Acetaldehyde electrooxidation: The influence of concentration on the yields of parallel pathways

Parallel pathways forming CO₂ and acetic acid occur during the electrooxidation of acetaldehyde at Pt in acid medium. The yields of products depend on potential and acetaldehyde concentration. In the whole range of concentrations investigated (2.5 × 10⁻³ – 0.5 M) and at potentials below 0.6 V, CO₂ is the only product of acetaldehyde oxidation. Acetic acid is detected at potentials higher than 0.7 V. According to the analysis of products using FTIR spectroscopy, a maximum yield of CO₂ production is obtained for an acetaldehyde concentration of 0.01 M at 0.6 V. The pathway forming CO₂ is strongly inhibited for 0.5 M of acetaldehyde. It is suggested that, at high concentrations, a competition with water for active sites occurs, which inhibits the oxidation of adsorbed species, which probably follow a Langmuir–Hinshelwood mechanism.     

Hydrogen evolution reaction and formic acid oxidation by decorated nanostructural Pt/Pd on a copper-filled nanoporous stainless steel

In this work, a 304 stainless steel (SS) was anodized to prepare nanoporous SS (NPSS) with an average size of about 75 nm and then filled with copper (Cu/NPSS) using pulsed electrodeposition method. Afterward, a nanostructural Pt and Pd film was deposited by galvanic replacement (GR) on the Cu/NPSS to prepare modified electrode (PtPd/Cu/NPSS) for hydrogen evolution reaction (HER) and formic acid electrooxidation (FAO). The electrocatalytic activity of the modified electrode and its structural characterization have been studied by voltammetric methods, electrochemical impedance spectroscopy (EIS), inductively coupled plasma optical emission spectrometry (ICP-OES), and field emission scanning electron microscopy (FESEM). The results show that the nanostructural Pt₁Pd₁/Cu/NPSS composition, with low Pt loading and suitable stability, has a good electrocatalytic performance toward HER (E_Onset = + 12 mV vs. NHE) and FAO (E_Onset = ?180 mV vs. NHE). For HER observed a high mass activity of noble metals (87.54 mA cm^?2μg _Pd+Pt ^?1 ) in comparison with Pt deposited Cu/NPSS (41.5 mA cm^?2 μg _Pt ^?1 ) at the same applied potential of ? 0.25 V versus NHE. Also, the fabricated electrocatalysts with more electrochemically active surface area in comparison with Pd/Cu/NPSS and Pt/Cu/NPSS revealed more resisting to the poisoning components and good stability for FAO.     

DEMS study on the nature of acetaldehyde adsorbates at Pt and Pd by isotopic labelling

The formation of acetaldehyde adsorbates on Pt and Pd has been studied applying cyclic voltammetry and differential electrochemical mass spectrometry (DEMS). The adspecies were isolated on the metal surface at selected adsorption potentials (E _ad) applying a flow cell procedure under potential control, and the anodic stripping were performed for each E _ad. For Pt, two different contributions were established during oxidation: one at E < 0.80 V and the second in the range 0.80–1.50 V in the Pt oxide region. For Pd, the voltammetric profile resembles that for the oxidation of adsorbed CO. DEMS experiments have shown that CO₂ was the sole electro-oxidation product in both cases. The oxidation of each C atom in acetaldehyde adsorbates has been distinguished using the isotopic-labelled aldehyde in DEMS experiments at selected E _ad. It was observed that, on Pt, acetaldehyde molecules loose part of the CH₃ groups during adsorption at E _ad < 0.40 V, whereas the CHO groups are easily oxidized at E _ad > 0.40 V. Therefore, both C₁ and C₂ species are present on the surface, and their yields depend on E _ad. On the contrary, on Pd, most of the CH₃ groups are lost during adsorption at all E _ad, and the main adsorbed species seems to be CO_ad. Dedicated to Prof. Dr. Teresa Iwasita on the occasion of her 65th birthday in recognition of her numerous contributions to interfacial electrochemistry.     

