Origins of N₂O Selectivity Limits in Catalyzed Ammonia Oxidation

Ammonia (NH₃) oxidation to nitrous oxide (N₂O) is a promising route to obtain this selective oxidant, but controlling product distribution is inherently challenging because N₂O occupies an intermediate nitrogen oxidation state between N₂ and NO. Despite recent advances, leading CeO₂-based catalytic systems have consistently encountered a selectivity limit in the range of 80-85%. Herein, CeO₂-supported Mn single atoms are employed as a stable, selective benchmark to investigate the origins of the N₂O selectivity losses. Thorough kinetic analysis revealed that direct oxidation of NH₃ to N₂ is the main reason for incomplete N₂O selectivity. This reaction dominates in a thin upstream catalyst bed layer, driven by its strong dependence on the NH₃ partial pressure that ensures dense surface coverage by N-containing intermediates and promotes their irreversible coupling to N₂. However, due to the inhibiting effect of H₂O, this reaction is increasingly hindered along the catalyst bed, with N₂O becoming the dominant product. Based on these insights, N₂O selectivity could be increased from 81% to 90% while N₂ selectivity decreased to 6% by water cofeeding and adjusting reactant partial pressures to tune surface coverage by N-containing intermediates. Evaluation of side reactions revealed a negligible impact of N₂O decomposition or N₂O reduction on product distribution. Conversely, employing isotopic tracing, reduction of in situ-formed NO by NH₃ was established as a significant route to secondary N₂O, and to a lesser extent, N₂. This was shown to be a general feature of CeO₂-based catalysts, including Mn, Au, and Cr systems, providing a lever for selectivity control. This work demonstrates how kinetic analysis can disentangle complex reaction pathways and identify both catalyst- and process-level strategies to advance NH₃ oxidation to N₂O beyond current limits.