Nitric oxide oxidation catalyzed by microporous activated carbon fiber cloth: An updated reaction mechanism

New catalysts, in particular activated carbons, are being developed to improve NO oxidation kinetics and to minimize the negative impacts of water vapor and SO2 as an alternative NOx control strategy for flue gases. However, the literature is inconclusive about the pathway to achieve a stable downstream NO2 concentration, creating discrepancies in the NO oxidation mechanism. In this paper, a simple two-step mechanism is proposed and justified for NO oxidation catalyzed by microporous activated carbon. In the first mechanistic step, NO is rapidly oxidized to NO2 in activated carbon's micropores with constant NO conversion efficiency, prior to the formation of adsorbed intermediates (e.g., C-center dot -NO2, C-center dot -NO3 or C-center dot -NO-NO3). This conclusion challenges the idea that NO oxidation to NO2 requires decomposition of the C-center dot -NO-NO3 intermediate. Instead, we show that the physical properties of the catalyst (i.e., micropore width and volume) control steady-state NO oxidation kinetics, with carbon's chemical properties having no apparent impact for the conditions tested here. In the second mechanistic step, newly formed NO2 is chemically adsorbed on the carbon surface, resulting in formation of NO and C N or C-O complexes. Chemical disproportion of NO2 proceeds until the carbon surface is saturated with these complexes. It follows that the first step of the mechanism (NO oxidation in micropores) controls steady state NO oxidation kinetics while the second step (NO/carbon surface reactions) controls transient NO oxidation kinetics. This two-step mechanism is confirmed via in situ cyclic experiments and NO2 pre-adsorption tests. Pre-saturation of the carbon surface with functional groups decreases the impact of the second mechanistic step, affecting transient NO oxidation kinetics but having no impact on steady-state oxidation kinetics. A more complete understanding of the reaction mechanism allows us to better prepare tailored carbonaceous NO oxidation catalysts. (c) 2013 Elsevier B.V. All rights reserved.