CO₂ Methanation Reactivity On Ni Catalysts Supporting On CeO₂ Modified By Alkaline Earth Metal Cation

Catalytic hydrogenation of CO2 to methane is an effective method to reduce CO2 emissions and convert CO2 into high value-added low-carbon energy,which is one of great significance in environmental protection,energy and other fields.However,the CO2 methanation is a typical exothermic and volume reduction reaction.From the thermodynamic point of view,low temperature is more conducive to the forward reaction.During CO2 methanation,reducing CO2 to CH4 involves the eight-electron reduction process.Therefore,the reaction is subject to significant kinetic constraints,so it is very important to prepare a high activity catalyst for CO2 methanation at low temperature.This thesis focuses on the structure-activity relationship,active sites and reaction mechanism to explore the modified effect of alkaline earth metal cations on its supported Ni catalysts especially the reactivity at low-temperature.The distribution of the different alkaline earth metal cations on the surface of the catalyst,the effects of the alkaline earth metal cations on the Ni dispersion oxygen vacancy and medium-strength alkali center have been investigated.The main results are summarized as follows:(1)To explore the threshold effect of lattice capacity of Ca2+doping on Ni/CeO2,the Ni/CeO2 catalysts modified by different amounts of Ca cation(named Ni/CayCe1-yOx,y is molar ratio of Ca/Ca+Ce)have beed prepared by Sol-gel method and used for CO2 methanation.The lattice capacity of Ca2+into CeO2 is 0.404 g CaCO3/g CeO2,and the corresponding Ca/Ca+Ce molar ratio is 11%.The Ni/Ca0.1 Ce0.9Ox catalyst,possessing the close Ca/Ca+Ce molar ratio to the lattice capacity of Ca2+and the maximum amount of pure solid solution phasefinally shows the best activity for CO2 methanation.The results demonstrate that the reaction performance of the Ni/CayCe1-yOx catalysts presents the threshold effect of lattice capacity.In addition,Raman,XPS,EPR characterization results revealed that the oxygen vacancy is highly related to the medium strength alkali sites.In-situ FTIR results showed that the reaction path of CO2 methanation on Ni/CeO2 is CO3H*?HCOO*?CH4.(2)To understand the role of alkaline sites played for CO2 methanation and develop better catalysts,the promoted Ni/M0.1Ce0.9Ox catalysts,in which the CeO2 are modified by different alkaline earth metal oxides with a M/Ce(M=Mg,Ca,Sr,Ba)molar ratio of 1/9,have been prepared.With the Sol-gel method,different alkaline earth metal oxides present varied distribution states in the modified CeO2 support.MgO and CaO are predominantly dissolved into the CeO2 support lattice to form a pure solid solution structure,while SrO and BaO are dispersed as the carbonates on the CeO2 support surface.The modification of all the alkaline earth metal oxides on CeO2 can improve the intrinsic activity of the obtained supported Ni catalysts,whose activity follows the order of Ni/Ca0.1Ce0.9Ox>Ni/Sr0.1Ce0.9Ox>Ni/Mg0.1Ce0.9Ox>Ni/Ba0.1Ce0.9Ox>Ni/CeO2.H2-TPR,H2 adsorption-desorption and XPS results have revealed that the modification of alkaline earth metal oxides can improve Ni dispersion,induce more surface oxygen vacancies and increase both the amount and the strength of the moderate alkaline sites,which are the major factors determining the activity of the Ni/M0.1Ce0.9Ox catalysts for CO2 methanation.Ni/Ca0.1Ce0.9Ox,possessing the best Ni dispersion and the largest amount of the moderate alkaline sites,displays the highest activity among all the catalysts.