522—Catalytic oxydation of biological components on platinum electrodes modified by adsorbed metals: Anodic oxidation of glucose

The catalytic oxidation of glucose on Pt electrodes modified by adsorbed metals was studied in 1 M HClO₄ by linear sweep voltammetry. The adsorbed metals (denoted as M_ad, such as Bi_ad and Pb_ad) formed on Pt in the potential region more positive than the reversible potential of an M⁼⁺/M^o couple, lead to a marked increase in the anodic c?urrent of glucose by about one order of magnitude. The catalytic activity depends on the surface coverage by the M_ad. The strongly adsorbed species of lactone type, which are responsible for blocking the successive oxidation, are formed on the electrode surface in the anodic processes of glucose on a bare Pt electrode. The formation of such poisonous species is accelerated in the presence of adsorbed hydrogen on Pt. The effects of M_ad were discussed on the basis that M_ad plays its major role on the Pt electrode surface in removal of the adsorbed hydrogen which initiates the formation of the poisonous species.     

Catalytic oxidation of biological components on platinum electrodes modified by adsorbed metals: Anodic oxidation of catecholamines

The catalytic oxidation of catecholamines on Pt electrodes modified by adsorbed metals (denoted as M_ad) was studied in 1 M HClO₄) by linear sweep voltammetry. The anodic peaks of four catecholamines (adrenaline, noradrenaline, iso proterenol and DOPA) shift to the negative potential side in the presence of M_ad, such as Bi_ad and Pb_ad. The anodic oxidation of catecholamines proceeds without the production of poisonous species on a bare Pt electrode. The catalytic activity depends on the surface coverage by M_ad (denoted as θ_M). The maximal effect of M_ad is attained at θ_M = 0.5. The presence of M_ad promotes the formation of the final product through irreversible hydroxylation following the electron transfer. From these results it was suggested that in the catalytic processes M_ad plays the major role in the provision of effective sites to activate water molecules which take part in the subsequent hydroxylation step.     

Effect of Non‐Specifically Adsorbed Ions on the Surface Oxidation of Pt(111)

The oxidation processes of a Pt(111) electrode in alkaline electrolytes depend on non‐specifically adsorbed ions according to in situ X‐ray diffraction and infrared spectroscopic measurements. In an aqueous solution of LiOH, an OH_ad adlayer is formed in the first oxidation step of the Pt(111) electrode as a result of the strong interaction between Li⁺ and OH_ad, whereas Pt oxidation proceeds without OH_ad formation in CsOH solution. Structural analysis by X‐ray diffraction indicates that Li⁺ is strongly protective against surface roughening caused by subsurface oxidation. Although Cs⁺ is situated near the Pt surface, the weak protective effect of Cs⁺ results in irreversible surface roughening due to subsurface oxidation.     

Characterization and application of an electrode modified by mechanically immobilized copper hexacyanoferrate

A newly modified electrode was prepared by mechanical immobilization of copper hexacyanoferrate (CuHCF) on a graphite electrode. The modified electrode was characterized by cyclic voltammetric experiments. The effect of different background electrolytes, pHs and scan rates on the electrochemical behaviour of the electrode has been evaluated. In NH₄Cl two reversible redox peaks were observed. The first redox peak corresponding to Cu⁺/Cu²⁺ is observed only in this medium. The second redox peak corresponds to the Fe(CN)₆ ^4–/Fe(CN)₆ ^3– couple. Both anodic peaks were used for catalytic oxidation of ascorbic acid. As the anodic current for catalytic oxidation was proportional to the amount of ascorbic acid, an analytical method was developed for the determination of ascorbic acid in commercial samples.     

