On Extended X-Rays Absorption Fine Structure Characterisation of Iron Platinum Clusters
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ABSTRACT Present paper reports on Extended X-rays Absorption Fine Structure (EXAFS) Spectroscopy characterisation of Fe/Pt clusters. Samples of different nominal iron contents (FePt4 and FePt2) have been prepared and characterised by direct measurements of absorption coefficient at Pt L 3 and Fe K edges. Catalysts were prepared by precipitation from suitable mixing (in the appropriate stoichiometry) of mesitylene suspentions containing small clusters of either platinum or iron. Single species suspention, in turn, were prepared by vapor metal deposition. Catalitic activity of prepared samples was tested on cinnammic aldehyde. As a general result, platinum clusters exhibit activity in the hydrogenation of both carbon double bond and aldehyde group with a preference for the latter. Iron addition to the pure platinum catalyst reverts the selectivity while leaving the catalitic activity almost unchanged. The present paper represents a temptative to achieve structural information on Fe and Pt short range environments, thus providing insights on the dynamics of the catalytic process. INTRODUCTION Interest in nanoscale system has been growing fast during last decade. Both single particle and aggregates are appealing for structural and functional applications [1]. Nanometric metallic clusters exhibit a large surface to volume ratio, thus enhancing their species natural catalytic activity [2]. In particular, platinum represents a natural catalyst for hydrogenation process. Platinum clusters are also appealing for optical applications as their addiction to inert matrices may enhance the resulting non linear optical response. Among the established methods of cluster preparation, the technique which is variously known as metal atoms or metal vapor synthesis is taking a relevant position [3-4]. The former technique has been employed to prepare stable suspensions of solvated Fe and Pt small clusters. Starting from former precursor clusters, composite Fe/Pt catalists have been prepared. Lattice structure of powder and its macroscopic morphology have been studied by means of TEM (Transmission Electron Microscope) and HRTEM (High resolution TEM) measurements [5]. Since electron microscopy is a non chemical selective tool, it was not easy to achieve information on Fe laying in global structure of the sample. In order to obtain different structural information on Fe and Pt species, EXAFS measurements have been performed at the two different Pt L3 and Fe K edges. By this means one may obtain separate information on Fe and Pt structural short range environment and inferring possible dynamics for the catalized hydrogenation process. As it will be clarified later, presence of non trivial tungsten contamination complicates the EXAFS characterization. 115 Mat. Res. Soc. Symp. Proc. Vol. 497 ©1998 Materials Research Society
SAMPLE PREPARATION Samples have been prepared in two steps. First process is a typical vapor deposition: each species (Fe and Pt) has been evaporated in an arene (toluene or mesitylene) saturated atmosphere and deposite
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