Identification, stability and reactivity of NOx adsorbed species on titania-supported manganese catalysts

Abstract

The needs of improved fuel economy, and lower emission of green-house gases such as CO2, is projected to increase the demand for diesel engines through the world. These engines operate at air/fuel ratio higher than stoichiometric (lean-burn conditions). This results in relatively low hydrocarbon/NOx ratio in the exhaust and an additional amount of reductant (typically about 2-3% of additional fuel) should be fed upstream of de-NOx catalyst. For this reason, it is important to study the interaction of NOx species produced upon adsorption of NO/O2 mixtures on the catalyst surface with long chain saturate hydrocarbons, which are typical for diesel fuel.In recent years many de-NOx lean-burn catalysts have been proposed among which supported metal oxides have been taken in consideration for their potential thermal stability and large composition variability. Subjects of this study are titania (anatase)-supported manganese catalysts, prepared by impregnation and ion-exchange from aqueous solutions of Mn2+ ions. TiO2 (anatase) is stable in SO2 containing atmosphere, typical for the exhaust gases in diesel engines. The identification of the NOx species formed during the adsorption of NO, NO/O2 mixtures and NO2 is performed by in situ FTIR Spectroscopy. the thermal stability and reactivity of the surface NOx forms towards n-decane is followed by application of the same technique. It is established that adsorption of NO on the support and manganese-containing catalysts is reactive and leads to linearly adsorbed NO and formation of anionic nitrosyl, NO− and NO3 − species. Contrary to the impregnated catalyst, the ion-exchanged catalyst does not contain NO− species coordinated to Ti4+ ions. This experimental fact is in agreement with the high dispersion of Mn3+ ions concluded from the CO adsorption experiments.The NO/O2 co-adsorption on the anatase and catalysts studied results in formation of NO3 − species differing in the mode of their coordination. Under these conditions no NO− species are detected. The surface nitrates formed on the manganesecontaining catalysts possess lower thermal stability than those on the pure support. This difference explains the higher reactivity of the former toward the n-decane. The nitrates formed upon NO/O2 co-adsorption on the manganese-containing catalysts are able to activate and oxidize the hydrocarbon at temperatures as low as 373 K. The latter process gives rise to adsorbed CO2, formic acid and isocyanate species. The NCO species is considered as an important intermediate in the formation of nitrogen. The extent of oxidation of n-decane is higher on the ion-exchanged catalyst. It is concluded that this catalyst can be promising in the selective catalytic reduction of NO by longer-chain saturated hydrocarbons.