Gaseous Dioxygen Research Articles

Electrochemical water oxidation reaction by dinuclear Re(V) oxo complexes with a 1,4-benzoquinone core via the redox induced electron transfer (RIET) process.

We report two dinuclear rhenium(V) oxo complexes 1 and 2 types, [ReV(O)(Cl)3(L2-)ReV(O)(Cl)3][NBu4]2 (1, L2- = dianionic 2,5-dihydroxy 1,4-benzoquinone (DBQ2-)) and (2, L2- = dianionic chloranilic acid (CA2-) ligands), as a homogeneous electrocatalyst for water oxidation reactions in the acetonitrile-water mixture. The evolution of dioxygen gas at the anode was confirmed by a GC-TCD study. In controlled potential electrolysis (CPE), oxidation at 1.30 V (vs. Ag/AgCl) at neutral pH, 1 and 2 afforded 1+ [ReVI(O)(Cl)3(DBQ˙3-)ReVI(O)(Cl)3]- and 2+ [ReVI(O)(Cl)3(CA˙3-)ReVI(O)(Cl)3]- ions, respectively, via the redox induced electron transfer (RIET) process. Electrochemically generated species of 1+ and 2+ could be isolated in dry acetonitrile. 1+ and 2+ ions give strong EPR signals in fluid solution as well as under frozen glass conditions due to the [ReVI(O)(Cl)3(L˙3-)ReVI(O)(Cl)3]- ↔ [ReVI(O)(Cl)3(L2-)ReV(O)(Cl)3]- (where L2- = DBQ2- and CA2-) equilibrium. However, the continuation of the CPE study (1.30 V vs. Ag/AgCl) in the presence of acetonitrile-water mixture oxidised the in situ generated species of 1+ and 2+ to higher valent ReVIO species. These species (1+ and 2+) bound water through the water nucleophilic attack (WNA) to produce peroxide intermediate species of [ReV(OOH)(Cl)3(DBQ2-)ReV(OOH)(Cl)3] (A1) and [ReV(OOH)(Cl)3(CA2-)ReV(OOH)(Cl)3] (A2) for catalysts 1 and 2, respectively. Interestingly, A1 and A2 were authenticated and analysed by ESI mass spectrometry and infrared spectroscopy and were the active precursors of this water oxidation process. The extent of current generation under similar conditions suggested that complex 1 is superior to complex 2 for the water oxidation reaction. Notably, the maximum turnover frequency (TOFmax) of catalysts 1 and 2 were 2.1 and 1.6 s-1 at 0.27 V and 0.24 V over potential, respectively, which is very significant in WOR.

Investigation of the Reaction Mechanisms in La1-XSrxCo1-YFeyO3 Electrode by DFT Calculations

La1-xSrxCoy-1FeyO3-δ (LSCF) is a widely used oxygen electrode in Solid Oxide Cells (SOCs). Despite its high electrochemical activity combined with a good mixed ionic and electronic conductivity, it is prone to a phase decomposition upon operation decreasing the global cell performance. However, the underlying mechanisms for the reaction mechanism and the degradation are still not precisely understood and require investigations at atomic scale. Therefore, Density Functional Theory (DFT) calculations are used to better unravel the mechanisms occurring in the electrode. In particular, the interaction of the dioxygen gas with the AO-terminated (100) surface is studied. In this context, the oxygen reduction reaction is decomposed in a series of elementary steps including oxygen vacancy formation, adsorption of the dioxygen molecule on the bare surface and into a surface vacancy accompanied with the formation of intermediate oxygen species, followed by the dissociation and incorporation into the bulk. The calculations have been carried out at two different Cobalt contents (y=0.125 and y=0.25).

Size-controlled growth of ZnSe nanocrystals for high-performance photocatalytic H2O2 production in pure water.

Semiconductor photocatalysis is deemed as a novel and promising process that can produce H2O2 from earth-abundant water and gaseous dioxygen using sunlight as the energy supply. The searching of novel catalysts for photocatalytic H2O2 production has received increasing attention in the last few years. Herein, size-controlled growth of ZnSe nanocrystals was realized via a solvothermal method by varying the amount of Se and KBH4. The performance of the as-obtained ZnSe nanocrystals towards photocatalytic H2O2 production depends on the mean size of the synthesized nanocrystals. Under O2-bubbling, the optimal ZnSe sample presented an excellent H2O2 production efficiency (8.596 mmol g-1 h-1), and the apparent quantum efficiency for H2O2 production reaches as high as 2.84% at λ = 420 nm. Under air-bubbling, the accumulation of H2O2 was as high as 1.758 mmol L-1 after 3 h irradiation at the ZnSe dosage of 0.4 g L-1. The photocatalytic H2O2 production performance is far superior to the most investigated semiconductors such as TiO2, g-C3N4, and ZnS.