Chemical and electrochemical behavior of metallic technetium in acidic media

The chemical and electrochemical properties of technetium metal were studied in 1–6 M HX and in 1 M NaX (pH 1 and 2.5), X = Cl, NO₃. The chemical dissolution rates of Tc metal were higher in HNO₃ than in HCl (i.e. 8.63 × 10^?5 mol cm^?2 h^?1 in 6 M HNO₃ versus 2.05 × 10^?9 mol cm^?2 h^?1 in 6 M HCl). The electrochemical dissolution rates in HNO₃ and HCl were similar and mainly depended on the electrochemical potential and the acid concentration. The optimum dissolution of Tc metal was obtained in 1 M HNO₃ at 1 V/AgAgCl (1.70 × 10^?3 mol cm^?2 h^?1). The dissolution potentials of Tc metal in nitric acid were in the range of 0.596–0.832 V/AgAgCl. Comparison of Tc behavior with Mo and Ru indicated that in HNO₃, the dissolution rate followed the order: Mo > Tc > Ru, and for dissolution potential the order: E _diss(Ru) > E _diss(Tc) > E _diss(Mo). The corrosion products of Tc metal were analyzed in HCl solution by UV–Visible spectroscopy and showed the presence of TcO₄ ^?. The surface of the electrode was characterized by microscopic techniques; it indicated that Tc metal preferentially corroded at the scratches formed during the polishing and no oxide layer was observed.     

Kinetics of ethylene glycol electrooxidation on the noble metal-based nano-catalysts

A comparative electrooxidation of Eg in the alkaline solution was investigated over Pt, Pd and Au nanoparticle-modified carbon-ceramic electrode. The kinetic parameters of Eg oxidation, i.e., Tafel slope and activation energy (E _a), were determined on the modified electrodes. The lowest E _a value of 8.9 kJ mol^?1 was calculated on Pt|CCE. In continuation, the reaction orders with respect to the Eg and NaOH concentrations on Pd|CCE were found to be 0.4–0.2 and 0.6, respectively. An adsorption equilibrium constant (b) of 22.36 M^?1 and the adsorption Gibbs energy change (ΔG°) of ?7.7 kJ mol^?1 were obtained on Pd|CCE. The chronopotentiometry (CP) and chronoamperometry (CA) results showed that Pd|CCE and then Au|CCE have better performance stability than Pt|CCE for Eg electrooxidation. Additionally, the electrochemical impedance spectroscopy (EIS) suggested faster electron-transfer kinetics on Pt than that on the Pd and Au electrocatalysts.     

The influence of Pt-activation on the corrosion of carbon in gas diffusion electrodes—A dems study

Investigations of the dependence on the potential of the anodic oxidation of carbon electrodes using differential electrochemical mass spectroscopy (DEMS) show that pure carbon is oxidized only at potentials higher than 0.9 V (RHE) (with CO₂ and, to a lesser extent, CO being the main products), and that Pt activation catalyzes the oxidation of a CO_surf surface layer to CO₂ at potentials between 0.6 and 0.8 mV (RHE), with the CO_surf being formed on the carbon at E>0.3 V (RHE).It is assumed that the Pt-induced carbon corrosion occurs in the neighbourhood of the Pt-sites, thus damaging the Pt to carbon contact. Surface segregation of Pt-clusters and a loss of catalytic activity is the result.     

Nanocrystalline tungstic carbide/graphitic carbon composite: synthesis,characterization, and its application as an effective Pt catalyst support for methanol oxidation

Tungsten carbide and graphitic carbon (WC/GC) composite has been synthesized by a simple solid-state pyrolysis method from an in situ route. The results indicate that the synthesized sample has a large specific surface area (S _BET) of 198 m² g^?1, and the WC nanoparticles (NPs) with a narrow particle size are well dispersed on the graphitic carbon. After loading Pt nanoparticles, the prepared Pt/WC/GC catalyst exhibits a mass activity of 416.1 mA mg^?1 Pt toward methanol electrooxidation, which is much higher than that of commercial Pt/C (JM) (231.2 mA mg^?1 Pt). Moreover, the onset potential is 100 mV more negative than that on Pt/C (JM) electrocatalyst. In addition, the Pt/WC/GC catalyst has stronger resistance to CO poisoning than the commercial Pt/C (JM). Its superior electrochemical performance could be attributed not only to the synergistic effect between Pt and WC NPs but also to the excellent electrical conductivity of GC and proper porous structure for desirable mass transportation in a porous electrode.     

Voltammetric study of the nature of adsorbed residues arising from irreversible adsorption of acetaldehyde and ethanol on Pt(111) in acid media: first oxidation peak

Irreversible adsorption of ethanol and acetaldehyde on Pt(111) in acid medium results in different adsorbed compounds. This paper describes a study of the nature of the adsorbed species, giving an anodic voltammetric peak at 0.56 V (vs. RHE). These species could be associated with one carbon atom species, because (i) the existence of adsorbed CO (in the case of ethanol and acetaldehyde) implies the rupture of the C---C bond, and (ii) the electrochemical reduction of formaldehyde at potentials lower than or equal to 0.1 V gives the peak at 0.56 V. This reduction must give one carbon atom species.     

