共查询到20条相似文献,搜索用时 31 毫秒
1.
We investigated the singlet oxygen quenching ability of several derivatives of trans-resveratrol which have been reported to have significant antioxidant ability, including photoprotective activity. We measured the total rate constants of singlet oxygen removal ( kT) by the methylated resveratrol derivative 1,3-dimethoxy-5-[(E)-2-(4-methoxyphenyl)ethenyl]benzene, and the partially methylated resveratrol derivatives 4-((E)-2-(3,5-dimethoxyphenyl)ethenyl)phenol (pterostilbene), 5-[(E)-2-(4-methoxyphenyl)ethenyl]benzene-1,3-diol and (2 R,3 R)-3,5,7-trihydroxy-2-(3,4,5-trihydroxyphenyl)-2,3-dihydrochromen-4-one (dihydromyricetin). A protic solvent system results in higher kT values, except for the completely methylated derivative. We also investigated the ability of trans-resveratrol to directly act as a photosensitizer (rather than via secondary photoproducts resulting from other primary photochemical reactions) for the production of singlet oxygen but found that neither resveratrol nor any of its derivatives are able to do so. We then studied the chemical reactions of the methylated derivative with singlet oxygen. The main pathway consists of a [4 + 2] cycloaddition reaction involving the trans-double bond and the para-substituted benzene ring similar to what has been observed for trans-resveratrol. Unlike trans-resveratrol, the primary singlet oxygen product undergoes a second [4 + 2] cycloaddition with singlet oxygen leading to the formation of diendoperoxides. A second reactivity pathway for both trans-resveratrol and the methylated derivative leads to the formation of aldehydes via cleavage of a transient dioxetane. 相似文献
2.
The activation of oxygen in new synthetic procedures dramatically expands the scope of osmium-catalyzed dihydroxylations [Eq. (1)]. Even air can now be used as the cooxidant in the asymmetric version of this reaction with little loss of selectivity. 相似文献
3.
The dinuclear copper enzyme, tyrosinase, activates O 2 to form a (μ-η2:η2-peroxido)dicopper(II) species, which hydroxylates phenols to catechols. However, the exact mechanism of phenolase reaction in the catalytic site of tyrosinase is still under debate. We herein report the near atomic resolution X-ray crystal structures of the active tyrosinases with substrate l -tyrosine. At their catalytic sites, CuA moved toward l -tyrosine (CuA1 → CuA2), whose phenol oxygen directly coordinates to CuA2, involving the movement of CuB (CuB1 → CuB2). The crystal structures and spectroscopic analyses of the dioxygen-bound tyrosinases demonstrated that the peroxide ligand rotated, spontaneously weakening its O−O bond. Thus, the copper migration induced by the substrate-binding is accompanied by rearrangement of the bound peroxide species so as to provide one of the peroxide oxygen atoms with access to the phenol substrate's ϵ carbon atom. 相似文献
5.
Lithium–oxygen (Li–O 2) batteries have attracted extensive research interest due to their high energy density. Other than Li 2O 2 (a typical discharge product in Li–O 2 batteries), LiOH has proved to be electrochemically active as an alternative product. Here we report a simple strategy to achieve a reversible LiOH-based Li–O 2 battery by using a cation additive, sodium ions, to the lithium electrolyte. Without redox mediators in the cell, LiOH is detected as the sole discharge product and it charges at a low charge potential of 3.4 V. A solution-based reaction route is proposed, showing that the competing solvation environment of the catalyst and Li + leads to LiOH precipitation at the cathode. It is critical to tune the cell chemistry of Li–O 2 batteries by designing a simple system to promote LiOH formation/decomposition. 相似文献
7.
During the past decade, dense-type oxygen-permeable ceramic membranes have received considerable attention both in the fields of scientific research and industrial application1, 2. Such membranes have mixed oxygen-ionic and electronic conductivity, and they can permeate oxygen at high temperature under an oxygen gradient without the need of outside connecting line and electrodes. Envisioned applications range from small scale oxygen pumps for medical use to large scale integrated gasification … 相似文献
8.
