Formation and dissociation of C2 from high temperature pyrolysis of acetylene

Abstract

Kinetics of formation and decay of Cz produced by passing shock waves through dilute C2H2+Ar mixtures were studied at temperatures in the range 4860-6920 K. The kinetics of formation of Cz were in essential agreement with a postulated simple two-step dissociation of CzH2 by Ar. The rate constant for the dissociation of Ct by Ar was found to be ~c ~, A ~ = *'.'') exp( -583. &59 kJ mol-'/RT) cm3 molecufe-l s-l.The diatomic molecule of carbon, C 2 , is well known to be a product of the high temperature pyrolysis of simple organic molecules.1 The reactions of this species are important in many areas of high temperature chemistry.2 In an investigation of negative ion formation from carbon we have found that a proper kinetic treatment of C , formation necessitates an understanding of the rates and mechanism of C2 formation and dissociation.Acetylene is a usual initial reagent for Cz. The pyrolysis of acetone has been much studied at considerably lower temperatures than those at which C2 appreciably di~sociates.~-~ In these low temperature studies (< 2500 K) the importance of polymer intermediates has been demonstrated.* At sufficiently high temperatures ( > 4800 K) and at high dilutions in argon the formation and decay of C , from acetylene should be considerably simplified. Polymer products, at least at equilibrium, are negligible at these temperatures. There has been only one prior report of an attempt to measure the rate of dissociation of C2. Fairbairn' investigated the pyrolysis of several organic molecules at temperatures above 5000 K, but concluded that the observed decay of C2 somehow was coupled to the overall decomposition rate of the initial reagent. We report the kinetics of formation and dissociation of C2 in dilute mixtures of acetylene with argon. EXPERIMENTALThe high temperature pyrolysis of dilute mixtures of acetylene with argon was studied by passing a shock wave through the gas. The basic details of the shock tube have been described previously.lo All observations behind the incident shack were carried out 2.95 m from the diaphragm, giving a usable flow time of between 150-200~s (laboratory time). The emission spectrum of the Cz Swan system arising from the shock-heated gas was monitored by a Hamamatsu R213 photomultiplier at the exit slit of a Hilger and Watts monochromator. The voltage signaI from the photomultiplier was displayed on a storage oscilloscope. The monochromator was set at 514 nm and had a spectral bandpass of 1.0 nm. Initial photographic recording using a time-resolved spectrograph3 indicated that the emission at 514 nm arose only from Cz. By means of two narrow slits in the optical path, optical time resolution of better than 0.3 p s was achieved.C.I.G. special high purity acetylene and argon were used without further purification.

High-Temperature Fluorocarbon Chemistry Revisited

“…This identification of the absorption is supported by analyzing the decay of the signal. The rate constant of thermal dissociation of C 2 has been estimated as k 9 ≈ 10 14.57 (±0.27) exp(−69880 (±7100) K/ T ) cm 3 mol –1 s –1 in ref . Although the decay of the signal in Figure would correspond to an about a factor of 2 larger rate constant, within the uncertainty limits it agrees with the literature value for k 9 .…”

Elaboration of collisional–radiative models for flows related to planetary entries into the Earth and Mars atmospheres

“…dissociation rate coefficient of C 2 (X 1 + g ) close to the value of Beck and Mackie [79] when T v ≃ T A . For dissociation under impact of electron with energy ǫ, the collision cross section is assumed to behave like…”

Kinetics of C2 Reactions during High-Temperature Pyrolysis of Acetylene

“…The experiments on acetylene pyrolysis were performed with mixtures of argon containing 5-500 ppm C 2 H 2 at temperatures 2580 K e T e 4650 K and pressures between 1. 5 2 for three different temperatures and an initial C 2 H 2 concentration of 50 ppm diluted in Ar. The directly measured ring dye laser absorption profiles were converted via the Lambert-Beer law into concentrations based on f 00 ) 0.037.…”

“…Examples are soot formation in hot flames, diamond synthesis from gas phase pyrolysis, or the formation and decay of the recently discovered C 60 fullerene. − The pyrolysis of acetylene has been studied in the last decades by numerous investigators, but up to now, no quantitative measurement of C, C 2 , or C 3 radicals has been performed. Only in one kinetic study by Beck et al, C 2 radicals were measured during the acetylene pyrolysis, but in this investigation C 2 was monitored only by emission and thus only qualitatively. With the development of tunable and narrow bandwith CW ring dye lasers, the sensitive and quantitative detection of many di- and some triatomic molecules by direct absorption measurements became feasible.…”