Platinum oxygen evolution

On platinum, oxygen evolution occurs at +1.4V (SHE). No peaks corresponding to oxidation or reduction of the glycol (PG) or disulfide (SPS) are visible, because the potentials for these reactions are more positive than the potential for water electrolysis on platinum, i.e., they are outside the window of water stability. However, on diamond, two oxidative peaks are observed for each additive +2.05V and +2.4 V (SHE) for PG (Fig. 3.2a), and +1.65 V and +2.1 V (SHE) for SPS (Fig. 3.2h). The oxygen evolution observed at +2.4V on diamond does not interfere with detection of these compounds. Moreover, the relationship between the peak heights and the additive concentration is linear [31] as shown in Fig. 3.3. 

A mixture of A,A-dimethylcyclohexylmethylamine (49.4 g, 0.35 mole. Chapter 2, Section I), 30% hydrogen peroxide (39.5 g, 0.35 mole) and 45 ml of methanol is placed in a 500-ml Erlenmeyer flask, covered with a watch glass, and allowed to stand at room temperature. After 2 hours, and again after an additional 3 hours, 30% hydrogen peroxide (39.5-g portions each time) is added with swirling. The solution is allowed to stand at room temperature for an additional 30 hours, whereupon excess peroxide is destroyed by the cautious addition (swirling) of a small amount of platinum black. Cessation of oxygen evolution indicates complete decomposition of the excess peroxide. 

Platinum has also had its share of attention in recent years. The effect of phosphoric acid concentration on the oxygen evolution reaction kinetics at a platinum electrode using 0-7 m-17-5 m phosphoric acid at 25°C has been studied with a rotating disc electrode . The characteristics of the ORR are very dependent on phosphoric acid concentration and H2O2 is formed as an intermediate reaction. Also, platinum dissolution in concentrated phosphoric acid at 176 and 196°C at potentials up to 0-9 (SHE) has been reported . 

Dimethyl sulphoxide has also been oxidized electrochemically, using either a platinum anode or a dimensionally stable anode containing iridium and selenium in 1 M sulphuric acid solution158. The former electrode requires a potential close to that required for oxygen evolution whilst the latter needed a potential 0.5 volts lower. Thus the dimension-... 

The oxygen reactions occur at potentials where most metal surfaces are covered by adsorbed or phase oxide layers. This is particularly true for oxygen evolution, which occurs at potentials of 1.5 to 2.2 V (RHE). At these potentials many metals either dissolve or are completely oxidized. In acidic solutions, oxygen evolution can be realized at electrodes of the platinum group metals, the lead dioxide, and the oxides of certain other metals. In alkaline solutions, electrodes of iron group metals can also be used (at these potentials, their surfaces are practically completely oxidized). 

FIGURE 15.5 Polarization curves for anodic oxygen evolution at a platinum electrode in perchloric acid solutions with various concentrations (1) 1.34 (2) 3 (3) 5 (4) 9.8 M. 

FIGURE 15.9 Anodic polarization curves recorded at a platinum electrode in the region of high anodic potentials in the presence of acetate ions (1) total current (2) partial current of oxygen evolution (3) partial current of oxidation of adsorbed species. 

Oxides of Platinum Metals Anodes of platinum (and more rarely of other platinum metals) are used in the laboratory for studies of oxygen and chlorine evolution and in industry for the synthesis of peroxo compounds (such as persulfuric acid, H2S2O8) and organic additive dimerization products (such as sebacic acid see Section 15.6). The selectivity of the catalyst is important for all these reactions. It governs the fraction of the current consumed for chlorine evolution relative to that consumed in oxygen evolution as a possible parallel reaction it also governs the current yields and chemical yields in synthetic electrochemical reactions. 

The character of the oxide layers influences the kinetics and mechanism of the electrochemical reactions occurring on the platinum anode surface. The relation between the rate of oxygen evolution and oxide layer thickness is complex. In the region where the a-oxides exist, the reaction rate decreases with increasing oxide layer thickness. In the region where the P-oxides exist, the reaction rate depends little on oxide layer thickness or, according to some data, increases with increasing oxide layer thickness. 

Since the oxygen overvoltage on a platinum electrode is equal to 0.45 V, the minimum potential required for oxygen evolution is (+ 0.813 + 0.45) V or 1.263 V. Since the voltage required to implement this reaction is lower than that for a normal chlorine electrode, it follows that oxygen will be evolved at the anode in preference to chlorine. In contrast to the two possible reactions at the anode, there are five reactions that are possible at the cathode as shown below ... 

