Hydrogen evolution during the oxidation of formaldehyde on Au:: The influence of single crystal structure and Tl-upd

Hydrogen evolution during formaldehyde oxidation in alkaline solution has been monitored by Differential Electrochemical Mass Spectrometry on Au(111) and polycrystalline gold. The current efficiency for hydrogen evolution increases with higher concentration and is in the same range on both, polycrystalline Au and Au(111) electrode. The onset potentials and half-wave potentials are higher on Au(111). Reaction orders for the faradaic current on the bare gold electrodes have been determined as 0.21 for higher and 0.76 for lower concentrations. Reaction orders for hydrogen evolution during formaldehyde oxidation are 1.4 times higher in each case. Tafel slopes in the range of 140-160 mV are found. This signifies that the first reaction step involving the formation of adsorbed hydrogen is largely determining the overall reaction rate. In the presence of thallium adlayers hydrogen evolution from formaldehyde oxidation is largely suppressed. On the thallium modified polycrystalline Au, formaldehyde oxidation is shifted for 100 mV to higher potentials where Tl is partially desorbed and hydroxide is coadsorbed on the modified surface. On thallium modified Au(111), a similar process takes place, but in the same potential region as the onset of formaldehyde oxidation on the bare surface and therefore the formaldehyde oxidation is only slightly shifted. Tafel slopes are decreased to 80 mV/dec in the presence of thallium. In the presence of adsorbed thallium, the first reaction step is in equilibrium, the coverage with adsorbed hydrogen is smaller and its recombination to H₂ is largely suppressed.     

Kinetics of electrodeposition of nickel from watts baths

Tafel slopes and reaction orders for the electrodeposition of nickel from Watts type baths onto a vitreous carbon electrode have been determined by both steady state and transient methods. After correcting the currents for concurrent hydrogen evolution, and taking the composition of the deposit into account, the following kinetic parameters have been obtained: Tafel slope 120 mV decade^?1, reaction orders with respect to nickel and chloride both +1. The agreement between the two methods was excellent. No dependence of cathodic rate constants on pH could be found. The cathodic results do not therefore support the widely held opinion that an adsorbed complex involving a hydroxide ion is an intermediate in active dissolution.Results for the nucleation constant are given in the Appendix. The nucleation constant for nickel on vitreous carbon is found to depend on potential and concentration of nickel ions, and to be independent of pH and concentration of chloride ions.     

Electrochemical behaviour of indium in H2SO4

The kinetics of the electrodissolution of indium and the hydrogen evolution on indium in H₂SO₄ of various pH values have been investigated. For electrodissolution, Tafel slopes of 22 mV per decade and a reaction order of 1 with respect to OH^– have been obtained. For hydrogen evolution, Tafel slopes were 20 mV per decade; the reaction was first order with respect to H⁺. An electrodissolution mechanism based on three consecutive one-electron transfer steps with the last electron transfer step rate-determining is consistent with the experimental results.     

An application of the hammett equation to the cathodic reduction of aromatic acids

Nine aromatic acids were cathodically reduced in aqueous acid medium. Lead was the main cathode metal employed, though experiments on Cu, Fe, Cd, Sn and Cu/Hg as well as PbSn alloys are also reported. At all potentials, hydrogen evolution was the dominant electrode reaction. The rate of reduction of the organic species was measured by periodic sampling of the electrolyte and chemical analysis. The corresponding alcohol was the only determinable product. The reaction rates so derived were plotted according to the Hammett equation. Straight line plots were obtained and the ? value agreed with non-electrochemical studies in the literature. From this, the Tafel slopes and electrochemical reaction orders, a mechanism, largely in agreement with earlier hypothesis, is put forward.     

Oxidation of L-ascorbic acid on a gold electrode

Voltagrams of L-ascorbic acid on a gold electrode show the existence of two oxidation waves but no reduction wave. An analysis of the I—E curves was carried out together with the Tafel slopes and reaction orders. The products from the first oxidation were identified by paper chromatography.A scheme is proposed for the overall reaction of the first oxidation together with a reaction mechanism at potentials corresponding to the foot of the wave.     

