Diagnostics to study the vibrational excitation kinetics of CO 2 for renewable energy storage

Time-resolved in situ FTIR spectroscopy and spatiotemporally resolved in situ Raman spectroscopy are used to study the excitation and relaxation of the vibrations of CO 2 and the reduction of CO 2 to CO in a pulsed glow discharge

Excitation and relaxation of the asymmetric stretch mode of CO2 in a pulsed glow discharge

The excitation and relaxation of the vibrations of CO2 as well as the reduction of CO2 to CO are studied in a pulsed glow discharge. Two diagnostics are employed: (1) time-resolved in situ Fourier transform infrared spectroscopy and (2) spatiotemporally resolved in situ rotational Raman spectroscopy. Experiments are conducted within a pressure range of 1.3–6.7 mbar and a current range of 10–50 mA. In the afterglow, the rate of exponential decay from the asymmetric stretch temperature (T 3) to the rotational temperature (T rot) is found to be only dependent on T rot, in the conditions under study. The decay rate rho T 3 - T rot follows the relation rho T 3-T rot=388 s-1 exp(T rot - 273 K / 154 K). Pressure and varying concentrations of CO and (presumably) atomic oxygen did not show to be of significant influence. In the active part of the discharge the excitation of T 3 showed to be positively related to current and negatively to pressure. However, the contribution of current to vibrational excitation is ambiguous: the conversion of CO2 and therefore the fraction of CO in the discharge, is found to be strongly dependent on the current, with a conversion factor of 0.05–0.18 for 10–50 mA, while CO can contribute to the excitation through near-resonant collisions. A clear relation between the elevation of T 3 and the dissociation of CO2 could not be confirmed, though conversion peaks are observed in the near afterglow, which motivate future experiments on vibrational ladder-climbing directly after termination of the discharge

Vibrational kinetics of CO2 in non-thermal plasma

During the presentation I will discuss the development of two diagnostics to increase our current level of understanding of the vibrational kinetics within CO2 discharges, with the intention to ultimately contribute to a controlled and efficient dissociation process. The diagnostic techniques are (1) time resolved in situ Fourier transform infrared (FTIR) spectroscopy and (2) spatiotemporally resolved in situ rotational Raman spectroscopy. Both techniques are used to obtain information about the rovibrational density distributions in the electronic ground state of CO2 in a pulsed glow discharge. During the active part of the plasma pulse a clear non-equilibrium is observed between the rotational and the ν3, and the (ν1, ν2) and ν3 vibrational density distributions. The results provide ample experimental foundation to expand our knowledge on CO2 vibrations and dissociation, especially through comparison with numerical models. This work has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 81339

Spatiotemporally resolved rotational Raman spectroscopy in a pulsed CO<SUB>2</SUB> glow discharge

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Excitation and relaxation of the asymmetric stretch mode of CO2 in a pulsed glow discharge

The excitation and relaxation of the vibrations of CO2; as well as the reduction of CO2 to CO are studied in a pulsed glow discharge. Two diagnostics are employed, being (1) time-resolved in situ Fourier transform nfrared (FTIR) spectroscopy and (2) spatiotemporally resolved in situ rotational Raman spectroscopy. Experiments are conducted within a pressure range of 1.3-6.7 mbar and a current range of 10-50 mA. In the afterglow, the rate of exponential decay from the asymmetric stretch temperature (T3) to the rotational temperature (Trot) is found to be only dependent on Trot, in the conditions under study. The cay rate ρT3-Trot follows the relation ρT3-Trot = 388 s-1 exp(Trot - 273 K)/(154 K). Pressure and varying concentrations of CO and (presumably) atomic oxygen did not show to be of significant influence. In the active part of the discharge the excitation of T3 showed to be positively related to current and negatively to pressure. However, the contribution of current to vibrational excitation is ambiguous: the conversion of CO2 and therefore the fraction of CO in the discharge, is found to be strongly dependent on the current, with a conversion factor of 0.05 to 0.18 for 10 mA to 50 mA, while CO can contribute to the excitation through near-resonant collisions. A clear relation between the elevation of T3 and the dissociation of CO2 could not be confirmed, though conversion peaks are observed in the near afterglow, which motivate future experiments on vibrational ladder-climbing directly after termination of the discharge

Diagnostics to study the vibrational excitation kinetics of CO \u3csub\u3e2\u3c/sub\u3e for renewable energy storage

\u3cp\u3e Time-resolved in situ FTIR spectroscopy and spatiotemporally resolved in situ Raman spectroscopy are used to study the excitation and relaxation of the vibrations of CO \u3csub\u3e2\u3c/sub\u3e and the reduction of CO \u3csub\u3e2\u3c/sub\u3e to CO in a pulsed glow discharge. \u3c/p\u3

Kinetic study of CO\u3csub\u3e2\u3c/sub\u3e plasmas under non-equilibrium conditions. II. Input of vibrational energy

\u3cp\u3eThis is the second of two papers presenting the study of vibrational energy exchanges in non-equilibrium CO\u3csub\u3e2\u3c/sub\u3e 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 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\u3csub\u3e2\u3c/sub\u3e plasmas.\u3c/p\u3

A rotational Raman study under non-thermal conditions in a pulsed CO\u3csub\u3e2\u3c/sub\u3e glow discharge

\u3cp\u3eThe implementation of in situ rotational Raman spectroscopy is realized for a pulsed glow discharge in CO\u3csub\u3e2\u3c/sub\u3e in the mbar range and is used to study the rotational temperature and molecular number densities of CO\u3csub\u3e2\u3c/sub\u3e, CO, and O\u3csub\u3e2\u3c/sub\u3e. The polarizability anisotropy of these molecules is required for extracting number densities from the recorded spectra and is determined for incident photons of 532 nm. The spatiotemporally-resolved measurements are performed in the same reactor and at equal discharge conditions (5-10 ms on-off cycle, 50 mA plasma current, 6.7 mbar pressure) as in recently published work employing in situ Fourier transform infrared (FTIR) spectroscopy. The rotational temperature ranges from 394 to 809 K from start to end of the discharge pulse and is constant over the length of the reactor. The discharge is demonstrated to be spatially uniform in gas composition, with a CO\u3csub\u3e2\u3c/sub\u3e conversion factor of 0.15 ± 0.02. Rotational temperatures and molecular composition agree well with the FTIR results, while the spatial uniformity confirms the assumption made for the FTIR analysis of a homogeneous medium over the line-of-sight of absorption. Furthermore, the rotational Raman spectra of CO\u3csub\u3e2\u3c/sub\u3e are related to vibrational temperatures through the vibrationally averaged nuclear spin degeneracy, which is expressed in the intensity ratio between even and odd numbered Raman peaks. The elevation of the odd averaged degeneracy above thermal conditions agrees well with the elevation of vibrational temperatures of CO\u3csub\u3e2\u3c/sub\u3e, acquired in the FTIR study.\u3c/p\u3