Quasi-reversible two-electron reduction of oxygen at iodine submonolayer-coated gold electrode in alkaline media

Oxygen reduction reaction (ORR) was investigated using polycrystalline gold (Au (poly)) electrode modified with chemisorbed iodine (I_(ads)) submonolayer (sub I_(ads)) in O₂-saturated 0.1 M KOH solution. The sub I_(ads) was tailored by potential-dependent partial reductive desorption of I_(ads) from its full monolayer. The Au (1 1 1) facet of the Au (poly) electrode was considered to remain bared at the sub I_(ads)/Au (poly) electrode. The interesting finding of the present study is that (unlike the bare Au (poly) electrode) the sub I_(ads)/Au (poly) electrode exhibited a quasi-reversible two-electron reduction of O₂ in alkaline media. The probable origin of the observed quasi-reversible behavior of the ORR is discussed. Experimental investigations were performed using cyclic and steady-state voltammetric, amperometric and coulometric techniques.     

Short-time preparation and electrochemical properties of a single layer of tetraoctylammonium bromide capped Au nanoparticles on dithiol self-assembled monolayer-modified Au electrode

Short time immobilization of densely packed tetraoctylammonium bromide (TOAB) stabilized gold nanoparticles (AuNPs) were established on a Au electrode modified with a self-assembled monolayer (SAM) of 1,6-hexanedithiol (HDT) or 1,4-benzenedimethanethiol (BDMT). The quartz crystal microbalance experiment showed densely packed TOAB–AuNPs single layer formation on both SAMs was achieved within 20 min. AFM images demonstrated that the immobilized TOAB–AuNPs on the SAMs were densely packed and the AuNPs film thickness was 6–7 nm. The electronic communication between the immobilized AuNPs and the underlying bulk electrode was confirmed by cyclic voltammetry and electroreflectance spectroscopy. A reversible electron transfer reaction was observed for both [Fe(CN)₆]^4−/3− and [Ru(NH₃)₆]^2+/3+ at TOAB–AuNPs immobilized on HDT (Au/HDT/AuNPs) and BDMT (Au/BDMT/AuNPs) modified electrodes. The electroreflectance spectra show a red-shifted strong positive-going plasmon resonance bands at 551 nm and 584 nm, respectively, for the Au/BDMT/AuNPs and Au/HDT/AuNPs electrodes. The observed reversible redox response for the solution redox species and red-shifted plasmon resonance bands for the immobilized AuNPs again indicated that the AuNPs were immobilized on the SAMs in a densely packed manner. An advantage of TOAB–AuNPs modified electrode prepared by short time immersion over citrate-stabilized AuNPs modified electrode was demonstrated by the enhanced oxidation of ascorbic acid (AA) at these electrodes. The oxidation of AA was shifted to 90 mV less positive potential with higher oxidation current at TOAB–AuNPs modified electrode when compared to citrate-stabilized AuNPs modified electrode.     

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% HNO₃, 0.07 M H₃PO₄, 0.2 M KOH and 10% H₂O₂. The effect of treatment on the properties of the carbon support was studied by N₂ 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% HNO₃ and 10% H₂O₂ 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.     

Oxidation of sugar acids on polycrystalline platinum and gold electrodes modified with adsorbed bismuth oxide adlayers

Cyclic voltammetry and chronoamperometry have been employed for the preparation and characterization of noble metal electrode substrates modified with bismuth adlayers. The influence of bismuth ad-atoms on the electrocatalytic activity of the resulting platinum and gold modified electrode was investigated toward the electrooxidation of several sugar acid molecules in moderately alkaline medium. A direct comparative study regarding the catalytic activity between platinum and gold electrodes has been carried out in order to obtain useful information on the electrooxidation mechanism of sugar acids. It is found that different pre-adsorption processes of analyte on the platinum and gold electrodes were involved giving rise to different adsorption states in the oxide region and double region of potentials, respectively. The bismuth adsorbed species acts as a true catalyst on the direct electrooxidation pathway, providing the oxygen necessary for the oxidation of organic molecules adsorbed on the PtO_x or Au sites in the oxide and double region, respectively.     

