Density Functional Theory Study Of The Structures And Catalytic Roles Of Rh_x/CeO₂ Catalysts

Rh nanoparticles supported on ceria?Rh_x/CeO₂?have high stability,oxygen storage and release capacity and catalytic activity,which have been widely used in automotive exhaust purification,VOCs removal and solid fuel cell,etc.It is important to understand the structure and catalytic roles of Rh_x/CeO₂ at an atomic level.In this thesis,we have systematically studied the structures of Rh nanoparticles on CeO₂ and its electronic properties,catalytic roles and reaction mechanism by density functional theory?DFT?and artificial intelligence?AI?technology.This work provides an important insight into the development of catalysts with high efficiency at low temperature.Using DFT calculation and AI technology,the structures of Rh_x/CeO₂?111?were systematically investigated and the effects of oxygen vacancy on their stability were discussed.We built and optimized a series of Rh_x/CeO₂?111?modeling catalyst structures by three methods.The first method is that the initial structures of Rh_x/CeO₂?111?are generated by our previously reported rule,which the Rh number in different layers of Rh_x clusters is changed successively using the 2D Rh_x cluster as the initiation.The second method is that the most stable structure of metal clusters is optimized by global optimization algorithm using IMAGE package developed by Synfuel China;then the corresponding clusters adsorb at the different sites on CeO₂surface.The third one is Stochastic Surface Walking?SSW?method developed by Fudan University.The theoretical results indicate that a single Rh atom prefers to locate at the O-O bridge sites of CeO₂?111?;while it prefers to adsorb at the oxygen vacancy of the partially reduced CeO₂?111?surface.With the increase of Rh atoms,Rh_x/CeO₂?111??x=2-3?tend to form 2D structures wetting on ceria;while Rh_x/CeO₂?111??x>3?incline to form 3D clusters.Similar trend is also obtained for Rh_x/CeO_2-??111?.By comparing the optimized structures of Rh_x/CeO₂?111??x=7-10?with different methods,we find that the first method can approach the global minimum of Rh_x/CeO₂?111?since the interaction between metal and support is well considered in the method.The second method failed to obtain the globally stable Rh_x/CeO₂?111?structure.This might be attributed to the strong interactions between metal cluster and support for M_x/CeO₂catalysts.The SSW method can obtain the global optimum structure on the basis of massive computations.The effects of different index surfaces of CeO₂ on the structures of Rh_x/CeO₂modeling catalysts were also systematically investigated.For CeO₂?110?,a single Rh atom prefer to adsorb on the hollow site;while Rh₂ prefers to adsorbed at the short O-O bridge site.With the increase of Rh coverage?x>3?,Rh_x on CeO₂?110?trend to form 3D structures.Different from CeO₂?111?and?110?,Rh_x clusters?x?9,??1ML?tend to scatter on CeO₂?100?.When the coverage is higher than 1ML,Rh_x?x>9?clusters tend to form 3D structures on CeO₂?100?.According to the comparison of the stable structures of Rh_x clusters on different surfaces of CeO₂,it is found that the main factors affecting the structures of Rh_x/CeO₂ are the different structures of CeO₂ surface,the size of Rh_x clusters and arrangement of Rh_x clusters on ceria.In order to explore the catalytic effects of Rh-CeO₂ catalysts,the structures of Rh/CeO₂ catalysts and the reaction mechanism of CO oxidation were systematically studied in this thesis.The structures and electronic properties of Rh/CeO₂?111?,Rh/CeO_2-??111?and Rh_x Ce_1-xO_2-??111?,as well as the adsorption and reaction behaviors of CO on the three modeling catalysts,were systematically calculated.Based on the comparison of the electronic properties,adsorption energies of O₂ and desorption barriers of CO₂ on the three catalysts,it is found that Rh/CeO_2-??111?has the lowest CO oxidation barrier;meanwhile Rh/CeO₂?111?has the highest barrier for CO₂ desorption.