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

Abstract

Formation and stability of surface NO_x species related to the promotional effect of H₂ over Ag–Al₂O₃ as NO_x reduction catalyst were investigated with temperature-programmed desorption and DRIFT spectroscopy. Formation of two groups of surface NO_x 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 NO_x was attributable to the decomposition of surface NO_x species formed on the active sites where its elimination by addition of H₂ or thermal decomposition correlated with higher NO oxidation and NO_x reduction conversion. Under reaction conditions, these possibly inhibiting LT NO_x species were stable up to about 300 °C and their formation depended on donation of oxygen from surface oxides. Removal of LT nitrate species by H₂ accounted for only a fraction of the increased NO oxidation and NO_x reduction conversion by co-feeding H₂. Furthermore, it was also found that H₂ facilitates formation of HT NO_x 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 H₂ not only eliminated LT NO_x but also promoted formation of greater quantities of adsorbed hydrocarbons.