Efficient mediatorless superoxide sensors using cytochrome c-modified electrodes: surface nano-organization for selectivity and controlled peroxidase activity

Amperometric superoxide anion sensor electrodes were prepared by immobilizing cytochrome c (Cyt c) on COOH-terminated Au–alkanethiolate monolayers. Monolayers and mixed-monolayers of 3-mercaptopropionic acid (MPA) with the coadsorbate, 3-mercaptopropanol (MP), were constructed on the surface of Au electrodes. The electrochemical characteristics and the superoxide anion sensor activities of cytochrome c immobilized on the monolayers and mixed-monolayers of MPA were found to depend largely on the structure of the underlying alkanethiolate monolayer. While cytochrome c on a MPA monolayer, Au/MPA/Cyt c, showed a reversible redox wave at 0.08 V (vs. Ag ∣ AgCl ∣ NaCl (sat.)), cytochrome c on the mixed-monolayer of MPA and MP, Au/MPA+MP/Cyt c, showed a wave at ?0.01 V. Cytochrome c immobilized on the Au–alkanethiolate layer was supposed to acquire different conformations at each of the Au/MPA/Cyt c and Au/MPA+MP/Cyt c electrodes, leading to a difference in the thermodynamic potential of cytochrome c. The apparent heterogeneous electron-transfer rate constant, k_het, is determined by fast-scan cyclic voltammetry to be (2.8±0.4)×10³ s^?1 for the Au/MPA+MP/Cyt c electrode. Both Au/MPA/Cyt c and Au/MPA+MP/Cyt c electrodes show anodic currents to O₂^? anion generated by enzymatic reaction with a response time of ～15 s, and the magnitude of the current response is higher at Au/MPA+MP/Cyt c relative to that at Au/MPA/Cyt c. While H₂O₂ did not give any current response at the Au/MPA electrode at the applied potential of 0.15 V, cytochrome c immobilized on the Au–alkanethiolate layers was found to reduce H₂O₂; the cathodic current recorded for the reduction of H₂O₂ is very much higher at Au/MPA/Cyt c than that at Au/MPA+MP/Cyt c at all the applied potentials. The O₂^? anion sensor activity of the two electrodes was compared with that of Au/HS(CH₂)₁₀COOH/Cyt c. Among the cytochrome c-modified electrodes tested, Au/MPA+MP/Cyt c showed a high anodic current for O₂^? anion and low interferences, 17 and 7%, due to the electrochemical interferents, H₂O₂ and uric acid, respectively.     

Mixed platinum–gold electrocatalysts for borohydride oxidation prepared by the galvanic replacement of nickel deposits

Mixed platinum/gold coatings were prepared by partial galvanic replacement of nickel layers that had been electrodeposited onto glassy carbon electrode substrates. The process (termed “transmetalation”) involves the spontaneous replacement of surface Ni atoms by Pt and Au upon immersion of the former metal into equimolar mixed chloroplatinic and chlorolaurate acid solutions. The multimetallic deposits on glassy carbon substrates, PtAu(Ni)/GC, were characterised by SEM/EDS, XRD, AES and electrochemical techniques. Smooth films with a bulk composition enriched in Au and minimum quantities of Ni left were obtained. Pt and Au were found to be alloyed and both Pt and Au to co-exist with Ni down to the core of the layers. The PtAu(Ni) deposits displayed typical Pt and Au surface electrochemistry indicating the formation of a continuous noble metal shell; the estimated electroactive surface areas point again to an excess of Au on the surface. The activity of the PtAu(Ni)/GC electrodes towards the oxidation of borohydride was studied by voltammetry at a rotating disc electrode, RDE, and compared to that of bulk Pt and Au. As expected, the Pt–Au alloy catalyst showed behaviour intermediate to that of its pure components. However, in the limiting current potential range the alloy tended more to bulk Au behaviour (number of electrons was closer to eight) while at low overpotentials it resembled the catalytic activity of pure Pt.     

Electrochemical response of α-H4SiW12O40 on Ag and Au electrodes

Electrochemical measurements are presented for acidic solutions containing α-H₄SiW₁₂O₄₀ adsorbed on Ag(111) and Au(111) electrodes. These measurements show that this molecule passivates the Ag surface towards solution redox events. This passivation is unique to Ag, as it is not observed on Au or carbon electrodes. Depassivation can be accomplished by moving the potential of the Ag electrode into the hydrogen evolution region. These results are discussed in terms of formation of a reduced AgSiW₁₂O₄₀ complex that is stable only at negative potentials.     

