Understanding the Distinct Effects of Ag Nanoparticles and Highly Dispersed Ag Species on N2 Selectivity in NH3–SCO Reaction

Ag-based catalysts, especially Ag/Al2O3, show high NH3 oxidation activity in NH3-selective catalytic oxidation (NH3–SCO). However, the low N2 selectivity limits their further application. To improve N2 selectivity, herein, a series of Ag/Al2O3-X catalysts (X = calcination temperature) are designed by simple calcination. Only Ag nanoparticles (Ag NPs) were observed on the Ag/Al2O3-400 catalyst, while highly dispersed Ag species (Ag HDs) were dominated on the Ag/Al2O3-800 catalyst. NH3–SCO results showed that Ag NPs/Al2O3 forms much N2O by-products, whereas Ag HDs/Al2O3 achieves >99% N2 selectivity over the Ag/Al2O3-800 catalyst at <200 °C. More importantly, various intermediates (NO3 ads, NH2 ads, N═N–M, and N═N–O–M) and their internal transformations were detected, and the reaction pathways for the formation of N2O and N2 are evidenced by in situ NH3-Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). This study not only provides a convenient and effective approach to obtain excellent N2 selectivity in NH3–SCO but also affords a systematic insight into the reaction pathways over Ag/Al2O3 catalysts.