A cyclic reaction pathway triggered by ammonia for the selective catalytic reduction of NOx by ethanol over Ag/Al2O3

Alumina-supported silver catalysts (Ag/Al2O3) prepared using nordstrandite as a precursor exhibited excellent activity for the selective catalytic reduction of NOx by ethanol (ethanol-SCR). During the ethanol-SCR, the formation of NH3 was observed over all Ag/Al2O3 catalysts, the amount of which was related to silver loading, reaction temperature, and the concentration of ethanol used, even when water was excluded from the inlet gas. Transient activity testing and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis showed that the reaction between isocyanate species (–NCO) and water is the main pathway of NH3 formation. Addition of NH3 enhanced the NOx conversion for ethanol-SCR and the oxidation of ethanol over 3 wt% Ag/Al2O3. DRIFTS analysis revealed for the first time that NH3 reacted with enolic species to form –NCO over Ag/Al2O3, thus resulting in a novel and cyclic reaction pathway for ethanol-SCR.

Ammonia originating from the hydrolysis of –NCO species can in turn react with enolic species to produce –NCO, creating a cyclic pathway for the reduction of NOx by ethanol over Ag/Al2O3.Figure optionsDownload as PowerPoint slideHighlights
► Ag/Al2O3 prepared from nordstrandite shows high activity for ethanol-SCR.
► Ammonia was inevitably produced by the hydrolysis of–NCO over Ag/Al2O3.
► In turn, the produced ammonia can react with enolic species to produce –NCO.
► This reaction involving ammonia creates a cyclic pathway for ethanol-SCR.