The catalytic activity of monoatomically dispersed Pd supported on TiO2 toward propylene epoxidation in the presence of H-2 and O-2 was studied at ambient temperature, and both propylene oxide (PO) and propane were obtained. Short-chain alkanes also reacted but epoxide formation was not observed in those reactions except in the case of isobutane, which formed isobutylene oxide at a low rate. The optimum surface concentration of Pd on TiO2 was 0.005-0.01 atom/nm(2); because the supported amount of Pd is extremely small relative to the surface area of the support, the supported Pd is thought to be monoatomically dispersed. Pd/TiO2 catalysts prepared from a tetraphenylporphyrin-Pd chloride complex showed almost the same product distribution for propylene epoxidation as did catalysts prepared from Pd(NO3)(2). Isotope exchange between H-2 and D-2 proceeded over Pd/TiO2 with a low surface concentration (0.0001 atom-Pd/nm(2)), and chemical potential calculations suggested that H-2 molecules could dissociatively adsorb onto the monoatomically dispersed Pd/TiO2. A PO formation mechanism over the catalyst is proposed on the basis of these results. The results presented here may provide the first clear evidence of catalysis by monoatomically dispersed noble metals. (C) 2012 Elsevier B.V. All rights reserved.