On the role of H2 to modify surface NOx species over Ag-Al2O3 as lean NOx reduction catalyst: TPD and DRIFTS studies

Formation and stability of surface NOx species related to the promotional effect of H-2 over Ag-Al2O3 as NOx reduction catalyst were investigated with temperature-programmed desorption and DRIFT spectroscopy. Formation of two groups of surface NOx species was found: a less thermally stable group of "low temperature (LT) species" and a more thermally stable group of "high temperature (HT) species". The LT NOx was attributable to the decomposition of surface NOx species formed on the active sites where its elimination by addition of H-2 or thermal decomposition correlated with higher NO oxidation and NOx reduction conversion. Under reaction conditions, these possibly inhibiting LT NOx species were stable up to about 300 degrees C and their formation depended on donation of oxygen from surface oxides. Removal of LT nitrate species by H-2 accounted for only a fraction of the increased NO oxidation and NOx reduction conversion by co-feeding H-2. Furthermore, it was also found that H-2 facilitates formation of HT NOx that primarily corresponded to the decomposition of spectator species on the Al2O3 support identified as monodentate nitrate species. From TPD studies of C3H6-SCR, it was shown that H-2 not only eliminated LT NOx but also promoted formation of greater quantities of adsorbed hydrocarbons.