Plasma-Catalysis of Nonoxidative Methane Coupling: A Dynamic Investigation of Plasma and Surface Microkinetics over Ni(111)

Plasma Catalysis Modeling: How Ideal Is Atomic Hydrogen for Eley-Rideal?

Plasma catalysis is an emerging technology, but a lot of questions about the underlying surface mechanisms remain unanswered. One of these questions is how important Eley-Rideal (ER) reactions are, next to Langmuir-Hinshelwood reactions. Most plasma catalysis kinetic models predict ER reactions to be important and sometimes even vital for the surface chemistry. In this work, we take a critical look at how ER reactions involving H radicals are incorporated in kinetic models describing CO2 hydrogenation and NH3 synthesis. To this end, we construct potential energy surface (PES) intersections, similar to elbow plots constructed for dissociative chemisorption. The results of the PES intersections are in agreement with ab initio molecular dynamics (AIMD) findings in literature while being computationally much cheaper. We find that, for the reactions studied here, adsorption is more probable than a reaction via the hot atom (HA) mechanism, which in turn is more probable than a reaction via the ER mechanism. We also conclude that kinetic models of plasma-catalytic systems tend to overestimate the importance of ER reactions. Furthermore, as opposed to what is often assumed in kinetic models, the choice of catalyst will influence the ER reaction probability. Overall, the description of ER reactions is too much "ideal" in models. Based on our findings, we make a number of recommendations on how to incorporate ER reactions in kinetic models to avoid overestimation of their importance.

Technical Challenges and Prospects in Sustainable Plasma Catalytic Ammonia Production from Methane and Nitrogen

Ammonia is crucial for human life as an important ingredient for fertilizer, industrial and household chemicals, and is considered as a future fuel alternative and hydrogen storage molecule. There remain no viable alternatives to the energy-and capital-intensive Haber-Bosch (H-B) process. Efforts in the development of novel catalytic processes operated at milder conditions (low temperatures and ambient pressure), prominently electrochemistry and non-thermal plasma (NTP), and utilization of lower-cost H sources for ammonia formation than the ultrapure H₂ have been witnessed in the last few years. Yet, limited progress from these routes has been made to date given unresolved low ammonia yield and technical challenges. Several rare works attempted to activate methane (CH₄ ) and nitrogen (N₂ ) by non-thermal plasma to produce ammonia and valued-added hydrocarbons have proven to be a promising research direction, rivalling the reaction between N₂ and ultrapure H₂ or water. The direct conversion of CH₄ and N₂ to ammonia is still at the beginning level, and it remains unclear that what extent these technologies must be improved to develop a commercial process. Toward this goal, this Perspective critiques current steps and miss-steps of sustainable plasma catalytic ammonia production from CH₄ and N₂ in terms of technology, plasma-catalyst synergy, mechanistic insights, and experimental protocols. We discuss mechanistic understandings of catalyst-promoted ammonia production and translate such discussions as well as key metrics achieved in the field into recommendations of feasible processes for ammonia and value-added hydrocarbons formation from CH₄ and N₂ .