Siobhan Brown

Post‐microwave plasma catalysis combines plasma preactivation of the gas phase followed by a fixed bed catalyst. This process has the potential to be energy efficient, modular, versatile and to generate high numbers of active species compared with other plasma generation technologies. Specific applications of these post‐microwave plasma systems include the dissociation and activation of stable molecules and the modification of catalyst materials. This concept article will address the current findings in microwave plasma catalysis research and propose future questions that need to be addressed to further develop the technology.

This chapter introduces the applications of microwave energy combined with heterogeneous catalysis for the conversion of stable molecules, such as CH4, N2 and CO2, to value added chemicals. Experimental results show that methane and N2 can be simultaneously activated in a single-stage microwave-heated reactor to form ammonia, ethylene, and acetylene at atmospheric pressure. Microwave irradiation can also induce oxidative dehydrogenation of methane and ethane using CO2 as a mild oxidant. The chapter illustrates that microwave catalysis as a transformational technology has the potential to increase energy efficiency across various industries and decarbonize high GHG-emitting industrial subsectors including ammonia, ethylene and BTX.

Ammonia is emerging as a potential decarbonized H₂ energy carrier when produced from renewable energy. The on-site production of liquid ammonia from stranded renewable energy can solve the current energy transportation challenges. The employment of microwave technology can produce the desired ammonia product at milder conditions with the supply of intermittent renewable energy sources. Our previous studies have indicated that the Cs-Ru/CeO₂ catalyst is a promising catalyst for microwave-driven ammonia synthesis. In this study, the Cs-Ru/CeO₂ catalyst mechanically mixed with carbon nanotubes (CNT) and chemically synthesized using coprecipitation and a hydrothermal method is investigated systematically at low temperatures and atmospheric pressure for microwave-assisted ammonia synthesis. Additionally, the combination of two Ru-based catalysts (Cs-Ru/CeO₂ and Cs-Ru/CNT) is studied as well. Mechanical mixing of Cs-Ru/CeO₂ with CNT exhibited superior activity as compared to the chemically synthesized Cs-Ru/CeO₂-CNT catalyst. Besides the enhancement in dielectric property, the probable synergistic effect leads to increased interfacial polarization at the interface of the mechanically mixed catalyst, improving the overall heating and ammonia production rate. Moreover, the combined Ru-based catalyst also exhibited higher activity as compared to their individual activity toward ammonia synthesis. Numerous characterization techniques were performed, including thermal imaging camera and dielectric measurements, to better understand microwave interaction with the composite catalysts.

A novel reactor methodology was developed for chemical looping ammonia synthesis processes using microwave plasma for pre-activation of the stable dinitrogen molecule before reaching the catalyst surface. Microwave plasma-enhanced reactions benefit from higher production of activated species, modularity, quick startup, and lower voltage input than competing plasma-catalysis technologies. Simple, economical, and environmentally benign metallic iron catalysts were used in a cyclical atmospheric pressure synthesis of ammonia. Rates of up to 420.9 μmol min^-1 g^-1 were observed under mild nitriding conditions. Reaction studies showed that both surface-mediated and bulk-mediated reaction domains were found to exist depending on the time under plasma treatment. The associated density functional theory (DFT) calculations indicated that a higher temperature promoted more nitrogen species in the bulk of iron catalysts but the equilibrium limited the nitrogen converion to ammonia, and vice versa. Generation of vibrationally active N₂ and, N₂⁺ ions is associated with lower bulk nitridation temperatures and increased nitrogen contents versus thermal-only systems. Additionally, the kinetics of other transition metal chemical looping ammonia synthesis catalysts (Mn and CoMo) were evaluated by high-resolution time-on-stream kinetic analysis and optical plasma characterization. This study sheds new light on phenomena arising in transient nitrogen storage, kinetics, effect of plasma treatment, apparent activation energies, and rate-limiting reaction steps.

The main challenge in simultaneous conversion of methane and nitrogen to ethylene and ammonia under plasma conditions is the low selectivity. This is largely due to the difficulties in controlling the reactions among the plasma-excited reactive intermediates. To address this challenge, we explored an innovative strategy – post-plasma catalysis outside a microwave-enhanced plasma zone (MWP). Experimentally, the post-plasma species, such as CH_x, NH_x, CN, and C₂H_x, from a feed-gas mixture of methane, nitrogen and argon were identified using an optical emission spectrometer. In the absence of a catalyst but under plasma conditions, methane and nitrogen could only convert to hydrogen cyanide, acetylene, and hydrogen. After placing the thermally heated Ag–Pd/CeO₂ catalyst in the post plasma region, the selectivity to hydrogen cyanide was reduced by 9%, while the active nitrogen-based species simultaneously generated a steady amount of ammonia (selectivity = 9%). The selectivity to ethylene increased from 3% to 37%, the highest among the C₂ products. Meanwhile under conventional thermal heating, in the absence of plasma, the same catalyst over-hydrogenated acetylene to ethane and no ammonia was produced from nitrogen. Theoretically, density functional theory calculations (DFT) determined that the post-plasma species-induced positive charge over the Pd–Ag catalytic surface could enhance the energetics of forming ethylene but suppress the energetics of its further hydrogenation to ethane. Overall, this combined experimental and theoretical study advanced the fundamental understanding of the effects of post-plasma species on optimizing the selectivity in heterogeneous catalysis.

Chemical looping ammonia synthesis (CLAS) materials were subjected to a three cycle ammonia synthesis test under both conventional thermal heating and microwave heating. Microwave heating was found to outperform conventional heating for the first cycle on both Fe and CoMo materials. Principles of microwave catalysis and the heating of metallic particles may be generalized from this study; the dielectric loss tangent for optimum heating must fall within a realtively small range, ∼0.8–1.4, and the penetration depth of the microwave into the particle must be considered and optimized for efficient heating, penetration depth should be on the same order as the particle size. The active phases of both catalysts, Fe₄N and Co₃Mo₃N, were found to be responsive to microwave irradiation. Finally, the deactivation and regeneration of the materials was studied by examining the BET surface areas by reforming surfaces with a low concentration gas phase oxidation reaction. This study sheds light on more general principles of microwave catalysis and on the scale-up of CLAS reactions.