Adsorption of mercaptopropionic acid onto Au(1 1 1): Part I. Adlayer formation, structure and electrochemistry

The adsorption of 3-mercaptopropionic acid (MPA) on Au(1 1 1) in 0.1 M H₂SO₄ was studied by cyclic voltammetry, capacitance measurements and in situ scanning tunneling microscopy (STM). The formation of a self-assembled monolayer under electrochemically controlled conditions was monitored for the first time with an STM. A two-step adsorption process has been found, beginning with the adsorption of MPA to form a disordered film, which lifts the () reconstruction of Au(1 1 1), followed by an ordering of the MPA adlayer into two different dense structures. Concomitantly with the second step, formation of the well-known vacancy islands is observed. This formation is explained by an adsorbate-induced compression of the gold substrate. Two distinctly different structures of the dense MPA monolayer on Au(1 1 1) were observed. One () structure with p=3.6-4.8 and a second structure consisting of two unit cells of () and () symmetry. In part II, the effect of the MPA on the copper electrodeposition onto Au(1 1 1) is discussed.     

Hydrogen evolution during the oxidation of formaldehyde on Au:: The influence of single crystal structure and Tl-upd

Hydrogen evolution during formaldehyde oxidation in alkaline solution has been monitored by Differential Electrochemical Mass Spectrometry on Au(111) and polycrystalline gold. The current efficiency for hydrogen evolution increases with higher concentration and is in the same range on both, polycrystalline Au and Au(111) electrode. The onset potentials and half-wave potentials are higher on Au(111). Reaction orders for the faradaic current on the bare gold electrodes have been determined as 0.21 for higher and 0.76 for lower concentrations. Reaction orders for hydrogen evolution during formaldehyde oxidation are 1.4 times higher in each case. Tafel slopes in the range of 140-160 mV are found. This signifies that the first reaction step involving the formation of adsorbed hydrogen is largely determining the overall reaction rate. In the presence of thallium adlayers hydrogen evolution from formaldehyde oxidation is largely suppressed. On the thallium modified polycrystalline Au, formaldehyde oxidation is shifted for 100 mV to higher potentials where Tl is partially desorbed and hydroxide is coadsorbed on the modified surface. On thallium modified Au(111), a similar process takes place, but in the same potential region as the onset of formaldehyde oxidation on the bare surface and therefore the formaldehyde oxidation is only slightly shifted. Tafel slopes are decreased to 80 mV/dec in the presence of thallium. In the presence of adsorbed thallium, the first reaction step is in equilibrium, the coverage with adsorbed hydrogen is smaller and its recombination to H₂ is largely suppressed.     

Ash chemistry and mineralogy of an Indonesian coal during combustion: Part 1 Drop-tube observations

The paper reports a systematic and comprehensive laboratory investigation into the ash chemistry and mineralogical changes undergone by a low-rank Indonesian coal during combustion. Combustion experiments conducted in a drop-tube furnace included ash formation experiments (using cyclone and filter arrangement) under closely controlled conditions in the range of 1200–1400 °C and deposition experiments at a probe temperature of 750 °C. Tests conducted with raw coal, coal/additive mixtures and washed coal indicated significant changes in ash characteristics. Of the ash formation and deposit samples examined, the raw coal + bauxite showed the lowest glass content and high contents of corundum indicating low ash deposition propensities. When compared to the ash formation samples, the deposit samples showed even significantly lower glass contents and were enriched in quartz. With the exception of the raw coal + bauxite sample, all are characterized by high silica and iron and moderate aluminium contents. In contrast, the raw coal + bauxite sample have low silica and much higher alumina contents which is in agreement with XRD observations. QEMSCAN™ results showed that the ash particles are sparsely distributed suggesting lack of a deposit initiation layer. Experimental observations suggest that the use of raw coal with bauxite would appear to offer the best performance with respect to handling ash-related issues. Present findings are of practical significance to power utilities employing Indonesian coal as there is no comprehensive work reported in the literature on ash chemistry and mineralogy of such coals.     

Water-entrained cement-based materials : II. Experimental observations

This paper concerns a new concept for the prevention of self-desiccation in hardening cement-based materials. The concept is based on using fine, superabsorbent polymer (hydrogel, SAP) particles as a concrete admixture. This permits a controlled formation of water-filled macropore inclusions—water entrainment—in the fresh concrete. Consequently, the pore structure is actively designed to control self-desiccation. In the paper, experimental observations in relation to this technique are described and discussed. The observations show that self-desiccation can be controlled by water entrainment. The paper forms the second part of a series. In the first part, the theoretical background was presented [Cem. Concr. Res. 31(4) (2001) 647].     

