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-an atase supported chromium, manganese, iron, cobalt, nickel and copper o xides, has been investigated using FT-IR spectroscopy. These materials are models of catalysts active in the Selective Catalytic Reduction o f NOx by ammonia (SCR process) and in the Selective Catalytic Oxidatio n of ammonia to dinitrogen (SCO process). For comparison, the adsorpti on of ammonia and hydrazine over the TiO2-anatase support has also bee n studied. CrOx-TiO2 adsorbs ammonia both in a co-ordinated form over Lewis acid sites and in a protonated form over Bronsted acid sites, in volving high-valence chromium (chromyl species), However, simple outga ssing at r.t. causes the desorption of ammonia from Bronsted acid site s showing that they are very weak. All other catalysts do not present any Bronsted acidity. Co-ordinated ammonia gives rise to several oxida tion products over Fe2O3-TiO2, CrOx-TiO2, CoOx-TiO2 and CuO-TiO2, amon g which hydrazine is likely present. Other species have been tentative ly identified as imido species, NH, nitroxyl species, HNO, and nitroge n anions, N-2(-). NiOx-TiO2 and MnOx-TiO2 appear to be even more activ e in ammonia oxidation, because the adsorbed species disappeared compl etely at lower temperature (473 K) than in the other cases. However, p ossibly just due to their excessive activity, no adsorbed species diff erent from co-ordinated ammonia can be found in significant amounts ov er these surfaces. Based on these data, the mechanism of the SCR and S CO processes over these catalytic materials is discussed. In particula r, it is concluded that Bronsted acidity is not a requirement for SCR and SCO activity.