Synthesis Of Defective Hydrotalcite Nanosheets And Their Application In Photocatalysis CO₂ Reduction

The rise in the atmospheric carbon dioxide(CO2)levels and the depletion of fossil fuel reserves raise serious concerns regarding global climate change and the future of energy supplies.It is an important challenge to convert CO2 into high value-added hydrocarbons to realize the artificial carbon cycle.Compared with electro-reduction of CO2,photocatalytic CO2 reduction is believed to be an appealing approach because of its promising features for practical applications,including low-cost,mild-reaction conditions,and low energy consumption due to a direct conversion of solar energy into chemical energy.However,due to the extremely inferior separation of photogenerated electron holes,the efficiency of CO2 photoreduction is relatively low,which is far below the demands of practical application.In particular,converting CO2 and water into CH4 while relying on the low-energy visible light remains challenging mainly due to the sluggish kinetics caused by the complex eight-electron reduction processes.Moreover,the competitive hydrogen evolution reaction(HER)by photo-generated electrons always suppresses the hydrocarbon production,leading to poor selectivity.Thus,an appropriate catalyst is necessary to selectively promote the generation of CH4 while suppressing HER under wider light irradiation.Layered double hydroxide(LDH)represents an important class of 2D layered materials,which has been widely used as photocatalysts.Defects in catalysts can effectively tune the photoactivity/selectivity in CO2 conversion by modulating the bandgap and electronic structure.Therefore,based on 2D layered double hydroxide(2D),some appropriate structural models were constructed to gain an in-depth explanation about the structure-activity relationship between microstructure and photocatalytic CO2 reduction performance through various spectral characterization and theoretical calculation.The main results of this dissertation are as follows:1.Ni-based ternary layered double hydroxides nanosheets boosting photoreduction of CO2 to CH4 under irradiation above 500 nm:NiCoFe-LDH nanosheets were synthesized via a one-pot coprecipitation method by incorporating the Ni species into the CoFe-LDH layers.This material showed a favorable performance of CO2 photoreduction to CH4.NiCoFe-LDH nanosheets under the long-wavelength(λ>500 nm),the selectivity of CH4 in NiCoFe-LDH could be increased to as high as 78.9%,and the H2 evolution reaction could be inhibited to as low as 1.7%.X-ray absorption fine structure(XAFS)demonstrated an increase in the concentration of defects(oxygen and metal vacancies)in NiCoFe-LDH due to the addition of the Ni species,which not only improved the absorption of visible light but also increased the separation efficiency of the photoinduced electron-hole pairs as evidenced in the calculations of the density functional theory(DFT).This explained the high selectivity of photocatalytic CO2 reduction to CH4.This work offers a valid approach to the generation of earth-abundant photocatalysts for CO2 photoreduction under visible light.2.Ultrafine NiFe nanosheets achieving highly selective photocatalysis CO2 reduction under visible light:Based on the adjustable size/thickness of LDHs,NiFe nanosheets with the size of 3nm and the thickness of 1 nm were prepared by the ultrasonic method for photoreduction of CO2.The H2 selectivity could be completely suppressed under irradiation above 400 nm,while the selectivity of CH4 was up to 81.93%.X-ray absorption fine structure(XAFS)spectroscopy tests proved the formation of metal and oxygen defects in NiFe nanosheets DFT+U calculations unveiled that the driving forces of NiFe nanosheets can overcome the Gibbs free energy barrier of CO2 reduction to CH4,rather than that of H2 evolution.The above experimental and theoretical results manifested that introducing defect could tune the band edge placement of conduction band minimum while the selectivity of the products can be adjusted in catalytic reaction.This work provides a facile approach to the rational design photocatalysts for inhibiting the production of H2 as a by-product in CO2 photoreduction under visible light.