Synergy of the successive modification of cryptomelane MnO2 by potassium insertion and nitrogen doping for catalytic formaldehyde oxidation

Defect engineering is a powerful way to achieve the structural manipulation of MnO2 with rich surface active oxygen species for formaldehyde elimination under ambient conditions. Herein, a two-step protocol for consecutive modification of ɑ-MnO2 structure by potassium insertion and ammonium treatment was successfully implemented, leading to a synergetic enhancement of formaldehyde oxidation efficiency. For K modification alone, only slight activity increase was realized at moderate K level and an even higher K addition jeopardized the catalytic performance. Neither did NH4+ treatment alone work well for promoting formaldehyde oxidation. However, for the sample containing a higher K content (>3.07 wt%), the following NH4+ treatment could induce structural deformation via substitution of lattice oxygen by N doping, resulting in the formation of abundant reactive oxygen vacancies that could continuously transform inert gaseous dioxygen into oxidative surface adsorbed oxygen. Catalyst characterizations proposed that activity of the oxygen vacancy was the main factor contributing to the excellent formaldehyde oxidation performance. The best-performed K-MnO2-NH sample with 76% removal efficiency for ∼ 400 mg/m3 HCHO at 30 °C not only surpassed a benchmark commercial purifier in terms of formaldehyde mineralization, but also was much more resistant to the interference from the concomitant benzene pollutant than the commercial catalyst. Reaction pathways were proposed through comprehensive experimental investigations on the surface intermediates of the spent catalysts.

Inorganic Metal-Oxide Photocatalyst for H2 O2 Production.

Hydrogen peroxide (H2 O2 ) is a mild but versatile oxidizing agent with extensive applications in bleaching, wastewater purification, medical treatment, and chemical synthesis. The state-of-art H2 O2 production via anthraquinone oxidation is hardly considered a cost-efficient and environment-friendly process because it requires high energy input and generates hazardous organic wastes. Photocatalytic H2 O2 production is a green, sustainable, and inexpensive process which only needs water and gaseous dioxygen as the raw materials and sunlight as the power source. Inorganic metal oxide semiconductors are good candidates for photocatalytic H2 O2 production due to their abundance in nature, biocompatibility, exceptional stability, and low cost. Progress has been made to enhance the photocatalytic activity toward H2 O2 production, however, H2 O2 photosynthesis is still in the laboratory research phase since the productivity is far from satisfaction. To inspire innovative ideas for boosting the H2 O2 yield in photocatalysis, the most well-studied metal oxide photocatalysts are selected and the modification strategies to improve their activity are listed. The mechanisms for H2 O2 production over modified photocatalysts are discussed to highlight the facilitating role of the modification methods. Besides, methods for the quantification of H2 O2 and associated radical intermediates are provided to guide future studies in this field.

Coupling between oxidation kinetics and anisothermal oil flow during deep-fat frying

Deep-fat frying is a cooking technique that has been used continuously since prehistoric times. A domestic deep-fryer heated from the bottom develops significant convection inside the bath cavity. It is responsible for very high heat transfer coefficients and the exposure of the deep-frying oil to the atmospheric oxygen. The continuous conversion of gaseous dioxygen into unstable and reactive hydroperoxides and their subsequent advection throughout the bulk volume is at the origin of the main complaints made of frying which includes issues such as odors, fouling, and generation of several toxic compounds. This study analyzes the coupling between natural convection of triacylglycerols and the combinatorial chemistry controlling their autoxidation. At macroscopic scale, anisothermal flow within a typical kitchen appliance, including a cold zone beneath the heating element, was simulated in 3D under the Boussinesq approximation with a Eulerian description during initial heating and temperature holding. At molecular and intermediate scales, the endothermic decomposition of fatty-ester hydroperoxides in various mixtures was simulated over several hours using a Lagrangian description. Simulations and the proposed description of the combinatorial generation of hydroperoxides highlighted the complex relationship between deep-fryer design, residence times at different temperatures, and oil composition. The half-life of hydroperoxides (>15 min) is intermediate between vertical mixing times (<3 min) and longitudinal ones (>2 h). Hydroperoxides of unsaturated fatty esters are generated at the surface, which are preferentially decomposed in heated plumes and accumulate between production cycles in cold stagnant regions. Experimental validation is proposed for the eddy structure, temperature fluctuations, and hydroperoxides decomposition kinetics.

