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Ind. Eng. Chem. Res. 2008, 47, 4358–4362
Reaction of NO2 with Activated Carbon at Ambient Temperature Wei-Jun Zhang,* Andrey Bagreev, and Firooz Rasouli Philip Morris U.S.A. Research Center, 4201 Commerce Road, Richmond, Virginia 23234
NO2 adsorption on activated carbons at ambient temperature was studied. The adsorbed species after various adsorption times were examined by analyzing the evolution of NO, NO2, CO, CO2, and O2 during thermal desorption. Emission of NO but not NO2 was observed during the early stage of NO2 adsorption. From the nitrogen and oxygen balance data, we conclude that NO2 adsorption involves the reduction of NO2 to NO driven by oxidation of the carbon surface. The formed NO is either adsorbed on the carbon surface or released as exhaust gas depending on the surface conditions. The adsorbed NO species are released along with O2 and a small amount of CO2 and CO at temperatures below 150 °C during heating. NO2 becomes stable when the carbon surface has been extensively oxidized. 1. Introduction The emission control of nitrogen oxides has become an increasingly important environmental concern. Removal of NOx using activated carbons (ACs) is one of the potential techniques being extensively studied. Although NO is the main component of NOx in the exhaust gases, it was suggested in previous studies that NO adsorption on ACs involves the oxidation of NO to NO2 in the presence of oxygen.1–3 Moreover, NO2 has been found recently to be a more efficient oxidant than O2 for burning soot particles collected on particulate filters.4–6 Gasification of soot particles by NO2 occurs at temperatures 200 °C lower than that by O2 alone.6 Therefore, it is of great interest to study the interaction between NO2 and activated carbons, which is the objective of this study. The NO2-carbon interaction has been studied using spectroscopic and temperature-programmed desorption (TPD) measurements in the temperature range 25-200 °C.6–8 It was reported that exposure of carbon to NO2 leads to both adsorption of NO2 and reduction of NO2 to NO. Jeguirim et al.6 postulated that NO2 adsorption involves a sequence of reactions that result in the formation of C(NO2), C(O), and C(ONO2) surface complexes. On the other hand, Shirahama et al.7 suggested a mechanism of dual adsorption sites, with one site adsorbing NO2 weakly while the other site adsorbs NO2 strongly to form NO3 species. When heated, these species desorbed as NO and NO2. In the present study, NO2 reaction with activated carbons was examined at ambient temperature. The adsorbed species were evaluated by measuring the amounts of evolved gases during TPD at different adsorption stages. The results suggest that NO2 decomposition dominates the initial stage of adsorption and the adsorbed species are not NO2 but NO-O2 complex on the carbon surface before the carbon surface is extensively oxidized. 2. Experimental Section The AC samples used in this study are as-received SKC carbon molecular sieves (SKC-AR) (Anasorb CMS produced by SKC Inc., PA), further activated SKC carbon (SKC-360), and commercial coconut shell based activated carbon (PICA). The as-received carbon sieves (SKC-AR) have a bulk density of 0.56 g/cm3, a density functional theory (DFT) surface area of 1357 m2/g, and an average micropore size of ∼5 Å. The * Corresponding author.
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SKC-360 carbons were prepared by further activation of SKCAR under a flow of 30% CO2 (vol %) in N2 at 950 °C for 360 min. The SKC-360 and PICA carbons have DFT surface areas of 1901 and 1045 m2/g, respectively. The details of the texture properties of these carbons and the characterization methods are reported in ref 3. NO2 adsorption and TPD tests were carried out in a quartz flow tube reactor with an inner diameter of 9 mm. A sample of 200 mg of carbon was packed into the reactor, and a K-type thermocouple was inserted next to the carbon to monitor the sample temperature. The inlet and effluent gases were measured by an NGA2000 MLT multigas analyzer (Rosemount Analytical, Orrville, OH), which monitors simultaneously the concentrations of NO, NO2, N2O, NH3, CO, CO2, and O2 at ppm levels. NO2 adsorption was carried out at ambient temperature after purging the carbon bed with argon at a flow rate of 0.5 L/min for 20 min. The adsorption gas mixture was adjusted to a flow rate of 0.5 L/min, stabilized in the bypass line to the desired concentration, and then switched to the carbon bed in the reactor for adsorption. Unless specified otherwise, the adsorption gas is typically a mixture of 500 ppm NO2 in argon with an oxygen impurity of ∼1600 ppm and a NO impurity of
