Unusual Activity Enhancement of NO Conversion over Ag/Al2O3 by Using a Mixed NH3/H2 Reductant Under Lean Conditions

Addition of H₂ to a NO/NH₃/O₂/H₂O feed for selective catalytic reduction of nitrogen oxide over Ag/Al₂O₃ catalysts causes an unusual enhancement of activity, e.g., the marginal activity (<10%) of 1 wt% Ag impregnated on γ-Al₂O₃ or mesoporous Al₂O₃ modifications is boosted to nearly 100% over a broad temperature range from 200 to 550°C at a space velocity of 30,000cm³g^?1h^?1). Contrary, silver on SiO₂ or α-Al₂O₃ shows no improvement of activity in the presence of H₂. The effect is tentatively attributed to a higher percentage of intermediary nano-sized Ag clusters on high-surface area Al₂O₃ in the presence of hydrogen. This promotes oxygen activation and hence NO oxidation to reactive intermediate nitrite species. The required dispersion of Ag cannot be stabilized on SiO₂ or α-Al₂O₃.