Synthesis And Catalytic Properties Of Novel Catalysts For Carbon Dioxide Methanation

As the global population and economy grow,the environmental problems caused more and more prominent due to CO2.In order to reduce the concentration of CO2 in the atmosphere,utilization of CO2and its transformation into other chemical products or renewable energy sources are an urgent problem to be solved.CO2 conversion seems to be a more attractive and promising approach.Ni-based catalysts are preferred as promising catalysts for this reaction due to their high intrinsic activity and low cost.However,supported Ni catalysts frequently suffer from severe deactivation due to particle sintering,interaction of metal particles with carbon monoxide,formation of mobile nickel subcarbonyls,and coke deposition.It is an important key to develop effective catalysts for CO2 methanation.So this thesis mainly focuses on the following aspects:(1)Ni-Ce-Zr mixed oxides were prepared through one-pot hydrolysis of mixed metal nitrates with ammonium carbonate.Effects of Ce/Zr molar ratio and Ni content on catalysts physical and chemical properties,and reduction degree of Ni2+were systematically investigated by XRD,N2 adsorption,H2-TPR.Their catalytic behavior for CO2 methanation with H2 in the range of 150-350? was intensively investigated to design an effective catalyst for C02 methanation.The catalysts showed excellent low-temperature activity and stability in CO2 methanation in the temperature range of 150-350?.The catalytic performance not only depended on the Ni crystallite sizes or the number of surface Ni atoms,but also had a strong correlation with interaction of Ni with supports.Zr addition to Ni-CeO2 affected on reducibility of Ni2+ ions,the Ni crystallite sizes formed,interaction between Ni and supports,resulting excellent catalytic activity,stability and anti-coking ability.40Ni-Ce0.9Zr0.1O2 catalyst exhibited optimal catalytic properties.The maximal levels of CO2 conversion in the presence of the 40Ni-Ce0.9Zr0.1O2 catalyst was 97.0%at 275?,which reached chemical equilibrium value.The stability test showed that 40Ni-Ce0.9Zr0.1O2 retained long-term stability without deterioration during the 500 h reaction period.These results will be helpful to develop highly effective Ni-based catalysts for low temperature CO2 methanation.(2)Mesoporous y-alumina was prepared via one-pot template-free partial hydrolysis route.Highly-dispersed Ni-Ce-Zr mixed oxides supported on mesoporous?-alumina(Ni-Ce1-rZrxO2/?-Al2O3)were prepared by a citric acid(CA)-assisted impregnation method.The effects of CA contents and Ce/Zr molar ratio in the reaction solutions on the physicochemical properties and the catalytic performance of the Ni-Ce-Zr/y-MA catalysts were investigated in detail.Addition of CA significantly promoted dispersion of Ni-Ce-Zr oxide species on the surface of?-alumina and improved the interaction between Ni oxide species and support,resulting in the formation of homogeneously-dispersed Ni nanoparticles in y-MA frameworks upon reduction with H2.(3)Ni-Ce oxides(NiO-xCeO2/?-Al2O3)supported on m,esoporous ?-alumina with various mass percentage contents of CeO2(x = 0,1,2,3,4 and 5)were prepared through co-impregnation with citric acid.The influences of CeO2 content on the catalyst structure,surface characteristics,interaction between Ni species and the support,reducibility of Ni2+ ions and Ni particle dispersion were investigated in detail.The Ni-Ce oxides were highly dispersed on mesoporous y-alumina and producing uniform Ni nanoparticles throughout y-alumina frameworks after H2 reduction.The addition of CeO2 could lower metallic Ni particle sizes and generate oxygen vacancies.The reduced Ni-xCeO2/?-Al2O3 catalysts were investigated for their catalytic behaviors in CO2 methanation and exhibited excellent catalytic performance at low temperature in the range of 150-350?.Equilibrium conversion of CO2 was achieved at a low temperature of 300 0C over the optimal CeO2 content of 3 wt.%.(4)In order to further impove catalytic performance,Ru doped Ni-xCeO2/?-Al2O3 catalysts was prepared for CO2 methanation.The influences of Ru content on the physicochemical properties and catalytic activities of prepared catalysts were investigated.The addition of Ru could lower metallic Ni particle sizes and reduce reducibility of Ni2+ ions.As a result,CO2 conversion was improved.Equilibrium conversion of CO2 was achieved(95.9%)at a low temperature of 250? over the optimal Ru content of 0.5 wt.%,which was higher than that of Ni-3Ce02/?-Al2O3.