New insights on the adsorption, thermal decomposition and reduction of NOx over Pt- and Ba-based catalysts

In this study mechanistic aspects related to the adsorption of NOx over alumina-supported Pt, Ba and PtBa catalysts are deepened, that are of interest for NOx Storage-Reduction (NSR) catalysts and other applications involving the adsorption of NOx (e.g. Passive NOx Adsorbers, PNAs). NOx adsorption is investigated at low and high temperatures (150 degrees C and 350 degrees C, respectively); the thermal decomposition and the reactivity with H-2 of the adsorbed species is also addressed. By coupling FT-IR spectroscopy and microreactor studies, new insights on the adsorption, decomposition and reactivity of the stored NOx, are derived. In particular it is found that at 150 degrees C nitrites are formed on all surfaces when starting from NO/O-2; different species are formed onto the different storage sites (Ba vs. Al) that can be distinguished spectroscopically. Different routes for the storage of nitrites are highlighted, i.e. direct oxidative NO uptake and nitrite formation involving NO2 produced by oxidation of NO. Clear and novel evidence is herein provided that the direct NO oxidative uptake is much faster than the route involving the NO to NO2 oxidation. Both routes are catalyzed by Pt, although Ba is also able to store nitrites; the role of the interaction between Pt and the storage sites is discussed. When the NOx storage is carried out at higher temperatures (350 degrees C), both routes are greatly favored although nitrites can hardly be observed being readily transformed into nitrates. Besides NO2 is also formed in significant amounts that may participate to the formation of nitrites/nitrates adspecies following a NO2 disproportion pathway for which clear and new spectroscopic evidences are herein provided. The adsorbed species (nitrites or nitrates) start to decompose above the adsorption temperature, i.e. 150 degrees C and 350 degrees C, respectively. In the presence of Pt, the decomposition of the stored nitrites leads to the formation of nitrates and NO due to the occurrence of a nitrite disproportion reaction. The formed nitrates show high thermal stability and decompose only at high temperatures, thus making these systems unappropriated for PNA applications. In the presence of a reductant, Pt catalyzes the reduction of the stored NO at much lower temperatures than the adsorption; the role of the interaction between Pt and the storage sites on this step is herein discussed.