ARTICLE pubs.acs.org/IECR
Adsorption and Reaction of NO2 on Carbon Black and Diesel Soot at Near-Ambient Temperatures Christopher. J. Tighe,† Martyn V. Twigg,‡ Allan N. Hayhurst,† and John S. Dennis*,† † ‡
Department of Chemical Engineering and Biotechnology, University of Cambridge, Pembroke Street, Cambridge, CB2 3RA, England Orchard Laboratories, Johnson Matthey Plc., Orchard Road, Royston, Hertfordshire, SG8 5HE, England ABSTRACT: NO2 interacts with soot in the atmosphere and, at higher temperatures, is used as an oxidant to regenerate filters on the exhausts of diesel cars. This work investigates the adsorption and reaction of NO2 (forming NO) with either carbon black or soot (from a diesel engine) held at near ambient temperatures (2075 °C) in a packed bed. It was found that initially NO2 very rapidly adsorbed; approximately half the NO2 adsorbed was converted to NO, leaving behind O-containing functionalities on the surface of the carbon. These adsorbed species can be made to produce CO2 and CO by heating the carbon to a temperature above 100 °C. At 20 °C, the number of O-containing species, C(O), formed on the carbons was found, surprisingly, to be equal to the amount of NO2 remaining stably adsorbed. A mechanism of elementary reactions is postulated, in which NO2 is only adsorbed on the newly formed C(O). A model of adsorption and reaction in a packed bed was used to estimate kinetic parameters for the surface reactions giving overall C(_) + 2NO2 f C(O + NO2) + NO. The measurements of [NO2] and [NO] against time were represented best by the reactions Cð_Þ þ NO2 f CðNO2 Þ, CðNO2 Þ f CðOÞ þ NO, and CðOÞ þ NO2 f CðO þ NO2 Þ; each is first order with respect to NO2. Excellent agreement between the results of modeling and experiment is obtained by assuming that there are two types of active site, C(_), with different reactivities.
’ INTRODUCTION Studies of the interaction of NO2 with soot at near-ambient temperatures are relevant to processes occurring in the atmosphere as well as in flue gases: they may also help to elucidate the elementary mechanisms underpinning the regeneration of filters for soot fitted to the exhausts of diesel cars. These filters use NO2 as an oxidant at higher temperatures, typically above 300 °C. Exposing different types of carbon (e.g., diesel soot, carbon black, and activated carbon) below 25 °C to NO2 results in its rapid adsorption and, in some cases, subsequent reaction to form NO (plus HONO if water vapor is present).1 A reaction to form NO Cð_Þ þ NO2 f CðOÞ þ NO
ðR1Þ
has been proposed.1 Here, C(_) represents an active site on the surface of the soot and C(O) represents an O-containing functionality: both have been assumed1 to be present, but their existence has not been confirmed experimentally. Kirchner et al.2 used transmission spectroscopy to investigate in situ the species produced during the reaction of NO2 with carbon black and diesel soot at 25 °C. The IR absorption spectra showed peaks attributed to CH and COH groups and different types of C(O) functionalities (e.g., quinones), whereas new peaks, attributed to RNO2, RONO, and RONO2, were detected when soot was exposed to NO2. Here, R represents the soot, probably linked to the adsorbed species by a C atom. Zawadzki et al.3 also used transmission spectroscopy to investigate in situ the surface species formed on microporous carbon on exposure to NO2 at 22 and 275 °C. Peaks attributed to CO, RONO, and RNO2 were observed, in agreement with Kirchner et al.2 Azambre et al.4 used diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to probe surface complexes formed when diesel soot at either 40 or 100 °C was exposed to r 2011 American Chemical Society