Characterization and application of an electrode modified by mechanically immobilized copper hexacyanoferrate

A newly modified electrode was prepared by mechanical immobilization of copper hexacyanoferrate (CuHCF) on a graphite electrode. The modified electrode was characterized by cyclic voltammetric experiments. The effect of different background electrolytes, pHs and scan rates on the electrochemical behaviour of the electrode has been evaluated. In NH₄Cl two reversible redox peaks were observed. The first redox peak corresponding to Cu⁺/Cu²⁺ is observed only in this medium. The second redox peak corresponds to the Fe(CN)₆ ^4–/Fe(CN)₆ ^3– couple. Both anodic peaks were used for catalytic oxidation of ascorbic acid. As the anodic current for catalytic oxidation was proportional to the amount of ascorbic acid, an analytical method was developed for the determination of ascorbic acid in commercial samples. Received: 26 May 1998 / Revised: 15 March 1999 / Accepted: 20 March 1999     

Spontaneous Bi-modification of polycrystalline Pt electrode: fabrication, characterization, and performance in formic acid electrooxidation

Spontaneous modification of polycrystalline Pt by irreversibly adsorbed bismuth was performed in BiCl₃ solution in concentrated hydrochloric acid under open-circuit conditions. After spontaneous modification, followed by extensive rinsing with water and drying, the surface was characterized using X-ray photoelectron spectroscopy and electrochemistry. Bi-oxy(chloride), oxide species, and metallic Bi were found at a submonolayer coverage on the Pt surface after spontaneous modification. The electrochemical response of Bi-modified polycrystalline Pt electrode in sulfuric acid solution exhibits a complex multi-peak feature, which is resulting in about constant redox charge (Bi species coverage) in the potential region from 0 to 0.9 V (vs. a standard hydrogen electrode). The spontaneously Bi-modified Pt catalyst in model studies exhibits a superior activity towards formic acid oxidation at fuel cell anode relevant potentials. The catalytic effect of bismuth oxy-species is explained in terms of both inhibition of CO_ad formation and oxidation of CO_ad in reaction with Bi-oxy-species.     

Meldola's Blue Immobilized on a SiO2/SnO2/Phosphate Xerogel,a New Sensor for Determination of Ascorbic Acid in Medicine and Commercial Fruit Juice

A modified carbon paste electrode with SiO₂/SnO₂/Phosphate/Meldola's blue, SSPMelB, was used to study the electrocatalytic oxidation of ascorbic acid by cyclic voltammetry and chronoamperometry. The adsorbed dye mediates ascorbic acid oxidation at an anodic potential of 0.04 V vs. saturated calomel electrode (SCE) at pH 7.0, in 0.5 mol L^?1 solution. The linear range of the sensor is between 4.0×10^?7 and 2.0×10^?3 mol L^?1, with a limit of detection of 4.0×10^?7 mol L^?1. This novel electrode shows good analytical performance for determination of ascorbic acid in medicine and commercial fruit juice.     

Formic acid oxidation at platinized platinum electrodes: Part II. Effect of temperature on steady state oxidation process

A study has been made of the effect of temperature on the steady state oxidation of formic acid on platinized platinum electrodes in 0.5 M HCOOH+0.5 M H₂SO₄. The activation energy and entropy of activation for the anodic oxidation process decrease sharply with increase in electrode potential. The results provide additional evidence that the weakly adsorbed intermediate pathway proceeds through a rate-determining step involving the dissociation of weakly adsorbed formic acid molecule.     

The formation and behavior of Ag adsorbates at Pd electrodes and their influence on electrocatalytic oxidation of formic acid at Pd

The electrochemical behavior of Ag ions at smooth Pd electrodes and Pd/Pt deposits is investigated. At adsorption potentials, which are more anodic than the reversible potential of the Ag⁺/Ag electrode, an underpotential deposition of Ag⁺ ions occur.We can distinguished two types of Ag ad-atoms at Pd. The first type of Ag is irreversibly adsorbed. The second, which exists only in the presence of Ag ions in the solution, is reversibly adsorbed.The influenced of various coverages of Ag ad-atoms at smooth Pd and Pd/Pt deposits on the electrocatalytical oxidation of formic acid was investigated. Even small coverages of Ag ad-atoms lower the rate of formic acid oxidation. With higher coverages this inhibiting influence grows continously.     