Application of the electrochemical quartz crystal microbalance technique to copper sonoelectrochemistry: Part 1. Sulfate-based electrolytes

The applicability of the electrochemical quartz crystal microbalance technique in an ultrasonic field created by an ultrasound probe is demonstrated for the electrodeposition of copper. Cyclic voltammetry and potentiostatic depositions in acidic sulfate-based copper electrolytes were performed at different ultrasonic intensities. The electrochemical quartz crystal microbalance was operated in ultrasonic fields with intensities up to 30 W cm⁻². For cyclic voltammetry, potential resolved and averaged (apparent) current efficiencies were calculated from mass and charge data in function of the amplitude of the ultrasonic horn. Ultrasound slightly affected the current efficiencies during copper deposition in cyclic voltammetry, but did not change the efficiencies during dissolution. During potentiostatic depositions the current efficiency increased from 84% to almost 100% upon application of ultrasound. Morphology of deposits prepared by potentiostatic depositions was analyzed by scanning electron microscopy, and found to be different at high ultrasonic intensities.     

Electrokinetic remediation of copper mine tailings: Implementing bipolar electrodes

Important process parameters to optimize in electrokinetic soil remediation are those influencing remediation time and power consumption since these directly affect the cost of a remediation action. This work shows how the electrokinetic remediation (EKR) process could be improved by implementing bipolar electrodes in the porous material. The bipolar electrodes in EKR meant two improvements: (1) a shorter migration pathway for the contaminant, and (2) an increased electrical conductivity in the remediation system. All together the remediation proceeded faster with lower electrical resistance than in similar experiments but without the bipolar electrodes.The new electrokinetic remediation design was tested on copper mine tailings with different applied electric fields, remediation times and pre-treatment. The results showed that the copper removal was increased from 8% (applying 20 V for 8 days in sulphuric acidified tailings) without bipolar electrodes to 42% when bipolar electrodes were implemented.Furthermore, the results showed that in this system sulphuric acid addition prior to remediation was better than citric acid addition. In addition, applying a too strong electric field (even with bipolar electrodes) could cause a severe polarization (e.g. a high electrical resistance) in the remediation system.     

Mass transport studies at rotating cylinder electrode (RCE): Influence of using plates and concentric cylinder as counter electrodes

This study shows the effect of using four-plate, six-plate and concentric cylinder as counter electrodes on the RCE mass transport characterization. The Cu(II)/Cu(0) process in sulfuric acid is employed as the model reaction. Based on the analysis of Sh = aRe^bSc^0.356 correlation, the values of the constant a, associated with shape and cell dimensions, were 0.012, 0.014, and 0.022 for the four-plate, six-plate and concentric cylinder, respectively. This behavior suggests that both, the shape and area of the counter electrode, influence the values of a. The values of b, associated with the hydrodynamic regime, were similar for the six-plate and the concentric counter electrodes, b = 0.91; whereas for the four-plate counter electrode, b = 0.95. The fact that the four-plate device gives greater values of b suggests that this device has a higher turbulence-promoting action than the others. The mass transport at the RCE with four plates has a performance similar to the concentric counter electrode, which is important from the technical and economics standpoints.     

Crown ether bridged homo- and heterodinuclear copper(II) and nickel(II) cyclidene complexes: Interaction with anions

Homodinuclear (3CuCu, 3NiNi) and heterdionuclear (3CuNi) cyclidene complexes linked by two 1,10-diaza-18-crown-6 ethers were synthesized and their properties characterized. For the 3CuCu in the negative potential range the mixed-valence state Cu^I-Cu^II was observed and the comproportionation constants were determined. The redox potentials of M^II/III showed a good correlation with DN of the solvent. In acetonitrile solution containing 0.1 mol dm⁻³ Bu₄NPF₆ reversible M^II/III redox processes were observed. However, in the presence of stronger ion pairing anions (e.g. BF₄⁻ and ClO₄⁻) the “potential inversion” occurred. Redox potentials of dinuclear complexes were compared with mononuclear cyclidene complexes. The influence of selected anions (Cl⁻ and NO₃⁻) on the redox process M^II/III was studied. The effect of Cl⁻ anion was different for 3CuCu and 3NiNi. In 3CuCu coordination of two chloride anions took place after oxidation of copper centers. In the 3NiNi complex two Cl⁻ anions were coordinated to one of the nickel(II) centers facilitating oxidation, at different potentials, of both nickel(II) cations. The behaviour of heteronuclear complex with Cl⁻ anions was similar to 3NiNi. All dinuclear complexes interacted with NO₃⁻ anions and the observed potential shifts were larger for nickel(II) than for copper(II) cations.     

