Four-electron Step Research Articles

Self-driving photothermal anode electrocatalyst towards the robust OER for water electrolysis

The hydrogen production of water electrolysis is a potential technique route of clean energy exchange, but suffering from the sluggish oxygen evolution reaction (OER) owing to its four-electron step. In this work, we presented a significant improvement in the OER activity by photothermally triggering P-doped NiFe2O4 (NFO–P) electrocatalyst. As compared to the NiFe2O4 (NFO) catalyst, the excellent PEC performance of robust NFO–P is a result of more readliy generated active sites (Ni(III)), evidencing by the in-situ Raman spectra and polarization curves. With the near-infrared (λ = 808 nm) irradiation, the NFO–P surface temperature increases from 37 to 73 °C, and the NFO–P delivers a current density of 10 mA cm−2 at a pretty low overpotential of 191 mV (10 W/cm2), outperforms most OER catalysts including noble oxides. Photothermal enhancement in OER is attributed to photogenerated hot carriers and thermal activation. The thermal effect accelerates the kinetics, while with the assistance of the hot carrier, the rate-determining step (RDS) transforms from the O* to OOH* formation, reducing the activation energy. This study offers a thorough understanding of the combined contributions of hot carriers and thermal effects to the OER and also opens avenues for enhancing electrochemical efficiency and diminishing techno-economic costs in future applications.

Dealloyed Pt2Os nanoparticles for enhanced oxygen reduction reaction in acidic electrolytes

Carbon-supported Pt2Os (Pt2Os/C) nanoparticles in 3.55nm sizes are synthesized from a wet chemical reflux process. Subsequently, the Pt2Os/C undergoes a dealloying treatment in which multiple cyclic voltammetric scans are imposed to dissolve the Os atoms selectively from the surface of the Pt2Os nanoparticles. X-ray diffraction signals from the dealloyed sample (DA–Pt2Os/C) indicate a fcc phase and composition analysis suggests Pt4Os. Line scans from the scanning transmission electron microscope confirm that the surface of Pt4Os is depleted with the Os atoms. This agrees with our quantum mechanics (Density Funtional theory) calculations, which predict for the Pt3Os composition that the surface skin layer is pure Pt. The DA–Pt2Os/C shows impressive electrocatalytic behaviors (0.29mAμgPt−1 in mass activity and 1.03mAcmPt−2 in specific activity) for the oxygen reduction reaction (ORR) in oxygen-saturated 0.1M aqueous HClO4 solution, as compared to those of commercially available Pt/C and as-synthesized Pt2Os/C. In stability test, the DA–Pt2Os/C demonstrates a better retention of ORR activities and a smaller loss of electrochemical active surface area. We verify experimentally that a four-electron step is responsible for the ORR process occurring on the DA–Pt2Os/C.

The effect of pretreatment of Vulcan XC-72R carbon on morphology and electrochemical oxygen reduction kinetics of supported Pd nano-particle in acidic electrolyte

The influence of chemical pretreatment of carbon support for oxygen reduction on palladium nano-particles in acidic electrolyte was studied. Vulcan XC-72R carbon as catalyst support for palladium nano-particles was pretreated with 5% HNO 3, 0.07 M H 3PO 4, 0.2 M KOH and 10% H 2O 2. The effect of treatment on the properties of the carbon support was studied by N 2 adsorption and X-ray photoelectron spectroscopy (XPS). It was found that chemical treatment significantly changed the surface chemical properties and surface area of the carbon support. The surface area and pore volume of 5% HNO 3 and 10% H 2O 2 treated carbon supports were drastically decreased due to the oxidative nature of treatment. Ethylene glycol (EG) reduction method was used to synthesise 20% Pd on pr-treated and un-treated carbon supports. Differences in catalyst morphology were characterized using X-ray diffraction, energy dispersive X-ray analysis and transmission electron microscope techniques. It was observed that by using a mild reducing agent, namely EG, well-dispersed and nano-size Pd particles could be achieved during catalyst synthesis. The electrocatalytic activity of different Pd/C catalysts towards the oxygen reduction reaction (ORR) was examined by cyclic voltammetry (CV) on a rotating ring-disc electrode (RRDE) and compared with E-Tek 20% Pd/C catalyst under identical experimental conditions. The kinetics of ORR on these electrocatalysts predominantly involved a four-electron step reduction with the first electron transfer being the rate-determining step. However, the observed specific activity, mass activity and amount of hydrogen peroxide produced during ORR were greatly influenced by the pretreatment employed for carbon support.

