ARTICLE pubs.acs.org/JPCC
Effects of the Synthesis Parameters on the Size and Composition of Pt Sn Nanoparticles Prepared by the Polyalcohol Reduction Method Ferenc Somodi, Zhenmeng Peng, Andrew “Bean” Getsoian, and Alexis T. Bell* Department of Chemical and Biomolecular Engineering, 201 Gilman Hall, University of California—Berkeley, Berkeley, California 94720-1462, United States
bS Supporting Information ABSTRACT: Platinum tin alloy nanoparticles with cubic and hexagonal structure have been prepared by the “heating up” method using metal acetylacetonates as precursors and 1,2-hexadecanediol as the reducing agent dissolved in dioctyl ether containing oleyl amine and oleic acid as capping agents. The influence of the principal reaction parameters—the concentration of the capping agents, final reaction temperature, reaction time, and ratio of metal precursors—on the size and composition of the Pt Sn nanoparticles was investigated. Transmission electron microscopy and X-ray diffraction results show that decreasing the amount of capping agents increases not only the size of the nanoparticles but also the extent of alloying. It is proposed that the reaction between the metal precursors is the primary step of the nucleation process leading to Pt Sn bimetallic particles. In a competitive reaction that depends on the concentration of capping agents, metal oleate complexes are formed. The balance between the rates of these processes affects the relative rates of particle nucleation and growth as well as the composition of the bimetallic nanoparticles. The preparation method described is suitable for controlled formation of Pt Sn nanoparticles with cubic and hexagonal crystalline structure, which are excellent candidates for investigation of the structure activity relationship for a number of catalyzed reactions.
’ INTRODUCTION Platinum tin bimetallic catalysts have been investigated extensively because of their high activity and stability for numerous reactions.1 3 These include the reforming of hydrocarbons, dehydrogenation of light alkanes,4,5 selective hydrogenation of aldehydes to alcohols,6 oxidation of carbon monoxide,7 and electro-oxidation of ethanol.8 As a consequence, there has been considerable interest in developing means to control the interactions between the two components by appropriate catalyst preparation. The simplest ways to prepare supported Pt Sn catalysts are coimpregnation or sequential impregnation of the support material with metal precursors or coprecipitation of the metal salts from solution followed by high-temperature reduction. The main disadvantage of these methods is that formation of bimetallic particles is quite unlikely. Much better results can be achieved by surface modification of supported platinum particles through their reaction with organometallic tin compounds. This technique enables exclusive modification of the platinum surface and by carefully choosing reaction parameters avoids deposition of tin onto the support, thereby increasing the amount of anchored tin.9 Pt Sn complexes can also be used to provide intimate contact between the two metals after decomposition of the complex.4,10 Coreduction of the metal ions from the liquid phase at moderate temperatures by applying strong11 or weak12,13 reducing agents (generally ethylene glycol) and subsequent deposition of the prepared colloid onto a support can also be used to produce bimetallic catalysts. A limitation of all of the above-mentioned preparation methods is the inability to selectively form stoichiometric alloy cubic r 2011 American Chemical Society
Pt3Sn or hexagonal PtSn phases. It has been reported that both alloy phases were present in catalysts prepared by coprecipitation and high-temperature treatment.14 M€ossbauer spectroscopy has revealed that use of surface organometallic chemistry leads to formation of PtxSn (3 < x
