A SHOCK-TUBE STUDY OF THE PYROLYSIS OF NO2

NO2 concentration profiles in shock-heated NO2/Ar mixtures were measur ed in the temperature range of 1350-2100 K and pressures up to 380 atm using Ar+ laser absorption at 472.7 nm, IR emission at 6.25 +/- 0.25 mu m, and visible emission at 300-600 nm. in the course of this study, the absorption coefficient of NO2 at 472.7 nm was measured at tempera tures from 300 K to 2100 K and pressures up to 75 atm. Rate coefficien ts for the reactions NO2 + M --> NO + O + M (1), NO2 + NO2 --> 2NO + O -2 (2a), and NO2 + NO2 --> NO3 + NO (2b) were derived by comparing the measured and calculated NO2 profiles. For reaction (1), the following low- and high-pressure limiting rate coefficients were inferred which describe the measured fall-off curves in Lindemann form within 15%: k (10) = 10((15.6+/-0.2))exp(-(251 +/- 5) kJ mol(-1)/RT) cm(3) mol(-1) s (-1) k(10) = 10((14.6+/-0.3))exp(-300 kJ mol(-1)/RT) s(-1) The inferre d rate coefficient at the low-pressure limit, k(1o) is in good agreeme nt with previous work at higher temperatures, but the energy of activa tion is lower by 20 kJ/mol than reported previously. The pressure depe ndence of k(1) observed in the earlier work of Tree [1] was confirmed. The rate coefficient inferred for the high pressure limit, k(1 infini ty) is higher by a factor of two than Tree's value, but in agreement w ith data obtained by measuring specific energy-dependent rate coeffici ents. For the reactions (2a) and (2b), least-squares fits of the prese nt data lead to the following Arrhenius expressions: k(2a) = 10(12.3+/ -0.2)exp(-(105+/-7) kJ mol(-1)/RT) cm(3) mol(-1) s(-1) k(2b) = 10(13.0 +/-0.3)exp(-(108+/-8) kJ mol(-1)/RT) cm(3) mol(-1) s(-1) For reaction (2), the new data agree with previously recommended values of k(2a) an d k(2b), although the present study suggests a slightly higher preexpo nential factor for k(2a). (C) 1997 John Wiley & Sons, Inc.