Ammonia oxidation over Au/MOx/γ-Al2O3 -: activity, selectivity and FTIR measurements

The selective catalytic oxidation (SCO) of NH3 was studied over various beta-Al2O3-supported gold-based catalysts at 2% NH3 + 2% O-2 (balance He). The effects of different kinds of oxidic additives, MOx, (M = Cu, Fe, Ce, Li, and Ti) were examined. The mixed metal oxides, MOx-Al2O3, were used as supports of 5% An catalysts. The presence of Au enhances the SCO activity. The following order of per-gram activity was found in the temperature range from 473 to 673 K: Au/Ti-Al2O3 < Au/Al2O3 approximate to Au/TiO2 < Au/Li-Al2O3 < Au/FeOx approximate to Au/Fe-Al2O3 < Cu-Al2O3 Au/Ce-Al2O3 < Au/Li-Ce-Al2O3 < Au/Cu-Al2O3. The selectivity is strongly dependent on the type of MO, used. The most active catalyst Au/Cu-Al2O3 shows a conversion of 50% at 483 K and a N-2 selectivity over 90%. On the other hand, Au/Li-Ce-Al2O3, which has an only slightly lower activity than Au/Cu/Al2O3 is very selective to NO. FTIR spectroscopy shows bands following NH3 adsorption that can be attributed to imide-like (-NH) adspecies, in addition to NH4+ (on Bronsted acid sites) and coordinated NH3 (on Lewis acid sites). The presence of An enhances the band intensity of imide-like adspecies, which may be responsible for the improved SCO activity in the presence of An. (C) 2004 Elsevier B.V All rights reserved.