Ni Supported On LaBO₃（B=Al,Cr,Fe） Perovskite For CH₄ Reforming To Produce Hydrogen

The fast industrialization process has led to increasing energy consumption.Fossil fuels are still the major energy sources in the world,whose large-scale use has caused serious environmental pollution.As the main component of natural gas,methane is a strong greenhouse gas,which has a tremendous contribution to global warming.CH₄ molecule contains only four potent C-H bonds,which are very difficult to be activated.To realize its chemical conversion successfully,it is not only necessary to design a reasonable conversion route,but also develop a highly active catalyst.Dry reforming（DRM）and steam reforming of methane（SRM）can convert it into higher value hydrogen and syngas,which can effectively solve the energy crisis problem,and also improve the ecological environment of mankind.Due to the abundant source and low cost of Ni,supported Ni catalysts have been widely used for reforming processes.The advantage of Ni-based catalysts are their high initial activity,but their Ni sites can be sintered during high temperature reaction process and generate coking,which leads to catalyst deactivation.With the purpose to solve these problems,a series of LaBO₃（B=Al,Cr,Fe）perovskite supports have been designed and prepared to support Ni O/Ni.The metal-support interaction,the redox property,the surface basic and active oxygen sites,and their influence on the reforming performance,anti-coking ability and stability have been explored.The main research contents are summarized here:Part Ⅰ:A series of LaBO₃（B=Al,Cr,Fe）perovskite supports were prepared by sol-gel method,and the 2%Ni/LaBO₃ catalysts were obtained by impregnation method.The activity results show that the 2%Ni/LaAlO₃ has the best activity,and 2%Ni/La CrO₃ has the highest stability.XPS results prove that Ni interacts with both the A and B site of LaAlO₃ support,but it interacts only with the B site for the other two perovskite supports.H₂ sorption results have demonstrated that the dispersion degree of the active Ni follows the order of 2%Ni/LaAlO₃>2%Ni/La CrO₃>2%Ni/La FeO₃,hence 2%Ni/LaAlO₃ exhibits the best activity.O₂-TPD and XPSO_1sresults have proved that the amount of surface active oxygen sites obeys the sequence of 2%Ni/La CrO₃>2%Ni/LaAlO₃>2%Ni/La FeO₃,thus 2%Ni/La CrO₃ has the best anti-coking ability.The Ni dispersion and the amount of surface active oxygen sites are the factors to decide the reaction performance and anti-coking ability of 2%Ni/LaBO₃ catalysts.Part Ⅱ:Based on the work of part I,LaAlO₃ support with a specific surface area of 26m²/g was prepared by the urea-ammonia precipitation method.The monolayer dispersion behavior of Ni O on the support,and its influence on the DRM performance was investigated.XRD extrapolation method has quantified that the monolayer dispersion capacity of Ni O on the support is 1.42mmol/100m² LaAlO₃,which is equivalent to 2.7%Ni O loading.3%Ni O/LaAlO₃,the catalyst with a Ni O loading around the capacity,shows the best activity.Above the capacity,the activity of the catalysts is nearly constant,which displays apparent threshold effect.The characterization results have testified that for the catalysts with Ni O loadings below the capacity,the Ni sites are highly dispersed after reduction.However,for the catalysts with Ni O loadings above the capacity,Ni crystallites are generated,whose sizes grow with the increase of the loading,which can lead to evident coking,especially graphic type carbon deposits.Therefore,the catalysts demonstrate also anti-coking threshold effect.In summary,Ni O/LaAlO₃ catalysts with the best reaction performance and anti-coking ability can be obtained by loading accurately the monolayer capacity amount of Ni O on the support surface.Part Ⅲ:By uing sol-gel method,a series of La_0.9M_0.1AlO₃ perovskite（M=Mg,Ca,Sr）supports modified by alkaline earth metals were prepared.5%Ni was loaded onto the supports with impregnation method,and used for DRM reaction.XRD results have demonstrated that Mg,Ca and Sr cations enter completely into the A-sites of the bulk phase of LaAlO₃ successfully.It is revealed that all the modified 5%Ni/La_0.9M_0.1AlO₃ catalysts display a slightly better activity than the unmodified 5%Ni/LaAlO₃catalyst,but the anti-coking ability is remarkably enhanced.CO₂-TPD results have testified that the surface basicity of the catalysts prepared by the alkaline earth metal-modified La_0.9M_0.1AlO₃ supports is much stronger,which is favorable to the adsorption and activation of CH₄and CO₂ molecules.XPS results have proved that the alkaline earth metal-modified 5%Ni/La_0.9Sr_0.1AlO₃ own also a larger amount of surface active oxygen species,which is beneficial to removing carbon deposits.5%Ni/La_0.9Sr_0.1AlO₃,the catalysts modified by Sr,possesses the largest amount of both kinds of surface active sites,thus showing the very good activity and the best anti-coking ability.