Selective Catalytic Reduction of NOx Over Ag/Alumina: Investigation of Alcohols and Fuel Blends On Performance and Selectivity

Introduction and Experimental

Ag/Alumina has been shown to be an effective catalyst for selectively reducing NOx to N₂ in a wide temperature range with various hydrocarbons and alcohols.  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.  It has also been demonstrated that HC-SCR with ethanol vapor (and other HCs) over Ag-alumina can form NH₃ under lean conditions [2,3], which can then be used with a downstream zeolite-based NH₃-SCR catalyst to achieve higher NOx conversion. This important finding may offer an alternative to urea-SCR systems at a lower cost.  In our work we report a synergy between NO and ethanol that lowers the ethanol oxidation lightoff temperature and the NOx conversion lightoff relative to other HCs.  DRIFTS studies suggest that surface acetate formation plays a role in this enhancement.  All of these recent findings point to the importance of determining the mechanism and a predictive model of this reaction over a silver catalyst that includes this synergy and 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 140k h^-1.  Feed gas conditions were 500 ppm NO, 10% O₂, 5% H₂O, and either 750 ppm ethanol at 140k h^-1 GHSV or 1500 ppm ethanol at 30k h^-1 GHSV (balance Ar).  Gas flows from mass flow controllers incluced H₂O and ethanol introduction using impingers. 

Results and Discussion

Peak NOx conversions reached 85% at 400°C (Fig. 1–right), and an activation energy was determined to be 57 kJ/mol with a feed of ethanol with HC₁/NOx =3.  Up to 80% of the NO is oxidized to NO₂ at 250°C, but overall NOx conversion is only 15%.  Interestingly, ethanol oxidation occurs at much lower temperatures than NOx reduction; at 250°C, ethanol oxidation is 80% when flowing ethanol+NO+O₂.  This increased reactivity, compared to only 15% when flowing only ethanol+O₂, combined with the observation that NO is not oxidized to NO₂ in the absence of ethanol illustrates a synergistic relationship between the reactants.

 

To further investigate this chemistry, a series of DRIFTS experiments were performed.  To form nitrates/nitrites on the catalyst it was necessary to include ethanol in conjunction with NO+O₂ which also formed NO₂ (Fig. 1-right).  It is proposed that ethanol adsorbs through an ethoxy-intermediate on its way to a stable acetate, that provides atomic hydrogen to the surface.  This hydrogen aids the release of NO₂ from Ag to the gas-phase which, can subsequently react on Ag or be adsorbed at alumina sites away from Ag.  The disappearance of nitrates/nitrites at higher temperatures proceeds in parallel with the increase in NOx reduction reactivity between 300 and 350°C observed in the kinetic study (Fig. 1-left).  It is therefore proposed that the consumption of nitrates is involved in the rate determining step for this reaction.  In addition, the acetate formed on the surface may lead to the synergy between NO and ethanol that causes a lower ethanol oxidation lightoff and the release of NO₂ to initiate NOx conversion.

Figure 1.  Ethanol light-off precedes NOx light-off for 2 wt.% Ag/alumina catalyst.  Ethanol presence results in high NO to NO₂ oxidation and nitrate formation.  Nitrate consumption parallels NOx light-off.

 

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).