Controllable Preparation Of Two-Dimensional Oxygen-containing Metal Compounds And Their Application In Photo/Electrocatalytic CO₂ Reduction

The massive use of fossil fuels not only triggers energy crisis,but also releases excess CO2,making serious greenhouse effect.To address these challenges,researchers have developed a green energy conversion method that converts CO2 molecules into clean energy by photocatalysis or electrocatalysis.However,due to the structure stability of CO2 molecule and the uncontrollable reduction process,it is difficult to reach the level of practical application at the yield and selectivity of CO2 reduction reaction.In recent years,2D materials have attracted extensive attention due to their superior structural advantages,namely,larger specific surface area,high exposure ratio of surface atoms,and shorter carrier transport distance compared to bulk materials.In this dissertation,based on the two-dimensional oxygen-containing metal compounds,the geometric/electronic structure of catalytic materials was regulated to improve their photo/electrocatalytic CO2 reduction performance by various strategies.At the same time,the reaction mechanism was explored by combining in-situ characterization methods and density functional theory calculations,helping to provide new insights for the design of efficient catalysts.The main research contents of this dissertation are as follows:1.Industrial-Current-Density CO2-to-Formate Conversion with Low Overpotentials Enabled by Disorder-Engineered Metal Sites:To achieve industrialcurrent-density CO2-to-formate electroreduction,2D metallic nanosheets with different degrees of lattice distortion were designed to create abundant disorder-engineered metal sites with highly localized electrons.As a typical example,Bi nanosheets with rich lattice distortion were successfully fabricated by a fast in-situ electrochemical topotactic transformation strategy.In-situ XRD patterns and in-situ Raman spectra revealed the phase transformation from Bi2O2CO3 to Bi,while HRTEM image and Raman spectra clearly illustrated the higher degree of lattice distortion over the richly lattice-distorted Bi nanosheets.More importantly,HAADF-STEM image directly observed the lattice-distorted Bi sites.Meanwhile,the CO2 adsorption isotherms and CO2-TPD measurements indicated that lattice distortion could improve the CO2 adsorption capability,which could facilitate the subsequent CO2 activation and reduction.In-situ FTIR spectra disclosed the CO2·-*radical was the key intermediate during CO2 reduction process,while theoretical calculations suggested the lattice distortion caused electron localization on the disorder-engineered Bi sites,which decreased the formation energy barrier of the rate-limiting CO2·-intermediates from 0.49 eV to 0.39 eV.On the strength of these advantages,the richly lattice-distorted Bi nanosheets realized the high formate selectivity of 91%at the current density of 800 mA cm-2 and exhibited a very low overpotential of ca.570 mV at the current density of 200 mA cm-2 with formate faradaic efficiency of nearly 100%,outperforming most other reported metal-based electrocatalysts.2.Defective ZnGa2O4 nanosheets drive high-efficiency visible-light photocatalytic CO2 reduction to CO:To enhance the visible-light photocatalytic CO2 property of wide-bandgap two-dimensional semiconductor nanosheets,oxygen vacancy was designed to optimize the band structure for improve CO2 photoreduction activity.Taking Ov-rich ZnGa2O4 nanosheets as an example,the catalyst was synthesized by calcining the pristine ZnGa2O4 nanosheets in 5%H2/Ar atmosphere,and the successful introduction of oxygen vacancies was demonstrated by electron paramagnetic resonance spectroscopy and X-ray photoelectron spectroscopy.Meantime,the UV-vis absorption spectrum showed that the existence of oxygen vacancies greatly improved the light absorption capacity of the pristine ZnGa2O4 nanosheets,and the room-temperature photoluminescence spectrum indicated that oxygen vacancies could enhance the carrier separation efficiency,which was beneficial to the subsequent activation and transformation of CO2 molecules.The Gibbs free energy calculation suggested that the rate-determining step energy barrier for CO2 reduction to CO on defect ZnGa2O4 nanosheets was reduced from 1.99 eV to 1.63 eV compared to pristine ZnGa2O4 nanosheets.In addition,the calculated differential charge densities on the adsorbed COOH*intermediates showed more obvious charge interaction on the defective ZnGa2O4 nanosheet slab,which further promoted the CO2 reduction process.Finally,the Ov-rich ZnGa2O4 nanosheets greatly improved the performance of CO2 reduction to CO under visible light,and its CO evolution rate reached 17.6 μmol g-1 h-1,which was much higher than that of the pristine ZnGa2O4 nanosheets.3.Selective CO2 Photoreduction to CH4 via Pdδ+-assisted Hydrodeoxygenation over CeO2 Nanosheets:To tailor the activity and selectivity of CO2 photoreduction to CH4,positively charged noble metal doped two-dimensional metal oxides nanosheets were designed to provide sufficient protons by facilitating water activation reaction.Taking the Pd-CeO2 nanosheets as example,HAADF image and Pd K edge of EXAFS spectra demonstrated positively charged Pd has been doped into the CeO2 nanosheets.In-situ EPR measurements presented the increasing peak intensities of Ov and Ce3+during CO2 photoreduction,while the quasi in-situ XPS spectra revealed that Pd sites could obtain the photoexcited electrons and be reduced to lower valence of +δ(2<δ<4)in the photocatalytic process,which implied the active sites could be Ce3+-Ov and Pdδ+.Moreover,in-situ FTIR spectra of D2O photoactivation affirmed the existence of Pd-OD bond,strongly confirming that Pdδ+ sites could be involved in water activation reaction to provide more active H*species,thus promoting the protonation of the intermediates to realize selective CO2 conversion to CH4.Furthermore,Gibbs free energy calculations suggested that Pd incorporation could be beneficial for the water oxidation to form abundant active H*species,which could regulate the energy barrier of the key intermediates in CO2 reduction reaction.In detail,the energy barrier of CO*to CO could be largely increased from-0.20 eV to 1.08 eV,which converted the step of CO desorption from endothermic to exothermic,causing the lower CO selectivity.Meantime,the energy barrier of CH3O*to CH3OH enhanced from-0.46 eV to 0.26 eV,altering CH3OH formation and desorption step from spontaneous to non-spontaneous,which resulted in CH4 formation be the most predominant process.In addition,the calculated differential charge densities on the adsorbed CH3O*intermediates elucidated that Pd doping could conduce to the break of C-O bond in Ce-O-CH3 intermediates by lengthening C-O bond from 0.139 nm to 0.140 nm,which would be helpful for the subsequent hydrogenation to form CH4.Consequently,the Pd-CeO2 nanosheets exhibited the nearly 100%selectivity for CO2 photoreduction to CH4,with the promoted CH4 evolution rate of 41.6 μmol g-1 h-1.