The Selectivity-Conversion Tradeoff in Partial Methane Oxidation Using Non-Equilibrium Plasmas

The direct oxidation of methane to methanol (DOMtM) is a utilization pathway that enables feasible conversion at low temperatures, pressures, and over distributed scales. However, the poor selectivity toward methanol, and unwanted formation of CO and CO₂, continue to limit its broader adoption. Much like catalytic pathways, plasmas have been used extensively with DOMtM to reduce activation barriers and drive methane conversion but are constrained by their selectivity toward desired products. In this work, we show the influence of product breakdown, transport effects, and non-equilibrium excitations on the selectivity and conversion of DOMtM. We develop scaling laws to predict plasma processes using a selectivity-conversion power law that depends on the specific energy input (SEI) of the process. Unlike conventional thermal equilibrium processes, we show how a spark discharge can break selectivity-conversion thermodynamic constraints. Plasma gives a degree of freedom to vary conversion independently. Using a spark discharge and a fixed SEI, we measure a 500% increase in conversion with the same selectivity by varying the applied electric field. Although we are trading off energy efficiency to achieve this, the method allows to alter the product production dynamically whenever required. Transport effects including diffusion, convection, and reaction kinetics are shown to enable dynamic control in product distributions including H₂, methanol, formic acid, and formaldehyde.