M. Faraday was the first to observe an electrocatalytic process, in 1834, when he discovered that a new compound, ethane, is formed in the electrolysis of alkali metal acetates (this is probably the first example of electrochemical synthesis). This process was later named the Kolbe reaction, as Kolbe discovered in 1849 that this is a general phenomenon for fatty acids (except for formic acid) and their salts at higher concentrations. If these electrolytes are electrolysed with a platinum or irridium anode, oxygen evolution ceases in the potential interval between +2.1 and +2.2 V and a hydrocarbon is formed according to the equation... 

Walton DJ, Burke LD, Murphy MM (1996) Sonoelectrochemistry chlorine, hydrogen and oxygen evolution at platinised platinum. Electrochim Acta 41 2747-2751... 

As it follows from Table 5, many catalysts contain metallic platinum. We have developed bi-layer porous hydrophobic air electrodes, which do not contain platinum metals, are active and can be cycled [24, 25] (Figures 4-6). These bifunctional catalysts are pyrolized Co - macrocyclic compounds. Said catalyst has high catalytic activity for the oxygen reduction and also features acceptable stability, however its activity for the oxygen evolution is not high enough. 

FIGURE 3.2. Oxygen evolution for 2E6 platinum/gold nanorods in 10 ml of a 3.74% w/w aqueous solution of hydrogen peroxide. The autocatalytic decomposition of hydrogen peroxide was less than 1 lE-9 mol/min and is therefore insignificant. 

Smooth platinum, lead dioxide and graphite are anode materials commonly used in electrooxidation processes. All show large overpotentials for oxygen evolution in aqueous solution. Platinum coated titanium is available as an alternative to sheet platinum metal. Stable surfaces of lead dioxide are prepared by electrolytic oxidation of sheet lead in dilute sulphuric acid and can be used in the presence of sulphuric acid as electrolyte. Lead dioxide may also be electroplated onto titanium anodes from lead(Il) nitrate solution to form a non-porous layer which can then be used in other electrolyte solutions [21],... 

Platinum and carbon are frequently used as counter electrode materials for both anode and cathode. Platinum is resistant to corrosion while carbon is cheap and can be discarded after use. Nickel is a suitable counter cathode material in aqueous solution because of the low overpotential for hydrogen evolution. Titanium coated with platinum and then over coated with mthenium dioxide is a stable counter anode material with a low overpotential for oxygen evolution. 

The non-Kolbe reaction of trichloroacetic acid at platinum shows con etition with oxygen evolution. The formation of trichloromethy trichloroacetate only begins when the anode potential exceeds 2.35 V vs, see [63]. At lower anode potentials oxygen only is evolved. 

At IREQ, besides the participation in the field tests run by the engineers of Hydro-Quebec (12), the main effort has been to tackle fundamental problems in the field of electrocatalysis (18-22) and of anodic oxidation of different potential fuels (23-26). A careful and extensive study of the electrochemical properties of the tungsten bronze has been carried out (18-20) the reported activity of these materials in acid media for the oxygen reduction could not be reproduced and this claim by other workers has been traced back to some platinum impurities in the electrodes. Some novel techniques in the area of electrode preparation are also under study (21,22) the metallic deposition of certain metals on oriented graphite show some interesting catalytic features for the oxygen reduction and also for the oxygen evolution reaction. 

The oxygen cathode—for which platinum catalyst due to its outstanding structural and catalytic capability is the rule—is not used as an oxygen evolution anode in the electrolyzer operation mode because oxidation of Pt and fast catalyst deterioration would be the consequence. Therefore an oxygen cathode based on a platinum catalyst must operate as a -evolving cathode in the regenerative mode. 

Platinum is the most catalytically active substrate employed by the clectro-voltammogram chemist and it shows no dissolution between the potentials of hydrogen and n in acid oxygen evolution in aqueous solution. Hence, it finds common application... 

Electrocatalytic oxidation of glyoxal on platinum in perchloric acid medium at fixed potential in the oxygen evolution region. 

Fig. 5. Distribution of the reaction products of glyoxal electrolysis on platinum in the oxygen evolution region in acidic medium (pH=l), after 7 hours of operation. (B) E=2.03 V/RHE, E=2.13 V/RHE.

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