Plasma-Catalytic Ammonia Synthesis beyond the Equilibrium Limit

Abstract

We explore the consequences of non-thermal plasma activation on product yields in catalytic ammonia synthesis, a reaction that is equilibrium-limited at elevated temperatures. We employ a minimal microkinetic model that incorporates the influence of plasma activation on N 2 dissociation rates to predict NH 3 yields into and across the equilibrium-limited regime. NH 3 yields are predicted to exceed bulk thermodynamic equilibrium limits on materials that are thermal-rate-limited by N 2 dissociation. In all cases, yields revert to bulk equilibrium at temperatures at which thermal reaction rates exceed plasma-activated ones. Beyond-equilibrium NH 3 yields are observed in a packed bed dielectric-barrier-discharge reactor and exhibit sensitivity to catalytic material choice in a way consistent with model predictions. The approach and results highlight the opportunity to exploit synergies between non-thermal plasmas and catalysts to affect transformations at conditions inaccessible through thermal routes. File list (2) download file view on ChemRxiv Chemrxiv-manuscript.pdf (504.40 KiB) download file view on ChemRxiv supporting-info.pdf (396.20 KiB)

The Catalytic Mechanics of Dynamic Surfaces: Stimulating Methods for Promoting Catalytic Resonance

“…The techniques to stimulate catalyst change are distinct from other reactor technologies that merely supply energy to a reactor. While techniques such as microwave irradiation (131) , plasma (116) , and pulsed heating (132) or pulse pressure (133) can be used continuously or dynamically with catalysts leading to unique, beneficial, reactor behavior, they do not manipulate the catalyst itself and are not the focus of this perspective. Catalyst stimulation exists in three categories related to the general approach of mechanical, electrical, or photochemical perturbations from their resting structures.…”

“…(111,112) A third strategy is the application of work; added work to a system can perturb to a steady state away from equilibrium. (113,114,115,116) A dynamic catalyst surface with oscillating binding energies provides work to adsorbates to move the steady-state reaction away from equilibrium. (52) As depicted in Figure 5a, the simulated A-to-B surface-catalyzed reaction in a batch reactor operating under dynamic conditions approaches a steady state different from equilibrium, independent of the starting composition of the batch reactor.…”

Fundamental Properties and Applications of Dielectric Barrier Discharges in Plasma‐Catalytic Processes at Atmospheric Pressure

“…Promising results were reported using packed-bed DBD reactors, operating at ambient conditions. Moreover, the plasma process allows to surpass the equilibrium limitation of thermal catalysis at elevated temperatures by kinetically trapping ammonia [116]. A wide variety of different catalysts has been applied in plasma-assisted ammonia synthesis, ranging from the semimetal gallium [117] over diamond-like carbon structures [118] and zeolites [119] to traditional ammonia synthesis catalysts such as Ru, Fe and other metals supported on Al 2 O 3 or MgO [117,[120][121][122][123].…”

“…This is followed by sequential hydrogenation by adsorbed hydrogen to NH 3 (Eq. (20)), which involves several elementary steps not shown here [116].…”

Plasma-driven catalysis: green ammonia synthesis with intermittent electricity

“…131,156,159,276,277 Such metals have less ammonia desorption limitations than the classical Fe and Ru catalysts for thermal-catalytic ammonia synthesis. Mehta et al 131,293 proposed that plasma-activation of N 2 via vibrational excitation leads to a lower barrier for N 2 dissociation, resulting in an enhancement for late-transition metals which are typically rate-limited by N 2 dissociation. On the other hand, Wang et al 156 proposed that the introduction of metal nanoparticles on γ-Al 2 O 3 changes the acid site strength, and thereby the ammonia synthesis rate on γ-Al 2 O 3 acid sites.…”