I2. Kinetics of the Reaction Between Propargyl Radical and Acetylene
Vadim D. Knyazev* and Irene R. Slagle
The
Catholic University of America
Department of Chemistry
Washington, DC 20064, USA
Formation of soot in the combustion of hydrocarbon fuels is an important area of research. Numerous efforts have been concentrated on the elucidation of chemical pathways leading to the formation of the first aromatic ring. A review of many of these studies can be found in [1]. Recently, Moriarty et al.[2,3] have identified the reaction between propargyl radical and acetylene as a potentially important pathway leading to the formation of the highly stable cyclopentadienyl radical (cyclo-C5H5). These authors used quantum Monte Carlo and density-functional-theory methods and a master equation approach to explore the mechanism of the C3H3 + C2H2 reaction, which has not been experimentally studied before.
In the current work, we present the first direct experimental investigation of the reaction
C3H3 + C2H2 ® C5H5 (1)
between the propargyl radical and acetylene. Reaction 1 was studied by the Laser Photolysis / Photoionization Mass Spectrometry technique in the temperature range 800 – 1100 K and bath gas (helium) densities [He] = (1.2 – 2.4)´ 1017 atom cm-3. Propargyl radicals (C3H3) were produced by the pulsed 248-nm laser photolysis of 1,3-butadiene and the kinetics of their decay due to reaction 1 was monitored in real time by photoionization mass spectrometry. Initial conditions were selected to provide low radical concentrations so that potential side reactions such as reactions between radical products have negligible rates compared to that of reaction 1. Experiments were conducted under pseudo-first-order conditions with acetylene ([C2H2] = (4.1 – 52.3)´ 1015 molecule cm-3 ) in large excess over propargyl ([C3H3]0 = (0.8 – 18)´ 1010 molecule cm-3 ). The photolysing laser intensity was kept low (0.11 – 5.33 mJ pulse-1 cm-2) to ensure that products of the photodissociation of acetylene do not interfere with the kinetics of reaction 1. It was demonstrated that the values of the rate constant of reaction 1 obtained are independent of the photolysing laser intensity, initial concentration of C3H3, concentration of the precursor of propargyl radicals, and bath gas pressure within the ranges used in the experiments.
The obtained temperature dependence of the rate constant of reaction 1 (Figure 1) can be described by the following Arrhenius expression:
k1 = 3.9´ 10-13 exp(- 5030 K/T) cm3 molecule-1 s-1
The low value of the preexponential factor is consistent with the formation of C5H5 in a chemically activated mechanism characterized by a "loose" entrance and a "tight" exit transition states.
C5H5 was identified as a primary product of reaction 1 with the characteristic growth time of the C5H5 signal matching that of the C3H3 decay. Formation of C7H7 was observed with kinetics corresponding to that of a secondary reaction and formation of C9H8 was also observed at even longer reaction times. The kinetics of product formation suggest the following sequence of reactions occurring under the conditions of excess acetylene used in the current work:
C3H3 + C2H2 ® C5H5 (1)
C5H5 + C2H2 ® C7H7 (2)
C7H7 + C2H2 ® C9H8 + H (3)
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Figure 1. Arrhenius temperature dependence of the rate constant of the reaction between C3H3 and C2H2. |
Acknowledgement. This research was supported by Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U.S. Department of Energy under Grant No. DE-FG02-94ER1446.
References.
1. Richter, H.; Howard, J. B. "Formation of polycyclic aromatic hydrocarbons and their growth to soot - a review of chemical reaction pathways." Progress in energy and combustion science, 2000, 26, 565.
2. Moriarty, N. W.; Krokidis, X.; Lester, W. A., Jr.; Frenklach, M. "Formation of cyclopentadienyl: A theoretical study." 16th International Symposium on Gas Kinetics, Cambridge, UK, July 2000, Abstract PA7.
3. Moriarty, N. W.; Krokidis, X.; Lester, W. A., Jr.; Frenklach, M. "The Addition Reaction of Propargyl and Acetylene: Pathways to Cyclic Hydrocarbons." 2-nd Joint Meeting of the US Sections of the Combustion Institute, Oakland, California, March 2001. Abstract 102.