Platinum anode contaminants

In this case a platinum anode wich is not sensitive to organic contamination can be used giving a current yield for Cr(VI) of 94%. Electrolyses can be performed in a two-phase medium making an in-cell process possible This method has been applied to the synthesis of benzaldehyde from benzylic alcohol giving 100% current yields at 10 % conversion while the current yield drops to 60 % at high conversions. 

Furthermore, the restrictions on operating voltage that apply to titanium in a marine enviroment are not always relevant to titanium in soils free of chloride contamination. Coke breeze is, however, an integral part of the groundbed construction and ensures a lower platinum consumption rate. However, for some borehole groundbeds, platinised niobium is preferred, particularly in the absence of carbonaceous backfill or in situations where the water chemistry within a borehole can be complex and may, in certain circumstances, contain contaminants which favour breakdown of the anodic Ti02 film on titanium. In particular, the pH of a chloride solution in a confined space will tend to decrease owing to the formation of HOCl and HCl, and this will result in an increase in the corrosion rate of the platinum. 

As expected, H ionizes from the surface of a platinum electrode previously used to evolve H2, when the electrode is swept anodically. However, two peaks, each at a characteristic anodic potential, are always observed (even after the electrode has been subjected to an H2-02 flame to remove contamination) (Fig. 7.78). 

Experiments on the 111 planes of platinum have shown a detailed structure in the anodic sweep (0.05 to 0.6 RHE),70 but this structure is extinguished if contamination from the solution is present. Another interesting effect occurs if the HjSC solution is replaced by HC104. In the potential range 0.05 to 0.5 (RHE) on the anodic side, the voltammogram splits into two parts in the presence of C104. 

Carbon-supported Pt can also be used as the anode catalyst. However, this requires pure H2. Contaminants such as carbon monoxide (CO) poison the catalyst, because CO can strongly adsorb on Pt, blocking the catalytic sites and reducing platinum s catalytic activity. In H2 produced from the reforming of other fuels, CO is always present. Thus, to improve contaminant tolerance, carbon-supported PtRu was developed and now is always used as the anode catalyst. Ru can facilitate the oxidation of CO, releasing the catalytic sites on Pt through the following reactions ... 

Finally, it should be realized that CO is not the only fuel (or fuel-derived) contaminant expected to affect anode performance in the PEFC. In a test of other possible contaminants that could result, in principle, from methanol reforming, Seymour et al. [27] reported strong and irreversible effects of formic acid at a PEFC platinum (high-loading) anode, whereas methanol, formaldehyde, and methyl formate were found to have much smaller and reversible effects. The fuel impurity aspects of coupling between natural gas (or gasoline) reformers of various types and a PEFC stack are even wider, and make it essential to probe and address, either by removal upstream or by use of modified catalysts, the possible detrimental effects of low levels of sulfur, H2S, COS, and NH3 [28]. 

Fig. 7,8. Reversible mercury-calomel electrode with the parts consisting of a 250 ml wide-mouthed reagent bottle, a vented and drilled rubber stopper, a soft glass tube with platinum wire sealed into the end, mercury, a wire to the output of the power supply, saturated KCl, and a glass tube containing 5% polyacrylamide gel made up in 1 M KCl. The tube dips into the buffer reservoir of the electrophoresis apparatus. The anode and cathode should be interchanged after each run. This arrangement will prevent all contamination of the buffer with electrode products even in very long runs (J. C. Finder, Ph.D. Thesis, London University, 1974).

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