Ammonia is one of the most produced chemicals, mainly synthesized from fossil fuels for fertilizer applications. Furthermore, ammonia may be one of the energy carriers of the future, when it...

Plasma catalysis is an emerging new technology for the electrification and downscaling of NH3 synthesis. Increasing attention is being paid to the optimization of plasma catalysis with respect to the plasma conditions, the catalyst material, and their mutual interaction. In this work we use microkinetic models to study how the total conversion process is impacted by the combination of different plasma conditions and transition metal catalysts. We study how plasma-generated radicals and vibrationally excited N2 (present in a dielectric barrier discharge plasma) interact with the catalyst and impact the NH3 turnover frequencies (TOFs). Both filamentary and uniform plasmas are studied, based on plasma chemistry models that provided plasma phase speciation and vibrational distribution functions. The Langmuir–Hinshelwood reaction rate coefficients (i.e., adsorption reactions and subsequent reactions among adsorbates) are determined using conventional scaling relations. An additional set of Eley–Rideal reactions (i.e., direct reactions of plasma radicals with adsorbates) was added and a sensitivity analysis on the assumed reaction rate coefficients was performed. We first show the impact of different vibrational distribution functions on the catalytic dissociation of N2 and subsequent production of NH3, and we gradually include more radical reactions, to illustrate the contribution of these species and their corresponding reaction pathways. Analysis over a large range of catalysts indicates that different transition metals (metals such as Rh, Ni, Pt, and Pd) optimize the NH3TOFs depending on the population of the vibrational levels of N2. At higher concentrations of plasma-generated radicals, the NH3 TOFs become less dependent on the catalyst material, due to radical adsorptions on the more noble catalysts and Eley–Rideal reactions on the less noble catalysts.

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)

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.

The combination of catalysis and nonthermal plasma holds promise for enabling difficult chemical conversions. The possible synergy between both depends strongly on the nature of the reactive plasma species and the catalyst material. In this paper, we show how vibrationally excited species and plasma-generated radicals interact with transition metal catalysts and how changing the catalyst material can improve the conversion rates and product selectivity. We developed a microkinetic model to investigate the impact of vibrational excitations and plasma-generated radicals on the nonoxidative coupling of methane over transition metal surfaces. We predict a significant increase in ethylene formation for vibrationally excited methane. Plasma-generated radicals have a stronger impact on the turnover frequencies with high selectivity toward ethylene on noble catalysts and mixed selectivity on non-noble catalysts. In general, we show how the optimal catalyst material depends on the desired products as well as the plasma conditions.

N2 fixation into NH3 is one of the main processes in the chemical industry. Plasma catalysis is among the environmentally friendly alternatives to the industrial energy-intensive Haber-Bosch process. However, many questions remain open, such as the applicability of the conventional catalytic knowledge to plasma. In this work, we studied the performance of Al2O3-supported Fe, Ru, Co and Cu catalysts in plasma-catalytic NH3 synthesis in a DBD reactor. We investigated the effects of different active metals, and different ratios of the feed gas components, on the concentration and production rate of NH3, and the energy consumption of the plasma system. The results show that the trend of the metal activity (common for thermal catalysis) does not appear in the case of plasma catalysis: here, all metals exhibited similar performance. These findings are in good agreement with our recently published microkinetic model. This highlights the virtual independence of NH3 production on the metal catalyst material, thus validating the model and indicating the potential contribution of radical adsorption and Eley-Rideal reactions to the plasma-catalytic mechanism of NH3 synthesis.

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.

We developed a microkinetic model to reveal the effects of plasma-generated radicals, intermediates, and vibrationally excited species on the catalytic hydrogenation of CO 2 to CH 3 OH on a Cu(111) surface. As a benchmark, we first present the mechanisms of thermal catalytic CH 3 OH formation. Our model predicts that the reverse water-gas shift reaction followed by CO hydrogenation, together with the formate path, mainly contribute to CH 3 OH formation in thermal catalysis. Adding plasma-generated radicals and intermediates results in a higher CH 3 OH turnover frequency (TOF) by six to seven orders of magnitude, showing the potential of plasma-catalytic CO 2 hydrogenation into CH 3 OH, in accordance with the literature. In addition, CO 2 vibrational excitation further increases the CH 3 OH TOF, but the effect is limited due to relatively low vibrational temperatures under typical plasma catalysis conditions. The predicted increase in CH 3 OH formation by plasma catalysis is mainly attributed to the increased importance of the formate path. In addition, the conversion of plasma-generated CO to HCO* and subsequent HCOO* or H 2 CO* formation contribute to CH 3 OH formation. Both pathways bypass the HCOO* formation from CO 2 , which is the main bottleneck in the process. Hence, our model points toward the important role of CO, but also O, OH, and H radicals, as they influence the reactions that consume CO 2 and CO. In addition, our model reveals that the H pressure should not be smaller than ca. half of the O pressure in the plasma as this would cause O* poisoning, which would result in very small product TOFs. Thus, plasma conditions should be targeted with a high CO and H content as this is favorable for CH 3 OH formation, while the O content should be minimized.