Theoretical Study On Structural Regulation Of Supported Transition Metal Catalysts And Their Effects On CO₂ Reduction Reaction

The massive emission of CO2 has broken the balance of carbon cycle in nature and caused many serious environmental problems.Reducing the emission of CO2 and its resource utilization has become the focus of attention all over the world.In this situation,China has also put forward the goals of "carbon neutrality" and "carbon peaking".The reduction of CO2 into high value-added fuels and chemical products can not only reduce the CO2 concentration,but also reduce the dependence on traditional fossil fuels,which is an effective way to curb the continuous massive emission of CO2 and alleviate the energy crisis.However,in the process of catalytic conversion of CO2,due to the high thermodynamic stability and strong kinetic inertness of CO2 itself,the paths and products of CO2 conversion reaction are complicated,so that the activity and selectivity of CO2 reduction conversion are not high enough,and the conversion efficiency is low.Therefore,the design and preparation of corresponding efficient catalysts is the focus of research in this field.Due to the complex structure of composite catalysts and the unclear structure-activity relationship,most studies are still limited to phenomenological studies.In recent years,with the development of quantum chemistry and computational science,the theoretical simulation has become a crucial research method.Theoretical research can analyze some data at the atomic level that are difficult to obtain in experiments,which is important for the regulation of the microstructure in chemical reactions and the exploration of the reaction mechanism.Therefore,in this paper,we use density functional theory calculation method to systematically study the influence of structural regulation of transition metal catalysts supported by reducing oxides on CO2 reduction reaction,which provides theoretical basis for rational design and synthesis of related efficient composite catalysts in the experiment.Firstly,the effect of alloy composition on the supported alloy catalyst PdAu/TiO2 was investigated,and the effect of the interaction between alloy composition and H2O on the CO2 reduction reaction was further considered.Secondly,in Pd/TiO2 system,the influence of carrier defect engineering(oxygen vacancy and nitrogen doping)on CO2 reduction reaction was discussed.Then,the effect mechanism of the crystal plane structure of CeO2 support on the loading of transition metal single atoms(transition metal atoms in Group VIII and Group IB)and on the catalytic activity and selectivity was investigated.It was revealed that the stepped CeO2 plane could provide a second active site to influence the adsorption mode of intermediates.The synergistic interaction with transition metal single atoms has an important effect on the activity and selectivity of CO2 reduction reaction.Finally,the catalytic reaction system was extended to the study of more complex C2 reduction products,and the diatomic Cu catalyst supported on the stepped CeO2 surface was used as a model to clarify the action law of the local environment of the active site and the important role of asymmetric active sites for the reduction of CO2 to C2H4.The main research contents and conclusions are as follows:The first chapter outlines the research background and research significance of this paper,introduces the basic principle of CO2 reduction reaction,focuses on analyzing the research progress and dynamics of CO2 reduction reaction,and summarizes the main challenges and opportunities in this field.The second chapter mainly expounds the significance of theoretical research,and summarizes the theoretical basis of the first-principles calculation based on density functional theory.Finally,the VASP calculation software package used in this paper is introduced.In the third chapter,we use the density functional theory calculations to study the effects of alloy composition and water promotion over supported alloy catalyst PdAu/TiO2 on CO2 reduction.The research results show that the dominant reaction path of CO2 reduction will change with the change of alloy composition.The dominant pathway is the "formate"pathway(referred to as the "CO2 HYD”pathway)on Pd8-xAux/TiO2(x=0-3,Au is only distributed in the upper layer of the alloy cluster).The dominant pathways is the "reverse water gas conversion and CO hydrogenation" pathway(referred to as the "RWGS+CO HYD" pathway)on Pd8-xAux/TiO2(x=4-6,Au is distributed in both the upper and lower layers of the alloy cluster).Further research found that there are different determinants of the activity of different pathways.In the "CO2 HYD" pathway,there is a volcano-shaped relationship between the binding energy of CO2*and the energy barrier,while in the "RWGS+CO HYD",pathway,the catalytic activity and the difference between the adsorption energies of CHO*and CO*are volcano curve relationship.In addition,water molecules can affect the electronic structure of the supported PdAu catalyst and further affect the CO2 reduction reaction,and the promoting effect of water is closely related to the alloy composition.These results provide important insights for maximizing the synergistic effect of alloy catalysts by regulating the composition of supported alloy catalysts,and illustrate the microscopic mechanism of environmental water molecular promoting effect,thus providing useful theoretical reference for rational design of relevant efficient catalysts and catalytic systems.In the fourth chapter,we pay attention to the effects