(753c) 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).