Platinum electrodes problem

Another issue with platinum catalysts is that their capacity sometimes fades over time. Several factors are responsible, including a phenomenon similar to the side effects described for medications in chapter 3. Side effects occur when a medication acts on healthy tissue instead of the intended target. With platinum electrodes, the problem is that sometimes unwanted reactions occur at the electrodes. In the oxygen reactions taking place at the cathode, for example, hydroxide (OH) and other molecules sometimes form and bind to the platinum atoms. These molecules cover the platinum atoms and block access to the desired reactant, thereby reducing the catalytic activity. Sometimes the molecules even pull platinum atoms away from the surface, causing serious electrode degradation. 

This problem is exacerbated when power requirements fluctuate. For instance, a fuel cell in an automobile would experience frequent stops, especially in city traffic, and platinum electrodes would rapidly lose their catalytic function. 

If (23) is selected as the dihalosilane, a convenient way of modifying the nickel surface is available.64 The electrochemical properties of the treated nickel electrode are very similar to those of a similarly derivatized platinum electrode for example, both are equally effective in the elec-trocatalytic oxidation-reduction of solution ferrocene. Normally oxidation of the nickel surface would be a competing process ultimately rendering the electrode passive. The surface modification clearly eliminates this problem and opens up the possibility of using surface modified inexpensive metals as electrodes. 

Platinum electrodes are sometimes utilized to obtain Eh measurements on water samples. However, they are often ineffective, especially with natural waters (Drever, 1997), 136, 180-182 (Lindberg and Runnells, 1984). Besides problems with chemical disequilibria in many natural waters, platinum electrodes usually fail to quickly respond to the kinetics of the couples that commonly control redox conditions in water samples, including O2-H2O, S02 -H2S, CO2-CH4, NO -N2, and N2-NH (Drever, 1997), 136. For arsenic, large discrepancies may exist between redox values calculated from the As(V)/As(III) couple and Eh measurements with platinum electrodes (Ryu et al., 2002), 2989-2990. Additionally, when collecting... 

But the film thickness doesn t change, so how can the coercive voltage increase And the same capacitor can show its true coercive voltage of e.g. 2 Volts after a few measurements. So it appears to be a contact problem, though a pure platinum electrode is contacted to a pure Ptlr alloy coated cantilever, so an excellent contact should be expected. To investigate the reason of this poor contact behavior, afm conductivity scans were performed on a pure Pt coated... 

Activation (of noble metal electrodes) — Noble metal electrodes never work well without appropriate pretreatment. Polycrystalline electrodes are polished with diamond or alumina particles of size from 10 pm to a fraction of 1 pm to obtain the mirror-like surface. The suspensions of polishing microparticles are available in aqueous and oil media. The medium employed determines the final hydrophobicity of the electrode. The mechanical treatment is often followed by electrochemical cleaning. There is no common electrochemical procedure and hundreds of papers on the electrochemical activation of -> gold and platinum (- electrode materials) aimed at a particular problem have been published in the literature. Most often, -> cyclic and - square-wave voltammetry and a sequence of potential - pulses are used. For platinum electrodes, it is important that during this prepolarization step the electrode is covered consecutively by a layer of platinum oxide and a layer of adsorbed hydrogen. In the work with single-crystal (- monocrystal) electrodes the preliminary polishing of the surface can not be done. 

EMIRS studies of ethanol on platinum electrodes have demonstrated the presence of linearly bonded carbon monoxide on the surface [106]. An important problem in the use of EMIRS to study alcohol adsorption is the choice of a potential window where the modulation is appropriate without producing faradaic reactions involving soluble products. Ethanol is reduced to ethane and methane at potentials below 0.2 V [98, 107] and it is oxidized to acetaldehyde at c 0.35 V. Accordingly, a potential modulation would be possible only within these two limits. Outside these potential region, soluble products and their own adsorbed species complicate the interpretation of the spectra. The problem is more serious when the adsorbate band frequencies are almost independent of potential. In this case, the potential window (0.2-0.35 V) is too narrow to obtain an appropriate band shift and spectral features can be lost in the difference spectrum. 

System Condition Redox and Metastabilitv. Two major sources of uncertainty in modeling aqueous systems are the redox potential and metastability these are frequently acknowledged as conceptual problems, but discussion of the error which results from improper assumptions and calculations is generally avoided. The Eh or electrochemical potential which is computed from a potential measured with a platinum electrode, is used in almost all geochemical models as a system parameter. 

The problem was solved (2, 7) by splitting a straight horizontal platinum electrode into a vertical array with each member attached to a diode and then onto a common bus (hence named diode isolated electrodes). A diode can be thought of as an electrical ratchet because it confines electrical current to unidirectional flow. This new electrode design permits more than one electrode pair to coexist simultaneously in the same conductive media without any appreciable interaction. 

Iodide in the solution in Problem 11.5 can also be determined by controlled-potential oxidation to iodine at a platinum electrode. What potential should be used for this oxidation (see Figure 11.10.1)7 How many coulombs will be passed ... 

The measurement of redox conditions by means of a cell voltage, where one electrode has a fixed reference potential and the other is expected to react reversibly with natural systems, is attended by a number of problems. The platinum electrode only works well under certain conditions, it is difficult to get the electrode into... 

Under an in situ condition, a platinum electrode is inserted into the soil at a predetermined depth. Usually platinum electrodes are left in place for subsequent measurements. At the time of measurements, the reference electrode is inserted at a short distance into the wet soil. It is not necessary that the reference electrode be placed at the same depth as the platinum electrode, as long as there is enough soil moisture to ensure a good electrical contact. However, the electrical resistance of the measuring circuit can be affected by the distance between the platinum electrode and the reference electrode. This may not be a serious problem in saturated soils. 

Experimental difficulties, theoretical uncertainties, and poor planning have so conspired together as to frustrate most attempts to determine the conductances or excess conductances of the electrons in amine solvents. One of the main problems in the laboratory has been the low chemical stability of the alkali metal solutions. Their blue colour gradually fades as the solutions decompose with the formation of hydrogen, a process catalysed by impurities and especially by the platinum electrodes of the cell itself. Pyrex vessels, it was recently discovered, cause sodium contamination, and for this reason much of the early research is now of doubtful worth. The experimental problems are exacerbated in the case of methylamine, whose volatility demands the use of low temperatures at which the metals dissolve but slowly. A further problem arises in the extrapolation of the data to infinitesimal ionic strength, for the appropriate conductance function to be applied depends upon the kind of species which the solution contains. And when, after all these hazards, the limiting conductance of an alkali metal solution has finally been obtained, it turns out as often as not that it can neither be compared with values for other metals because each experimenter has worked at a different temperature, nor with the conductances of normal salts because in the excitement their measurement has been overlooked. 

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