Thermal Dissociation of SO₃ at 1000−1400 K

Abstract

The thermal dissociation of SO 3 has been studied for the first time in the 1000-1400 K range. The experiments were conducted in a laminar flow reactor at atmospheric pressure, with nitrogen as the bath gas. On the basis of the flow reactor data, a rate constant for SO 3 + N 2 f SO 2 + O + N 2 (R1b) of 5.7 × 10 17 exp(-40000/T) cm 3 /(mol s) is derived for the temperature range 1273-1348 K. The estimated uncertainty is a factor of 2. The rate constant corresponds to a value of the reverse reaction of k 1 ≈ 1.8 × 10 15 cm 6 mol -2 s -1 . The reaction is in the falloff region under the investigated conditions. The temperature and pressure dependence of SO 2 + O (+N 2 ) was estimated from the extrapolation of low temperature results for the reaction, together with an estimated broadening parameter and the high-pressure limit determined recently by Naidoo, Goumri, and Marshall (Proc. Combust. Inst. 2005, 30, 1219-1225. The theoretical rate constant is in good agreement with the experimental results. The improved accuracy in k 1 allows a reassessment of the rate constant for SO 3 + O f SO 2 + O 2 (R2) based on the data of Smith, Tseregounis, and Wang (Int. J. Chem. Kinet. 1982, 14, 679-697), who conducted experiments on a low-pressure CO/O 2 /Ar flame doped with SO 2 . At the location in the flame where the net SO 3 formation rate is zero,A value of 6.9 × 10 10 cm 3 mol -1 s -1 is obtained for k 2 at 1269 K with an uncertainty a factor of 3. A recommended rate constant k 2 ) 7.8 × 10 11 exp(-3065/T) cm 3 mol -1 s -1 is consistent with other flame results as well as the present flow reactor data.

Reactions of SO3 with the O/H Radical Pool under Combustion Conditions

“…The current recommendation for k 2 is the sum of both channels, k 2 ) 2.8 × 10 4 T 2.57 exp(-122.3 kJ mol -1 /RT) cm 3 mol -1 s -1 . It is several orders of magnitude lower than values derived from flames, 12,[20][21][22] but a low value is in agreement with the recommendation of Alzueta et al 10 based on batch reactor data. 31 The flame and batch reactor results are discussed further below.…”

“…On the basis of a more accurate value of k 4 , the authors 12 recently reanalyzed the results of Smith and co-workers 21,22 and Merryman and Levy 20 to obtain a value of k′ 2 ) 7.8 × 10 11 exp(-3065/T) cm 3 /(mol s), in support of the earlier estimates. The flame interpretations 12,[20][21][22] were all based on the assumption that the SO 3 level resulted from a balance between formation by SO 2 + O(+M) (4) and consumption by SO 3 + O (2), i.e., that the SO 3 + H and SO 3 + OH reactions could be disregarded due to a high O/H ratio in these flames. The present work indicates that the SO 3 + O reaction is too slow to be an important consumption step for SO 3 , even at the high temperatures in flames.…”

“…7,23,43 The flame results have consistently been interpreted in terms of a fast rate constant for the SO 3 + O reaction (2). On the basis of a more accurate value of k 4 , the authors 12 recently reanalyzed the results of Smith and co-workers 21,22 and Merryman and Levy 20 to obtain a value of k′ 2 ) 7.8 × 10 11 exp(-3065/T) cm 3 /(mol s), in support of the earlier estimates. The flame interpretations 12,[20][21][22] were all based on the assumption that the SO 3 level resulted from a balance between formation by SO 2 + O(+M) (4) and consumption by SO 3 + O (2), i.e., that the SO 3 + H and SO 3 + OH reactions could be disregarded due to a high O/H ratio in these flames.…”

“…12,17 The reaction mechanism consists of a H 2 /O 2 subset and a full description of the H/S/O reaction system. The mechanism contains the elementary steps that could conceivably influence the SO 2 /SO 3 ratio at high temperatures.…”

“…7,[10][11][12]16,17 In addition to k 1 , k 2 , and k 3 , only one rate constant has been modified in the present work. For the SO 2 + SO 2 reaction (6), we use an estimate, based on batch reactor results on SO 2 + O 2 , 31 discussed below, and shock tube results on SO 2 decomposition.…”