Au electrodes

The temperature dependence of the electrical double-layer parameters has been determined for real393,398 as well as quasi-perfect Ag planes.382,394 For quasi-perfect Ag electrodes, the value of 3 ffa0/9rhas been found to be higher for Ag(100) than for Ag(lll), and so it was concluded that Ag(lll) is more hydrophilic than Ag(100). For real surfaces,382,385,386 dEff=0/BT increases in the order (110) < (100) <(111). The same order of planes has been observed for Au 446-448 BEa /BT linearly increases as AX (interfacial parameter) decreases, i.e., as the hydrophilicity of Ag and Au electrodes decreases.15 32 393 397 398 446 48... 

The electrical double-layer structure at Au(l 11), Au(110), Au(100), and Au(210) faces and at a pc-Au electrode has been studied in 5 x 10 3 and 1 x 10-2 M LiC104 solutions in DMSO by cyclic voltammetry and impedance methods.477 The electrodes were cleaned by heating in a flame ... 

The pc-Au/propylene carbonate (PC) + NaC104 interface has been studied by Nguyen Van Huong.481 A flame-annealed (02 + H2) pc-Au sphere was used. Before each experiment the pc-Au electrode was cleaned in an NaC104 aqueous solution by a few potential cycles involving oxidation-reduction of the surface until the i,E and C,E curves exhibited stable character. The C,E curves were recorded in the interval 15 150... 

Figure 5.26. Effect of catalyst potential on the oxygen desorption activation energy, Ed, calculated from the modified Redhead analysis for Pt, Ag and Au electrodes deposited on YSZ.44,46 Reprinted from ref. 44 with permission from the Institute for Ionics.

O.A. Mar ina, V.A. Sobyanin, V.D. Belyaev, and V.N. Parmon, The effect of electrochemical oxygen pumping on catalytic properties of Ag and Au electrodes at gas-phase oxidation ofCH4, Catalysis Today 13, 567-570 (1992). 

Similar studies utilizing Au electrodes on YSZ showed again that the selectivity and yield of C2 hydrocarbons can be significantly affected by applying currents or potentials to the cell.40,41,53 The behaviour with Au appears to be qualitatively similar with that obtained with Ag electrodes although electrophilic behaviour is also reported.40,41... 

As shown in Figure 12.4 this finely dispersed Pt catalyst can be electrochemically promoted with p values on the order of 3 and A values on the order of 103. The implication is that oxide ions, O2", generated or consumed via polarization at the Au/YSZ/gas three-phase-boundaries migrate (backspillover or spillover) on the gas exposed Au electrode surface and reach the finely dispersed Pt catalyst thereby promoting its catalytic activity. 

The experimental setup used for the first bipolar or wireless NEMCA study is shown in Figure 12.6.8 An YSZ disc with two terminal Au electrodes and one Pt catalyst film deposited on one side and a reference Au electrode on the other side is placed in a single-chamber reactor. Ethylene oxidation on the Pt catalyst film was chosen as a model reaction.8... 

As shown in Fig. 12.7 application of a potential between the two terminal Au electrodes induces NEMCA on the Pt film which is not connected to any metal wire but is simply in contact with the YSZ solid electrolyte. 

Figure 12.6. Pt catalyst and Au electrode geometry (left) corresponding electrical connection in four wireless and two conventional configurations (right). Electrical connections to the reference (R) electrode not shown for simplicity.8 Reprinted with permission from Academic Press.

As in aqueous electrochemistry it appears that application of a potential between the two terminal (Au) electrodes leads to charge separation on the Pt film so that half of it is charged positively and half negatively8 thus establishing two individual galvanic cells. The Pt film becomes a bipolar electrode and half of it is polarized anodically while the other half is polarized cathodically. The fact that p is smaller (roughly half) than that obtained upon anodic polarization in a classical electrochemical promotion experiment can be then easily explained. 

An obvious extension of the bipolar design idea presented in the previous section is the induction of NEMCA using multi-stripe or multi-dot Pt catalysts placed between two terminal Au electrodes, as shown in Figs. 12.8 and 12.9. Both designs have been successfully tested as shown in these figures.10 Larger terminal voltages are applied here between the two Au electrodes, so that the potential difference in each individual cell formed between the Pt stripes or dots is of the order of IV.10... 

A monolith was made from YSZ and the surface of the monolith channels was covered with a Ru02 catalyst. Two terminal Au electrodes were deposited on the outside surface of the monolith. Potential application between the two terminal Au electrodes was found to induce NEMCA on the Ru02 catalyst which is not in electrical contact with any metal wire.9... 

