(244e) NOx Reduction and NH3 Production Over Silver-Alumina Catalysts With Oxygenated Hydrocarbon Fuels

Introduction

       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 the 10% level in all gasoline and may increase to 15% or higher levels as higher alcohol content gasoline blends are evaluated up to 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 determining the important factors in alcohol promotion of ammonia formation.  Using DRIFTS along with reactor studies we are able to describe this chemistry in more detail and report the first results that correlate the formation of ammonia with ethanol concentration under lean NOx conditions.    

Materials and Methods

        A 2 wt.% Ag/gamma-Al₂O₃ catalyst was synthesized using the incipient wetness technique with a AgNO₃ precursor.  Additionally, a washcoated commercial Ag/gamma-Al₂O₃ was obtained from Catalytic Solutions.  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, with variable amounts of ethanol (250-2000 ppm) 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. 

Results and Discussion

      The product distribution for ethanol SCR of NOx under lean conditions for the temperature window of 200-550 °C with Ag/gamma-Al₂O₃ at 30k h^-1 GHSV (HC₁/NOx=3) shows clear evidence of three regions of correlated products being formed for N- and C-containing species up to 450°C in the following order:

 EtOH + NO -> CH₃CHO + NO₂ -> [CO₂ + CO] + [N₂ + NH₃]

 Ethanol and NO react in the presence of oxygen starting at 200 °C to form acetaldehyde and NO₂.  Between 275 and 450°C, ethanol and NO form CO₂, CO, N₂, and NH₃.  Above 450 °C, behavior depends on HC/NOx the ratio. As we have discussed recently [5], the NO₂ and acetaldehyde release correlates with the decrease in adsorbed nitrate and acetate monitored by DRIFTS.  At higher temperatures CO₂, CO, N₂, and NH₃ are all formed via much less stable intermediates. The selectivity to NH₃ (versus N₂) increases greatly as the HC/NOx ratio increases from 1 to 8, reaching more than 40% at 350 °C for a HC/NOx ratio of 8.

       Additional efforts are being made to investigate the ability of ethanol and gasoline-ethanol blends to promote the NOx removal activity of hydrocarbons that would be found in gasoline exhaust, similar to the way H₂ has been shown to promote low temperature NOx removal with hydrocarbons.

Significance

      Several factors suggest that higher ethanol content gasoline fuels will rise in importance as transportation fuels.  Understanding their ability to provide a potential non-urea pathway to NOx reduction with alcohol-based fuels may facilitate the implementation of a non-precious metal emissions control system that is applicable in lean burn vehicles.

References

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. Johnson, W. J., Fisher, G. B., and Toops, T. J., Catalysis Today 184, 166 (2012).