Mechanistic Study of Ethanol SCR of NOx over Ag/Al2O3: Spatial Distribution of Products
Galen Fisher, University of Michigan
Mechanistic Study of Ethanol SCR of NOx over Ag/Al2O3: Spatial Distribution of Products
Galen B. Fisher*, Amin Reihani, Andrew Brown, Patricia de Sousa Pestana, John W. Hoard
University of Michigan, Ann Arbor, Michigan 48109 (USA)
*gbfisher@umich.edu
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. In addition, acetaldehyde (C2H4O) and NO were also tested to see the impact of acetaldehyde as a direct reactant. Mechanistic insights from these data 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 or 1500 ppm C2H4O (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. 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.
Results and Discussion
The spatial distributions of NO and ethanol concentrations at 13 points before, after, and within the catalyst were measured over a range of 11 inlet flow temperatures from 131°C to 561°C. The data 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. At elevated temperatures, there is already some NO conversion at the inlet face of the monolith indicating a fast surface reaction. The same general behavior is observed for ethanol. The overall NOx and ethanol conversions show good agreement with previous studies [2, 3]. The acetaldehyde SCR of NOx measurements may be the first taken.
The axial distribution of a range of products has been measured at the same time as the NO and ethanol. They include N2, NO2, NH3, N2O, C2H4O, C2H4 (ethylene), and CO. Of special note is that about 50% of the NO forms the maximum concentration of ammonia seen immediately at the inlet of the monolith in the temperature window of 300 to 560°C. Only small amounts of ammonia are then consumed moving towards the outlet. The other 50% of the NO makes N2 that grows to near saturation about 5-7 mm from the front of the catalyst at temperatures above 400°C. Significant amounts of acetaldehyde are also observed, which is formed with a slower rate over a wider 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.
The conversion of NO in this reaction proceeds in parallel with the formation of ammonia and then nitrogen. The growth and disappearance of acetaldehyde inside the monolith appears connected to NOx reduction. And with acetaldehyde as the initial HC reactant the formation of ammonia and nitrogen are generally like that found with ethanol. Further analysis and studies are underway to further provide mechanistic insight into this important HC-SCR reaction.
Significance
These first 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
- 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.
- Pihl, A., Toops, T.J., Fisher, G.B., and West, B.H., Catalysis Today231 (2014): 46-55.
- Johnson, W.L., Fisher, G.B., and Toops, T.J., Catalysis Today1 (2012): 166-177.