A comprehensive kinetic study of thermal reduction of NO₂ by H₂

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)T^2.76 exp(-14 980/D 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 NO_x and CO_x (in CO-added experiment), x = 1 and 2, and for NO₂ decay rates reported earlier by Ashmore and Levitt (ref 22).