CO and methanol oxidation at platinum-tin electrodes

Electrooxidation of dissolved CO and methanol at platinum-tin electrodes with different phase composition (a two-phase Sn/Pt-catalyst, Pt-Sn alloy, and Pt₃Sn intermetallic compound) is studied. All studied catalysts show higher catalytic activity in the CO oxidation at lower potentials (0.3–0.5 V against reversible hydrogen electrode (RHE)), as compared with platinum; no catalysis is observed at higher potentials (0.7 V); moreover, inhibiting is observed in some cases. The catalyst with the most strongly ordered structure (Pt₃Sn) demonstrated the highest catalytic activity; however, it appeared being less stable against oxidation at potentials more positive than 1.0 V. Catalytic effects were practically absent in the CO-adsorbate oxidation process. The sequence of catalyst activities in the methanol oxidation process differed from that in the CO oxidation; in particular, Pt3Sn appeared being the least active. The observed difference can be associated with the difference in the CO and methanol adsorption mechanisms. The effect of the carbonaceous support dispersion on the current-voltage curves is discussed.     

Carbon supported electrocatalysts prepared by the sol–gel method and their utilization for the oxidation of methanol in acid media

One of the key objectives in fuel-cell technology is to improve the performance of the anode catalyst for the alcohol oxidation and reduce Pt loading. Here, we show the use of six different electrocatalysts synthesized by the sol–gel method on carbon powder to promote the oxidation of methanol in acid media. The catalysts Pt–PbO _x and Pt–(RuO₂–PbO _x ) with 10% of catalyst load exhibited significantly enhanced catalytic activity toward the methanol oxidation reaction as compared to Pt–(RuO₂)/C and Pt/C electrodes. Cyclic voltammetry studies showed that the electrocatalysts Pt–PbO _x /C and Pt–(RuO₂–PbO _x )/C started the oxidation process at extremely low potentials and that they represent a good novelty to oxidize methanol. Furthermore, quasi-stationary polarization experiments and cronoamperometry studies showed the good performance of the Pt–PbO _x , Pt–(RuO₂–PbO _x )/C and Pt–(RuO₂–IrO₂)/C catalysts during the oxidation process. Thus, the addition of metallic Pt and PbO _x onto high-area carbon powder, by the sol–gel route, constitutes an interesting way to prepare anodes with high catalytic activity for further applications in direct methanol fuel cell systems.     

Nickel foam-based composite electrodes for electrooxidation of methanol

Nickel foam and five nickel foam-based composite electrodes were prepared for being used as anode materials for the electrooxidation of methanol in KOH solution containing 0.1 and 1.0 M of methanol. The layered electrodes composed of nickel foam, platinum nanoparticles, polyaniline (PANI) and/or porous carbon (C) prepared in various assemblies. As shown by SEM analysis, depending on the preparation conditions, the electrodes of different morphologies were obtained. Using the cyclic voltammetry method, the oxidation of methanol on nickel foam electrode was observed in the potential range 0.4 V ↔ 0.7 V, where the Ni(OH)₂/NiOOH transformation occurred. The presence of Pt particles in electrode gave rise to the increase in electrocatalytic activity in this potential range. For electrodes containing dispersed platinum catalyst (Ni/Pt, Ni/PANI/Pt and Ni/C/Pt), the oxidation of methanol was noted also in the potential range −0.5 V ↔ 0.1 V. The electrocatalytic activities of the examined electrodes toward methanol oxidation at low potentials were in order Ni/Pt > Ni/C/Pt > Ni/PANI/Pt, whereas at high potentials in order Ni/PANI/Pt > Ni/Pt> Ni/C/Pt > Ni. Among the examined electrodes, the most resistant to cyclic poisoning appeared to be the Ni/C/Pt electrode. Presented at the 4Th Baltic Conference on Electrochemistry, Greifswald, March 13–16, 2005     

In situ transmission difference FTIR spectroscopic investigation on anodic oxidation of methanol in aqueous solution

In situ transmission difference FTIR spectroscopy method was introduced for studying the anodic oxidation of methanol in acid aqueous solution. A minigrid Pt optically transparent thin layer electrode was used as working electrode. This method has the ability to clarify the identity of species involved in the oxidation process both in solution and adsorbed at the surface of electrode. From the results of in situ transmission difference FTIR spectroscopy measurement it was found that HCHO, HCOOH, HCOOCH₃ and CH₂(OCH₃)₂ could be formed in the oxidation process of methanol. The final product was CO₂. The adsorbed poisonous intermediate CO was detected. It was formed at near 0.6 V and became significant at 0.9 V, where the oxidation current was inhibited. The in situ transmission difference FTIR spectroscopy method is a very convenient, relative simplicity and efficient method for investigating the electrochemical process, and could be as a good candidate for further application.     