Electrocatalysis on binary alloys: II. Oxidation of molecular hydrogen on supported Pt+Ru alloys

The electrocatalytic properties of Pt+Ru alloys supported on graphitized carbon have been studied using oxide-free metal alloys that have been well characterized for phase identification, specific metal surface area, and surface composition. The CO tolerance of the Pt+Ru alloys for the oxidation of CO contaminated hydrogen in hot concentrated H₃PO₄ increases monotonically with Ru content of the surface and is a direct result of a decreasing coverage of the alloy by adsorbed CO. Furthermore, the strength of bonding of adsorbed CO with the metal surface decreases dramatically with increasing Ru content in the surface. The absolute activity of Pt+Ru alloys for the oxidation of CO contaminated hydrogen is a complex function of temperature and electrode potential. At 160°C, pure Pt is the most active catalyst at all potentials, but at temperatures lower than 120°C the reaction-limiting current for pure Ru exceeds that of pure Pt. At any temperature from 110–160°C or any electrode potential from 0–0.3V (HE), the variation of electrocatalytic activity with alloy composition indicates only dilution of the activity of the more active component.     

Determination of trace amounts of plutonium in low-active liquid wastes from spent nuclear-fuel reprocessing plants by flow injection-based solid-phase extraction/electrochemical detection system

A flow injection-based electrochemical detection system coupled to a solid-phase extraction column was developed for the determination of trace amounts of plutonium in low-active liquid wastes from spent nuclear-fuel reprocessing plants. The oxidation state of plutonium in a sample solution was adjusted to Pu(VI) by the addition of silver(II) oxide. A sample solution was made up in 3 mol L^?1 HNO₃ and loaded onto a column packed with UTEVA^® with 3 mol L^?1 HNO₃ as the carrier. Plutonium(VI) was adsorbed onto the resin, and interfering elements were removed by rinsing the column with 3 mol L^?1 HNO₃. Subsequently, the adsorbed Pu(VI) was eluted with 0.01 mol L^?1 HNO₃, and then introduced directly into the flow-through electrolysis cell with boron-doped diamond electrode. The eluted Pu(VI) was detected by an electrochemical amperometric method at a working potential of 0.1 V (vs. Ag/AgCl). The current produced on reduction of Pu(VI) was continuously monitored and recorded. The plutonium concentration was calculated from the relationship between the peak area and concentration of plutonium. The relative standard deviation of ten analyses was 1.1% for a plutonium solution of 25 μg L^?1 containing 50 ng of Pu. The detection limit calculated from three-times the standard deviation was 0.82 μg L^?1 (1.6 ng of Pu).     

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.     

Magneli phase Ti n O2n − 1 as corrosion-resistant PEM fuel cell catalyst support

Magneli phase titanium suboxide, Ti _n O_{2n ? 1}, with Brunauer–Emmett–Teller surface area up to 25 m² g^?1 was prepared using the heat treatment of titanium oxide (rutile) mixed with polyvinyl alcohol in ratios from 1:3 to 3:1. XRD patterns showed Ti₄O₇ as the major phase formed during the heat treatment process. The Ti _n O_{2n ? 1} showed excellent electrochemical stability in the potential range of ?0.25 to 2.75 V vs. standard hydrogen electrode. The Ti _n O_{2n ? 1} was employed as a polymer electrolyte membrane fuel cell catalyst support to prepare 20 wt% platinum (Pt)/Ti _n O_{2n ? 1} catalyst. A fuel cell membrane electrode assembly was fabricated using the 20 wt% Pt/Ti _n O_{2n ? 1} catalyst, and its performance was evaluated using H₂/O₂ at 80 °C. A current density of 0.125 A?cm^?2 at 0.6 V was obtained at 80 °C.     

Silica veils-conductive diamond powder composite as a new propitious substrate for platinum electrocatalysts

An innovative composite was obtained by a straightforward sol-gel procedure, involving boron-doped diamond powder (BDDP) incorporation into a SiO₂ veil (SiO₂V) matrix. Composite-coated glassy carbon plates were used as substrate for Pt electrochemical deposition, and the electrodes thus obtained (Pt/BDDP–SiO₂V) were compared on a relative basis with those prepared in the absence of the silica matrix (Pt/BDDP). SEM measurements have shown that a BDDP substrate promotes Pt cluster formation, whereas on BDDP–SiO₂V, particles are much smaller (ca. 45 nm to ca. 140 nm). The activity for CH₃OH oxidation was checked by cyclic voltammetry, and it was found that at Pt/BDDP–SiO₂V, the main anodic peak is shifted with ca. 0.35 V toward lower potentials, indicating a considerable improvement in the overall process kinetics. Stripping experiments together with long-term polarization measurements demonstrated that when deposited on the BDDP–SiO₂V support, Pt particles are less susceptible to CO poisoning and this behavior was tentatively ascribed to the presence of a higher relative surface concentration of more stable, oxidized platinum species, as evidenced by XPS.     

