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USER: amr69
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DATE: August 21, 2002
J. Phys. Chem. A 2002, 106, 8291-8295
8291
Wide-Temperature Range Kinetics and CN(B-X) Violet Chemiluminescence of the C2(a3Π) + NO Reaction† Aleksandra Ristanovic, Abel Fernandez, and Arthur Fontijn* High-Temperature Reaction-Kinetics Laboratory, The Isermann Department of Chemical Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180-3590 ReceiVed: December 18, 2001; In Final Form: February 13, 2002
The C2(a3Π) + NO(X2Π) f CN (A2Π, B2Σ) + CO (X1Σ) reaction is studied in an HTP (high-temperature photochemistry) reactor. C2 is produced by the 193 nm multiphoton photolysis of C2Cl4. The CN (B-X) (0,0) and (1,1) chemiluminescence is used as the diagnostic. The C2 consumption rate coefficients are measured under pseudo-first-order conditions and are found to be independent of pressure in the 5-200 mbar domain. They yield k(292-968 K) ) 4.3 × 10-11 exp(89 K/T) cm3 molecule-1 s-1 with a 2σ precision limit of (5% and a corresponding estimated accuracy limit of (20%. The slight negative activation energy is indicative of intermediate complex formation. The results are compared to the C2 + NO measurements by Kruse and Roth for the 3150-3950 K temperature range and confirm that a mechanism change occurs between the two respective temperature ranges. In the present work, under otherwise constant conditions the CN(B-X) emission intensity increases with bath gas Ar pressure in the 5 to 100 mbar domain and decreases with further Ar addition. This increase in CN(B) population is likely, at least in part, due not only to collision-induced internal conversion from CN A and X state molecules, but also collision-induced intersystem crossing from CN quartet state molecules.
Introduction The reaction between C2 and NO is of interest to NO reburning for the reduction of NOx emissions from combustion.1 The reaction has been observed to produce CN(B-X) Violet System and CN(A-X) Red System chemiluminescence, which is attributed2-4 to
C2(a3Π) + NO(X2Π) f CN (A2Π, B2Σ) + CO(X1Σ) (1) ∆H° 298 for reaction 1 is -501 and -303 kJ mol-1, respectively for A and B states formation.5-7 In addition to laboratory observations, the CN(B-X) emission has been found to be the major emitter in the ultraviolet at the exit plane of several rocket motors. There too it has been attributed to reaction 1.8 The strong (0,0) band at 388.7 nm is the dominant feature in both types of observations. The kinetics of and spectral distributions resulting from reaction 1 have been extensively studied by Reisler, Mangir, and Wittig (RMW)2,3 at room temperature and total pressures in the range of 0.1 to 5 mbar. These workers produced C2 by CO2 laser ir photolysis of C2H3CN, C2HCl3, or C2H4 and made observations on both the C2(X1Σg+) and the C2(a3Πu) molecules. The latter have an excitation energy of 7 kJ mol-1. These C2 states are hereafter referred to as 1C2 and 3C2. RMW found essentially identical rate coefficients of 7.5 × 10-11 cm3 molecule-1 s-1 whether using CN(B-X), CN(A-X) chemiluminescence, or 3C2(d3Πg - a3Πu) laser-induced fluorescence LIF as the diagnostic tool.2 However, 1C2(A1Πu - X1Σg+) LIF observations yielded a considerably faster rate coefficient, 2.1 × 10-10 cm3 molecule-1 s-1.3 From this, it may be concluded †
Part of the special issue “Donald Setser Festschrift”. * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: (518)-276-4030.
that the emissions are predominantly due to the 3C2 reaction, which is in accord with the crossed molecular beams study by Krause.4 The 1C2 reaction produces mainly CN(X) and 3C2 does not correlate with CN(X) + CO production. This state selectivity is in accord with adiabatic state correlation arguments.3,4 RMW also showed that reaction 1 is a direct reaction, in the sense that the CN is not produced from the possible intermediates C2O or C2N.2 Production of these intermediates would also be exothermic and spin-allowed. By contrast, Kruse and Roth1 studied C2 + NO in a shock tube from 3150 to 3950 K at pressures from 1.6 to 2.2 bar and obtained
k(3150-3950 K) ) 1.3 × 10-10 exp(-4350 K/T) cm3 molecule-1 s-1 (2) They produced C2 from the photolysis of acetylene and monitored it by laser absorption spectroscopy, LAS, also on the 3C2(d - a) transition. Equation 2 extrapolates to a value 6 orders of magnitude lower than that of RMW at room temperature, suggesting a change in mechanism. This is in accord with their1 O, N, and CN product formation measurements, which indicate that the reactions
C2 + NO f C2N + O ∆H˚298, see footnote (9)
3
(3)
C2 + NO f C2O + N ∆H˚298 ) -64 kJ mol-1 (refs 5,10) (4)
3
dominate with no discernible contribution (
