Important Roles Of Excited-State Hydrogen Bonding In Typical Photocatalytic CO₂ Reduction Systems

Currently,the human is facing two major issues,environment and energy.CO₂ is not only the main greenhouse gas,but also is a kind of abundant Cl resource.Among the many CO₂ conversion technologies,photocatalytic CO₂ reduction is utilizing the abundant solar energy and using the photocatalyst,which can reduce CO₂ into valuable hydrocarbon fuels,is one of the most promising ways to convert CO₂.CO₂ photoreduction still faces many challenges and its low catalytic efficiency is the biggest bottleneck.One of the main reasons is the research on CO₂ photoreduction mechanism is insufficient:the first is the lack of research on the recombination of electrons and holes during the photophysical process;secondly,photochemical process occurs in the excited state,and most of the current researches are conducted in the ground state using the first principle.Therefore,only by studying the nature of the photophysical and photochemical processes of CO₂ reduction in the excited state can we further understand the mechanism of its photocatalytic process.More importantly:H2O is the reactant and medium in CO₂ photoreduction,and the role of hydrogen bonding in the excited state cannot be ignored.In this paper,combining the photocatalysis theory and molecular photochemistry theory,three representatives(carbon quantum dots,g-C₃N₄ and anatase TiO₂)photoreduction CO₂ systems are selected.Time-dependent perturbation theory and Fermi golden rule are used to study the photophysical process and time-density functional theory(TDDFT)is used to study the photochemical process.And combined with the experiment to deeply understand the important role of excited-state hydrogen bonding in the photocatalytic CO₂ reduction.The main research contents are as follows:(1)Pure material 9-hydroxyphenal-l-one(HPHN)with known molecular and crystal structures was used to prepare into carbon quantum dots(HPHN-CQDs).HPHN-CQDs used as the photocatalyst to reduce CO₂ to CO in water without sacrificial agents and photosensitizers.In the calculation,the model of the hydrogen bond complex formed by the catalyst,CO₂ and H2O was constructed and the photophysical and photochemical processes of the hydrogen bond complex are explored.The calculation results show that the behavior of excited-state hydrogen bond is closely related to the activation of CO₂ and the proton and electron transfer processes.The formation of COOH radical in the photochemical process is the rate-limiting step of the entire reaction,and this step is closely related to the electron donating ability of the catalyst.It is found that Zn²⁺ ions have the best electron donating ability through the screening of volcano map.The calculation results reveal that the existence of Zn²⁺ ions not only increase the rate constant of the rate-limiting step in the photophysical process,thereby reducing the recombination of electrons and holes,but also act as the catalytic active center and decrease the barrier of the formation of cOOH radical in the photochemical process.In addition,the experimental data shows that the activity of CO₂ photoreduction of HPHN-CQDs increased by approximately 2.9 times after the addition of Zn²⁺ions,which verifies the theoretical prediction results.(2)Single-atom Fe-g-C₃N₄ photocatalyst was prepared based on the graphitic carbon nitride,which can reduce CO₂ and generate CO in water without the presence of sacrificial agents and photosensitizers.The participation of Fe atom can increase the activity toward CO production from CO₂ photoreduction of g-C₃N₄ by approximately 1.9 times.In order to explore the mechanism of the role of single-atom Fe supported on the g-C₃N₄ catalyst in CO₂ photoreduction,the hydrogen bond complex formed by the catalyst(g-C₃N₄ and Fe-g-C₃N₄),CO₂ and H2O was selected as our calculation model.The calculation results show that the participation of Fe atoms not only makes the activation of CO₂ occur in the ground state and also increases the rate constant of rate-limiting step in the photophysical process,thereby reducing the recombination of electrons and holes;it also acts as the catalytic active center of the photochemical process and decreases the rate-limiting step(the cleavage of the C-O bond in COOH·)in the photochemical process,thereby improving the photocatalytic activity of the whole CO₂ photoreduction reaction.In addition,the calculation results show that the hydrogen bond in the excited state induces the process of proton and electron transfer in the photophysical and photochemical processes.(3)Single-Atom photocatalyst Fe-TiO₂ was prepared in which Fe atom was anchored on the nano-anatase TiO₂ substrate.The participation of single-atom Fe can increase the activity of anatase TiO₂ photoreduction of CO₂ to CO by about 2.4 times without any sacrificial agents and photosensitizers.In the calculation,the hydrogen bond complex composed of catalyst and reactants was constructed based on the cluster structure of anatase TiO₂,and the electron transfer and proton transfer induced by the excited state hydrogen bond in photophysical and photochemical processes were investigated.DFT and TDDFT calculations show that the enhancement of the hydrogen bond in the excited state during the photophysical process will promote the electron transfer from TiO₂ to CO₂,facilitating the activation of CO₂ and also reduce the recombination of electrons and holes;the formation of COOH· during the photochemical process involves the proton transfer and electron transfer,and the breaking of the C-O bond in COOH· in the rate-limiting step involves electron transfer,which are all closely related to the hydrogen bond in the excited state.Excited-state hydrogen bonds dominate the entire process of photocatalytic reduction of CO₂,which can enrich and perfect the mechanism of CO₂ photoreduction.This dissertation systematically studies three different types of typical photocatalytic reduction CO₂ systems by combining theory and experiment.Although the reaction mechanism is slightly different,the common point is the excited-state hydrogen bond plays a vital role in the recombination of electrons and holes,the activation of CO₂,and the transfer of electron and proton.