Redox

About: Redox is a research topic. Over the lifetime, 26853 publications have been published within this topic receiving 862368 citations. The topic is also known as: reduction-oxidation & reduction-oxidation reaction.

Copper-Catalyzed Electrochemical C–H Amination of Arenes with Secondary Amines

Abstract: Electrochemical oxidation represents an environmentally friendly solution to conventional methods that require caustic stoichiometric chemical oxidants. However, C-H functionalizations merging transition-metal catalysis and electrochemical techniques are, to date, largely confined to the use of precious metals and divided cells. Herein, we report the first examples of copper-catalyzed electrochemical C-H aminations of arenes at room temperature using undivided electrochemical cells, thereby providing a practical solution for the construction of arylamines. The use of n-Bu4NI as a redox mediator is crucial for this transformation. On the basis of mechanistic studies including kinetic profiles, isotope effects, cyclic voltammetric analyses, and radical inhibition experiments, the reaction appears to proceed via a single-electron-transfer (SET) process, and a high valent Cu(III) species is likely involved. These findings provide a new avenue for transition-metal-catalyzed electrochemical C-H functionalization reactions using redox mediators.

Correlative operando microscopy of oxygen evolution electrocatalysts

Abstract: Transition metal (oxy)hydroxides are promising electrocatalysts for the oxygen evolution reaction1–3. The properties of these materials evolve dynamically and heterogeneously4 with applied voltage through ion insertion redox reactions, converting materials that are inactive under open circuit conditions into active electrocatalysts during operation5. The catalytic state is thus inherently far from equilibrium, which complicates its direct observation. Here, using a suite of correlative operando scanning probe and X-ray microscopy techniques, we establish a link between the oxygen evolution activity and the local operational chemical, physical and electronic nanoscale structure of single-crystalline β-Co(OH)2 platelet particles. At pre-catalytic voltages, the particles swell to form an α-CoO2H1.5·0.5H2O-like structure—produced through hydroxide intercalation—in which the oxidation state of cobalt is +2.5. Upon increasing the voltage to drive oxygen evolution, interlayer water and protons de-intercalate to form contracted β-CoOOH particles that contain Co3+ species. Although these transformations manifest heterogeneously through the bulk of the particles, the electrochemical current is primarily restricted to their edge facets. The observed Tafel behaviour is correlated with the local concentration of Co3+ at these reactive edge sites, demonstrating the link between bulk ion-insertion and surface catalytic activity. Mapping the operational chemical, physical and electronic structure of an oxygen evolution electrocatalyst at the nanoscale links the properties of the material with the observed oxygen evolution activity.

Effect of the synthesis conditions on the redox and catalytic properties in oxidation reactions of LaCo1−xFexO3

Abstract: The aim of this work was to study the effect of the synthesis conditions of LaCoO3 on the redox properties and the catalytic activity for the CH4 and CO oxidation reactions. Five LaCoO3 samples were prepared by different methods: solid state (SS), coprecipitation (COP), citrate complexation (CIT) and reactive grinding of the single oxides (RG) and of an amorphous precursor (COPRG). TPD-O2 experiments showed three kinds of oxygen. First, two surface oxygen species (designated as α-oxygens) were distinguished. The amounts of oxygen desorbing in the two α-oxygen desorption peaks were found to be mainly dependant on the specific surface area of the samples and stay lower than one monolayer (0.7–0.8). The last desorption peak, which is generally attributed to the desorption of some lattice oxygens (β-oxygens), was found to be more dependant on the morphology of the sample. H2 reduction experiments (TPR-H2) showed the usual two-step reduction. The reduction temperatures were found to be directly related to the calcination temperatures. H2 uptakes and XRD analysis suggest a reduction of slightly more than one e− in the first step and the formation of some Co0 before the beginning of the second reduction step. Kinetic analysis for the CH4 oxidation reaction showed that the specific reaction rate is significantly affected by the synthesis procedure. Grinding an amorphous precursor results in a perovskite which combines a high specific surface area (made possible by the use of grinding) and a high activity. The RG sample presents a lower activity than COPRG, due to a higher iron contamination. Nevertheless, the differences observed in the oxidation reaction rate could not be correlated to the redox properties, measured by TPD-O2 and TPR-H2, of the catalysts. Then, these methods are not adequate to evaluate the reactivity of the solids. When tested in the CO oxidation reaction, the samples present different comportments than in the CH4 oxidation reaction. These differences are discussed on the basis of the oxidation mechanisms involved in the two reactions.

Kinetics of the catalytic liquid-phase hydrogenation of aqueous nitrate solutions

Abstract: Liquid-phase reduction using a solid Pd/Cu bimetallic catalyst provides a potential technique for the removal of nitrates from waters. Kinetic measurements were performed for a wide range of reactant concentrations and reaction conditions in an isothermal semi-batch slurry reactor operating at atmospheric pressure. The effects of catalyst loading and initial nitrate concentration on the reaction rate were also investigated. The proposed intrinsic rate expression for nitrate disappearance is based on the conventional Langmuir-Hinshelwood kinetic approach, considering both equilibrium nitrate as well as dissociative hydrogen adsorption processes to different types of active sites, and assuming an irreversible bimolecular surface reaction between adsorbed reactant species to be the rate-controlling step. The apparent activation energy for catalytic liquid-phase nitrate reduction and the heat of nitrate adsorption, in the temperature range 280.5–293 K, were found to be 47 and 22 kJ/mol, respectively. It is confirmed that the process of catalytic liquid-phase hydrogenation of aqueous nitrate solutions undergoes a redox mechanism.

A superior active and stable spinel sulfide for catalytic peroxymonosulfate oxidation of bisphenol S

Abstract: In this paper, for the first time the spinel sulfide carrollite (CuCo2S4) was applied to activate peroxymonosulfate (PMS) for the abatement of an endocrine disrupting compound (EDC) bisphenol S (BPS) in water. The hydrothermally synthesized CuCo2S4 outperformed conventional cobalt and copper oxides/sulfides in both catalytic activity and stability (i.e. metal leaching) in this oxidation process. The spinel sulfide was most active under neutral pH conditions, which is preferred for water/wastewater treatment. Sulfate radicals (SO4 −) which was proved to be the prominent oxidant species in this oxidation process, attacked on BPS producing multiple hydroxylated degradation intermediates. Cu(II)/Cu(I) and Co(III)/Co(II) synergetic surface redox couples are found responsible for the catalytic activation of PMS producing SO4 −; furthermore, the thermodynamically favorable reaction between surface Cu(I) and surface Co(III) guarantees the electron transfer between the surface metal sites. As confirmed by turnover frequency calculation, CuCo2S4 has higher catalytic effect per surface cobalt atom than the counterpart oxides for PMS activation, probably due to the lower electronegativity of S2− than O2−. CuCo2S4 could be a potentially useful catalyst for water/wastewater treatment.