Sputtering deposition of Pt nanoparticles on vertically aligned multiwalled carbon nanotubes for sensing L-cysteine

A highly sensitive electrochemical sensor for determination of L-cysteine (CySH) is presented. It is based on vertically aligned multiwalled carbon nanotubes modified with Pt nanoparticles by magnetron sputtering deposition. The morphology of the nanocomposite was characterized by scanning electron microscopy, transmission electron microscopy and energy-dispersive. The electrochemistry of CySH was investigated by cyclic voltammetry, differential pulse voltammetry and chronoamperometry. The mechanism for the electrochemical reaction of CySH at the modified electrode at different pH values is discussed. The electrode exhibits a higher electrocatalytic activity towards the oxidation of CySH than comparable other electrodes. It displays a linear dependence (R ²?=?0.9980) on the concentration of CySH in the range between 1 and 500 μM and at an applied potential of +0.45 V, a remarkably low detection limit of 0.5 μM (S/N?=?3), and an outstandingly high sensitivity of 1.42?×?10³ μA?mM^?1?cm^?2, which is the highest value ever reported. The electrode also is highly inert towards other amino acids, creatinine and urea. The sensor was applied to the determination of CySH in urine with satisfactory recovery, thus demonstrating its potential for practical applications.

Simple Electrodeposition of Dendritic Pd Without Supporting Electrolyte and Its Electrocatalytic Activity Toward Oxygen Reduction and H2O2 Sensing

Metallic palladium (Pd) electrocatalysts for oxygen reduction and hydrogen peroxide (H₂O₂) oxidation/reduction are prepared via electroplating on a gold metal substrate from dilute (5 to 50 mM) aqueous K₂PdCl₄ solution. The best Pd catalyst layer possessing dendritic nanostructures is formed on the Au substrate surface from 50 mM Pd precursor solution (denoted as Pd‐50) without any additional salt, acid or Pd templating chemical species. The Pd‐50 consisted of nanostructured dendrites of polycrystalline Pd metal and micropores within the dendrites which provide high catalyst surface area and further facilitate reactant mass transport to the catalyst surface. The electrocatalytic activity of Pd‐50 proved to be better than that of a commercial Pt (Pt/C) in terms of lower overpotential for the onset and half‐wave potentials and a greater number of electrons (n) transferred. Furthermore, amperometric i–t curves of Pd‐50 for H₂O₂ electrochemical reaction show high sensitivities (822.2 and ?851.9 µA mM^?1 cm^?2) and low detection limits (1.1 and 7.91 µM) based on H₂O₂ oxidation H₂O₂ reduction, respectively, along with a fast response (<1 s).     

Pd black decorated by Pt sub-monolayers as an electrocatalyst for the HCOOH oxidation

Pd black was modified by a very low amount of Pt corresponding to a sub-monolayer (ML). Spontaneous displacement method was employed. The catalysts with 0.02–0.12 ML were characterized by cyclic voltammetry and CO_ads stripping and were tested for HCOOH oxidation under the potentiodynamic and potentiostatic conditions. All the Pt@Pd catalysts were more active for HCOOH oxidation than Pd black. The Pt@Pd with 0.08 ML of Pt exhibited the highest activity with the maximum current density under the potentiodynamic conditions of 8 mA cm^?2 (vs. 2.7 mA cm^?2 on Pd black). Contrasting HCOOH oxidation kinetics on Pt@Pd and Pt@Au catalysts revealed that the current densities are higher, and the poisoning rate is lower on Pt@Pd catalyst. This was ascribed to an optimal strength of the Pt–adsorbate bond when Pt is supported on Pd and to a possible influence of the Pt atoms on the Pd substrate.     

