Polycrystalline Platinum Electrode Research Articles

Observed electrochemical oscillations during the oxidation of aqueous sulfur dioxide on a sulfur modified platinum electrode

The electrochemical oxidation of aqueous sulfur dioxide has been investigated on a sulfur modified polycrystalline platinum electrode. It has been found that time and potential-based current oscillations occur on a modified electrode in the oxidation region >0.7 V vs. SHE. The oscillation is specific to catalytic oxidation on sulfur modified platinum and is not observed on an unmodified electrode. Using i–R compensation techniques, the oscillation has been found to stem from electrochemical origins; the interfacial potential being an essential variable. The influence of other controllable variables including holding potential, sulfur dioxide concentration and sulfuric acid concentration have also been investigated for their effects on the oscillation. The electrochemical oscillation is explained here in part by the intermediate formation of dithionate. Two possible mechanisms, including a dual pathway mechanism, are explored.

Impact of SO 2 poisoning of platinum nanoparticles modified glassy carbon electrode on oxygen reduction

An extraordinary recovery characteristic of Pt-nanoparticles from SO 2 poisoning is introduced in this study. Platinum nanoparticles (nano-Pt) modified glassy carbon electrode (nano-Pt/GC) has been compared with polycrystalline platinum (poly-Pt) electrode towards SO 2 poisoning. Two procedures of recovery of the poisoned electrodes were achieved by cycling the potential in the narrow potential range (NPR, 0–0.8 V vs. Ag/AgCl/KCl (sat.)) and wide potential range (WPR, −0.2 to 1.3 V). The extent of recovery was marked using oxygen reduction reaction (ORR) as a probing reaction. SO 2 poisoning of the electrodes changed the mechanism of the oxygen reduction from the direct reduction to water to the stepwise reduction involving the formation of H 2O 2 as an intermediate, as indicated by the rotating ring-disk voltammetry. Using the WPR recovery procedure, it was found that two potential cycles were enough to recover 100% of the activity of the ORR on the nano-Pt/GC electrode. At the poly-Pt electrode, however, four potential cycles of the WPR caused only 79% in the current recovery, while the peak potential of the ORR was 130 mV negatively shifted as compared with the fresh poly-Pt electrode. Interestingly, the NPR procedure at the nano-Pt/GC electrode was even more efficient in the recovery than the WPR procedure at the poly-Pt electrode.

Interaction of Mass Transport and Reaction Kinetics during Electrocatalytic CO Oxidation in a Thin-Layer Flow Cell

The interplay between electrocatalytic CO oxidation kinetics and mass transport in a thin-layer flow cell with a polycrystalline platinum electrode was experimentally studied and numerically simulated. The experiments were performed in a flow cell under controlled electrolyte flow. The computations are based on four different models for mass transport, coupled with a three-step reaction mechanism for CO oxidation that includes surface coverage effects. A zero-dimensional model neglecting mass transport effects on the overall reaction rate is applied to verify the effect of the kinetic parameters on the Faradaic current qualitatively. Transport models of increasing complexity, one-, two-, and three-dimensional, are used to analyze the impact of diffusion and convection on the total reaction rate in the flow cell. The numerical simulation, based on a time-resolved three-dimensional transport model coupled with the electrocatalytic kinetics, resolves profiles of temporal and spatial concentrations, velocities, and surface coverages. Mass transfer limitations of the electrocatalytic reaction rate are primarily caused by CO diffusion normal to the electrode. Both the two- and the three-dimensional models can quantitatively predict the Faradaic current as a function of potential measured in the experimental setup.

Underpotential deposition of cobalt onto polycrystalline platinum

An electrochemical study of cobalt electrodeposition onto a polycrystalline platinum electrode from an aqueous solution (10−2 M CoCl2 + 1 M NH4Cl (pH 9.5)) was carried out through cyclic voltammetry and potential step techniques. Analysis of the voltammetric data clearly showed that a cobalt adlayer is formed during the application of potential in the underpotential deposition (upd) region. Formation of this cobalt adlayer involved the simultaneous presence of both adsorption and 2D nucleation processes. Cobalt adlayers obtained by linear voltammetry in upd region were analyzed employing diffuse reflectance spectroscopy (DRS). By using theoretical quantum studies at PM6//HF/LANL1MB level, it was possible to assign the peaks obtained by DRS at 328 and 337 nm to the cobalt adsorption on Pt(111) and Pt(100), respectively, while the signals recorded at 355 and 362 nm were related with the clean platinum surfaces Pt(100) and Pt(111). Also, quantum calculations at the PM6 level indicated that the energy formation order is Co-Pt(100) > Co-Pt(111) > Co-Pt(110) > Co-Co(surface).

