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

The Ag/Al 2 O 3 catalyst possesses high activity for selective catalytic oxidation of ammonia (NH 3 –SCO), and its activity could be significantly improved by H 2 pretreatment. It is generally accepted that the enhancement of Ag/Al 2 O 3 activity after H 2 reduction is due to the promoted O 2 activation by metallic Ag species. Here, we show that the recovery of Brønsted acid sites via H 2 reduction is also an important factor. Ag/Al 2 O 3 and H 2 -pretreated Ag/Al 2 O 3 (Ag/Al 2 O 3 –H 2 ) catalysts were prepared and tested for the NH 3 –SCO performances, and also carefully characterized. It is revealed that Ag species were anchored on the Al 2 O 3 surface through exchange between Ag + and H + in AlOH groups during the preparation process, and therefore occupied the Brønsted acid sites of Al 2 O 3 . H 2 reduction could break the Ag–O bonds and thus recover the Brønsted acid sites. In situ DRIFTS results show that NH 4 + species adsorbed on Brønsted acid sites is much more active for reaction with O 2 than NH 3 species adsorbed on Lewis acid sites. DFT calculation results identified the different activation processes for NH 4 + and NH 3 species, confirming that Brønsted acid sites have markedly higher activity than the Lewis acid sites. Therefore, the recovery of Brønsted acid sites after H 2 reduction is also responsible for the improved activity of Ag/Al 2 O 3 –H 2 for NH 3 –SCO.