Synthesis of Pd Nanoframes by Excavating Solid Nanocrystals for Enhanced Catalytic Properties.

Synthesis of metal nanoframes has been of great interest for their open structures and high fractions of active surface sites, which gives rise to outstanding performance in catalysis. In this work, Pd nanoframes with well-defined structures have been successfully prepared by directly excavating solid nanocrystals. The success of this synthesis mainly relies on the fine control over the oxidative etching and regrowth rates. Due to the different regrowth rates at three typical types of surface sites (e.g., corners, edges, and faces), the removal of Pd atoms can be controlled at a certain site by carefully tuning the rates of the oxidative etching and regrowth. Without the presence of the reducing agent, etching dominates the process, resulting in the shape transformation of nanocrystals with well-defined shapes (e.g., octahedra) to cuboctahedra. In contrast, when a certain amount of the reducing agent (e.g., HCHO) is added, the regrowth rate at the corner and edge sites can be controlled to be equivalent to the etching rate, while the regrowth rate at the face sites is still smaller than the etching rate. In this case, the etching can only take place at the faces; thus, Pd nanoframes could be obtained. On the basis of this approach, solid Pd nanocrystals with different shapes, including cubes, cuboctahedra, octahedra, and concave cubes, have been successfully excavated to the corresponding nanoframes. These nanoframes can unambiguously exhibit much enhanced catalytic activity and improved durability toward formic acid oxidation reaction due to their three-dimensional (3D) open frameworks compared with solid Pd octahedra catalysts.