Ammonia oxidation over Au/MOx/(gamma)-Al2O3—activity, selectivity and FTIR measurements

The selective catalytic oxidation (SCO) of NH3 was studied over various (gamma)-Al2O3-supported gold-based catalysts at 2% NH3+2% O2 (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% Au 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/TiAl2O3 < Au/Al2O3 (almost equal) Au/TiO2 < Au/Li-Al2O3 < Au/FeOx (almost equal) Au/Fe-Al2O3 < Cu-Al2O3 (almost equal) Au/Ce-Al2O3 <= Au/Li–Ce-Al2O3 <= Au/Cu-Al2O3. The selectivity is strongly dependent on the type of MOx used. The most active catalyst Au/Cu–Al2O3 shows a conversion of 50% at 483 K and a N2 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 N2O. FTIR spectroscopy shows bands following NH3 adsorption that can be attributed to imide-like (–NH) adspecies, in addition to NH4+ (on Br?nsted acid sites) and coordinated NH3 (on Lewis acid sites). The presence of Au enhances the band intensity of imide-like adspecies, which may be responsible for the improved SCO activity in the presence of Au.