Mechanistic study of ethanol SCR of NOx over Ag/Al2O3 with a spatial distribution of products

Galen  Fisher, University of Michigan

Galen B. Fisher1*, Amin Reihani2, John W. Hoard2

1Chemical Engineering Dept., 2Mechanical Engineering Dept., University of Michigan, Ann Arbor, MI 48109

NOx reduction in lean exhaust by ethanol SCR of NOx over Ag/Al2O3 catalyst has been studies in previous work as a system that achieves 100% NOx conversion over a moderate temperature widow 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 and acetaldehyde was observed during this reaction which might be active species for NOx reduction. The focus of this study is to obtain mechanistic insights on this reaction by investigating the spatial distribution of reaction products in different temperature regimes. Such insights may allow for further improvements in the Dual SCR aftertreatment system [1] that can use the ammonia from the ethanol-NOx reaction in this silver catalyst.

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 both MS and FTIR simultaneously. 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 by a 6:1 ratio to achieve the desired time response from the MS and FTIR.

Fig.2 shows the spatial distribution of NO and ethanol at different inlet flow temperatures, which indicate slow reduction and long reaction zone of 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 which indicates a very 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).

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 and 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.

The conversion of NO in this reaction proceeds in parallel with the consumption of ammonia and acetaldehyde inside the monolith which suggests them as 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.

References 

[1] G. B. Fisher, C. L. DiMaggio, D. Trytko, K. M. Rahmoeller, and M. Sellnau, “Effects of fuel type on Dual SCR aftertreatment for lean NOx reduction, SAE Paper #2009-01-2818.

[2] J. A. Pihl, T. J. Toops, G. B. Fisher, and B. H. West, “Selective catalytic reduction of nitric oxide with ethanol/gasoline blends over a silver/alumina catalyst.” Catalysis Today 231 (2014): 46-55.

[3] W. L. Johnson, G. B. Fisher, and T. J. Toops. “Mechanistic investigation of ethanol SCR of NOx over Ag/Al 2 O 3.” Catalysis Today 184.1 (2012): 166-177.