H2O2 synthesis directly from H-2 and O-2 over supported Pd-Pt alloy catalysts was carried out using a semibatch reactor under ambient conditions. As compared to pure Pd, the performance of Pd-Pt catalysts was enhanced significantly. The promotional role of Pt was studied systematically by using in situ diffuse reflectance infrared Fourier transform spectroscopy of CO adsorption (DRIFTS), quantitative powder X-ray diffraction (XRD), X-rays photoelectron spectroscopy (XPS), and temperature-programmed desorption of H-2/O-2 (H-2/O-2-TPD). The spectra of DRIFT, XPS, and XRD demonstrate the formation of Pd-Pt alloy particles, which surfaces are enriched by Pt accompanying with possible electron transfer from Pd to Pt. The addition of Pt into Pd phase was proposed to impact on reactants adsorption, stabilization of intermediates such as OOH center dot and OH center dot radicals, and the formation and decomposition of H2O2. (C) 2011 Elsevier Inc. All rights reserved.