Electrochemical waste water treatment using high overvoltage anodes. Part I: Physical and electrochemical properties of SnO2 anodes

The electrochemical performance of SnO₂ as an anode material with high oxygen gas-evolution overpotential was investigated in view of its application for electrochemical oxidation of bio-refractory organics in waste waters. The influence of the doping agent (Sb, F, Cl) and doping level on the oxygen-evolution reaction was studied in terms of Tafel slope, oxygen-overpotential and exchange current densities for the Ce³⁺Ce⁴⁺ reaction. Tafel slopes of about 300 mV decade^–1 were found and the oxygen evolution overpotential was 600mV higher than that of platinum. While the doping level had no significant influence on Tafel slopes and oxygen overpotentials the stability of the SnO₂ electrode increased with charge carrier density. The oxidation of phenol was investigated as a test for the oxidizing power of the new anode material when compared to Pt or PbO₂. The rate of phenol removal was much higher for SnO₂ than for PbO₂ or Pt.     

Oxidation mechanism of allyl alcohol on an Au-electrode in basic solutions

A study was carried out on the oxidation reaction of allyl alcohol molecules on an Au electrode in alkali medium. The kinetic study gives rise to three different reaction mechanisms taking place at different potential ranges and under different experimental conditions. All the mechanisms studied are characterized by the interaction of the allyl alcohol molecules with the AuOH coverage, which produces radicals adsorbed on the surface and influencing, in this way, the reaction pathway. Under the conditions specified in the text, a polymerization reaction appears to take place on the gold surface, which can account for the rather high Tafel slope values obtained (b = 280 mV). The different mechanisms are discussed and compared with results obtained for Pt and gold—platin alloys[9].     

鸟嘌呤和腺嘌呤吸附和电氧化行为的电化学石英晶体微天平研究

基于压电电化学石英晶体微天平(EQCM)技术,研究了腺嘌呤(A)和鸟嘌呤(G)在裸金电极和多壁碳纳米管(MWCNTs)修饰金电极上的吸附和电氧化行为。结果表明,在金电极上,A的吸附量和氧化电流均比G大,而在MWCNTs/Au电极上,A和G可类似地吸附,但其氧化峰电位负移且氧化峰电流增大,表明MWCNTs对A和G的氧化具有催化作用。实验发现,A和G在裸金电极上氧化的电子转移数分别为5.4(RSD=±2.3%)和1.9(RSD=±1.3%),而在MWCNTs/Au电极上的氧化电子转移数分别为5.1(RSD=±1.8%)和1.5(RSD=±1.6%)。     

甲醛在Pd(111)与Pd(100)晶面上氧化的计算化学研究

使用密度泛函理论对甲醛分子在Pd(111)晶面与Pd(100)晶面的氧化反应机理进行研究,结果表明,Pd(100)晶面相比于Pd(111)晶面更有利于甲醛分子的氧化,为进一步合理设计甲醛氧化贵金属催化剂提供了理论依据。     

The electrodeposition and dissolution of zinc and amalgamated zinc in alkaline solutions

The reaction mechanism of zinc and amalgamated zinc was investigated with the galvanostatic transient technique in the concentration range 1.5–10 M KOH. The Tafel slopes of the zinc electrode were 40 mV anodically and 120 mV cathodically. The cathodic reaction order in zincate was found to be +1. From the Tafel slopes and the dependence of the exchange current density on the activity of the KOH, the reaction orders in OH^? were calculated, yielding values of 2.3± 0.8 in the anodic and ?0.8 ± 0.2 in the cathodic direction. These results are consistent with the suggested mechanism of Bockris et al.[16] for the zinc electrode. The Tafel slope in cathodic direction of the amalgamated zinc electrode was a function of the KOH concentration (120 mV at KOH concentrations up to 3 M; about 60 mV in 10 M KOH); the anodic Tafel slope was 30 mV over the whole concentration range. These results and measurements at constant ionic strength suggest a mechanism which involves the participation of water. The difference in behaviour of the zinc electrode and the amalgamated zinc electrode is probably caused by changes in the adsorption characteristics due to amalgamation.     