Mechanism of soft solution processing formation of alkaline earth metal tungstates: an electrochemical and in situ AFM study

The mechanism of alkaline earth metal tungstate formation during soft solution processing was studied by cyclic voltammetry, electrochemical impedance spectroscopy and by direct in situ observation of the surface changes using atomic force microscopy. The electrochemical oxidation of W to WO₃ was followed by dissolution of WO₃ and, with some delay, by precipitation of tungstates at the metal surface. The same Tafel slopes observed in Li⁺, Ba²⁺, Sr²⁺ and Ca²⁺ containing solutions indicate that the course of the oxidation process is independent of the cation present in solution. The observed differences in the current-voltage curves outside the Tafel region are accounted for by the different film-forming tendencies of the various alkaline earth metal cations. The growth of tungstate layers at the W substrate decreases the electrochemically active area and limits the production of WO₄ ^2– at later stages of deposition. At low potentials (E<0.2 V) the oxidation of W is the rate-controlling step. At higher potentials, however, the dissolution process slows down due to a relative decrease of the pH in the electrode vicinity and dissolution becomes the rate-limiting step. Electronic Publication     

Hydrous oxide formation on platinum—A useful route to controlled platinization

The formation of thick hydrous oxide films on platinum under triangular potential cycling conditions was investigated as a function of sweep-rate, sweep limits, and solution pH. A significant improvement in the procedure outlined earlier was the discovery that increasing the cycling rate (from ca. 5 to 100 V s^?1) decreased the optimum upper limit (from ca. 2.8 to 2.2 V in acid) for thick film growth. Hydrous oxide growth was observed in both acid and base but not at intermediate pH values (ca. 4.0 to 9.0). For reasonably short cycling time (<15 min at 100 V s^?1) loss of platinum due to dissolution was virtually negligible. One immediate practial application of the work is the activation (or reactivation) of platinum surfaces. A brief investigation of the methanol electrooxidation reaction at initially smooth platinum, activated by potential cycling, followed by cathodic reduction of the hydrous film, demonstrated that excellent control of the surface roughness and, hence, the level of electrocatalytic activity of the electrode surface was possible using this approach. An interesting fundamental point emerging from the work is that from a thermodynamic viewpoint platinum may be susceptible to oxidation to form an insoluble, highly hydrated species at quite low potentials (less than 0.0 V vs. RHE) in base. The reason why such species are not normally observed under positive, single-sweep conditions is probably due to the inability of the ligands involved (O, OH and OH₂ species) to coordinate in a symmetrical manner about a platinum atom that is partially imbedded in a metal lattice.     

Electrochemical oxidation of chlorinated benzenes

The electrochemical oxidation of benzene and its chloro derivatives (n=1–3) has been studied in acetonitrile, water, and an aqueous emulsion at both metallic electrodes and dimensionally stable anodes. In acetonitrile all substrates underwent a six-electron oxidation near +3 V versus the standard hydrogen electrode. Low yields of organic products, principally phenols, were obtained; electrochemical combustion to CO₂ was a major reaction pathway, but current efficiencies were low.     

Mechanistic – Kinetic Scheme of Oxidation/Reduction of TEMPO Involving Hydrogen Bonded Dimer. RDE Probe for Availability of Protons in Reaction Environment

Mechanistic – kinetic studies on the electrochemical oxidation/reduction process of radical TEMPO (2,2,6,6‐tetramethylpiperidine‐1‐oxyl) under ionic strength (0.1 M, 1.0 M) and pH (0, 7) of aqueous perchlorate electrolyte (NaClO₄‐HClO₄) have been undertaken. Analytical and/or digital simulation methods for voltammetry at stationary (CV) and rotating electrode (RDE) have allowed one to determine numerical values of twelve parameters characterizing two electrode reactions (oxidation and reduction of the radical) and three chemical reactions (protonation, disproportionation, dimerization involving the radical and/or electrogenerated species). A potential window of the measurements was 0.6 V and it corresponded to that where the oxidation wave of TEMPO in neutral aqueous solution is situated. To account for the observed pH effect, the hydrogen bonded dimer resulting from the radical reactant and the protonation product of its reduction has been postulated to form in solution near the electrode surface. The RDE voltammetric discernables of the TEMPO process (i.e., absolute RDE wave current, zero RDE current potential, oxidation and reduction limiting RDE currents) can be considered good candidates for a use to follow acidity of complex reactive media.     