Direct numerical simulation (DNS) modeling of PEFC electrodes: Part II. Random microstructure

The direct numerical simulation (DNS) method, developed for modeling the cathode catalyst layer (CL) of a polymer electrolyte fuel cell (PEFC) in Part I, is further extended wherein the catalyst layer is described as a random three-dimensional porous structure. A random CL microstructure is obtained using a computer-generated random number with specified porosity and pore size as the input structural parameters. Some statistical features of the CL and their dependence on the porosity are identified and demonstrated. The charge and species conservation equations are solved directly on this microscopically complex structure. The results from the DNS calculation are compared with the one-dimensional macrohomogeneous predictions and the Bruggeman factor for transport property correction is evaluated, which can be used as direct input into the macroscopic fuel cell models.     

Gel polymer electrolytes based on PMMA : III. PMMA gels containing cadmium

Gel polymer electrolyte containing cadmium perchlorate was prepared by polymerisation of methylmethacrylate with a solution containing Cd perchlorate in propylene carbonate (PC). Electric conductivity, fundamental electrochemical properties and Cd electrodeposition were investigated. The deposition of Cd was fairly homogenous.     

Investigation of zinc chromatation: Part II. Electrochemical impedance techniques

A mechanism to explain the formation of a chromate layer on zinc is proposed. It assumes that a ZnO inner film blocks the zinc surface on which the chromate layer grows. This layer has gel-like properties. The diffusion of the protons across the chromate layer and across the solution is supposed to be the kinetically limiting steps. This model was derived and experimentally tested in terms of impedance. The influences of the immersion time, mass transport, and pH of the chromatation solution were examined. A rather good agreement was found between the predictions of the model and the experimental results.     

New quality criteria in wastewater reuse : The case of Gran Canaria

Wastewater reuse plays a key role in this vital cycle of water because it is able to reduce the wastewater spilled. Simultaneously, the supply of water for specific uses is increased. A new regulation (Royal Decree 1620/2007) came into force in Spain in December 2007 and regulates the basic conditions for the wastewater reuse and establishes the required criteria of quality to waters according to use.In the present paper, different kinds of tertiary treatments in reuse of wastewater are described. We focused on the applied tertiary treatments in most wastewater treatment plants of Gran Canaria Island. Particularly, we analysed Hoya del Pozo wastewater treatment plant due to its wide number of technologies and processes.Finally, we reviewed the quality criteria used in the regenerated waters, in agreement with the new regulations.     

Water electrolysis under microgravity: Part 1. Experimental technique

Water electrolysis was conducted in both alkaline (25 wt.% KOH, 2 wt.% KOH) and acid (0.1N H₂SO₄) solutions for 8 s under microgravity environment realized in a drop shaft. The gas bubble formation of hydrogen and oxygen on platinum electrodes was observed by CCD camera. In alkaline solutions, a bubble froth layer grew on the electrode surface. Hydrogen bubble size was smaller than that of oxygen. The current density at constant potential decreased continually with time. In spite of the growth of a bubble froth layer on the electrode, the electrolysis never stopped, apparently because fresh electrolyte is supplied to the electrode surface by microconvection induced by the gas bubble evolution. In acid solution, hydrogen gas bubbles frequently coalesced on the cathode surface, yielding a larger average bubble than that of oxygen. The current density did not vary at constant potentials from -0.4 to −0.8 V versus reversible hydrogen electrode (RHE), because the effective electrode surface area was significantly reduced by the larger bubble size compared to alkaline electrolyte. The present experiments indicate that, especially in a microgravity environment, the bubble evolution behavior and the resultant current-potential curves are significantly influenced by the wettability of the electrode in contact with the electrolyte.     

