From N2 Excitation to NOx Formation at Atmospheric Pressure: Energy Flow Mechanisms of Vibrational and Electronic Excitation Pathways in Plasma Nitrogen Fixation

Abstract Atmospheric-pressure nonequilibrium plasma (APNP) has attracted attention for nitrogen fixation (NF) because it enables N2 activation under mild conditions. Vibrationally excited N2 has been considered a key intermediate for efficient NOx formation, since a large fraction of electron energy can be selectively transferred into N2 vibrational excitation at low reduced electric fields. However, most existing studies have mainly focused on the initial energy transfer from electrons to different excitation channels, while the subsequent energy transfer from excited N2 states to NOx formation has not been quantitatively clarified. In this work, a zero-dimensional plasma chemical model based on ZDPlasKin is used to quantitatively investigate the energy flow from N2 excitation to NOx formation in atmospheric-pressure air (78% N2 and 22% O2) and to compare the fractions of electron energy transferred into N2 vibrational and electronic excitation channels with the fractions ultimately contributing to NOx formation through these pathways. The results show that although most electron energy is transferred into N2 vibrational excitation at low reduced electric fields, only a small fraction contributes to NOx formation. At approximately 40 Td, nearly 95% of the electron energy is deposited into N2 vibrational excitation, whereas less than 4% of the total input energy finally contributes to NOx production. Most vibrational energy is ultimately converted into heat. In contrast, at 100 Td, approximately 35% of the input energy is transferred into N2 electronic excitation, while about 8.9% contributes to NOx. Further analyses of vibrational distribution functions and reaction pathways reveal that the contribution of vibrational excitation to NOx formation strongly depends on the vibrational population distribution, whereas electronically excited N2 gradually becomes the dominant pathway for NOx formation at high reduced electric fields. These results demonstrate that maximizing vibrational excitation alone does not necessarily guarantee high NF efficiency.