(141g) Mechanistic Investigation of Ethanol SCR of NOx Over Ag/Al2O3

Supported Ag on gamma-Al₂O₃ has been shown to be an effective catalyst for selectively reducing NOx to N₂ in a wide temperature range with various hydrocarbons and oxygenates typically found in fuels used in mobile systems.  Additionally, this reactivity has been demonstrated at a range of hydrocarbon to NOx (HC₁/NOx) ratios that would not introduce a severe fuel penalty [1].  A particularly effective HC for this selective catalytic reduction (SCR) of NOx is ethanol, which is found at low levels in all gasoline and is becoming more prevalent in concentration and availability in domestic automotive fuels (e.g., E85).  Fisher et al. [2,3] have demonstrated that flowing NOx and ethanol vapor (and other HCs) over these same Ag-based catalysts can form NH₃ under lean conditions, which can then be used with a downstream zeolite-based NH₃-SCR catalyst to produce NOx conversions in excess of 90% for realistic exhaust streams in lean burn engine systems. This important finding may offer an alternative to urea dosing currently being implemented with several NH₃-based SCR systems using either zeolite- or vanadia-based catalysts [4].  All of these recent findings point to the importance of studying this chemistry and establishing a predictive model of the reaction in the silver catalyst that may also account for NH₃ formation. 

A 2 wt.% Ag/g-Al₂O₃ catalyst was synthesized using the incipient wetness technique with a AgNO₃ precursor.  Catalysts were calcined at 700 °C for 15 h in flowing air and evaluated at gas hourly space velocities (GHSV) ranging from 30k to 180k h^-1 in  temperatures from 200 to 550°C.  Feed gas conditions were 500 ppm NO, 10% O₂, 5% H₂O, and either 750 ppm ethanol at 180k h^-1 GHSV or 1500 ppm ethanol at 30k h^­1 GHSV (balance Ar).  All gas flows were controlled by mass flow controllers with H₂O and ethanol introduction using impingers submersed in temperature controlled baths.  Chemiluminscent NOx analyzers were used in conjunction with a quadrupole mass spectrometer for gas-species analysis. 

At 30k h^-1 and a HC₁/NOx=6 ethanol conversion increases very rapidly from 0% at 200 °C to over 80% at 250 °C; this contrasts with the low NOx conversion at 250°C, less than 20%, despite the high HC₁/NOx ratio of 6.  However, this ethanol oxidation does not occur in the absence of NOx; in fact, there is 0% conversion at 250°C when flowing ethanol vapor and O₂ to the reactor.  This new result is consistent with NOx initiating the ethanol oxidation pathway, especially considering that Ag/Al₂O₃ is an excellent NO to NO₂ oxidization catalyst, greater than 80% NO₂/NOx at 200°C.  This finding suggests that the high ethanol oxidation rate at 250°C may be due to NO₂ initiating the reaction.  However, this NO₂ is only reduced to NO, which can be re-oxidized to NO₂, and the cycle can continue.  At higher temperatures the NO can be dissociated and its reduction pathway can be followed as evidenced by the higher NOx conversion rates. 

To further study the kinetics and mechanisms at work in this catalyst system it is necessary to investigate the reactions in the kinetic regime.  Differential conversion of both NOx and ethanol is achieved between 250-290°C at a GHSV of 180k h^-1.  This allows for the determination of the activation energy for NOx conversion of 63 kJ/mol, which is comparable to the 67 kJ/mol reported by Shimizu et al. for NOx reduction with n-hexane over Ag/Al₂O₃ [5].  Additional efforts are being performed to determine the dependence of the reaction rate on the partial pressure of the reactants. These tests will help with kinetic rate law development that includes the rate of NH₃ formation.

1.  Wu, Q., Hong, H., and Yu, Y. Appl. Catal. B:  Environmental 61, 67 (2005).
2.  Fisher, G. B., DiMaggio, C. L., Trytko, D., Rahmoeller, K. M., and Sellnau, M. SAE Paper 2009-01-2818 (2009).
3.  DiMaggio, C. L., Fisher, G. B., Rahmoeller, K. M., and Sellnau, M. SAE Paper 2009-01-0277 (2009).
4.  Diewald, R., 2010 Directions in Engine-Efficiency and Emissions Research (DEER) Conference, September, 30, 2010, (2010).
5.  Shimizu, K., Shibata, J., Yoshida, H., and Hattori, T. Appl. Catal. B:  Environmental 30, 151 (2001).