Variable Temperature Study of Au and Au-Pt Nanoparticles on Selected Oxide Supports
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Variable Temperature Study of Au and Au-Pt Nanoparticles on Selected Oxide Supports D. Barrett1, P. Franklyn1, M. Scurrell1 1 School of Chemistry, University of the Witwatersrand, Johannesburg, South Africa, P O Wits, South Africa, 2050. E-mail: [email protected] Abstract We report on the size relationship of Au and Au-Pt nanoparticles that were synthesised on silica and anatase phase titania supports. Deposition-precipitation (DP) of metal chlorides with the addition of urea and ammonium hydroxide was used to produce the nanoparticles. The relative particle size relationship of the Au and Au-Pt nano particles (NP’s) was investigated, relating the Pt concentration and the support polymorph over a temperature range. It was found, with the use of in-situ variable temperature powder X-ray diffraction (VT-PXRD) and transmission electron microscopy (TEM), that the addition of Pt to the Au system corresponded to a reduction in particle size over a broad temperature range. Keywords: Au-Pt nanoparticles; Deposition-precipitation; in situ XRD, Rietveld. INTRODUCTION The need to stabilize Au nanoparticles from the effects of thermal sintering has been shown to be crucial in controlling the catalytic activity of Au catalysts2. Catalytic activity is optimum when the Au particle size is less than 5 nm1-4. Supported nano-Au is particularly susceptible to sintering, and at non ambient temperatures this sintering results in the agglomeration of the Aunanoparticles until the catalyst deactivates5. Susceptibility of Au catalysts to sintering makes the catalyst very difficult to use when reaction temperatures increase5. As with alloys and bimetallics in the bulk state, nano alloys and nano bimetallics have been shown to improve the characteristics of the parent counterparts that make up the nanomaterial6. For example, when Cu was added to Au the bimetallic showed an ability to stabilize and limit the growth of the Au-Cu particles4. This stabilizing effect has also existed in Au-Ag systems7. The study of such nano alloys and bimetallics is still very much in its infant stage. Work by Mott et al. showed that Au-Pt nanoparticles not only show single phase alloy character in the nano crystal but also bimetallic alloy properties on the surface8. Further they found that the nano crystal and surface alloy properties are directly correlated with the bimetallic composition. Their findings also showed that these bimetallic alloys can occur in the miscibility gap that is known for the bulk alloy counterparts. This miscibility gap occurs between approximately 20-90% Au, again showing the differences between the bulk and nano structures2. In this work the particle sizes of supported Au nanoparticles were compared to those of supported gold systems where small amounts of platinum were added to the gold in order to stabilize the nanoparticles at non-ambient temperatures. The experimental method was chosen so as to minimize the number of experimental parameters and thus allowing the focus to be placed on the addition of platinum to the gold
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