An FT-IR study of ammonia adsorption and oxidation over anatase-supported metal oxides

The adsorption and the oxidation of ammonia over sub-monolayer TiO2-anatase supported chromium, manganese, iron, cobalt, nickel and copper oxides, has been investigated using FT-IR spectroscopy. These materials are models of catalysts active in the Selective Catalytic Reduction of NOx by ammonia (SCR process) and in the Selective Catalytic Oxidation of ammonia to dinitrogen (SCO process). For comparison, the adsorption of ammonia and hydrazine over the TiO2-anatase support has also been studied. CrOxTiO2 adsorbs ammonia both in a co-ordinated form over Lewis acid sites and in a protonated form over Brønsted acid sites, involving high-valence chromium (chromyl species). However, simple outgassing at r.t. causes the desorption of ammonia from Brønsted acid sites showing that they are very weak. All other catalysts do not present any Brønsted acidity. Co-ordinated ammonia gives rise to several oxidation products over Fe2O3TiO2, CrOxTiO2, CoOxTiO2 and CuOTiO2, among which hydrazine is likely present. Other species have been tentatively identified as imido species, NH, nitroxyl species, HNO, and nitrogen anions,N−2. NiOxTiO2 and MnOxTiO2 appear to be even more active in ammonia oxidation, because the adsorbed species disappeared completely at lower temperature (473 K) than in the other cases. However, possibly just due to their excessive activity, no adsorbed species different from co-ordinated ammonia can be found in significant amounts over these surfaces. Based on these data, the mechanism of the SCR and SCO processes over these catalytic materials is discussed. In particular, it is concluded that Brønsted acidity is not a requirement for SCR and SCO activity.