Mocvd Palladium and Platinum Supported on Alumina Catalysts: Preparation and Characterization
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ABSTRACT An horizontal hot-wall MOCVD reactor was used to prepare palladium and platinum catalysts supported on alumina. A conventional impregnated Pt on alumina catalyst was prepared as comparison. The solids were characterized by XRD, Auger spectroscopy, HREM and H2 -TPR. The operation conditions of the MOCVD reactor were fixed preparing several Pd catalysts until to find the appropriate deposition zone. The particle size of Pt catalysts prepared by MOCVD was at about 7 nm compared with 6 nm obtained with the Pt impregnated catalyst, measured by XRD. The HREM image of the Pt MOCVD catalyst showed a narrower particle size ranging from I to 4 nm. After calcination three Pt compounds were detected by TPR, which were attributed to PtO, PtO2 and Pt-A120 3 interaction in MOCVD preparation. Additionally, a clear reduction of surface oxygens of alumina was also observed. Keywords: MOCVD, Pt catalysts on alumina, XRD, Auger spectroscopy, HREM, TPR. INTRODUCTION Metallic supported catalysts are widely used in dehydrogenation, reforming and other industrial reactions [1]. They are usually prepared by conventional methods such as impregnation, coprecipitation and ion interchange [2]. The main disadvantage using these methods is that after thermal treatments a broad particle size distribution is obtained. In the last decade, efforts have been made in order to prepare catalysts with more homogeneity to improve their catalytic properties. Decomposition of a metal cluster compound, chemical deposition, ion implantation, vapor-phase deposition and sol-gel routes have been reported as novel preparation methods for supported metallic particles [2,3]. Platinum and palladium supported on zeolite catalysts have been already prepared by a modified metal organic chemical vapor-phase deposition combined with a separate step of decomposition under a reduction atmosphere. A small size and thermal stability of metallic particles with the desired topology were obtained by this two-step technique [4]. Decomposition of Pt-acetylacetonate on alumina and on silica in a fluidized-bed reactor was studied in order to correlate the effect of the surface properties and dispersion of metallic particles. The deposited particles were more mobile on silica and more readily agglomerate; on the other hand, dispersion of platinum particles on alumina did not change at high metal loading. After the final thermal treatment the metallic particles were contaminated with carbon [5]. In the present work, we report the structure and catalytic performance of platinum and palladium supported on alumina catalysts prepared by MOCVD. A conventional impregnated platinum supported catalyst was prepared and analyzed for comparison. 87 Mat. Res. Soc. Symp. Proc. Vol. 497 ©1998 Materials Research Society
EXPERIMENT Platinum (Pt) and palladium (Pd) catalysts were prepared in a horizontal hot-wall MOCVD apparatus which is depicted in Fig. 1.A commercial y-A120 3 powder (HaldorTopsoe), 80-100 mesh, was used as a support. It was first thermally pretreated to remove water at 4
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