Electrocatalytic Reduction Reaction Research Articles

Lean Cu-immobilized Pt and Pd films/–H+ Conducting Membrane Assemblies: Relative Electrocatalytic Nitrate Reduction Activities

Asymmetric reactors having the configuration (Pt|Nafion|M–Cu, M=Pt, Pd) were designed and applied to the electrocatalytic nitrate reduction reaction (NRR) with no supporting electrolyte. The electrochemically–fabricated Cu–M (M=Pt, Pd) catalysts showed a highly-efficient catalytic activity by effectively reducing more than ca. 99% of nitrate ions, under static-reactor conditions, with first-order rate constants of ca. 0.040min−1 and ca. 0.057min−1 for Cu–Pt and Cu–Pd, respectively. Similar reactivity trends were observed under flow mode irrespective of flow rates while NRR rates were predominantly controlled by diffusion-limited mass-transfer. A maximum of ca. 93% nitrate removal at Cu–Pd catalyst was achieved within ca. 60min. The comparatively lower CuO content of ca. 9%, as confirmed by XANES, resulted in a greater number of free NRR active sites inducing the superior catalytic activity of the Cu–Pd catalyst. On the other hand, NRR product selectivity investigations showed that Cu–Pd catalyst was highly ammonia–selective with a maximum of ca. 24% selectivity at a flow rate of 0.1mLmin−1, while Cu–Pt catalyst was much selective to intermediate nitrite ions reaching a maximum of ca. 35% selectivity at the same flow rate.

Influence of electrode assembly on catalytic activation and deactivation of a Pt film immobilized H+ conducting solid electrolyte in electrocatalytic reduction reactions

Symmetric assembly deactivated the electrocatalytic nitrate reduction process due to oxidation of copper atoms on the anode surface. In case of assymetric assembly, H2 evolution eroded copper oxides causing catalytic activation.

Cobalt oxide nanoparticles anchored to multiwalled carbon nanotubes: Synthesis and application for enhanced electrocatalytic reaction and highly sensitive nonenzymatic detection of hydrogen peroxide

Cobalt (II) dicobalt (III) oxide nanoparticles anchored to multiwalled carbon nanotubes were synthesized for the first time by a microwave decomposition method using cobalt nitrate as a precursor and multiwalled carbon nanotubes as a scaffold. The nanostructure was then employed as the modifier of a carbon paste electrode. The kinetics of the charge transfer process across the modified electrode/solution interface was studied. The modified electrode was then applied to fabricate an enzyme-less hydrogen peroxide biosensor. The mechanism and kinetics of the electrocatalytic reduction and oxidation reactions of hydrogen peroxide on the modified electrode surface were studied by cyclic voltammetry and chronoamperometry. The catalytic rate constants of the hydrogen peroxide electroreduction and electrooxidation processes by an active cobalt species, and the diffusion coefficient of hydrogen peroxide were reported. An amperometric method was developed for determination of hydrogen peroxide based on its electrocatalytic reduction with a sensitivity of 1002.8mAmol−1 dm3cm−2 and a limit of detection of 2.46μmol dm−3. The sensor had the advantages of high electrocatalytic activity and sensitivity, with a simple fabrication method without complications of immobilization steps and using any enzyme or reagent.

Selective Electrocatalytic Activity of Ligand Stabilized Copper Oxide Nanoparticles

Ligand stabilization can influence the surface chemistry of Cu oxide nanoparticles (NPs) and provide unique product distributions for electrocatalytic methanol (MeOH) oxidation and CO2 reduction reactions. Oleic acid (OA) stabilized Cu2O and CuO NPs promote the MeOH oxidation reaction with 88% and 99.97% selective HCOH formation, respectively. Alternatively, CO2 is the only reaction product detected for bulk Cu oxides and Cu oxide NPs with no ligands or weakly interacting ligands. We also demonstrate that OA stabilized Cu oxide NPs can reduce CO2 into CO with a ∼1.7-fold increase in CO/H2 production ratios compared to bulk Cu oxides. The OA stabilized Cu oxide NPs also show 7.6 and 9.1-fold increases in CO/H2 production ratios compared to weakly stabilized and nonstabilized Cu oxide NPs, respectively. Our data illustrates that the presence and type of surface ligand can substantially influence the catalytic product selectivity of Cu oxide NPs.

