Study On The Hydrogen Storage Performance Of N-ethylcarbazole Over Ni-based Catalysts

As a new liquid organic hydrogen storage carrier,N-ethylcarbazole（NEC）can be completely hydrogenated to perhydro N-ethylarbazole（12H-NEC）,which can be stored and transported in the same way as common oil products used.Because of the high mass density and the lower dehydrogenation temperature compared with the traditional materials,NEC has been considered as one of the most promising hydrogen carriers.The hydrogen storage in NEC achieved by hydrogenation over metal catalysts.Therefore,designing efficient catalysts is the key to storage hydrogen in NEC.In this thesis,nickel,one of the inexpensive transition metals,was selected as active component and silicon-based materials as the supports.The incipient wetness impregnation method was used for preparation the supported nickel catalyst.Combined with XRD,N₂ adsorption-desorption,H₂-TPR,TEM,CO-IR and other characterizations,the crystalline structures,particle sizes,physical properties and morphologies of supported Ni-based catalysts were investigated.Besides,their performances for NEC hydrogenation were also studied in a fixed bed reactor.The following three aspects were mainly discussed in this thesis:（1）The effects of the final reduction temperature of catalyst precursor on the structure of nickel catalyst on different support surfaces and its hydrogen storage performance of NEC.（2）Different solvents（water or ethanol）were used to dissolve the nickel precursor,and the distribution of nickel particles in the MCM-41 carrier channels was controlled.The effect of nickel particles distribution on the hydrogenation performance of NEC was investigated.（3）The effect of acidity constructed by adding Al³⁺into MCM-41 framwork on NEC hydrogenation performance of nickel catalyst was investigated.The results are as follows:（1）The carrier has significant influence on the particle size and electronic characteristics of nickel particles.On MCM-41 support,nickel particles are evenly distributed on the inside and outside surface of the channels,which the particle size of nickel is remarkably smaller than that on Si O₂ support,and the electron density of nickel particles on Si O₂ surface is higher than that on MCM-41.As the reduction temperature increases,the electron density of nickel particles on MCM-41 surface increases.The turnover frequency（TOF）of Ni particles on Si O₂ surface is higher than that on MCM-41 surface,but Ni/MCM-41 catalyst has more active centers,making NEC conversion distinctively higher than that of Ni/Si O₂.The selectivity of 12H-NEC is improved by increasing the electron density of nickel particles.（2）The distribution of nickel on the surface of MCM-41 channels has a prominent impact on the particle size of nickel and NEC hydrogenation.When nickel is located in the channels of MCM-41,the particle size is obviously smaller than that in and out of the pore,forming more active centers.Due to the limitation of MCM-41 mesoporous channels,the concentration of intermediate products（8H-NEC）is increased,and the reaction rate of hydrogenation of 8H-NEC to 12H-NEC is accelerated,and the selectivity of 12H-NEC is obviously promoted.Therefore,the catalyst with Ni located in the of channels MCM-41shows the highest NEC hydrogenation conversion.（3）With the decrease of Si/Al molar ratio,the acidity intensity increases slightly,and the specific surface area and pore volume decrease significantly.NEC hydrogenation conversion increases firstly and then decreases with the increase of catalyst acidity strength.Under the condition of 180℃,5 MPa H₂ partial pressure,optimal Si/Al ratio of 19 and space time of 9.52 g·min·mol^-1,the conversion of NEC reached 96%and the hydrogen storage density reached 5.3 wt.%.