The effect of size and morphology of Ag catalysts supported on ZrO2 on the soot oxidation activity was investigated. Spherically shaped Ag nano-particles with the size of ca. 16 nm, prepared by liquid-phase chemical reduction, supported on ZrO2 (Ag/ZrO2(NP)) exhibited outstanding performance for soot oxidation with gaseous O-2 in tight and loose contact modes, compared with hemispherically shaped Ag particles prepared by impregnation (Ag/ZrO2(IMP)). The exceptional performance of Ag/ZrO2(NP) for soot oxidation in loose contact mode was explained by relatively good catalyst-soot contact state and effective migration of active oxygen species from the catalyst to soot. In addition to morphological effect, Ag nano-particles supported on ZrO2 were found to be stabilized in the surface valence state of Ag+, and then oxygen species participating in soot oxidation can be effectively activated on Ag+ sites. We concluded that the presence of Ag+ sites on the surface of Ag nano-particles is responsible for high soot oxidation activity. (C) 2014 Elsevier B.V. All rights reserved.