An understanding of active species-metal oxide support interaction is a necessary prerequisite for finely tuning the catalytic performance of Ag-based catalysts. To shed light on the interrelationship between Ag structures and acidic sites of acidic supports, a series of Ag/Al-SBA-15 catalysts with different Si/Al ratios were prepared using in situ "pH-adjusting" method and tested for catalytic CO oxidation. Of all the catalysts employed, the one with a Si/AI molar ratio of 200 (Ag/MAS 200) outperformed other catalysts in terms of activity and durability. Al-27 NMR and H-1 NMR results showed that the octahedrally coordinated extra-framework Al connected to the framework via oxygen atoms suppressed the formation of H-bonded SiOH groups, resulting in the formation of highly dispersed Ag nanoparticles with high catalytic activity. In the meantime, the presence of Ag2O particles formed over the framework Al with bridging hydroxyl groups considerably deteriorated the catalytic performance. However, the Ag/MAS 200 catalyst exhibited a striking feature: that is, the bulk Ag2O species are easily reduced by CO at room temperature, leading to "in situ formation" of metallic Ag particles during CO oxidation even in the presence of high concentration of O-2. This unique property caused the induction period (ca. 270 min) and gradually enhanced the catalytic activity, eventually reaching a high and steady-state CO conversion (85%). (c) 2014 Elsevier Inc. All rights reserved.