The transition toward a carbon-neutral society and the growing demand for sustainable nitrogen resources have stimulated the development of alternative nitrogen-fixation technologies powered by renewable electricity. In this context, plasma-assisted nitrogen fixation offers a promising route for the direct conversion of N2 and O2 in air into nitrogen oxides (NOx), which can subsequently be converted into nitric acid. However, plasma-generated NOx is typically dominated by NO, requiring an additional oxidation step to convert NO into NO2 for efficient downstream nitric acid production. Considering the economic feasibility and eventual commercialization of such a process, the development of non-noble-metal catalysts capable of replacing conventional noble-metal-based NO oxidation catalysts is highly desirable.
Manganese oxides (MnOx) have been extensively investigated as non-noble oxidation catalysts because of their multiple oxidation states and favorable redox properties. Nevertheless, most previous studies on catalytic NO oxidation have focused on relatively dilute NO concentrations relevant to automotive and industrial exhaust treatment. In contrast, post-plasma gas streams can contain substantially higher NO concentrations, making the direct extrapolation of catalytic behavior established under conventional dilute-NO conditions questionable. Increasing the NO concentration may alter surface NO coverage, competitive interactions between NO and O2, the availability and consumption of reactive oxygen species, and the redox behavior of MnOx, potentially resulting in a distinct kinetic response toward NO oxidation.
In this study, commercial MnOx was employed as a reference material to investigate the concentration-dependent catalytic behavior of MnOx under dilute- and high-NO conditions as a fundamental step toward the development of non-noble catalysts for post-plasma NO oxidation. In addition to NO conversion, reaction rates and their temperature dependence were evaluated to determine how the kinetic response of MnOx evolves with increasing NO concentration. This comparative approach provides insight into whether catalytic characteristics established under conventional dilute-NO conditions remain applicable to the high-NO environment encountered downstream of plasma nitrogen fixation. The resulting understanding provides a basis for the subsequent design of supported MnOx catalysts through controlled Mn oxidation state, dispersion, and cluster structure for efficient post-plasma NO-to-NO2 oxidation.

NO oxidation; MnOx catalyst; High-concentration NO; Reaction kinetics; Plasma nitrogen fixation