Oxygen reduction reaction dissociative mechanism

In attempts to simulate the experimentally observed oxygen reduction reaction, several kinetic models have been reported in the literature, based on various proposed ORR mechanisms [38—41]. These models employed either the associative or the dissociative model. The former was used by Antoine et al. [38] in their simulation of a polarization curve and impedance spectra, and by Du et al. [40] in a model-based electrochemical impedance spectroscopy study. Antoine et al. considered the following reaction steps ... 

The lower the slope, the lower the activation loss for the O2 reduction reaction in acid media, the observed slope is 0.060 V/ decade and it can be obtained theoretically by assuming the oxygen discharge step as the slowest one (the rate determining step, the r.d.s. ( 7)). For hydrogen the mechanism is different and the r.d.s. is the atomic dissociation and in this case a Tafel slope of 0.030... 

The kinetic data obtained were interpreted on the basis of a redox mechanism, with two main steps (1) dissociative adsorption of 02 (surface oxidation) and (2) interaction of NH3 with oxygen adsorbed in atomic form (surface reduction). Both steps are complex, comprising several elementary reactions. This mechanism is again consistent with the rate equation (16.1). 

Tetraneopentyltitanium [36945-13-8] Np Ti, forms from the reaction of TiCl and neopentyllithium ia hexane at —80° C ia modest yield only because of extensive reduction of Ti(IV). Tetranorbomyltitanium [36333-76-3] can be prepared similarly. When exposed to oxygen, (NpO)4Ti forms. If it is boiled ia ben2ene, it decomposes to neopentane. When dissolved ia monomers, eg, a-olefins or dienes, styrene, or methyl methacrylate, it initiates a slow polymerisation (211,212). Results from copolymerisation studies iadicate a radical mechanism (212). Ultraviolet light iacreases the rate of dissociation to... 

The interesting point is that only the ratio of the partial pressures of the reactants enters this equation, but not the total pressure. The physical reason is obvious. Both reactants are competing for free adsorption sites on the surface and since CO inhibits the dissociative adsorption of oxygen the partial pressure of the latter compound enters into the denominator of the rate expression. This result indicates that the actual surface composition and, in addition, the reaction mechanism will not be affected by the total pressure. The only effect of increasing the total pressure (at a fixed p0l pco ratio) will be that rmax will be shifted towards higher temperatures since reduction of... 

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