Polyaniline fibres as electrodes.: Electrochemical characterisation in acid solutions

Polyaniline fibre microelectrodes prepared from a doped solution of polyaniline protonated with 2-acrylamido-2-methyl-1-propanesulphonic acid in dichloracetic acid were characterized electrochemically for the first time. Low scan rate cyclic voltammetry was used for characterisation in different acid electrolyte solutions, hydrochloric, nitric, perchloric, sulphuric and phosphoric, at low pH values with varying positive potential limits. Electrochemical impedance spectroscopy was also utilised. The electrochemical behaviour of polyaniline (PANI) fibres was found to be similar to that of PANI films obtained by electropolymerisation on metallic electrode substrates. The conduction potential window was found to be from +0.20 to +0.60 V versus SCE, with small variations in the different acid solutions as well as with pH. The standard electrochemical redox couple hexacyanoferrate(III), was found to behave quasi-reversibly in the conduction potential region and rate constants were evaluated.     

A novel flow battery: A lead acid battery based on an electrolyte with soluble lead(II): Part VII. Further studies of the lead dioxide positive electrode

Extensive cycling of the soluble lead flow battery has revealed unexpected problems with the reduction of lead dioxide at the positive electrode during discharge. This has led to a more detailed study of the PbO₂/Pb²⁺ couple in methanesulfonic acid. The variation of the phase composition (XRD) and deposit structure (SEM) have been defined as a function of current density, Pb²⁺ and H⁺ concentrations, deposition charge and temperature as well as the consequences of charge cycling. Pure α-PbO₂, pure β-PbO₂ and their mixtures can be deposited from methanesulfonic acid media. The α-phase deposits as a more compact, smoother layer, which is well suited to charge cycling. While the anodic deposition of thick layers of PbO₂ is straightforward, their reduction is not; the complexities are explained by an increase in pH within the pores of the deposit. The results suggest that operating the battery at lead(II) concentrations <0.3 M and elevated temperatures should be avoided.     

Potassium sorbate—A new aqueous copper corrosion inhibitor: Electrochemical and spectroscopic studies

This work presents the novel nature of 2,4-hexadienoic acid potassium salt (potassium sorbate (KCH₃CHCHCHCHCO₂)) as an effective copper aqueous corrosion inhibitor. The influence of pH and potassium sorbate concentration on copper corrosion in aerated sulfate and chloride solutions is reported. Degree of copper protection was found to increase with an increase in potassium sorbate concentration; an optimum concentration of this inhibitor in sulfate solutions was found to be 10 g/L. Copper is highly resistant to corrosion attacks by chloride ions in the presence of potassium sorbate. X-ray photoelectron spectroscopy (XPS) studies suggest that copper protection is achieved via the formation of a mixed layer of cuprous oxide, cupric hydroxide and copper(II)-sorbate at the metal surface.     

Nucleation and diffusion-controlled growth of electroactive centers: Reduction of protons during cobalt electrodeposition

A theory is presented describing, for the first time, the temporal evolution of the fractional surface area, S(t), of 3D non-interacting nuclei growing at a rate limited by diffusion of electrodepositing ions onto substrates of a different nature. Likewise, an equation has been derived describing the potentiostatic current-time transients arising from the formation and growth of such nuclei with redox reactions occurring simultaneously on their surfaces. An equation is also proposed to describe the current due to redox reactions taking place on the surface of interacting growing nuclei. The latter is used to describe the experimental current transients recorded during nucleation and growth of cobalt at applied potentials where the proton reduction reaction occurs simultaneously with the electrocrystallization process.     

Template electrosynthesis of aligned Cu2O nanowires: Part I. Fabrication and characterization

Large arrays of aligned copper oxide nanowires were produced by electrodeposition, using anodic alumina membranes as template. We have studied the effect of two fundamental parameters involved in fabrication process: potential perturbation and bath composition. Performing electrodeposition from a copper acetate/sodium acetate bath (pH 6.5), we found that chemical composition of nanowires varied in dependence on the shape of the applied potential perturbation: pure copper oxide nanowires were produced by pulsed potential, whilst continuous electrodeposition resulted in a co-deposition of Cu and Cu₂O. In a copper lactate bath, buffered at pH 10, the shape of perturbation did not influence the chemical composition of nanowires, consisting of pure copper oxide. Besides, bath composition influenced the crystallographic structure of nanowires: in the acetate bath polycrystalline nanowires, having a maximum length of about 2.5 μm, were obtained whilst in the lactate electrolyte Cu₂O nanowires with a preferential orientation along the (2 0 0) plane were deposited. In every case, nanowire diameters were uniform with an average value of about 200 nm. Growth rate of nanowires was influenced by the shape of potential pulse.