# Origins of N2O Selectivity Limits in Catalyzed Ammonia Oxidation

## TL;DR

This paper explains why ammonia oxidation to nitrous oxide (N2O) has a selectivity limit and shows how adjusting reaction conditions can improve N2O production.

## Contribution

The study identifies direct NH3-to-N2 conversion as the main cause of N2O selectivity loss and proposes strategies to enhance selectivity.

## Key findings

- Direct oxidation of NH3 to N2 is the main reason for incomplete N2O selectivity.
- Water cofeeding and adjusting reactant partial pressures increased N2O selectivity from 81% to 90%.
- Reduction of in situ-formed NO by NH3 is a significant route to secondary N2O.

## Abstract

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.