Theoretical Study On The Catalytic Activation Of CO₂ And H₂ By Ceria-based Materials And Its Crystal-plane Effect

The development of modern society is facing serious environmental problems such as resource shortages.The consumption of fossil fuels has led to a large amount of CO2 emissions,which intensifies the greenhouse effect.Realizing the efficient,clean conversion and utilization of carboncontaining resources such as CO2 is of great significance to the construction of a sustainable economic development framework.CeO2 has excellent oxygen storage performance,and its surface oxygen vacancies provide abundant active sites for activating carbon-containing molecules.It shows great development prospects in the application of CO2 conversion and hydrogenation catalysts.This thesis takes the CeO2-based materials to catalyze and activate the reaction of H2 and CO2 as the research object,and a series of theoretical studies have been carried out on the crystal-plane effect,metal doping effect,oxygen vacancy generation mechanism,and the metal-support interaction of the CeO2-based catalytic systems.The obtained progress is as follows:1.As key structural parameter,a crystal plane has a distinguished impact on catalytic performance.Different exposed crystal planes exhibit different reactivity.CeO2 nanostructured powders are usually exposed to three low-index surfaces,which are(111),(110)and(100)surfaces,respectively.Due to the unique structure and low stability of(100)surface,the investigation of the catalytic reaction mechanism on this surface is rarely involved.Here,the density functional theory(DFT)calculations suggest that CeO2(100)surface exhibits the strongest reactivity for H2 oxidation,attributed to the coordination unsaturation of surface oxygen atoms.For the hydrogenation of CO2 to methanol on defective CeO2(100)surface,CO2 prone to adsorb at oxygen vacancy in a nearly linear configuration,and formate pathway was verified as the dominant one.The bi-H2COO*can easily convert to bi-H2CO*with vacancy site filled,in which bi-H2CO*sever as the key intermediate,and methanol is synthesized by the steps of the bi-H2CO*? H2COH*? H3COH*.This study aims at providing a better understanding of the catalytic reactivity of CeO2(100)surface and theoretical insights into the experimental design of thermal CO2-to-methanol conversion.2.Surface doping is a common method to improve the performance of nanostructured materials.Different dopants will affect the structure and catalytic reactivity of the support.For comprehensive understanding the doping effects of metal doped into CeO2,the DFT studies were conducted on the stability and geometry structures of transition metal atoms(M=Fe,Co,Ni,Cu;Ru,Rh,Pd,Ag;Os,Ir,Pt,Au)doped into CeO2(111)(110)and(100)surfaces.Moreover,the reactivity for H2 dissociation and oxygen vacancies formation are systematic investigated on M doped CeO2(100)surfaces.The greater the binding energies of doped M atoms at CeO2 surface,the more difficult to form oxygen vacancy.The doped Co and Ir atoms do not directly participate in H2 activation,but serve as the promotor to make the H-H bond broken easily.The Cu,Ru,Pd,Ag,Pt,Au atoms could act as the catalytic active center for H2 dissociation and greatly reduce the activation energy barrier.Besides,it is easier to generate H2O(WM)and a surface oxygen vacancy from the intermediate H2M/H4M than from H3M/H5M,which is related to the acid-base interaction between the HCe/M*and Ho*in H2M/H4M.This work could provide theoretical insights into the atomic structure characteristics of the transition metal-doped CeO2(100)surface and give ideas for the design of hydrogenation catalysts.3.The CeO2-supported metals catalysts are widely used in the process of CO2 reduction and conversion.Their catalytic behavior is closely related to the interaction between the metals and the support.Exploring the stability and structural properties of metal clusters on the surface of different supports is crucial to the design of CeO2-supported metals catalysts and their reactivity regulation.In this part,the DFT calculation and ab initio molecular dynamics(AIMD)simulation were carried out to explore the morphology differences of supported Ru10 clusters on CeO2(111)and(100)surfaces.The stable configurations and dynamic features of Ru10 clusters are significantly different under vacuum and CeO2 supported conditions.The stability of the Ru10 cluster is also affected by the different crystal planes of CeO2.Electrons can be transferred from the Ru10 clusters to the CeO2 surfaces,resulting the positively charged of Ru clusters and reduced CeO2 surfaces.AIMD simulation results shown that the Ru-Ru bond of the Ru10 clusters are elongated on CeO2 surfaces,and the O atoms at interface could diffuse to the Ru10 clusters to form oxygen vacancies.The Ru10(c)cluster stably exists on CeO2(100)surface in the form of(3,7)configuration with no significant deformation.However,on CeO2(111)surface,the original configuration of Ru10(c)was distorted and prone to transform into another(4,6)configuration.In addition,the charge transfer of Ru10/CeO2 system was also significantly affected by the different configurations of Ru10 clusters.4.In order to better understand the role of metal-support interaction for CeO2-supported metal catalysts,the reaction mechanism of dry reforming of methane catalyzed by Ru10/CeO2(100)surface was explored.The DFT results shown that Ru cluster and the oxygen vacancy at the interface of Ru/CeO2 provided dual-functional sites for the activation and reforming of methane and CO2.During the reaction process,methane was more tend to gradually dehydrogenated at Ru cluster to form adsorbed C*and H*species,while the dehydrogenation of CH2*to form CH*was the rate determining step(RDS)for the complete dissociation of methane,with an energy barrier of 1.39 eV.Due to the higher energy barrier(1.42 eV)for the formation of CHOH*,CH*was more prone to directly dehydrogenated on Ru cluster to form C*and H*,with an energy barrier of 0.87 eV.In addition,CO2 could dissociated directly at the oxygen vacancy of interface and at the Ru cluster sites in curved and linear adsorption configurations,and the activation energy barriers are 0.30 eV and 0.72 eV,respectively.While the polydentate carbonate species was not easy to be activated due to its higher stability.Moreover,CO was more likely formed by direct oxidation of surface C*and O*at Ru cluster site.Additionally,the desorption of CO*from Ru cluster was highly endothermic,thus high temperature was needed.