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

In light of the increasing relevance of alternative energy carriers, the established domain of emission control is confronted with novel challenges. Among these carriers, ammonia (NH$_3$) has emerged as particularly promising due to its high energy density and carbon-free nature. This study focuses on the selective catalytic oxidation (SCO) of NH$_3$ to elemental nitrogen (N$_2$) under lean conditions. While conventional platinum-based catalysts exhibit high activity, they suffer from poor N$_2$ selectivity and significant formation of nitrous oxide (N$_2$O), a potent greenhouse gas. As an alternative, palladium-based catalysts are investigated due to their inherently higher selectivity toward N$_2$. Although palladium exhibits limited activity under static feed conditions, its performance is markedly enhanced under forced dynamic operation (FDO), wherein short, intermittent reducing pulses are introduced through oxygen cut-off. This operational mode not only boosts catalytic activity beyond that of platinum but also preserves palladium’s superior selectivity. Operando time-resolved X-ray absorption spectroscopy (XAS) and ex situ Raman spectroscopic analysis of spent catalyst samples indicate that FDO promotes the formation of metallic palladium, which exhibits higher activity compared to palladium oxide that is the predominant phase under lean conditions. ... mehrOperando XAS uncovers 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 300 °C 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.