Insights into the Activation Effect of H₂ Pretreatment on Ag/Al₂O₃ Catalyst for the Selective Oxidation of Ammonia

Abstract

The Ag/Al2O3 catalyst possesses high activity for selective catalytic oxidation of ammonia (NH3–SCO), and its activity could be significantly improved by H2 pretreatment. It is generally accepted that the enhancement of Ag/Al2O3 activity after H2 reduction is due to the promoted O2 activation by metallic Ag species. Here, we show that the recovery of Brønsted acid sites via H2 reduction is also an important factor. Ag/Al2O3 and H2-pretreated Ag/Al2O3 (Ag/Al2O3–H2) catalysts were prepared and tested for the NH3–SCO performances, and also carefully characterized. It is revealed that Ag species were anchored on the Al2O3 surface through exchange between Ag+ and H+ in AlOH groups during the preparation process, and therefore occupied the Brønsted acid sites of Al2O3. H2 reduction could break the Ag–O bonds and thus recover the Brønsted acid sites. In situ DRIFTS results show that NH4+ species adsorbed on Brønsted acid sites is much more active for reaction with O2 than NH3 species adsorbed on Lewis acid sites. DFT calculation results identified the different activation processes for NH4+ and NH3 species, confirming that Brønsted acid sites have markedly higher activity than the Lewis acid sites. Therefore, the recovery of Brønsted acid sites after H2 reduction is also responsible for the improved activity of Ag/Al2O3–H2 for NH3–SCO.