Conversion of CO 2 into CO and O is studied in a flowing gas surfaguide pulsed microwave discharge operating with CO 2 and CO 2 + N 2 gas mixtures under different conditions. Optical emission spectroscopy, including actinometry (using N 2 ), vibrational (N 2 molecule) and rotational (CO and N 2 molecules) analysis are utilized. Both time-and space-resolved measurements are performed. The results show the essential changes of the CO 2 conversion rate, its energetic efficiency, and the gas and vibrational temperatures along the gas flow direction in the discharge. The spatial distribution of the power absorbed in the plasma is analyzed. It is also confirmed that the vibrational excitation is a key factor in the CO 2 dissociation process in this type of plasma. It is suggested that the obtained dissociation rates can be further optimized by varying the gas composition, as well as the power applied to the discharge.

A kinetic model describing the time evolution of ∼70 individual CO 2 (X 1 Σ + ) vibrational levels during the afterglow of a pulsed DC glow discharge is developed in order to contribute to the understanding of vibrational energy transfer in CO 2 plasmas. The results of the simulations are compared against in situ Fourier transform infrared spectroscopy data obtained in a pulsed DC glow discharge and its afterglow at pressures of a few Torr and discharge currents of around 50 mA. The very good agreement between the model predictions and the experimental results validates the kinetic scheme considered here and the corresponding vibration-vibration and vibration-translation rate coefficients. In this sense, it establishes a reaction mechanism for the vibrational kinetics of these CO 2 energy levels and offers a firm basis to understand the vibrational relaxation in CO 2 plasmas. It is shown that first-order perturbation theories, namely, the Schwartz-Slawsky-Herzfeld and Sharma-Brau methods, provide a good description of CO 2 vibrations under low excitation regimes.

This is the second of two papers presenting the study of vibrational energy exchanges in nonequilibrium CO 2 plasmas in low-excitation conditions. The companion paper addresses a theoretical and experimental investigation of the time relaxation of ∼70 individual vibrational levels of ground-state CO X 2 1 S + ( )molecules during the afterglow of a pulsed DC glow discharge, operating at pressures of a few Torr and discharge currents around 50mA, where the rate coefficients for vibration-translation (V-T) and vibration-vibration (V-V) energy transfers among these levels are validated (Silva et al 2018 Plasma Sources Sci. Technol. 27 015019). Herein, the investigation is focused on the active discharge, by extending the model with the inclusion of electron impact processes for vibrational excitation and de-excitation (e-V). The time-dependent calculated densities of the different vibrational levels are compared with experimental data obtained from time-resolved in situ Fourier transform infrared spectroscopy. It is shown that the vibrational temperature of the asymmetric stretching mode is always larger than the vibrational temperatures of the bending and symmetric stretching modes along the discharge pulse-the latter two remaining very nearly the same and close to the gas temperature. The general good agreement between the model predictions and the experimental results validates the e-V rate coefficients used and provides assurance that the proposed kinetic scheme provides a solid basis to understand the vibrational energy exchanges occurring in CO 2 plasmas.

A chemical kinetics model is developed for a CO 2 /N 2 microwave plasma, focusing especially on the vibrational levels of both CO 2 and N 2 . The model is used to calculate the CO 2 and N 2 conversion, as well as the energy efficiency of CO 2 conversion, for different power densities and for N 2 fractions in the CO 2 /N 2 gas mixture ranging from 0 till 90%. The calculation results are compared with measurements, and agreements within 23% and 33% are generally found for the CO 2 conversion and N 2 conversion, respectively. To explain the observed trends, the destruction and formation processes of both CO 2 and N 2 are analyzed, as well as the vibrational distribution functions of both CO 2 and N 2 . The results indicate that N 2 contributes in populating the lower asymmetric levels of CO 2 , leading to a higher absolute CO 2 conversion upon increasing N 2 fraction. However, the effective CO 2 conversion drops, because there is less CO 2 initially present in the gas mixture, and thus also the energy efficiency drops with rising N 2 fraction.

In this experimental study, a flowing dielectric barrier discharge operating at atmospheric pressure is used for the splitting of CO2 into O2 and CO. The influence of the applied frequency and plasma power on the microdischarge properties is investigated to understand their role on the CO2 conversion. Electrical measurements are carried out to explain the conversion trends and to characterize the microdischarges through their number, their lifetime, their intensity and the induced electrical charge. Their influence on the gas and electrode temperatures is also evidenced through optical emission spectroscopy and infrared imaging. It is shown that, in our configuration, the conversion depends mostly on the charge delivered in the plasma and not on the effective plasma voltage when the applied power is modified. Similarly, at constant total current, a better conversion is observed at low frequencies, where a less filamentary discharge regime with a higher effective plasma voltage than that at a higher frequency is obtained.