Plasma gas conversion in non-equilibrium conditions

The Gibbs free energy is a valuable thermodynamic potential to predict the spontaneity of reactions and the feasibility of chemical processes. At thermodynamic equilibrium a system's Gibbs free energy reaches its minimum, and its entropy is maximized. At this point, the entropy becomes unavailable for performing useful chemical work. Hence, temperature, <em>T</em>, is generally used to drive chemical reactions, especially in gas conversion processes. However, in systems far from equilibrium, changes in entropy can be harnessed to drive chemical reactions. In a non-thermal plasma, non-equilibrium conditions can be sustained with electron temperature exceeding gas temperature (<em>T<sub>e</sub></em> ≫ <em>T<sub>gas</sub></em>). More importantly, the chemical reaction rates are affected by the energy input into the plasma while the observed temperatures remain constant. Here, a new concept is introduced proposing the effective (average) energy input – which is related to a change in entropy at different (constant) temperatures under non-equilibrium conditions – as the fundamental parameter controlling gas phase reactions. Using this theoretically derived formalism, electron-driven reactions related to <em>T<sub>e</sub></em> can be distinguished from thermally driven reactions by plasma gas heating related to <em>T<sub>gas</sub></em>. This approach can explain observed conversion and efficiency trends, as demonstrated for plasma-based methane pyrolysis, and point towards further efficiency improvements for gas conversion processes under non-equilibrium plasma conditions e.g. as an important source for hydrogen.