Photocatalytic oxidation of acetaldehyde by hybrid Pt/WO3–MOR photocatalysts under visible or sunlight irradiation

A hybrid photocatalyst was prepared from visible light-responsive Pt/WO₃ and siliceous mordenite (MOR) zeolite by simple impregnation with an aqueous solution of (NH₄)₁₀W₁₂O₄₁·5H₂O. Unmodified Pt/WO₃ had low photocatalytic activity in gas phase oxidation of acetaldehyde because of its low surface area (3–5 m²/g). In contrast, the Pt/WO₃–MOR had higher photocatalytic activity under focused sunlight and Xe lamp irradiation. Pt/WO₃–MOR with low WO₃ content (<20 wt%) adsorbed sufficient acetaldehyde, but absorption of light in the visible region was low. The optimum zeolite content enhancing the photocatalytic activity of Pt/WO₃ was estimated to be 30–50 % (w/w). Adsorption of the gaseous reactants and the efficiency of absorption of incident light are both important aspects of high photocatalytic activity.     

Hydrogen diffusivity in the massive LaNi5 electrode using voltammetry technique

The Randles–Sev?ik relationship has been applied to evaluate atomic hydrogen diffusivity in massive LaNi₅ intermetallic compound. The electrode was cathodically hydrogenated in 6 M KOH solution (22 °C), and then voltammetry measurements were carried out at various, very slow potential scan rates (υ?=?0.01–0.1 mV?·?s^?1). At potentials more noble than the equilibrium potential of the H₂O/H₂ system, the anodic peaks were registered as a consequence of oxidation of hydrogen absorbed in cathodic range. The peak potentials linearly increase with the logarithm of the scan rate with a slope of 0.059 V. The slope testifies to a symmetric charge transfer process with symmetry factor α?=?½. The peak currents linearly increase with the square root of the potential scan rate, and the straight line runs through the origin of the coordinate system. The slope of the I _a ^(peak) ?=?f(υ ^1/2) straight line is a measure of the atomic hydrogen diffusion coefficient. Assuming the hydrogen concentration in the LaNi₅ material after cathodic exposure to be C _0,H?=?0.071 mol?·?cm^?3 (63 % of theoretical value), the hydrogen diffusion coefficient equals D _H?=?2.0?·?10^?9 cm²s^?1. Extrapolation of rectilinear segments of potentiodynamic polarization curves with Tafel slopes of 0.12 V and linear polarization dependencies from voltammetry tests allowed the exchange current densities of the H₂O/H₂ system on the tested material to be determined. The exchange current densities on initially hydrogenated LaNi₅ alloy are close to 1 mA?·?cm^?2, irrespective of the electrode potential scan rate.     

Structural evolution of irreversibly adsorbed Bi on Pt(111) under potential excursion

This work presents a structural evolution of irreversibly adsorbed Bi on Pt(111) studied by electrochemical scanning tunneling microscopy and electrochemistry. The irreversibly adsorbed Bi showed a stable redox couple at 0.33 V whose maximum charge corresponded to the coverage of 0.33. The pristine layer of irreversibly adsorbed Bi grew from islands to large domains of the first monolayer, eventually to large domains of the monolayer with scattered protrusions of the second layer. During an electrochemical treatment from open circuit potential (～0.25 V) to 0.1 V, the domains of the pristine Bi layer shrunk and the Bi in the second layer moved to the first layer to form a compressed layer of elemental Bi. When the elemental Bi was re-oxidized at 0.35 V, there was no structural change to denote that the structures of the pristine and re-oxidized layers of oxygenated Bi differ from each other. The structural evolution during the electrochemical treatments is discussed in terms of removal and reinsertion of oxygen species.     

Electrochemical reduction of carbon dioxide with lead cathode and zinc anode in dry acetonitrile solution

The electrochemical reduction of carbon dioxide (CO₂) is investigated in acetonitrile with tetrabutylammonium perchlorate as an electrolyte using a lead cathode and a sacrificial zinc anode, and the product under such a setup is insoluble zinc oxalate at potentials between ?2.2 and ?2.8 V vs. Ag rod electrode. Preelectrolysis is an effective method to remove the water in the electrolyte, which makes a distinct reduction peak of CO₂ appear at ?2.6 V vs. Ag on cyclic voltammogram. Even trace amounts of water in the electrolyte can interfere with the faradaic efficiency of reduction of CO₂ to oxalate, and the product could be β-ZnC₂O₄ (in anhydrous solution) or ZnC₂O₄?·?2H₂O (if water exists). The faradaic efficiency for oxalate production also depends on the cathode potential and the temperature, and the maximum is 96.8 % at ?2.6 V vs. Ag and 5 °C. This is the highest value of CO₂ electrochemical reduction found in the literature under ambient pressure.