(544em) Rapid Cycling to Achieve High NOx Conversion on Pt/CeO2/Al2O3

Authors: 
Zhou, Z., University of Houston
Harold, M., University of Houston
Luss, D., University of Houston
Fast Lean-Rich Cycling to Achieve High 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 new deNOx system,
Di-Air (Diesel NOx aftertreatment by Adsorbed Intermediate Reductants), was
recently invented by Toyota researchers [1]. The Di-Air system involves rapid fuel
injection into the exhaust fed to the NSR converter and exhibits superior deNOx
performance at high temperature [1]. 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 [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 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
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].

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.

Reference

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

2.      Y. Bisaiji et al., SAE
Int. J. Fuels Lubr.
5 (2012) 1310-1316

3.      A. Ting et al., Chem.
Eng. J.
, 326 (2017) 419-435

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

5.      T., Montini et al., Chem.
Rev.
, 116 (2016), 5987-6041

6.      Y., Ji et al., Catal.
Lett.
110 (2006), 29-37

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