Spatial Profiling of a Pd/AlO Catalyst during Selective Ammonia Oxidation.

The utilization of spectroscopy has allowed us to watch the dynamic nature of supported metal nanoparticles. However, the realization that subtle changes to environmental conditions affect the form of the catalyst necessitates that we assess the structure of the catalyst across the reactant/product gradient that exists across a fixed bed reactor. In this study, we have performed spatial profiling of a Pd/AlO catalyst during NH oxidation, simultaneously collecting mass spectrometry and X-ray absorption spectroscopy data at discrete axial positions along the length of the catalyst bed. The spatial analysis has provided unique insights into the structure-activity relationships that govern selective NH oxidation-(i) our data is consistent with the presence of PdN after the spectroscopic signatures for bulk PdN disappear and that there is a direct correlation to the presence of this structure and the selectivity toward N; (ii) at high temperatures, ≥400 °C, we propose that there are two simultaneous reaction pathways-the oxidation of NH to NO by PdO and the subsequent catalytic reduction of NO by NH to produce N. The results in this study confirm the structural and catalytic diversity that exists during catalysis and the need for such an understanding if improvements to important emission control technologies, such as the selective catalytic oxidation of NH, are to be made.