(617dj) Effect of Fast Cycling on the Formation of HC Intermediates and NOx Conversion Using H2, Propene or Propane As Reductants

Effect of Fast Cycling on
the Formation of HC Intermediates and NOx Conversion Using H₂,
Propene or Propane as Reductants

Allen Wei-Lun Ting, MengMeng
Li, Michael P. Harold and Vemuri Balakotaiah

Department of Chemical & Biomolecular Engineering, University
of Houston, Houston, TX 77204, USA

Abstract

A combined
experimental and modeling study of fast cycling NOx storage and reduction for
emissions control of lean burn gasoline and diesel vehicles is conducted to
provide mechanistic insight.  Experiments reveal notable NOx conversion
enhancement for cycle times less than 10 seconds (fast cycling) compared to
long cycle times of a minute (Fig. 1, Perng et al. Catalysis Today 231 (2014) 125–134 [HMP1] [TA2] ).

The mechanism
of the fast cycling has been analyzed through both experiments and simulations
with H₂, propene, or propane as the reductant and ceria-free and
ceria-containing NOx storage catalyst (Pt/BaO/Al₂O₃). For
H₂, more frequent storage and regeneration under fast cycling
increases the storage capacity available proportionally when storage and
reaction is capacity limited, with negligible effect when kinetically limited.
The mechanism explains the increase in NOx conversion over a wide temperature
range and corresponding decrease in the NH₃ selectivity. The
cycle-averaged catalyst temperature under fast cycling increases because of the
combined effect of dispersion effect between lean and rich feed and the
enhancement of O₂ storage ability by ceria.

The “Di-Air”
system proposed by Toyota researchers shows an overall increase in NOx
conversion when using propene as the reductant under fast cycling[HMP3] [TA4]  (Bisaiji et al., SAE
2011-01-2089). Modeling
results show that fast cycling enhances the formation of a thermally-stable HC
intermediates from propene and NO.  This results in a substantial increase
the conversion at high temperature (Fig. 2). However, using propane as the
reductant shows a negative effect. Modeling results predict this inverse
behavior by treating the dehydrogenation reaction from propane to propene to be
the rate limiting step to reduce NOx. 

Fig. 1 Cycle averaged NOx conversion and
catalyst temperature vs. feed temperature for two different frequencies. Feed:
700 ppm NO 5% O₂, rich phase contain 1.75% C₃H₆.

Fig. 2 Simulated isothermal cycle
averaged NOx conversion vs. catalyst temperature for two different frequencies.
Feed: 700 ppm NO 5% O₂, rich phase contain 1.75% C₃H₆.