The ethanol electrooxidation reaction at Pt (1 1 1): The effect of ethanol concentration

In this work, we investigate the effects of ethanol concentration on the pathways involving the formation of CO, CO₂, acetic acid and acetaldehyde as electrooxidation products of ethanol for Pt (1 1 1). Voltammetric and spectroscopic results (FTIR in situ) show that the beginning of the oxidation pathways occurs at more anodic potentials when the ethanol concentration is increased, which corroborates those results previously founded for polycrystalline Pt and attest that ethanol adsorption is a self-poisoning process. Also, FTIR results suggest that the production of acetic acid can proceed by two distinct pathways, one of them being independent of the acetaldehyde formation.     

Electrochemical characterisation of gold on Pt{h k l} for ethanol electrocatalysis

The cyclic voltammetric (CV) behaviour of gold adsorbed on Pt{h k l} electrodes is reported. It is found that stepped surfaces vicinal to Pt{1 1 1} exhibit preferential adsorption of gold at step sites. However, for stepped surfaces containing Pt{1 0 0} terraces, adsorption of gold at step and terrace site occurs with equal probability, although for Pt{n 1 1} electrodes with n < 7 (i.e. electrodes containing short Pt{1 0 0} terraces), slight preference for step site adsorption is still observed. This behaviour is similar to that reported previously for Bi adsorption on Pt{h k l} and is interpreted in terms of slower rates of gold atom surface diffusion across Pt{1 0 0} vs. Pt{1 1 1} terraces. By comparing the electrochemical response of gold adlayers on Pt{h k l} with previously published bulk Au{h k l} voltammetry, it is deduced that the gold overlayers on Pt{h k l} are pseudomorphic in nature. This finding is in agreement with previous UHV and electrochemical results for gold deposition on platinum. Using such gold modified Pt{h k l} electrodes as model surfaces, we also report, for the first time, the electrooxidation of ethanol on Au/Pt{h k l} in alkaline aqueous solution. The ethanol oxidation reaction (EOR) is shown to be structure sensitive for submonolayer gold coverages. In fact, a maximum in the rate of EOR at platinum sites adjacent to gold islands is observed between approximately 0.4–0.6 monolayers of gold on Pt{1 0 0}, Pt{1 1 1} and Pt{1 1 0} in 0.1 M NaOH(aq). A second electrooxidation state at more positive potentials for 0 < θ_Au < 1 ascribed to EOR at pure gold sites is also observed, at least for Pt{1 1 1} and Pt{1 0 0}. On Pt{1 1 1}, the potential range and activity of EOR in this second process is similar to bulk gold whereas for submonolayer gold films on Pt{1 0 0}, the peak is shifted to more negative potentials.     

Electrochemical behaviors of dimethyl ether on platinum single crystal electrodes. Part II: Pt(1 0 0)

The electrochemical behaviors of dimethyl ether (CH₃–O–CH₃, DME), which is a promising fuel for the fuel cell, on Pt(1 0 0) electrode in 0.5 M H₂SO₄ solution have been investigated in detail by electrochemical and in situ infrared (IR) measurements. As the potential is swept from 0.05 V (vs. RHE) to positive direction at 50 mV s⁻¹, the dehydrogenation peak of DME is observed around 0.33 V to generate a reaction intermediate and is further converted to carbon monoxide (CO) in more positive potential region. The main peak for DME bulk oxidation locates around 0.80 and 0.72 V in the positive- and negative-going potential sweep, respectively. The positions of these peaks strongly depend on the scan rate. The in situ IR observations show that (CH₃OCH₂–)_ad is an intermediate for the first dehydrogenation step of DME on Pt(1 0 0) surface and can serve as a precursor of the subsequent intermediate of adsorbed CO (CO_ad). Cyclic voltammograms of Pt high index single crystal planes Pt(hkl) show that the direct oxidation of DME is suppressed by decreasing the (1 0 0) terrace width. Based on these results, a possible reaction mechanism for DME electro-oxidation on the platinum single crystal electrode surface is proposed.     

