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

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 Bronsted 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 Bronsted acid sites of Al2O3. H2 reduction could break the Ag–O bonds and thus recover the Bronsted acid sites. In situ DRIFTS results show that NH4+ species adsorbed on Bronsted acid sites is much more active for reaction with O2 than NH3 species adsorbed on Lewis acid sites...