Evolution of the Structure and Chemical State of Pd Nanoparticles during the in Situ Catalytic Reduction of NO with H2

An in-depth understanding of the fundamental structure of catalysts during operation is indispensable for tailoring future efficient and selective catalysts. We report the evolution of the structure and oxidation state of ZrO2-supported Pd nanocatalysts (∼5 nm) during the in situ reduction of NO with H2 using X-ray absorption fine-structure spectroscopy and X-ray photoelectron spectroscopy. Prior to the onset of the reaction (≤120 °C), a NO-induced redispersion of our initial metallic Pd nanoparticles over the ZrO2 support was observed, and Pdδ+ species were detected. This process parallels the high production of N2O observed at the onset of the reaction (>120 °C), while at higher temperatures (≥150 °C) the selectivity shifts mainly toward N2 (∼80%). Concomitant with the onset of N2 production, the Pd atoms aggregate again into large (6.5 nm) metallic Pd nanoparticles, which were found to constitute the active phase for the H2-reduction of NO. Throughout the entire reaction cycle, the formation and stabilization of PdO x was not detected. Our results highlight the importance of in situ reactivity studies to unravel the microscopic processes governing catalytic reactivity.