Reductions oxygen layer

Reductive removal of these oxygen layers is a slow kinetic process, commencing at potentials well below the characteristic potential for the layer formation on each metal. Thus, adsorption results based on the commonly used triangular potential sweep method can depend on the anodic potential excursions, the frequency of potential cycling and the number of cycles, that is, the catalyst surface history. Similarly, kinetic studies of oxygen reduction can be influenced by the dependence of the oxygen layer formation... 

Several surface oxygen species have been postulated based on kinetic parameters, stable intermediates, and in situ observations during oxygen reduction or evolution. As in the case of hydrogen, platinum is the most well-studied electrocatalyst. Optical measurements (122-126) show that a freshly developed oxygen layer on platinum behaves reversibly up to 0.95 V. However, rapid aging yields an irreversibly bound layer. Chemisorption of OH is assumed to occur in this potential region (123,124,127,128) formed by z. + + H- + e (27)... 

Because of the irreversible and not well-understood change of the electrocatalyst surface above 1.0 V, early mechanistic studies were conducted under ill-defined conditions. Thus, while anodic evolution of Oj takes place always in the presence of oxygen-covered electrodes, the cathodic reaction proceeds on either oxygen-covered or oxygen free surfaces with different mechanisms (77,158). The electrochemical oxide path, proposed for oxide-covered platinum metals in alcaline electrolytes (759,160), has been criticized by Breiter (7), in view of the inhibition of oxygen reduction by the oxygen layers. Present evidence points to the peroxide-radical mechanism (77,... 

Apart from poisoning by adsorbing impurities, the working electrode potential can also contribute to suppress electrocatalytic activity. Platinum metals, for instance, passivate or form surface oxygen and oxide layers above 1 V (Section IV,D), which inhibit Oj reduction (779,257,252) and oxidation of carbonaceous reactants (7, 78, 253, 254) however, decomposition of hydrogen peroxide on platinum is accelerated by oxygen layers (255). Some electrocatalysts may corrode or dissolve, especially in acidic electrolytes, while reactants may contribute to dissolution. Thus, ethylene oxidation on palladium to acetaldehyde proceeds via a Pd-ethylene complex, which releases colloidal palladium in solution (28, 29). Equivalent to this is the surface roughening and the loss of Pt in gas phase ammonia oxidation (256, 257). 

By quantification of the oxygen content it was possible to estimate the thickness of the oxide layer, related to the exposed surface of Pd (whose size was estimated after complete reduction of the sample). It was observed that the more active catalysts had a larger number of oxygen layers which suggested the participation of bulk Pd in the reaction. Therefore, for methane oxidation, the Pd/Al2C>3 catalytic properties seem to be controlled by a redox mechanism, where the oxidation and reduction rates of the active phase depend on the size and stability of the Pd and PdO species. The small PdO particles seemed to be poorly reactive, due to their higher stability or to a greater interaction with the support. In fact, analysis of TPO and TPD data revealed that the small particles of PdO decompose at... 

Fig. 4a shows the TPD spectra after dosing NO at room temperature on a pre-reduced Pt-Rh/BaO/AbOs catalyst. As can be seen the NO is reduced on the surface as manifested in Na and N2O desorption peaks around 200°C. The only trace of NO is a small peak around 100°C. The integrated amount from these curves correspond to an adsorbed amount of 7.1-10 moles NO. The result of a similar experiment but with a pre-oxidised sample is shown in Fig. 4b. In this case there is no reduction of NO taking place. There is a small NO peak at around 90°C and a larger one at about 500°C. There is also an O2 peak around 500°C. It is likely that a chemisorbed oxygen layer on the noble metals prevents the dissociation of NO as observed by Lo6f et al. [6]. When NO2 rather than NO is dosed at room temperature, there is a much larger quantity adsorbed. Further, pre-reduced and pre-oxidised samples show similar TPD spectra indicating that the strong oxidising agent NO2 oxidises the sample at room temperature.

