(14e) Fast Lean-Rich Cycling for Enhanced NOx Conversion on Pt/CeO2/Al2O3

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 more stringent emission standards
pose a challenge for the NOx (NO+NO₂) abatement. A potential route
to meet these standards is through fast lean-rich cycling on NOx storage and
reduction (NSR) catalyst. The Di-Air (Diesel NOx aftertreatment by Adsorbed Intermediate
Reductants), introduced by Toyota researchers [1] involves rapid fuel injection
into the exhaust to a NSR converter. Their pioneering work revealed superior
deNOx performance at high temperature [1]. There is an active debate about the underlying
mechanism for NSR at high cycle frequency. Previous studies have advanced mechanisms,
including reaction between hydrocarbon intermediates and adsorbed NOx [2], enhanced
utilization of fast NOx storage sites [3], and NO decomposition on reduced
ceria [4]. Ceria has been studied as an oxygen storage component [5] as well as
a low temperature NOx storage component [6] in automobile catalytic converters
for many years. Previous study showed that ceria enhances the deNOx performance
of the Di-Air system at both low [7] (<300°C) and high [4] (>550°C)
temperatures.

In order to isolate the role of
ceria, we are evaluating the performance of both  CeO₂/Al₂O₃
and Pt/CeO₂/Al₂O₃ washcoated monolith catalysts
in a bench-scale flow reactor system over 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 450°C. However, above 450°C, 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 cycle 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
500°C.

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].

These and other data will
be presented which provide insight into the underlying mechanism and
performance features of ceria-containing NSR catalysts for enhanced NOx
abatement for lean burn engines. The results which also provide critical data
for model development needed for the development of improved catalyst
formulations and operating strategies.

Reference

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