A series of size-controlled Pd/hydroxyapatite (HAp) catalysts ranging from single sites (Pd clusters) to nanoparticles (dpd: similar to 30 nm) was prepared and examined for the H2O2 synthesis directly from H-2 and O-2. A Pd/HAp (dpd: similar to 1.4 nm) showed a high selectivity of 94% toward H2O2 under mild conditions (283 K and atmospheric pressure). The crystal phase, morphology, surface electronic states and coordination number of Pd particles from atomic level to nano scale were characterized in detail using multiple techniques such as X-ray diffraction, scanning transmission electron microscopy, and extended X-ray absorption fine structure. Density functional theory calculations indicated that Pd clusters of subnano size have the most effective active sites for the selective activation of oxygen hydrogenation, thus resulting in high catalytic efficiency for H2O2 synthesis. This work elucidates why those smaller Pd particles in a proper size range show the best catalytic performance for H2O2 synthesis. (C) 2016 Elsevier Inc. All rights reserved.