Active Sites Regulation For Highly Selective Photocatalytic CO₂ Reduction And Its Synchrotron Radiation Spectroscopies Investigation

The excessive utilization of traditional fossil fuels has not only caused an energy crisis but also led to a significant release of carbon dioxide(CO2),resulting in series of environmental problems such as greenhouse effect.In response to these challenges,various energy-saving and emission-reducing technologies have been developed for efficient utilization of fossil fuels,and development of sustainable and clean energy.One promising approach is the photocatalytic conversion of CO2 into high-value-added chemical products,which utilizes clean and renewable solar energy under mild conditions.However,the CO2 photoreduction faces challenges due to the lack of reaction sites on the photocatalyst surface and the difficulty in activating stable CO2 molecules,resulting in low activity and selectivity,thereby constraining its widespread application.Therefore,it is vital to develop highly efficient and selective CO2 conversion photocatalyst for its potential practical application.To address the issue of lacking effective active sites,this study precisely regulates the active sites of two classic photocatalysts(TiO2 and CdS)through anion/cation defects regulation and/or the introduction of atomic dispersed Cu sites with different local structure.By employing advanced tools such as synchrotron radiation X-ray absorption spectroscopy(XAS),Xray photoelectron spectroscopy(XPS),in situ diffuse reflectance infrared Fourier transform spectroscopy(in situ DRIFTS),synchrotron radiation photoionization timeof-flight mass spectrometry and density function theory(DFT)calculations,the active site structures of different photocatalyst systems are accurately identified,thus established the corresponding structure-performance relationships and explored mechanism of photocatalytic CO2 reduction,providing guidance and help for the design and development of efficient and selective photocatalysts.The main research and results of this dissertation are as follows:Firstly,we constructed cationic vacancies to introduce unpaired electrons on TiO2 to investigate the impact in the activity and selectivity of photocatalytic CO2 reduction.The introduced unpaired electrons accelerated carrier separation and enhanced CHO*adsorption,leading to an efficient and selective CO2 photoreduction to CH4.The existence of Ti vacancies and unpaired electrons was confirmed by XAS,high-angle annular dark field scanning transmission electron microscope(HAADF-STEM),and electron paramagnetic resonance(EPR).Femtosecond transient absorption spectroscopy showed that the introduction of unpaired electrons created mid-gap states in the photocatalyst bandgap,accelerating charge carrier separation and enhancing the lifetime of active electrons.The cationic vacancy-regulated TiO2 exhibited a higher electron consumption yield than the normal TiO2,with a rate 15.7 times greater.Furthermore,in situ DRIFTS and DFT calculations reveals that cationic vacancyregulated TiO2 introduced unpaired electrons,which could promote the adsorption of the crucial reaction intermediate CHO*on active sites,resulting in a transition from a thermodynamically limited step of CO*hydrogenation to CHO*to a thermodynamically spontaneous step,finally leading to a significant increase in the selectivity of CH4 to 97%,much greater than the 34%for the normal TiO2.Subsequently,,we introduced atomic dispersed metal sites and anion vacancies(Cu+-Vs)composite active sites on CdS surface to investigate the effect on the CO2 photoreduction.By one-step cation exchange,we introduced atomic dispersed Cu sites and coexisting S vacancies onto CdS surface simultaneously,reducing the energy barrier of CO2 dissociation and adsorption significantly,and promoting the adsorption and dissociation of CO2 effectively,thus achieving high efficient and selective CO2 photoreduction.Through synchrotron radiation XAS,XPS and HAADF-STEM,the coordination environment and electronic structure of atomic dispersed Cu+sites were accurately identified,and EPR confirmed the introduction of Vs by the one-step cation exchange method,differential charge density calculation indicates the electronic modulation effect of Vs on the neighboring Cu+and Cd2+ sites.In situ DRIFTS was unveiled that Cu+ sites and VS could synergistically promote the adsorption and dissociation of CO2 thus accelerate its conversion to CO product.Combined with theoretical calculations,it was found that the construction of Cu+-VS composite active sites accelerated the separation of photogenerated charges and lowered the energy barrier for CO2 dissociation and adsorption,thereby promoting the photocatalytic CO2 reduction reaction,and finally exhibited 92%CO product selectivity.Finally,on the basis of the above work,we investigated the influence of the atomic dispersed Cu2+ composite sites mediated by anion/cation vacancies on CdS about the photocatalytic CO2 reduction performance.The results showed that the introduction of atomic dispersed Cu2+sites on the CdS surface mediated by Cd vacancies(VCd)and S vacancies(Vs),respectively,can significantly enhance the activation and dissociation of the proton donor H2O for overall photocatalytic CO2 conversion on the CdS surface.Compared with Vs mediated Cu2+composite active sites,Vcd mediated Cu2+composite active sites have a more significant effect on improving the photocatalytic CO2 reduction performance of CdS.The atomic coordination environment and electronic structure of atomic dispersed Cu2+sites mediated by VCd and Vs were determined by synchrotron radiation XAS,soft X-ray absorption spectroscopy and HAADF-STEM.In situ DRIFTS unveiled the promotion of H2O dissociation by atomic dispersed Cu2+sites mediated by VCd and Vs,respectively.According to the above experimental results and combined with the Gibbs free energy calculation,the mechanism of the influence of VCd and VS mediated atomic dispersed Cu2+composite sites on the photocatalytic CO2 reduction performance of CdS was proposed.The defect-mediated atomic dispersed Cu2+ sites can significantly promote the dissociation of H2O and accelerate the generation of protons,and the Cu2+composite active sites mediated by VCd are more conducive to the dissociation of H2O compared to the Cu2+composite active sites mediated by Vs.The generated protons transferred rapidly to the surrounding sites,thereby promoting the reduction of CO2 to CO by the coupling of two protons and two electron transfers.This work revealed the regulation of different defect-mediated atomic dispersed Cu2+ composite active sites on the dissociation of proton donor H2O in the photocatalytic CO2 reduction reaction,thereby improving the performance of overall photocatalytic CO2 reduction,which provides new insight for the design and development of highly efficient and selective CO2 reduction photocatalysts.