(762g) Selective Catalytic Reduction (SCR) of NOx By Ethanol over Ag/Al2O3: A Mechanistic Study of the Product Distribution with Spaci-MS/FTIR

Introduction

NOx reduction in lean
exhaust by ethanol SCR of NOx over Ag/Al₂O₃ catalyst has been studied in previous work and found to achieve
100% NOx conversion over a moderate temperature window of roughly 125°C [1, 2].
Previous studies have suggested a mechanism where ethanol adsorbs through an
ethoxy-intermediate which forms atomic hydrogen and adsorbed acetaldehyde on
the surface. The hydrogen is viewed as helping the release of NO₂ to
the gas phase which was reported to improve the activity [3]. In addition, a
high selectivity to NH₃ (>40%) and to acetaldehyde was observed
during this reaction both of which can be active species for NOx reduction. The
focus of this study is to obtain mechanistic insights into this reaction by
investigating the spatial distribution of reaction products in different
temperature regimes.  Such insights should allow for further improvements in
Dual SCR exhaust aftertreatment [1] that uses the ammonia from the ethanol-NOx
reaction over a silver catalyst.  

Materials and Methods

A 2 wt% Ag/γ-Al₂O₃
catalyst coated on a 400 cpsi cordierite monolith was tested in the temperature
range of 150 to 550°C at a gas
hourly space velocity of 35,000. The inlet gas composition was 500 ppm NO, 8% O₂,
5% H₂O, and 1500 ppm C₂H₅OH (C/N ratio = 6) in
Ar. A spatially-resolved capillary inlet mass spectrometer/FTIR (Spaci-MS/FTIR)
system was used to measure the spatial distribution of products using simultaneous
MS and FTIR measurements. As shown in Fig. 1, the setup consists of four
0.3mm/0.15mm OD/ID capillaries and a thermocouple in five adjacent monolith
channels. A pressure differential of 35 kPa was always maintained across the
capillaries to obtain the desired flow rate which was then diluted with Ar by a
6:1 ratio to achieve the desired time response from the MS and FTIR.

Figure 1. Schematic
of Spaci-MS/FTIR setup

Results and Discussion

The
spatial distributions of NO and ethanol at different inlet flow temperatures
are shown in Fig. 2, which indicate slow reduction and a long reaction zone for
NO at close to light-off temperatures,
~240°C. As the temperature increases, the
reaction zone becomes smaller due to the increased rate of reaction. Notice
that, at elevated temperatures, there is already some NO conversion at the
inlet face of the monolith indicating a fast surface reaction or possibly some
gas phase reactions. The same general behavior is observed for ethanol. The
overall NOx and ethanol conversions show good agreement with previous studies [2,
3].

font-family:" times new roman>Figure 2. Spatial distribution of NO (a) and ethanol (b) at different
inlet flow temperatures

0%">The axial distribution of a range of products
has been measured at the same time as the NO and ethanol in Fig. 2.  They are N₂,
NO₂, NH₃, N₂O, C₂H₄O
(acetaldehyde), C₂H₄ (ethylene), and CO.  Significant
amounts of ammonia are formed at the inlet of the monolith in the temperature
window of 350 to 460°C. Ammonia is then
partially consumed moving towards the outlet in a relatively small reaction
zone near the front of the catalyst.  Significant amounts of acetaldehyde are
also observed, which is formed with a slower rate over a longer reaction zone,
but as the temperature increases above 350°C most of the formed acetaldehyde is
consumed and only small concentrations are observed at the outlet.

0%"> 

0%">The conversion of NO in this reaction proceeds
in parallel with the consumption of ammonia and acetaldehyde inside the
monolith which suggests they may be active species for NOx reduction. On the
other hand, NO₂ shows a much slower and monotonic formation and is
not consumed moving downstream of the monolith. This might indicate a less
significant role for NO oxidation in the overall ethanol SCR of NOx reaction. 
Further analysis and studies are underway to further provide mechanistic
insight into this important HC-SCR reaction. 

Significance

These ongoing
Spaci measurements of the ethanol SCR of NOx reaction are needed for detailed
modeling of this unique reaction that makes ammonia with a high selectivity
(>40%) in the presence of high oxygen concentrations.  Resulting improved
reactor design (i.e., shorter length) could lead to using this catalyst in some
areas of lean exhaust emissions control.   

References

1.       
Fisher, G.B., DiMaggio, C.L.,
Trytko, D., Rahmoeller, K.M., and Sellnau, M., SAE International Journal of
Fuels and Lubricants, March 2010, vol. 2, no. 2, 313-322.

2.       
Pihl, J.A., Toops, T.J.,
Fisher, G.B., and West, B.H., Catalysis Today 231 (2014): 46-55.

3.       
Johnson, W.L., Fisher,
G.B., and Toops, T.J., Catalysis Today 184.1
(2012): 166-177.