Stability of carbon monoxide adsorbed on nanoparticle Pt and Pt/Ru electrodes in sulfuric acid media

Pt and Pt/Ru nanoparticle electrodes were used as substrates for studies of stability of chemisorbed CO and its interactions (exchange) with carbon monoxide and hydrogen gas admitted to sulfuric acid solution, which served as the supporting electrolyte for these measurements. The surface bound, radioactive CO was obtained after decomposition of ¹⁴C labeled methanol or formic acid from the same solution. The stability and/or exchange of the surface bound CO was examined after (i) the ¹⁴CCO containing electrolyte was replaced by the clean supporting electrolyte (to remove the CO solute from the bulk) and (ii) after hydrogen and, separately, non-radioactive CO was admitted to the cell. We found no displacement of adsorbed CO due to the exchange of electrolyte or introduction of hydrogen to the cell. However, non-radioactive CO rapidly displaced the adsorbed ¹⁴CCO on the surface leaving behind a monolayer of chemisorbed but non-radioactive CO. Both results were independent of the electrode potential applied and they neither dependent on the source of surface CO, that is, whether it was deposited from methanol or formic acid solutions.     

Anodic formaldehyde oxidation on Pt,Pd, Au and Pd-Au alloy electrodes in NaOH and Na2CO3 solutions

Electro-oxidation of HCHO on Pt, Pd, Au and Pd-Au alloy electrodes was carried out. In 0.2 M HCHO with 1 M NaOH, Au was electrocatalytically the most active, much more so than Pt or Pd, and was not seriously poisoned by brief contact with CO. The activity of the Pd-Au alloys lay between Au and Pd. In 0.2 M HCHO with 0.4 M Na₂CO₃, Pt and Au exhibited moderate activities. The possible use of HCHO as a fuel for fuel cell uses was favourably indicated.     

Carbon monoxide electrooxidation on porous Pt–Ru electrodes in sulphuric acid

CO electrooxidation on a Pt–Ru/C catalyst was investigated in sulphuric acid electrolyte. The physico-chemical properties of the Pt–Ru/C catalyst were studied by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The influence of temperature, CO partial pressure and proton concentration on the electrochemical oxidation rate was investigated by steady-state galvanostatic polarization measurements. The apparent activation energy decreased from 70 to 30kJmol–1 as the overpotential increased from 0.5 to 0.9V vs NHE. The reaction order with respect to carbon monoxide increased, passing from 0.4 to 1, with the increase of the overpotential from 0.5 to 0.7V vs NHE; a reaction order close to –1 with respect to the protonic concentration was observed, irrespective of the potential. Tafel slopes of about 136mVdec–1 were determined for oxidation of CO and CO/N2 mixtures.     

Electrochemical characterization of SnO2 electrodes doped with Ru and Pt

Antimony-platinum doped tin dioxide electrodes supported on titanium have been prepared by thermal decomposition. The effect of the progressive replacement of Sb with Ru (x = 0.00; 3.25; 6.50; 13.00 at.%) on their electrochemical response in acid medium has been analysed by cyclic voltammetry. The morphology of the coatings was observed by scanning electron microscopy. Ti/SnO₂-Sb-Pt electrodes without Ru presented a cracked-mud structure, typical of oxide electrodes prepared by thermal decomposition. The introduction of Ru in the oxide layer modified the coating morphology. The roughness increased and passed through a maximum with the increase of Ru content. A relation between the surface morphology, the roughness factor, voltammetric charge and the electrochemical activity has been established. The mechanism and electrocatalytic activity towards the oxygen evolution reaction has been studied from Tafel measurements. The progressive introduction of Ru in the electrodes increased their electrocatalytic activity for the oxygen evolution reaction with a change on the mechanism from non-active to active electrodes. The electrocatalytic activity mainly depends on electronic factors.     

Anodic oxidation of butan-1-ol on Pd and Pt electrodes in alkaline medium

Pure Pd is investigated as electro-catalyst for oxidation of butan-1-ol in aqueous NaOH solution and compared with the conventional catalyst, pure Pt. The steady-state anodic current density increases on increasing the potential up to ?52 and ?3 mV for Pt and Pd, respectively, and it reveals that Pd is a better electro-catalyst than Pt above ?0.18 V versus MMO [Hg/HgO/OH^?(0.1 M) having reversible electrode potential of 0.1 V vs. NHE]. Multiple scan cyclic voltammetry also reveals faster poisoning of Pt than Pd electrode. The Tafel slope for Pd (0.511 V dec^?1) is lower than Pt (2.30 V dec^?1) where as poisoning potential and current density of the former are greater than Pt indicating a relatively higher catalytic activity of Pd electrode. The FTIR study reveals the presence of aldehyde group in the products obtained with Pt unlike Pd electrode indicating different mechanistic pathways followed by them. With increase of potential in the range ?200 to 300 mV, the order with respect to butan-1-ol gradually decreases for Pt due to poisoning in the process of formation of complete oxidation product, CO₃ ^2?, but increases for Pd due to faster removal of adsorbed intermediates with the formation of mainly butyrate. In this study, different mechanisms are proposed for Pt and Pd which conform to the experimental results.     

