(544em) Rapid Cycling to Achieve High NOx Conversion on Pt/CeO2/Al2O3
Conversion on Pt/CeO2/Al2O3
Zhiyu Zhou, Michael P. Harold*, Dan Luss**
Chemical and Biomolecular Engineering, University of Houston, Houston, TX 77204
The more stringent emission standards
pose a challenge for the NOx (NO+NO2) abatement. A new deNOx system,
Di-Air (Diesel NOx aftertreatment by Adsorbed Intermediate Reductants), was
recently invented by Toyota researchers . The Di-Air system involves rapid fuel
injection into the exhaust fed to the NSR converter and exhibits superior deNOx
performance at high temperature . There is an active debate about the
underlying NOx reduction mechanism for conventional NSR catalysts in Di-Air
system, particularly at high temperatures and fast cycle frequency. Previous
studies discussed several mechanisms, including reaction between hydrocarbon
intermediates and adsorbed NOx , enhanced utilization of fast NOx storage
sites  and NO decomposition on reduced ceria . Ceria has been studied as
an oxygen storage component  as well as a low temperature NOx storage
component  in automobile catalytic converters for many years. Previous study
showed that ceria enhances the deNOx performance of the Di-Air system at both
low  (<300°C) and high  (>550°C) temperatures.
In order to isolate the role of
ceria, we examined the performance of a Pt/CeO2/Al2O3
washcoated monolith catalyst in a bench-scale flow reactor system for a wide
range of operating conditions, including cycle time, feed temperature and
composition, and reductant type. A lean to rich feed
time ratio 6 to 1 of was fixed to maintain the same reductant penalty
(reductant to oxidant ratio). The lean/rich cycling conditions included
lean/rich times (in seconds) of 90/15, 60/10, 30/5 and 6/1. Several sets of feed conditions were applied to study the favorable
and adverse conditions. 300 ppm NO was added to both rich and lean phases. 6.21% H2
or 0.69% C3H6 was only added to rich phase and 0.5% or 5%
O2 was only added to lean phase.
Figure 1 shows the cycle-averaged NOx
conversion for four cycle times from 150°C to 600°C using two sets of feeds.
With a cycle-averaged lean feed, faster cycling of lean/rich promotes NOx
conversion below 450C. However, above 450C, different cycling times have almost
negligible impact on deNOx performance and NOx conversion drops to a low level (~15%).
In contrast, with a cycle-averaged rich feed, the enhancement of NOx conversion
due to faster cycling exists for the whole temperature range (150°C ~ 600°C). Above
450°C, NOx conversion obtained under a fixed cycling time remains an asymptotic
value but the asymptotic value is elevated by faster cycling.
Figure 1. Cycle-averaged NO
conversion as a function of feed temperature and lean/rich switching frequency.
[Conditions: lean/rich switching frequency: 90/15s, 60/10s, 30/5s, 6/1s; rich feed:
6.21% H2, 300ppm NO; lean feed: (a) 300ppm NO, 5% O2; (b)
300ppm NO, 0.5% O2].
Figure 2 shows the cycle-averaged NO
conversion for a fixed cycling time of 6/1s with H2 or C3H6
as the reductant. With either cycle-averaged lean or cycle-averaged
stoichiometric feed, H2 is the better reductant than C3H6
below 400°C. Above 400°C, C3H6 surpasses H2 when
a cycle-averaged stoichiometric feed is applied. However, when a cycle-averaged
lean feed is applied, the promotional impact from C3H6
disappears and NOx conversion diminishes to the asymptotic value (~15%) above
Figure 2. Cycle-averaged NO
conversion as a function of feed temperature with fastest cycling frequency and
H2 or C3H6 as reductant. [Conditions:
lean/rich switching frequency: 6/1s; rich feed: 6.21% H2, or 0.69% C3H6,
300ppm NO; lean feed: (a) 300ppm NO, 5% O2; (b) 300ppm NO, 0.5% O2].
This study on NOx
abatement on Pt/CeO2/Al2O3 provides insight
into the beneficial function of ceria on NOx reduction, which also provide
guidance for optimization of catalyst formulation and operation strategies.
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