Hexakis(4-(N-butylpyridylium))benzene: A Six-Electron Organic Redox System

The cobalt-catalyzed cyclotrimerization of bis(4-pyridyl)acetylene affords hexakis(4-pyridyl)benzene in moderate yield. Alkylation with n-butyltriflate gives hexakis(4-(N-butylpyridylium))benzene triflate (1 6+), which can be reduced with Na/Hg in DMF to neutral 10. A single-crystal X-ray diffraction structure reveals that 1 0 has a chair-cyclohexane-like core and a [6]radialene structure. Cyclic voltammetry shows that 1 6+ is reversibly reduced to 1 2+ in one four-electron step and 1 2+ is reversibly reduced to 1 0 in one two-electron step. A reduction by four electrons at one potential is unprecedented for a molecule in which the electrochemically active centers are in electronic communication. The large structural transformation from 1 6+ to 1 0 is responsible for the "potential inversion" in the cyclic voltammetry, and DFT calculations suggest a possible structure for the stable intermediate 1 2+. A comparison is made to the electrochemistry and structural transformations in a previously prepared [4]radialene analogue of 1 0.

Formation of Ni-Zr Alloys in Molten Fluorides

The electrochemical behavior of Zr4+ was investigated in LiF-NaF between 923 and 1123 K. With an inert tungsten or molybdenum electrode, a well-defined oxydo-reduction wave was observed during cyclic voltammetry experiments. The reduction of Zr4+ is a single four-electron step and quasi-reversible, and the process is diffusion-controlled. Chronopotentiometry was used to measure the diffusion coefficient of Zr4+ ions. For instance, a value 1.54 × 10-5 cm2 s-1 was found at of at 979 K. The reduction of Zr4+ ions was also studied using nickel electrode. In that case, Ni- Zr alloys were formed as pointed out on cyclic voltammograms which exhibit several characteristic peaks. SEM and EDX analyses have revealed the presence of a multi-layered Ni-Zr alloy deposited film on Ni electrode. At -1.65 V vs Ni/Ni2+, three successive layers were pointed out, corresponding to three different Ni-Zr alloy compositions.

Dinuclear vanadium and tetranuclear iron complexes obtained with the open Wells–Dawson [Si2W18O66]16– tungstosilicate

Abstract Reaction of two transition metal cations M (M = V V , Fe III ) on the open Wells–Dawson anion α-[{K(H 2 O) 2 }Si 2 W 18 O 66 ] 15– leads to dinuclear and tetranuclear complexes, respectively. The molecular anions [{KV 2 O 3 (H 2 O) 2 }(Si 2 W 18 O 66 )] 11– and [{Fe 4 (OH) 6 }(Si 2 W 18 O 66 )] 10– have been structurally characterized by single crystal X-ray diffraction. The oxo/hydroxometallic clusters [KV 2 O 3 (H 2 O) 2 ] 5+ and [Fe 4 (OH) 6 ] 6+ are included in the pocket between the two subunits of [Si 2 W 18 O 66 ] 16– . The Fe III atoms of the iron complex can be reduced to Fe II by a single four-electron step. To cite this article: N. Leclerc-Laronze et al., C. R. Chimie 9 (2006) .

