The catalytic performance of supported heterogeneous catalysts is significantly affected by the particle size and morphologies of the active phase. In this work, we studied how metal-support interaction affected the morphology and size of Pt clusters on Al2O3. Under oxidizing atmosphere, when the specific interaction between oxidized Pt and Al2O3 is dominant, highly dispersed 2D PtO2 rafts (1 nm) and atomically dispersed Pt atoms formed. At high Pt loadings, when only a fraction of the metal atoms can interact with the alumina support, or on weakly interacting supports (e.g., SiO2), 3D PtO2 particles (>2 nm) and large Pt crystallites (>10 nm) formed. Due to the weak interaction with the support, these 3D PtO2 particles can be reduced at a lower temperature (-40 degrees C) than 2D PtO2 rafts (similar to 110 degrees C) and atomically dispersed Pt (>300 degrees C). The catalytic behavior for CO oxidation is also affected by the reduction properties of supported Pt catalysts. The activity of Pt/Al2O3 stepwise increases with the reduction temperature, which is closely related with the reduction of these three types of oxidized Pt species. In contrast, Pt/SiO2 didn't show drastic activity change with the subsequent reduction treatments up to 500 degrees C because it contains mainly 3D PtO2 particles. All these results demonstrate that specific metal-support interactions guide the geometric and chemical properties and the corresponding catalytic performances of oxidized Pt in Pt/Al2O3 catalysts, underlining the importance of careful catalyst activation for the efficient utilization of metallic Pt in real-world catalytic applications. (C) 2020 Elsevier Inc. All rights reserved.