A strategy for immobilisation of carbon nanotubes homogenised in room temperature ionic liquids on carbon electrodes

Functionalised multi-walled carbon nanotubes (MWCNT) were homogenised in the room temperature ionic liquids (RTILs) 1-butyl-3-methylimidazolium bis(trifluoromethane)sulfonimide (BmimNTF₂), 1-butyl-1-methylpyrrolidinium bis(trifluoromethane)sulfonimide (BmpyNTF₂) and 1-butyl-3-methylimidazolium nitrate, and the composite obtained was applied on the top of glassy carbon or carbon film electrode substrates. The modified electrodes were characterised electrochemically in aqueous electrolytes and ionic liquids using cyclic voltammetry and electrochemical impedance spectroscopy and the model redox couples potassium hexacyanoferrate and ferrocene were used for electrode characterisation in aqueous and RTIL media, respectively. It was found that the combination of MWCNT with BmimNTF₂ gave the best composite which augurs well for application in sensors and biosensors.     

Electrochemical behavior of Pd–Pt–Au alloys

Pd–Pt–Au alloys were prepared as limited volume electrodes by metal codeposition at constant potential from chloride solutions. Alloy surface state was examined by scanning electron microscopy (SEM) and Auger electron spectroscopy (AES). The alloys were characterized electrochemically in acidic solution (0.5 M H₂SO₄) by cyclic voltammetry (CV). Potential regions of hydrogen electrosorption and surface oxidation are clearly distinguishable. The influence of electrode potential and alloy composition on the amount of absorbed hydrogen was investigated. Hydrogen absorption capacity of Pd–Pt–Au alloys increases with decreasing potential and the α–β phase transition is shifted positively with increasing Au content and negatively with increasing Pt content in the bulk. At relatively high potentials the amount of absorbed hydrogen exhibits a maximum for ca. 15–25 at.% Au in the bulk, while for low potentials a monotonic decrease in hydrogen absorption capacity with increasing bulk concentration of both alloying metals (Pt, Au) is observed.     

Nanoporous gold particles modified titanium electrode for hydrazine oxidation

A nanoporous gold particles modified titanium electrode (Au/Ti) was prepared by using a hydrothermal method. Gold nanoparticles were stably immobilized on the Ti surface from a mixture of aqueous HAuCl₄/polyethylene glycol (PEG) to form a nanoporous network texture. Electrocatalytic activity of the Au/Ti towards hydrazine oxidation in 1 M NaOH solution was assessed utilizing cyclic voltammetry (CV), linear scanning voltammetry (LSV) and electrochemical impedance spectroscopy (EIS). At the Au/Ti electrode, hydrazine oxidation in 1 M NaOH took place at a potential of −0.55 V (vs. Ag, AgCl) which was 0.53 V less than polycrystalline Au electrode. The Au/Ti electrode also presented much larger current density of hydrazine oxidation than Au electrode. Cyclic voltammetric responses of the Au/Ti electrode showed an irreversible electro-oxidation process of hydrazine. Linear plots of the reduction peak current density from the CVs of the Au/Ti vs. hydrazine concentration provided a potential detection of low concentration hydrazine. The kinetic parameters such as the number of electrons transferred in rate-determining step and total numbers of electrons involved in the hydrazine oxidation were determined using CVs and LSVs. CVs at the Au/Ti electrode also illustrated (relatively) weak interactions of hydrazine with electrode surface at all stages of the hydrazine oxidation process. Further, EIS data showed significantly high electrocatalytic activity of the Au/Ti electrode for hydrazine oxidation in alkaline solutions.     

Iron(III) octaethylporphyrin chloride supported on glassy carbon as an electrocatalyst for oxygen reduction

Oxygen reduction was investigated at iron(III) octaethylporphyrin chloride adsorbed on a glassy carbon electrode. The title porphyrin was adsorbed irreversibly and strongly on the surface of a glassy carbon electrode. The electrochemical behavior and stability of the modified electrode were investigated using cyclic voltammetry, chronoamperometry and rotating disk electrode methods. The modified electrode showed clear but modest electrocatalytic activity for the reduction of oxygen to a mixture of water and hydrogen peroxide in buffered solutions on both the acid and basic sides of neutral with the domination of an overpotential of about 690 mV and an increase in peak current. The heterogeneous rate constant for the reduction of O₂ at the surface of the modified electrode and the diffusion coefficient of oxygen were determined by rotation disk electrode voltammetry using the Koutecký–Levich plots. In addition, iron(III) octaethylporphyrin chloride exhibited strong catalytic activity toward the reduction of H₂O₂.     

