A Comprehensive Kinetic Study of Thermal Reduction of NO₂ by H₂

Abstract

A comprehensive study of the kinetics and mechanism for the thermal reduction of NO2 by H2 has been carried out in the temperature range 602−954 K by pyrolysis/FTIR spectrometry employing different mixtures containing (1) NO2/Ar, (2) NO2/NO/Ar, (3) NO2/H2/Ar, (4) NO2/H2/CO/Ar, and (5) NO2/H2/CO/NO/Ar. The results of kinetic modeling for the data obtained from mixtures 1 and 2 gave the rate constant for the key reaction responsible for NO formation and NO2 decay, 2NO2 → 2NO + O2 (4): k4 = 4.16 × 1012 exp(−13 840/T) cm3/(mol s). Combination of our data with those of Röhrig et al. (ref 27) measured at high temperatures in shock waves led to k4 = (4.51 ± 0.15) × 1012 exp[-(13 890 ± 27)/T] cm3/(mol s) for 600 K < T < 1450 K. Kinetic modeling of the measured data from mixtures 3−5 indicated that the existing rate constant for H2 + NO2 → HONO + H (−1) was too large; on the other hand, our theoretically predicted rate constant, k-1 = (1.30 × 104)T2.76 exp(−14 980/T) cm3/(mol s), obtained by high-level ab initio MO/TST calculations with a small adjustment for the computed barrier from 32.5 to 33.0 kcal/mol, can quantitatively account for measured concentrations of NOx and COx (in CO-added experiment), x = 1 and 2, and for NO2 decay rates reported earlier by Ashmore and Levitt (ref 22).