In this work, we demonstrate plasma-catalytic synthesis of hydrogen and acrylonitrile (AN) from CH4 and N2. The process involves two steps: 1) plasma synthesis of C2H2 and HCN in a nominally 1:1 stoichiometric ratio with high yield up to 90% and high methane conversion > 90%; and 2) downstream thermocatalytic reaction of these intermediates to make AN. The effect of process parameters on product distributions and specific energy requirements are reported. If the catalytic conversion of C2H2 and HCN in the downstream thermocatalytic step to AN were perfect, which will require further improvements in the thermocatalytic reactor, then at the maximum output of our 1 kW radiofrequency 13.56 MHz transformer, a specific energy requirement of 73 kWh kgAN-1was determined. The expectation is that scaling up the process to higher throughputs would result in decreases in specific energy requirement into the predicted economically viable range less than 10 kWh kgAN-1.

In this work, stationary states in nonequilibrium plasmas of chemical reactions that can produce hydrogen are explored, namely the water splitting and water gas shift reactions. For both reactions, the effluent from the reactor at long gas residence times in the plasma was found to be independent of the influent speciation. In other words, feeding the reactor either 0.1 H2O or 0.1H2+0.05O2 by mole produced the same effluent composition, and similarly, feeding the reactor 0.1CO+0.1H2O produced nominally the same effluent as 0.1CO2+0.1H2. For both reactions, the effluent from the plasma was found to be very far from local equilibrium at the total pressure and background temperature of the reactor. An important conclusion of this work is that special attention must be paid to the recombination zone in plasma chemical processes. The recombination zone tends to drive the gas composition from plasma stationary states back towards local equilibrium.

In this work, methods based upon nonequilibrium thermodynamics are elucidated to predict stationary states of chemical reactions in nonequilibrium plasma, and limits for energy conversion efficiency. Two example reactions are used: CO2 splitting and NH3 synthesis, with emphasis on CO2 splitting. Expectations from the theoretical framework are compared to experimental results for both reactions, and reasonable agreement is obtained. The conclusion is that the probability of observing either reactants or products increases with the amount of energy dissipated by that side of the reaction as heat through collisions with hot electrons. The side of the reaction that dissipates more energy as heat has a higher probability of occurrence. Furthermore, endergonic chemical reactions in nonequilibrium plasma, such as CO2 splitting at low temperature, require an intrinsic energy dissipation to satisfy the 2nd law of thermodynamics – a sufficient and necessary waste. This intrinsic dissipation limits the maximum theoretical energy conversion efficiency