In situ DRIFTS-MS studies on the oxidation of adsorbed NH3 by NOx over a Cu-SSZ-13 zeolite

DRIFT spectroscopy combined with mass spectrometry was used to investigate the oxidation of adsorbed ammonia by NO2, NO + O2 and NO2 + O2 on a copper ion exchanged SSZ-13 (Cu-SSZ-13) zeolite. Compared with both NO2 and NO, the adsorption of ammonia is much stronger on the Cu-SSZ-13 zeolite. Two adsorbed ammonia species were found over the Cu-SSZ-13 zeolite studied here, notably ammonia on Brønsted acid sites (proton) and ammonia on Lewis acid sites (copper ions). These adsorbed ammonia species present different activity profiles and selectivity to N2 during NH3 oxidation. The results obtained suggest that ammonia adsorbed onto copper ions in Cu-SSZ-13 is more active at low temperatures than proton-adsorbed NH3, and give rise to a higher selectivity to N2. The formation of N2O is associated primarily with the reaction of NOx with proton-adsorbed NH3 via the formation and subsequent thermal decomposition of NH4NO3.

Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (235 K)Download as PowerPoint slideHighlights► Ammonia adsorbed to both Brønsted and Lewis acid sites of Cu-SSZ-13 can react with NOx molecules. ► Copper-bound ammonia reacts faster with NOx than proton-adsorbed one and produces N2 selectively. ► High N2 formation selectivities were observed with NO + O2, and NO2 + O2 oxidizing gas mixtures, while NH3 oxidation also produced N2O when NO2 was the only oxidizing agent present.