Microkinetic modelling of heterogeneous recombination in CO2 plasmas

I 05 31 Microkinetic modelling of heterogeneous recombination 2plasmas P. Viegas1, B. Berdugo2, A. Filipe1, O. Guaitella2, A. S. Morillo-Candás2 and V. Guerra1 1Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisboa, Portugal 2Laboratoire de Physique des Plasmas, CNRS, Sorbonne Université, École Polytechnique, Institut Polytechnique de Paris, 91120 Palaiseau, France [email protected] CO₂ plasmas are currently an active area of research due to their relevance in various applications, such as the production of CO₂-neutral fuels on Earth and oxygen generation for future Mars missions [1]. A key aspect to the understanding of these plasmas is the atomic oxygen kinetics [2], particularly the heterogeneous recombination of O-atoms on reactor walls. While a well-established description exists for O-atom recombination in pure O₂ plasmas [3], experimental studies in CO₂ plasmas have shown a significantly lower recombination probability, suggesting that existing models may fail to fully capture the recombination mechanisms in CO₂-containing plasmas. In this work we develop a microkinetic model to study atomic oxygen recombination in CO₂ plasmas, validated against experimental measurements of O-atom loss frequencies. The experiments were conducted in DC discharges at pressures around 1 Torr, discharge currents in the range of a few tenths of milliamperes, in a Pyrex cylindrical tube with a 1 cm radius, and wall temperatures varied between -20°C and +50°C. The model describes the elementary steps of physisorption, thermal desorption, chemisorption, surface diffusion, and both Eley-Rideal and Langmuir-Hinshelwood recombination mechanisms. Our results indicate that the lower recombination probability observed in CO₂ plasmas can be attributed to the partial passivation of chemisorption sites by CO molecules. This passivation reduces the efficiency of O-atom recombination compared with pure O₂ plasmas, explaining the differences observed. Additionally, our simulations reveal that the creation and destruction of atomic oxygen in the gas phase must be accounted for to properly analyse and interpret the measured total O loss frequency. Acknowledgements: This work was supported by the Portuguese FCT-Fundação para a Ciência e a Tecnologia, projects UIDB/50010/2020 (https://doi.org/10.54499/UIDB/50010/2020), UIDP/50010/2020 (https://doi.org/10.54499/UIDP/50010/2020), LA/P/0061/2020 (https://doi.org/10.54499/LA/P/0061/2020), PTDC/FIS-PLA/1616/2021 (https://doi.org/10.54499/PTDC/FISPLA/1616/2021) and 2023.15276.PEX (https://doi.org/10.54499/2023.15276.PEX),and by the European Union under Horizon Europe project CANMILK (DOI:10.3030/101069491). PV acknowledges support by project CEECIND/00025/2022 of FCT. [1] L. Pietanza et al, Eur. Phys. J. D 75, (2021), 237 [2] A. S. Morillo-Candas et al, Plasma Sources Sci. Technol. 28, (2019), 075010 [3] J. Afonso et al, J. Phys. D: Appl. Phys. 57, (2024), 04LT01