(582cl) Fast Cycling to Achieve High NOx Conversion in Exhaust: Role of Ceria

Zhiyu Zhou, Michael P. Harold^*, Dan Luss^**

Department
of Chemical and Biomolecular Engineering, University of Houston, Houston, TX 77204

*
mharold@uh.edu, ** dluss@central.uh.edu

The abatement of NOx (NO+NO₂)
remains a challenge under the continuously more stringent emission standards. Commercialized
deNOx systems, NOx storage and reduction (NSR) and selective catalytic
reduction (SCR), function well from ~250 ^oC to ~400 ^oC, but have unsatisfactory performance outside that
temperature range. Toyota researchers recently invented a new deNOx system,
Di-Air (diesel NOx aftertreatment by adsorbed intermediate reductants), which
involves fast injection of fuel into the exhaust feed to the NSR converter [1].
Previous studies show that ceria increases the deNOx performance of the Di-Air
system in both low [2] and high [3] temperature regions. Data suggest that NOx
conversion occurs via the reaction of an intermediate produced through the
reaction with O₂ and NO [1, 2]. However, a recent study [3] suggests
that catalysts containing ceria may involve an alternative pathway of NOx abatement
via NO decomposition at high temperatures. We studied in a bench-scale flow
reactor system the NO decomposition and reduction on ceria under different frequencies
with CO as sole reductant.

In our study, a lean to rich feed
time ratio 6 to 1 of was controlled to keep the same fuel penalty (fuel to
oxidant ratio). The lean/rich cycling conditions included lean/rich times (in
seconds) of 90/15, 60/10, 30/5 and 6/1. Different reaction conditions were
applied to study NO conversion behavior under favorable and adverse conditions.
500 ppm NO was added to both rich phase and lean phase. 2.5% CO was only added
to rich phase and O₂ with varied concentration was only added to
lean phase. In some cases, CO₂ and H₂O were introduced continuously
along with NO, O₂ and CO to simulate lean exhaust.

When CO is used as the sole
reductant and no H₂O is introduced in the feed, NO is mainly converted to N₂ with only trace amounts of N₂O and NO₂ detected. Figure
1 shows the cycle-averaged NO conversion with four cycle times from 450 ^oC
to 625 ^oC in two feed conditions. In the first case, when 1400 ppm O₂
is introduced in lean feed, Figure 1(a) shows that increase of lean/rich
switching frequency enhances NO conversion. The optimized lean/rich switching
frequency is always 6/1 above 500 ^oC. In the second case, when
2100ppm O₂ is introduced in the lean feed, Figure 1(b) shows that a different
optimal lean/rich switching frequency exists for different temperatures. The
optimal lean/rich switching frequency is 30/5 at intermediate temperatures (500
^oC and 550 ^oC) and 6/1 at high temperatures (600 ^oC
and 625 ^oC) respectively. Figure 1 shows that the optimal lean/rich
switching frequency changes with reaction conditions (e.g., temperature and
feed condition).

Figure 1. (a) NO conversion with
different lean/rich switching frequency; (a) rich: 2.5%CO, 500ppm NO; lean:
500ppm NO, 1400ppm NO; (b) rich: 2.5%CO, 500ppm NO, lean: 500ppm NO, 2100ppm O₂

Figure 2 shows the
cycle-averaged NO conversion with the same four cycle times  at 600 ^oC
with fixed  ratio of lean and rich feeds. The short cycle limit represents NO
conversion obtained with total mixed lean and rich feeds. The long cycle limit
represents weight averaged NO conversion in lean phase and rich phase. The
optimal lean/rich switching frequency of 30/5 exceeds those of both short and
long cycle time limit. This shows that the lean/rich cycling operation is
beneficial to NO conversion.

Figure
2.
NO conversion at 600C, with a fixed switching frequency as 90s lean/15s rich; rich:
2.5%CO, 500ppm NO, lean: 500ppm NO, 3600ppm O₂

Figure 3 shows that NO conversion
changes when CO₂ and H₂O are fed. By comparing the blue
line (reference) with either red line (with H₂O only) or purple line
(with CO₂ only), it is obvious that both H₂O and CO₂
inhibit NO decomposition on ceria but H₂O has a smaller detrimental
effect than CO₂. The green line (with both CO₂ and H₂O)
lies between the red line (with H₂O only) and the purple line (with
CO₂ only), which indicates that H₂O can mitigate the
detrimental effect of CO₂.

Figure
3.
NO conversion at 550C, with a fixed switching frequency as 90s lean/15s rich; (rich:
2.5%CO, 500ppm NO, lean: 500ppm NO, varied O₂ concentration, CO₂
and H₂O in both lean and rich if introduced)

This study on NO
decomposition on ceria during lean and rich cycling helps explain the beneficial
function of ceria on NOx reduction at high temperatures, which also provide
guidance for optimization of catalyst formulation and operation strategies.

Reference

1.      Y. Bisaiji et
al., SAE Int. J. Fuels Lubr. 5 (2012) 380-388

2.      Y. Zheng et al., Catal.
Today, 276 (2016), 192-201.

3.     
Y.
Wang et al., Top. Catal., 59 (2016), 854-860