| With rapid development of global economy,the limited fossil source(such as coal and petroleum)has become one of the main factors to hinder the social sustainable progress.As a kind of efficient clean energy,hydrogen is going to play a critical role in 21st century.At present,industrial H2 production mainly comes from CH4 reforming reactions,which can convert CH4 into CO and H2.Methane is the main component of natural or shale gas,which is abundant everwhere on the earth.The CH4 added-value transformation can be accomplished by CH4 reforming to produce H2.On the other hand,the growing emission of CO2 caused a series of environmental issues,thus CO2 methantion and CO2 reforming of methane reactions are efficient ways to utilize CO2.For these reactions,the low cost Ni-based catalysts display very good initial activity.However,due to Ni aggregation and severe coking during the reaction process,these Ni-based catalysts can be deactivated.Therefore,after clarifying the mechanism of CH4 reforming and CO2 methanation,and from the point of view to strengthen the interaction between the metal and pyrochlore supports,several series of catalysts with considerable activity have been synthesized in this work.The main results are summaried here:Part 1:To explore the interaction between NiO and Y2Ti2O7 support,and fabricate improved catalysts for SRM,several Ni/Y2Ti2O7 catalysts have been prepared by an impregnation method with the assistance of dielectric barrier discharge(DBD)plasma treating in different atmospheres.It is found that both the reaction performance and anti-coking ability are improved for the catalysts in comparison with the untreated sample,following the sequence of Ni/Y2Ti2O7-H2P>Ni/Y2Ti2O7-ArP>Ni/Y2Ti2O7-AirP>Ni/Y2Ti2O7.H2-TPR and XPS results have demonstrated that plasma treatment effectively strengthens the interaction between NiO and Y2Ti2O7 support,and Ni2+cations preferentially interact solely with the Y3+cations in the A site of Y2Ti2O7.As a consequence,the agglomeration of the metallic Ni species during the reduction process can be impeded,thus obtaining catalysts possessing higher Ni dispersion.As evidenced by Raman,EPR and XPS results,superoxide O2-anions induced by the intrinsic 8a oxygen vacancies of Y2Ti2O7 pyrochlore phase is the only kind of active surface oxygen sites,which might play a critical role for coking removing,and whose formation can be significantly facilitated by plasma treating.In conclusion,the improved active Ni surface area and the surface active superoxide O2-amount by DBD plasma treating are the major factors responsible for the enhanced reaction performance and coking resistance of the catalysts.Part 2:A series of NiO/Y2Ti2O7 with various NiO loading were synthesized.The monolayer dispersion capacity of NiO on the Y2Ti2O7 supports calculated by the XRD extrapolation method is 1.20 mmol NiO/100m2 Y2Ti2O7,which equals to 0.025g NiO/g Y2Ti2O7.This result is also confirmed by XPS extrapolation method.According to the dry reforming of methane results,the coking amount of the catalysts grew with the increasing of the NiO loading.Interestingly,2%NiO/Y2Ti2O7 and 3%NiO/Y2Ti2O7,whose NiO loadings are close to the monolayer dispersion capacity,display competitive activity to those high loading catalysts,but show much better anti-coking ability.H2-TPR and XPS results have confirmed that the NiO in sub-monolayer or monolayer state interacts strongly with the Y2Ti2O7 support,which originates from that the close interaction between Ni2+and both Y and Ti cations of the Y2Ti2O7 support.As a result,better Ni dispersion and higher active metallic Ni surface area can be obtained.Furthermore,2%NiO/Y2Ti2O7 and 3%NiO/Y2Ti2O7 possess more active surface superoxide O2-species and stronger ability to adsorb/activate CO2 moleculars,which is regared as the main reasons for their superior anti-coking ability.In brief,the Ni-support interaction influence evidently the surface oxygen properties and Ni dispersion after reduction,hence affecting the reaction performance of the catalysts.Part 3:Three dimensional ordered macropore(3DOM)Y2Zr2O7 pyrochlore supports with mesoporous pore walls have been synthesized by colloidal crystal templating method(PMMA as hard template and F127 as soft template).The 3DOM supports possess high surface areas,abundant pore structures and ordered macropores,which are identified by the N2 adsorption-desorption and SEM techniques.After supporting Ni on the Y2Zr2O7 supports,the catalysts were applied to both CO2 methanation and dry reforming of CH4,the reaction performances follow the order of Ni/Y2Zr2O7-3DOM700>Ni/Y2Zr2O7-3DOM800>Ni/Y2Zr2O7-CP800.H2-TPR,TEM and H2 sorption results have demonstrated that the Ni/Y2Zr2O7-3DOM700 and Ni/Y2Zr2O7-3DOM800 with a 3DOM structure have stronger Ni-supports interaction compared with the Ni/Y2Zr2O7-CP800,which was prepared by the traditional co-pricipitation method.As a consequence,smaller Ni crystallite/particle sizes and higher Ni dispersion can be achieved.Moreover,the richer oxygen vacancies on the surface of Ni/Y2Zr2O7-3DOM700 and Ni/Y2Zr2O7-3DOM800 promote CO2 molecule adsorption and activation.Last but not the least,In situ DRIFT results have certified that for CO2 methanation on Ni/Y2Zr2O7-3DOM700,the best catalyst in this work,CO species are the essential intermediates during the reaction. |
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