(27a) Kinetic and Reactor Modeling of the NH3-SCR Converter: Fe- and Cu-Exchanged Zeolites As Single or Dual Layers

Kinetic
and Reactor Modeling of the NH₃-SCR Converter:  

Fe-
and  Cu-exchanged Zeolites as Single or
Dual Layers

Pranit S. Metkar, Michael P. Harold and Vemuri
Balakotaiah

Department of Chemical & Biomolecular Engineering,
University of Houston, Houston, TX-77204

Abstract

The lean selective
catalytic reduction of NOx is a critical challenge in diesel and lean burn
gasoline vehicles.  The development of
predictive kinetic models is essential for identifying and optimizing improved
catalysts.  For example, we recently
showed that the combination of Fe- and Cu-exchanged zeolites in the form of
either sequential monoliths or dual layers can significantly increase the
temperature window for which high NOx conversion is achieved [1]. In the
current study we describe the development of global kinetic models for NH₃-SCR
on both Fe- and Cu-exchanged zeolite monolith catalysts (Fe-ZSM-5 and Cu-SSZ-13
(chabazite)).  To this end, the general
SCR kinetic model accounts for the following reactions: NH₃
adsorption, NH₃ oxidation, NO oxidation, standard SCR (NO + NH₃
+ O₂), fast SCR (NO + NH₃ + NO₂), NO₂
SCR (NH₃ + NO₂), ammonium nitrate formation and its
decomposition to N₂O_, N₂O decomposition and N₂O
reduction by NH₃. We incorporate the kinetic models into a monolith
reactor model to simulate the dual-layer catalysts and in so doing help to
interpret the performance of SCR monoliths over a wide range of conditions. The
two-dimensional, two-phase reactor model includes the contribution of both
external and internal mass transport processes so that  conditions are determined when diffusion
limitations are present.

Results to date are
described as follows.  The Cu-chabazite
catalyst showed a higher NH₃ storage capacity and higher activities
for NH₃ oxidation and standard SCR reactions compared to Fe-ZSM-5.
The NOx reduction activity on the Fe-ZSM-5 catalyst was found to be strongly
dependent on the feed NO₂. For the Cu-chabazite catalyst, the NOx
conversions were less sensitive to the feed fraction of NO₂. In the presence
of excess NO₂, both N₂O and ammonium nitrate byproducts are
formed on both the catalysts. The Fe-ZSM-5 catalyst showed a higher selectivity
towards these byproducts compared to Cu-chabazite. For different feed
conditions (NO₂/NOx = 0-1), Cu-chabazite was found to be more a more
active NOx reduction catalyst at lower temperatures (≤ 350 ^oC)
while Fe-ZSM-5 was more active at higher temperatures (≥ 400 ^oC).
The model accurately predicts the steady state NOx/NH₃ conversions
and selectivity for different products formed during these reactions.

Finally, a systematic
study of various SCR reactions was carried out on a combined system of Fe- and
Cu-zeolite monolithic catalysts to determine if a high NOx conversion could be
sustained over a wider temperature range than with individual Fe- and
Cu-zeolite catalysts. Amongst various configurations of these combined catalyst
systems, a dual-layer catalyst with a thin Fe-zeolite layer on top of a thick
Cu-zeolite layer and a sequential arrangement of short Fe-ZSM-5 brick followed
by longer Cu-chabazite brick resulted in achieving a very high NOx removal
efficiency over a broad temperature range of practical interest. The model
could successfully predict all the experimental data with combined Fe- and
Cu-zeolite catalysts.  A typical
comparison of experimental and modeling results is shown in Figure 1. The model
captures the main trends in the data and can be used to quantify the optimal
Fe/Cu ratio to maximize the NOx conversion.

Figure
1: Steady state NOx conversions obtained during the
standard SCR reaction studied on Cu-chabazite, Fe-ZSM-5, and combined system of
series arrangements of Fe-ZSM-5 (in front) followed by Cu-chabazite. a)
Experiments b) model predictions.

References

1.       Metkar, P., V. Balakotaiah, and M.P. Harold, ?Selective Catalytic
Reduction of NOx on Combined Fe- and Cu-Zeolite Monolithic Catalysts:  Sequential and Dual Layer Configurations,? Applied Catalysis B: Environmental, 111?
112, 67? 80 (2012).