizer The effect of Au-Pd molar ratio on the catalyst performance,the decomposition mechanism and kinetics of the FA-AF system,the stability of the catalyst and the purity of the hydrogen were investigated.At the same time,the morphology,crystal structure,composition,elemental valence states of the PdAu/C catalysts were characterized by TEM,HR-TEM,XRD,EDX,ICP-OES and XPS.The results showed that the PdiAu3/C had the best catalytic performance.The Pd,Au3 nanoparticles(NPs)with the least particle size were uniformly distributed on the carbon support.The reaction order of AF and FA was 0.55 and 0.25,respectively.The activation energy of catalytic dehydrogenation for FA-AF mixture over the PdiAu3/C catalyst was 23.3±1.3 kJ mol-1.The maximum H2 volume for the 4th run remains about 88.0%of the first run.This means that the stability of the Pd/C catalyst has been greatly improved by introduction of Au.The content of CO was less than 5 ppm,but the NH3 content was relatively high.This indicates that the FA-AF mixture is not a promising system for production of high-purity hydrogen at this condition.(2)The dehydrogenation of FA-SF mixture over PdNi/C catalysts was studied.Firstly,Ni NPs were prepared by a impregnation-reduction method using sodium borohydride as a reducing agent.Then,the PdNi/C catalyst denoted as the PdNi/Cd catalyst was prepared by a displacement reaction of Ni and Pd2+.The effects of the amount of sodium citrate,the molar ratio of Ni-Pd during the preparation of PdNi/Cd catalysts and different preparation methods on the performance of PdNi/C catalysts were investigated.The morphology,composition,crystal structure and elemental valence states of the catalysts were characterized by TEM,EDX,ICP-OES,XRD and XPS.The results showed that Pd1Ni2/Cd had the best catalytic performance when the amount of sodium citrate was 1 mmol.The initial TOF for the dehydrogenation of the FA-SF mixture over the Pd1Ni2/Cd catalyst can reach 678.1 h-1 at 325 K.The Pd1Ni2 NPs obtained by displacement reaction are evenly distributed on the carbon support with an average particle size of 2.72 nm.At 325 K,the hydrogen volume produced from FA-SF system over Pd1Ni2/Cd catalyst was significantly reduced in the 3rd run.The Pd1Ni2/Cc catalyst prepared by co-reduction of Pd and Ni precursors with sodium borohydride can be recycled three times without significant decrease in the dehydrogenation performance.This suggests that with comparison to Pd1Ni2/Cd catalyst prepared by displacement method,the Pd1Ni2/Cc catalyst prepared by co-reduction method has better catalytic performance.