Anodic polarization of silicon electrodes in water solutions of NaNO3

The polarization curves of silicon electrodes in sodium nitrate, ferric chloride, ferric sulphate and ferric nitrate obtained experimentally are presented. The curves calculated theoretically assuming the anodic passivation and the carrier generation in the space charge layer are compared with an empirical one derived from the equation of the generation in the space charge layer. The experimental results are in excellent agreement with the empirical curve providing that a suitable coefficient of the current multiplication is introduced.     

CO and methanol electrooxidation on Pt/Ir/Pt multilayers electrodes

This work describes the CO and methanol electrooxidation over an Ir/Pt bilayer electrodeposited on a platinum polycrystalline substrate. In the blank acidic solution it was observed that the electrochemical behavior of both the polycrystalline Pt and Pt/Ir/Pt nanostructured electrodes were very similar. The electroactive area, calculated using the hydrogen desorption method, are the same for both materials. In order to investigate the effect of the thickness change of Ir interlayer, two different samples were prepared. One with 1 Ir monolayer and the second with 3 monolayers thick. CO stripping voltammograms showed a shift in the anodic peak potential towards the negative direction of 160 and 180 mV for Pt/Ir/Pt 3:1 and 1:1 ML, respectively, compared to polycrystalline Pt. Besides, for methanol electrooxidation, the Pt/Ir/Pt electrodes presented an increase of 170% in the peak current density compared to polycrystalline Pt. These results are in agreement with the calculated activation energies which were 31.5, 39.0 and 43.5 kJ mol⁻¹ for Pt/Ir/Pt 1:1, 3:1 ML and polycrystalline Pt electrodes, respectively. Using the electrochemical impedance spectroscopy, surprisingly, the Pt/Ir/Pt electrodes, did not exhibit the inductive arc which means that the poisoning of the electrode surface is not important in this case.     

Reduction of nitrogen oxide on platinum electrodes in HClO4 and HCl

Reduction of NO in 2–8M HClO₄ and 2 M HCl at E ? 0.35V on platinized and smooth platinum electrodes is a diffusion-controlled process. On freshly activated electrodes the reduction in this potential region involves one-electron reaction to form N₂O, as well as multiple-electron reduction yielding NH₄⁺ and NH₃OH⁺. The product composition is largely dependent on the degree of electrode deactivation.     

The anodic evolution of oxygen on Co3O4 film electrodes in alkaline solutions

The Co₃O₄ film anodes for oxygen evolution were prepared by a thermal decomposition method and their anodic polarization characteristics were investigated in alkaline solutions. As a result, the polarization characteristics were found to be greatly affected by the kind of metal substrates used. Especially when Fe was used as a substrate, some Fe species were proved to be doped in the Co₃O₄ layer during the electrode preparation and the resulting electrodes showed the attractive character for oxygen evolution.     

Electrochemical reduction of oxygen on thin-film Pt electrodes in acid solutions

The electrochemical reduction of oxygen on thin-film platinum electrodes in 0.1 M HClO₄ and 0.05 M H₂SO₄ solutions has been investigated using the rotating disk electrode (RDE) method. Thin films of Pt (0.25-20 nm thick) were prepared by vacuum evaporation onto glassy carbon substrate. The surface morphology of Pt films was examined by transmission electron microscopy (TEM). The specific activity of O₂ reduction was higher in HClO₄ and decreased with decreasing film thickness. In H₂SO₄, the specific activity was lower and appeared to be independent of the Pt loading. The values of Tafel slopes close to −120 mV dec⁻¹ in high current density range and −60 mV dec⁻¹ in low current density range were obtained for all electrodes in both solutions, indicating that the mechanism of O₂ reduction is the same for thin-film electrodes as for bulk Pt. The number of electrons transferred per O₂ molecule was close to four for all thin Pt films studied.     

Kinetics of film growth on scratched bismuth electrodes

Data have been obtained for the kinetics of growth of the oxide film on scratched bismuth electrodes in 1.0 M NaOH solution. Growth of the oxide film occurs by a succession of two high field ion conduction mechanisms given by i = Aexp (BV/x). The results are compared with those obtained by potentiostatic pulsing of unscratched bismuth electrodes and the two types of data are in quantitative agreement.     