The Role of Reactive Reaction Intermediates in Two‐Step Heterogeneous Electrocatalytic Reactions: A Model Study

AbstractThis paper summarises the result of previous experimental investigations of heterogeneous electrocatalytic reactions performed in flow cells which provide an environment with controlled parameters. Measurements of the oxygen reduction reaction in a flow cell with an electrode consisting of an array of Pt nanodisks on a glassy carbon substrate exhibited a decreasing fraction of the intermediate H2O2 in the overall reaction products with increasing density of the nanodisks. A similar result is true for the dependence on the catalyst loading in the case of a supported Pt/C catalyst thin‐film electrode, where the fraction of the intermediate decreases with increasing catalyst loading. Similar effects have been detected for the methanol oxidation.In order to give a possible explanation to the observed effect, we present a model of multistep heterogeneous electrocatalytic oxidation and reduction reactions based on an adsorption‐reaction‐desorption scheme using the Langmuir assumption and macroscopic transport equations. A continuum based model problem in a vertical cross‐section of a rectangular flow cell is proposed in order to explain basic principles of the experimental situation. It includes three model species A, B, C, which undergo adsorption and desorption at a catalyst surface, as well as adsorbate reactions from A to B to C. These surface reactions are coupled with diffusion and advection in the Hagen Poiseuille flow in the flow chamber of the cell. High velocity asymptotic theory and a finite volume numerical method are used to obtain approximate solutions to the model. Both approaches show a behaviour similar to the experimentally observed. Working in more general situations, the finite volume scheme was applied to a catalyst layer consisting of a number of small catalytically active areas corresponding to nanodisks. Good qualitative agreement with the experimental findings is established for this case as well.

Electrocatalytic reactions of dioxygen and nitric oxide with reduced (nitrosyl) cobalt porphyrins — cyclic voltammetry and computational chemistry

Water-soluble cationic cobalt(tetra(N-methyl-pyridyl)porphyrins, CoTMpyP(4) and CoTMpyP(2) and the β-pyrrole octabrominated derivative CoTMpyP(4)Br8 have been studied by cyclic voltammetry under aqueous aerobic conditions at pH of 4.0 with and without addition of nitrite to explore their ability to catalyze the reduction of dioxygen. The porphyrins and their nitro/nitrosyl derivatives were studied in aqueous acetate buffer solution at a solid silver electrode and then also as they were immobilized in Nafion® film loaded with silver nanoparticles which provided electrical conductivity. Under these reducing conditions with nitrite in solution, coordinated nitrite appears to undergo an oxo-transfer/reduction reaction resulting in the formation of the nitrosyl ligand. The reaction of the nitrosyl complex allows a short-lived electrocatalytic reduction reaction with dioxygen before NO is lost from the complex by dissociation. The thermodynamics of the reactions and possible catalytic intermediates in the reduction of dioxygen were studied by DFT (BP/6-31G*) computations. An unusual N -bound cyclic NO3- structure was obtained in the optimized geometries for the product of reduced nitrosyl porphyrins and dioxygen in the model complexes and for [CoTMpyP(2)(NO-O2)]- . These structures are tentatively proposed to represent an intermediate in the mechanism of activation of dioxygen in catalytic reduction. DFT (BP/6-31G*) computations were also applied to probe thermodynamics and intermediates in the known catalytic reduction of NO to N2O by the reduced porphyrin under anaerobic conditions. Thermodynamics estimates suggest that reduction occurs through coordinated NO in the reduced porphyrin species similar to cytochrome P450nor (nitric oxide reductase), but the detailed mechanism is not clear.

Iodide Modified Electrodes for the Electrochemical Detection of Environmentally Hazardous Materials

Electrochemical deposition and chemisorption studies of iodide film have been done on the glassy carbon electrode (GCE) and polycrystalline gold electrode (poly GE). The iodide film preparation under different conditions and its stability were examined. Further, the redox activity of the iodide modified film on GCE and poly GE was examined in different pH solutions. Here, the electrochemically deposited iodide film on GCE showed good stability and was electrochemically active at pH 1.5 and 4 solutions, while the chemisorbed iodide film on poly GE was found active and stable at pH 9 and 13 solutions. The iodide modified GCEs exhibits a good electrocatalytic activity for the oxidation of As (III) and the reduction of Cr (VI), respectively. A detailed study for As (III) oxidation has been done by using the rotating ring-disk electrode technique. Meanwhile, chemisorbed iodide film on poly GE shows electrocatalytic reduction reactions of by using different techniques like cyclic voltammetry and differential pulse voltammetry. To examine the practical analytical utility of the proposed electrode, the real sample analyses for arsenic and chromate detection have been examined and the results were found satisfactory.