Oxygen evolution reaction (OER) is the most critical step in water splitting, still limiting the development of efficient alkaline water electrolyzers. Here we investigate the OER activity of Au–Fe nanoalloys obtained by laser-ablation synthesis in solution. This method allows a high amount of iron (up to 11 at %) to be incorporated into the gold lattice, which is not possible in Au–Fe alloys synthesized by other routes, due to thermodynamic constraints. The Au 0.89Fe 0.11 nanoalloys exhibit strongly enhanced OER in comparison to the individual pure metal nanoparticles, lowering the onset of OER and increasing up to 20 times the current density in alkaline aqueous solutions. Such a remarkable electrocatalytic activity is associated to nanoalloying, as demonstrated by comparative examples with physical mixtures of gold and iron nanoparticles. These results open attractive scenarios to the use of kinetically stable nanoalloys for catalysis and energy conversion. 相似文献
9.
Kinetic features are studied of the chemisorption and reduction of molecular oxygen from water by metal–ion exchanger nanocomposites that differ in the nature of the dispersed metal and state of oxidation. In the Pd < Ag < Cu series, the increasing chemical activity of metal nanoparticles raises the degree of oxygen sorption due to its chemisorption and subsequent reduction, while the role of the molecular chemisorption stage increases in the Cu < Ag < Pd series. Metal particles or their oxides are shown to act as adsorption sites on the surface and in the pores of the ion-exchanger matrix; the equilibrium sorption coefficient for oxygen dissolved in water ranges from 20 to 50, depending on the nature and oxidation state of the metal component. 相似文献
10.
Water could be decomposed into hydrogen and oxygen over Ga-modified ZSM-5 zeolite under UV irradiation. The photocatalytic activity was elevated significantly by supporting the gallium species and was sensitive to the loading amount of gallium species on the ZSM-5 zeolites. 相似文献
11.
Iron–carbonized aerogel nanocomposite was prepared from highly porous polyacrylonitrile microcellular foams containing a salt of iron, followed by carbonization. The electrochemical reduction of oxygen at this material was studied by using the rotating disk electrode method. In common with Pt/C, iron–carbonized aerogel nanocomposite presented excellent electrocatalytic activity for the oxygen reduction under experimental conditions close to those of a fuel cell cathode, that is, at the catalyst/Nafion interface in acidic solutions. 相似文献
12.
HeterometalicClusterCatalyzingCoordinativeHydrogenationPromotedbyOxygenTransferReagentLIQingshanDINGErrunWUShulinYINYuanqi1)(... 相似文献
13.
The oxidation of styrene with molecular oxygen catalyzed by PdCl2 CuCl2 has been investigated in supercritical CO2 with a batch reactor. The oxidative system of styrene contains four components at the beginning and seven components during the reaction. The critical temperature, critical pressure, and critical density at different conversions are determined by using a high-pressure view cell. The effect of phase behavior on the conversion and selectivity were studied. Experimental results showed that the critical parameters of the reaction mixture at fixed initial molar ratio changed with the conversion of reactant. The conversion of styrene reached maximum near the critical density of the reaction mixture. Product selectivity also varied with density of reaction mixture and could be tuned to some degree. 相似文献
14.
The viscosity property of Cr~(3 ),Al~(3 ),and compound ion cross-linked polymer gel solution in the anaerobic and aerobic environment was investigated aiming at meeting the practical demand of the oil field.The viscosity reserving effect after adding the biocide and the gelation in the anaerobic and aerobic environments was also studied in the paper.The results indicate that the viscosity of the cross-linked polymer gel solution caused by the water produced in aerobic environment is higher than that in anaerobic environment,and that the viscosity value of the cross-linked polymer gel solutions after adding biocides has improved to some extent and polymer gel has gelated well in anaerobic environment. 相似文献
15.
Iron(Ⅱ) tetra-(1,4-dithin)-porphyrazine, (FePz(dm)4) is able to activate molecule oxygen for oxygenation degradation of rhodamine B (RhB) in an extensive pH region without light excitation. Experiments indicate that the RhB can be degraded nearly 52% in alkaline aqueous solution, bubbling with dioxygen for seven hours in the presence of FePz(dtn)4 and the hydrogen peroxides as an active intermediate were determined by DPD method. The catalyst is recyclable and the catalyst activity was maintained after 10 recycles. 相似文献
16.
