▎ 摘　　要

Supported metal clusters containing only a few atoms are of great interest. Progress has been made in synthesis of metal single-atom catalysts. However, precise synthesis of metal dimers on high-surface area support remains a grand challenge. Here, we show that Pt-2 dimers can be fabricated with a bottom-up approach on graphene using atomic layer deposition, through proper nucleation sites creation, Pt-1 single-atom deposition and attaching a secondary Pt atom selectively on the preliminary one. Scanning transmission electron microscopy, x-ray absorption spectroscopy, and theoretical calculations suggest that the Pt-2 dimers are likely in the oxidized form of Pt2Ox. In hydrolytic dehydrogenation of ammonia borane, Pt-2 dimers exhibit a high specific rate of 2800 mol(H2) mol(Pt)(-1) min(-1) at room temperature, similar to 17- and 45-fold higher than graphene supported Pt single atoms and nanoparticles, respectively. These findings open an avenue to bottom-up fabrication of supported atomically precise ultrafine metal clusters for practical applications.