In practical terms, large-scale cracking in the produced films, detrimental to photoelectric applications, was the main drawback of the above method. In order to prevent the appearance of cracks, propylene carbonate (PC) has been used as a solvent, with encouraging results [51]. The mechanism of electrodeposition of CdS in PC solutions containing Cd(II) ions and elemental sulfur has been studied by performing cyclic voltammetry at stationary Pt and Au electrodes [52]. 

Solaliendres MO, ManzoU A, Salazar-BandaGR, Egmluz KIB, Tanimoto ST, Machado SAS (2008) The processes involved in the Se electrodeposition and dissolution on Au electrode the H2Se formation. J Solid State Electrochem 12 679-686... 

In acidic media, the reactivity of ethanol on Au electrodes is much lower than in alkaline media. The main product of the oxidation of ethanol on Au in an acidic electrolyte was found to be acetaldehyde, with small amounts of acetic acid [Tremiliosi-FiUio et al., 1998]. The different reactivities and the product distributions in different media were explained by considering the interactions between the active sites on Au, ethanol, and active oxygen species absorbed on or near the electrode surface. In acidic media, surface hydroxide concentrations are low, leading to relatively slow dehydrogenation of ethanol to form acetaldehyde as the main oxidation pathway. In contrast, in alkaline media, ethanol, adsorbed as an ethoxy species, reacts with a surface hydroxide, forming adsorbed acetate, leading to acetate (acetic acid) as the main reaction product. 

Electrochemical experiments have been carried out on materials deposited by PVD on silicon microfabricated arrays of Au pad electrodes [Guerin et al., 2006a]. The substrate is made up of a square silicon wafer capped with silicon nitride (31.8 mm x 31.8 mm), which has an array of 100 individually addressable Au pad electrodes. These electrodes make up a square matrix on the wafer, which can be masked when placed in a PVD chamber, allowing deposition of thin films on the Au electrodes. Figure 16.3 is a schematic drawing of the configuration. Small electrical contact pads in Au for the individual addressing of electrodes (0.8 mm x 0.8 mm) are placed on the boundaries. 

Figure 16.3 Silicon nitride pacified silicon wafer with an array of 100 individually addressable square Au electrodes.

Figure 17.11 Schematic representation of an approach for achieving efficient electrocatalysis of glucose oxidation by glucose dehydrogenase on Au nanoparticles tethered on an Au electrode. The nanoparticles are modified with a PQQ-capped linker that interacts with the unoccupied PQQ site of cofactor-deficient glucose dehydrogenase [Zayats et al., 2005].

The Fe-only form of these metalloporphyrins is a highly selective ORR catalyst when adsorbed on a graphite or Au electrode. It operates at an overpotential of about 0.55 V at pH 7 and av >3.9 (Fig. 18.19) and retains these characteristics for >10 mrnovers the catalytic selectivity is independent of the amount of deposited catalyst. 

Cyclic voltammograms of the [Fe(CN)6] /Fe[(CN)g] redox couple with the bare and the DNA-modified electrodes are shown in Fig. 5 [14a]. The peak currents due to the reversible electrode reaction of the redox system on the bare Au electrode were significantly suppressed by the treatment with DNA. In contrast, the treatment with unmodified, native DNA made no suppression, and that with HEDS caused only a slight one, as seen in Fig. 

FIG. 5 Cyclic voltammograms of [Fe(CN)6] redox couple on bare (solid line), DNA-modified (heavy line), and HEDS-modified Au electrodes (dotted line). Electrol5de solution, aqueous each 5 mM of K4[Fe(CN)g] and K3[Fe(CN)g] containing 10 mM KCl scan rate 25 mV s temperature, 25°C electrode area, 0.02 cm (geometrical). 

These results indicate that the surface-anchored DNA blocks the electrochemical reaction of [Fe(CN)6]" with the underlying Au electrode, due to the electrostatic repulsion between the polyanionic DNA and the anionic redox couple ions. 

Under this electrochemical configuration, it is commonly accepted that the system can be expressed by the Randles-type equivalent circuit (Fig. 6, inset) [23]. For reactions on the bare Au electrode, mathematical simsulations based on the equivalent circuit satisfactorily reproduced the experimental data. The parameters used for the simulation are as follows solution resistance, = 40 kS2 cm double-layer capacitance, C = 28 /xF cm equivalent resistance of Warburg element, W — R = 1.1 x 10 cm equivalent capacitance of Warburg element, IF—7 =l.lxl0 F cm (

charge-transfer resistance, R = 80 kf2 cm. Note that these equivalent parameters are normalized to the electrode geometrical area. On the other hand, results of the mathematical simulation were unsatisfactory due to the nonideal impedance behavior of the DNA adlayer. This should... 

Here, i ct.hare and i ct.oNA indicate the charge transfer resistance for the redox couple on the bare and the DNA-modilied Au electrodes, respectively and 6 was calculated to be 80%. 

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