Vertically Aligned Porous Nickel(II) Hydroxide Nanosheets Supported on Carbon Paper with Long‐Term Oxygen Evolution Performance

Vertically aligned Ni(OH)₂ nanosheets were grown on carbon paper (CP) current collectors through a simple and cost‐effective hydrothermal approach. The as‐grown nanosheets are porous and highly crystallized. If used as a monolithic electrode for electrochemical water oxidation in alkaline solution, the carbon paper supported Ni(OH)₂ nanosheets [CP@Ni(OH)₂] exhibit high electrocatalytic activity and excellent long‐term stability. The electrode can attain an anodic current density of 20 mA cm⁻² at a low overpotential of 338 mV, comparable to that of state‐of‐the‐art RuO₂ nanocatalysts supported on CP (CP/RuO₂) with the same catalyst loading. Significantly, CP@Ni(OH)₂ shows much better long‐term stability than CP/RuO₂ upon continuous galvanostatic electrolysis, particularly at a high industry‐relevant current density such as 100 mA cm⁻². CP@Ni(OH)₂ can sustain water oxidation at 100 mA cm⁻² for 50 h without any degradation, whereas the performance of CP/RuO₂ is much poorer and deteriorates gradually over time. CP@Ni(OH)₂ electrodes hold substantial promise for use as low‐costing water oxidation anodes in electrolyzers.     

Effects of thermal activation on the oxidation pathways of methanol at bulk Pt–Ru alloy electrodes

The competition between pathways that lead to adsorbed CO and CO₂ during the electrochemical oxidation of 1.0 M methanol in 0.1 M HClO₄ on two bulk Pt–Ru alloys (10 at.% Ru (X_Ru≈0.1) and 90 at.% Ru (X_Ru≈0.9)) was investigated for temperatures in the range of 25–80°C. On the high Ru content alloy studied (X_Ru≈0.9), the dissociative chemisorption of methanol was inhibited below 70°C; the faradaic current for methanol oxidation was low, and only small quantities of adsorbed CO and CO₂ were detected with infrared spectroscopy between 0.2–0.8 V (vs. RHE). At 80°C, strong infrared bands from CO₂ and adsorbed, atop coordinated CO were observed over the potential ranges of 0.4–0.8 V and 0.2–0.8 V, respectively. The infrared measurements are consistent with the observation that bulk, high Ru content alloy electrodes appear passivated toward methanol oxidation below 70°C. On the low Ru content alloy studied (X_Ru≈0.1), the methanol surface chemistry was similar to that of pure, polycrystalline Pt, but the electrode was more poison resistant than Pt. For both alloys, the persistence of strong adsorbed CO bands and rapid CO₂ production between 0.4–0.8 V suggests CO functions as a reactive species with high steady-state coverages at these potentials.     

RuO<Subscript>2</Subscript>–TiO<Subscript>2</Subscript> mixed oxide composite coating for improvement of Al-alloy sacrificial anodes

It has been recently proved that RuO₂ can act as an effective surface activator of aluminum alloy sacrificial anodes. TiO₂ has the property of stabilizing RuO₂ coating and resisting biofouling on metal surfaces. Hence, a mixed oxide catalytic coating of TiO₂ and RuO₂ can enhance the galvanic performance of aluminum alloy sacrificial anodes and resists biofouling on the anode surface. In the present work RuO₂–TiO₂ mixed oxide was coated on aluminum alloy sacrificial anodes. The large and uniform porous nature of the coating was found to facilitate efficient ion diffusion. The coating was found to persist on the anode even after 3 months of galvanic exposure. The anode having an optimum combination of the mixed oxide had 70% TiO₂ as the major component in the coating. The catalytic coating significantly improved the performance of the anodes to a large extent.     

Electroanalysis of Dopamine at RuO2 Modified Vertically Aligned Carbon Nanotube Electrode

Electrochemical behavior of dopamine at the RuO₂‐modified vertically aligned carbon nanotubes electrode was investigated by cyclic voltammetry, differential pulse voltammetry and chronoamperometry. The RuO₂‐modified carbon nanotube electrode showed higher electrocatalytic activity towards the oxidation of dopamine than the MWNTs electrode in 0.10 M phosphate buffer solution. At an applied potential of +0.4 V, the RuO₂/MWNTs electrode exhibited a wide detection range up to 3.6×10^?3 M with detection limit of 6.0×10^?8 M (signal/noise=3) for dopamine determination. Meanwhile, the optimized sensor for dopamine displayed a sensitivity of 83.8 μA mM^?1 and response time of 5 s with addition of 0.20 mM dopamine. In addition, DPV experiment revealed that interfering species such as ascorbic acid and uric acid could be effectively avoided. The RuO₂/MWNTs electrode presents stable, highly sensitive, favorable selectivity and fast amperometric response of dopamine.     

