Unraveling the Mechanism of Ammonia Selective Catalytic Oxidation on Ag/Al2O3 Catalysts by Operando Spectroscopy

Ammonia leakage from the aftertreatment of diesel vehicles arouses serious environmental concerns, thus demanding a high-efficiency ammonia elimination catalyst. In the present work, the mechanism of ammonia selective catalytic oxidation (NH3-SCO) on Ag/Al2O3 catalysts was investigated by operando spectroscopy (DRIFTS-MS and DR-UV–vis). Characterization results show that silver cations are predominant on Ag/Al2O3 samples with a low silver loading, whereas many silver nanoparticles are present on 10 wt % Ag/Al2O3. Operando DR-UV–vis analysis confirms that silver nanoparticles are the active sites for NH3 oxidation, while silver cations are inert in this reaction. During NH3 oxidation, oxygen activation on the silver nanoparticles induces the oxidation of metallic silver, whereas NH3 dehydrogenation, in turn, reduces the oxidized silver species. Notably, the NH3-SCO reaction on 10 wt % Ag/Al2O3 could be divided into three regions: the light-off region, high-efficiency region, and high-temperature region. The reaction pathway of NH3 oxidation is nearly related to oxygen activation on the silver nanoparticles; that is, a slow activation rate results in the generation of N2O, whereas rapid activation induces the quick dehydrogenation of NH3 to produce NO. Instead, appropriate rates for O2 activation and NH3 dehydrogenation are essential for N2 formation during NH3 oxidation.