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

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, τ_f, 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 τ_f and [NO₃⁻] at τ_f are independent of [NO₃⁻]₀, the initial concentration of NO₃⁻. There is an observed inverse dependence of τ_f 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 k_eff. The activation energy for the surface diffusion of H⁺ is calculated to be 30 ± 30 kJ/mol from the temperature dependence of τ_f, 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 τ_f for a chemical reaction.