Electrodes, noble metal

Electromagnetic flow meters ate avadable with various liner and electrode materials. Liner and electrode selection is governed by the corrosion characteristics of the Hquid. Eor corrosive chemicals, fluoropolymer or ceramic liners and noble metal electrodes are commonly used polyurethane or mbber and stainless steel electrodes are often used for abrasive slurries. Some fluids tend to form an insulating coating on the electrodes introducing errors or loss of signal. To overcome this problem, specially shaped electrodes are avadable that extend into the flow stream and tend to self-clean. In another approach, the electrodes are periodically vibrated at ultrasonic frequencies. 

For example, the reaction enthalpy for the reduction of PC proceeding at lithium amalgam to form propylene gas and lithium carbonate is estimated to be -I41kcal (molPC)-1 [149]. PC is reduced at noble-metal electrodes at potentials below 1.5 V vs. Li, and yields lithium alkyl carbonates when lithium salts are the supporting electrolytes. Reduction occurs at 0.7-0.8 V vs. Li with Bu4NC104as supporting electrolyte [150],... 

Pulsed amperometric detection (PAD), introduced by Johnson and LaCourse (64, 65) has greatly enhanced the scope of liquid chromatography/electrochemistry (66). This detection mode overcomes the problem of loss of activity of noble metal electrodes associated with the fixed-potential detection of compounds such as carbohydrates, alcohols, amino acids, or aldehydes. Pulsed amperometric detection couples tlie process of anodic detection with anodic cleaning and cathodic reactivation of a noble metal electrode, thus assuring a continuously cleaned and active... 

Beden, B. Electrocatalytic Oxidation of Oxygenated Aliphatic Organic Compounds at Noble Metal Electrodes 22... 

Beden, C. Lamy, and J.-M. Leger, Electrocatalytic Oxidation of Oxygenated Aliphatic Organic Compounds at Noble Metal Electrodes, in Modem Aspects of Electrochemistry, Vol. 22, Ed. by J. O M. Bockris, B. E. Conway, and R. E. White, Plenum Press, New York, 1992, pp. 97-264. 

Aliphatic Organic Compounds at Noble Metal Electrodes... 

Beden B, Lamy C, Leger JM. 1992. Electrocatalytic oxidation of oxygenated aliphatic organic compounds at noble metal electrodes. In Bockris JO M, Conway BE, White RE, eds. Modem Aspects of Electrochemistry. Volume 22. New York Plenum Press, p 97-264. 

Capon A, Parsons R. 1973a. Oxidation of formic acid at noble metal electrodes. I. Review of previous work. J Electroanal Chem 44 1-7. 

The anodic evolution of oxygen takes place at platinum and other noble metal electrodes at high overpotentials. The polarization curve obeys the Tafel equation in the potential range from 1.2 to 2.0 V with a b value between 0.10 and 0.13. Under these conditions, the rate-controlling process is probably the oxidation of hydroxide ions or water molecules on the surface of the electrode covered with surface oxide ... 

X-ray photoelectron spectroscopy (XPS) of electrodes was first applied to the oxidation of noble metal electrodes. Kim and Winograd investigated in 1971 the electrochemical formation of anodic oxides on Pt [10] and later on Au electrodes [60]. The electrochemical parameters of oxide formation on these noble metal electrodes were well characterized and enabled a direct correlation between ex situ XPS and in situ electrochemical analysis. 

It is clear from the calculated limiting-current curves in Fig. 3a that the plateau of the copper deposition reaction at a moderate limiting-current level like 50 mA cm 2 is narrowed drastically by the surface overpotential. On the other hand, the surface overpotential is small for reduction of ferri-cyanide ion at a nickel or platinum electrode (Fig. 3b). At noble-metal electrodes in well-supported solutions, the exchange current density appears to be well above 0.5 A/cm2 (Tla, S20b, D6b, A3e). At various types of carbon, the exchange current density is appreciably smaller (Tla, S17a, S17b). 

Metal UPD on noble metal electrodes has been well reviewed and documented [74, 83-85, 87, 88], Metals of obvious relevance to the EC-ALE formation of compounds include Hg, Cd, Zn, In, and Ga, which are components of II-VI, III-VI and III-V compounds. 

The intimate relationship between double layer emersion and parameters fundamental to electrochemical interfaces is shown. The surface dipole layer (xs) of 80SS sat. KC1 electrolyte is measured as the difference in outer potentials of an emersed oxide-coated Au electrode and the electrolyte. The value of +0.050 V compares favorably with previous determinations of g. Emersion of Au is discussed in terms of UHV work function measurements and the relationship between emersed electrodes and absolute half-cell potentials. Results show that either the accepted work function value of Hg in N2 is off by 0.4 eV, or the dipole contribution to the double layer (perhaps the "jellium" surface dipole layer of noble metal electrodes) changes by 0.4 V between solution and UHV. 

