J.H. Kwak et al. / Journal of Catalysis 275 (2010) 187–190
189
temperatures; i.e., 27 ppm at 200 °C and 24 ppm at 450 °C, respec-
a
tively. The Cu-ZSM-5 catalyst produced a similar N2O formation
profile to Cu-beta, but the amounts of N2O formed were much
smaller. These N2O formation profiles are likely related to the reac-
tion mechanisms of the NOx reduction reactions. For example, our
results demonstrate that reaction intermediates (e.g., NOx–NH3 ad-
sorbed complexes) on Cu-SSZ-13 take a more selective reaction
route toward the production of N2 than do the complexes on the
Cu-beta and Cu-ZSM-5 catalysts.
The differences in activity and selectivity of the three zeolites
studied may be related to fundamental differences in the known
structures of these zeolites, i.e., the pore sizes and locations of
the copper ions. The order of high-temperature NH3 SCR reactivity
discussed earlier is the inverse of the order in pore size, i.e., SSZ-13
having the smallest pores (ꢀ4 Å, 8-membered ring) being the most
active, ZSM-5 with medium size pore opening (ꢀ5.5 Å, 10-mem-
bered ring) having medium activity, and beta with the largest
pores (ꢀ7 Å and ꢀ5.5 Å, 12-membered ring) having the lowest
activity and N2 selectivity. For these three catalysts, the smaller
size pores seem to be preferred for the desirable reaction path-
ways; however, detailed mechanistic studies need to be conducted
to substantiate the correlation between pore size and activity/
selectivity. In summary, both the activity and selectivity of NOx
SCR with NH3 for Cu-SSZ-13 are superior to those of Cu-ZSM-5
and Cu-beta over the entire temperature range studied (up to
550 °C).
b
The differences observed in the ammonia SCR reactivities and
N2 formation selectivities for the three catalysts studied may also
be related (at least in part) to their abilities to oxidize ammonia.
Therefore, we performed NH3 oxidation reactions over the three
different Cu–zeolite catalysts in the absence of NO and the results
are presented in Fig. 3. Ammonia conversions (Fig. 3a) reveal that
the light-off temperature for NH3 oxidation is the lowest for Cu-
SSZ-13, indicating its superior intrinsic NH3 oxidation ability. For
this catalyst, the NH3 oxidation reaction lights off at around
200 °C and reaches a conversion level of more than 90% at
ꢀ300 °C. The NH3 conversion profiles for Cu-beta and Cu-ZSM-5
are shifted to higher temperatures by ꢀ50 and ꢀ100 °C, respec-
tively, relative to that of Cu-SSZ-13.
The concentrations of NOx (NO + NO2 + N2O) in the reaction
effluent, which are regarded as by-products during NH3 oxida-
tion to N2, are plotted in Fig. 3b. The Cu-beta catalyst produced
relatively higher levels of these by-products, with a maximum of
about 55 ppm at 350 °C, while the Cu-ZSM-5 catalyst produced
significant amounts of by-products at 550 °C. The relative lack
of NOx formation during ammonia oxidation on the Cu-SSZ-13
catalyst implies that most of the NH3 is converted to N2 over
a wide temperature range for this catalyst. The near absence
of further oxidization to N2O, NO, or NO2, as was the case for
the Cu-beta and Cu-ZSM-5 catalysts, suggests again that the
environment within the Cu-SSZ-13 catalyst may provide opti-
mum conditions for selective conversion of reaction intermedi-
ates to N2.
According to the results of previous studies, noble metal cata-
lysts, including Pt [12], have been found to be very active in
ammonia oxidation, but rather non-selective to N2 formation,
while transition metal oxides such as MnO2 and CuO [13] have
higher N2 selectivity, but require significantly higher tempera-
tures. Cu-SSZ-13, on the other hand, can meet the two important
requirements: excellent NH3 oxidation activity and N2 selectivity
over a wide temperature range. Thus, for example, the use of Cu-
SSZ-13 as an NH3 oxidation catalyst at the downstream end of a
NOx SCR with NH3 unit might provide flexibility for controlling
the dose of urea introduced before the SCR catalyst, since any ex-
cess of NH3 can perhaps be removed more easily over the catalyst
bed.
Fig. 3. (a) NH3 conversion profiles and (b) NOx product distributions during the NH3
oxidation reaction on Cu-SSZ-13 (squares), Cu-beta (circles), and Cu-ZSM-5
(triangles) at various temperatures in a gas mixture containing 350 ppm NH3,
14% O2, and 2% H2O with a balance of N2.
4. Conclusions
Under the same reaction conditions for NOx SCR with NH3,
Cu-SSZ-13 demonstrates superior activity and N2 formation
selectivity in comparison with Cu-beta and Cu-ZSM-5 zeolites.
We find that Cu-SSZ-13 is more active for NOx conversion over
the entire temperature range studied (160–550 °C). Moreover,
the Cu-SSZ-13 is also more selective toward the formation of
N2, producing lower amounts of undesired by-products such
as NO2 and N2O. Our results also demonstrate that Cu-SSZ-13
has superior performance for NH3 oxidation (lower light-off
temperature) than Cu-beta and Cu-ZSM-5 zeolites, while also
producing significantly lower amounts of (over-oxidized) NOx
species. These results suggest that Cu-SSZ-13 is an excellent
candidate catalyst for use in practical NH3 SCR of NOx and/or
NH3 oxidation applications (the after-treatment systems of var-
ious mobile or stationary sources). Detailed mechanistic studies
are currently under way in our laboratory to understand the
origin of the different activities and selectivities observed for
these three catalysts in both the NOx SCR and NH3 oxidation
reactions.
Kwak, Ja Hun