Temperature effects in the electrochemical behaviour of cycled lead electrodes in HCl solutions

The electrochemical behaviour of cycled lead electrodes has been investigated in HCl solutions in the temperature range ?6° to 30°C. Because of the low ionic strength of solution below ~0.8 M, and the associated ir drop problem, solutions stronger than 1.0 M were principally considered. In weaker solution (0.1–0.8 M) only derived current functions were analysed, whilst all potential-dependent parameters were rejected. Results are reported for both stationary and rotatiang disc electrodes.Because of the chemical dissolution of the surface layer of PbCl₂ formed on anodic scan, measurements were confined to time-scales of < 1 s. By applying these strict criteria, the overall reaction scheme was deduced. In particular, the kinetics of passivation were found to involve the three-dimensional growth of isolated PbCl₂ nuclei, which slowly dissolve into solution. At extreme anodic potentials, or at low temperature, passivation occurs by progressive, two-dimensional growth under diffusion control.A simultaneous electrochemical dissolution of Pb²⁺ at somewhat more negative potentials is noted, for which the rate-determining step is the diffusion of lead ion complexes away from the electrode surface. The nature of the reactivation of the electrode at extreme anodic potentials is also discussed.     

Corrosion and corrosion inhibition of nickel in HClO4 solutions using the EQCM technique

Application of the electrochemical quartz crystal microbalance (EQCM) to the study of metal corrosion and its inhibition is rather recent. Among the advantages of this technique are its very high sensitivity and the possibility of simultaneous mass variations and voltammogram recording. These characteristics suggest the use of the EQCM for research in very low corrosion rate conditions. This paper reports the results of EQCM measurements on the corrosion inhibition rates of Ni in 0.1m HClO4, in the absence and presence of different inhibitors in free corrosion conditions, for following inhibitors: acridine (A), benzyl quinolinium chloride (BQCl), dodecyl quinolinium bromide (DDQBr), tributylbenzyl ammonium iodide (TBNI) and potassium iodide (KI). The corrosion rate was reduced considerably by KI and TBNI. DDQBr showed a good inhibitive efficiency, while BQCl had only a small effect, and A stimulated corrosion of the Ni. Voltammograms at different scanning rates and the mass variation in the same solutions were recorded. Comparison of the current density and the mass changes provided the basis for a qualitative interpretation of the passivation of Ni and the mechanism of action of the different inhibitors.     

Corrosion and corrosion inhibition of nickel in HClO4 solutions using the EQCM technique

Application of the electrochemical quartz crystal microbalance (EQCM) to the study of metal corrosion and its inhibition is rather recent. Among the advantages of this technique are its very high sensitivity and the possibility of simultaneous mass variations and voltammogram recording. These characteristics suggest the use of the EQCM for research in very low corrosion rate conditions. This paper reports the results of EQCM measurements on the corrosion inhibition rates of Ni in 0.1m HClO4, in the absence and presence of different inhibitors in free corrosion conditions, for following inhibitors: acridine (A), benzyl quinolinium chloride (BQCl), dodecyl quinolinium bromide (DDQBr), tributylbenzyl ammonium iodide (TBNI) and potassium iodide (KI). The corrosion rate was reduced considerably by KI and TBNI. DDQBr showed a good inhibitive efficiency, while BQCl had only a small effect, and A stimulated corrosion of the Ni. Voltammograms at different scanning rates and the mass variation in the same solutions were recorded. Comparison of the current density and the mass changes provided the basis for a qualitative interpretation of the passivation of Ni and the mechanism of action of the different inhibitors.     

Anodic oxidation of bismuth in H2SO4 solutions

Anodic charging curves have been measured on polycrystalline Bi in H₂SO₄ solutions (0·01–6·0 N) at 30°C. The effect of various experimental procedures, eg electrode treatment and stirring, has been established. The anodic behaviour of Bi depends markedly on the acid concentration. In the high range, the results indicate dissolution which follows a Tafel relation. However, in dilute solutions, growth of oxide film occurs, and the potential rises rapidly with time, reaching over 100 V at high cds. Sparking and oxide breakdown start at about 150 V. Oxide growth follows the high field ionic conduction. The reciprocal capacitance is linearly related to the logarithm of cd. The constants of the exponential law, the half-barrier width, and the field strength have been calculated The last lies between 1 and 3 × 10⁶ V/cm at 1 mA/cm². The effect of F⁻ and Cl⁻ ions on oxide growth in H₂SO₄ has been also investigated.     

