Plasma Radicals as Kinetics-Controlling Species during Plasma-Assisted Catalytic NH <sub>3</sub> Formation: Support from Microkinetic Modeling
Tsung-Wei Liu, Fnu Gorky, Maria L. Carreon, Diego A. Gómez‐Gualdrón
Abstract
About 1% of the world’s CO 2 emissions are tied to the standard method to produce NH 3 (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 NH 3 . 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 NH 3 synthesis such as the (i) similar catalytic activity for Fe and Ag (two metals at the opposite ends of N 2 dissociation capabilities), (ii) activity increase in Fe (a metal that readily dissociates N 2 ) relative to thermal catalysis, and (iii) detection of catalyst-bound N 2 H Y 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 N 2 H Y -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.