Protonation and deprotonation of cysteine and cystine monolayers probed by impedance spectroscopy

The desorption mechanism for electrochemically-formed self-assembled monolayers from cysteine and cystine on a Au(1 1 1) electrode was investigated. The monolayer desorption in 0.1 M KClO₄ is found to occur in two steps with differing potentials. At potentials more positive than the desorption potential (circa −0.65 V vs. SCE), a quasi-reversible process, assigned to protonation/deprotonation, is observed. This process is found to occur at very discreet potentials when cysteine monolayers are probed by impedance spectroscopy. Monolayers derived from the disulfide (cystine) are found to protonate/deprotonate over a wider potential range. This suggests that electrochemically-formed monolayers obtained from disulfides do not afford the same level of organization as those derived from thiol species. The value of the double layer capacitance increases from 15 to 30 μF cm⁻² after deprotonation. This suggests that the adsorption of cysteine weakens following the deprotonation. As expected, the protonation/deprotonation step is pH dependent and it is not observed in strong basic medium (0.1 M NaOH).     

The purification of an ionic liquid

The purification of commercially available 1-butyl-3-methyl-imidazolium hexafluorophosphate (BMIPF₆) is described, which is still required in addition to the usual vacuum drying for detailed double-layer studies. By using a molecular sieve of 0.3 nm pore size, the level of impurities originating either from an omnipresent humidity or from IL synthesis, is reduced to a point, where cyclic voltammograms in essence represent only double-layer behaviour of a Au(1 1 1)/BMIPF₆ interface. It is demonstrated that pore sizes larger than 0.3 nm are counterproductive; the situation is worse than without any sieve.     

Adsorption of tetraethylammonium ions on Cd(0 0 0 1) single crystal plane from solutions in ethanol

The adsorption of tetraethylammonium (TEA⁺) ions on the basal (0 0 0 1) plane of the cadmium single crystal from solutions in ethanol has been investigated by impedance measurement method. The experimental data were obtained in 0.02 M mixed-electrolyte solutions and the calculations performed with electrode potential as the independent electrical variable. The Gibbs energy of adsorption of TEA⁺ ions has been calculated using the simple virial adsorption isotherm and it was found that the adsorption of TEA⁺ cations is weaker than the adsorption of halide anions. The electrosorption valency evaluated has a nearly constant value in the potential region studied. It was concluded that the formed effective surface dipole is significantly screened by the solvent molecules and the metal electron gas. Comparing the adsorption characteristics with the parameters obtained for bismuth single crystal electrodes, it was found that the Gibbs adsorption energy on uncharged electrode surface is close for these metals. The results obtained indicate that only weak interaction between TEA⁺ ions and cadmium surface takes place and there is no remarkable partial charge transfer from the adsorbed ions to the Cd(0 0 0 1) surface atoms.     

Electrochemical formation of Se atomic layers on Au(1 1 1) surfaces: the role of adsorbed selenate and selenite

We report an investigation of the electrodeposition of Se atomic layers on Au(1 1 1) surfaces from aqueous SeO₂ solutions. The behavior of this system is complex, and characterized by two major surface-limited reduction waves in the potential window positive of the onset of Se bulk deposition. These two waves occur at potentials negative of the known E° for the Se(IV)/Se(0) couple due to the large overpotential for Se deposition on Au surfaces, and therefore cannot be considered true underpotential deposition. On the basis of potential switching and concentration dependence experiments, the first surface wave is unambiguously assigned as arising from the two-electron reduction of an adsorbed selenate species to adsorbed selenite. In some earlier reports, this feature has been assigned as corresponding to the four-electron reduction of selenite to elemental selenium. The second major surface-limited wave corresponds to the four-electron reduction of adsorbed selenite to elemental selenium. Coulometric measurements indicate that the selenium coverage corresponds to approximately 0.41 monolayers, in good agreement with previous scanned probe microscopy investigations which indicate that Se forms a (√3×√3)R30° adlayer on Au(1 1 1) surfaces.     

Electrochemical growth and characterization of size-quantized CdTe thin films grown by underpotential deposition

Size-quantized thin films of CdTe were electrosynthesized onto Au(1 1 1) substrates by underpotential deposition (UPD) from an acidic aqueous solution of CdSO₄, and TeO₂. The electrodeposited thin films were characterized by scanning tunneling microscopy (STM), atomic force microscopy (AFM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and UV–vis absorption spectroscopy. The electrodeposition mechanism and the codeposition potential of CdTe were explored by cyclic voltammetry. The voltammetric analysis indicates that the electrodeposition of CdTe is a two-step codeposition process under the conditions of UPD. Morphological investigations demonstrate that the film growth proceeds via the formation of a few layers, growth of spherical nanoparticles on the overlayer and then continues by growth to form smooth spherical crystallites of CdTe. XRD results show that the growth of films are highly preferential with (1 1 1) orientation. Optical studies revealed blue shift in band gap energy due to quantum confinement.     

