(440g) Hydrocarbon Deactivation of a Cu/Zeolite SCR Catalyst

Hydrocarbon
Deactivation of a Cu/Zeolite SCR Catalyst

A Cu-exchanged zeolite catalyst
was evaluated for selective catalytic reduction (SCR) performance before and
after exposure to propylene at different temperatures, ranging from 150 to
500°C. A transient response method was used to characterize the standard, fast
and NO₂ SCR reactions, as well as NH₃ and NO oxidation,
in the presence and absence of propylene.

The results clearly show propylene
inhibition of the standard and fast SCR reactions between 200 and 400°C. With
the addition of propylene there is an immediate increase in outlet NO_X.
According to the DRIFTS data obtained, propylene oxidation intermediates, for
example acrolein-like species, rapidly form on the
catalyst surface upon propylene introduction at these temperatures, explaining
this immediate drop in performance. At 300°C there was a slower decrease in
performance with prolonged exposure to propylene, after the initial rapid loss,
which was not observed at 250°C. In the 300 to 400°C range, acrolein
species were again observed, resulting in the rapid loss in performance
observed, but coke formation was also observed, and was slow to build-up. The
coke formation explains the slow decrease in performance noted. At 150°C,
inhibition was not observed, nor was any propylene consumption via oxidation,
suggesting that some extent of propylene oxidation is required for the onset of
inhibition, further verifying the propylene oxidation intermediates causing
inhibition.

In terms of NO oxidation, in all
tests, the addition of propylene led to reduced NO₂ formed via NO₂
being used as an oxidant for propylene oxidation, and thus reduction of NO₂
to NO. NH₃ was found to inhibit propylene oxidation, due to the
relatively stronger adsorption of NH₃ to the surface. For NO₂
SCR, the addition of propylene resulted in improved performance, through HC-SCR
as well as reduction of NO₂ to NO, allowing some fast SCR reaction
to occur.