Self-assembly of (3-mercaptopropyl)trimethoxysilane on iodine coated gold electrodes

The adsorption of (3-mercaptopropyl)trimethoxysilane (MPS) has been studied on iodine coated gold electrodes. The MPS adsorption from alcoholic solution on Au(111) and iodine coated Au(111) was studied by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The electrochemical formation of MPS monolayers was studied by cyclic voltammetry on polycrystalline uncoated and coated gold electrodes with different MPS pre-treatment conditions. Lead electrochemical deposition was used to probe the defect sites of the surfaces created. The MPS-over-iodine coated gold surface produces a lower-density monolayer than the MPS over pure gold. The MPS monolayer formed electrochemically on the iodine coated gold is chemically equal to its counterpart after the iodine desorption. The MPS adsorption occurs via an AuS bond, after the partial reductive-desorption of the iodine monolayer from the iodine coated gold electrode, and produces an ordered composite monolayer of MPS/iodine. The size of the defects can be controlled by varying the electrochemical preparation conditions, using the following reaction: AuI_(ads)+MPSH_(ac)+e⁻↔AuMPS_(ads)+H_(ac)⁺+I_(ac)⁻.     

Effects of the molecular structure of fluorinated additives on the kinetics of cathodic oxygen reduction

The kinetics of oxygen reduction at a gold electrode was studied in 0.5 M sulfuric acid, in which different kinds of straight-chain [CF₃(CF₂)₂CH₂OH, CF₃CF₂CH₂OH, and CF₃CH₂OH] and branched [(CF₃)₂CHOH] fluorinated alcohols were added. The adsorbed layers of the fluorinated alcohols were used as models of the fluorocarbon phase of the perfluorinated polymer electrolyte in gas-diffusion electrodes in proton-exchange membrane fuel cells. A rotating ring-disk electrode was used to determine kinetic parameters for O₂ reduction and to detect intermediate H₂O₂ formation. The kinetics of oxygen reduction were strongly dependent on the molecular structure of fluorinated additives. The addition of the straight-chain fluorinated alcohols enhanced the kinetic current density while addition of the branched alcohol did not. The linear C₃ fluorinated alcohol, CF₃CF₂CH₂OH, gave the maximum enhancement effect. Oxygen is reduced predominantly via the two-electron series path in the range of 0.4 to 0.0 V at Au, on which no effect of fluorinated additives was observed. The rate constant for intermediate H₂O₂ reduction, k₃, was negligible in the range 0.40–0.25 V, whereas it increased with decreasing E_D in the range 0.25–0.0 V. In the lower potential range, k₃ decreased with an increase in the concentration of fluorinated alcohol and this decreasing tendency was greatly dependent on the molecular structure of the fluorinated alcohol.     

Electrocatalytic oxidation and reduction of H2O2 on vertically aligned Co3O4 nanowalls electrode: Toward H2O2 detection

Single crystal and vertically aligned cobalt oxide (Co₃O₄) nanowalls were synthesized by directly heating Co foil on a hot-plate under ambient conditions. The vertically aligned Co₃O₄ nanowalls grown on the plate show excellent mechanical property and were facilely attached to the surface of a glassy carbon (GC) electrode using conductive silver paint. The prepared Co₃O₄ nanowalls electrode was then applied to study the electrocatalytic oxidation and reduction of hydrogen peroxide (H₂O₂) in 0.01 M pH 7.4 phosphate buffer medium. Upon the addition of H₂O₂, the Co₃O₄ nanowalls electrode exhibits significant oxidation and reduction of H₂O₂ starting around +0.25 V (vs. Ag/AgCl), while no obvious redox activity is observed at a bare GC electrode over most of the potential range. The superior electrocatalytic response to H₂O₂ is mainly attributed to the large surface area, minimized diffusion resistance, high surface energy, and enhanced electron transfer of the as-synthesized Co₃O₄ nanowalls. The same Co₃O₄ nanowalls electrode was also applied for the amperometric detection of H₂O₂ and showed a fast response and high sensitivity at applied potentials of +0.8 V and −0.2 V (vs. Ag/AgCl), respectively. The results also demonstrate that Co₃O₄ nanowalls have great potential in sensor and biosensor applications.     

