Ultrafast Dynamics Of Photocatalytic Reduction Of CO₂ By Perovskite/Metal-Organic Frameworks

The overuse of fossil fuels has caused a surge in greenhouse gases,posing a threat to life systems.Inspired by plant photosynthesis,the photocatalytic conversion of carbon dioxide into value-added chemical fuel is one of the effective strategies to alleviate global warming and energy crisis.Halide perovskite has attracted people’s attention because of its excellent optoelectronic properties,such as good visible light capturing ability and large carrier mobility.However,the recombination speed of photogenerated electron-hole pairs on single perovskite is fastand the material is oftentimes chemically unstable,which seriously affect the photocatalytic efficacy.On the other hand,Metal Organic Frameworks（MOFs）have the advantages of porous crystal structure,large specific surface area and high metal center activity,which facilitate CO₂ reduction.Therefore,the combination of perovskite and MOFs provides promising photocatalysts for CO₂ reduction.However,the charge transfer mechanism of this system for CO₂ reduction is still unclear,and the ultrafast conversion process of CO₂ reduction intermediates remains to be explored.To solve these problems,we have carried out the following work:1.The“cascade electron transfer”catalyst was designed to improve the photocatalytic performance.CsPbBr₃ QDs and 2D CuTCPP MOF were combined in situ to form 0D/2D composite photocatalyst,which showed high catalytic activity(the yields of CO and CH₄ were 11.8 and 2.95μmol.g^-1.h^-1,respectively).Femtosecond transient spectra reveal that the“cascade electron transfer”stimulates the interfacial and internal electron transfer process of the catalyst.It takes only 1.4 ps for electrons to transfer from the quantum dots to the MOFs,and then completes the electron transfer from ligands to nodes in the MOFs within 21 ps.This ultra-fast electron transfer is the key to improve the catalytic performance.This work provides a useful insight for improving the efficiency of photocatalysis from perspective of kinetics.2.The above catalysts were further upgraded to prepare lead-free perovskite-based catalysts to significantly improve and regulate the product selectivity of photocatalytic reduction of CO₂.We chose lead-free perovskite-based Cs₃Bi₂Br₉/MOF 525 Co composite for photocatalytic CO₂ reduction under gas-solid phase conditions,which showed high CO selectivity（99.5%）.Femtosecond transient absorption spectra reveal that the kinetic time of charge transfer between MOF 525 Co and Cs₃Bi₂Br₉is 136 ps.The conversion process and reaction path of the intermediate products reduced from CO₂ to CO were confirmed by in situ infrared absorption spectroscopy.COOH^*is the main intermediate in the conversion of CO₂.In addition,CH₄ is a thermodynamically favorable product,but the photoreduction of CO₂ to CH₄ has the problem of low activity and selectivity.In the other work,to achieve high selectivity for the conversion of CO₂to CH₄,Cs₃Bi₂I₉ quantum dots were selected as the light harvesting unit and compounded with In₂S₃ derived from In-MIL-68 to improve the selectivity of CH₄.The ultrafast spectral results show the kinetic process of Cs₃Bi₂I₉and In₂S₃/Cs₃Bi₂I₉ during the photocatalytic reaction and display rapid charge separation is beneficial to the improvement of catalytic performance.These two work provide guideline on how to improve the selectivity of photocatalytic CO₂ reduction products.3.Further design and synthesis of space radiation-resistant photocatalysts for application in extraterrestrial artificial photosynthesis.The effect of space irradiation environment on UiO-66-NH₂-2 MOF and its photocatalytic performance was investigated.Femtosecond transient absorption spectra reveal that the dynamic process of UiO-66-NH₂-2 MOF before and after irradiation is similar,and the charge transfer time from Cs₂Ag Bi Br₆ quantum dots to UiO-66-NH₂-2 is 32 ps.In-situ infrared spectroscopy showed that COOH^*and CH₃O^*intermediates appeared in the composite catalyst in the photocatalytic CO₂,which confirmed the source of CO and CH₄.This work reveals that space radiation-resistant MOFs are promising for use in space artificial photosynthesis.