Palladium Monolayer and Palladium Alloy Electrocatalysts for Oxygen Reduction

We investigated the oxygen-reduction reaction (ORR) on Pd monolayers on various surfaces and on Pd alloys to obtain a substitute for Pt and to elucidate the origin of their activity. The activity of Pd monolayers supported on Ru(0001), Rh(111), Ir(111), Pt(111), and Au(111) increased in the following order: Pd/Ru(0001) < Pd/Ir(111) < Pd/Rh(111) < Pd/Au(111) < Pd/Pt(111). Their activity was correlated with their d-band centers, which were calculated using density functional theory (DFT). We found a volcano-type dependence of activity on the energy of the d-band center of Pd monolayers, with Pd/Pt(111) at the top of the curve. The activity of the non-Pt Pd2Co/C alloy electrocatalyst nanoparticles that we synthesized was comparable to that of commercial Pt-containing catalysts. The kinetics of the ORR on this electrocatalyst predominantly involves a four-electron step reduction with the first electron transfer being the rate-determining step. The downshift of the d-band center of the Pd "skin", which constitutes the alloy surface due to the strong surface segregation of Pd at elevated temperatures, determined its high ORR activity. Additionally, it showed very high methanol tolerance, retaining very high catalytic activity for the ORR at high concentrations of methanol. Provided its stability is satisfactory, this catalyst might possibly replace Pt in fuel-cell cathodes, especially those of direct methanol oxidation fuel cells (DMFCs).

Reduction of diprotonated form of aryl hydrazones

Hydrazones derived from aromatic aldehydes and ketones are reduced at pH 2 to about 8 in a four-electron step. The species reduced in this step bears two positive charges on adjacent nitrogen atoms. This has been proved by the pH-dependence of half-wave potentials of N,N,N-trialkylhydrazonium ions, which indicates a protonation of the azomethine nitrogen prior to the first electron uptake. Similar species with two adjacent positive charges is generated by diprotonation of hydrazones, adsorbed at the electrode surface. The existence of such species as reactive intermediates at electrode surface has been in some instances confirmed, based on steep plots of imax/id = f(pH). The steep shape of these plots has been confirmed for acetophenone (II) and fluorenone (III) hydrazones using conventional buffers and for benzophenone hydrazone (IV) after extrapolation of buffer concentration to zero. The shape of the imax/id = f(pH) can be namely achieved by protonation not only by H+ ions, but also by acid buffer components, as in general acid catalysis.

Multielectron Redox Chemistry of Iron Porphyrinogens

Iron octamethylporphyrinogens were prepared and structurally characterized in three different oxidation states in the absence of axial ligands and with sodium or tetrafluoroborate as the only counterions. Under these conditions, the iron- and ligand-based redox chemistry of iron porphyrinogens can be defined. The iron center is easily oxidized by a single electron (E(1/2) = -0.57 V vs NHE in CH(3)CN) when confined within the fully reduced macrocycle. The porphyrinogen ligand also undergoes oxidation but in a single four-electron step (E(p) = +0.77 V vs NHE in CH(3)CN); one of the ligand-based electrons is intercepted for the reduction of Fe(III) to Fe(II) to result in an overall three-electron oxidation process. The oxidation equivalents in the macrocycle are stored in C(alpha)-C(alpha) bonds of spirocyclopropane rings, formed between adjacent pyrroles. EPR, magnetic and Mossbauer measurements, and DFT computations of the redox states of the iron porphyrinogens reveal that the reduced ligand gives rise to iron in intermediate spin states, whereas the fully oxidized ligand possesses a weaker sigma-donor framework, giving rise to high-spin iron. Taken together, the results reported herein establish a metal-macrocycle cooperativity that engenders a multielectron chemistry for iron porphyrinogens that is unavailable to heme cofactors.