Electrocatalytic oxidation of formic acid and formaldehyde on platinum nanoparticles decorated carbon-ceramic substrate

Electrochemical characteristics of formic acid (FM) and formaldehyde (FM) oxidation on a potent catalyst, platinum nanoparticles supported on carbon-ceramic substrate (CC/Pt), were investigated via cyclic voltammetric and chronoamperometric analysis in mixed 0.75 M FM (or 0.75 M FM) and 0.1 M H 2SO 4 solutions. The results were compared to those at a polycrystalline platinum electrode and platinum particles deposited on platinum and glassy carbon electrodes. It was found that CC/Pt was catalytically more active than smooth platinum and platinum particles supported on platinum and glassy carbon electrodes. On the other hand, such nanoparticles on CC substrate exhibit better catalytic behavior towards FA and FM than the corresponding platinum and glassy carbon electrodes, which is raised form high porosity of CC substrate for better distribution of platinum particles and to produce of platinum particles in nano size. The effect of some experimental factors was studied and optimum conditions were suggested. Finally, the long-term stability of the modified electrode has also been investigated. These results indicate that the system studied in the present work is the most promising system for use in fuel cells.

The Electrochemical Oxidation of Aqueous Sulfur Dioxide: I. Experimental Parameter Influences on Electrode Behavior

The oxidative behavior of aqueous sulfur dioxide has been assessed on a polycrystalline platinum electrode using cyclic voltammetry with variation in applied experimental parameters. Parameters varied include the acid concentration, reaction temperature, and lower potential limit . Three distinctly different oxidation scenarios were identified. These are defined by and, at , acid concentration as well. Observed oxidation scenarios include a limited response (scenario 1), catalytic oxidation (scenario 2), and reaction inhibition (scenario 3). Scenario 1 required an activation energy of with a Tafel slope of and transfer coefficient of . Scenario 2 was found to have a different reaction mechanism to scenario 1 , displaying faster reaction kinetics and catalytically improved oxidation onset . The onset of scenario 2 oxidation was dependent on oxidation kinetics, not thermodynamics, with . The variable response of the electrode is discussed in terms of sulfur species adsorption, including the reductive formation of adsorbed sulfur at potentials less than 0.5 V, and the adsorption of sulfur dioxide with varying adsorption strength.

The effect of hydrocarbon chain length in normal aliphatic alcohols on their reaction with oxygen adsorbed on polycrystalline platinum electrode

The reaction of adsorbed oxygen (Oads) with aliphatic alcohols n-CnH2n + 1OH with n = 2–5 is studied by the method of transients of open-circuit potential in combination with potentiodynamic pulses. It is shown that these alcohols react with Oads by a mechanism the same as for CH3OH. Kinetic parameters of these reactions are determined in ranges of high and medium surface coverages with Oads. These data together with analogous results obtained earlier for CH3OH were studied with the aim of elucidating how the length of the hydrocarbon chain affects the kinetics of interaction of alcohols with Oads. The complex variations of the reaction rate with n (with a maximum) are explained by several factors among which the energy of the C–H bond at α-carbon atom and the degree hydration of alcohols should be singled out.

Comparative Study of Tungsten Monocarbide and Platinum as Counter Electrodes in Polysulfide-Based Photoelectrochemical Solar Cells

The stability and the electrocatalytic activity of polycrystalline tungsten monocarbide (WC) and platinum (Pt) electrodes in a concentrated polysulfide electrolyte were examined in an electrochemical half-cell and an integrated photoelectrochemical cell (PEC) device. Cyclic voltammetry and linear sweep voltammetry measurements found WC electrodes to be approximately twice as active as Pt electrodes toward polysulfide reduction. Electrochemical and X-ray photoelectron spectroscopy (XPS) measurements indicated that WC was stable in a polysulfide electrolyte, with no appreciable W sulfide formation. In contrast, XPS showed significant levels of sulfide species on the Pt surface, which might explain the differences in the electrochemical behavior of the two electrode materials. The performance of integrated PEC devices comprised of thin-film CdSe photoanodes and WC or Pt counter electrodes in a polysulfide electrolyte was consistent with that predicted from electrochemical half-cell measurements.

