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Issue 2, 2012
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Direct measurements of the high temperature rate constants of the reactions NCN + O, NCN + NCN, and NCN + M

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Abstract

The rate constant of the reaction NCN + O has been directly measured for the first time. According to the revised Fenimore mechanism, which is initiated by the NCN forming reaction CH + N2 → NCN + H, this reaction plays a key role for prompt NOx formation in flames. NCN radicals and O atoms have been quantitatively generated by the pyrolysis of NCN3 and N2O, respectively. NCN concentration–time profiles have been monitored behind shock waves using narrow-bandwidth laser absorption at a wavelength of λ = 329.1302 nm. Whereas no pressure dependence was discernible at pressures between 709 mbar < p < 1861 mbar, a barely significant temperature dependence corresponding to an activation energy of 5.8 ± 6.0 kJ mol−1 was found. Overall, at temperatures of 1826 K < T < 2783 K, the rate constant can be expressed as kNCN + O = 9.6 × 1013 × exp(−5.8 kJ mol−1/RT) cm3 mol−1 s−1 (±40%). As a requirement for accurate high temperature rate constant measurements, a consistent NCN background mechanism has been derived from pyrolysis experiments of pure NCN3/Ar gas mixtures, beforehand. Presumably, the bimolecular secondary reaction NCN + NCN yields CN radicals hence triggering a chain reaction cycle that efficiently removes NCN. A temperature independent value of kNCN + NCN = (3.7 ± 1.5) × 1012 cm3 mol−1 s−1 has been determined from measurements at pressures ranging from 143 mbar to 1884 mbar and temperatures ranging from 966 K to 1900 K. At higher temperatures, the unimolecular decomposition of NCN, NCN + M → C + N2 + M, prevails. Measurements at temperatures of 2012 K < T < 3248 K and at total pressures of 703 mbar < p < 2204 mbar reveal a unimolecular decomposition close to its low pressure limit. The corresponding rate constants can be expressed as kNCN + M = 8.9 × 1014 × exp(−260 kJ mol−1/RT) cm3 mol−1 s−1(±20%).

Graphical abstract: Direct measurements of the high temperature rate constants of the reactions NCN + O, NCN + NCN, and NCN + M

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Publication details

The article was received on 29 Jun 2011, accepted on 11 Aug 2011 and first published on 30 Aug 2011


Article type: Paper
DOI: 10.1039/C1CP22123J
Phys. Chem. Chem. Phys., 2012,14, 1030-1037

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    Direct measurements of the high temperature rate constants of the reactions NCN + O, NCN + NCN, and NCN + M

    J. Dammeier, N. Faßheber and G. Friedrichs, Phys. Chem. Chem. Phys., 2012, 14, 1030
    DOI: 10.1039/C1CP22123J

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