Structure-relationships in electrocatalysis: oxygen reduction and hydrogen oxidation reactions on Pt(111) and Pt(100) in solutions containing chloride ions

Kinetics of the oxygen reduction reaction (orr) and the hydrogen evolution–oxidation reactions (her/hor) were studied on the Pt(111) and Pt(100) surfaces in 0.05 M H₂SO₄ containing Cl⁻. The orr is strongly inhibited on the (100) surface modified by adsorbed Cl⁻ (Cl_ad), and it occurs as a 3.5e⁻ reduction via solution phase peroxide formation. In the hydrogen adsorption (H_upd) potential region, the orr is even more inhibited, and corresponds only to a 2 e⁻ reduction at the negative potential limit where the electrode is covered by one monolayer of H_upd and some (unknown) amount of Cl_ad. On the Pt(111)---Cl_ad surface, the orr is inhibited relatively little (in addition to that caused by strong bisulfate anion adsorption on this surface), and the reaction pathway is the same as in Cl⁻ free solution. The kinetics of the hor on Pt(111) are the same in pure solution and in a solution containing Cl⁻, since Cl_ad does not affect platinum sites required for the breaking of the H---H bond. A relatively large inhibition of the hor is observed on the (100) surface, implying that strongly adsorbed Cl_ad is present on the surface even near 0 V.     

抗坏血酸在PPy-HEImTfa/Pt电极上的电催化氧化及其机理

以铂为基底电极,在1-乙基咪唑三氟乙酸盐(HEImTfa)离子液体中电化学合成导电聚吡咯(PPy),制得PPy-HEImTfa/Pt电极;采用循环伏安法研究了PPy-HEImTfa/Pt电极对抗坏血酸的电催化氧化性能.结果表明:PPy-HEImTfa/Pt电极对0.1mo·lL-1抗坏血酸具有较高的电催化氧化活性,与相同条件下硫酸溶液中在铂表面修饰的聚吡咯(PPy-H2SO4/Pt)电极和裸铂电极相比,其氧化峰电位分别降低了0.10和0.19V,氧化峰电流分别增加了3.0和3.6mA.同时采用原位傅里叶变换红外(insitu FTIR)光谱技术对抗坏血酸在PPy-HEImTfa/Pt电极上的电氧化机理进行了研究,结果表明:抗坏血酸在PPy-HEImTfa/Pt电极上首先被氧化为脱氢抗坏血酸,在水溶液中脱氢抗坏血酸迅速发生水合作用形成水合脱氢抗坏血酸,它进一步水解并发生内酯开环反应生成2,3-二酮古洛糖酸;在较高电位下,部分抗坏血酸最终被氧化成CO2.     

In situ FTIRS studies of kinetics of HCOOH oxidation on Pt(110) electrode modified with antimony adatoms

The electrochemical behavior of irreversibly adsorbed antimony on a Pt(110) electrode (Pt(110)/Sb) with various coverages was studied using cyclic voltammetry. The kinetics of HCOOH oxidation via reactive intermediates on Pt(110)/Sb were investigated quantitatively by employing the potential step technique and in situ FTIR spectroscopy. The results demonstrated that Sb adatoms were stable on Pt(110) when the electrode potential was below 0.45 V (SCE). It has been revealed that the dissociative adsorption of formic acid can be inhibited by the presence of Sb_ad on the Pt(110) surface. The electrocatalytic effects of Sb_ad towards HCOOH oxidation consist in a negative shift of the oxidation potential (about 350 mV) and the enhancement of the oxidation current. Based on the data processing method of integration transform developed in our previous papers, the kinetics of HCOOH oxidation on Pt(110)/Sb electrodes of different θ_Sb have been investigated quantitatively, and both the rate constant k_f and the transfer coefficient β were determined and reported.     

