(14b) Ion-Exchanged Zeolites for Hydrocarbon Traps and Passive NOx Adsorption Applications
on vehicle emissions are becoming more stringent worldwide due to increasing
concerns of the impact of air pollution on the environment and public health .
The current emission control technologies involve the use of three-way and diesel
oxidation catalysts in gasoline and diesel vehicles, respectively. However, in
most cases these catalysts are relatively ineffective until they reach their
operating temperature (~200°C) . In this context, foremost attention is
required for low temperatures associated with engine cold starts that take
place within the first 1-2 minutes of a driving cycle and emit around 80% of
the polluting gases into the environment . A very promising strategy to
mitigate cold start hydrocarbon (HC) emissions is to a HC trap (HCT)
that can adsorb the HC emissions at low temperatures and then oxidize the
stored HCs to carbon dioxide (CO2) and water (H2O) at
higher temperatures. To treat the NOx emissions during the cold start period,
a passive NOx adsorber (PNA) can adsorb the NOx at low temperatures . Zeolites were
evaluated using ethylene (C2H4), toluene (C7H8)
as model HC compounds and NOx in simulated exhaust conditions relevant to
vehicle cold starts . The specific objective was to investigate in detail
trends observed between C2H4, C7H8
and NOx trapping-release performance, zeolite framework structure (SSZ-13 vs. ZSM-5
vs. BEA), exchanged cation (e.g. H+ vs. Ag+ vs Pd2+),
and gas composition. Our results show that Ag/SSZ-13 and Pd/SSZ-13 are able to
adsorb C2H4 and NOx, respectively. Toluene is not
adsorbed by either Ag or Pd/SSZ-13, because its kinetic diameter is larger than
the pore size of SSZ-13. Ethylene and NOx are desorbed at temperatures higher
than the operating temperature of conventional catalysts. Specifically, C2H4
and NOx release is observed at temperatures > 200 oC. Larger
pore ion-exchanged zeolites, e.g. Ag/ZSM-5, are able to adsorb both C2H4
and C7H8. However, cracking of toluene to CO is observed
during desorption over Ag/ZSM-5. The adsorption/desorption properties of
ion-exchanged Ag/SSZ-13 are retained after hydrothermal aging at 800 oC
for 10h. Moreover, the NOx adsorption efficiency of Pd/SSZ-13 is improved
after aging under the same conditions. These results indicate that Ag and Pd
ion-exchanged SSZ-13 zeolites are suitable as potential HCT and PNA materials.
Figure 1. (a)
C2H4 adsorption and (b) desorption profile over fresh and
hydrothermally aged (800oC-10h) Ag/SSZ-13, and (c) comparison of its
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