Steady-state Kinetics Of Oxidation Research Articles

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*NO2, C*NO3 or C*NONO3). This conclusion challenges the idea that NO oxidation to NO2 requires decomposition of the C*NONO3 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 CN or CO 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.

Oxidation behavior of extruded Mo 5Si 3B x–MoSi 2–MoB intermetallics from 600°–1600 °C

The oxidation behavior of Mo 5Si 3B x (T1)–MoSi 2–MoB composites fabricated by green extrusion processing was investigated over the temperature range of 600–1600 °C in air. Low temperature experiments from 600 to 1000 °C revealed that unoxidized material initially forms a mixed molybdenum and silicon oxide scale. The unoxidized material undergoes rapid mass loss over a narrow temperature range of 660–760 °C, with complete disintegration of the material occurring at about 730 °C. At temperatures above about 760 °C, a coherent glassy scale forms to protect the alloy from further oxidation. High temperature oxidation tests from 1150 to 1600 °C show that the material forms a continuous glassy scale, 10–20 μm in thickness, which protects the underlying alloy. Steady state oxidation kinetics were not achieved until exposure at 1600 °C, due to the effect of contamination introduced during green extrusion processing. The calculated parabolic rate constant at 1600 °C is 9.7×10 −3 mg 2/cm 4/h, which compares favorably to that of MoSi 2-based materials. The observed oxidation behavior and kinetics over the entire temperature range were not significantly influenced by sample porosity (79% versus 96% of theoretical density). Boron depletion of the near-surface scale was observed at 1150 °C, leading to the formation of pure silica in the outer portion of the scale. The extent and rate of boron evaporation increased significantly as temperature increased to 1600 °C. Formation of pure silica in the scale implies that oxygen transport through silica must be the rate-limiting step in the high temperature oxidation of these alloys.

Procedure for Studying the Steady-State Kinetics of Oxidation of Gaseous Substrates by Hydrogen Peroxide in a Liquid Phase

A procedure was developed for studying the steady-state kinetics of oxidation of various gaseous substrates by hydrogen peroxide under conditions of the continuous release of gaseous products. The steady state of a system was provided by continuously injecting hydrogen peroxide at low conversions of the oxidized substrate. The process was monitored by measuring substrate losses in a gas phase rather than the buildup of products. This allowed us to avoid difficulties associated with further oxidation of the products.