Plasma-Catalytic Ammonia Synthesis in a DBD Plasma: Role of Microdischarges and Their Afterglows

Abstract

Plasma-catalytic ammonia synthesis is receiving ever increasing attention, especially in packed bed dielectric barrier discharge (DBD) reactors. The latter typically operate in the filamentary regime when used for gas conversion applications. While DBDs are in principle well understood and already applied in the industry, the incorporation of packing materials and catalytic surfaces considerably adds to the complexity of the plasma physics and chemistry governing the ammonia formation. We employ a plasma kinetics model to gain insights into the ammonia formation mechanisms, paying special attention to the role of filamentary microdischarges and their afterglows. During the microdischarges, the synthesized ammonia is actually decomposed, but the radicals created upon electron impact dissociation of N 2 and H 2 and the subsequent catalytic reactions cause a net ammonia gain in the afterglows of the microdischarges. Under our plasma conditions, electron impact dissociation of N 2 in the gas phase followed by the adsorption of N atoms is identified as a rate-limiting step, instead of dissociative adsorption of N 2 on the catalyst surface. Both elementary Eley−Rideal and Langmuir−Hinshelwood reaction steps can be found important in plasma-catalytic NH 3 synthesis.

Plasma-catalytic ammonia synthesis beyond thermal equilibrium on Ru-based catalysts in non-thermal plasma

“…At higher plasma power, however, ammonia may decompose in micro-discharges. 55,61,62 Fig. 8 shows good correlations between the NH 3 concentration measured at 450°C and plasma power.…”

“…However, the density of NH is orders of magnitude lower than that of plasmaactivated N 2 . 55 This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.…”

“…Secondly, an Eley-Rideal type of reaction, e.g. N + H ads → NH ads , without adsorbing the N radical first, may contribute, as proposed by Engelmann et al 30,55 with DFT calculations and Yamijala et al 57 with ab initio calculations.…”

“…The fact that NH 3 forms in the plasma phase or on the reactor wall implies that N, H, and NH X radials are present in the plasma, as previously reported by various authors. 55,56 NH 3 synthesis is catalyzed on the Ru metal in the presence of a plasma when operating above the apparent onset hydrogenation of N ads and subsequent NH 3 desorption from the Ru/MgO and Ru-K/MgO catalysts, 175°C and 125°C, respectively (see Fig. 3).…”

Al2O3-Supported Transition Metals for Plasma-Catalytic NH3 Synthesis in a DBD Plasma: Metal Activity and Insights into Mechanisms

“…Plasma catalysis (PC) for NH 3 production is of particular interest because of the synergy between the two techniques [25,26]: plasma-enabled excitation and dissociation of N 2 molecules allows NH 3 yields beyond values attainable by thermal catalysis [27]. Dielectric barrier discharge (DBD) plasma reactors are the most common choice for PC NH 3 synthesis [28]. DBD plasma is a type of non-thermal plasma, in which the electrons are at high temperature (10 4 -10 5 K), while the gas remains at 300-1000 K [13].…”