Electrodeposited indigotetrasulfonate film onto glutaraldehyde-cross-linked poly-l-lysine modified glassy carbon electrode for detection of dissolved oxygen

The nano composited film of indigotetrasulfonate (ITS) electrodeposited onto poly-l-lysine (PLL)–glutaraldehyde (GA) (ITS/PLL–GA) was modified on glassy carbon electrode (GCE) by multiple scan cyclic voltammetry. Composited of the proposed film was characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), electrochemical quartz crystal microbalance (EQCM), electrochemical impedance spectroscopy (EIS), and UV–vis spectrum for the absorption at λ_max at 566 nm. For the electrocatalytic reduction of dissolved oxygen, ITS/PLL–GA film modified electrodes was determined in 0.1 M acetate buffer solution (pH 5.6) by cyclic voltammetry and rotating disk electrode voltammetry. This dissolved oxygen electrochemical sensor exhibited a linear response range (from 0 to 178.4 μM, R² = 0.9949), lowest detection limit (2.2 μM), lowest overpotential at −0.09 V, high sensitivity (906 μA mM⁻¹) and relative standard deviation (RSD) for determining dissolved oxygen (n = 3) was 4.2%. In addition, the ITS/PLL–GA/GCE was advantageous in terms of its simple preparation, specificity, stability and the ability of regeneration.     

Bismuth UPD on the modified Au(1 1 1) electrode

The bismuth UPD was studied on Au(1 1 1) electrodes modified by silver intermediates and adsorbed thymine. Cyclic voltammetry, potential step experiments and X-ray photoelectron spectroscopy were used for investigation. The results were compared to the results of the analogous experiments performed on bare Au(1 1 1) and on bulk silver. In the first part it is shown that the Bi UPD on Ag_ML/Au(1 1 1) differs completely from the UPD on Au(1 1 1) as well as on bulk Ag due to changed electronic properties of the substrate. The 2nd part focuses on the Bi UPD in presence of the nucleobase thymine. Thymine undergoes a reorientation from the chemisorbed state to the physisorbed state on Au(1 1 1) at potentials where the Bi UPD takes place. During the UPD adsorbed thymine is replaced by Bi species on the electrode surface. It does not re-adsorb on top of the deposited Bi layer. An energetic advantage due to re-adsorption on topmost layer as described for the Cu UPD on thymine modified Au(1 1 1) does not occur.     

Direct electron transfer in the system gold electrode–recombinant horseradish peroxidases

The kinetics of the bioelectrocatalytic reduction of hydrogen peroxide has been studied at gold electrodes modified with different forms of horseradish peroxidase (HRP). Native HRP, wild type recombinant HRP (rec-HRP) and its two mutant forms containing a six-histidine tag at the C- or N-terminus, C_Hisrec-HRP and N_Hisrec-HRP, respectively, have been used for an adsorptive modification of the gold electrodes. The histidine sequences, i.e. histidine tags, were introduced into the peroxidase structure by genetic engineering of non-glycosylated rec-HRP using an Escherichia coli expression system. Experiments with a gold rotating disc electrode demonstrated that electrodes with the adsorbed rec-HRP forms exhibited high and stable current response to H₂O₂ due to its bioelectrocatalytic reduction based on direct (mediatorless) ET between gold and the active site of HRP. The heterogeneous ET rate constants were evaluated to be in the order of 20 or 33 s^?1 between rec-HRP or its histidine mutants and gold, respectively, in 0.01 M phosphate buffer (pH 7.4) containing 0.15 M NaCl. The increase in the heterogeneous ET rate found for C_Hisrec-HRP and N_Hisrec-HRP is probably due to the interaction of the histidine tag with the electrode surface. The kinetic data demonstrate that new possibilities for enhancing the catalytic activity of the enzyme at the electrode ∣ solution interface can be achieved by genetic engineering design of the enzyme molecules.     

