(14b) Ion-Exchanged Zeolites for Hydrocarbon Traps and Passive NOx Adsorption Applications

Regulations
on vehicle emissions are becoming more stringent worldwide due to increasing
concerns of the impact of air pollution on the environment and public health [1].
 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) [2].  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 [3].  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 [4].  Zeolites were
evaluated using ethylene (C₂H₄), toluene (C₇H₈)
as model HC compounds and NOx in simulated exhaust conditions relevant to
vehicle cold starts [5].  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 (800^oC-10h) Ag/SSZ-13, and (c) comparison of its
adsorption/desorption capacity.  

[1]
A. Westermann, B. Azambre, M. Chebbi, A. Koch, Microporous and Mesoporous
Mater. 230 (2016) 76−88.

[2]
M.J Hazlett, W.S Epling, Catal. Today 267 (2016) 157−166.

[3]
I. Daldoul, S. Auger, P. Picard, B. Nohair, S. Kaliaguine, Can. J. Chem. Eng.
94 (2016) 931−937

[4]
Y. Zheng, L. Kovarik, M.H. Engelhard, Y. Wang, Y. Wang, F. Gao, J. Szanyi, J.
phys. Chem. C 121 (2017) 15793−15803.

[5]
J. Lee, E.A. Kyriakidou, submitted 2018.