Hydrogen adsorption and hydrogen evolution reaction on a polycrystalline Pt electrode studied by surface-enhanced infrared absorption spectroscopy

Hydrogen evolution reaction (HER) on a polycrystalline Pt electrode has been investigated in Ar-purged acids by surface-enhanced infrared absorption spectroscopy and electrochemical kinetic analysis (Tafel plot). A vibrational mode characteristic to H atom adsorbed at atop sites (terminal H) was observed at 2080-2095 cm⁻¹. This band appears at 0.1 V (RHE) and grows at more negative potentials in parallel to the increase in hydrogen evolution current. Good signal-to-noise ratio of the spectra enabled us to establish the quantitative relation between the band intensity (equivalently, coverage) of terminal H and the kinetics of HER, from which we conclude that terminal H atom is the reaction intermediate in HER and the recombination of two terminal H atoms is the rate-determining step. The quantitative analysis of the infrared data also revealed that the adsorption of terminal H follows the Frumkin isotherm with repulsive interaction.     

Electrochemical studies with a Cu-5wt.%Ni alloy in 0.5 M H2SO4

The electrochemical behavior of the annealed Cu-5wt.%Ni alloy in 0.5 M H₂SO₄ was studied by means of open-circuit potential (E_OCP) measurements, cyclic voltammetry, electrochemical impedance spectroscopy (EIS), and quasi-stationary linear potential sweep. The hydrodynamics of the system was also studied. This material is constituted by a single α₁ phase. The anodic behavior of a Cu-Ni alloy in H₂SO₄ consists fundamentally on the electrodissolution of Cu, its main component, and the formation of a sulfur-containing passive layer. The presence of Ni decreases the rate of Cu oxidation, mostly at high positive potentials. The impedance spectra, obtained for the unrotating electrode, can be interpreted in terms of a simple charge-transfer reaction across a surface layer. When the electrode is rotated, the occurrence of an inductive loop evidenced the existence of an adsorbed layer. All the resistance estimated from the proposed equivalent circuits diminished with the electrode rotation rate, emphasizing the influence of ion transport in the overall electrode process. The system presented two anodic Tafel slopes: 40 mV dec⁻¹ for E<255 mV and 67 mV dec⁻¹ for E>275 mV. A Tafel slope of 40 mV dec⁻¹ evidences that copper dissolution can be interpreted in terms of the mechanism proposed by Mattsson and Bockris. The second Tafel suggests that at potentials more positive than 275 mV, copper dissolves according to a mechanism that considers the disproportionation of adsorbed Cu(I) species.     

Indium as a cathodic material: catalytic reduction of formaldehyde

The performance of indium as cathodic material for the electroreduction of small organic molecules is considered. The cathodic reduction of formaldehyde (FA) is an ideal model reaction for this purpose since indium has a very large overpotential for the hydrogen evolution reaction with and without FA. Kinetic sets of the reaction pathways, with respect to the Tafel slope and reaction order, are considered on the basis of quasi-potentiostatic measurements and cyclic voltammetry. The value of the Tafel slope bc60mVdec-1 indicates that the protonation of the adsorbed radical is the rate determining step in the proposed CECE mechanism. The reaction order with respect to FA is close to one in the limiting current regions but smaller in the Tafel region. The existence and kinetics of the radicals adsorbed during FA reduction are evidenced by very fast potentiodynamic experiments, with scan rates between 40 and 80Vs-1. Electrochemical measurements are carried out on freshly in situ prepared In-electrodes. During cathodic polarization, the surface oxide film is reduced to In-metal via a solid-state mechanism. The crystallization kinetics of indium in the oxide matrix is also discussed.     

Electrocatalytic oxidation of nitrite on a vitreous carbon electrode modified with cobalt phthalocyanine

The oxidation of nitrite on a vitreous carbon electrode modified with different surface concentrations of cobalt phthalocyanine (CoPc) was investigated using cyclic voltammetry and rotating disk electrode techniques. The voltammetric curves show well-defined anodic peak, which is indicative of the electrocatalytic oxidation of nitrite. The activity of the modified electrodes does not increase with the amount of CoPc adsorbed on the electrode in a wide range of surface coverages and the order of the electro-oxidation reaction is one in nitrite. Tafel plots obtained from rotating disk measurements give two linear regions of slopes of 0.071 and 0.14 V/decade, respectively. This was discussed in terms of a catalytic process and a mechanism is proposed.     

