Oxygen reduction reaction kinetic model

RRDE measurements with X-ray diffrac- cells. This gives tion results to investigate the detailed nature of the surface structures that are formed, particularly in coadsorption studies, for example, the influence of anion species on the UPD process. In Sect. 4.1.5, the oxygen reduction reaction (ORR) is used as a model electrochemical reaction to demonstrate the relation between the metal-O2 energetics and reaction pathway/kinetics as well as the importance of the local symmetry of surface atoms in determining the electrocatalytic properties of metal surfaces. 

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]). 

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 ... 

Reports by Li and Zuberbuhler were in support of the formation of Cu(I) as an intermediate (16). It was confirmed that Cu(I) and Cu(II) show the same catalytic activity and the reaction is first-order in [Cu(I) or (II)] and [02] in the presence of 0.6-1.5M acetonitrile and above pH 2.2. The oxygen consumption deviated from the strictly first-order pattern at lower pH and the corresponding kinetic traces were excluded from the evaluation of the data. The rate law was found to be identical with the one obtained for the autoxidation of Cu(I) in the absence of Cu(II) under similar conditions (17). Thus, the proposed kinetic model is centered around the reduction of Cu(II) by ascorbic acid and reoxidation of Cu(I) to Cu(II) by dioxygen ... 

In alkaline solution (pH 11), the complex is present as a p-oxo dimer and ascorbic acid is fully deprotonated. In the absence of oxygen, kinetic traces show the reduction of Fe(III) to Fe(II) with a reaction time on the order of an hour at [H2A] =5xlO-3M. The product [Fen(TPPS)] is very sensitive to oxidation and is quickly transformed to Fe(III) when 02 is added. This leads to a specific induction period in the kinetic traces which increases with increasing [02]. The net result of the induction period is the catalytic two-electron autoxidation of ascorbic acid in accordance with the following kinetic model (23) ... 

Co-limited kinetics with a significant utilization region. As with platinum, the model predicts that the chemical portion of the reaction will be co-limited by molecular dissociation and transport. Values of k calculated from the model for the analyzed conditions vary from 0.4 to 20 gvc depending on Pq2 temperature, and electrode surface area, with typical values in the 3—5 gm range. This result indicates that a significant portion of the electrode surface is active for oxygen reduction, which explains Takeda s (and other s) observation that the performance of LSG electrodes on YSZ improves with thickness up to a... 

The replenishment of the vacancy can be directly from the gas phase or indirectly from the catalyst. In the latter case, the oxygen mobility within the catalyst is so large that bulk oxygen can diffuse to the vacancy. Then oxygen from the gas phase reoxidizes the lattice on sites which differ from hydrocarbon reaction sites. In a steady state, the rate of catalyst oxidation will be equal to the rate of reduction by the substrate. The steady state degree of reduction, equivalent to the surface coverage with oxygen, is determined by the ratio of these two rates. Kinetic models based on these principles are called redox models, for which the simplest mathematical expression is... 

The reactions considered were simultaneous CO oxidation and NO reduction, using a kinetic model for rhodium catalyst [25]. Initial and boundary conditions for the concentrations of reactants were the same as given by Lie et al., NO oscillating in phase with oxygen. Kinetic and reactor parameters used were mentioned in Section 8.IV.A. 

The kinetics and mechanism of oxygen reduction and evolution reactions at oxide-covered platinum electrodes was studied by Damjanovic in the 1990s, as a continuation of their previous work. The role of the oxide film in these processes was analyzed and model assumptions were discussed. 

The deactivation of bulk iron oxide during methane combustion has been studied. The observed deactivation behaviour has been explained as the result of two simultaneous deactivation mechanisms. In the initial phase of reaction both are in operation and the activity drops rapidly as a consequence of both catalyst sintering and of the depletion of lattice oxygen in the outer layers, due to a partial reduction of the catalytic surface. In later stages, catalyst deactivation is almost exclusively due to sintering imder reaction conditions. A kinetic model of deactivation is presented, together with the physicochemical characterization of fresh and partially deactivated catalysts. 

In a catalytic reaction, all steps do not equally depend on the surface structure. So, for example, the rate of simple desorption processes is often not markedly affected by the structure of the surface. In catalysis, therefore, reactions are classified into "structure sensitive" and "structure insensitive" [5], usually on the basis of the variation of reactivity with particle size. As an example, the electrocatalytic oxygen reduction at platinum (which is of importance for fuel cells) will be mentioned, where a decrease of specific activity with increasing particle size was reported [6,7]. In a theoretical analysis [8], the kinetics was treated on the (111), (10 0), and (211) facets of several transition metals, and the results were combined with simple models for the geometries of catalytic nanoparticles. Thus, the experimentally observed trend could be well reproduced. 

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