The MnOx/TiO2 catalysts have been proved very active in the catalytic oxidation of NO to NO2, but the mechanism of this heterogeneous reaction was still not clear. In this study, through a systematic in situ diffuse reflectance infrared Fourier transform spectroscopy investigation, the main pathway of NO oxidation over MnOx/TiO2 was revealed. Experimental results indicated that the NO was first coordinated to Mn sites, forming nitrosyls, which were readily oxidized to nitrates by lattice oxygen, and then the nitrates were decomposed to NO2 at high temperatures. However, when SO2 was present, the formation of nitrates was significantly hindered because the SO2 would permanently occupy the active Mn sites as sulfates. We believe that this revealed the mechanism of NO oxidation and SO2 deactivation would provide useful guidance for the development of better catalysts toward fast selective catalytic reduction, NOx storage-reduction, continuously regenerating trap, and NOx wet scrubbing.

Mechanism Study of NO Catalytic Oxidation over MnOx/TiO2 Catalysts

Removal of SO2 and NO from flue gas by wet scrubbing using an aqueous NaClO2 solution

The rate of uptake of NO2 by liquid water accordingo (R1), 2NO2(g) + H20(I) - 2H + + NO3- + NO2-, is shown to be unaffected by O2 (0.2 atm). Hence the rate constant and Henry's law solubility constant of NO2 previously obtained may be employed to evaluate the rates of aqueous phase reactions of NO2 in the ambient atmosphere. Reactions (R1) and (R2), NO2(g) + NO(g) + H20(l) -- 2H + + 2NO2-, are quite slow at representative atmospheric partial pressures and cloud liquid water content; the characteristic times range upward from 103 - 104 hours at 10 -7 atm, increasing with decreasing partial pressures of the gases. Direct acidification of cloud liquid water by (R1) or (R2) is also unimportant. Catalytic enhancement of (R 1) is potentially important for catalyst concentrations of order 10 -7 M, assuming sufficiently fast rate constants (---108 M - s-). Iron-catalyzed reaction in particular, however, is found to be unimportant. Reaction of NO2 with dissolved S(IV) is potentially important, based upon an assumed upper limit rate constant of 2.5 x 10 7 M-  s- I. Deposition of NO2 to surface (ocean or lake) water is shown to be controlled by aqueous phase mass transport and/or reaction and is much slower than heretofore assumed.

Evaluation of the Rate of Uptake of Nitrogen Dioxide by Atmospheric and Surface Liquid Water

A three-part experimental program was carried out to investigate possible methods to improve sorbent reversibility during cyclic calcination/carbonation using one limestone and one dolomite. In the...

Investigation of Attempts to Improve Cyclic CO2 Capture by Sorbent Hydration and Modification

Removal of NO from flue gas may be achieved by its oxidation to NO2 followed by absorption in sulfite solution in existing scrubbers for desulfurization. Rates of NO2 absorption and sulfite oxidation were measured in a highly characterized stirred-cell contactor. The results were interpreted by the theory of mass transfer with fast reaction in the boundary layer. The reactions between NO2 and sulfite (SO32-), bisulfite (HSO3-), and thiosulfate (S2O32-) were first order in NO2 concentration, and their respective rate constants at 55 °C were 11.2 × 105, 2.8 × 104, and 5.4 × 103 M-1 s-1. The NO2 hydrolysis reaction was second order in NO2 concentration, with a rate constant of 1.6 × 107 M-1 s-1 at 55 °C. NO2 absorption initiates sulfite oxidation in the presence of oxygen, and this study quantified the effect of sulfite and oxygen concentration, the rate of NO2 absorption, and the presence of thiosulfate and gas-phase SO2 on the rate of sulfite oxidation. With typical conditions for limestone slurry, 10 mM t...

Nobuo. Kizawa

Papers

Absorption of Nitrogen Oxides in Aqueous Sodium Sulfite and Bisulfite Solutions

Absorption of Nitrogen Dioxide in Sodium Sulfite Solution from Air as a Diluent