Room Temperature COad Desorption/Exchange Kinetics on Pt Electrodes—A Combined In Situ IR and Mass Spectrometry Study

The room temperature desorption and exchange of CO in a saturated CO adlayer on a Pt electrode, at potentials far below the onset of oxidation, was investigated by isotope labeling experiments, using a novel spectroelectrochemical setup, which allows the simultaneous detection of adsorbed species by in situ IR spectroscopy and of volatile (side) products and reactants by online mass spectrometry under controlled electrolyte flow conditions. Time‐resolved IR spectra show a rapid, statistical exchange of pre‐adsorbed ¹³CO_ad by ¹²CO_ad in ¹²CO containing electrolyte; mass spectrometric data reveal first‐order exchange kinetics, with the rate increasing with CO partial pressure. The increasing CO_ad desorption rate in equilibrium with a CO containing electrolyte is explained by a combination of an increasing CO_ad coverage upon increasing the CO pressure, and a decrease of the CO adsorption energy with coverage, due to repulsive CO_ad–CO_ad interactions.     

Preparation and voltammetric characterisation of bismuth-modified mesoporous platinum microelectrodes. Application to the electrooxidation of formic acid

In this paper mesoporous platinum microeletrodes (Pt-ME) modified with submonolayers of adsorbed bismuth (Bi-PtME) were prepared and characterised by cyclic voltammetry (CV). The mesoporous platinum films were electrodeposited from hexachloroplatinic acid dissolved in the aqueous domain of the lyotropic liquid crystalline phase of Brij 78^®, to form metal films with hexagonal arrays of nanometer-sized channels. Bismuth deposition was performed by different procedures involving either the spontaneous adsorption of bismuth onto the Pt surface from Bi³⁺ solutions, or by under potential deposition (UPD) of bismuth from Bi³⁺ solutions, by cycling the potential over an useful range, or applying a constant potential for a given time. The latter procedures provided high bismuth coverage (θ_Bi), whereas only small amounts of bismuth could be adsorbed from the simple immersion of the Pt-ME at open circuit. The coverage by irreversibly adsorbed bismuth was checked in a 0.5 M H₂SO₄ solution free of Bi³⁺ ions and exploiting the charge involved in the hydrogen adsorption/desorption, which decreased in proportion to the amount of platinum sites covered by bismuth. The ability of the prepared Bi-PtME towards the oxidation of formic acid was also investigated. It was found that Bi-PtME with θ_Bi = 0.6 provided stationary voltammograms characterised by a low hysteresis between the anodic and cathodic scans. The onset of the waves resulted shifted by about 150 mV towards less positive potentials with respect to that of the corresponding Pt-ME. At 0.1 M HCOOH current densities of about 70 mA cm⁻² were achieved. These results were discussed in terms of high tolerance towards the intermediate poisons of the Bi-PtME investigated here. Bi-PtME with much lower real surface area and bismuth coverage displayed both lower catalytic activity and tolerance to poisons.     