On the basis of the quantum-chemical method of density functional theory (DFT) in cluster approximation binary nanoclusters
of platinum-cobalt are simulated and their interactions with molecular and atomic oxygen are calculated. It was shown that
a binary nanocluster Pt 37Co 18, constructed of platinum atom outer shell, has the lowest adsorption heat for both molecular and atomic oxygen. This may
contribute favorably to accelerate oxygen reduction on catalytic electrodes. The results obtained are in good satisfaction
with known experimental and theoretical data, indicating the perspectives of the proposed approximation for theoretical search
of chemical composition and structure of effective binary nanocatalysts with platinum as cathode materials in low temperature
membrane fuel cells. 相似文献
17.
This paper, for the first time, reports a method that can be used as a highly sensitive probe for singlet oxygen (^1O2) and superoxide anion (O2^-) in vitro or in vivo. FCLA(3,7-dihydro-6-{4-[2-(N‘-(5-fluoresceinyl)thioureido)ethoxy]phenyl}-2-methylimidazo{1,2-a}pyrazin-3-one sodium salt), a chemiluminescence (CL) analysis reagent, has been reported to sensitively react with ^1O2 and O2^- to emit photons with a spectral peak of 525nm. In this work,when human serum albumin (HSA) was added into FCLA solution to enhance the CL intensity,approximately 20 times, compared to that without HSA. The enhanced CL had the same 525 nm spectral peak, identical to that without HSA. By gradually reducing the molecular oxygen content in the solution, we find that the auto-oxidation of oxygen molecules dissolved in the solution plays an important role in the CL process. Based on these experimental evidences, we propose a novel and highly sensitive detection method of ^1O2 and O2^- which may have a great potential in chemical and medical applications. 相似文献
18.
Electrochemical water splitting requires efficient water oxidation catalysts to accelerate the sluggish kinetics of water oxidation reaction. Here, we designed an efficient Co 3O 4 electrocatalyst using a pyrolysis strategy for oxygen evolution reaction (OER). Morphological characterization confirmed the ultra-thin structure of nanosheet. Further, the existence of oxygen vacancies was obviously evidenced by the X-ray photoelectron spectroscopy and electron spin resonance spectroscopy. The increased surface area of Co 3O 4 ensures more exposed sites, whereas generated oxygen vacancies on Co 3O 4 surface create more active defects. The two scenarios were beneficial for accelerating the OER across the interface between the anode and electrolyte. As expected, the optimized Co 3O 4 nanosheets can catalyze the OER efficiently with a low overpotential of 310 mV at current density of 10 mA/cm 2 and remarkable long-term stability in 1.0 mol/L KOH. 相似文献
19.
The oxygen vacancies of defective iron–cobalt oxide (FeCoO x-Vo) nanosheets are modified by the homogeneously distributed sulfur (S) atoms. S atoms can not only effectively stabilize oxygen vacancies (Vo), but also form the Co−S coordination with Co active site in the Vo, which can modulate the electronic structure of the active site, enabling FeCoO x-Vo-S to exhibit much superior OER activity. FeCoO x-Vo-S exhibits a mass activity of 2440.0 A g −1 at 1.5 V vs. RHE in 1.0 m KOH, 25.4 times higher than that of RuO 2. The Tafel slope is as low as 21.0 mV dec −1, indicative of its excellent charge transfer rate. When FeCoO x-Vo-S (anode catalyst) is paired with the defective CoP 3/Ni 2P (cathode catalyst) for overall water splitting, current densities of as high as 249.0 mA cm −2 and 406.0 mA cm −2 at a cell voltage of 2.0 V and 2.3 V, respectively, can be achieved. 相似文献
20.
Hybrid electrodes with improved O 2 tolerance and capability of CO 2 conversion into liquid products in the presence of O 2 are presented. Aniline molecules are introduced into the pore structure of a polymer of intrinsic microporosity to expand its gas separation functionality beyond pure physical sieving. The chemical interaction between the acidic CO 2 molecule and the basic amino group of aniline renders enhanced CO 2 separation from O 2. Loaded with a cobalt phthalocyanine-based cathode catalyst, the hybrid electrode achieves a CO Faradaic efficiency of 71 % with 10 % O 2 in the CO 2 feed gas. The electrode can still produce CO at an O 2/CO 2 ratio as high as 9:1. Switching to a Sn-based catalyst, for the first time O 2-tolerant CO 2 electroreduction to liquid products is realized, generating formate with nearly 100 % selectivity and a current density of 56.7 mA cm −2 in the presence of 5 % O 2. 相似文献
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