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Plasma-assisted catalysis is an emerging technology for the atmospheric pressure and low bulk gas temperature synthesis of ammonia from molecular nitrogen and hydrogen. Direct evidence for plasma-i...

Ammonia is a critically important industrial chemical and is largely responsible for sustaining the growing global population. To provide ammonia to underdeveloped regions and/or regions far from industrial production hubs, modular systems have been targeted and often involve unconventional production methodologies. These novel approaches for ammonia production can tap renewable resources at smaller scales located at the point of use, while decreasing the CO2 footprint. Plasma-assisted catalysis and electrochemical ammonia synthesis have promise owing to their atmospheric pressure and low-temperature operation conditions and the ability to construct units at scales desired for modularization. Fundamental and applied studies are underway to assess these processes, although many unknowns remain. In this review, we discuss recent developments and opportunities for unconventional ammonia synthesis with a focus on plasma-stimulated systems.

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 N2 dissociation rates to predict NH3 yields into and across the equilibrium-limited regime. NH3 yields are predicted to exceed bulk thermodynamic equilibrium limits on materials that are thermal-rate-limited by N2 dissociation. In all cases, yields revert to bulk equilibrium at temperatures at which thermal reaction rates exceed plasma-activated ones. Beyond-equilibrium NH3 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.

Plasma-assisted catalysis is the process of electrically activating gases in the plasma-phase at low temperatures and ambient pressure to drive reactions on catalyst surfaces. Plasma-assisted catalytic processes combine conventional heterogeneous surface reactions, homogeneous plasma phase reactions, and coupling between plasma-generated species and the catalyst surface. Herein, we perform kinetically controlled ammonia synthesis measurements in a dielectric barrier discharge (DBD) plasma-assisted catalytic reactor. We decouple contributions due to plasma phase reactions from the overall plasma-assisted catalytic kinetics by performing plasma-only experiments. By varying the gas composition, temperature, and discharge power, we probe how macroscopic reaction conditions affect plasma-assisted ammonia synthesis on three different γ-alumina-supported transition metal catalysts (Ru, Co, and Ni). Our experiments indicate that the overall reaction and plasma-phase reaction are first-order in both N2 and H2. In ...

Nonthermal plasma-driven catalysis is an emerging subfield of heterogeneous catalysis that is particularly promising for the chemical transformation of hard-to-activate molecules (e.g., N2, CO2, CH4). In this Review, we illustrate this promise of plasma-enhanced catalysis, focusing on the ammonia synthesis and methane dry reforming reactions, two reactions that have received wide attention and that illustrate the potential for plasma excitations to mitigate kinetic and thermodynamic obstacles to chemical conversions. We highlight how plasma activation of reactants can provide access to overall reaction rates, conversions, product yields, and/or product distributions unattainable by thermal catalysis at similar temperatures and pressures. Particular emphasis is given to efforts aimed at discerning the underlying mechanisms at play in these systems. We discuss opportunities for and challenges to the advancement of the field.

Correlations between the energies of elementary steps limit the rates of thermally catalysed reactions at surfaces. Here, we show how these limitations can be circumvented in ammonia synthesis by coupling catalysts to a non-thermal plasma. We postulate that plasma-induced vibrational excitations in N2 decrease dissociation barriers without influencing subsequent reaction steps. We develop a density-functional-theory-based microkinetic model to incorporate this effect, and parameterize the model using N2 vibrational excitations observed in a dielectric-barrier-discharge plasma. We predict plasma enhancement to be particularly great on metals that bind nitrogen too weakly to be active thermally. Ammonia synthesis rates observed in a dielectric-barrier-discharge plasma reactor are consistent with predicted enhancements and predicted changes in the optimal metal catalyst. The results provide guidance for optimizing catalysts for application with plasmas. Plasma catalysis holds promise for overcoming the limitations of conventional catalysis. Now, a kinetic model for ammonia synthesis is reported to predict optimal catalysts for use with plasmas. Reactor measurements at near-ambient conditions confirm the predicted catalytic rates, which are similar to those obtained in the Haber–Bosch process.

Ammonia, a precursor for fertilizers, is crucial to feed the world's population but also it has the potential to be used as an alternative fuel or as a chemical store for renewable energy technologies. The most common process of ammonia production is the Haber-Bosh (H-B) process, a well-optimized method that requires high temperature (~ 700 K) and pressure (~ 1 00 atm) to operate under equilibrium conditions (using thermal catalysis). Alternatively, ammonia can be produced less efficiently close to standard conditions by combining non-thermal plasmas and catalysts (plasma catalysis). Unlike thermal catalysis, in plasma catalysis the non-equilibrium state of the plasma $(T_{electrons}\gg T_{ions},T_{neutrals}$ produces reactive species, such as excited species, that may play an important role in the production of ammonia. The interaction between the plasma and the catalyst can be characterized in two categories: the effect of the plasma on catalysis and the effect of the catalyst on the plasma state. This work focuses on the latter. We use a laboratory-scale, packed bed, dielectric barrier discharge (DBD) reactor to investigate the effects of different supported metal catalysts on the plasma. Optical emission spectroscopy (OES) and electrical measurements are used to estimate various system parameters, including rotational and vibrational temperatures and electron densities, for various supported catalyst configurations (e.g., alumina supported nickel). These parameters are correlated with measurements of ammonia synthesis under identical conditions to assess whether there are significant differences in the plasma under conditions where ammonia synthesis is enhanced.

Plasmas can enhance the selectivity, activity, and product yields for many chemical reactions. In the presence of a catalyst, synergy between the catalyst and the plasma exists to assist in the activation of reactants to promote the chemical transformation. We have been particularly interested in the use of non-thermal plasmas for the catalytic production of ammonia from nitrogen/hydrogen gas mixtures. The industrial Haber-Bosch process for ammonia synthesis from nitrogen and hydrogen is carried out at high pressures (100-200 bar) and temperatures (400-500 °C). N2 dissociation is the fundamental barrier for this reaction, and plasma-catalysis has been suggested as viable way to activate N2 under ambient conditions. We have demonstrated that is possible to produce ammonia at atmospheric pressure and temperatures between 100 - 200 °C by coupling the catalyst with a non-thermal plasma. It is speculated that the plasma assists in activating the source gas by generating reactive species, such as vibrationally or electronically excited N2*, ions, and radicals, which interact on the catalyst surface to produce ammonia. In this presentation, we will discuss our experimental results from varying the gas composition, bulk gas temperature, plasma input power, and reactor space velocity and how these conditions influence the catalyst-plasma interactions using a variety of catalysts (Fe/Al2O3, Ru/Al2O3, Pt/Al2O3, Ni/Al2O3, and Co/Al2O3).