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Supported Ag catalysts on silica and corundum have been synthesized applying an improved impregnation technique. The resulting Ag particle sizes can be divided into three categories concerning: (I) bulk-like, (II) nanoparticles of 1–6 nm, and (III) in situ created Ag clusters below 1 nm. Ag nanoparticles and bulk-like Ag are investigated concerning their pretreatment dependence for CO oxidation showing that harsher pretreatment conditions need to be applied for smaller particle sizes, based on their tendency to form Ag2CO3. A particle size effect for Ag in oxidation reactions is investigated using CO oxidation as a test reaction. The CO oxidation performance is increasing with decreasing particle size with Ag clusters showing the highest activity. A novel method based on the adsorption of ethylene (C2H4) as sensor molecule is further used to discriminate the silver–oxygen (Ag–O) interaction strength of bulk-like Ag, Ag nanoparticles, and Ag clusters, showing a distinct Ag–O chemistry for the three individual particle size regimes. By application of C2H4 breakthrough curve measurements, the available Ag surface area is determined which enables a correlation of Ag surface area and CO oxidation rate. Correlations of Ag–O interaction strength, Ag surface area, and CO oxidation activity are discussed within the scope of this work.
