Mechanistic investigation of ethanol SCR of NO _x over Ag/Al ₂O ₃

A 2 wt.% Ag/γ-Al ₂O ₃ catalyst was studied for the ethanol selective catalytic reduction of NO _x from 200 to 550°C and space velocities between 30,000 h ^-1and 140,000 h ^-1. Peak NO _x conversions reached 85% at 400°C, and an activation energy was determined to be 57 kJ/mol with a feed of ethanol to NO _x or HC ₁/NO _x = 3. Up to 80% of the NO is oxidized to NO ₂ at 250°C, but overall NO _x conversion is only 15%. Interestingly, ethanol oxidation occurs at much lower temperatures than NO _x reduction; at 250°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 NO ₂ 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 NO ₂ 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 NO ₂ from Ag to the gas-phase which, can be subsequently adsorbed at γ-Al ₂O ₃ sites away from Ag. The disappearance of these nitrates/nitrites at higher temperatures proceeds in parallel with the increase in NO _x 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.