1,2-Diethoxyethane catalyzed oxidative cleavage of gem-disubstituted aromatic alkenes to ketones under minimal solvent conditions

Aerobic oxidation using pure dioxygen gas as the oxidant has attracted much attention, but its application in synthetic chemistry has been significantly hampered by the complexity of catalytic system and potential risk of high-energy dioxygen gas. By employing 1,2-diethoxyethane as a catalyst and ambient air as an oxidant, an efficient protocol for the construction of various aryl-alkyl and diaryl ketones through oxidative cleavage of gem-disubstituted aromatic alkenes under minimal solvent conditions has been achieved.

Cucurbit[5]uril derivatives as oxygen carriers

ABSTRACTCucurbit[n]urils are rigid cage-molecules of pumpkin-like shape, made of n-glycoluril units, able to bind mainly neutral molecules and cations. In this work, we investigate the binding of three cucurbit[5]uril derivatives with dioxygen O2 and show that one of them, namely per-hydroxylated cucurbit[5]uril, (OH)10CB[5], is able to significantly bind dioxygen gas at physiological temperature, even in the presence of sodium chloride at the concentration of injectable solution in blood. As cucurbit[n]urils studied up to now reveal low toxicity, per-hydroxylated cucurbit[5]uril appears as a promising precursor to design a host able to transport O2 in a haemoglobin substitute solution.

Stainless steel surface structure and initial oxidation at nanometric and atomic scales

The durability of passivable metals and alloys is often limited by the stability of the surface oxide film, the passive film, providing self-protection against corrosion in aggressive environments. Improving this stability requires to develop a deeper understanding of the surface structure and initial surface reactivity at the nanometric or atomic scale. In this work we applied scanning tunneling microscopy to unravel the surface structure of a model stainless steel surface in the metallic state and its local modifications induced by initial reaction in dioxygen gas. The results show a rich and complex structure of the oxide-free surface with reconstructed atomic lattice and self-organized lines of surface vacancies at equilibrium. New insight is brought into the mechanisms of initial oxidation at steps and vacancy injection on terraces leading to Cr-rich oxide nuclei and locally Cr-depleted terraces, impacting the subsequent mechanism of chromium enrichment essential to the stability of the surface oxide.

DABCO‐Catalyzed Double Cascade Cycloaddition of Maleimides with Bisarylhydrazones: Access to Fused Pyrazolo[5,1‐c][1,2,4]triazole Derivatives

AbstractA DABCO‐catalyzed double cascade cycloaddition reaction of diverse maleimides with bisarylhydrazones has been developed for the synthesis of a variety of synthetically challenging pyrazolo[5,1‐c][1,2,4]triazole derivatives. Dioxygen gas is employed as the sole oxidant in this transformation. It proceeds with high step‐ and atom‐efficiency and shows a broad substrate scope and functional group tolerance, making it a practical approach for the preparation of fully substituted pyrazolo[5,1‐c][1,2,4]triazole derivatives.magnified image

Chemical Activity of the Peroxide/Oxide Redox Couple: Case Study of Ba5Ru2O11 in Aqueous and Organic Solvents.

The finding that triggering the redox activity of oxygen ions within the lattice of transition metal oxides can boost the performances of materials used in energy storage and conversion devices such as Li-ion batteries or oxygen evolution electrocatalysts has recently spurred intensive and innovative research in the field of energy. While experimental and theoretical efforts have been critical in understanding the role of oxygen nonbonding states in the redox activity of oxygen ions, a clear picture of the redox chemistry of the oxygen species formed upon this oxidation process is still missing. This can be, in part, explained by the complexity in stabilizing and studying these species once electrochemically formed. In this work, we alleviate this difficulty by studying the phase Ba5Ru2O11, which contains peroxide O22– groups, as oxygen evolution reaction electrocatalyst and Li-ion battery material. Combining physical characterization and electrochemical measurements, we demonstrate that peroxide groups can easily be oxidized at relatively low potential, leading to the formation of gaseous dioxygen and to the instability of the oxide. Furthermore, we demonstrate that, owing to the stabilization at high energy of peroxide, the high-lying energy of the empty σ* antibonding O–O states limits the reversibility of the electrochemical reactions when the O22–/O2– redox couple is used as redox center for Li-ion battery materials or as OER redox active sites. Overall, this work suggests that the formation of true peroxide O22– states are detrimental for transition metal oxides used as OER catalysts and Li-ion battery materials. Rather, oxygen species with O–O bond order lower than 1 would be preferred for these applications.

