Kinetic Modeling of the NO x Operating Cycle in a Pd/CHA Passive NO x Adsorber

A global kinetic model is developed that predicts the reported NO uptake trends on Pd/CHA in the presence of H2O and CO at various adsorption temperatures. NO storage decreases with temperature, both with and without H2O. The reoxidation rate of Pd1+ to [Pd–O–Pd]2+ increases with adsorption temperature, but H2O suppresses it at low temperatures, resulting in a less pronounced decrease in NO storage. NO adsorption on the [Pd–O–Pd]2+ site results in the release of NO2 and Pd1+ formation, which functions as an additional NO adsorption site. H2O facilitates NO adsorption but negatively impacts the NO2 release at low temperatures, with NO2 being released during the desorption phase. CO is found to be a stronger reductant than NO, preferentially reducing [Pd–O–Pd]+2 to Pd1+ with CO2 formation. The relatively high NOx storage in the presence of CO can be attributed to the coadsorption of NO and CO on [Pd–O–Pd]2+ sites.