Bioelectrocatalytic application of titania nanotube array for molecule detection

A bioelectrocatalysis system based on titania nanotube electrode has been developed for the quantitative detection application. Highly ordered titania nanotube array with inner diameter of 60nm and total length of 540nm was formed by anodizing titanium foils. The functionalization modification was achieved by embedding glucose oxidases inside tubule channels and electropolymerizing pyrrole for interfacial immobilization. Morphology and microstructure characterization, electrochemical properties and bioelectrocatalytic reactivities of this composite were fully investigated. The direct detection of hydrogen peroxide by electrocatalytic reduction reaction was fulfilled on pure titania nanotube array with a detection limit up to 2.0×10−4mM. A biosensor based on the glucose oxidase–titania/titanium electrode was constructed for amperometric detection and quantitative determination of glucose in a phosphate buffer solution (pH 6.8) under a potentiostatic condition (−0.4V versus SCE). The resulting glucose biosensor showed an excellent performance with a response time below 5.6s and a detection limit of 2.0×10−3mM. The corresponding detection sensitivity was 45.5μAmM−1cm−2. A good operational reliability was also achieved with relative standard deviations below 3.0%. This novel biosensor exhibited quite high response sensitivity and low detection limit for potential applications.

Electron Transfer Behavior of Multi‐Iron Sandwich‐Type Polyoxometalates and Electrocatalytic Reduction Reactions

AbstractThe physicochemical and electrocatalytic behaviors of eight multi‐iron Wells−Dawson sandwich‐type polyoxometalates were studied with specific emphasis on the FeIII centers. Magnetization measurements were used to identify and quantify the antiferromagnetic interactions between the edge‐sharing FeIII units. Electrochemical studies show the stepwise reduction of the FeIII centers, in complete agreement with magnetization conclusions. The location of the potential of each wave depends on the pH of the solution, as well as the concentration and composition of the electrolyte. In addition, ion‐pairing studies show there is a positive shift of the FeIII centers with an increase in ion pairing (i.e. K+ > Na+ > Li+). The electrocatalytic reduction of dioxygen and hydrogen peroxide is efficient for all the complexes. However, there is a pronounced increase in efficiency as the number of FeIII centers in the complex increases (i.e. 4 FeIII > 3 FeIII > 2 FeIII). The mixed‐metal complexes [αββα‐Na14(MnIIOH2)2(FeIII)2(As2W15O56)2 and αββα‐Na14(MnIIOH2)2(FeIII)2(P2W15O56)2] are also efficient catalysts for the reduction of NO and HNO2. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Voltammetric and amperometric studies of selected thiols and dimethyldisulfide using a screen‐printed carbon electrode modified with cobalt phthalocyanine: Studies towards a gas sensor

AbstractCyclic voltammetry was used to investigate the electrochemical behavior of hydrogen sulfide, methanethiol, ethanethiol and dimethyl‐disulfide at a cobalt phthalocyanine modified screen‐printed carbon electrode (SPE) in phosphate electrolytes of different pH values. The thiols and hydrogen sulfide were found to undergo electrocatalytic oxidation while dimethyldisulfide exhibited an electrocatalytic reduction reaction. These processes were considered to involve the central cobalt(II) ion in the phthalocyanine macrocycle. The possibility of monitoring the thiols by exploiting the electrocatalytic oxidation process and DMDS by the electrocatalytic reduction process was examined by amperometry. In one approach a bulk phosphate electrolyte, pH 12.0, was employed. The amperometric behavior of the low boiling liquids ethanethiol and DMDS were studied by making standard additions to a stirred solution; both species showed rapid response times at applied potentials of +0.15 V and −0.40 V, respectively. The gaseous thiols, hydrogen sulfide and methanethiol, were examined by bubbling these through the same bulk electrolyte and again rapid, reproducible and reversible signals were obtained. In another approach a gel was prepared by adding agarose to the same phosphate electrolyte and a thin layer was deposited over the working and counter/reference electrodes of a screen‐printed strip. This sensor showed favorable electrochemical characteristics for the gaseous species examined which suggested that the device had promise as an amperometric gas sensor.