First Principle Study On The Mechanism And Product Selectivity Of Photocatalytic Reduction Of CO₂ By Modified Cobalt Aluminum Hydrotalcite

Due to the increasing demand for fossil fuels in modern society,the greenhouse effect caused by carbon dioxide（CO₂）as one of the combustion products is an environmental problem that we need to solve urgently.Simulating artificial photosynthesis and using effective catalysts to reduce CO₂to organic products can not only alleviate energy shortage,but also solve the problem of environmental pollution.Therefore,it is particularly important to explore available photocatalysts.Ultra-thin LDHs structure makes its low coordination surface atoms easy to separate from the lattice to form defects,which is conducive to the improvement of CO₂adsorption and electronic structure.Because of its strong controllability,it has attracted extensive attention in the field of catalysis.In this paper,defective CoAl-LDHs and CuO supported CoAl-LDHs were constructed based on CoAl-LDHs,and their photocatalytic reduction performance and product selectivity were studied by density functional theory（DFT）.The contents are as follows:（1）Electronic structure of defective CoAl-LDHs and mechanism of photocatalytic reduction of CO₂Based on density functional theory plus U（DFT+U）method,three single structural defects including cobalt defect(V_Co),aluminum defect(V_Al)and hydroxyl defect(V_OH)and two CoAl-LDHs models including hydroxyl and cobalt defect mixture(V_OH&Co)and hydroxyl and aluminum defect mixture(V_OH&Al)are constructed based on CoAl-LDHs hydrotalcite.We systematically studied the effects of defects on the structure,electronic properties,band edge position and photocatalytic CO₂reduction performance of CoAl-LDHs.All possible intermediates were constructed,and the Gibbs free energy change of each reaction step was calculated to determine the potential energy determining step,and further understand the best reaction path and mechanism of CO₂reduction to CH₄.The research shows that V_Cois not conducive to the photocatalytic reduction of CO₂,V_OH&Aleffectively reduces the Gibbs free energy barrier of the potential energy determination step and enhances the photocatalytic performance because of the synergistic effect of the OH defects and Al defects.（2）Selectivity of CuO/CoAl-LDHs photocatalytic CO₂productsCu₄O₄clusters were loaded on CoAl-LDHs,and the binding mode of the load was studied.Through the comparison of electronic properties,optical properties and CO₂adsorption,CuO/CoAl-LDHs with the best photocatalytic performance was obtained.Through the Gibbs self use energy calculation of the reaction transition state,it is found that CuO/CoAl-LDHs changes the reaction path,prevents the formation of CH₃OH products and increases the desorption energy of intermediate products.This is because CuO/CoAl-LDHs can construct binary electron channels with intermediates,which enhances the binding of intermediates.Finally,the calculation of product desorption kinetics shows that*CO on CoAl-LDHs will desorb in the form of intermediate product,and*CH₃OH will become the final product to prevent the formation of CH₄.The*CO on CuO/CoAl-LDHs will stably combine with the catalyst and then react,so as to enhance the selectivity of CH₄products on CuO/CoAl-LDHs.