Polarographic and Voltammetric Behavior of the Antibiotic Cefetamet; Reduction of the Methoxyimino Group

Evaluation of DC polarographic i-E curves and analogous curves obtained by cyclic voltammetry in aqueous buffers as well as in buffers containing 30% v/v ethanol indicated that the methoxyimino group of cefetamet is reduced on DME and HDME in the following way: the methoxyimino group can be at the electrode surface rapidly protonated up to pH about 9. In the protonated form of the grouping (&gt;C=NH-OCH3)+, the OCH3 is cleaved off in the first two-electron step. The resulting imine is in equilibrium with its protonated form, which is reduced in a second two-electron step. At pH higher than about 10, the unprotonated form of the methoxyimino group is reduced in a single four-electron step to amine. Studies using cyclic voltammetry were complicated by adsorption phenomena.

Electrochemical behavior and voltammetric determination of the herbicide metribuzin at mercury electrodes.

The electrochemical behavior of the herbicide metribuzin (4-amino-6-tert-butyl-4,5-dihydro-3-methylthio-1,2,4-triazin-5-one) at mercury electrodes was studied in aqueous solutions by direct current (DC) and tast polarography, differential pulse (DPV) and cyclic voltammetry (CV), and controlled-potential coulometry. The electrolysis products were separated and identified by chromatographic techniques combined with mass spectrometric detection. The reduction process in acid media includes two irreversible steps. In the first four-electron step the N-NH2 and the 1,6-azomethine bonds are reduced. The second step leads to the formation of 5-tert-butyl-2,3,4,5-tetrahydroimidazol-4-one at the mercury-pool electrode. The first reduction step combined with adsorptive accumulation of the herbicide molecule at the mercury electrode surface was used for its determination by differential pulse adsorptive stripping voltammetry (DPAdSV). Calibration curves were linear in the range 1-30 microg L(-1) with a detection limit of 0.27 microg L(-1) (1 nmol L(-1)) under the conditions used (buffer pH 4.5, Eacc = -0.45 V relative to Ag/AgCl and tacc = 10 s). Preconcentration on solid-phase extraction columns (SPE-phenyl) was used for the determination of very small amounts of metribuzin in river water samples. Recovery was approximately 97%. The reproducibility of the analytical procedure including SPE treatment and DPV determination was expressed as relative standard deviations of 2.53 and 3.66% for 2 and 6 microg L(-1) metribuzin, respectively.

Two-step reduction of the O-methyloxime group in the antibiotic cefetamet

Most oximes as well as their N-alkyl and O-alkyl derivatives are electrochemically reduced in a single four-electron step to the amine. Some exceptions have been reported, where the reduction occurs in two two-electron steps. The reduction of the O-methyloxime grouping in the antibiotic cefetamet occurs at pH 5–9 in two steps, corresponding to a reduction to the imine and amine, respectively. It has been shown that the separation of the two two-electron processes is caused by differences in position and rate of establishment of acid–base equilibria resulting in protonations of the oxime and imino grouping.

Electrochemical reduction of nitrotetralones

The electrochemical reduction of three nitrotetralone derivatives by tast and differential pulse polarography and cyclic voltammetry in a wide pH range was studied. In aqueous media, the reduction of mono- and dinitrotetralone follows the general pattern of reduction of aromatic nitro compounds: the nitro group is reduced in a four-electron step to a hydroxylamine group. However, in mixed media this reduction differs from that of other nitrobenzenes, due to the fact that the formation of the nitroradical anion was not observed. The reduction of the acetoxy derivative was studied only at alkaline pH, because it suffered acid hydrolysis. The ionization p K of the protonatable groups were polarographically obtained.

