Research On Catalytic Methane Decomposition For High-concentration Hydrogen Production With Amorphous Carbon Modulation

In recent years,the shortage of fossil energy and environmental pollution has become more and more serious.In order to establish a clean,safe,and efficient energy system,it has become an inevitable trend of future energy development to develop new energy vigorously.As an energy carrier with a wide range of sources,flexible utilization forms,high energy density,and zero carbon emissions,hydrogen energy is of positive significance for meeting some of the world’s energy needs and reducing carbon dioxide emissions.Catalytic methane decomposition is an environmentally friendly technology with broad application prospects,which has the advantages of one-step hydrogen production and basically no COx emission.However,it has problems such as easy catalyst deactivation,low methane conversion rate,and difficult separation of solid by-product carbon and the catalyst.For these problems,this paper proposed a scheme of catalytic methane decomposition for high-concentration hydrogen production with amorphous carbon modulation.Based on Ni/MgAl2-xFexO4 materials,the lattice distortion was controlled and the low-temperature and efficient conversion of methane was achieved by adjusting the Fe doping amount of Ni/MgAl2-xFexO4 materials.The separation of carbon deposit and catalyst at a relatively low temperature was realized by controlling the amorphous degree of by-product carbon material using carbon dioxide as a gasification agent.Firstly,a series of Fe-doped Ni/MgAl2-xFexO4 catalysts(x=0.5-2.0)were prepared by the sol-gel method combined with the impregnation method for hydrogen production from catalytic methane decomposition.The effect of Fe doping amount on the structure and physical properties of the catalysts was investigated by characterization analysis,and the evolution of catalytic performance was investigated by performance tests.The results show that appropriate doping of Fe can enhance the metalsupport interaction,promote the dispersion of nickel particles,and inhibit the agglomeration and deactivation of the catalysts.In addition,it contributes to increasing the degree of lattice distortion of the catalyst,which plays a role in facilitating the activity of lattice oxygen.Combined with the test results of the performance tests in a fixed bed reactor,the catalyst Ni/MgAlFeO4(S3)exhibited excellent performance of both catalytic activity and stability when the Fe doping amount x is 1.0(Ni/MgAlFeO4).The H2 concentration and CH4 conversion rate remained at 96.99 vol.%and 100%,respectively,as the methane decomposition reaction continues for 40 minutes.Secondly,the catalytic activity of S3 in different reaction parameters and methane decomposition-carbon gasification cycle was explored in a fixed bed reactor,while the physical properties of S3 at each stage of the cycle were analyzed by XRD,H2-TPR,TEM,etc.The results show that the S3 catalyst has higher catalytic activity.In the methane decomposition stage,a methane conversion of 97.82%can be achieved at 700℃.In the carbon gasification stage,the rapid conversion of carbon dioxide and the separation of deposited carbon can be achieved at 800℃.In the stability test of 20 cycles,the H2 concentration in the methane decomposition stage and the CO concentration in the carbon gasification stage were stable at a relatively high level.S3 catalyst showed superior catalytic activity and cycle stability.The formation and disassembly of Ni-Fe alloy during the process of methane decomposition-carbon gasification cycles promoted the dispersion of particles and improved the stability of the catalyst.In addition,the results of phase analysis also confirmed that the S3 catalyst can maintain the basic stability of the phase within 20 cycles.Finally,the graphitization degree of carbon products was characterized through Raman,and the correlations of carbon products with catalysts and reaction parameters were constructed.The analysis results indicate that the structure and graphitization degree of the deposited carbon is affected by the Fe doping amount in the catalyst,the methane decomposition temperature and time.An appropriate doping amount of Fe can improve the lattice oxygen activity by increasing the lattice distortion of the catalyst,thereby reducing the degree of graphitization of the deposited carbon.Excessive Fe doping can lead to the reduction of the lattice distortion and the methane decomposition performance of the catalyst,which will bring out an increase in the degree of graphitization of carbon products.In addition,relatively low decomposition temperature and short reaction time are favorable for the preparation of carbon products with a lower graphitization degree.Based on the above discussion,this research has realized efficient conversion of methane and production of high-concentration hydrogen at low temperatures.Furthermore,the separation of the by-product carbon from the catalyst is achieved by gasification of the deposited carbon.This provides a reference and theoretical guidance for the development of highefficiency and low-consumption catalytic methane decomposition technologies for hydrogen production.