Preparation Of Pt/WO 3 Powders and Thin Coatings on Carbon Black and Metal Supports by The Complex Sol-Gel Process (CSGP
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2. Precipitation on the support from similar solutions [6,7,11], 3. Precipitation by electrodeposition from solutions [5]. Freshly prepared catalysts are subjected to thermal treatment in nitrogen (argon) or /and hydrogen atmosphere. For preparation of similar type catalysts the method of impregnation has been widely used [13]. However there is no information on its application to the preparation of catalyst for DMFC. There are two main advantages of the impregnation process: the relative simplicity and possibility of precisely obtaining materials with the desired composition. The goal of the present work was the application of this method for the preparation of PtWO. catalysts on carbon black. Sols prepared with a very strong complexing agent (ascorbic acid-ASC) were used as impregnating solutions. The application of ASC to sol preparation by the Complex Sol Gel Process (CSGP) has been patented by INCT (Poland) and ANL (USA) teams [14] and successfully applied to the synthesis of Li.Mn 204±8 [15]. Because this process was also successfully used for preparation of ceramic coatings e.g. hydroxyapatite on Ag or Ti supports [16] we decided to apply it for preparing thin layers of Pt/W0 3 on metallic supports. Similar coatings were obtained by electrodeposition [17]. EXPERIMENTAL Starting saturated tungsten solutions were prepared by dissolving (NH4) 4W 50 17'2.5 H 20 (POCh, Gliwice, Poland) in concentrated aqueous solution of ASC containing 200 g/dm3 , (solution No I) or in the solution diluted 10 times (solution No II). In both cases the concentration of W (0. 15M) was similar. To both solutions an aqueous solution of H 2PtCI6 237 Mat. Res. Soc. Symp. Proc. Vol. 496 © 1998 Materials Research Society
(1 lgPt/dm3) was added to the molar ratio of Pt:W=1:1. The resulting products are respectively denoted I+Pt and II+Pt. The solutions were immediately subjected to the next steps because after several minutes the precipitation was observed, despite the complexing action of ASC. Solution I+Pt, diluted with ethanol, was used for preparing coatings on silver or titanium substrates by the immersion technique with ultrasonic mixing described in detail in [16]. Homogeneous yellowgreen powders were obtained from this solution by evaporation at boiling point and further heating at 700TC for 2h. Solution No II+Pt was used for impregnation (method IM-2) of carbon black Vulcan XC72 (Carbochrom, Milan, Italy). The amounts of starting sols were calculated as to obtain 10 and 20% of Pt+W in the final product. Dilution of the starting solutions with water to a total volume of -170 cm 3 per 10g of carbon black was necessary for complete wetting of the carbon black. The impregnation was carried out in an ultrasonic bath supported by mechanical stirring until black, creamy, homogeneous, very viscous slip was obtained. In some experiments (denoted method IM-1) W-ASC solution (II) diluted with water was added first to the carbon black followed by the addition of respective Pt solutions. The obtained slips were initially dried under an infra
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