Polarographic and electrochemical studies of some aromatic and heterocyclic nitro compounds Part 9. Substitutent effects on protonation of the radical ArNO2H· and its reactions with hydroxylamino and nitroso compounds in buffered mixtures of water and DMF

Abstract Whereas in aqueous solutions the polarographic reduction of most nitrobenzenes occurs in a single four-electron step, in the presence of 20% or 40% dimethylformamide (DMF) separation of two waves can be observed. The decrease of the first reduction step from a four-electron to a one-electron process with increasing pH is attributed to a decrease in the rate of protonation of the radical ArNO 2 H · . The decrease from a four-electron to a one-electron wave has the shape of a dissociation curve with an inflexion point at pH = p K′ . The dependence of the values of p K′ on the electron-withdrawing substituents indicates the predominant role of adsorption on the rate of protonation of the radical ArNO 2 H · . The effects of a change in concentration of the co-solvent (DMF) on the values of p K′ are attributed to changes in the solvation energies of ArNO 2 H + 2 and ArNO 2 H · . The ratio of currents of the reduction of the nitro group in the forward sweep in cyclic voltammetry to those of the oxidation of the resulting arylhydroxylamine on the reverse sweep and to the peak current of the reduction of the nitro group on the second sweep indicate deactivation of the arylhydroxylamine at the electrode surface. The effects of electron-withdrawing substituents on such current ratios indicate the role of a surface reaction of arylhydroxylamine with an intermediate of the reduction of the nitro group, probably ArNO 2 H · .

Molecular Biology of the Alternative Oxidase

Possibly the most easily recognized component of mitochondrial respiration specific to plants is a second terminal oxidase that has been variously described as being responsible for cyanide-insensitive 'cyanide-resistant or pathway respiration.' This type of respiration has been best defined in thermogenic inflorescences of the Araceae, such as Sauromatum guttatum, the lily' (Meeuse, 1975). Alternative pathway respiration has also been identified in other types of organisms; for example, in protista such as trypanosomes, in fungi such as Neurospora crassa and Hansenula anomala, and in green algae such as Chlamydomonas (Moore and Siedow, 1991). Although some reports exist for such activity in animal tissues, they have never been reliably confirmed. What is the alternative pathway of respiration, how does it work, and, perhaps more importantly, what is its purpose? Alternative pathway respiration branches from the Cyt pathway at ubiquinone and donates electrons directly to oxygen to form water, apparently in a single four-electron step (Siedow and Moore, 1993). An important feature of this pathway is that it does not contribute to a transmembrane potential; electrons that flow down this pathway lose two of three potential coupling sites for proton transport, and thus ATP production (Fig. 1). The only confirmed function for alternative pathway respiration is to supply heat in thermogenic blooms of the Araceae, such as S. guttatum, the voodoo lily (Meeuse, 1975). In this organism heat is produced in ephemeral inflorescences during a single day of flowering to better volatilize scents to attract insect pollinators.

Electroreduction of Nifedipine

At pH > 6, the nitro group of nifedipine is reduced in a four-electron step to a hydroxylamino group. At pH < 6, the hydroxylamino derivative undergoes an acid-catalyzed dehydration, and the rate of this reaction governs the limiting current. Resulting quinonemethide is further reduced, and a six-electron reduction step results. Formation and reducibility of the quinonemethide is favored by conjugation with the 1,4-dihydropyridine ring. Further reduction of the hydroxylamino derivative is confirmed by the absence of the anodic oxidation peak on the reverse scan at pH < 6.5. At potentials more negative than that of the nitro group, the protonated form of the hydroxylamino derivative is reduced between pH 4 and 8 in direct current (dc) polarography and between pH 6 and 11 in cyclic voltammetry, the difference resulting from the difference in the measurement time window. The yields and fate of the hydroxylamino derivative obtained in controlled potential electrolysis with a hanging mercury drop electrode (HMDE) differ from those in dc polarography, indicating the presence of different competing reactions. For analyses, pH 2 and 11 are most suitable.

