Elementary reactions of formyl (HCO) radical studied by laser photolysis—transient absorption spectroscopy

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Abstract

Several elementary reactions of formyl radical of combustion importance were studied using pulsed laser photolysis coupled to transient UV–Vis absorption spectroscopy: HCO  H + CO (1), HCO + HCO → products (2), and HCO + CH3  products (3). One-pass UV absorption, multi-pass UV absorption as well as cavity ring-down spectroscopy in the red spectral region were used to monitor temporal profiles of HCO radical. Reaction (1) was studied over the buffer gas (He) pressure range 0.8–100 bar and the temperature range 498–769 K. Reactions (2a), (2b), (2c), (3a), (3b) as well as the UV absorption spectrum of HCO, were studied at 298 and 588 K, and the buffer gas (He) pressure of 1 bar. Pulsed laser photolysis (308, 320, and 193 nm) of acetaldehyde, propionaldehyde, and acetone was used to prepare mixtures of free radicals. The second-order rate constant of reaction (1) obtained from the data at 1 bar is: k1(He) = (0.8 ± 0.4) × 10−10exp(−(66.0 ± 3.4) kJ mol−1/RT) cm3 molecule−1 s−1. The HCO dissociation rate constants measured in this work are lower than those reported in the previous direct work. The difference is a factor of 2.2 at the highest temperature of the experiments and a factor of 3.5 at the low end. The experimental data indicate pressure dependence of the rate constant of dissociation of formyl radical 1, which was attributed to the early pressure fall-off expected based on the theory of isolated resonances. The UV absorption spectrum of HCO was revised. The maximum absorption cross-section of HCO is (7.3 ± 1.2) × 10−18 cm2 molecule−1 at 230 nm (temperature independent within the experimental error). The measured rate constants for reactions (2a), (2b), (2c), (3a), (3b) are: k2 = (3.6 ± 0.8) × 10−11 cm3 molecule−1 s−1 (298 K); k3 = (9.3 ± 2.3) × 10−11 cm3 molecule−1 s−1(298 and 588 K).

Introduction

The unimolecular dissociation of formyl radical (HCO) is of considerable importance in the hydrocarbon combustion mechanisms [1], [2], [3]:HCOH+COReaction (1) leads to the chain branching via interaction of hydrogen atoms with molecular oxygen. Competition between the unimolecular dissociation of formyl radicals (reaction (1)) and reactions of HCO with O2, OH, and H controls the chain branching rate, and, therefore, a number of important characteristics of combustion (such as the autoignition thresholds and delays, the flame propagation speed, etc.) [1], [2], [3].

Formyl radical also received significant attention in recent years also due to its “non-classical RRKM” behavior. Due to relatively weak C–H bond and large vibrational frequencies, the radical has isolated resonances at energies above the dissociation threshold. These resonances have been extensively studied both using spectroscopic techniques and theoretically [4], [5], [6], [7], [8], [9]. This feature is expected to have a significant impact on the pressure fall-off curve for reaction (1) [10]. Due to the long lifetime of the resonances (compared to the lifetime of a collision complex that obeys classical RRKM behavior) the characteristic transition pressure is expected to be lowered from the RRKM value (ca. 3000 bar) to ca. 10 bar at ambient temperature [10].

Until recently there was only a few direct studies of reaction (1) under the conditions of thermal activation [11], [12]. The previous knowledge on the kinetics of this reaction was obtained either from complex reactions (such of formaldehyde [13] and acetylene [14] flames, photooxidation of acetone [15] and pyrolysis of formaldehyde [16]), or from the kinetics of the reverse reaction [17], [18], [19], [20], [21], [22], [23], [24] combined with the reaction thermochemistry. The first direct study of reaction (1) employed laser photolysis combined with the photoionization mass spectrometry [11], at the buffer gas pressure less than 0.015 bar, where the reaction is well in the low-pressure region.

