Mechanistic investigation of ethanol SCR of NOx over Ag/Al2O3

A 2 wt.% Ag/γ-Al2O3 catalyst was studied for the ethanol selective catalytic reduction of NOx from 200 to 550 °C and space velocities between 30,000 h−1and 140,000 h−1. Peak NOx conversions reached 85% at 400 °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 °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 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 γ-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 °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.