Meteoroids entering the Earth's atmosphere can create meteor trail irregularity seriously disturbing the background ionosphere. Although numerous observations of meteor trail irregularities were performed with VHF...Meteoroids entering the Earth's atmosphere can create meteor trail irregularity seriously disturbing the background ionosphere. Although numerous observations of meteor trail irregularities were performed with VHF/UHF coherent scatter radars in the past, no simultaneous radar and optical instruments were employed to investigate the characteristics of meteor trail irregularity and its corresponding meteoroid. By installing multiple video cameras near the Sanya VHF radar site, an observational campaign was conducted during the period from November 2016 to February 2017. A total of 242 optical meteors with simultaneous non-specular echoes backscattered from the plasma irregularities generated in the corresponding meteor trails were identified. A good agreement between the angular positions of non-specular echoes derived from the Sanya radar interferometer and those of optical meteors was found,validating that the radar system phase offsets have been properly calibrated. The results also verify the interferometry capability of Sanya radar for meteor trail irregularity observation. The non-specular echoes with simultaneous optical meteors were detected at magnetic aspect angles greater than ~78°. Based on the meteor visual magnitude estimated from the optical data, it was found that the radar nonspecular echoes corresponding to brighter meteors survived for longer duration. This could provide observational evidence for the significance of meteoroid mass on the duration of meteor trail irregularity. On the other hand, the simultaneous radar and video common-volume observations showed that there were some cases with optical meteors but without radar non-specular echoes. One possibility could be that some of the optical meteors appeared at extremely low altitudes where meteor trail irregularities rarely occur.展开更多
Photoelectrochemical (PEC) small-molecule oxidation can selectively transform substrates into high-value-added fine chemicals and increase the rate of cathode hydrogen evolution. Nevertheless, achieving high-selectivi...Photoelectrochemical (PEC) small-molecule oxidation can selectively transform substrates into high-value-added fine chemicals and increase the rate of cathode hydrogen evolution. Nevertheless, achieving high-selectivity PEC oxidation of small molecules to produce specific products is a very challenging task. In general, selectivity can be improved by changing the surface catalyticsites of the photoanode and modulating the interfacial environments of the reactions. Herein, recent advances in approaches to improving selective PEC oxidation of small molecules are introduced. We first briefly discuss the basic concept and fundamentals of small-molecule PEC oxidation. The reported approaches to improving the performance of selective PEC oxidation of small molecules are highlighted from two aspects: (1) changing the surface properties of photoanodes by selecting suitable materials or modifying the photoanodes and (2) mediating the oxidation reactions using redox mediators. The PEC oxidation mechanism of these studies is emphasized. We also discuss the challenges in this research direction and offer a perspective on the further development of selective PEC-based small-molecule transformation.展开更多
Alkenes difunctionalization reaction has been recognized as a highly important organic transformation.However,its application is significantly hindered by the reliance on homogeneous transition metal-assisted radical ...Alkenes difunctionalization reaction has been recognized as a highly important organic transformation.However,its application is significantly hindered by the reliance on homogeneous transition metal-assisted radical addition,as well as the need for additional oxidants.In this study,we report that azido-hydroxylation of alkenes can be efficiently achieved by a heterogeneous photoelectrochemical(PEC)oxidation on the BiVO_(4)photoanode,by using KN_(3)as the azido source in the presence of HCO_(3)^(-).A wide range of alkene substrates featuring diverse substituent groups can be effectively oxidized to the corresponding product with an impressive product selectivity.Electrochemical and electron paramagnetic resonance(EPR)experiments confirmed that the high performance is assigned to the interaction of hetero-radicals produced from the oxidation of N_(3)^(-)and HCO_(3)^(-).Transient spectroscopy dynamics experiments and density functional theory(DFT)analysis demonstrate that the addition order of two radical groups is attributed to their distinctive characteristics.The high reactivity of transient·N_(3)is preferentially added to the C=C double bond of alkene to form the carbon radical intermediate both kinetically and thermodynamically.The extended lifespan of persistent·CO_(3)^(-)can greatly increase the probability of paring the carbon radical intermediate,thus improving the selectivity of the azido-hydroxylation product.The present work provides a promising strategy for the difunctionalization of alkenes with the tandem addition of two radicals in the absence of homogeneous metal catalysis.展开更多
