First-principles Studies On The Reaction Mechanism Of Photocatalytic Reduction Of CO₂ On The Surface Of Catalyst

With the rapid development of the global economy and science and technology,it has brought convenience and rapidness to people,and has also brought many problems.Among them,energy shortages and environmental pollution are particularly prominent.On the one hand,people hope to find new energy to replace the increasingly depleted fossil energy as soon as possible.On the other hand,they must try to deal with large amounts of greenhouse gas CO₂ emissions.Photosynthesis in nature converts CO₂ into O₂ while producing the energy（organic matter）needed for plant growth.Inspired by the photosynthesis in nature,scientists proposed the concept of“artificial photosynthesis”,which uses light to drive energy,and uses photocatalyst to adsorb and convert CO2 and H2O in the air into energy fuel（store solar energy and convert it into energy.The process of converting solar power into chemical energy in organic）.Therefore,photocatalytic reduction of CO₂ has also become a material,and energy and environmental issues have attracted attention in recent years.After the photocatalyst absorbs photons,the photogenerated electrons and holes must migrate to the surface of the material to react with the molecules adsorbed on the surface.Therefore,the surface structure and properties of the catalyst/cocatalyst and the adsorption and activation of the reaction molecules on the surface are important factors that determine the rate of the catalytic reaction and catalytic selectivity.Photocatalytic reduction of CO2 has a lot of research both experimentally and theoretically.However,it is difficult to provide all reaction information on the surface in the existing experimental characterization methods,and the photocatalytic reaction is very complicated and has many factors.It is difficult to study a single specific influencing factor in the experiment.First-principles calculations provide a method for the study of surface photocatalytic reduction of CO2.In this paper,we use the first-principles calculation method to study the adsorption and activation of CO2 on the catalyst/cocatalyst surface.A series of work has started:The first chapter summarizes the research background and research progress in the field of photocatalytic.First,the basic principles and physical models of photocatalytic reactions are outlined.Next,the basic principle of photocatalytic reduction of CO2 is introduced.Finally,the experimental and theoretical research progress of the role of surface in photocatalytic reduction of CO₂ is described in detail.The second chapter introduces the theoretical basis and calculation methods.The classification of theoretical calculations and simulations,the approximation of the first principles,the density functional theory and its development are introduced.Finally,we also introduced the commonly used method for calculating transition states CI-NEB and the theoretical model for searching for transition states,the first-principles calculation package VASP.In chapter three,the microscopic mechanism of CO₂ adsorption and activation on the surface of photocatalyst is studied.For the photocatalytic reduction of CO₂ reaction,the first step of adsorption and activation is very important.However,CO₂ is a very stable molecule and requires high energy to activate it.Some experimental reports have found that chemisorption on the surface of the material can lead to partial charge transfer between CO₂and the surface,thereby shifting CO₂ from a linear structure to a curved structure and lowering the activation barrier.However,there is no systematic study on which materials or surfaces exposed to adsorption bends.Using a method based on density functional theory,we have systematically studied the adsorption of CO₂ molecules at different adsorption sites by selecting different photocatalytic materials and different surfaces.It was found that not all materials and surfaces have the characteristic of adsorption bending,and only an oxide material with a certain geometric structure（metal-oxygen-metal）can easily form this spontaneous absorption bend.The study also found that the adsorption and bending of the TiO₂（001）and MgO（100）surfaces are relatively easier than other surfaces,explaining the experimentally-found cause of the strong catalytic activity of these two exposed surfaces.Our conclusions also provide theoretical support for the design of new surface structures.In the fourth chapter,the microcosmic mechanism of CO₂ adsorption and activation on the surface of photocatalyst modified by metal co-catalyst was studied.Three kinds of reaction pathways for surface photocatalytic reduction of CO₂ were first introduced,and a formaldehyde pathway with good selectivity was selected as the research object.Next,the effects of single atom Au and metal cluster Ag₅ modified rutile TiO₂ catalysts on the catalytic reduction pathway of CO2 were discussed.The study found that the metal modification has a great influence on the adsorption site,adsorption configuration and adsorption energy of CO₂and reaction products.Further,we found through the search of the transition state method that the metal modification will make the photocatalytic CO2 reaction.The reaction barriers and reaction paths have changed,and our conclusions will also provide important information for understanding the role of cocatalyst.The fifth chapter summarizes the main conclusions of the full text,and puts forward the future research.