Modification Of Ceria-based Oxide Materials By Doping And Two-step Thermochemical CO₂ Splitting Reaction

Finding pathways to clean and renewable generation of fuels is a crucial step toward mitigating the greenhouse effect and environmental pollution in the process of burning fossil fuel.Splitting CO₂ into CO and O₂ utilizing concentrated solar energy via two-step thermochemical cycles has drawn much attention recently,because it is a promising option to produce long-term storable and transportable chemical fuel.Thermochemical CO₂ splitting reactions have only been investigated over the ferric oxide,ZnO,Ce O₂,V₂O₅ and so on.CeO₂ has come to a hot research for the two-step cycle reaction because of its involatile,faster CO generation rate,good cycling stability,excellent ability to reversibly store and release lattice oxygen and resistant sintering.The main drawback of CeO₂ is that the available oxygen vacancy is very limited,which shows restricted chemical yield.Based on the previous studies,the citric acid complex method was used to modify ceria-based materials by doping other cations(Zr⁴⁺,In³⁺)to improve the fomer reactivity of splitting CO₂.Structures,the oxygen storage/release capacities and thermal cycling stablility of modified materials were investigated in this study by X-ray diffraction?XRD?,N₂ adsorption,Raman spectroscopy,H₂-TPR,and self-designed in-situ CO-CO₂ looping test.For the cerium-zirconium solid solution materials by citric acid complex method,the effects of the content of Zr⁴⁺ on structures,redox properties and releasing rules of CO₂ decomposition product in the two-step cycle reaction were detailedly studied in this work.The results showed that the introduction of Zr play a role of inert carrier,which can limit the sintering during cycling.When Zr content is less than 20%,more content of Zr is conductive to maintain porous channel as well as the specific surface area.Especially,the sample Ce_0.8Zr_0.2O₂ has the highest specific surface area,which can reach 58.3 m2/g.However,the specific surface area decrease with Zr content continously increasing due to the changing of the crystalline symmetry from cubic to tetragonal.Doping a certain content of Zr is able to reduce the activation energy of oxygen migration,which is beneficial to the migration and diffusion of lattice oxygen.Materials doping 10% Zr showed a good performance in two-step thermochemical CO₂ splitting.The amount of lattice oxygen releasing continue to increase when Zr content is more than 20%,but the generation rate and the production of CO decrease obviously.Further rearch was done to support different content of In on the Ce_0.8Zr_0.2O₂,to study the effect of In doping content on the structure,redox properties,thermal cycle stability and activation of CO₂ splitting reaction.Results showed that Ce_0.8Zr_0.2O₂ doped by In is able to form two different solid solution,substitutional solid solution and interstitial solid solution,that depends on the doped content of In.Thermal stability and anti-sintering ability of the materials doped excess content of In will become worse,lead to lower production of O₂ than Ce_0.8Zr_0.2O₂.It is noted that CO₂ decomposition rates of materials are improved through the doping modification of In,but the generation rate of CO decreases significantly when the content of In is more than 4%.Materials doped less than 6% performed better in the CO-CO₂ cycling stability test and that doped more content of In will become gradually worse.Different contents of In loaded on the surface of Ce_0.8Zr_0.2O₂ were prepared by the impregnation method.CO-CO₂ cycling stability test was carried out to study the thermal cycling stability.Results showed that all materials loaded In were able to maintain the pore structure and the thermal stability and have an excellent redox property than Ce_0.8Zr_0.2O₂.InOx dispersed on the Ce_0.8Zr_0.2O₂ plays an important role in the CO evolution reaction.But the production of CO is lower than the ternary solid solution Ce0.8-yZr0.2InyO₂.