Mechanism of catalytic ascorbic acid oxidation system Cu2+–ascorbic acid–O2

Analysis is made of reported results on the kinetics and mechanism of ascorbic acid oxidation with oxygen in the presence of cupric ions. The diversities due to methodological reasons are cleared up. A kinetic study of the mechanism of Cu²⁺ anaerobic reaction with ascorbic acid (DH₂) is carried out. The true kinetic regularities of catalytic ascorbic acid oxidation with oxygen are established at 2.7 ≤ pH < 4, 5 × 10^?4 ≤ [DH₂] ≤ 10^?2M, 10^?4 ≤ [Cu²⁺] ≤ 10^?3M, and 10^?4 ≤ [O₂] ≤ 10^?3M: where??₁ (25°C) = 0.13 ± 0.01 M^?0.5˙sec^?1. The activation energy for this reaction is E₁ = 22 ± 1 kcal/mol. It is found by means of adding Cu⁺ acceptors (acetonitrile and allyl alcohol) that the catalytic process is of a chain nature. The Cu⁺ ion generation at the interaction of the Cu²⁺ ion with ascorbic acid is the initiation step. The rate of the chain initiation at [Cu²⁺] ± 10^?4M, [DH₂] ± 10^?2M, 2.5 < pH < 4, is where??_i,1 (25°C) = (1.8 ± 0.3)M^?1˙sec^?1, E_i,1 = 31 ± 2 kcal/mol. The reaction of the Cu⁺ ion with O₂ is involved in a chain propagation, so that the rate of catalytic ascorbic acid oxidation for the system Cu²⁺? DH₂? O₂ is where??₁ (25°C) = (5 ± 0.5) × 10⁴ M^?1˙sec^?1. The Cu⁺ ion and a species interacting with ascorbate are involved to quadratic chain termination. By means of photochemical and flow electron spin resonance methods we obtained data characteristic of the reactivities of ascorbic acid radicals and ruled out their importance for the catalytic chain process. A new type of chain mechanism of catalytic ascorbic acid oxidation with oxygen is proposed: .     

The influence of hydrogen peroxide on carbon monoxide electrooxidation at Pt/C and Pt:Ru/C electrodes

Polymer electrolyte fuel cells constitute one of the most important efficiency energy converters for non-centralised uses. However, the use of fuels arising from reformate processes significantly lowers the current efficiency because of anodic catalytic poison coming from adsorbed carbon monoxide (CO_ad). In this work, the influence of the addition of hydrogen peroxide in the flow current is studied, considering the adsorption and electrochemical oxidation of carbon monoxide on carbon-supported Pt (20% Pt/Vulcan) and Pt:Ru (1:1, 20% Pt:Ru/Vulcan) catalysts in 2 M sulphuric acid. The investigation was conducted applying cyclic voltammetry and on-line differential electrochemical mass spectrometry. A series of experiments has been performed to investigate the influence of the temperature as well as the time of contact and the concentration of hydrogen peroxide. Oxidation of CO_ad to carbon dioxide occurs at lower potentials in the presence of hydrogen peroxide. Moreover, it is possible to remove ca. 70% of CO_ad on Pt/C electrodes. On the other hand, for PtRu/C electrodes, similar charge values to those of Pt/C electrodes were obtained for the CO stripping, but the process occurs at more negative potentials. In this case, the effect of partial desorption for CO_ad by interaction with hydrogen peroxide is added to the bifunctional mechanism usually considered for this alloy. This paper is dedicated to Prof. Francisco Nart, in memoriam.     

Electrocatalytic oxidation of nickel amalgam in aqueous solutions of halogen and pseudohalogen ions

The voltammetric oxidation of nickel amalgam from the hanging mercury drop electrode in aqueous solutions of F^?, Cl^?, Br^?, I^?, N₃^?, SCN^?, and ClO₄^? ions have been investigated. Concentrations of these anions were sufficiently low to depress the formation of complexes with nickel(II) in the bulk of the solution.An increase in the rate of anodic oxidation with increase of concentration of anions was observed both without and with correction for the φ₂ potential. This increase is explained as due to a catalytic effect of anions adsorbed on the electrode surface.Using the concept of changes of the activity coefficient of the activated complex it was possible to show that the oxidation of the nickel amalgam in thiocyanates and azides proceeds by the formation of the activated complex with bound SCN^? and N₃^? anions. These complexes form only in the activated state and decompose when products leave the double layer.In chlorides and bromides a similar mechanism is suggested only at larger surface concentration of anions. At lower surface concentration and in iodides the oxidation proceeds by the activated complex with no anions bound to the nickel, only long-range interactions of adsorbed anions with activated complex then exist.The order of these electrode reactions was calculated using the concept of the surface activity.The two-step mechanism of the charge transfer is also discussed.     

