(405b) Fast Lean-Rich Cycling for Enhanced NOx Conversion

Abstract

Conventional
NO_x storage and reduction (NSR) has emerged as an important emission
aftertreatment technology for reducing NO_x (NO and NO₂)
to N₂ under lean conditions typical of diesel or lean-burn gasoline
vehicles [1]. Typical cycle timing is 1-2 minutes for the lean phase and 3-20
seconds for the rich phase. In the past year, Toyota Inc. has presented the ?Di-Air?
technology which involves much faster cycling and the use of hydrocarbon
reductant. Published data show a large expansion in the temperature operating
window for achieving high NO_x conversion [2]. In this study we have
carried out NSR over a wide range of cycle timing to examine the generality of
rapid cycling and to understand causative mechanisms for conversion
enhancement.

NO_x
reduction experiments have been carried out using the rapid injection of C₃H₆
over a NSR catalyst containing Pt/Rh/BaO/Ce/Al₂O₃ combined
with a fast switching valve in close proximity to the catalyst. The injection
frequency was varied for a fixed cycle-averaged propylene concentration. In
contrast to traditional NSR cycling systems, NO_x conversion was
observed to increase as injection frequency increases, up to an injection
frequency of 0.5 Hz. The shorter cycle times result in a steadier rate of
exothermic catalytic oxidation of propylene, characterized by nearly pseudo
steady state values for the catalyst temperature and O₂
concentration. This also allows a higher conversion of propylene to be obtained
throughout the length of the cycle. In addition, the effect of the lean phase
length and thermal durability were studied. High conversions were obtained
using less reductant (>90% conversion for a cycle-averaged stoichiometric
number of 6) and over a wider temperature range (>90% conversion for a
catalyst temperature of 250-550°C) than previous literature values. Figure 1
shows typical results. The extension of the high conversion temperature window
implies that the decomposition of barium nitrate is not the limiting factor,
likely due to the diminished role of NO_x storage. Furthermore, the
inhibitory effect of Ba carbonate formation on NO_x conversion was
determined to be less significant at high injection frequencies. This can be
attributed to low utilization of dynamic storage sites, which reduces the
impact of carbonate formation. These results are consistent with hydrocarbon
intermediates on the catalyst surface providing competition for the propylene
oxidation reaction and reducing NO_x.

References

[1] Harold, M.P., ?NOx Storage and Reduction in Lean
Burn Vehicle Emission Control:  A Catalytic Engineer's Playground,? Current
Opinion in Chemical Engineering, 1, 303-311 (2012).

[2]
Bisaiji, Y., Yoshida, K., Inoue, M., Umemoto, K., Fukuma, T., "Development
of Di-Air - A New Diesel deNOx System by Adsorbed Intermediate
Reductants," SAE Int. J. Fuels Lubr. 5(1):380-388, 2012,
doi:10.4271/2011-01-2089.

Figure 1. Comparison of temperature effect on NO_x
conversion using conventional NSR and fast injection method.