Dynamics of CO Oxidation on a Polycrystalline Platinum Electrode: A Time-Resolved Infrared Study

Electro-oxidation of CO adsorbed on a polycrystalline Pt electrode in the potential region of hydrogen absorption is examined by fast time-resolved surface-enhanced infrared absorption spectroscopy coupled to voltammetry or chronoamperometry. Oxidation dynamics at a weak preoxidation peak around 0.5 V (vs RHE) and the main oxidation peak around 0.7 V observed in stripping voltammetry are focused. IR spectra show that the shift of bridge-bonded CO to atop sites triggers the partial oxidation of CO adsorbed at atop sites to yield the preoxidation peak. It is also shown that CO on terraces becomes very mobile in the main oxidation region after some amount of CO being oxidized via a nucleation-and-growth mechanism and that terrace CO is oxidized faster than CO adsorbed at step edges. The result is interpreted in terms of a Langmuir−Hinshelwood type mechanism involving adsorbed CO and an oxygen-containing species (most likely OH) adsorbed at steps. The oxidation mechanism is essentially identical to that proposed in some earlier studies, but more convincing spectroscopic evidence of the mechanism is presented.

Role of adsorbed species in methanol oxidation on Pt studied by ATR-FTIRAS combined with linear potential sweep voltammetry

Methanol adsorption/oxidation on a polycrystalline platinum electrode was investigated by ATR-FTIRAS combined with linear potential sweep voltammetry after initial adsorption at 0.05 V in 0.05–1.0 M HClO 4 containing 0.04–1.0 M methanol. Methanol adsorption exhibits a pronounced maximum around 0.56 V, which coincides with the onset potential of the oxidation. This suggests that adsorbed CO is not a catalytic poison but an intermediate of the oxidation. The low methanol oxidation activity on Pt is not ascribed to the poisoning action by the adsorbed CO but to inability of Pt to dissociate water to adsorbed OH species needed for the CO and methanol oxidation. It was found for the first time that methanol adsorption at 0.05 V is significantly diminished in dilute perchloric acids. This was interpreted in terms of reduced physisorption of methanol at the adsorption potential which becomes more negative with respect to the pzc. Band intensity of the formate species with a symmetric O–C–O vibration around 1320 cm −1 remains unchanged with increase of methanol concentration from 0.04 M to 1 M, while the peak current of methanol oxidation increases by ca. three times in 1 M HClO 4. The result suggests a minor role of the formate in the methanol oxidation mechanism.

Electrochemical processes of adsorbed chlorobenzene and fluorobenzene on a platinum polycrystalline electrode

The electrode processes of chlorobenzene (CB) and fluorobenzene (FB) on polycrystalline platinum (Pt-pc) electrode in sulfuric acid were studied by differential electrochemical mass spectrometry (DEMS). Contrary to the oxidation of adsorbed benzene on Pt surface, the oxidation of adsorbed CB and FB in the oxygen adsorption region does not provide solely CO2 as the final product. At negative polarization potentials CB and FB were desorbed under dehalogenation. While in the case of FB only benzene was detected, CB gave intermediates besides benzene. The final product of stepwise hydrogenation of these species was cyclohexane.

QSPR Study of Passivation by Phenolic Compounds at Platinum and Boron-Doped Diamond Electrodes

Blocking polycrystalline platinum (Pt) and boron-doped diamond (BDD) electrodes by 20 phenolic compounds was studied by means of chronoamperometric and theoretical methods [i.e., quantitative structure-property relationships (QSPRs)] and chemometric methods. The difference between the current densities after 15 and of oxidation time was proposed for the first time as a quantitative measurement of passivation on the electrodes. Structures of phenolic molecules and their hydrogen-bonding complexes with fluoride ion were modeled and geometry optimized with the B3LYP method and the basis set. Several molecular descriptors were calculated and correlated with the passivation measurements using the partial least-squares (PLS) regression method. A PLS model with one latent variable from five descriptors was built with high-level predictivity for phenolic passivation on the Pt electrode, and then was externally validated with four phenolics. The Pt model statistical parameters obtained were: , , and standard error of validation . A BDD model with one latent variable and four descriptors was built and validated in the same way; however, the statistical parameters (, , and ) were of inferior quality with respect to the model for the Pt electrode. Both models were applied for prediction of 10 phenolic compounds. The Pt model showed to be suitable for predictive purposes. It was observed that passivation was much weaker on the BDD electrode than on the Pt electrode. Different interactions and reactions involving phenolics at the electrodes are the main reasons for such large differences between the models. Exploratory analyses were also performed and interpreted in terms of chemical concepts, such as phenolic reactivity, size/shape, hydrogen bonding, and electronic features. These findings can be useful to explore the possibility to predict phenolic passivation and to design electrochemical experiments involving different phenolic compounds. Furthermore, these PLS models aid in understanding electrode inactivation by phenolic compounds.

