Abstract
Dielectric barrier discharge (DBD) plasmas and plasma catalysis are becoming an alternative procedure to activate various gas phase reactions. A low-temperature and normal operating pressure are the main advantages of these processes, but a limited energy efficiency and little selectivity control hinder their practical implementation. In this work, we propose the use of isotope labelling to retrieve information about the intermediate reactions that may intervene during the DBD processes contributing to a decrease in their energy efficiency. The results are shown for the wet reforming reaction of methane, using D2O instead of H2O as reactant, and for the ammonia synthesis, using NH3/D2/N2 mixtures. In the two cases, it was found that a significant amount of outlet gas molecules, either reactants or products, have deuterium in their structure (e.g., HD for hydrogen, CDxHy for methane, or NDxHy for ammonia). From the analysis of the evolution of the labelled molecules as a function of power, useful information has been obtained about the exchange events of H by D atoms (or vice versa) between the plasma intermediate species. An evaluation of the number of these events revealed a significant progression with the plasma power, a tendency that is recognized to be detrimental for the energy efficiency of reactant to product transformation. The labelling technique is proposed as a useful approach for the analysis of plasma reaction mechanisms.
Highlights
Plasma and plasma catalysis with dielectric barrier discharge (DBD) reactors have been widely utilized for a large variety of chemical processes, including the reforming of hydrocarbons [1,2,3], the abatement of contaminants [4,5,6], or the synthesis of ammonia [7,8,9]
We attributed this relatively low reaction yield, in comparison with that attained in conventional catalytic processes [21], to the existence of back reactions leading to the formation of nitrogen and hydrogen from the formed ammonia (i.e., the inverse of Reaction (1)) or other intermediate processes, which result inefficient in rendering ammonia molecules
We present a systematic description of the isotope labelling technique to study plasma
Summary
Plasma and plasma catalysis with dielectric barrier discharge (DBD) reactors have been widely utilized for a large variety of chemical processes, including the reforming of hydrocarbons [1,2,3], the abatement of contaminants [4,5,6], or the synthesis of ammonia [7,8,9]. The second refers to the selectivity, which is still an unsolved challenge when trying to favor the formation of a particular product in detriment to others [10]. These limitations stem from the same nature of the plasma processes where kinetics control the reaction pathways and thermodynamics is a secondary player in determining the final reaction outputs. Much attention has been paid to the influence of electrical operating conditions (voltage, frequencies, etc.), there is still limited knowledge about the influence of other working parameters, such as the residence time of the reactants, Catalysts 2019, 9, 45; doi:10.3390/catal9010045 www.mdpi.com/journal/catalysts
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