Toward Phase and Catalysis Control: Tracking the Formation of Intermetallic Nanoparticles at Atomic Scale

？ Consecutive formation of Pt 3Sn and PtSn was observed in real time at atomic scale ？ The surface-mediated Sn diffusion is key to controlling overall reaction dynamics ？ A coherent Pt 3Sn/PtSn interface lowers activation energy of phase transformation ？ Controlled phase selection was achieved, resulting in tunable catalytic performanc