Investigating Oxygen Vacancies On Metal Oxides And Its Photocatalytic Reduction Of CO₂

The rising CO₂ emission from the burning of fossil fuels is deem to the culprit causing green-house effect and global warming.Solar energy-driven high-efficiency conversion of carbon dioxide to fuels and other high-value hydrocarbons provides a new―green‖approach for the prevention of environmental pollution and carbon circling utilization.There are three pivotal processes in photocatalytic CO₂conversion:（1）Adsorption of carbon dioxide on the catalyst surface and water oxidation,（2）solar-light absorption,（3）photo-generated charging carriers separation and,migration.Based on the photocatalytic mechanism,an excellent photocatalyst with merits of harvesting sunlight,efficient photo-generated charge carrier separation and abundant active sites for CO₂ adsorption,is expected.A large number of reported literature studies on band regulation of semiconductor catalysts have shown that surface defects on metal oxide play an important role in promoting the separation of charge carriers and enhancing light absorption,thereby improving the photocatalytic efficiency of CO₂ reduction.Herein,oxygen vacancy defects are introduced into MgO and CeO modulate the electronic structure and surface basic sites,aiming to achieve the the efficient photocatalytic CO₂conversion to high-valued chemicals and hydrocarbons.The involved research contents are as follows:（1）Oxygen vacancies（Vos）enriched MgO nanosheets are prepared by a solvothermal method in mixed solvent of benzyl alcohol and methanol,combing a post-calcination.XPS and positron annihilation spectrum results show that the calcination temperature has a great influence on the content and type of oxygen vacancies on the surface of MgO.When the calcination temperature is 500℃,MgO nanocrystals with rich oxygen vacancy clusters on the surface can be obtained.Vos-rich MgO shows dramatically enhanced absorption across entire UV-Vis region in comparison of commercial MgO.The unique and strong PL emission interprets the presence of different types of oxygen vacancies on MgO.Photocurrent response results indicate electrons and holes can be effective separation on Vos-rich MgO,due to surface oxygen vacancies forming a medium energy level to transfer electrons to conduction.In addition,the CO₂-TPD results show that MgO contains a large number of strong basic sites,and the chemisorption amount of CO₂ reaches 707μmol·g^-1,which provides a premise for the photocatalysis of CO₂.As a result,Vos-rich MgO shows high photocatalytic activity for CO₂ reduction.The yield rates of CO and CH₄ attains to 17.66μ mol.g^-1.h^-1 and 1.9μ mol.g^-1.h^-1,respectively,which were significantly higher than P25 TiO₂ activity.（2）The precursor was prepared by solvothermal method using magnesium acetate and CQDs as raw materials,and then calcined at high temperature to successfully prepare C-doped MgO nanosheets（C-MgO）with enhanced crystallinity and uniform size.The effect of increasing the specific surface area of the catalyst after C doping on the CO₂adsorption and the effect of the reduction of bulk oxygen vacancies on the photocatalytic CO₂ reduction activity after C doping were investigated.The results show that the study found that the C-doped MgO is larger than the surface,which is 1.93 times that before the doping.The specific surface area increased,and the contact area between the catalyst and CO₂ increased.In addition,the oxygen vacancies in the C-MgO bulk phase are reduced,which suppresses the recombination of carriers and promotes the separation of charge carriers.Eventually C-MgO improves the CO₂ reduction activity.The CO and CH₄ reduction rates of the CO₂ reduction products increased by 2.2 times and 4.3 times,respectively.（3）CeO_2-x is prepared by solvothermal method using cerium nitrate as raw material.There are two chemical states of Ce³⁺and Ce⁴⁺,which convert each other to generate oxygen vacancies.The role of oxygen vacancy in CeO_2-x photocatalytic CO₂ reduction was investigated CeO_2-x,which is rich in oxygen vacancies on the surface,improves light absorption and inhibits carrier recombination.The results show that the rates of CO and CH₄in the photocatalytic CO₂ reduction products of CeO_2-x-2 with rich oxygen vacancies on the surface were 55.80μ mol.g^-1.h^-1 and 20.66μ mol.g^-1.h^-1,respectively,and the photocatalytic CO₂ reduction activity is proportional to the oxygen vacancy concentration.