Direct synthesis of H2O2 from H-2 and O-2 on Pd/TiO2 catalysts has been systematically studied to gain understanding of the origin of active sites. For the first time, the structure of Pd ensembles has been demonstrated to be finely tuned by the variation of Pd loading without change in the particle size. The structure of Pd/TiO2 catalysts has been thoroughly characterized using multiple techniques such as HRTEM, XRD, in situ DRIFTS, XPS, XANES, and EXAFS. With the variation of Pd loading in the range 1.0-5.0 wt%, the size (ca. 2.4 nm in diameter) and crystallinity of Pd particles have shown no discernible differences, while the surface configuration of Pd atoms changed significantly. Performance (a selectivity of 61% to H2O2 and a rate in TOF of 630 h(-1)) was optimal for a catalyst of 1.0 wt% Pd at 283 K and 1 atm using a semibatch reactor. We demonstrate that part of the surface Pd atoms can be oxidized (47.6% for 1.0 wt% Pd and 35.2% for 5.0 wt% Pd) by the adsorbed oxygen with the assistance of TiO2 even at 283 K, and form Pd-PdO ensembles. The electronic structure of the surface Pd atoms has proved to change dynamically in different atmospheres. We assume that H2O2 synthesis occurs at the interfaces of the Pd and PdO domains. In addition, the deep insight into the origin of active sites from this paper, as a case study, reminds us that the structure of active sites of support catalysts can be potentially tuned by varying the metal loading. (c) 2014 Elsevier Inc. All rights reserved.