基金carried out as a part of the project funded by the National Natural Science Foundation of China (41422404 and 41727803)
文摘Meteoroids entering the Earth's atmosphere can create meteor trail irregularity seriously disturbing the background ionosphere. Although numerous observations of meteor trail irregularities were performed with VHF/UHF coherent scatter radars in the past, no simultaneous radar and optical instruments were employed to investigate the characteristics of meteor trail irregularity and its corresponding meteoroid. By installing multiple video cameras near the Sanya VHF radar site, an observational campaign was conducted during the period from November 2016 to February 2017. A total of 242 optical meteors with simultaneous non-specular echoes backscattered from the plasma irregularities generated in the corresponding meteor trails were identified. A good agreement between the angular positions of non-specular echoes derived from the Sanya radar interferometer and those of optical meteors was found,validating that the radar system phase offsets have been properly calibrated. The results also verify the interferometry capability of Sanya radar for meteor trail irregularity observation. The non-specular echoes with simultaneous optical meteors were detected at magnetic aspect angles greater than ~78°. Based on the meteor visual magnitude estimated from the optical data, it was found that the radar nonspecular echoes corresponding to brighter meteors survived for longer duration. This could provide observational evidence for the significance of meteoroid mass on the duration of meteor trail irregularity. On the other hand, the simultaneous radar and video common-volume observations showed that there were some cases with optical meteors but without radar non-specular echoes. One possibility could be that some of the optical meteors appeared at extremely low altitudes where meteor trail irregularities rarely occur.
基金the National Natural Science Foundation of China (No. 22136005)the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB36000000).
文摘Photoelectrochemical (PEC) small-molecule oxidation can selectively transform substrates into high-value-added fine chemicals and increase the rate of cathode hydrogen evolution. Nevertheless, achieving high-selectivity PEC oxidation of small molecules to produce specific products is a very challenging task. In general, selectivity can be improved by changing the surface catalyticsites of the photoanode and modulating the interfacial environments of the reactions. Herein, recent advances in approaches to improving selective PEC oxidation of small molecules are introduced. We first briefly discuss the basic concept and fundamentals of small-molecule PEC oxidation. The reported approaches to improving the performance of selective PEC oxidation of small molecules are highlighted from two aspects: (1) changing the surface properties of photoanodes by selecting suitable materials or modifying the photoanodes and (2) mediating the oxidation reactions using redox mediators. The PEC oxidation mechanism of these studies is emphasized. We also discuss the challenges in this research direction and offer a perspective on the further development of selective PEC-based small-molecule transformation.
基金supported by the National Natural Science Foundation of China(22136005,22188102,22321004)the Strategic Priority Research Program of the Chinese Academy of Sciences(XDB36000000)。
文摘Alkenes difunctionalization reaction has been recognized as a highly important organic transformation.However,its application is significantly hindered by the reliance on homogeneous transition metal-assisted radical addition,as well as the need for additional oxidants.In this study,we report that azido-hydroxylation of alkenes can be efficiently achieved by a heterogeneous photoelectrochemical(PEC)oxidation on the BiVO_(4)photoanode,by using KN_(3)as the azido source in the presence of HCO_(3)^(-).A wide range of alkene substrates featuring diverse substituent groups can be effectively oxidized to the corresponding product with an impressive product selectivity.Electrochemical and electron paramagnetic resonance(EPR)experiments confirmed that the high performance is assigned to the interaction of hetero-radicals produced from the oxidation of N_(3)^(-)and HCO_(3)^(-).Transient spectroscopy dynamics experiments and density functional theory(DFT)analysis demonstrate that the addition order of two radical groups is attributed to their distinctive characteristics.The high reactivity of transient·N_(3)is preferentially added to the C=C double bond of alkene to form the carbon radical intermediate both kinetically and thermodynamically.The extended lifespan of persistent·CO_(3)^(-)can greatly increase the probability of paring the carbon radical intermediate,thus improving the selectivity of the azido-hydroxylation product.The present work provides a promising strategy for the difunctionalization of alkenes with the tandem addition of two radicals in the absence of homogeneous metal catalysis.
