Structural Modulation Of Cu-CeO₂-TiO₂ Catalysts And Study Of Their Catalytic Performance For CO₂ Hydrogenation To Methanol

CO₂ as rich,non-toxic and low-cost C1 resource,its utilization has attracted a lot of attention.Methanol is not only directly used as fuel,but also is an important basic chemical raw material.Hydrogenation of CO₂ to methanol can not only help reduce CO₂ emission,but also alleviate energy crisis.Therefore,hydrogenation of CO₂ to methanol has been widely studied.The methanol selectivity of traditional Cu/ZnO-based catalysts is low due to reverse water gas shift（RWGS）reaction as the presence of product water in the reaction process which accelerates the sintering of the active component.Therefore,it is very necessary to develop a new and efficient catalyst.Ce O₂,a reducible oxide with special electronic structure,is easy to be reduced.As a reducible oxide with many adsorption sites including oxygen vacancies and different types of coordination unsaturated Ti and O atoms,Ti O₂ is also a potential catalyst carrier for CO₂ hydrogenation to methanol.The metal-oxide interface is considered as the active site for CO₂ hydrogenation to methanol over Cu-Ce O₂-Ti O₂ catalyst.Therefore,in the third chapter of this thesis,different Cu-Ce O₂-Ti O₂catalysts were synthesized through the selection of different solvents during the process of loading Ce O₂ to find the best preparing condition and figure out the factors that affect CO₂ adsorption,activation and further conversion.On the basis of the results in Chapter 3,Cu-Ce O₂,Cu-Ti O₂ and Ce O₂-Ti O₂ catalysts containing only two-phase interfaces were constructed in Chapter 4 to further explore the catalyst structural and electronic property evolution from the loading of Cu to the calcination,as well as H₂ reduction,disclosing the special oxide-oxide interactions and metal-oxide interactions at three-phase interface,revealing the important roles of Ce O₂ on the structure-activity relationship of catalysts.The main contents and conclusions are as follows:（1）The highly exposed（001）surface anatase Ti O₂ nanosheets were prepared by hydrothermal method while Ce O₂ was loaded through the alcohol thermal method（methanol,ethylene glycol and glycerol）,following by loading Cu through Na BH₄ reduction method.The Cu-Ce O₂-Ti O₂catalyst prepared using methanol/water as mixed solvent has the best performance for CO₂ hydrogenation to methanol.At 280℃and 3 Mpa,the methanol yield is as high as 8.8%.XRD,XPS and EPR characterizations were used to study the distribution of defects such as Ce³⁺,oxygen vacancies and Ti³⁺on the supports and catalysts.XRD and H₂-TPR were used to study the distribution and particle size of Cu species on the surface and CO₂-TPD was used to study the adsorption capacity of catalysts for CO₂.The results show that the particle size of Cu particles and the content of Ce³⁺,oxygen vacancies and Ti³⁺on the catalyst surface are very important for CO₂ adsorption,activation and further hydrogenation.（2）On the basis of the above investigation,Cu-Ce O₂,Cu-Ti O₂ and Ce O₂-Ti O₂ samples containing only two-phase interfaces were successfully constructed.Combined with the results of SEM,TEM,XRD,XPS,and H₂-TPR,Ce O₂ supported on Ti O₂ is rod-like defect-rich Ce O_2-x,which dependends strongly on the presence of Ti O₂.The defective Ce O_2-x supported on Ti O₂maintains relatively high stability at high temperature in O₂ and H₂ atmospheres.XRD,H₂-TPR,EPR and XPS characterizations show that the existence of defective Ce species is beneficial to improve Cu dispersion and the stability of Cu species during the calcination and reduction.The Cu loading in turn enhances the reducibility of Ce species due to the strong Cu-Ce O₂ interaction and the Ce³⁺-rich species on Cu-Ce O₂-Ti O₂ three-phase interface leads to high CO₂ conversion and methanol selectivity.