Formation of Bi2S3 thin films on Au(111) by electrochemical atomic layer epitaxy: kinetics of structural changes in the initial monolayers

The kinetics and growth mechanism of the first and second monolayers of Bi₂S₃ by electrochemical atomic layer epitaxy (ECALE) has been studied using voltammetry and chronoamperometry techniques. Scan rate dependent cyclic voltammetry experiments reveal that the peak current in the stripping of the S layer from a Bi modified Au(111) electrode is a linear function of a 2/3 power of the scan rate, ν^2/3. Similar behavior is observed for the deposition of Bi on to one atomic layer of an S modified Au(111) electrode. The dissolution of the Bi atomic layer from the S modified Au(111) electrode involves an initial Langmuir-type adsorption–desorption accompanied by a nucleation and two-dimensional (2-D) growth process. When Bi atomic layers are used as a first layer, the nucleation and 2-D growth process was observed for both deposition and stripping. Analysis of the reduced variables demonstrates that the stripping of S from Au(111)Bi and stripping of Bi from Au(111)S fit the progressive model. Our results suggest that the nucleation and 2-D growth mechanism is the dominant process in the formation of Bi₂S₃ atomic layers if Bi is used as the first layer in the ECALE method.     

Short-time preparation and electrochemical properties of a single layer of tetraoctylammonium bromide capped Au nanoparticles on dithiol self-assembled monolayer-modified Au electrode

Short time immobilization of densely packed tetraoctylammonium bromide (TOAB) stabilized gold nanoparticles (AuNPs) were established on a Au electrode modified with a self-assembled monolayer (SAM) of 1,6-hexanedithiol (HDT) or 1,4-benzenedimethanethiol (BDMT). The quartz crystal microbalance experiment showed densely packed TOAB–AuNPs single layer formation on both SAMs was achieved within 20 min. AFM images demonstrated that the immobilized TOAB–AuNPs on the SAMs were densely packed and the AuNPs film thickness was 6–7 nm. The electronic communication between the immobilized AuNPs and the underlying bulk electrode was confirmed by cyclic voltammetry and electroreflectance spectroscopy. A reversible electron transfer reaction was observed for both [Fe(CN)₆]^4−/3− and [Ru(NH₃)₆]^2+/3+ at TOAB–AuNPs immobilized on HDT (Au/HDT/AuNPs) and BDMT (Au/BDMT/AuNPs) modified electrodes. The electroreflectance spectra show a red-shifted strong positive-going plasmon resonance bands at 551 nm and 584 nm, respectively, for the Au/BDMT/AuNPs and Au/HDT/AuNPs electrodes. The observed reversible redox response for the solution redox species and red-shifted plasmon resonance bands for the immobilized AuNPs again indicated that the AuNPs were immobilized on the SAMs in a densely packed manner. An advantage of TOAB–AuNPs modified electrode prepared by short time immersion over citrate-stabilized AuNPs modified electrode was demonstrated by the enhanced oxidation of ascorbic acid (AA) at these electrodes. The oxidation of AA was shifted to 90 mV less positive potential with higher oxidation current at TOAB–AuNPs modified electrode when compared to citrate-stabilized AuNPs modified electrode.     

Electrochemical behavior of Pd–Pt–Au alloys

Pd–Pt–Au alloys were prepared as limited volume electrodes by metal codeposition at constant potential from chloride solutions. Alloy surface state was examined by scanning electron microscopy (SEM) and Auger electron spectroscopy (AES). The alloys were characterized electrochemically in acidic solution (0.5 M H₂SO₄) by cyclic voltammetry (CV). Potential regions of hydrogen electrosorption and surface oxidation are clearly distinguishable. The influence of electrode potential and alloy composition on the amount of absorbed hydrogen was investigated. Hydrogen absorption capacity of Pd–Pt–Au alloys increases with decreasing potential and the α–β phase transition is shifted positively with increasing Au content and negatively with increasing Pt content in the bulk. At relatively high potentials the amount of absorbed hydrogen exhibits a maximum for ca. 15–25 at.% Au in the bulk, while for low potentials a monotonic decrease in hydrogen absorption capacity with increasing bulk concentration of both alloying metals (Pt, Au) is observed.     

