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Kinetics of NO + H+ + NO3 → NO2 + HNO2 on BaNa−Y: Evidence for a Diffusion-Limited A + B → 0 Reaction on a Surface

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journal contribution
posted on 09.12.2010, 00:00 authored by Aditya Savara, Eric Weitz
The reaction NO + NO3 + H+ → NO2 + HNO2 on BaNa−Y, between 100 and 300 °C, displays evidence for low dimensionality diffusion-limited kinetics that can be modeled as an A + B → 0 reaction (DLAB0 reaction), where H+ and NO3 are “A” and “B”, and the diffusion of H+ is rate-limiting. DLAB0 reactions are described by fractal kinetics when the reactants are confined to dimensionalities ≤4. Appropriate plots (alpha plots) of the depletion of surface nitrates versus time show evidence for a time regime in the kinetics that is termed the “Zeldovich regime”, during which reaction occurs at the edges of segregated regions of reactants. The Zeldovich regime manifests itself as a linear region in alpha plots, with a slope characteristic of the dimensionality to which the reactants are confined. Here alpha is ∼0.5. The end time of the Zeldovich regime, τf, was obtained by spline-fitting the kinetic data to extract the linear region of the alpha plot. Less reactive or unreactive nitrates remain at the end-time. The experimental data are consistent with the theoretical prediction that both τf and [NO3] at τf are independent of [NO3]0, the initial concentration of NO3. There is an observed inverse dependence of τf on [NO], which can be rationalized by incorporating [NO] as a “coefficient”, p, for the probability of a reaction following each encounter of H+ and NO3, rather than directly into keff. The activation energy for the surface diffusion of H+ is calculated to be 30 ± 30 kJ/mol from the temperature dependence of τf, which is within the expected range for such a process. Reaction-limited models with up to three site types were also considered, but did not provide physically realistic results. To our knowledge, this is only the second instance of kinetic evidence for a DLAB0 reaction on a surface, and the first opportunity to experimentally test predictions for τf for a chemical reaction.