J.M. Macak, F. Schmidt-Stein, P. Schmuki, Efficient oxygen reduction on layers of ordered Ti02 nanotubes loaded with Au nanoparticles . Electrochemistry Communications, 9, 1783-1787, (2007). 

The clean iron surface exhibits a tendency to minimize an excess of the surface energy by reduction of the surface area. It is assumed in the model that the surface is covered by an oxygen layer (there are promoters on top). In this case the surface energy is compensated by Fe-O bonds. The greater the oxygen... 

Relatively good interface between Pt-MoOx could be established on that support with very low concentrations of the promoter NP, molybdenum oxide (MoO c) 2system prepared using 5% MoOx additive displayed reasonably high active Pt area and excellent oxygen reduction [96], Layered poly(3,4-ethylenedioxythiophene) in PEDOT/VS2 nanocomposite offers enhanced discharge capacity as a cathode material for rechargeable Li batteries. This activity is attributed to the electrochemical intercalation of Li into the PEDOT/VS2 nanocomposite [97],... 

Pretreatment of platinum in hot chromic acid was shown [83] to be equivalent to anodic activation. The electrode has a large reactivity after reduction of the oxygen layer that was formed at open circuit. In contrast to the pretreatment with hot chromic acid, the i — U curve of the first sweep after treatment of the platinum electrode in hot nitric acid does not have the shape characteristic for a clean surface. An intermediate product in the reduction of nitric acid seems to be strongly adsorbed on platinum. It takes several hours of continuous cycling between 0.05 V and 1.4 V at 30mV/sec before the i- U curve regains the regular shape. 

Formation and Reduction of Oxygen Layers on Platinum Metals and Some Alloys... 

The i— U curves measured on the platinum-gold alloys, which consist of a platinum-rich phase and a gold-rich phase 2, demonstrate [51] that the formation and reduction of the oxygen layers on patches of these two phases on the surface occur independently of each other (compare section 8 in chapter VI). On the platinum-chromium alloys the formation and reduction of oxygen layers on patches of chromium overshadows that on platinum patches. 

Here reactions 3 and 4 stand for the formation of the hydroxide M(OH) and the oxide MO respectively. Similar reactions apply in alkaline electrolytes. It is conceivable that the chemisorbed layers and metal hydroxide or oxide are formed simultaneously on different parts of the surface in a certain potential range. The conditions [42] under which the voltammetric i — U curves give evidence for the occurrence of reaction 1 and 2 are discussed subsequently. The electrolyte concentrations are assumed such that concentration gradients of OH or H are negligible in the diffusion layer during the formation or reduction of the oxygen layer. 

Estimates of the ratio 6oxide/6o,cath between the charge for oxide formation and the charge for the reduction of the total oxygen layer yielded [42] about 0.12 for Pt at 1.5 V and 0.20 for Ir at 1.4 V. The oxygen layers in these studies [18, 23, 41, 42] consisted mainly of chemisorbed layers. The ratio Goxide/6o,cath be different for other studies in... 

The preceding considerations may also be applied to the discussion of the formation and reduction of oxygen layers on Pt, Rh, and Ir in alkaline electrolytes since the respective i—U curves display similar properties in acid and alkaline solutions. The extent of phase hydroxides and oxides on platinum metals is small in the potential region which is of interest for the anodic oxidation of fuels and for the oxygen electrode in fuel cells. The formation of thicker oxide layers at more positive potentials will not be considered here. 

The mechanism of the oxygen electrode is discussed for the above metals in this chapter. Depending upon the potential and the platinum metal, the oxygen reduction may occur on surfaces free of oxygen layers or on surfaces covered partially with layers. The two cases are treated separately since the reduction mechanism is affected by oxygen layers. As discussed in chapter VI, adsorbed impurities decrease the ability of platinum metals to adsorb hydrogen. Similarly, the O2 reduction depends strongly upon the surface state. For two reasons, the emphasis is on clean surfaces of platinum metals, obtained for instance by previous anodic and subsequent cathodic pretreatment ... 

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