Oxygen reduction reaction acid adsorption

Baker WS, Pietron JJ, Teliska ME, et al. 2006. Enhanced oxygen reduction activity in acid by tin-oxide supported Au nanoparticle catalysts. J Electrochem Soc 153 A1702-A1707. Blizanac BB, Lucas CA, Gallagher ME, et al. 2004a. Anion adsorption, CO oxidation, and oxygen reduction reaction on a Au(lOO) surface The pH effect. J Phys Chem B 108 625-634. 

Adsorption inhibitors act by forming a film on the metal surface. The action of traditional oil-based red lead paint formulations presumably involves the formation of soaps and the precipitation of complex ferric salts that reinforce the oxide film. There has been substantial interest in recent years in development of replacements for lead-based and chromate-based inhibitor systems. Adsorption inhibitors based on pol3rmers have been of particular interest. In this volume, Johnson et al. and Eng and Ishida discuss inhibitors for copper 2-undecylimidazole is shown to be effective in acid media, where it suppresses the oxygen reduction reaction almost completely. Polyvlnyllmidazoles are shown to be effective oxidation inhibitors for copper at elevated temperatures. Also in this volume, Chen discusses the use of N-(hydroxyalkyl)acrylamide copolymers in conjunction with phosphate-orthophosphate inhibitor systems for cooling systems. 

More recently, Wang et al. [28] derived an intrinsic kinetic equation for the four-electron (4e ) oxygen reduction reaction (ORR) in acidic media, by using free energies of activation and adsorption as the kinetic parameters, which were obtained through fitting experimental ORR data from a Pt(lll) rotating disk electrode (RDE). Their kinetic model consists of four essential elementary reactions (1) a dissociative adsorption (DA) (2) a reductive adsorption (RA), which yields two reaction intermediates, O and OH (3) a reductive transition (RT) from O to OH and (4) a reductive desorption (RD) of OH, as shown below [28] (Reproduced with permission from [28]). 

Because of the phosphoric acid-hlled catalyst layer, the HT-PEMFC encounters the same limitations as the phosphoric acid fuel cell (PAFC) in developing high performance MEAs. The sluggish oxygen reduction reaction (ORR) kinetics which are caused by the low O2 solubility in phosphoric acid and the strong adsorption... 

Nowadays, it has been demonstrated that the reaction is indeed structure sensitive with a multielectron transfer process that involves several steps and the possible existence of several adsorption intermediates [93-96]. The main advantage that we have with the new procedures with respect to cleanliness is that we have well-ordered surfaces to study a complex mechanism such as the oxygen electroreduction reaction [96-99]. In aqueous solutions, the four-electron oxygen reduction appears to occur by two overall pathways a direct four-electron reduction and a peroxide pathway. The latter pathway involves hydrogen peroxide as an intermediate and can undergo either further reduction or decomposition in acid solutions to yield water as the final product. This type of generic model of a reaction has been extensively studied since the early 1960s by different authors [100-108]. 

The study observed fliat the O2 adsorption energy for one-fold end-on was 0.43 eV and for two-fold was 0.94 eV. Two-fold bonded oxygen was more stable than one-fold. The dissociation energy for two-fold bonded O2 was 0.74 eV, while the activation barrier for the first reduction step to OOH was less than 0.60 eV at 1.23 V electrode potential. In other words, the first electron transfer has a smaller barrier than that of O2 dissociation. Furthermore, the dissociation barrier for the first electron transfer product OOH was much smaller, 0.06 eV. So, the authors eoncluded that O2 did not dissociate before the first reduction step, and OOH easily dissociated once formed after the first electron transfer step. The paper also demonstrated that the electronic field of the proton increased the electron affinity of the reactant complex and therefore facilitated the reaction. Thus, they proposed that for oxygen reduction on Pt in acid, proton transfer would be involved in the rate determining step because of the ability of its electric field to enhance the electron attracting capability of flic surface-coordinated O2. The authors concluded... 

The NOx emitted into the atmosphere creates serious environmental issues, such as acid rain and excessive ground-level ozone with leads to photochemical smog, which is common in the urban environment [2,34], The continual exposure of NO, to the PEM fuel cell, even at low concentration, can be detrimental to cell performance. It is estimated that the NO2 concentration in a PEM fuel cell should remain below 0.05 ppm in order that its performance and lifetime not be unduly compromised [35,361. Although the adsorption of NO on Pt (platinum) is weak, it can block sites on the platinum surface that would otherwise be available for oxygen reduction [37]. The following reaction mechanism for the detrimental effect of NO , has been proposed based on a study by [38] ... 

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