Flower-like Cobalt Hydroxide/Oxide on Graphitic Carbon Nitride for Visible-Light-Driven Water Oxidation

Direct water oxidation via photocatalysis is a four-electron and multiple-proton process which requires high extra energy input to produce free dioxygen gas, making it exacting, especially under visible light irradiation. To improve the oxygen evolution reaction rates (OERs) and utilize more visible light, flower-like cobalt hydroxide/oxide (Fw-Co(OH)2/Fw-Co3O4) photocatalysts were prepared and loaded onto graphitic carbon nitride (g-C3N4) by a facile coating method in this work. Influenced by the unique three-dimensional morphologies, the synthesized Fw-Co(OH)2 or Fw-Co3O4/g-C3N4 hybrids reveal favorable combination and synergism reflected by the modified photoelectric activities and the improved OER performances. Attributed to its prominent hydrotalcite structure, Fw-Co(OH)2 shows better cocatalytic activity for g-C3N4 modification compared with that of Fw-Co3O4. Specifically, 7 wt % Fw-Co(OH)2/g-C3N4 photocatalyst exhibits photocurrent density 4 times higher and OER performance 5 times better than pristine g-C3N4. This work unambiguously promotes the application of sustainable g-C3N4 in water oxidation.

Metal enzymes in "impossible" microorganisms catalyzing the anaerobic oxidation of ammonium and methane.

Ammonium and methane are inert molecules and dedicated enzymes are required to break up the N-H and C-H bonds. Until recently, only aerobic microorganisms were known to grow by the oxidation of ammonium or methane. Apart from respiration, oxygen was specifically utilized to activate the inert substrates. The presumed obligatory need for oxygen may have resisted the search for microorganisms that are capable of the anaerobic oxidation of ammonium and of methane. However extremely slowly growing, these "impossible" organisms exist and they found other means to tackle ammonium and methane. Anaerobic ammonium-oxidizing (anammox) bacteria use the oxidative power of nitric oxide (NO) by forging this molecule to ammonium, thereby making hydrazine (N2H4). Nitrite-dependent anaerobic methane oxidizers (N-DAMO) again take advantage of NO, but now apparently disproportionating the compound into dinitrogen and dioxygen gas. This intracellularly produced dioxygen enables N-DAMO bacteria to adopt an aerobic mechanism for methane oxidation.Although our understanding is only emerging how hydrazine synthase and the NO dismutase act, it seems clear that reactions fully rely on metal-based catalyses known from other enzymes. Metal-dependent conversions not only hold for these key enzymes, but for most other reactions in the central catabolic pathways, again supported by well-studied enzymes from model organisms, but adapted to own specific needs. Remarkably, those accessory catabolic enzymes are not unique for anammox bacteria and N-DAMO. Close homologs are found in protein databases where those homologs derive from (partly) known, but in most cases unknown species that together comprise an only poorly comprehended microbial world.

Synthesis of Indazoles and Azaindazoles by Intramolecular Aerobic Oxidative CN Coupling under Transition‐Metal‐Free Conditions

A transition-metal-free oxidative C-N coupling method has been developed for the synthesis of 1H-azaindazoles and 1H-indazoles from easily accessible hydrazones. The procedure uses TEMPO, a basic additive, and dioxygen gas as the terminal oxidant. This reaction demonstrates better reactivity, functional group tolerance, and broader scope than comparable metal catalyzed reactions.

Critical role of water in the direct oxidation of CO and hydrocarbons in diesel exhaust after treatment catalysis

CO and C3H6 oxidation have been carried out in the absence and presence of water over a Pd/Al2O3 catalyst. It is clear that water promotes CO and, as a consequence, C3H6 oxidation takes place at much lower temperatures compared with the dry feed. The significant increase in the catalyst's activity with respect to CO oxidation is not simply associated with changes in surface concentration as a result of competitive adsorption effects. Utilising 18O2 as the reactant allows the pathways whereby the oxidation due to gaseous dioxygen and where the water activates the CO and C3H6 to be distinguished. In the presence of water, the predominant pathway is via water activation with C16O2 and C16O18O being the major species formed and oxidation with dioxygen plays a secondary role. The importance of water activation is further supported by the significant decrease in its effect when using D2O versus H2O.

Direct evidence of the role of dispersed ceria on the activation of oxygen in NaX zeolite by coupling the 17O/16O isotopic exchange and 17O solid-state NMR

The possibility to understand the phenomena of oxygen diffusion in the solids may be decisive to explain the role of framework oxygen atoms in a catalytic mechanism or in catalyst regeneration steps. In this work, two highly efficient characterization techniques have been associated: the 17O/16O isotopic exchange and the 17O high field solid-state NMR spectroscopy. This combination leads to a powerful tool to investigate the interaction between gaseous dioxygen molecule and lattice oxygen atoms. Here, it permits to demonstrate the enhancement of the participation of zeolitic oxygen atoms in the exchange with the gas phase when ceria is simply mixed with NaX.