1000 ppm NO2; O- and N-containing species were detected on the surface of the soot, although the measurements were hampered by a low signal-to-noise ratio. Studies of the adsorption and reaction of NO2 on diesel soot4 and carbon black,5 held in a fixed bed at temperatures < 100 °C, indicated that the NO2 was adsorbed rapidly and that a proportion of this adsorbed NO2 reacted, forming NO. In subsequent temperature-programmed-desorption (TPD) experiments, Azambre et al.4 and Jeguirim et al.5 observed significant amounts of NO2, released soon after heating started; this suggests that the NO2 was physisorbed, or only weakly chemisorbed, on the soot. Above 100 °C, [NO] and [CO2] were also measured.4,5 Jeguirim et al.5 found that the total number of N atoms disappearing from the gas phase in the adsorption step was in close agreement with the number subsequently desorbed as NO2 and NO during TPD; i.e., all the N-containing species on the surface were desorbed below ∼250 °C. A closed balance on O atoms was obtained; Jeguirim et al.5 interpreted this as showing that the formation of NO during the adsorption experiment was associated with the formation of C(O) by reaction R1. Jeguirim et al.5 also showed that the total amounts of NO and CO2 desorbed below 250 °C were quite similar, thus concluding that these were the products of the decomposition of C(ONO2) in CðONO2 Þ f CO2 þ NO
ðR2Þ
Here, the surface species C(ONO2) was assumed to have been formed by the adsorption of NO2 on an existing C(O) site during the adsorption phase. In fact, the measured profiles of [NO] and Received: May 9, 2011 Accepted: August 2, 2011 Revised: July 27, 2011 Published: August 02, 2011 10480
dx.doi.org/10.1021/ie2009982 | Ind. Eng. Chem. Res. 2011, 50, 10480–10492
Industrial & Engineering Chemistry Research
ARTICLE
[CO2] presented by Jeguirim et al.5 show [CO2] < [NO] below 250 °C, whereas reaction R2 must give [CO2] = [NO]. Thus, it is also possible that some of the measured [NO] and [CO2] might have been formed from the oxidation of carbon by NO2 in C þ 2NO2 f CO2 þ 2NO
ðR3Þ
rather than by the decomposition of chemisorbed intermediates as claimed. Chughtai et al.6,7 studied the reactions of mixtures of 82100 mol % NO2 in N2 with soot produced in a hexane flame; the soot was held in a thermogravimetric balance and measurements of [NO2], [NO], [CO2], and [CO] were made during burnout. When soot was exposed to NO2, an initial, short-lived reaction was observed at temperatures as low as 25 °C, accompanied by the evolution of trace amounts of NO, CO2, and CO. This reaction, termed the “minor redox reaction”,6,7 possibly arose from the desorption of existing C(O) functionalities in the presence of NO2, or the consumption of the most reactive components of the soot. On heating above 60 °C, a further reaction occurred, termed the “major redox reaction”, during which significant [CO], [CO2], [NO], and [N2O] were measured; in fact 75 mg of soot almost completely disappeared in ∼10 min at 75 °C. Probably owing to the very high [NO2]in used, the observed reactions were quite different from those of other studies below 100 °C,15,8 in which carbon was generally not consumed to any significant extent. Gray and Do8 have investigated the rate of adsorption of NO2 on microporous activated carbon in a thermogravimetric balance between 25 and 100 °C; measurements of gaseous products were not made. The adsorption isotherms measured at different temperatures were of the favorable type and were described either by a Freundlich correlation or by the sum of two independent Langmuir isotherms. Gray and Do8 assumed that the pores in their activated carbon were bidisperse and that the rate of adsorption of NO2 was controlled by three resistances in series: diffusion in the macropores, diffusion in the micropores, and diffusion on the surface of the carbon. Their model was solved numerically, and its parameters were determined by comparison with measurements of the rate of increase of the mass of carbon during exposure to NO2. Perhaps unsurprisingly, since it contained many adjustable parameters, the model represented the experiments well. In other work, a rate expression consistent with a two-site Langmuir isotherm was proposed by Akhter et al.,9 based on thermogravimetric measurements of the rate of adsorption of pure NO2 on soot from a hexane flame. Three studies35 involved mixtures of NO2 in argon or helium flowing over different carbons in a fixed bed at