Oxygen reduction on copper in neutral NaCl solution

The reduction of oxygen on copper in neutral unbuffered 1 mol dm^?3 NaCl has been studied using rotating ring-disc electrodes at six oxygen concentrations equivalent to atmospheres of 2% O₂ + N₂ to 100% O₂. Steady-state potentiostatic measurements show that the reaction is first order with respect to [O₂] and that, following adsorption of O₂, the first electron transfer is rate determining. In 50% O₂ + N₂ and 100% O₂, a cathodic oxygen reduction peak is observed in both potentiodynamic and potentiostatic experiments at a disc potential of ?0.3 to ?0.4 V/SCE. The reaction is dominated by the overall four-electron reduction to OH^?, with only small amounts of peroxide detected by the ring electrode at disc potentials corresponding to the formation of the cathodic oxygen reduction peak. Tafel slopes increase with [O₂] and vary from ?0.13₅ V in 2% O₂ + N₂ to a limiting value of ?0.16 V to ?0.18 V in air, 50% O₂ + N₂ and 100% O₂.The results are explained by a mechanism involving oxygen reduction on two types of surface site with different reactivities. The most catalytic surface is believed to comprise Cu(0) and Cu(I) sites, where the Cu(I) species is stabilized as Cu(OH)_ads and/or submonolayer Cu₂O. The less catalytic site consists of Cu(0) only. Oxygen reduction is believed to proceed by a series pathway involving an adsorbed peroxide intermediate on both sites. Peroxide is reduced to OH^? prior to desorption at Cu(0) sites, but some is released before being reduced at Cu(0)/Cu(I) sites. Surface coverage by catalytic Cu(0)/Cu(I) species is favoured by a higher interfacial pH and more positive disc potentials.     

Electrochemical reduction of (1R,2r,3S,4R,5r,6S)-hexachlorocyclohexane (Lindane) at carbon cathodes in dimethylformamide

Direct reduction of Lindane (1R,2r,3S,4R,5r,6S-hexachlorocyclohexane, 1) at carbon cathodes in dimethylformamide (DMF) containing 0.10 M tetra-n-butylammonium tetrafluoroborate (TBABF₄) has been explored by means of cyclic voltammetry and controlled-potential (bulk) electrolysis. Cyclic voltammograms for reduction of 1 at a glassy carbon electrode exhibit two cathodic peaks at −1.40 V and −2.10 V as well as an anodic peak at −1.93 V; the first cathodic peak is attributed to reduction of 1 itself, whereas the second cathodic peak is due to reduction of chlorobenzene that is derived from 1. Controlled-potential (bulk) electrolyses conducted with reticulated vitreous carbon electrodes held at −1.70 or −2.20 V reveal that reduction of 1 is essentially a six-electron process that affords benzene as major product (80–100% yield) along with small amounts of chlorobenzene (3–10% yield). To account for these products, a mechanism is proposed that is supported by the results of theoretical computations based on density functional theory.     

Effect of pH and temperature on carbon-supported manganese oxide oxygen reduction electrocatalysts

Manganese oxides nanoparticles were chemically deposited on a high area (ca. 300 m² g⁻¹) carbon black substrate to act as electrocatalysts for oxygen reduction. The morphology and chemistry of the carbon-supported MnO_x nanoparticles was characterised by Transmission Electron Microscopy), X-ray Diffraction, and chemical analysis. The oxygen reduction reaction (ORR) catalytic activity was studied in the 7–10 pH range using a rotating disk electrode (RDE). High activity towards oxygen reduction and very good stability in neutral and slightly basic solution were obtained. At low current densities, at 25 °C, MnO_x/C displayed a reaction order with respect to OH⁻ ions of −0.5 and Tafel slopes of −0.153 and −0.167 V dec⁻¹ at pH 7 and 10 respectively; showing that the ORR mechanism on MnO_x/C is unchanged in the 7–10 pH range. From the data, we propose that the first electrochemical step of the 4-electron ORR mechanism, in the 7–10 pH range, is the quasi equilibrium proton insertion process in MnO₂ yielding MnOOH (insoluble in neutral or slightly basic solution). The ORR activity of the MnO_x/C materials increased with increasing temperatures from 5 to 40 °C. The 2-electron pathway of oxygen reduction, yielding hydrogen peroxides as intermediates, may however be favoured over the 4-electron O₂ reduction at higher temperatures.