Pitzer and Pitzer–Simonson–Clegg modeling approaches: Ternary HCl + ethanol + water electrolyte system

We report in this work, for the first time, the results obtained from both Pitzer, Pitzer–Simonson–Clegg (PSC) and an extended PSC ion-interaction approaches for modeling the non-ideal behavior of HCl electrolyte in the mixed ethanol + water solvent systems. In this context, a review of literature shows that the use of an exact and complete form of PSC ion-interaction approach for modeling electrolyte in mixed solvent systems was only limited to the works of two research groups. In this work, the modeling purposes were achieved based on the experimental potentiometric data of a galvanic cell containing a pH glass membrane and Ag/AgCl electrodes. The measurements were achieved over the HCl electrolyte molality ranging from 0.02 up to about 5 mol kg⁻¹ in mixed ethanol (x%) + water (100 − x%) solvent with different solvent mass fractions percent (x% = 10%, 20%, 30%, 40% and 50%), at 298.15 ± 0.05 K.     

Liquid membrane system for extraction and electrodeposition of silver(I)

The process of Ag⁺ ions extraction from nitric solutions by bulk liquid membranes containing di(2-ethylhexyl)phosphoric acid and tri-n-octylamine accompanied with silver(I) electrodeposition from perchloric acid solutions is studied at galvanostatic electrodialysis. The effects of the current density as well as of composition of the liquid membrane and aqueous solutions on the rate of the silver(I) transport are determined. More than 90% of silver ions was extracted from the feed solution containing 0.01 mol l⁻¹ AgNO₃ and about 40% of metal was electrodeposited during 1 h of electrodialysis. Sound adherent silver coatings have been deposited on platinum and copper cathodes.     

Influence of deposition current density on the composition and properties of electrodeposited Ni + RuO2 and Ni + IrO2 composites

Ni + IrO₂ composites were deposited from suspensions of fine powders of the noble metal oxide in two Ni²⁺ containing electrolytes (a mildly acid NiCl₂, NH₄Cl solution and a basic NiSO₄, (NH₄)₂SO₄, NH₄OH solution) using current densities varying from −5 mA cm⁻² to −2.5 A cm⁻². The composition, morphology and surface roughness of the deposits were studied as a function of the deposition current density (j_DEP) and compared to that of Ni + RuO₂ composites deposited from the chloride electrolyte. For all systems, the volume fraction of the dispersed phases (α_IrO2${\alpha }_{{\text{IrO}}_2}$ or α_RuO2${\alpha }_{{\text{RuO}}_2}$) decreased as j_DEP increased. At the high j_DEP values usually employed to deposit porous Ni, α_IrO2${\alpha }_{{\text{IrO}}_2}$ was very low, irrespective of the deposition bath, whereas α_RuO2${\alpha }_{{\text{RuO}}_2}$ remained as high as 0.2. At the same j_DEP values, the surface roughness of the Ni + RuO₂ composites was ca. four times higher than that of the Ni + IrO₂ composites, which was comparable to that of porous Ni devoid of dispersed particles. Since composites simultaneously possessing large surface roughness and large concentration of catalytic sites were obtained only when RuO₂ was used as dispersed phase, the Ni + RuO₂ cathodes had much higher catalytic activity in the hydrogen evolution reaction than Ni + IrO₂ cathodes, the latter being only slightly more active than porous Ni cathodes.     

Kinetics of oxygen reduction on gold nanoparticle/multi-walled carbon nanotube hybrid electrodes in acid media

In this paper, the electrochemical reduction of oxygen has been studied on gold nanoparticle/multi-walled carbon nanotube (AuNP/MWCNT) modified glassy carbon (GC) electrodes in 0.5 M H₂SO₄ using the rotating disk electrode (RDE) method. The AuNP/MWCNT catalysts were prepared by chemical deposition of AuNPs onto MWCNTs spontaneously grafted with 4-nitrophenyl groups. The composite electrode was characterised by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV). The oxygen reduction behaviour of these electrodes was compared with that of a bulk gold electrode. The AuNP/MWCNT catalyst showed a pronounced electrocatalytic activity towards O₂ reduction in acid media. The half-wave potential of O₂ reduction on the AuNP/MWCNT catalyst shifted ca 80 mV to more positive potentials as compared to that of a polished Au electrode. The kinetic parameters of oxygen reduction were determined and the specific activity of the hybrid electrode was slightly higher than that of the bulk Au electrode.     