Electrocatalytic Oxidation of Hydrogen Peroxide on Poly(m‐toluidine)‐Nickel Modified Carbon Paste Electrode in Alkaline Medium

The poly(m‐toluidine) film was prepared by using the repeated potential cycling technique in an acidic solution at the surface of carbon paste electrode. Then transition metal ions of Ni(II) were incorporated to the polymer by immersion of the modified electrode in a 0.2 M NiSO₄, also the electrochemical characterization of this modified electrode exhibits stable redox behavior of the Ni(III)/Ni(II) couple. The electrocatalytic ability of Ni(II)/poly(m‐toluidine)/modified carbon paste electrode (Ni/PMT/MCPE) was demonstrated by electrocatalytic oxidation of hydrogen peroxide with cyclic voltammetry and chronoamperometry methods in the alkaline solution. The effects of scan rate and hydrogen peroxide concentration on the anodic peak height of hydrogen peroxide oxidation were also investigated. The catalytic oxidation peak current showed two linear ranges with different slopes dependent on the hydrogen peroxide concentration and the lower detection limit was 6.5 μM (S/N=3). The catalytic reaction rate constant, (k_h), was calculated 5.5×10² M^?1 s^?1 by the data of chronoamperometry. This modified electrode has many advantages such as simple preparation procedure, good reproducibility and high catalytic activity toward the hydrogen peroxide oxidation. This method was also applied as a simple method for routine control and can be employed directly without any pretreatment or separation for analysis cosmetics products.     

A Prussian blue-based reactive electrode (reactrode) for the determination of thallium ions

A reactive electrode (reactrode) made of Prussian blue (PB), graphite and paraffin can be used for a selective determination of thallium ions down to a concentration of 2 · 10^–8 mol 1^–1. The working principle of the reactrode is that thallium ions can be pumped into Prussian blue during alternating oxidation-reduction cycles. After a preconcentration of thallium ions in PB, the voltammetric determination follows as usually in anodic stripping voltammetry, i.e. the thallium ions are reduced to thallium metal which is subsequently oxidized to give the anodic stripping signal. The peculiarity of the Prussian blue-thallium system is that the thallium ions are situated in the holes of the PB matrix. When reduced to metallic thallium, they are substituted by potassium ions. Cd²⁺, Fe³⁺, Zn²⁺, Cu²⁺ and Ni²⁺ do not interfere up to a hundredfold excess, NH₄⁺ does not interfere up to a thousandfold – and Bi³⁺ up to tenfold excess. The interference by Pb²⁺ can be suppressed with EDTA.     

Electrochemical Characterization of Pyrocatechol Sulfonephthalein-Modified Glassy Carbon Electrode and Separation of Electrocatalytic Responses for Ascorbic Acid and Dopamine Oxidation

A pyrocatechol sulfonephthalein- (PS-) modified glassy carbon (PS/GC) electrode has been prepared by adsorption of PS on a glassy carbon electrode surface. Cyclic voltammograms of the PS/GC electrode indicate the presence of a couple of well-defined redox peaks, and the formal potential shifts in the negative direction with increasing solution pH. The relation between formal potential,E^0′, and solution pH can be fit to the equationE^0′(mV) = −51.4 pH + 538.7. The PS/GC electrode shows high electrocatalytic activity toward ascorbic acid oxidation, with an overpotential ca. 380 mV less than that of the bare electrode and a drastic enhancement of the anodic currents. The electrocatalytic reaction rate constant (k), which was decreased with increasing concentration of H₂A, was determined using rotating disk electrode measurements. The values ofkwas also affected by the solution pH. The electrode can also separate the electrochemical responses of ascorbic acid and dopamine. The separation between the anodic peak potentials of ascorbic acid and dopamine is more than 50 mV by the differential pulse voltammetry.