Abstract
Interest in Pd/zeolites as automobile exhaust catalysts has driven research focused on understanding the chemical environment of Pd active centers. Here, we studied the effects of hydrothermal deactivation on a Pd/BEA hydrocarbon trap. Hydrothermal aging (HTA) led to decreased ethylene storage capacity and oxidation activity. Characterization by x-ray absorption spectroscopy (XAS) and NOx adsorption indicated that for lower Si/Al ratio samples, the amount of ion exchanged Pd increased after HTA. Temperature programmed desorption of NOx after NO exposure revealed two desorption peaks, with a high temperature peak that increased in area with increased aging temperature. Based on infrared spectroscopy, XAS and reactor results, the high temperature desorption peak is consistent with NO stored on Pd+. The Pd+ formed during NO adsorption, existing originally as [Pd(OH)]+. This species was less active in ethylene storage and oxidation than the original Pd2+ species. [Pd(OH)]+ stored NOx to a higher temperature than Pd2+.
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