(580f) Low Temperature Pd Mobility in Ion-Exchanged Zeolite Hydrocarbon Traps

Ion-exchanged zeolites are currently studied for a wide variety of applications from chemical production to automotive exhaust clean-up catalysis. Catalyst deactivation is an important consideration for any application. Previous work in the area of metal loaded zeolites has highlighted the mobility of metal ions in zeolites under reaction conditions. In some cases, this mobility leads to irreversible deactivation when metals migrate outside the pores and agglomerate to form large external particles. In this presentation, we will discuss the effects of H₂O and CO on Pd²⁺ in BEA zeolite. Pd/BEA is being examined for hydrocarbon trapping in automotive exhaust, where Pd²⁺ at ion-exchange sites serves to adsorb unsaturated hydrocarbons such as ethylene at low temperatures, and larger hydrocarbons are trapped in the zeolite pores. By utilizing benchtop reactor studies, hydrogen temperature programmed reduction (TPR), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), we were able to characterize the Pd speciation and identify a shift from ionic Pd²⁺ to a reduced Pd phase when the catalyst was exposed to CO and H₂O simultaneously at 80°C, even in an oxidative environment (10% O₂). In the fresh catalyst, hydrogen TPR provides evidence for the existence of ion-exchanged Pd species, as well as the absence of large external Pd particles (>2.8 nm). After exposure to CO and H₂O, infrared features indicating CO bound to Pd at hollow and bridging sites suggest the existence of Pd in the form of particles. Other features related to the zeolite framework internal vibrations suggest that Pd migrates away from ion-exchange sites simultaneously as the reduced Pd phase forms. These two findings support the hypothesis that ion-exchanged Pd is reduced by CO and subsequently agglomerates to form Pd particles. Finally, reactor studies demonstrate the impact of this transformation on hydrocarbon trap efficacy.