Mechanisms of Oxide-Catalyst-Assisted Nitrogen Fixation via \(\text {NO}_x\) Formation in Glow Discharges

Abstract

Coupling oxide catalysts with plasma discharges has emerged as a highly effective strategy to enhance the efficiency of \(\text {NO}_x\)-based nitrogen fixation. However, the fundamental mechanisms by which oxide catalysts promote \(\text {NO}_x\) formation in plasma-assisted processes remain poorly understood. In this paper, several common oxides were investigated to evaluate their influence on \(\text {NO}_x\) formation efficiency under atmospheric DC glow plasma. The term “catalyst” is used here in a broader sense to describe oxides that participate in plasma-assisted surface reactions, rather than conventional promotion of \(\text {NO}_x\) formation. By combining XPS, FTIR, and TPD characterizations, the role of oxide catalysts in plasma \(\text {NO}_x\) production was explored. All oxide catalysts were found to enhance \(\text {NO}_x\) production, with the maximum production rate increasing by up to 25%. In parallel, the lowest energy cost decreased by around 10%, reaching a minimum of 4.5 MJ/mol. Notably, \(\text {NO}_2\) production is more significantly affected by oxides, accounting for up to 90% of the overall increase in \(\text {NO}_x\) production. XPS and FTIR analyses reveal the formation of nitrate (\(\mathrm {NO_3^-}\)) or nitrite (\(\mathrm {NO_2^-}\)) on all oxides except \(\text {SiO}_2\) after discharge, while TPD results confirm the \(\text {NO}_x\) adsorption and storage capabilities for all oxides used in this experiment. Low-oxygen concentration discharge experiments indicate that direct \(\text {NO}_x\) formation on oxide surfaces is negligible, whereas the \(\text {NO}_2\) production varies significantly among different oxides. These findings suggest a possible “\(\text {NO}_x\) adsorption–oxidation–desorption” cycle occurring on oxide catalysts during plasma-assisted \(\text {NO}_x\) formation. In particular, the catalytic oxidation of NO to \(\text {NO}_2\) is proposed to facilitate the progression of nitrogen fixation and contribute to improved energy efficiency. Further thermal desorption analysis of CuO after discharge and TPD results demonstrate that effective \(\text {NO}_x\) desorption is a key step in the enhancement provided by oxides. Although elevated temperatures favor \(\text {NO}_x\) desorption, they also suppress NO oxidation, thereby limiting the improvement in \(\text {NO}_x\) formation efficiency by oxides.