Effects of Zeolite Framework Type on Mononuclear Pd(II) Species

Presentation Title: Effects of zeolite framework type on mononuclear Pd(II) species

Research Focus: Passive NO_x Adsorption

School: Purdue University

Presentation Type: Poster and Oral Presentations

Abstract:

Effects of zeolite framework type on mononuclear Pd(II) species

Harrison Lippie, Trevor Lardinois, Rajamani Gounder, Purdue University

The majority of government-regulated, engine tailpipe NO_x emissions occur during cold-start or low-load when exhaust temperatures are below (<473 K) the operating window of NO_x catalytic converters. Pd-exchanged zeolites have emerged as promising candidates for passive NO_x adsorption, where NO_x species adsorb at <473 K and desorb at temperatures compatible with downstream catalytic converters. Mononuclear Pd²⁺ cations are the purported adsorption precursor sites for NO_x adsorption, but the influence of zeolite material properties on the formation of these species is incompletely understood. Here, a series of Pd-exchanged zeolites of different topologies (CHA, MFI, FER, MOR) with fixed framework Al and extraframework Pd densities was synthesized to study the influence of topology. Additionally, a series of Pd-MFI zeolites was synthesized with similar bulk Pd contents but varied framework Al amounts to investigate the influence of acid site density. All samples were characterized with H₂ temperature programmed reduction (TPR) after high temperature air pretreatments (T>823 K) to quantify the amounts of agglomerated PdO and mononuclear Pd²⁺ species. Finally, comparing H₂ TPR profiles of dehydrated and hydrated Pd-zeolites demonstrates that hydrating Pd-zeolites ameliorates the deconvolution of TPR features for more accurate quantification of Pd sites. Altogether, the influence of framework topology and bulk Al density on the formation of mononuclear Pd²⁺ was deduced.