Study On Mechanism Of Catalytic Reforming Of Methane Over Modified Nickel Based Carbon Fibers

Biomass as a renewable energy resource,can be converted into syngas by thermochemical method.In the pyrolysis and gasification of biomass,H₂,CO,CO₂,CH₄,H₂O as well as a small amount of tar can be generated.How to alter the proportion of biomass-derived gas at appropriate conditions resulting in desirable hydrogen production,so as to maximize their utilization has always been the focus of attention.The synthesis of syngas from carbon dioxide reforming methane(CRM)can be used to optimize the pyrolysis gas of biomass.Obstacle for the industrial applications is the catalyst deactivation.The study investigated the application of carbon fibers(CF)with large specific surface area and special fibrous structure in CRM reaction.Specifically,a series of oxidant coated nickel based carbon fibers catalysts were synthesized by different methods to improve the catalytic performance of Ni/CF.Combined with a suite of characterization techniques,it demonstrated the role of different oxide-coated carbon fibers in the CRM reaction process,thus illustrating the relationships among catalytic performance,the coating methods,the properties of the coated oxidants.Additionally,the relationship between the physi-chemical properties of the catalyst and its activity and stability was established by DFT calculation.Finally,the prepared modified nickel based carbon fibers were used in the study of rice husk pyrolysis-reforming.According to all the results obtained from the study,it was found that:The activity of in-situ Ni/CF prepared by electrospinning method is higher than that of the catalyst which was prepared by impregnation method.Besides,a preliminary exploration of the CRM reaction process on 5%Ni/CF was found that CO₂ was dissociated and activated at the interface between the metal and the support to produce two activated intermediate species,namely carbonate intermediates and COO^-intermediates.The intermediate products of CH₄ activation were mainly CH₂ and CH_xO.It was also found that the dissociation activation of CH₄ is the rate controlling step of the CRM reaction on Ni/CF.However,carbon fibers were consumed by partial reaction of CO₂ during the process,thereby affecting the application of carbon fibers-supported catalysts in CRM reactions.Based on this,varies modified nickel based carbon fibers were synthesized and applied to improve the catalytic performance of Ni/CF.Firstly,SiO₂ modified Ni based carbon fibers was prepared and the gasification of carbon fibers was restrained to a great extent after SiO₂ coating.It was found that the modified Ni exists on the surface of carbon fibers,the surface of SiO₂ and the junction of SiO₂ and carbon fibers at the same time.CH₄ is dissociated and activated at these three positions to produce CH_x,while CO₂ is activated on the surface of some carbon fibers to form CO and O atoms.Then the formed O atoms reacts with CH_x,the intermediate produced by CH₄ pyrolysis,to form another transition state CH_xO,which is followed by the dissociation and activation of CH_xO to form CO and H₂.DFT calculation further shows that the SiO₂ modified Nickel based carbon fibers can adsorb CO₂ and CH₄ easily.After adsorption,CH₄ is activated and dissociated to form 2H₂ and C gradually on the modified Nickel based carbon fibers,in which the process of CH₃ dissociation to CH₂ is the speed control step of CH₄ dissociation.Then,to further improve the catalyst stability,Ni₃Si₂O₅(OH)₄ modified nickel based carbon fibers was prepared and it showed greatly stability improvement under reaction conditions at 700°C,where simultaneously adding the Ni and SiO₂(SiO₂Ni-CF)during synthesis delivered the best stability over 24 h.CO₂ conversion over SiO₂Ni-CF was maintained at approximately 80%after 24 h time on stream.the superior performance of SiO₂Ni-CF is attributed to the formation of Ni₃Si₂O₅(OH)₄ species.It is difficult for Ni₃Si₂O₅(OH)₄ to be reduced,favoring a decrease in the Ni particle size to?6 nm.Moreover,the presence of the Ni₃Si₂O₅(OH)₄ species improves the MSI via a confinement effect.In addition,Ni_xMg_3-xSi₂O₅(OH)₄,as one Ni₃Si₂O₅(OH)₄ derived species,was also investigated in CRM reaction with the same conclusion observed,where the existence of this kind of phyllosilicate structure can effectively increase the interaction between metal and support and the unreduced part can fix the nickel particles via a confinement effect,thus improving the sinter resistence.To further improve the catalytic activity of Ni/CF,a layer of metal oxides Fe₂O₃,Mn O₂,CeO₂ and Zn O,which contain movable lattice oxygen was introduced on the surface of Ni/CF via different methods.It was found that the catalyst,45CeO₂10Ni-HCF,which was pretreated by H₂O₂ showed a best coating morphology and the occurrence of the CeO₂ enhanced the the carbon deposition resistance of Ni-based CF in CRM.Based on the results obtained above,a series of modified nickel based carbon fibers were applied in the actual rice husk pyrolysis-gas reforming after exploring the application of Ni based carbon fibers in catalytic CRM reaction.It was found that the proportion of syngas(H₂+CO)after pyrolysis-reforming on 5%Ni/CF was 67.7%,reaching to 25 mmo L/g.Besides,the syngas yield and the H₂/CO over 45CeO₂10Ni-HCF were all the highest with the value of 34.21 mmo L/g and 1.8 respectively.It was indicated that the cracking of CH₄ to produce hydrogen was improved on 45CeO₂10Ni-HCF.Moreover,the gas production increased gradually,especially the syngas,which increased from 6.23 mmo L/g to 52.37mmo L/g with the increase of catalyst amount from 0 g to 1 g,revealing the availability of45CeO₂10Ni-HCF in the actual rice husk pyrolysis-reforming system.