RuO2 electrode surface effects in electrocatalytic oxidation of glucose

We have studied the electrocatalytic activity of RuO₂-PVC film electrodes, fabricated using RuO₂ powders prepared at five different temperatures, viz., 300, 400, 500, 600 and 700°C, for the oxidation of glucose in high alkaline media, 1 to 3 M NaOH. The RuO₂-PVC film electrodes have been first characterized in 1 to 3 M NaOH solution by cyclic voltammetry (CV) and rotating disc electrode (RDE) techniques in a wide potential range −1,100 to 450 mV (SCE), and three redox pairs representing Ru(IV)/Ru(III), Ru(VI)/Ru(IV) and Ru(VII)/Ru(VI) transitions have been identified. The voltammetric peaks at low sweep rates have been analyzed using surface activity theory formulated for interacting electroactive adsorption sites, and interaction terms have been evaluated. The total voltammetric surface charges have been analyzed as per Trassatti’s formalism with respect to their dependence on potential sweep rate, and charges associated with less accessible and more accessible surface sites have been calculated. For glucose oxidation, the results have indicated that RuO₂ (700°C)-PVC electrode shows two oxidation peaks in contrast to RuO₂ (300°C)-PVC electrode. Also, RuO₂ (700°C)-PVC electrode exhibits higher intrinsic electrocatalytic activity than the 300°C electrode, although the former possesses lower electrochemically active surface area. Additionally, kinetic analyses made from RDE results with reference to Michealis–Menten (MM) enzyme catalysis has shown that RuO₂ (700°C) electrode possesses extended glucose-sensing range in terms of MM kinetic constant, K _M , compared to other electrodes. Possible reasons for such differences in the behavior of the electrodes of different temperatures towards glucose oxidation are identified from studies on oxidation of glucose in solutions of different pH, oxidation of different glucose derivatives, and also from physicochemical results from BET, XRD, SEM, DTGA, XPS analysis of RuO₂ powder samples.     

Aqueous Photoelectrochemical CO2 Reduction to CO and Methanol over a Silicon Photocathode Functionalized with a Cobalt Phthalocyanine Molecular Catalyst

We report a precious-metal-free molecular catalyst-based photocathode that is active for aqueous CO₂ reduction to CO and methanol. The photoelectrode is composed of cobalt phthalocyanine molecules anchored on graphene oxide which is integrated via a (3-aminopropyl)triethoxysilane linker to p-type silicon protected by a thin film of titanium dioxide. The photocathode reduces CO₂ to CO with high selectivity at potentials as mild as 0 V versus the reversible hydrogen electrode (vs RHE). Methanol production is observed at an onset potential of −0.36 V vs RHE, and reaches a peak turnover frequency of 0.18 s⁻¹. To date, this is the only molecular catalyst-based photoelectrode that is active for the six-electron reduction of CO₂ to methanol. This work puts forth a strategy for interfacing molecular catalysts to p-type semiconductors and demonstrates state-of-the-art performance for photoelectrochemical CO₂ reduction to CO and methanol.     

Polarographic reduction of benzylidenemalononitrile

The mechanism of reduction of benzylidenemalononitrile (1), C₆H₅CH=C(CN)₂, at the dropping mercury electrode in aqueous solutions containing 50% methanol changes with pH. In acidic solutions the reduction proceeds by a four-electron transfer to the monoprotonated species, yielding an aminomethyl derivative. In alkaline solutions two one-electron steps involving the vinyl double bond reduction are observed. Hydrolysis of dinitrile 1 to form benzaldehyde complicates the study at high pH values. The system is unique in that at pH 5–6 the unprotonated form is reduced at more positive potentials than the protonated form, reflecting a change in the mechanism of the reduction process.     

