Mechanistic investigation of ethanol SCR of NOx over Ag/Al2O3

A 2 wt.% Ag/gamma-Al2O3 catalyst was studied for the ethanol selective catalytic reduction of NOx from 200 to 550 degrees C and space velocities between 30,000 h(-1) and 140,000 h(-1). Peak NOx conversions reached 85% at 400 degrees C, and an activation energy was determined to be 57 kJ/mol with a feed of ethanol to NOx or HC1/NOx = 3. Up to 80% of the NO is oxidized to NO2 at 250 degrees C, but overall NOx conversion is only 15%. Interestingly, ethanol oxidation occurs at much lower temperatures than NOx reduction; at 250 degrees C, ethanol oxidation is 80% when flowing ethanol + NO + O-2. This increased reactivity, compared to only 15% when flowing only ethanol + O-2, 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 catalysts it was necessary to include ethanol in the feed with NO. These nitrates/nitrites were readily formed when flowing NO2 over the catalyst. It is proposed that ethanol adsorbs through an ethoxy-intermediate that results in atomic hydrogen on the surface. This hydrogen aids the release of NO2 from Ag to the gas-phase which, can be subsequently adsorbed at gamma-Al2O3 sites away from Ag. The disappearance of these nitrates/nitrites at higher temperatures proceeds in parallel with the increase in NOx reduction reactivity between 300 and 350 degrees C observed in the kinetic study. It is therefore proposed that the consumption of nitrates is involved in the rate determining step for this reaction. (C) 2011 Elsevier B.V. All rights reserved.