State-Selected Reactivity of Carbon Dioxide Cations ( CO 2 + ) With Methane

Abstract

The reactivity of ${\text{CO}}_2^{+}$ with CD₄ has been experimentally investigated for its relevance in the chemistry of plasmas used for the conversion of CO₂ in carbon-neutral fuels. Non-equilibrium plasmas are currently explored for their capability to activate very stable molecules (such as methane and carbon dioxide) and initiate a series of reactions involving highly reactive species (e.g., radicals and ions) eventually leading to the desired products. Energy, in the form of kinetic or internal excitation of reagents, influences chemical reactions. However, putting the same amount of energy in a different form may affect the reactivity differently. In this paper, we investigate the reaction of ${\text{CO}}_2^{+}$ with methane by changing either the kinetic energy of ${\text{CO}}_2^{+}$ or its vibrational excitation. The experiments were performed by a guided ion beam apparatus coupled to synchrotron radiation in the VUV energy range to produce vibrationally excited ions. We find that the reactivity depends on the reagent collision energy, but not so much on the vibrational excitation of ${\text{CO}}_2^{+}$ . Concerning the product branching ratios ( ${\text{CD}}_4^{+}$ / ${\text{CD}}_3^{+}$ /DOCO⁺) there is substantial disagreement among the values reported in the literature. We find that the dominant channel is the production of ${\text{CD}}_4^{+}$ , followed by DOCO⁺ and ${\text{CD}}_3^{+}$ , as a minor endothermic channel.

Keywords: CO2 dissociation; Mars atmosphere; astrochemistry; ion-molecule reaction; plasma; synchrotron radiation; vibrational excitation.