The current work summarizes direct studies of reaction (1) performed during the last five years using laser pulsed photolysis combined with transient absorption spectroscopy in the visible and UV regions [25], [26]. Reaction (1) was directly studied over an extended buffer gas pressure range (1–100 bar) using a variety of experimental techniques. Single-pass and multi-pass UV absorption [25] as well as cavity ring down spectroscopy (CRDS) detection [26] using the red transition of the radical was employed. The results of these measurements are in considerable discrepancy (factor of 2–3) with previous data [11]. In addition, a pressure dependence of the rate constant of reaction (1) was observed and attributed to the earlier pressure fall-off due to the “non-RRKM” behavior of HCO radical.

In the course of the current study, it was found that the UV absorption cross-sections [27] of formyl radical, the rate constant for the HCO self-reaction (2a), (2b), (2c) as well as the reaction of HCO with methyl radical (3) should be revised:HCO+HCOH2CO+CO(HCO)2H2+2COHCO+CH3CH4+COCH3CHONew UV absorption cross-sections as well as rate constants of reactions (2a), (2b), (2c), (3a), (3b) were obtained.

Section snippets

Experimental

Two experimental facilities were used to study reactions (1), (2a), (2b), (2c), (3a), (3b). Pulsed laser photolysis–transient UV–Vis absorption using a single-pass and a multi-pass cell was used in the experiments performed at NJIT. CRDS on the red transition of HCO was used to monitor the temporal profiles of the radical in the laser photolysis experiments performed at NOAA. The experimental facility at NJIT consists of excimer laser photolysis coupled to UV–Vis transient absorption

One-pass experiments

Transient absorption profiles were measured at 14 combinations of temperature and pressure. Examples of temporal absorption profiles at 230 nm (HCO radical) and at 216.5 nm (CH3 radical) are shown in Fig. 1. Two approaches were used to account for the role of radical–radical processes. In the first approach, the temporal absorption profiles were fitted by an exponential decay function. The decay parameters (the apparent rate constants) were determined at different laser energies. Extrapolation of

Summary

Reaction of unimolecular dissociation of formyl radicals 1 was studied over an extended temperature and buffer gas pressure ranges. The measured rate constant of this reaction is ca. 2–3 times lower than the value currently accepted in the combustion models. The data indicate an early pressure fall-off in reaction (1) in accord with the theoretical predictions.

UV absorption spectrum of HCO was revised. The measured UV cross-sections are ca. factor of 2 higher than the previously reported.

Acknowledgment

This work was supported by the Petroleum Research Fund administered by the American Chemical Society (Grant No. 31640-AC6).

References (38)

  • G.W. Adamson et al.

    J. Mol. Spectrosc.

    (1993)
  • W.G. Browne et al.

    Proc. Combust. Inst.

    (1969)
  • J.E. Bennett et al.

    Proc. Combust. Inst.

    (1971)
  • E.B. Gordon et al.

    Chem. Phys.

    (1978)
  • J.E. Baggott et al.

    Chem. Phys. Lett.

    (1986)
  • J.A. Miller et al.

    Ann. Rev. Phys. Chem.

    (1990)
  • W. Tsang et al.

    J. Phys. Chem. Ref. Data

    (1986)
  • B. Eiteneer et al.

    J. Phys. Chem.

    (1998)
  • K.-T. Lee et al.

    J. Chem. Phys.

    (1986)
  • A.D. Sappey et al.

    J. Chem. Phys.

    (1990)
  • D.W. Neyer et al.

    J. Chem. Phys.

    (1995)
  • J.D. Tobiason et al.

    J. Chem. Phys.

    (1995)
  • U. Brandt-Pollmann et al.

    J. Chem. Phys.

    (2001)
  • A.F. Wagner et al.

    J. Phys. Chem.

    (1987)
  • R.S. Timonen et al.

    J. Phys. Chem.

    (1987)
  • G. Friedrichs et al.

    Phys. Chem. Chem. Phys.

    (2002)
  • L.O. De Guertechin et al.

    Bull. Soc. Chim. Belg.

    (1983)
  • G.S. Pearson

    J. Phys. Chem.

    (1963)
  • Y. Hidaka et al.

    Int. J. Chem. Kinet.

    (1993)
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