We present a zero-dimensional kinetic model of CO2 splitting in non-equilibrium plasmas. The model includes a description of the CO2 vibrational kinetics (25 vibrational levels up to the dissociation limit of the molecule), taking into account state-specific VT and VV relaxation reactions and the effect of vibrational excitation on other chemical reactions. The model is applied to study the reaction kinetics of CO2 splitting in an atmospheric-pressure dielectric barrier discharge (DBD) and in a moderate-pressure microwave discharge. The model results are in qualitative agreement with published experimental works. We show that the CO2 conversion and its energy efficiency are very different in these two types of discharges, which reflects the important dissociation mechanisms involved. In the microwave discharge, excitation of the vibrational levels promotes efficient dissociation when the specific energy input is higher than a critical value (2.0 eV/molecule under the conditions examined). The calculated energy efficiency of the process has a maximum of 23%. In the DBD, vibrationally excited levels do not contribute significantly to the dissociation of CO2 and the calculated energy efficiency of the process is much lower (5%).

https://iopscience.iop.org/article/10.1088/0963-0252/23/4/045004/pdf

Splitting of CO2 by vibrational excitation in non-equilibrium plasmas: a reaction kinetics model

Many aspects of the collision dynamics of vibrational energy transfer are presented. Special emphasis is placed on three broad areas within this field:  (1) vibrational energy transfer in large molecules (>10 modes) at low excitation, (2) vibrational energy transfer in large molecules at high vibrational excitation, and (3) vibrational energy transfer of highly excited small molecules. Advances in laser methods have revolutionized experimental investigations of all of these areas. Recent results are presented, and directions for the future are discussed.

Vibrational energy transfer

This paper addresses the analysis, validation, and application of analytic, nonperturbative, semiclassical vibration-translation (V-T) and vibration-vibration-translation (V-V-T) rate models for atom-diatom and diatom-diatom vibrational molecular energy transfer collisions. These forced harmonic oscillator (FHO) rate models are corrected and validated by comparison with recent experiments, and with three-dimensional semiclassical trajectory calculations for N 2 -N 2 , O 2 -O 2 , and N 2 -O 2 , which are considered to be the most reliable theoretical data available. A remarkably good overall agreement is shown for both the temperature and quantum number dependence of single-quantum and double-quantum V-V-T transitions in the temperature range 200 < T < 8000 K and for vibrational quantum numbers 0 < ν < 40. It is demonstrated that the multiquantum vibrational energy transfer processes occur via a sequential FHO mechanism, as a series of virtual single-quantum steps during one collision. An important exception, asymmetric multiquantum V-V exchange at low temperatures, that occurs via a direct first-order mechanism, is discussed. Analytic thermally averaged FHO V-T and V-V rates are suggested. The FHO model gives new insight into vibrational kinetics and may be easily incorporated into kinetic modeline calculations under conditions when first-order theories are not applicable.

Vibrational Energy Transfer Rates Using a Forced Harmonic Oscillator Model

Vibration—vibration (V—V) and vibration—translation/rotation (VT/R) rate coefficients for H 2 H 2 collisions are calculated by a semi-classical 3-dimensional model. The model used treats the translational and rotational motion classically while treating the vibrational motion quantum mechanically. The effects of both the long range and short range interaction potential is included in the model. The results obtained for the VT/R processes are in good agreement with available experimental data over the temperature range of 200–800 K and predicts considerably faster scaling with increasing vibrational level than expected from harmonic oscillator models. Non-resonant and multi-quantum VV rate coefficients show substantial disagreement with earlier collinear collision models both in magnitude and scaling with increasing vibrational level.

VV and VT rate coefficients in N2 by a quantum-classical model

The rate constants associated with the deactivation of vibrationally excited CO2*(ν3) by collision with either CO2 or N2 have been experimentally determined from 300°–1000°K by a laser fluorescence method. The reactions investigated were CO2*(ν3)+N2⇆ lim k−5k5CO2+N2*, CO2*(ν3)+N2→ lim k5CO2*(ν1, ν2)+N2, CO2(ν3)+CO2→ lim k1CO2*(ν1, ν2)+CO2, N2*+CO2→ lim k7CO2*(ν1, ν2)+N2, where the asterisk denotes a vibrationally excited molecule, and the quantities in parentheses represent the specific excited modes of CO2. The rate constant k5 varies with temperature T (°K) as 8.60 × 107 / T3/2torr−1·sec−1. From 400°–1000°K the rate constant k1 (per torr−1 ·second−1) varies with temperature as log10k1 = A − BT−1 / 3 with A = 6.79 and B = 30.8. From 300°–400°K the measured values of k1 are greater than those corresponding to the above relation. The rate constant k6 increases from a value of 110 torr−1·sec−1 at room temperature to a value of 2700 torr−1·sec−1 at 1000°K, but the variation of k6 with temperature cannot be s...

Deactivation of Vibrationally Excited Carbon Dioxide (ν3) by Collisions with Carbon Dioxide or with Nitrogen

The cross section for the near‐resonant transfer of vibrational energy from CO2(001) to N2(0), CO2(001) + N2(0)→CO2(000) + N2(1) + ΔE, is calculated for the isotopes 14N2 (ΔE = 18 cm−1) and 15N2 (ΔE = 97 cm−1). The impact parameter (semi‐classical) approximation is used, and it is assumed that the vibrational‐energy transfer is caused by the interaction of the instantaneous CO2 dipole moment with the N2 quadrupole moment. When proper account is taken of the rotational motions of the molecules it is found that in collisions of CO2 with 14N2 only the low rotational levels of the CO2 and N2 molecules contribute to Reaction (1). In collisions of CO2 with 15N2, only those rotational levels contribute which undergo transitions cancelling most of the relatively large (97 cm−1) vibrational‐resonance defect. For 14N2 below about 1000°K, where the cross section displays a negative temperature dependence, the results are in excellent qualitative and quantitative agreement with available experimental data, with no ad...

Energy Transfer in Near‐Resonant Molecular Collisions due to Long‐Range Forces with Application to Transfer of Vibrational Energy from ν3 Mode of CO2 to N2

C. A. Brau

Papers

Energy Transfer in Near‐Resonant Molecular Collisions due to Long‐Range Forces with Application to Transfer of Vibrational Energy from ν3 Mode of CO2 to N2