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

Abstract

The nature of the NOx species obtained on NO adsorption and its coadsorption with O2 at room temperature on TiO2 and MnOx/TiO2 catalysts with two different manganese loadings has been studied by means ofin situFourier transform infrared spectroscopy. In order to obtain information about the potentials of titania-supported manganese materials as catalysts for selective catalytic reduction (SCR) of NO by hydrocarbons, the stability and reactivity of the adsorbed NOx species toward decane has been investigated. The adsorption of NO on the support and the catalysts leads to disproportionation of NO to anionic nitrosyl, NO−, its protonated form, NOH, and NO2. On coadsorption of NO and O2 at room temperature, various kinds of surface nitrates are observed differing in the mode of their coordination. The nitrates on the manganese-containing samples are characterized by significantly lower thermal stability than the NO− 3 species on the pure support, titania. The difference in the thermal stability of the nitrates parallels their reactivity toward the reducer (decane). The monodentate and bridged nitrates formed on the manganese catalysts studied are able to oxidize the adsorbed hydrocarbon at temperatures as low as 373 K. A mechanism for the interaction between the surface nitrates and the adsorbed decane is proposed in which the NO− 3 and NCO− species are considered as important intermediates leading to dinitrogen formation. The concentration of the reactive surface nitrates on the MnOx/TiO2 catalyst with manganese content corresponding to a monolayer is considerably greater than that on the sample with higher manganese loading. The former catalyst is promising for the SCR of NO by longer chain saturated hydrocarbons. °c 2001 Elsevier Science