Quasi-reversible two-electron reduction of oxygen at iodine submonolayer-coated gold electrode in alkaline media

Oxygen reduction reaction (ORR) was investigated using polycrystalline gold (Au (poly)) electrode modified with chemisorbed iodine (I_(ads)) submonolayer (sub I_(ads)) in O₂-saturated 0.1 M KOH solution. The sub I_(ads) was tailored by potential-dependent partial reductive desorption of I_(ads) from its full monolayer. The Au (1 1 1) facet of the Au (poly) electrode was considered to remain bared at the sub I_(ads)/Au (poly) electrode. The interesting finding of the present study is that (unlike the bare Au (poly) electrode) the sub I_(ads)/Au (poly) electrode exhibited a quasi-reversible two-electron reduction of O₂ in alkaline media. The probable origin of the observed quasi-reversible behavior of the ORR is discussed. Experimental investigations were performed using cyclic and steady-state voltammetric, amperometric and coulometric techniques.     

Nanoporous gold particles modified titanium electrode for hydrazine oxidation

A nanoporous gold particles modified titanium electrode (Au/Ti) was prepared by using a hydrothermal method. Gold nanoparticles were stably immobilized on the Ti surface from a mixture of aqueous HAuCl₄/polyethylene glycol (PEG) to form a nanoporous network texture. Electrocatalytic activity of the Au/Ti towards hydrazine oxidation in 1 M NaOH solution was assessed utilizing cyclic voltammetry (CV), linear scanning voltammetry (LSV) and electrochemical impedance spectroscopy (EIS). At the Au/Ti electrode, hydrazine oxidation in 1 M NaOH took place at a potential of −0.55 V (vs. Ag, AgCl) which was 0.53 V less than polycrystalline Au electrode. The Au/Ti electrode also presented much larger current density of hydrazine oxidation than Au electrode. Cyclic voltammetric responses of the Au/Ti electrode showed an irreversible electro-oxidation process of hydrazine. Linear plots of the reduction peak current density from the CVs of the Au/Ti vs. hydrazine concentration provided a potential detection of low concentration hydrazine. The kinetic parameters such as the number of electrons transferred in rate-determining step and total numbers of electrons involved in the hydrazine oxidation were determined using CVs and LSVs. CVs at the Au/Ti electrode also illustrated (relatively) weak interactions of hydrazine with electrode surface at all stages of the hydrazine oxidation process. Further, EIS data showed significantly high electrocatalytic activity of the Au/Ti electrode for hydrazine oxidation in alkaline solutions.     

Facile growth of flower-like Au nanocrystals and electroanalysis of biomolecules

Growth of flower-like Au nanocrystals at room temperature on a conducting support and electroanalysis of biomolecules are described. The Au nanocrystals are grown by seed-mediated growth approach using electrochemically deposited Au nanoseeds (nAuS) without any template or surfactants. The flower-like nanocrystals (nAuF) have the size ranging from 75 to 350 nm. XRD profile of nAuF reveals the existence of (1 1 1), (2 0 0) and (3 1 1) planes, corresponding to the face centered cubic lattice of Au. The nAuF on the electrode surface have significantly large surface area of 0.354 ± 0.02 cm². The electroanalysis of biomolecules such as ascorbic acid (AA), dehydroascorbic acid (DHA), dopamine (DA) and reduced nicotinamide adenine dinucleotide (NADH) is performed using nAuF electrode. The nanocrystals on the electrode facilitate the oxidation of these bioanalytes; decrease in the overpotential and enhancement in the peak current is observed. Unlike the bulk Au electrode, nAuF electrode displays unique inverted ‘V’ shape voltammogram for NADH and DHA. Oxidation of AA occurs in two steps and two voltammetric peaks are obtained for AA in neutral and alkaline pH. Increase in the peak current and decrease in peak-to-peak separation with respect to the unmodified electrode are obtained for DA on nAuF electrode. The nAuF electrode shows high sensitivity, stability and linear response for a wide concentration range towards these analytes. The high sensitivity is ascribed to the ensemble behavior of the nAuF electrode.