especially from internal combustion engine exhausts at low temperatures (< 420 K)
during cold-start, much attention has been directed to the use of zeolites containing
various transition metal ions as adsorbents [115–119]. Similar to NH 3 -SCR catalysis, the adsorption/desorption properties of NO x on such zeolites are severely
influenced by temperature, aging, types of the metal cation, zeolite support structure
employed, etc. Chen et al. have compared the passive NO x adsorption (PNA)
capacities of Pd-SSZ-13, Pd-ZSM-5, and Pd-beta (*BEA), respectively
[120]. When 1 wt% Pd was loaded and then hydrothermally treated at 1023 K, the
NO storage capacity at 373 K was measured to be 48, 58, and 64 NO x μmol
(g sorbent)
À1 or 0.52, 0.62, and 0.68 NO x mol (mole Pd)
À1 for Pd-SSZ-13,
Pd-ZSM-5, and Pd-beta, respectively. These authors also showed that the maximum
NO x desorption peak temperature increases with decreasing zeolite pore size:
Pd-beta (530 K) < Pd-ZSM-5 (550 K) < Pd-SSZ-13 (630 K).
Szanyi and co-workers studied the low-temperature NO x adsorption behavior of
Pd-SSZ-13, Pd-ZSM-5, and Pd-beta in order to understand how the metal oxidation
state, composition, and structure in these small-pore zeolites effect performance
[121]. They reported the coexistence of multiple Pd species, such as atomically
dispersed Pd in the cationic sites of zeolite supports, as well as PdO 2 and PdO
particles on the external surface of zeolite crystals. More recently, these researchers
have developed a simple and scalable route to prepare atomically dispersed highly
loaded (>0.3 wt%) Pd- and Pt-SSZ-13 materials, using the NH 4
+ form of the zeolite
support and the modified incipient wetness impregnation method [122]. The 1.9 wt%
Pd-SSZ-13 (Si/Al ¼ 6) material prepared in this way was found to abate 180 μmol
NO x g
À1 during cold-start, an unprecedented NO x storage capacity, while keeping
atomic dispersion.
6 Conclusions and Prospects
It is now clear that the small gas adsorption capacity and selectivity of small-pore
zeolites are strongly influenced by their pore structure and extra- and intraframework
compositions. A high degree of tunability in the surface selectivity could motivate
researchers in this field to consider novel zeolite structures and compositions that can
also be used for the selective adsorption of inert gases, e.g., He, Ne, Ar, etc. Of
course, the negative effect of water in their adsorption/separation needs to be
considered, as hydrophobic pure-silica zeolites can exhibit very low adsorption
capacities for the target adsorbate. There are also some limitations and concerns
that should be taken into account when designing zeolite adsorbents for a specific
application. For example, the use of highly sophisticated and thus expensive organic
SDAs and environmentally unfriendly fluoride anions in zeolite synthesis may be a
major hurdle to be overcome for the commercial applications of zeolites with
superior separation and adsorption properties.
With respect to energy consumption and alternative fuels, on the other hand, the
most urgent challenge is the discovery of new zeolitic adsorbents that are kinetically
Small Gas Adsorption and Separation in Small-Pore Zeolites
23
during cold-start, much attention has been directed to the use of zeolites containing
various transition metal ions as adsorbents [115–119]. Similar to NH 3 -SCR catalysis, the adsorption/desorption properties of NO x on such zeolites are severely
influenced by temperature, aging, types of the metal cation, zeolite support structure
employed, etc. Chen et al. have compared the passive NO x adsorption (PNA)
capacities of Pd-SSZ-13, Pd-ZSM-5, and Pd-beta (*BEA), respectively
[120]. When 1 wt% Pd was loaded and then hydrothermally treated at 1023 K, the
NO storage capacity at 373 K was measured to be 48, 58, and 64 NO x μmol
(g sorbent)
À1 or 0.52, 0.62, and 0.68 NO x mol (mole Pd)
À1 for Pd-SSZ-13,
Pd-ZSM-5, and Pd-beta, respectively. These authors also showed that the maximum
NO x desorption peak temperature increases with decreasing zeolite pore size:
Pd-beta (530 K) < Pd-ZSM-5 (550 K) < Pd-SSZ-13 (630 K).
Szanyi and co-workers studied the low-temperature NO x adsorption behavior of
Pd-SSZ-13, Pd-ZSM-5, and Pd-beta in order to understand how the metal oxidation
state, composition, and structure in these small-pore zeolites effect performance
[121]. They reported the coexistence of multiple Pd species, such as atomically
dispersed Pd in the cationic sites of zeolite supports, as well as PdO 2 and PdO
particles on the external surface of zeolite crystals. More recently, these researchers
have developed a simple and scalable route to prepare atomically dispersed highly
loaded (>0.3 wt%) Pd- and Pt-SSZ-13 materials, using the NH 4
+ form of the zeolite
support and the modified incipient wetness impregnation method [122]. The 1.9 wt%
Pd-SSZ-13 (Si/Al ¼ 6) material prepared in this way was found to abate 180 μmol
NO x g
À1 during cold-start, an unprecedented NO x storage capacity, while keeping
atomic dispersion.
6 Conclusions and Prospects
It is now clear that the small gas adsorption capacity and selectivity of small-pore
zeolites are strongly influenced by their pore structure and extra- and intraframework
compositions. A high degree of tunability in the surface selectivity could motivate
researchers in this field to consider novel zeolite structures and compositions that can
also be used for the selective adsorption of inert gases, e.g., He, Ne, Ar, etc. Of
course, the negative effect of water in their adsorption/separation needs to be
considered, as hydrophobic pure-silica zeolites can exhibit very low adsorption
capacities for the target adsorbate. There are also some limitations and concerns
that should be taken into account when designing zeolite adsorbents for a specific
application. For example, the use of highly sophisticated and thus expensive organic
SDAs and environmentally unfriendly fluoride anions in zeolite synthesis may be a
major hurdle to be overcome for the commercial applications of zeolites with
superior separation and adsorption properties.
With respect to energy consumption and alternative fuels, on the other hand, the
most urgent challenge is the discovery of new zeolitic adsorbents that are kinetically
Small Gas Adsorption and Separation in Small-Pore Zeolites
23