Electrocatalyst surface oxygen layers

Depending on potential, oxygen is bound on the surface of noble metal electrocatalysts as a chemisorbed species or a surface oxide (7, 99). The formation of these surface oxygen layers is independent of Oj presence (7,779) and irreversible, with the exception of iridium (97,120,121). Irreversibility becomes more pronounced in the order Os < Ru < Rh < Pd Pt... 

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)... 

PtO2 (752, 757), similar to some electrochemical oxygen layers. Figure 12 shows a possible structure of platinum oxides on various planes (752). The [1(X)] plane has a PtOj composition (752,757), while the bulk corresponds to a PtO oxide. Present information does not unambiguously point toward either surface oxide formation or chemisorption. Because of the apparent similarity of some surface oxygen species on catalysts and electrocatalysts, coordinated efforts in both fields using standardized techniques and procedures could resolve uncertainties. 

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). 

Several electrochemically important reactions occur on the surface of oxidized noble metal electrodes without reaction with the surface oxide. The oxide film then behaves as a new type of electrocatalyst surface on which the reaction proceeds, in distinction to the underlying metal. The oxide films are normally only one to three oxygen layers in thickness. The following inorganic reactions are of this class (a) O2 evolution, (b) CI2, (c) Br2 (to a small extent, since adsorbed Br blocks surface oxidation) and N2 evolution (from N3 ) while, in the case of organic reactions, (a) the Kolbe and Crum-Brown/Walker syntheses, as well as (b) the Hofer-Moest reaction, are of this type. ... 

Practical units of fuel cells could not operate without porous electrode structures. Porous electrodes with their large electrochemically active surface allow reasonable currents to be supplied at acceptable losses due to polarization (see section 2 of chapter II). Although a few properties, like maximum available surface of electrocatalyst and hydrogenation and dehydrogenation of carbonaceous species for Teflon-bonded platinum black electrodes, and formation of oxygen layers for Raney nickel electrodes, have been discussed in preceding chapters, a discussion of the parameters that determine the operation of porous electrodes had to be offered in a separate chapter. While the empirical aspects concerning the operation of porous electrodes are covered in this chapter, theoretical aspects are dealt with in chapter XVI. 

Pt-doped carbon aerogels have been used successfully in the preparation of cathode catalyst layers for oxygen reduction reaction (ORR) in PEMFC systems [83-86]. Thus, different Pt-doped carbon aerogels with a Pt content of around 20 wt% were prepared by impregnation [83]. Results obtained with these Pt catalysts were compared with others supported on carbon blacks Vulcan XC-72 and BP2000, which are commonly used as electrocatalysts. The accessibility of the electrolyte to Pt surface atoms was lower than expected for high-surface-area... 

Another theory claims that a protective complex between the metal and the CP is formed in the metal-polymer interface. Kinlen et al. [73] found by electron spectroscopy chemical analysis (ESCA) that an iron-PANl complex in the intermediate layer between the steel surface and the polymer coating is formed. By isolating the complex, it was found that the complex has an oxidation potential 250 mV more positive than PANI. According to Kinlen et al. [73], this complex more readily reduces oxygen and produces a more efficient electrocatalyst. 

Thus prepared layers have been further modified to develop electrocatalysts and sensors. Polypyridyl ruthenium-oxo complexes are of particular interest as efficient oxidants for a wide variety of organic molecules, including aromatic hydrocarbons, olefins, alcohols, and ketones. One such electrocatalyst was prepared by first electrografting bipyridine at an applied positive potential followed by treating the modified surface with [Ru tl2(DMSO)(terpyridine)] and then CFjSOjH/HjO [104]. Enhanced electrochemical activity has also been observed for the reduction of oxygen at anthraquinone-modified GC electrodes in 0.1 M KOH solution [105,... 

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