As the SPE is an acid electrolyzer, noble metal electrodes are preferred for high current density operation. 

Preparation of amalgams electrochemical reduction on an Hg cathode According to Guminski (2002), the electrochemical reduction of metallic ions on an Hg cathode from aqueous or non-aqueous solvents (as well as from molten salts) allows the introduction of both soluble and insoluble metals into the Hg phase. Some amalgams may be prepared by simultaneous reduction of Hg2+ and Men+ from their solutions. On the other side, noble metal (Pd, Pt, Ag, Au) amalgams may by obtained by reduction of Hg2+ on noble metal electrodes. 

Reductions at noble metal electrodes in acidic protic media often form adsorbed hydrogen, which is the actual reductant. For example, reduction of nitrobenzene at a Pd/C electrode in acetic acid-methanol mixtures affords aniline via adsorbed hydrogen28. This reaction is more closely related to catalytic hydrogenation of nitro groups than to the... 

In the cathodic regime the silicon atoms of the electrode do not participate in the chemical reaction. Therefore, an n-type or a strongly illuminated p-type silicon electrode behave like a noble metal electrode and hydrogen evolution or metal plating reactions are observed. For the case of an aqueous electrolyte free of metal ions the main reaction is electrochemical hydrogen evolution according to ... 

The lack of knowledge of precise values of the roughness factor makes it difficult to compare data reported from different studies. This applies in particular to the double-layer capacity data, the values of surface concentration of the adsorbates, and the rates of electrochemical reactions. Therefore, the question of how to determine the real surface of the electrode is of cmcial importance. A survey of various methods for determining roughness was given by Trasatti and Petrii. For noble metal electrodes, the charges of hydrogen deposition and surface oxide formation can be utilized in real-surface determination." ... 

The relative simplicity and low cost of STM instrumentation has contributed significantly to the rapid increase in the number of in situ electrochemical studies performed over the last decade. An excellent discussion of the general aspects of STM design and construction is available in a recent textbook [39], Beyond instrumentation, insightful experiments depend on the preparation of a flat, well-defined substrate and the formation of a stable tip capable of atomically resolved imaging. In this sense, the ability to reliably produce high-quality noble metal electrodes outside UHV has been central to the success of many STM studies [145-148]. In contrast, our knowledge of the structure, chemistry, and operation of the probe tip may be more aptly viewed as an art form. 

Oxide formation on noble metal electrodes usually proceeds by two-dimensional adsorption followed by place exchange between the metal and oxide species to form a three-dimensional overlayer. Subsequent reduction... 

To better understand the diffusion-limited school of thought mentioned above, it is worth digressing momentarily on another noble -metal electrode system silver on YSZ. Kleitz and co-workers conducted a series of studies of silver point-contact microelectrodes, made by solidifying small (200—2000 //m) silver droplets onto polished YSZ surfaces. Following in-situ fabrication, the impedance of these silver microelectrodes was measured as a function of T (600-800 °C), P02 (0.01-1.0 atm), and droplet radius. As an example. Figure 9a shows a Nyquist plot of the impedance under one set of conditions, which the authors resolve into two primary components, the largest (most resistive) occurring at very low frequency (0.01—0.1 Hz) and the second smaller component at moderately low frequency ( 10 Hz). 

Mostly noble metal electrodes [71-75] and modified glassy-carbon electrodes [76-81] have been used. In the case of platinum electrodes, the formation of four products characterized by the following equations was observed [74, 75]. 

In a series of publications by Attard and coworkers (see [101] and literature cited therein), the adsorption and reduction of N2O at single crystal and polycrystalline noble metal electrodes were studied. It was demonstrated that N2O adsorption can be a useful tool to probe the anion adsorption. According to the authors, the reduction of N2O can be described by the preequilibrium characterized by K equilibrium constant,... 

A significant part of these studies was carried out with noble metal electrodes mainly with Pt and Pd, thus information relevant to the subject of the present section can also be found in Chapter 17 dealing with the electrochemistry of Pt and Pd. 

The electrocatalytic reduction of NOs" ions on noble metal electrodes has been the subject of several studies. (The reduction of nitrate has recently gained renewed attention in view of its relevance to pollution control.)... 

G. Jerkiewicz, Surface oxidation of noble metal electrodes in Interfacial Electrochemistry. Theory, Experiment and Applications (Ed. A. Wi ckowski), Marcel Dekker, New York-Basel, 1999, p. 559. 

The oxygen/water half-cell reaction has been one of the most challenging electrode systems for decades. Despite enormous research, the detailed reaction mechanism of this complex multi-step process has remained elusive. Also elusive has been an electrode material and surface that significantly reduces the rate-determining kinetic activation barriers, and hence shows improvements in the catalytic activity compared to that of the single-noble-metal electrodes such as Pt or Au. 

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