XRD characterization of highly dispersed metal catalysts on carbon support
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Pier Luigi Antonuccib) CNR-Istituto di Ricerche sui Metodi e Processi Chimici per la Trasformazione e I'Accumulo deU'Energia, Via Salita S. Lucia sopra Contesse, 39-98126 S. Lucia, ME, Italy (Received 14 December 1992; accepted 7 April 1993)
Carbon-supported Pt catalysts were prepared from H 2 PtCl 6 or K2PtCl6 aqueous solutions. Particle size and structure after several thermal activation treatments were studied by X-Ray Diffraction (XRD), Transmission Electron Microscopy (TEM), and Cyclic Voltammetry (CV), and the results of the three techniques were compared. As the catalysts were highly dispersed on an amorphous support, a conventional XRD profile analysis for crystallite size determination could not be performed properly, because of the strong overlapping between the broad Pt peaks superposed to the halos of the amorphous phase. Thus, a new procedure of whole XRD pattern fitting, based on the Rietveld method, was used to have reliable data of Pt particle size (surface area) and lattice parameter. All structural and microstructural parameters were refined within the same procedure, also considering the transparency of the carbon supported catalysts and minimizing the effect of the amorphous background. The method can also take into account the presence of bimodal particle size distributions, which is difficult to study by CV or TEM.
I. INTRODUCTION Solid-state characterization of supported metal crystallites is of paramount significance for the knowledge of the structure-property relationships, with particular emphasis to that directly linked to the catalytic phenomenon. Information on crystallites' particle size and distribution is, in this respect, essential as these parameters strongly influence the degree of metal-support interaction due to structural sensitivity effects, especially when said catalytic materials are employed for demanding (structure sensitive) reactions1'2; in this case, a specific reaction rate is observed, depending upon particle size and other structural parameters, such as the lattice parameter of the active component. Carbon-supported Pt catalysts find large application for a number of such structure sensitive reactions3'4; thus, information related to the bulk properties of the material is a necessary complement to the more widespread knowledge obtained by surface spectroscopy techniques. Particle size of supported catalysts is usually determined by selective gas chemisorption (GC) methods, transmission electron microscopy (TEM), electrochemical techniques (CV, cyclic voltammetry), or by x-ray,
^Address correspondence to this author. b) Current address: Facolta di Ingegneria, Istituto di Chimica, Via E. Cuzzocrea 48, 89100 Reggio Calabria, Italy. J. Mater. Res., Vol. 8, No. 8, Aug 1993
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from diffraction line broadening or low angle scattering.3 As is well known, some drawbacks are connected with each of the above-mentioned techniques: a rather poor statistical significance is inherent to TEM, whereas experimental conditions ca
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