The hydrogen storage performance of Pd-doped oxidized activated carbon (Pd/AC-ox) with various oxygen contents or functional groups was investigated. The surface chemistry of the Pd/AC-ox sample was modified by treatment with hydrogen gas. Temperature-programmed desorption was performed to characterize the oxygen functional groups in each sample. In this study, low- and high-pressure hydrogen adsorption isotherm experiments were conducted using a static volumetric measurement at room temperature (RT) and pressures of up to 8 MPa. The results showed that increasing the oxygen content and functional groups on the surface of the Pd/AC-ox significantly improved the reversible RT hydrogen storage capacity due to the spillover effect. The hydrogen spillover enhancement factors at 0.12 MPa were greater than 100% for all samples. The hydrogen uptake of Pd/AC-ox1 at RT and 8 MPa with an oxygen content of 8.94 wt.% was 0.37 wt.%, which was 48% greater than that of Pd-free AC-ox (0.25 wt.%). In addition, the hydrogen uptake of Pd/AC-ox3 with lower oxygen contents demonstrates that the hydrogen spillover enhancement gradually disappears when the pressure is increased to more than 2 MPa (i.e., a transition from spillover to physisorption). The surface diffusion, or reversible adsorption, of the spiltover H atoms, which is enhanced by oxygen functional groups, was affected by a threshold amount of oxygen groups (such as hydroxyl groups). (C) 2014 Elsevier Inc. All rights reserved.