Reaction of electrogenerated arylhydroxylamines and nitrosobenzene in the course of reduction of nitrobenzene under conditions of cyclic voltammetry

Under quasi-potentiostatic conditions as they occur in dc polarography or when using rectangular voltage polarization (Kalousek commutator), reduction of aromatic nitro compounds usually follows a simple pattern [12]. The nitro group is reduced in a single four-electron step to a hydroxylamino group which, when protonated, is reduced at more negative potentials to an amino group. An increase in current in acidic media in the absence of surfactants was attributed to a reaction of the hydroxylamino derivative with a nitroso intermediate yielding an azoxy compound [3], but measurement of the first step was affected by the presence of streaming maxima. The situation is different in voltammetric methods which involve a voltage scan, where both the potential and the period of contact at the electrode surface are affected by the rate of scanning. Not only is the time available for a chemical conversion different from conditions in dc polarography but products formed at one potential can react with species generated or present at another potential. Voltammetric curves on mercury [4,5] show on the first scan from positive to negative potentials a single peak corresponding to a four-electron reduction (Fig. l), accompanied in acidic media at more negative potentials by an indistinct peak of the reduction of the protonated form of the hydroxylamino derivative. The voltage reversals both at potentials corresponding to an overall four-electron uptake and those corresponding to a six-electron transfer indicate that both overall processes

Contribution to the understanding of the reduction mechanism of nitrobenzene

Electrochemical reduction of nitrobenzenes has been studied extensively [1,2]. For the processes involved Haber [3] proposed a scheme which is generally accepted in a modified form [4,5]. Nitrobenzene was the first organic compound for which a current-voltage curve obtained with the dropping mercury electrode was reported [6]. At this electrode, where, because of the continuous surface renewal the short contact period excludes accumulation and side-reaction of products [7], the reduction occurs in a single four-electron step. In acidic media the monoand di-protonated forms of arylhydroxylamine [8-lo] formed in the first step are reduced at more negative potentials in a second, two-electron step. The overall four-electron process involves also a transfer of four protons (reaction 1): ArNO,+4e-+4H++ArNHOH+H,O (1) The sequence of transfer of protons and electrons cannot be deduced in aqueous media from the variation in the limiting current of the first step, as this is pH-independent. Mechanistic studies in non-aqueous media [11,12] do not offer information about the process in aqueous solutions because of the difference in solvation of all the species involved and in their acid-base properties. Similarly for mechanistic studies in the presence of surfactants [13-151 the role of micelles and other specific effects of adsorbed surfactants cannot be excluded. The first type of experimental evidence that can be used in the elucidation of the sequence of electron and proton transfer is the fact that the sequential reduction of the nitro group to a nitroso and a hydroxylamino group has never been reported to occur in two two-electron steps. The presence of a single four-electron wave

Electroreduction of Phenylhydrazones in Methanol–Water Mixtures

Abstract The electroreduction of a few substituted phenylhydrazones on mercury cathode has been studied in methanol–water mixtures using polarography, cyclic voltammetry, micro- and macrocontrolled-potential electrolysis. These compounds are reduced in a single four-electron step to yield the corresponding amino products. A linear relationship between the half-wave potentials (E1⁄2) and Hammett substituent constants (σ) has been observed.

Study of the electroreduction of dioxygen on thin films of platinum phthalocyanine in alkaline solutions

A detailed investigation of the electrochemical kinetics of dioxygen reduction on thin films of platinum phthalocyanine is reported. It has been found that the initial behaviour of the films is affected profoundly by the method of preparation, but that after potential cycling and/or lengthy exposure to air all the films investigated become highly active. Results from both RDE and RRDE studies show that the film reduces dioxygen by two parallel mechanisms, with the four-electron reduction process being dominant. A small amount of hydrogen peroxide could be detected, though the low activity of the film towards reduction of H 2 O 2 confirms that this is not an intermediate in the reduction of oxygen to water. The rate constants for the two- and four-electron pathways could be obtained by an analysis of the RRDE and RDE data. and the kinetic law for the four-electron step was derived. It takes the form: I k = 4 FAk 0 c O 2 ( c OH− ) m exp(−α FE/RT ) where, at low overpotentials, m ∼- 0.5 and α ∼- 1.5. whereas in the high overpotential region, m ∼- 0.15 and α∼-0.5.