Effect of Pd dispersion on the catalytic performance of Pd/Al2O3 for C3H6 and CO oxidation was investigated. Pd/Al2O3 with different Pd dispersion ranging from 0.11 to 0.41 was obtained by the treatment under reducing atmosphere at 400-800 degrees C for various hours. FT-IR spectroscopy following CO adsorption revealed that there is no significant difference in the surface electronic state of Pd particles for all the Pd/Al2O3 samples. Turnover frequency (TOF) for C3H6 oxidation increased with decreasing the Pd dispersion. The catalytic activity of Pd/Al2O3 with Pd dispersion of 0.41 was found to increase with decreasing the average oxidation number of Pd, suggesting that metallic Pd is the active species for C3H6 oxidation over Pd/Al2O3. The structure sensitivity of C3H6 oxidation over Pd/Al2O3 was explained by the difference in Pd/PdOx ratio depending on the Pd dispersion. In situ FT-IR spectroscopy suggested that acrylate species participates as a reaction intermediate in C3H6 oxidation on Pd/Al2O3, and the reaction of the acrylate species is effectively catalyzed on the metallic Pd sites, resulting in high TOF for C3H6 oxidation. The TOF for CO oxidation on Pd/Al2O3 catalysts was found to be almost identical irrespective of Pd dispersion. Kinetic studies suggested that metallic Pd sites, which would be created by partial reduction of PdOx by CO under the reaction condition, act as catalytically active sites for CO oxidation. (C) 2016 Elsevier B.V. All rights reserved.