Platinum electrodes electrode reaction problems

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. 

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. 

The low-temperature fuel cells described in Chapters 18 and 19 have (acidic) proton-conducting membranes as an electrolyte. On account of corrosion problems, only metals of the platinum group can be used as catalysts for the electrode reactions in such fuel cells. 

Along with the identity of the intermediate in the methanol oxidation reaction, the mechanism of the hydrogen evolution reaction, (HER), at Pt group metals has remained relatively ill understood. The key problem lies in reconciling the observed kinetic data with the known coverage of, for example, platinum electrodes by adsorbed hydride at the onset of hydrogen evolution. The HER may be most simply represented by ... 

This will reduce the efficiency of the alkaline membrane with time. One can avoid this problem via an alternative strategy which uses a proton exchange membrane, e.g. Nation, and electrodes containing a noble metal platinum as the catalyst The electrode reactions then are as follows ... 

Other methods for the preparation of rare earth cyanides are the reaction of metallic R with Hg(CN)2 in liquid ammonia and the precipitation with HCN in an ammonia solution of metallic Eu and Yb (McColm and Thompson, 1972). The problem with the first of these methods is the difficulty of removing metallic Hg and the possible excess of Hg(CN)2- The precipitation method has been extended to rare earths insoluble in ammonia, in which case the reaction is effected by electrolysis at —63°C in a concentrated NH4CN-NH3 solution, using rare earth and platinum electrodes. In all cases the crystallinity is poor and the particle size of the product is small. The colors of some rare earth cyanides already synthesized are presented in table 30. 

The problem of corrosion is immense. One can say with certainty that all metals, except the noble ones (gold, platinum etc.), corrode to a certain extent. Some of them form oxide layers on the surface which protects them against further attack, but some, notably iron, do not form such a layer. Iron corrosion alone costs every year millions of pounds in protection and replacement. There are two main mechanisms by which corrosion occurs. One is by direct oxidation of the metal by air oxygen. Metal oxides are, as a rule, thermodynamically more stable than the metal and oxygen in their elementary states. This is the mechanism by which metals become covered with oxide even in very dry atmosphere. The second mechanism is an electrochemical one two electrode reactions take place on the surface of the corroding metal their products are the undesirable corrosion products. It is easier to understand electrochemical corrosion by considering the corrosion of copper plated iron as an illustration if the plating is broken, the iron, which is in contact with the copper, is exposed to the atmosphere, to rain and often to traffic fumes. The atmosphere contains carbon dioxide and some-... 

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