Elimination of NH3 by Interfacial Charge Transfer over the Ag/CeSnOx Tandem Catalyst

Selective catalytic oxidation of NH3 is the most promising method for removing low-concentration NH3. However, achieving high activity and N2 selectivity remains a great challenge. A Ag/CeSnOx tandem catalyst with dual active centers was designed and synthesized, which couples the NH3 over-oxidation on the noble metal active sites with NOx reduction on the support. The tandem catalyst exhibited excellent NH3 selective catalytic oxidation (NH3–SCO) performance at 200–400 °C. Based on various characterization techniques and DFT calculations, it was identified that the silver species on the Ag/CeSnOx catalysts existed as AgO nanoparticles (AgO NPs), and the electrons on the support were more easily transferred to AgO NPs, which promoted the oxidation activity of AgO and the reduction performance of the CeSnOx support. The coupling between the AgO NPs and CeSnOx helped balance the NH3 oxidation rate and the NOx reduction rate. In addition, the uniform adsorption of gaseous NH3 on the oxidation and reduction sites was also demonstrated by theoretical calculations, which is a prerequisite for tandem catalysis. By in situ DRIFTS, we revealed that the NH3–SCO reaction over Ag/CeSnOx catalysts mainly follows the internal selective catalytic reduction mechanism. It was characterized by excessive oxidation of NH3 to NOx on AgO NPs. At a temperature lower than 200 °C, NOx was reduced to N2 by the adsorbed NH3 on the AgO. When the temperature was higher than 200 °C, NOx was reduced to N2 by NH3 or NH4+ adsorbed on the CeSnOx support. Therefore, the charge transfer at the Ag/CeSnOx catalyst interface and the coordination of atomic scale catalytic sites have realized the conversion of NH3 to N2 through NOx in a tandem catalytic mode.