Plasma assisted dry reforming of methane: Syngas and hydrocarbons formation mechanisms.*

 

Plasma reactions of CO₂ + CH₄ mixtures have been proposed as a suitable process for the dry reforming of methane. Without specific catalysts, most studies report the formation of CO and H₂ as main reaction products and arise the question whether CH_x radicals coming from CH₄ may interact with intermediate species formed by electron impact dissociation of CO₂, a critical step for the formation of high added value oxygenated compounds. We have addressed this question studying the CO₂ + CH₄ plasma reaction in a ferroelectric-moderated packed-bed reactor varying the reactants ratio. Analysis of the reaction products by mass spectrometry and the plasma reaction intermediates by optical emission spectroscopy suggest that few direct cross-link interactions exist between intermediate plasma species issued from CH₄ or CO₂. This preliminary evidence is corroborated by experiments using CO₂ instead CO₂ as reactant. The isotope labeling procedure has proved that plasma reaction mechanisms of CO₂ and CH₄ molecules proceed almost independently, with the formation of small amounts of water and the removal of carbon deposits resulting CH₄ plasma decomposition as sole evidences of cross reactions. These results highlight the need of using catalysts to promote specific surface reactions for a better control of the selectivity of the process.