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 Ag from Pt nanoparticles of well-defined shape, resulting in the formation of elementally-pure Pt cubes, cuboctahedra, or octahedra, with a largest vertex-to-vertex distance of {approx}9.5 nm from Ag-modified Pt nanoparticles. A nitric acid etching process was applied Pt nanoparticles supported on mesoporous silica, as well as nanoparticles dispersed in aqueous solution. The characterization of the silica-supported particles by XRD, TEM, and N{sub 2} adsorption measurements demonstrated that the structure of the nanoparticles and the mesoporous support remained conserved during etching in concentrated nitric acid. Both elemental analysis and ethylene hydrogenation indicated etching of Ag is only effective when [HNO{sub 3}] {ge} 7 M; below this concentration, the removal of Ag is only {approx}10%. Ethylene hydrogenation activity increased by four orders of magnitude after the etching of Pt octahedra that contained 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 [HNO{sub 3}] led to the decomposition of the capping agent, polyvinylpyrollidone (PVP); infrared spectroscopy confirmed that many decomposition products were present on the surface during etching, including carbon monoxide

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

2 A procedure has been developed for the selective etching of Ag from Pt nanoparticles of well-defined shape, resulting in the formation of elementally-pure Pt cubes, cuboctahedra, or octahedra, with a largest vertex-to-vertex distance of ~ 9.5 nm from Agmodified Pt nanoparticles. A nitric acid etching process was applied Pt nanoparticles supported on mesoporous silica, as well as nanoparticles dispersed in aqueous solution. The characterization of the silica-supported particles by XRD, TEM, and N 2 adsorption measurements demonstrated that the structure of the nanoparticles and the mesoporous support remained conserved during etching in concentrated nitric acid. Both elemental analysis and ethylene hydrogenation indicated etching of Ag is only effective when [HNO 3 ] ≥ 7 M; below this concentration, the removal of Ag is only ~ 10 %. Ethylene hydrogenation activity increased by four orders of magnitude after the etching of Pt octahedra that contained 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 [HNO 3 ] led to the decomposition of the capping agent, polyvinylpyrollidone (PVP); infrared spectroscopy confirmed that many decomposition products were present on the surface during etching, including carbon monoxide. and energy dispersive X-ray spectroscopy) indicated Ag was not present in the NPs after removing excess AgCl, it was later found that residual Ag of up to ~ 20 at %, is still present in the NP system (metal NP plus stabilizing polymer). In a subsequent paper, we found that Ag acts as a poison in the hydrogenation of ethylene by Pt and the amount of Ag in the NPs can be directly correlated to the decrease in catalytic activity. This is in agreement with studies of highly dispersed Pt-Ag catalysts on silica gel; in that study, rates of 1-hexene hydrogenation decreased exponentially with increasing Ag content . The dependence of etching on temperature, HNO 3 concentration, and silver particle size (mm-range) is well-documented For [HNO 3 ] of 4.9 -7.1 M, the stoichiometry is Platinum, on the other hand, is resistant to etching by nitric acid, usually being dissolved with aqua regia (70 % concentrated hydrochloric acid and 30 % concentrated nitric acid). Using this difference in reactivity towards nitric acid, we are able to modify the Pt-Ag NPs after synthesis to pure platinum nanoparticles of well-defined shapes