Plasma Radicals as Kinetics-Controlling Species during Plasma-Assisted Catalytic NH₃ Formation: Support from Microkinetic Modeling

Abstract

About 1% of the world’s CO2 emissions are tied to the standard method to produce NH3 (i.e., Haber–Bosch process); hence, there is a need to decarbonize the production of this chemical. To this end, plasma-assisted catalysis is emerging as a “green” alternative to synthesize NH3. However, the insufficient mechanistic understanding of this process has hindered significant improvements in its cost-effectiveness. Here we leverage “minimal plasma” microkinetic models and select experiments in a dielectric-barrier discharge (DBD) plasma reactor to look for missing mechanistic insights. Relatively robust to model assumptions, our modeling supports the thesis that plasma N and H radicals are the kinetics-controlling plasma species for reactions involving the catalyst. This support stems from the realization that only the inclusion of N and H radicals in our models can readily explain key experimental observations for plasma-assisted NH3 synthesis such as the (i) similar catalytic activity for Fe and Ag (two metals at the opposite ends of N2 dissociation capabilities), (ii) activity increase in Fe (a metal that readily dissociates N2) relative to thermal catalysis, and (iii) detection of catalyst-bound N2HY species. We also find the N radicals (a source of surface-bound N*) to be more important in nitrophobic metals and H radicals (a hydrogenating agent via Eley–Rideal reactions) to be more important in nitrophilic metals. On the other hand, other mechanistic aspects such as the kinetic relevance of N2HY-forming pathways and dissolution reactions are discussed as a function of model assumptions. Our modeling suggests that some of these assumptions could be potentially clarified through in situ compositional analysis of catalyst adlayers (e.g., the fraction of radicals from the plasma bulk that reach the catalyst surface), as the adlayer composition seems to be rather sensitive to the plasma environment assumed to be “seen” by the catalyst.