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 . 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 . 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 h1 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 . 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.
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