Synthesis of Tri- and Tetrasubstituted Pyrazoles via Ru(II) Catalysis: Intramolecular Aerobic Oxidative C–N Coupling

An unprecedented ruthenium(II)-catalyzed intramolecular oxidative C-N coupling method has been developed for the facile synthesis of a variety of synthetically challenging tri- and tetrasubstituted pyrazoles. Dioxygen gas is employed as the oxidant in this transformation. The reaction demonstrates excellent reactivity, functional group tolerance, and high yields.

Voltammetric Determination of Both Concentration and Diffusion Coefficient by Combinational Use of Regular and Microelectrodes

AbstractTwo Pt disks with 1.6 and 0.1 mm diameter, whose sizes were accurately evaluated, were applied to voltammetric diffusion‐controlled currents to determine the concentrations of redox species. Since voltammetric peak currents at a regular electrode, Ip, and limiting currents at a microelectrode, IL, are proportional to D1/2 and D, respectively, the ratio Ip2/IL is independent of the diffusion coefficient but dependents on the concentration. We used this fact to determine the concentrations of ferrocenyl derivatives, hemin, hexacyanoferrate(II), dioxygen gas and hydrogen gas. New insights are not only the absolute determination of concentrations but also the sensitive detection of adsorption of electrochemical products which blocked the voltammetric currents.

The first pentanuclear heterobimetallic coordination cation with CeIII, CeIV and MnII

[Mn2(μ-dipic)2(H2O)6]·2H2dipic (dipic = dipicolinate) reacts with (NH4)2[Ce(NO3)6] to produce the pentanuclear coordination species in [MnCe4(μ-dipic)6(H2O)20][Ce(dipic)3]2·11H2O (1). The coordination anion [Ce(dipic)]2− is formed by three tridentate O,N,O dipicolinate(2−) anionic ligands around the metal CeIV cation. The same pattern is found for two CeIV in the pentanuclear CeIII/CeIV/MnII/CeIV/CeIII chain-like coordination cation [MnCe4(μ-dipic)6(H2O)20]4+. The CeIV(dipic)3 moieties in the cation coordinatively bridge the terminal CeIII(H2O)8 with the central MnII(H2O)4 units. Each manganese(II) ion is thus 6-coordinated and each cerium(III, IV) ion is 9-coordinated. During the synthesis of 1, the gaseous dioxygen is evolved, due to water oxidation that is attributed to the presence of Ce and Mn in different oxidation states.

Reductive and Transition-Metal-Free: Oxidation of Secondary Alcohols by Sodium Hydride

The oxidation of alcohols into their corresponding carbonyl compounds represents a fundamentally important functional group transformation and occupies a prominent position in modern synthetic organic chemistry. Advances on the development of new oxidation reagents and methodologies and their applications in both targetand diversity-oriented synthesis have been regularly surveyed and constituted one of the most extensively and actively investigated areas of current organic synthesis. In this context, a plethora of oxidants, including those small organic molecule-based reagents (such as Dess-Martin periodinane, Swern oxidation, Moffatt oxidation, Corey-Kim oxidation, and SO3/pyridine) and metal-based systems (such as Jones reagent, Collins reagent, pyridinium chlorochromate, pyridinium dichromate, barium permanganate, manganese dioxide, ruthenium tetroxide, silver carbonate, and Oppenauer oxidation), have been identified to be powerful tools promoting conversion of various alcohols into their corresponding aldehyde or ketone products and thus evolved as a most important class of tools in synthetic chemists’ arsenal. A recent conceptually novel approach in alcohol oxidation chemistry, driven by the needs of process environmental compatibility and sustainability, has been exploring aerobic oxidation with highly active transition-metal catalysts (such as Pd, Ru, Fe, Cu, Pt, Au, Ir, Rh, etc.) and dioxygen gas as the terminal oxidant, their heterogeneously immobilized variants, and biomimetic systems. Despite these impressive advances, very few of the known methods are capable of offering truly economic and practical oxidation transformations across a broad spectrum of alcohol substrates. Many of these systems suffered from high reagent cost, air instability, employment of heavy metals or organic oxidants, stringent reaction conditions, operational complexity, functional group incompatibility, or production of wastes in their related processes. Thus, there is a continuing and increasing demand for new reagents that could help address the above-mentioned challenges. We herein report on an exceedingly simple secondary alcohol oxidation protocol that employs the widely available sodium hydride (NaH) as the oxidant.