Research On The Photocatalyst Modulated By Defect Engineering For Carbon Dioxide Reduction

In the past decades,intensive efforts have been working hard to solve the problem of emitting large amounts of CO₂into the atmosphere;These include developing and promoting renewable energy,improving energy efficiency,reducing carbon emissions,and developing efficient carbon capture and storage technologies.Solar energy is considered an inexhaustible natural energy.If CO₂ is reduced through sunlight and an efficient catalyst,it should be a very promising strategy to convert CO₂ into high-value chemicals,because the use of CO₂ feedstocks can help to close the carbon cycle and reduce petrochemical consumption.It also helps to achieve the strategic goal of carbon peaking and carbon neutralization.However,due to the chemical inertness and thermodynamic stability of CO₂,the reduction of CO₂ is very challenging.At the same time,due to the lack of effective active sites on the catalyst surface,the catalyst efficiency is often unsatisfactory.By controlling the catalyst through defect engineering,we can not only study the electronic structure and carrier dynamics of the catalyst,but also build efficient catalytic active sites.This paper aims at the efficient use of green energy and the realization of the"double carbon"strategy.Through the construction of active sites through defect engineering,a series of defective photocatalysts were synthesized to improve the catalytic conversion of CO₂into value-added green fuel.The main research results of this paper are as follows:（1）The zinc vacancy(V_Zn)was successfully introduced into 3-dimensional hierarchical Zn In₂S₄（3D-ZIS）via hydrothermal method and characterized by Laser confocal fluorescence microscopy（LCFM）,spherical aberration corrected Transmission Electron Microscope（ACTEM）.The UV-Vis diffuse reflectance spectra（DRS）suggested that V_Zn widened the photo-response of 3D-ZIS to near infrared absorption.The electrochemical and photo-electrochemical experiments also demonstrated that the charge separation and carrier transfer were more efficient in the3D-ZIS with rich V_Zn.Of note,for the first time,we found that V_Zn could decrease the carrier transport activation energy（CTAE）measured by in-situ impedance spectrum,from 1.14 e V for Bulk-ZIS to 0.93 e V for 3D-ZIS,which may provide a feasible platform for further understanding the mechanism of photocatalytic CO₂ reduction.In-situ FT-IR results revealed that the presence of rich V_Zn ensured the CO₂ chemical activation,promoting single-electron reduction of CO₂ to CO₂^-.In addition,in-situ FT-IR,XPS and CO₂ TPD results showed that V_Zn could promote the formation of surface hydroxyl.To the best of our knowledge,for one thing,there were no reports on the photo-reduction of CO₂ simply by virtue of 3D hierarchical structure Zn In₂S₄with V_Zn and scarce literature on the photocatalytic reduction of CO₂ concerned with CTAE.For another,this work found that surface hydroxyl may play a crucial role in the process of CO₂ photoreduction.The work might provide some novel ways to ameliorate solar energy conversion performance and better understanding of photo-reaction mechanism.（2）The influence of defects on quantitative carrier dynamics is still unclear.Therefore,full-spectrum responsive metallic Zn In₂S₄(V_In-rich-ZIS)rich in indium vacancies and exhibiting high CO₂ photoreduction efficiency was synthesized for the first time.The influence of the defects on the carrier dynamic parameters was studied quantitatively;the results showed that the minority carrier diffusion length（L_D）is closely related to the catalytic performance.In situ infrared spectroscopy and theoretical calculations revealed that the presence of indium vacancies lowers the energy barrier for CO₂ to CO conversion via the COOH*intermediate.Hence,the high rate of CO evolution reaches 298.0μmolg^-1h^-1,a nearly 28-fold enhancement over that with Zn In₂S₄(V_In-poor-ZIS),which is not rich in indium vacancies.This work fills the gaps between the catalytic performance of defective photocatalysts and their carrier dynamics and may offer valuable insight for understanding the mechanism of photocatalysis and designing more efficient defective photocatalysts.（3）It is of great importance to understand the relationship between structure and properties at the atomic level,which provides a solid platform for design of efficient heterogeneous catalysts.However,it remains a challenge to elucidate the roles of the structure of reaction sites in the catalytic activity of active sites,due to the lack of understanding of the structure of specific active site species.Herein,taking metal-organic frameworks（MOFs）Ui O-66（Zr）as a prototype,MOF catalysts with all-solid-state Frustrated Lewis Pairs（FLPs）Zr³⁺-OH were synthesized in situ by adding acetic acid（HAc）as modulator.By introducing missing linkers,Ui O-66（Zr）firstly becomes a visible-light responsive photocatalyst for CO₂ reduction.In-situ FTIR spectrum reveals that b-CO₃^2-is the key intermediate for activation of CO₂molecules through FLPs Zr³⁺-OH.Moreover,defective Ui O-66（Zr）could‘self-breathing’by surface hydroxyls.This finding not only provides a new avenue for utilizing UV responsive MOFs by defect engineering,but sheds light on its catalytic activity at atomic level.（4）Homogeneous frustrated Lewis pair（FLP）has become a promising strategy for activating"inert"molecules such as CO₂.However,the critical issues of homogeneous FLP catalytic systems increase the difficulty of product purification and catalyst recovery.Moreover,it is of great significance to accurately construct active sites of heterogeneous at the atomic level for studying the structure-activity relationship.Herein,we outline the strategy for these issues by incorporating oxygen defects into a Zr-based metal-organic layer（Zr-MOL-D）and employing Lewis basic proximal surface hydroxyls for the in-situ formation of all-solid-state heterogeneous FLP(Zr^4-δ-V_O-Zr^-OH).With CO₂ as feedstocks,Zr-MOL-D showed an excellent CO generation rate of 49.37μmolg^-1h^-1in water vapor without any sacrificing agent or photosensitizer,which is about 11.7 times than that of pure MOL（Zr-MOL-P）,and exhibited extremely stability.This research constructed CO₂ active sites of solid heterogeneous FLP at the atomic scale by defect engineering revealed the structure-activity relationship between defective materials and photochemical reaction and opened new prospects for developing efficient MOL-based photocatalysts in FLP chemistry.