Time Evolution of the Dissociation Fraction in rf CO₂ Plasmas: Impact and Nature of Back-Reaction Mechanisms

Abstract

The time evolution of the dissociation fraction in a pulsed radio frequency (rf) CO2 discharge is studied by infrared absorption. A large parametric study performed in a closed reactor brings valuable information about both dissociation and recombination processes. The CO2 conversion shows a time evolution initially controlled by electron impact dissociation. For longer plasma-on times, the dissociation fraction reaches a steady-state that corresponds to a balance between dissociation processes and back-reaction mechanisms. The characteristic times of vibrational and rotational excitation of CO and CO2 are measured during a single plasma pulse. The dependence of the CO2 conversion as a function of pulse duration and frequency is then analyzed, showing the influence of the plasma excitation conditions on the back-reaction mechanisms. The study of CO2–O2 and CO–O2 gas mixtures gives further insight into the impact of the oxygen content in these mechanisms. The global back-reaction rate observed in these experiments is compared with calculated values from rate coefficients available in the literature. The experimental results and preliminary calculations reveal a key role of molecular oxygen and of the metastable electronically excited state CO­(a 3Π r ) in the back-reaction. Competing processes involving vibrational excited CO are not dominant in our discharge conditions but may become relevant at slightly higher vibrational temperatures.

Advances in non-equilibrium $$\hbox {CO}_2$$ plasma kinetics: a theoretical and experimental review

“…A systematic mutual step-by-step validation procedure of their results allowed to separately investigate the role of V-V and V-T transfers [60,61], e-V excitation [62] and gas heating [63,64]. Two particularly revealing results of the benefit of a joint model / experiment approach are the identification of the best CO 2 dissociation cross section by electron impact for E/N values below 100 Td [89], and the evidence of the key role of CO(a 3 Π) in the 'back reaction mechanisms' with O 2 at low gas temperature giving back CO 2 [131,148]. A much more extended vibrational kinetic model for the CO 2 system was proposed by Armenise and Kustova [46,274], but applied to the investigation of the hypersonic entry of vehicles in Mars atmosphere.…”

“…by dissociative recombination of the CO + 2 ion as measured in the downstream of a MW plasma in FTE [147]. The large amount of energy carried by this metastable state can influence significantly both vibrational kinetics and chemistry [131,148]. The transfer from electronic to vibrational energy in collisions between (CO(a 3 Π) + CO) is found to happen with 89% efficiency in [149] leading to fast redistribution of energy on high vibrational levels and radiation.…”

“…However this reaction occurs mostly at high temperature, or at the walls with the surface acting as third body [130]. At low pressure and temperature, back reaction mechanisms are probably dominated by other reactions involving for instance CO(a 3 Π) state [131]. In any case, the avoidance of recombination of O atoms whether in O 2 or CO 2 is a key process to achieve an energy efficient conversion of CO 2 , and the use of membrane to extract O atoms from the plasma could enhance the conversion efficiency [37].…”

“…5 and the related discussion). Depending on pressure and temperature conditions, this last reaction can even be the main 'back reaction mechanisms' as shown in [131]. Even though CO(a 3 Π) appears to be a key specie in CO 2 plasma dynamics, its direct measurement is very challenging and has been done only in very diluted gas mixture for CO 2 laser study [155].…”

“…Important modifications in the EEDF are also brought by the changes in the mixture composition due to dissociation, as electrons start channeling their energy to the excitation of CO and, to a lesser extent, O 2 and O [148]. Following the experimental evidence of an important role of the electronically excited state CO(a 3 Π r ) in the decomposition of CO 2 [131], its kinetics is included and carefully discussed in the model. The addition of the stateto-state CO 2 vibrational kinetics within low-excitation conditions, i.e., accounting for the same ∼ 70 individual vibrational levels and corresponding reactions and rate coefficients as in [61,62], was achieved very recently [39].…”