Dimethoxymethane (DMM) electrooxidation on polycrystalline platinum electrode in acid media

The electrooxidation of dimethoxymethane (DMM) was studied on polycrystalline platinum electrode. Cyclic voltammetry has shown that the behaviour of DMM electrooxidation depends on scan rate, DMM concentration and the nature of anion of the supporting electrolyte. At a low scan rate or at a high DMM concentration, the electrode is strongly poisoned during DMM electrooxidation, which is an anion-sensitive reaction. Using electrochemical and in situ IR experiments, a multistep mechanism was proposed with DMM adsorption as the first and rate-determining step. At a low potential range, adsorbed CO is slowly formed. CO ads inhibits the DMM electrooxidation until it is full oxidized to CO 2. Conversely, for lower adsorbed CO coverage, oxidation starts earlier and CO 2 formation is observed. In situ infrared reflectance spectroscopy experiments were carried out to identify these adsorbed species intermediates and reaction products of DMM adsorption and oxidation. In addition to adsorbed CO L (linearly bonded CO), adsorbed HCO, CH 3O species and CO 2 were detected suggesting a complex mechanism of the electrooxidation of DMM, with parallel paths involving methanol and formaldehyde electrooxidation.

Temperature (Over)Compensation in an Oscillatory Surface Reaction

Biological rhythms are regulated by homeostatic mechanisms that assure that physiological clocks function reliably independent of temperature changes in the environment. Temperature compensation, the independence of the oscillatory period on temperature, is known to play a central role in many biological rhythms, but it is rather rare in chemical oscillators. We study the influence of temperature on the oscillatory dynamics during the catalytic oxidation of formic acid on a polycrystalline platinum electrode. The experiments are performed at five temperatures from 5 to 25 °C, and the oscillations are studied under galvanostatic control. Under oscillatory conditions, only non-Arrhenius behavior is observed. Overcompensation with temperature coefficient (q(10), defined as the ratio between the rate constants at temperature T + 10 °C and at T) < 1 is found in most cases, except that temperature compensation with q(10) ≈ 1 predominates at high applied currents. The behavior of the period and the amplitude result from a complex interplay between temperature and applied current or, equivalently, the distance from thermodynamic equilibrium. High, positive apparent activation energies were obtained under voltammetric, nonoscillatory conditions, which implies that the non-Arrhenius behavior observed under oscillatory conditions results from the interplay among reaction steps rather than from a weak temperature dependence of the individual steps.

Current oscillations during the electrochemical oxidation of sulfide in the presence of an external resistor

The electrochemical oxidation of sulfide on a polycrystalline platinum electrode was studied under potentiostatic condition when an external resistor is in series with the working electrode. Only two oscillatory regions can be obtained in the absence of the external resistance, but four oscillatory regions, including two new current oscillations, were found in this system by controlling the external resistance. It is demonstrated that three oscillatory regimes, which arise on the positive branch of current-potential curve, can be classified as HN-NDR (Hidden N-shaped Negative Differential Resistance) oscillators. For the first oscillatory region, various transient complex phenomena, which result from the change of the electrode/electrolyte interface by accumulation of adsorbed element sulfur on the electrode, have been observed. The dynamic behavior of NDR (Negative Differential Resistance) oscillations, appearing along with negative branch of polarization curve, can transform from oscillations into bistability with a sufficient large external resistance in series. Two oscillatory regions in high-potential region classified as HN-NDR type oscillations are separated by a saddle-loop bifurcation. They displayed a sequence of bursting oscillations and irregular oscillations, respectively. The electrochemical oxidation of sulfide provides a model system for studying complex dynamics and possible application in sulfur removal.

Nucleation and growth of tin-zinc electrodeposits on a polycrystalline platinum electrode in tartaric acid

Current transients measurements and scanning electron microscopy were used to characterize the electrocrystallization process and morphology of tin-zinc alloys electrodeposited on Pt, in the absence and in the presence of tartaric acid. The model of Scharifker and Hills was used to analyze the current transients and it revealed that Sn-Zn electrocrystallization process in the presence of tartrate, under the studied conditions, is governed by three-dimensional progressive nucleation controlled by diffusion. In the absence of the complexant, the results indicated that nucleation process changes from instantaneous to progressive when the deposition potential becomes more negative, or when the incorporation of zinc occured to the deposit. The microscopic results showed deposits with two layers with different morphologies and is also influenced by deposition potential.

