Controllable Preparation Of Two-dimensional Metal Oxides And Their Applications In Photocatalytic Conversion Of Carbon-based Small Molecules

Recently,a series of environmental problems caused by global warming have received great attention of the international community.The increasing greenhouse gases in the atmosphere have seriously hindered the sustainable development of human beings.In this regard,CO2 and CH4,as the most important greenhouse gases,have a strong greenhouse effect.Therefore,the photocatalytic conversion of such greenhouse gas molecules into higher-value chemicals could not only alleviate environmental pressure,but also bring additional economic value.However,limited by low photogenerated electron-hole separation efficiency and slow carrier transport speed under light irradition,most catalysts display low photocatalytic conversion efficiency of small molecules.How to design a photocatalyst with high-activity and high-selectivity is still the focus of current research.Two-dimensional materials show unique advantages in the field of catalysis,ascribed to large surface area,abundant active sites and short carrier transport distance.Based on two-dimensional metal oxides,this dissertation explores the relationship between microstructure and macro-catalytic performance by adjusting the atomic and electronic structure,and provides new insights to design more effective photocatalysts for catalytic conversion of CO2 or CH4.The main research contents of this dissertation are as follows:1.Selective CO2 Photoreduction into C2 Product Enabled by Charge-Polarized Metal Pair Sites:To promote C-C coupling and selective CO2 photoreduction into C2 products,the photocatalysts with charge-polarized metal pair sites are designed.Taking Co3O4 as an example,the partially reduced Co3O4 nanosheets with charge-polarized cobalt pair sites were successfully fabricated by calcining in a reduced atmosphere.X-ray photoelectron spectroscopy,synchrotron-radiation X-ray absorption near edge structure and electron paramagnetic resonance spectra demonstrated the formation of oxygen vacancies and increased amount of Co2+ ions relative to the pristine Co3O4 nanosheets,while the newly emerged Co2+ sites and the surrounding Co3+ sites formed the charge-polarized Co pair sites.Quasi in situ XPS spectra unveiled that these Co2+ions could donate electrons to the adsorbed CO2 molecules through being reoxidized into Co3+ ions,facilitating the CO2 activation process.In addition,theoretical calculations revealed that creating oxygen vacancies resulted in asymmetric charge distribution on the surface cobalt sites,while the formed charge-polarized cobalt pair sites accelerated the dimerization of COOH*intermediates,verified by the shortened C-C bond distance and the largely decreased energy barrier.In addition,the increased electron density of cobalt sites could weaken and cleave the C-O bonds in critical intermediate,finally benefiting the rate-limiting CH3COOH desorption process.As an outcome,the partially reduced Co3O4 nanosheets achieve 92.5%selectivity of CH3COOH in simulated air,while the CO2-to-CH3COOH conversion ratio is 2.75%.2.Improved Visible-light-driven CO2 Photoreduction Realized by Doping Cu in SrTiO3 Nanosheets:To expand the light absorption range of wide-bandgap semiconductors and improve the performance of CO2 photoreduction under visible-light,the perovskite two-dimensional photocatalysts with doped metal element were constructed.Taking SrTiO3 as an example,Cu doped SrTiO3 nanosheets were successfully fabricated via hydrothermal method.It was inferred that the doped Cu atoms would replace the Sr sites in the SrTiO3 lattice through X-ray photoelectron spectroscopy and X-ray near-edge absorption structure spectroscopy.Meanwhile,the surface charge density theoretical simulation showed that electrons were concentrated on doped Cu atoms.Thus,Cu atoms could be used as reactive sites to promote the adsorption of CO2 molecules on the surface of catalysts,thereby improving the CO2 reduction performance.Subsequently,CO2 temperature-programmed-desorption measurements proved that the ability of the catalyst for adsorbing CO2 on the surface had been improved after Cu doping.In addition,density functional theory calculations displayed that compared with the original SrTiO3 nanosheets,the energy barrier of the rate-limiting process for forming*CO intermediates was reduced from 2.245 to 0.505 eV over the Cu doped SrTiO3 nanosheets.Finally,the formation rate of CO could reach 10.4 ?mol g-1 h-1 under visible light over the SrTiO3 nanosheets doped with 1%Cu in CO2 reduction,which was 10 times higher than that over the initial SrTiO3 nanosheets.3.Efficient Photooxidation of Methane to Liquid Oxygenates over ZnO Nanosheets at Atmospheric Pressure and Near Room Temperature:To generate enough active O-species for activating the inert C-H bond in methane,two-dimensional metallic oxide semiconductors with abundant coordinated unsaturated surface oxygen atoms were designed.Taking the synthetic ZnO nanosheets as an example,in situ electron paramagnetic resonance spectra directly verified their lattice oxygen atoms could capture the photoexcited holes and generate abundant active O-species,which could efficiently abstract H from CH4 to generate·CH3-In addition,the formed CH3 initially combined with OH or-OOH radicals to produce CH3OH or CH3OOH,which could be further converted to HOCH2OOH and HCOOH through dehydrogenation reaction,confirmed by radical capture experiments and in situ Fourier-transform infrared spectroscopy.Gibbs free energy calculations corroborated that the rate limiting step was the first C-H bond activation process,where the relatively low energy barrier of 0.375 eV implied that CH4 molecules could be easily activated and transformed over the ZnO nanosheets.In relative to the endoergic over-oxidation to CO,the partial oxidation of*CHO to HCOOH was exoergic,suggesting that it is more inclined to produce liquid products rather than over-oxidation to gas products.As a result,the formation rate of liquid oxygenates over the ZnO nanosheets could reach 2.21 mmol g-1 h-1 with the selectivity of 90.7%at atmospheric pressure and approximately 50?,outperforming previously reported photocatalysts under similar conditions.