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Insight into the performance-boosting mechanism of Ag/MgAlOx for NH3 selective catalytic oxidation: Perspective from the support crystalline phase structures

Yin Che,Xuezhen Liu,Lifei Liu,Yali Du,Xia An,Xu Wu

Arranging an ammonia oxidation process after the selective catalytic reduction of NO x by NH 3 (NH 3 -SCR) system is an effective way for ensuring the DeNO x efficiency and controlling the slip ammonia. To improve operability and reduce operating costs in ammonia selective catalytic oxidation (NH 3 -SCO), developing feasible and efficient catalysts becomes meaningful. Herein, focusing on the dilemma of Ag/Al 2 O 3 catalysts in NH 3 -SCO applications, Ag/MgAlO x catalysts with different support different crystalline phase structures (5 %Ag/MgAl-LDO and 5 %Ag/MgAl 2 O 4 ) were innovatively synthesized and systematically evaluated application potential in NH 3 -SCO. Results indicated that the introduction of Mg apparently broadened reaction temperature window and catalytic performance of 5 %Ag/MgAl 2 O 4 was higher than 5 %Ag/MgAl-LDO, while the N 2 selectivity was completely opposite. The combination of series characterization and density functional theory (DFT) calculations revealed that 5 %Ag/MgAl 2 O 4 surface has more Ag 0 species with small particle size, which is favorable for ammonia adsorption and activated dehydrogenation to further promote reaction of NO x species, resulting in a higher NH 3 -SCO activity. In contrast, 5 %Ag/MgAl-LDO readily induces oxidized Ag species and Ag 0 is more inclined to be present in large particle sizes, which is more conducive to dissociation of O 2 to produce reactive O species (O*) leading to the peroxidation of ammonia to nitrate and thus, positively affects the improvement of N 2 selectivity. Model catalyst 5 %Ag/MgAl-650 (mixed-phase) was prepared by controlling the roasting conditions and exhibited excellent catalytic performance in NH 3 -SCO with almost 93 % of NH 3 oxidized at 250 °C (N 2 selectivity up to 71 %). This work could afford theoretical support for the optimization and application of catalysts with high performance at low temperatures in selective catalytic oxidation of slip ammonia.