Silver ion mediated shape control of platinum nanoparticles: Removal of silver by selective etching leads to increased catalytic activity

A procedure has been developed for the selective etching of silver from platinum nanoparticles of well-defined shape, resulting in the formation of nearly elementally pure Pt cubes, cuboctahedra, or octahedra, with a largest vertex-to-vertex distance of similar to 9.5 nm from Ag-modified Pt nanoparticles. The characterization of mesoporous silica-supported Pt nanoparticles by XRD, TEM, and N-2 adsorption measurements demonstrated that the structure of the nanoparticles and the mesoporous support was conserved after etching in concentrated nitric acid. Both elemental analysis and ethylene hydrogenation indicated that etching of Ag is only effective when [HNO3] >= 7 M; below this concentration, the removal of Ag is limited to similar to 10%. The activity for ethylene hydrogenation increased by four orders of magnitude after etching Pt octahedra containing the highest fraction of silver. High-resolution transmission electron microscopy of the unsupported particles after etching demonstrated that etching does not alter the surface structure of the Pt nanoparticles. High HNO3 concentration led to decomposition of the capping agent, polyvinylpyrollidone (PVP).