Abstract
Ammonia (NH(3)) oxidation to nitrous oxide (N(2)O) is a promising route to obtain this selective oxidant, but controlling product distribution is inherently challenging because N(2)O occupies an intermediate nitrogen oxidation state between N(2) and NO. Despite recent advances, leading CeO(2)-based catalytic systems have consistently encountered a selectivity limit in the range of 80-85%. Herein, CeO(2)-supported Mn single atoms are employed as a stable, selective benchmark to investigate the origins of the N(2)O selectivity losses. Thorough kinetic analysis revealed that direct oxidation of NH(3) to N(2) is the main reason for incomplete N(2)O selectivity. This reaction dominates in a thin upstream catalyst bed layer, driven by its strong dependence on the NH(3) partial pressure that ensures dense surface coverage by N-containing intermediates and promotes their irreversible coupling to N(2). However, due to the inhibiting effect of H(2)O, this reaction is increasingly hindered along the catalyst bed, with N(2)O becoming the dominant product. Based on these insights, N(2)O selectivity could be increased from 81% to 90% while N(2) 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 N(2)O decomposition or N(2)O reduction on product distribution. Conversely, employing isotopic tracing, reduction of in situ-formed NO by NH(3) was established as a significant route to secondary N(2)O, and to a lesser extent, N(2). This was shown to be a general feature of CeO(2)-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 NH(3) oxidation to N(2)O beyond current limits.