of defect engineering(oxygen vacancies and nitrogen doping)over supported Pd10/TiO2(101)catalysts on CO2 reduction and the transfer and distribution of photogenerated electron.The calculation results show that the introduction of oxygen vacancies(VOS)on the surface of the reducing carrier TiO2(101)can cause reverse charge transfer,making the interface rich in electrons,and VO and Pd atoms together form an active site,which promotes the adsorption and dissociation of CO2.However,the VO will be filled by the dissociated O atom during CO2 reduction.The co-modification of VO and substituted nitrogen doping atoms(Ns)could not prevent the VO from being filled by the dissociated O atom.When the doped nitrogen atom is located at the interstitial sites directly below the oxygen vacancy,the interstitial nitrogen atom(Ni)can promote the delocalization of highly localized charges near VO and form a new active site.The new active site is composed of Pd atoms located at the bottom edge of the Pd10 cluster above VO and the Tisc atom adjacent to VO,which not only promotes the adsorption and reduction of CO2,but also protects the oxygen vacancy and ensures the stability of the system.Furthermore,under the defect engineering control of the oxygen vacancy and interstitial nitrogen doping,photogenerated electrons can efficiently transfer from the oxide to the interface,which further promotes the adsorption and dissociation of CO2.This work clarifies the micro-mechanism of the effect of carrier defect engineering regulation on CO2 reduction reaction,at the same time shows that for photocatalysis,reasonable defect regulation,especially the optimal combination of different defect forms,can contribute to the directional transfer of photogenerated electrons,achieving multi-site functional integration,thus improving reactivity and stability of the catalyst.In the fifth chapter,based on first-principles calculations,we explore the crystal plane structure of the support on the loading behavior of transition metal single-atom catalysts and its effect on CO2 reduction.A series of transition metal single atoms(all elements in GroupⅧ and Group IB)catalytic systems supported on stepped CeO2 surface are explored,and the intrinsic microscopic mechanisms affecting the activity and selectivity of CO2 reduction are discussed.The research results show that the exposed low-coordination O atoms and Ce atoms on the stepped CeO2 surface are beneficial to the bonding of the single atom and improve the stability of the supported single atom.The single atom and stepped CeO2 surface jointly provide active sites,which change the adsorption mode and strength of the intermediate,and synergistically improve the activity of CO2 reduction.Group Ⅷ and Group IB transition metal single atoms supported on the stepped CeO2 surface(TM1/CeO2-S-U)showed selectivity to CH3OH(TM=Cu,Pd,Ag,Au)and CH4(TM=Fe,Co,Ni,Ru,Rh,Ir),respectively.The activity and selectivity of CO2 reduction are closely related to the adsorption mode of key intermediates.This work provides a strategy for stabilizing transition metal single-atom catalysts using stepped reducing metal oxide surfaces,and utilizing the synergistic effect between multiple sites to realize the control of the activity and selectivity of CO2 reduction.Through the discussion of electronic structure and reaction process,the microscopic mechanism is clarified,which will provide some theoretical reference for the development of efficient CO2 reduction electrocatalyst.In chapter 6,this research focus on the reaction mechanism of the reduction of CO2 to C2 product(C2H4)over stepped CeO2 suppported diatomic catalyst Cu2/CeO2-S-U.The effects of the oxygen vacancy and nitrogen doping on the local environment of Cu2/CeO2-S-U catalyst and the activity and selectivity of CO2 reduction reaction were explored.The research results show that Cu2/CeO2-S-U can catalyze the reduction of CO2 to C2H4 through the coupling reaction of*CH2,and is a potential catalyst for the reduction of CO2 to C2H4.However,because the two Cu atoms are in the same local environment and the charge distribution is consistent,the charged situation of the two adsorbed*CH2 is consistent,and there is strong dipole-dipole interaction,which is not conducive to the occurrence of C-C coupling reaction.The modification of the oxygen vacancy makes the local environment of the two Cu atoms asymmetric,but the intermediates such as*CO/*CHO/*COH/*CH2 are not generated during the reaction,resulting in no selectivity for C2 products.However,nitrogen doping can make the local environment of the two Cu atoms different and the charge distribution is asymmetric,so the charged situation of the two*CH2 is not the same,and then the electrostatic interaction occurs.This effect can reduce the difficulty of*CH2 coupling reaction,promote the formation of C2 products,and improve the catalytic activity.This work revealed the reaction path of CO2 reduction to C2H4 over Cu2/CeO2-S-U catalyst,and clarified the mechanism of support surface structural regulation on CO2 reduction over Cu2/CeO2-S-U.Theoretically,the feasibility of nitrogen doping on Cu2/CeO2-S-U catalyst to improve the catalytic activity of CO2 reduction to C2H4 was proposed,and the importance of asymmetric active site for the preparation of C2 product by CO2 reduction was emphasized,which could provide some theoretical guidance for the design and synthesis of C2 product catalyst in experiment.Finally,the seventh chapter summarizes the main research contents and innovation points of this paper,and gives an outlook on the future research work.