Theoretical Calculation Study Of Typical Catalytic System In Carbon Dioxid Hydrogenation To Methanol

It is a common challenge for all mankind to face environmental problems caused by the excess emission of CO₂ ,such as global warming.The conversion and utilization of CO₂ through CO₂ hydrogenation to methanol can simultaneously alleviate energy shortages and environmental problems.The typical catalysts for CO₂ hydrogenation to methanol mainly include metals and metal oxides.However,the reaction process of CO₂ hydrogenation to methanol involves a complex reaction network composed of multiple parallel reaction paths.It contains multiple key intermediate steps such as H₂ and CO₂ activation,CO₂ stepwise hydrogenation,and C-O bond cleavage.There are different steps that determine the reaction rate and optimal pathways for different catalytic systems,which makes it difficult to understand the reaction mechanism.At present,there are great challenges in understanding the reaction mechanisms of different catalytic systems by means of experimental characterization.In contrast,theoretical calculations allow the study of the active site structure and reaction mechanism of catalytic reactions at the atomic level.The aim of this study is to focus on the typical catalytic systems for CO₂ hydrogenation to methanol reaction:main group metal oxide（Ga₂O₃ and In₂O₃ ）catalysts and Cu-based catalysts.The main findings are as follows:1.For the main group metal oxide catalytic system,machine learning-based intelligent sampling method of surface sites was used to systematically investigate surface structure under reaction conditions,adsorption and dissociation of H₂ molecules and CO₂ activation and their electronic structure changes,surface site structure and activity during hydrogenation to methanol.The study concluded as follows:（1）Thermodynamic phase diagrams show that under the reaction conditions of CO₂ hydrogenation,the main group metal oxides are reducible from difficult to easy:Al₂O₃ >Ga₂O₃ >In₂O₃ .Ga₂O₃ and In₂O₃ are at the threshold of reducibility and oxidizability.Al₂O₃ will not be reduced.（2）H₂ is physically adsorbed in molecular form on the surface of Al₂O₃ ;Ga-H and O-H are formed on the surface of Ga₂O₃ to reach dynamic equilibrium;In-H and O-H are formed on the surface of In₂O₃ ,and O-H is generated after surface migration.（3）There are 2 M-H generated by H₂ homogeneously dissociation at the oxygen vacancies on both Ga₂O₃ and In₂O₃ surfaces.Oxygen vacancies reduce the activation energy of H₂ on Ga₂O₃ surface.However,oxygen vacancies increase the activation energy of H₂ on In₂O₃ surface.（4）Different sources of H species react with CO₂ to produce different intermediates.The reaction of linear CO₂ with H from metal sites to form HCOO;the reaction of curved adsorbed CO₂ with H from oxygen sites to form COOH is a favorable reaction path.Ga₂O₃ surface has low oxygen vacancy concentration due to its low catalytic activity.In₂O₃ surface has high oxygen vacancy concentration,but H₂ COO generation and C-O bond breaking are difficult to occur.2.For Cu-based catalytic systems,Cu/ZnO catalysts are of great interest due to their wide source and good catalytic performance.The catalytic activity was boosted further by the addition of the second metal Au.For unraveling the mechanism of Au doping,three models,ZnO/Cu（111）,ZnO_1-x/Cu（111）and ZnO_1-x/Au-Cu（111）,were constructed to calculate the CO₂ hydrogenation to methanol reaction.The results show that:（1）The addition of Au promotes the formation of oxygen vacancies at the ZnO/Cu（111）interface.The oxygen vacancy reduces the reaction potential energy surface and stabilizes the intermediate adsorption.（2）Experimental characterization and theoretical calculations show that the addition of Au stabilizes the adsorption of b-CH₃O at the oxygen vacancy at the ZnO/Cu interface.（3）CO₂ hydrogenation to methanol follows the formate mechanism.The results show that Au can enrich the active site of oxygen vacancy at the Cu-ZnO interface.Affirmation:All experimental parts in Chapter 5 of this paper were mainly completed by Guiming Xie of the group.Only some of the experimental conclusions are used in Chapter 5 of this paper.For more details of the experimental parts,see the papers published in this research work（AppliedCatalysisB:Environmental.DOI:10.1016/j.apcatb.2022.122233）.