Bench-flow
reactor and in situ DRIFTS experiments were carried
out to elucidate the features of the propylene + NO + O2 reaction system on Cu–SSZ13 (chabazite) monolithic catalyst.
Experiments were conducted under both steady state and transient conditions
for application-relevant feed conditions. Steady state conversion
data of the C3H6 light-off in the presence of
excess O2 (5%) shows inhibition by a much smaller amount
of NO (500 ppm). Corresponding data in the presence of NO2 reveals a transition in the C3H6 light-off
curve; for temperatures below 350 °C, the C3H6 conversion is higher in the presence of NO2 as
compared to O2 whereas above 350 °C the C3H6 conversion in the absence of NO2 is higher.
The reduction of NO2 to NO and N2 is enhanced
by the addition of C3H6, which facilitates the
reduction of Cu sites favorable for the NO2 reduction.
Spatially resolved concentration profiles using a series of monolith
pieces of different lengths provide insight into the reaction pathways.
The spatial profiles show that NO2 is completely reduced
in the front part of monolith at temperatures above 350 °C. On
the other hand, NO that is formed from the NO2 reduction
increases along the length in the temperature range of 200–350
°C and exhibits a maximum value above 350 °C. Similarly,
C3H6 oxidation is inhibited by NO at the light-off
temperature of the C3H6 oxidation reaction (∼350
°C) while there is no effect of NO on C3H6 oxidation reaction below the light-off temperature. The results
clearly suggest that inhibition is not due to a competitive adsorption
process but due to the generation of partially oxidized hydrocarbon
species that react with NO to form O- and N-containing surface intermediates
which serve to block the active catalytic sites. In situ DRIFTS measurements
confirm the formation and existence of −NCO like species over
catalyst surface on exposure to NO after a 45 min of exposure to C3H6 and O2. A phenomenological reaction
mechanism is proposed that involves the formation of inhibiting intermediates
through the reaction of NO with oxygenates, which then react further
to form N2.
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