Some aspects of limonene oxidation at Pt electrodes in aqueous and acetonitrile solutions

Limonene oxidation does not occur at reasonable rates in aqueous media, but it is found that in acidified acetonitrile solutions containing some water, steady-state currents can be maintained. Obtained data suggest that water addition to a double bond happens before electrochemical oxidation in the acetonitrile solution takes place.     

Impedance spectroscopy of oxidized polyaniline and poly(o-ethoxyaniline) thin film modified Pt electrodes

Impedance spectra of potentiodynamically formed polyaniline (PANI) and poly(o-ethoxyaniline) (POEA), thin film modified Pt electrodes measured at potentials of conductive, emeraldine form in H₂SO₄ electrolyte solution were reported. Specific features of spectra obtained for films of different voltammetric charge densities were modelled and interpreted according to “homogeneous” and “distributed” impedance models. The values of charging capacitances of different PANI and POEA films were found independent of the model used. Charging of polymer films was in terms of both models controlled by charge transport processes being ion diffusion and migration in the first case, and polymer film and ionic conductivities in the second case. Distortion from Warburg-like response observed for both polymer films was explained by either differences in relaxation times of diffusion and migration processes, or by impedance of ionic channels with “anomalous” frequency dispersion. There were some discrepancies between theories and results suggesting that “homogeneous” model was more appropriate for POEA films while “distributed” model was more appropriate for PANI films, at least if both films were formed and measured in presently described conditions.     

Electrochemical preparation of photosensitive porous n-type Si electrodes, modified with Pt and Ru nanoparticles

A novel electrochemical procedure for preparation of the very stable, thin modifying layer onto the n-type Si surface was elaborated. The modification consisted of platinum or/and ruthenium ultrafine particles etched into the porous Si film. A unique sequence of modifications was applied: at first the metal particles were evenly electrodeposited onto a flat silicon surface, and in the next electrochemical step the porous structure was produced. The platinum coverage and mean particle diameter were well controlled by the electrochemical programs. All the attempts and progress in modifications were monitored by scanning electron microscope (SEM) observations. Furthermore, the materials obtained were compared with the non-porous, Pt or/and Ru modified electrodes by testing them as anodes in the photoelectrochemical (PEC) cell with organic Br₂/2Br⁻ solution.In general, the porous photo-anodes gave higher output powers and the light-to-electricity conversion efficiencies. The best performance was observed for the PEC cell employing the porous anode with sequentially electrodeposited Ru and Pt particles, respectively (PS-Si/Ru/Pt).¹ This cell maintained good electrical parameter values during the 2-week tests, having a maximum output power equal to 0.23 mW/cm² and a cell conversion efficiency of 8.5%. The PS-Si/Pt photo-anode gained 0.21 mW/cm² and 7.8%, respectively.     

Methanol oxidation at Pt/MoO x /MoSe2 thin film electrodes prepared with exfoliated MoSe2

This paper presents results on the dispersion of Pt on MoO _x /MoSe₂ electrodes, which were prepared by an intercalation–exfoliation technique. The Pt/MoO _x /MoSe₂ electrodes were tested for methanol oxidation by cyclic voltammetry. Thin films of MoSe₂ can be oxidized electrochemically to form a MoO _x layer. Compared with pure platinum, Pt/MoO _x /MoSe₂ electrodes gave lower oxidation potentials and higher current densities. We believe that methanol adsorption and subsequent dehydrogenation occur at the Pt atoms, while the OH_ads nucleation occurs at Mo sites. The presence of the adsorbed —OH groups on Mo sites may catalyze CO oxidation to CO₂. This surface reaction is necessary for the improved electrocatalytic behavior of Pt/MoO _x /MoSe₂ electrodes.     

Electrochemical Reduction of Oxygen on Carbon Supported Pt and Pt/Ru Fuel Cell Electrodes in Alkaline Solutions

A study of O₂ reduction in 1 M NaOH solution at gas diffusion electrodes made from carbon supported Pt and Pt/Ru catalysts is reported. Two Tafel regions were observed for both the Pt and Pt/Ru electrodes. Although the same mechanism was suggested for oxygen reduction on both Pt and Pt/Ru catalysts, the O₂ reduction activity was lower on Ru. Electrochemical Impedance Spectroscopy (EIS) analysis was carried out at different potentials and showed the significant contribution of diffusion on the reaction process and kinetics. The effect of methanol on O₂ reduction was investigated in solutions containing various concentrations of methanol. The electrode performance deteriorated with increasing methanol concentration because of a mixed cathode potential. The methanol tolerance, i. e., the methanol concentration which polarises the O₂ reduction reaction for O₂ reduction, at the Pt/C electrode with a Pt loading of 1.2 mg cm^–2 is 0.2 M methanol in 1 M NaOH.