Electrocatalytic mechanism and kinetics of SOMs oxidation on ordered PtPb and PtBi intermetallic compounds: DEMS and FTIRS study

The electrocatalytic activities and mechanisms of PtPb and PtBi ordered intermetallic phases towards formic acid, formaldehyde and methanol oxidation have been studied by DEMS and FTIRS, and the results compared to those for a pure polycrystalline platinum electrode. While PtPb exhibits an enhanced electrocatalytic activity for the oxidation of all three organic molecules when compared to a Pt electrode, PtBi exhibits an enhanced catalytic activity towards formic acid and formaldehyde oxidation, but not methanol. FTIRS data indicate that adsorbed CO does not form on PtPb or PtBi intermetallic compounds during the oxidation of formic acid, formaldehyde and methanol, and therefore their oxidation on both PtPb and PtBi intermetallic compounds proceeds via a non-CO(ads) pathway. Quantitative DEMS measurements indicate that only CO(2) was detected as a final product during formic acid oxidation on Pt, PtPb and PtBi electrodes. At a smooth polycrystalline platinum electrode, the oxidation of formaldehyde and methanol produces mainly intermediates (formaldehyde and formic acid), while CO(2) is a minor product. In contrast, CO(2) is the major product for formaldehyde and methanol oxidation at a PtPb electrode. The high current efficiency of CO(2) formation for methanol and formaldehyde oxidation at a PtPb electrode can be ascribed to the complete dehydrogenation of formaldehyde and formic acid due to electronic effects. The low onset potential, high current density and high CO(2) yield make PtPb one of the most promising electrocatalysts for fuel cell applications using small organic molecules as fuels.

Impedance study of formic acid oxidation on platinum electrodes

The electrooxidation of formic acid on polycrystalline platinum electrodes in sulphuric acid is studied by a dynamic impedance method. Impedance spectra in the frequency range 0.7 Hz to 20 kHz, assembled from ac voltammograms, are interpreted in terms of a mechanism with three adsorbed species, although one of these may be free sites. The spectra show evidence for incipient oscillations in two potential regions, and observed oscillations were related to these spectra. The zeroes of the interfacial impedance were directly extracted from suitable equivalent circuits in distinct potential regions where oscillations may be triggered. The observation of only real zeroes of the interfacial impedance eliminates the possibility of pure potentiostatic oscillations arising from chemical reasons alone, and therefore potential is an essential variable.

Electrodeposition of platinum metal on titanium and anodised titanium from P salt: application to electro-oxidation of glycerol

Pt/Ti and Pt/TiO2 electrodes were made by galvanic deposition of platinum metal from an acid plating bath containing cis-dinitro-diammin-platinum(II) complex (P salt) onto titanium and anodic TiO2 layer on titanium substrates respectively. Titanium oxide nanotubes were fabricated by anodising titanium foil in acid solution. Pt/Ti and Pt/TiO2 electrodes were characterised using electrochemical methods and scanning electron microscopy. Cyclic voltammetric analysis of Pt/Ti electrodes in dilute sulphuric acid yields voltammograms with features characteristic of bulk polycrystalline platinum electrodes. In contrast, the voltammograms for Pt/TiO2 electrodes are not quantitatively similar to those for smooth polycrystalline platinum electrodes. Voltammetry in potassium ferricyanide yields reversible voltammograms, indicating the good electrical conductivity of and connectivity between the deposited platinum and substrates and hence the absence of any significant resistive surface oxide film. The electrocatalytic behaviour of this type of electrodes in the glycerol electro-oxidation was studied.

Electrochemical surface reshaping of polycrystalline platinum: Morphology and crystallography

A novel process of morphological evolution was observed on a polycrystalline platinum electrode surface during intensive potential perturbation in sulfuric acid electrolyte. The process was started from a smooth polycrystalline platinum surface, and ended up with a roughened overlayer which is composed of numerous morphological units. According to its appearance, the unit is named “feather-like morphology (FLM)” which contains finer structures at nanometer scale. The active surface area of platinum at different stages during the morphological evolution was estimated by cyclic voltammetry. Electron microscopic measurements provided both morphological details and crystallographic information.