Metal-exchanged zeolites are common catalysts, used in applications ranging from fossil fuel and biomass hydrocarbon transformations to CO2 capture to diesel engine exhaust emissions cleanup. Describing the active sites for many of these reactions, under reaction conditions, can be challenging, especially with growing recognition of mobility of the metal constituents within the zeolite. Depending on reaction conditions, ion-exchanged metal atoms, solvated ions, dimers, trimers, small clusters, and nanoparticles have all been observed and sometimes invoked as active sites. In this presentation, we will discuss the effects of high temperature exposure, simulating hydrothermal deactivation, on a Pd-exchanged BEA zeolite hydrocarbon trap. We used ethylene as a surrogate hydrocarbon molecule, and the results show how the Pd speciation changes impact ethylene uptake and oxidation. With high temperature exposure, decreased ethylene storage capacity and oxidation activity was observed. Sample characterization, primarily NO adsorption and desorption, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and x-ray absorption spectroscopy (XAS), indicates that the amount of ion exchanged Pd increased with high temperature exposure. The NO adsorption and desorption (TPD) studies revealed multiple NO bound species types and in combination with the DRIFTS, XAS and reactor results, one is consistent with NO stored on Pd+, with this species formed during NO adsorption. In correlating the species changes and hydrocarbon trapping, this Pd+ species was less active in ethylene storage and oxidation than the original Pd2+ species.

Pd speciation changes in Pd ion-exchanged zeolites with high temperature and CO/H2O exposures

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