Kinetic criteria for the mechanism of the hydrogen evolution reaction

Additional criteria for distinguishing between the Volmer—Heyrovský and the Volmer—Tafel reaction mechanisms for the hydrogen evolution reaction (HER) have been derived from a theoretical treatment of polarization curves under conditions of continuous surface renewal. From data published by Tomashov and Vershinina for the HER on Pd, Ni, Fe, Sn and Pb, estimates of hydrogen coverages and rate constants have been made. Approximate values of standard exchange current densities could thus be predicted for the two reaction mechanisms; these have been compared with experimental data. In order to reconcile the various kinetic criteria with each other, it has been proposed that the Volmer—Heyrovský reaction mechanisms is operative on the majority of metals. Alternative interpretations within the Volmer—Heyrovský mechanism have been presented for Tafel slopes in the region of −30 mV dec⁻¹ and for the relative positions of volcano curves, which might otherwise have been considered indicative of the Volmer—Tafel reaction mechanism.     

Cathodic behaviour of RuO2-doped Ni/Co3O4 electrodes in alkaline solutions: hydrogen evolution

RuO2-doped Co3O4 electrodes were prepared by thermal decomposition of the corresponding nitrates using Ni as a support. The RuO2 content was varied between 0 and 20mol%. The kinetics of hydrogen evolution from alkaline solution was studied by recording quasisteady state polarization curves at several NaOH concentrations. A mechanism is proposed on the basis of Tafel slopes and reaction orders. The electrocatalytic activity of the mixed oxides has been found to go through a maximum at intermediate RuO2 contents. Some evidence of instability has emerged.     

CO monolayer oxidation on stepped Pt(S) [(n − 1)(1 0 0) × (1 1 0)] surfaces

The electrochemical oxidation of CO has been studied on Pt(S)[(n − 1)(1 0 0) × (1 1 0)] electrodes to investigate the effect of the step density in the reaction. This series shows two different trends for long (n ≥ 7) and short terraces. For long terraces, the voltammetric peak shifts towards higher potential as the step density increases, unlike the behaviour observed for other stepped surfaces, which exhibit the opposite behaviour in agreement with the Smoluchowski effect. For short terraces, the “normal” behaviour is observed, that is, as the step density increases the peak shifts towards lower potentials. Chronoamperometric measurements were used to determine rate constants and Tafel slopes using the mean field Langmuir-Hinselwood kinetics. Rate constants follow the same trends as the peak potentials in voltammetry. A Tafel slope of 75 ± 4 mV has been obtained for the surfaces with long terraces whereas a value of the surfaces with short terraces showed a value of 100-120 mV is obtained. This change of slopes is interpreted as a change in the electrochemical behaviour of the species involved in the mechanism, probably, a change in the adsorption isotherm of adsorbed OH. Pt(5 1 0) electrode exhibits an intermediate behaviour between those of long and short terraces with two different peaks that can be associated with both behaviours previously described.     

Mechanism study of the ethanol oxidation reaction on palladium in alkaline media

In this work, the mechanism of the ethanol oxidation reaction (EOR) on a palladium electrode was studied using the cyclic voltammetry method. The dissociative adsorption of ethanol was found to proceed rather quickly and the rate-determining step was the removal of the adsorbed ethoxi by the adsorbed hydroxyl on the Pd electrode. The Tafel slope was found to be 130 mV dec⁻¹ at lower potentials, which suggests that the adsorption of OH⁻ ions follows the Temkin-type isotherm on the Pd electrode. In comparison, the Tafel slope increased gradually to 250 mV dec⁻¹ at higher potentials. The change in the Tafel slope indicated that, at higher potentials, the kinetics is not only affected by the adsorption of the OH⁻ ions, but also by the formation of the inactive oxide layer on the Pd electrode.     

Kinetic study of methanol oxidation on carbon-supported PtRu electrocatalyst

Methanol electrooxidation was investigated on the carbon-supported PtRu electrocatalyst (1:1 atomic ratio) in acid media. X-ray diffraction measurement indicated alloying of Pt and Ru. Cyclic voltammetry of the sample reflects the amount of Ru in the catalyst and its ability to adsorb OH radicals. Tafel plots for the oxidation of 0.02-3 M methanol in the solutions containing 0.05-1 M HClO₄ and in the temperature range 27-40 °C showed reasonably well-defined linear region with the slope of about 115 mV dec⁻¹ at the low currents, irrespective of the experimental conditions employed. Reaction order with respect to methanol was found to be 0.5. A correlation between methanol oxidation rate and pseudocapacitive current of OH adsorption on Ru sites was established. It was proposed that bifunctional mechanism is operative with the reaction between methanol residues adsorbed on Pt sites and OH radicals adsorbed on Ru sites as the rate-determining step.