X-ray photoelectron spectroscopic and electrochemical impedance spectroscopic analysis of RuO2-Ta2O5 thick film pH sensors

The paper reports on investigation of the pH sensing mechanism of thick film RuO₂-Ta₂O₅ sensors by using X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). Interdigitated conductimetric pH sensors were screen printed on alumina substrates. The microstructure and elemental composition of the films were examined by scanning electron microscopy and energy dispersive spectroscopy. The XPS studies revealed the presence of Ru ions at different oxidation states and the surface hydroxylation of the sensing layer increasing with increasing pH. The EIS analysis carried out in the frequency range 10 Hz–2 MHz showed that the electrical parameters of the sensitive electrodes in the low frequency range were distinctly dependent on pH. The charge transfer and ionic exchange occurring at metal oxide-solution interface were indicated as processes responsible for the sensing mechanism of thick film RuO₂-Ta₂O₅ pH sensors.     

Studies on the role of oxidation states of the platinum surface in electrocatalytic oxidation of small primary alcohols

The role of the oxidation state of a platinum polycrystalline surface in the electrocatalytic oxidation of C₁ to C₄ primary alcohols has been studied by using electrochemical techniques, in situ FTIR spectroscopy and X-ray photoelectron spectroscopy. The results revealed that the oxidation state of the Pt surface plays a key role in the oxidation of primary alcohols, and demonstrated that the oxidation of C₁ to C₄ primary alcohols on a Pt electrode is controlled by the formation of surface oxides on the Pt electrode at different potentials. It was found that the dependence of the reaction process on the oxidation states of the platinum surface yielded similar features in the cyclic voltammogram for oxidation of different primary alcohols at a Pt electrode. According to the effects in the oxidation of primary alcohols, the surface oxides of platinum may be classified as active and poison species. The Pt surface oxides of higher oxidation states (Pt(OH)₃ and PtO₂) formed at potentials above 1.0 V (SCE) were identified as poison species, while other lower oxidation states of Pt surface oxides such as PtOH, Pt(OH)₂ and PtO may be identified as the possible active species for primary alcohol oxidation.     

Visible light photoelectrocatalytic degradation of rhodamine B using a dye-sensitised TiO2 electrode

Titanium dioxide is a promising catalyst for application in the photodegradation of organic pollutants in water due to its powerful oxidising property and long-term photostability. This study presents the production of titanium dioxide using the sol-gel process, dye sensitisation of the TiO₂ electrode, and the performance of that cell. Sensitisation of titanium dioxide was performed using a dye, i.e., Fe(II)-polypyridyl complexes. The photoelectrocatalytic degradation of rhodamine B (RB) using ITO/TiO₂/dye as electrode was investigated via a series of potentials, from +1.0 V to ?1.0 V, and at various pH and NaCl concentration values (ITO is indium tin oxide conductive glass). The photoelectrocatalytic degradation of RB was performed with a visible light lamp. The change in the absorbance of RB with various potentials indicated that the absorbance of RB in solution systems with the sensitised TiO₂ electrodes decreased with increasing anodic potential bias. The degradation cell exhibited better performance when the positive anodic bias was applied. The pH values of RB in solution systems also influence the photoelectrodegradation process because of the different RB species present. NaCl concentration also affects the activity of RB photoelectrocatalytic degradation due to changes in the ionic strength character of the electrolyte.     

Electroanalytical Determination of Some Phenolic Acids by High‐Performance Liquid Chromatography at Gold Electrodes

The electrochemical and amperometric behavior of a gold electrode was investigated towards the oxidation of several common phenolic acids in neutral phosphate solutions. Au electrodes show an appreciable stability and reproducibility of the amperometric signals by using a constant applied potential of 1.0 V vs. Ag/AgCl. Separations of selected phenolic acids using a reverse phase C₁₈ analytical column with a mobile phase containing 10 mM NaH₂PO₄ plus 10 mM Na₂HPO₄ (pH 7) and methanol as organic modifier, are achieved isocratically in less than 30 min. The detection limits at the level of nmol/L and linear ranges of four‐five orders of magnitude are generally achieved. The proposed chromatographic strategy coupled with the electrochemical detection at the Au electrode was successful tested for the quantitative determination of phenolic acids in beer, red wine and brandy with good sensitivity and recovery.     

Degradation of the Pt-MoO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> composite catalyst in methanol solutions

Pt-MoO _x deposits synthesized by an electrochemical method are shown to be instable in methanol solutions at potentials ≥0.3 V. It is assumed that molybdenum oxides in the deposit composition react with methanol. The reduction of molybdenum compounds with methanol to form soluble Mo(+3) complexes in the presence of platinum is confirmed by spectrophotometric data. The MoO _x reaction with methanol leads to the removal of molybdenum compounds from the electrode surface, which is accompanied by the loss of its catalytic activity in the methanol oxidation reaction.