Experimental investigation of NO reduction by H2 on Pd using planar laser-induced fluorescence

This study investigates the NO reduction by H${}_2$ on a Pd/Al${}_2$O${}_3$ catalyst in a temperature range of 100–300 °C and NO/H${}_2$ ratios from 0.5–2, aiming to gain a deeper understanding of the reaction kinetics and its interaction with mass transfer. Planar laser-induced fluorescence (PLIF) is used to visualize the NO distributions over the catalyst, supplemented by end-of-pipe gas analysis of other components. The reduced Pd-based catalyst undergoes a slow deactivation after exposure to the reactive flow, leading to reduced overall NO conversion and decreased selectivity towards N${}_2$. The NO-PLIF measurements are only conducted on the reduced catalyst without considering the temporal evolutions. Despite the overall NO conversion varying only around 50%–65% across all the investigated conditions, the spatially resolved NO distributions reveal three distinct regimes that limit the overall NO conversion: the regime governed by intrinsic reaction rates, the regime constrained by H${}_2$ availability, and the regime restricted by NO diffusion. These findings, demonstrating the interaction between reaction kinetics and mass transfer over a heterogeneous catalyst, highlight the significance of analyzing spatially resolved concentration distributions obtained through PLIF measurements. This approach complements the conventional end-of-pipe analysis, offering a more comprehensive understanding of the underlying processes.