Vibrationally Excited Activation of N2 in Plasma-Enhanced Catalytic Ammonia Synthesis: A Kinetic Analysis

Abstract

Plasma-enhanced catalytic ammonia synthesis has been proposed as an alternative pathway for green nitrogen fixation in the case of medium- and small-scale operation. Recently, Mehta et al. [ Nat. Catal. 2018, 1 (4), 269-275 ] postulated that plasma-induced vibrational excitations of N2 decrease the dissociation barrier, without influencing the subsequent hydrogenation reactions and ammonia desorption at atmospheric conditions. In this paper, this postulation is substantiated with experimental data of unpromoted and promoted, alumina-supported ruthenium ammonia synthesis catalysts. Within the temperature regime for plasma-enhanced catalytic ammonia synthesis over ruthenium-based catalysts (>200 °C), synergy is experimentally observed between the catalyst and the plasma by a lowered apparent activation energy. While the apparent activation energy for thermal-catalytic ammonia synthesis typically ranges from ∼60 to ∼115 kJ mol-1 depending on the promoters, the apparent activation energy for plasma-enhanced catalytic ammonia synthesis ranges from ∼20 to ∼40 kJ mol-1, consistent with the hypothesis that ammonia synthesis is enhanced via plasma-induced vibrational excitations of N2. Further support follows from the observation that the effects of promoters and supports on activity are similar for thermal catalysis and plasma-enhanced catalysis. As promoter and support influence activity via enhancing dissociation of N2, it follows that breaking the N-N bond is still relevant in plasma-enhanced catalytic ammonia synthesis.