Recently it was proposed that plasma-catalytic NH3 synthesis with excited N2 allows for conversions beyond thermal equilibrium. We show that this is indeed possible with experimental data for Ru catalysts at temperatures above 300°C, resulting in significant thermal activity for NH3 synthesis. The resulting NH3 concentration is determined by competition between, at one hand, dissociative adsorption of ground-state N2 and adsorption of plasma-generated N radical species with subsequent hydrogenation to NH3, and at the other hand, thermal-catalytic decomposition of NH3. At temperatures below 300°C, plasma-catalytic ammonia synthesis is attributed to adsorption of N radicals, generated in the plasma, with subsequent hydrogenation to NH3. These findings imply that catalysts with thermal activity are not suitable for plasma catalysis, aiming at conversion beyond equilibrium, as these also catalyze the reverse decomposition reaction.