The Co-based Core-shell Catalysts And Their Application In Methane Carbon Dioxide Reforming Reaction

Rapid increase in population and significant energy consumption has been forecasted over the years.The dependence on fossil fuels to meet energy demand caused environmental issues by the production of greenhouse gases.Even though the concentration of methane in the atmosphere was lower than carbon dioxide,it kicked in about 20%of overall global warming.Therefore,it makes sense to utilized methane efficiently.Methane reforming in carbon dioxide is an important approach to utilization of natural gas,and reduction in emission of greenhouse gases,attracting extensive attention all over the world.However,due to the high endothermic nature of the process,high temperatures are required to reach high conversions.Under these conditions catalyst would experience rapid deactivation due to carbon deposition and metal particle sintering.So,it is necessary to develop a new type of catalyst that shows anti-sintering and coke-resistant in dry reforming of methane(DRM).The application of nanoparticles(NPs)in heterogeneous catalysis is highly desirable due to the intrinsic "surface effects".Unfortunately,the application of NPs is restricted in heterogeneous catalysis because NPs are unstable and aggregate easily especially at elevated temperatures.Core-shell structured materials have attracted great attentions in recent years because of the unique structural feature and physicochemical properties.Particularly SiO2-encapsulated nanostructures have been studied extensively.The NPs can be isolated by SiO2 layer,which would restrain the sintering of active metal particles.Moreover,the growth of carbon deposits on the core metal can be suppressed in DRM.Based on the characteristics of DRM and the advantages of nanostructured core-shell catalyst,in the present study,we prepared several core-shell catalysts through fine tuning the core constitution and particle size,shell thickness,as well as shell porosity,to establish a complicated structure-performance correlation for DRM upon detailed characterizations.The major conclusions can be obtained below:1.The core-shell feature can be affected by the applied TEOS concentration and the size/shape pf core nanoparticles.The oxide core particle size shows direct impact on the reduction behavior of Co3O4@SiO2 as well as core-shell interaction.The smaller core size is,the stronger the core-shell interaction will be.The strong core-shell interaction causes fine oxide core precursor rather difficult to be reduced and severe carbon deposition and hence quick sintering of catalyst.On the other hand,the weak core-shell interaction causes bulky oxide core precursor readily to be reduced and insignificant carbon deposition.However,the large Co0 core can be easily re-oxidized by the surface O*species upon CO2 activation at the core-shell boundary in DRM.A thick shell may cause an issue of mass transfer for DRM.With fine tuning core size,shell thickness,and core-shell interaction,27.8-Co@SiO2-14.3 exhibits superior activity and stability,as the result of optimized CH4/CO2 activation and minimized carbon deposition/metal oxidation.2.Ru distribution in binary Co-Ru core-shell system is crucial to achieve the optimal Co-Ru synergetic effect which can be accomplished through the employed hydrothermal approach.Ru addition enhanced the feature of coking-and oxidation-resistant of Co constituent.Moreover,a notable phase transformation of ?-Co to ?-Co was observed on Ru-Co@SiO2-P under the reaction atmosphere,favorable for DRM.The SiO2 shell effectively prevents the Ru-Co bimetallic core particles from sintering and suppresses the oxidation of Co constituent during the reaction.Tuning shell porosity was also found to be critical for mass transfer and catalyst performance.3.The microemulsion method is an effective way to prepare core-shell structured catalysts with small metal cores.The Ni dispersion as well as particle size can be affected by Ni content.The higher the Ni content is,the lower the Ni dispersion will be.Moreover,part of Ni particles are exposed on the outer shell of catalyst at a high Ni loading.The higher the content of Ni is,the easier the reduction of catalyst will be.The catalyst of a lower Ni content requires a higher initial reaction temperature in the DRM because small Ni particles result in a strong core-shell interaction.The carbon deposits anchor more strongly to the smaller Ni particles;and this is opposite to the situation observed over the Co-based catalysts.Ni10@SiO2 shows excellent catalytic activity and stability in DRM due to the features of better Ni dispersion,and anti-sintering/anti-coking.