posted on 2017-07-11, 00:00authored byYang Zheng, Libor Kovarik, Mark H. Engelhard, Yilin Wang, Yong Wang, Feng Gao, János Szanyi
Pd/zeolite
passive NOx adsorber (PNA)
materials were prepared with solution ion-exchange between NH4/zeolites (Beta, ZSM-5, and SSZ-13) and PdCl2 solutions.
The nature of Pd (dispersion, distribution, and oxidation states)
in these materials was characterized with Na+ ion exchange,
TEM imaging, CO titration with FTIR, and in situ XPS. The NOx trapping and release properties were tested
using feeds with different compositions. It is concluded that multiple
Pd species coexist in these materials: atomically dispersed Pd in
the cationic sites of zeolites and PdO2 and PdO particles
on the external surfaces. While Pd is largely atomically dispersed
in ZSM-5, the small pore opening for SSZ-13 inhibits Pd diffusion
such that the majority of Pd stays as external surface PdO2 clusters. NOx trapping and release are
not simple chemisorption and desorption events but involve rather
complex chemical reactions. In the absence of CO in the feed, cationic
Pd(II) sites with oxygen ligands and PdO2 clusters are
reduced by NO to Pd(I) and PdO clusters. These reduced sites are the
primary NO adsorption sites. In the presence of H2O, the
as-formed NO2 desorbs immediately. In the presence of CO
in the feed, metallic Pd, “naked” Pd2+, and
Pd+ sites are responsible for NO adsorption. For Pd adsorption
sites with the same oxidation states but in different zeolite frameworks,
NO binding energies are not expected to vary greatly. However, NO
release temperatures do vary substantially with different zeolite
structures. This indicates that NO transport within these materials
plays an important role in determining release temperatures. Finally,
some rational design principles for efficient PNA materials are suggested.