Supported metal single-atom catalysts (SACs) with maximized metal atom efficiency and unique catalytic properties have drawn tremendous attention in the catalysis field. In this work, we reported that in selective hydrogenation of 1,3-butadiene, graphene supported Pd-1 single atoms, synthesized by atomic layer deposition, exhibited expressively 100% butenes selectivity at 100% conversion at near ambient temperature, regardless of hydrogen partial pressures. The hydrogen reaction order was found to be about 1.2, indicating that hydrogenation dissociation is the rate-determining step. Combining with other structural characterization techniques, in situ X-ray absorption fine structure spectroscopy suggests that the Pd-1 single atom likely bonds to the graphene support through three Pd-C and one Pd-O-C coordinations. Density functional theory calculations show that 1,3-butadiene adsorbs on Pd-1 via a mono-pi adsorption mode (-0.98 eV), much stronger than that of H-2 (-0.30 eV), which makes Pd-1 to be predominantly covered with a 1,3-butadiene molecule during reaction, consistent very well with the results of H-D exchange reaction. Facilitated by the bridging O atom in the Pd-O-C coordination, H-2 dissociates at the 1,3-butadiene covered Pd-1 atom in the heterolytic way, then hydrogenates the adsorbed 1,3-butadiene molecule by following the pseudo Horiuti-Polanyi mechanism. Distinct from its adsorption on extended Pd surfaces, such mono-pi adsorption of 1,3-butadiene on Pd-1 favors 1-butene formation, but impedes the secondary hydrogenation to butane owing to intensified steric hindrance, thus yielding the expressively high butenes selectivity over Pd-1 SAC. These findings pave a new way to rational design SACs catalyst in selective hydrogenation reactions. (C) 2018 Elsevier Inc. All rights reserved.