Zeolite supported palladium catalysts have recently been reported to exhibit high storage capacity and trapping efficiency for NOx at temperatures below 200o C. The present study applies a combined Infrared and XAS approach under operando conditions to understand the NO storage mechanism. The samples employed in the study were 0.5 wt% palladium on chabazite (CHA) and beta zeolite (BEA) – representing small and large pore zeolites respectively – and a 0.5 wt% palladium on alumina refence for comparison. The samples were prepared by incipient wetness impregnation. Operando diffuse reflectance infrared experiments were performed in a Harrick Praying Mantis diffuse reflectance cell, using a Perkin Elmer Frontier spectrophotometer. Operando XAS measurements were performed in a capillary cell with a hot air blower, at the palladium K-edge (24.35 keV) on the SuperXAS beamline at the Swiss Light Source. Both sets of experiments were monitored by mass spectrometry. The properties of the materials were initially evaluated in a simplified gas mixture. Stepwise other gases were added, leading towards a synthetic gas mixture that simulated cold start exhaust emissions from typical diesel vehicles (NO; CO; O2; H2O). Thus influence of each gas feed component was monitored during storage at 100°C, and during release between 100 and 400°C. The combination of operando techniques confirmed our early finding that cationic palladium at the exchange sites of the zeolite supports is responsible for NO adsorption. Exchanged palladium cations maintain their location without chemical transformation during storage under the complex gas feed.

Operando Infrared and XAS study of NO adsorption on zeolite supported Pd catalysts under complex gas feeds

2019 Spring National Meeting

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