Are plasma discharges really “catalyst-free”?

Abstract

Many plasma chemistry studies assume that chemical reactions occur either within the volume of plasmas or on heterogeneous catalysts that are distributed in packed beds. This designation overlooks the catalytic role of electrodes that bound the plasma. Beyond sustaining the discharge through electron emission, electrodes can act as chemically active interfaces, undergoing erosion, film growth, and nanoparticle release that influence the reactivity of the plasma phase and surrounding surfaces or interfaces. Literature comparisons show that reactors with catalytically active electrodes often achieve similar or higher performance than packed-bed systems, while reactors with inert boundaries underperform. Here, we perform experiments using a nanosecond-pulsed spark discharge for ammonia synthesis to elucidate how electrodes contribute catalytic reactions in plasma systems. We find that electrode composition strongly affects both yield and energy efficiency: under identical plasma conditions, Ni electrodes produced 0.37% NH3, Cu 0.32%, and W only 0.11%, and on Cu, adjusting the feed ratio from N2:H2 = 1:3 to 1:7 increased NH3 yield sevenfold (0.3% to 2.2%). SEM–EDX and XPS analyses confirm that plasma operation generates Cu/Cu-N nanoparticles that coats surfaces within reactors and induces electrode nitridation, providing dynamic nitrogen storage and release and establishing hidden catalytic cycles. These electrode-driven processes influence yields, selectivity, and mechanistic pathways directly. Recognizing electrodes as catalytic elements is essential for accurate mechanistic interpretation, fair benchmarking, and deliberate design of plasma systems, ensuring that advances in plasma chemistry are based on realistic assessments of surface contributions.