▎ 摘　　要

In this paper, we report the synthesis of Pd nanoparticles (NPs) with controllable size and dispersion supported on graphene by controlling the reduction pathway without any surfactants. The small-sized and highly-dispersed Pd NPs on graphene could be obtained through an effective separated step chemical reduction route. By first introducing a partial dosage of the reducing agent, PdCl42- ions were reduced to form Pd NPs in the presence of graphene oxide (denoted as GO), while GO was incompletely reduced. Thereby, Pd NPs could be anchored and stabilized by the oxygenated group of GO (the sample was denoted as Pd/GO). In the second step, with the addition of the rest of the reducing reagent, Pd/GO was further transformed into Pd/graphene due to the mass removal of oxygenated group of GO. By separated step chemical reduction, the Pd NPs with small size (about 3 nm) were highly dispersed on graphene (denoted as Pd/GN2). In contrast, the Pd NPs obtained from the synchronous chemical reduction had a large size (about 10 nm) and poor dispersion (denoted as Pd/GN1). Notably, the Pd/GN2 catalyst prepared by the separated step chemical reduction exhibited excellent catalytic activity and stability towards formic acid electrooxidation compared with the Pd/GN1 catalyst obtained from the synchronous chemical reduction and the Pd/Vulcan XC-72R carbon catalyst. The enhanced performance of Pd/GN2 catalyst should be contributed to the small size and high dispersion of Pd NPs and the stabilizing effect of the graphene support.