Performance enhancement of Pd-based selective ammonia oxidation catalysts through forced dynamic operation

This operational mode not only boosts catalytic activity beyond that of platinum but also preserves palladium’s superior selectivity. \textit{Operando} time-resolved X-ray absorption spectroscopy (XAS) and \textit{ex situ} Raman spectroscopic analysis of spent catalyst samples indicates that FDO promotes the formation of metallic palladium, which exhibits higher activity compared to palladium oxide that is the predominant phase under lean conditions. \textit{Operando} XAS uncover that a finite fraction of metallic palladium is retained at the catalyst inlet, enhancing low‑temperature activity and likely contributing to improved long‑term stability as long as \qty{300}{\degreeCelsius} is not exceeded for extended periods.

These findings correspond directly to axially resolved concentration profiles of gas phase species, which reveal the development of a highly active front zone during FDO, where the majority of NH$_3$ conversion occurs selectively to N$_2$. This phenomenon is observed under both dry and humid conditions, underscoring the robustness of the approach. The results suggest that FDO enables the use of significantly shorter monolithic converters for selective NH$_3$ oxidation, i.e., a reduction of length by up to 50\% in realistic humid conditions, thereby offering substantial reductions in both reactor volume and noble metal demand for practical applications.