Influence of the preparation procedure and of the barium content on the physicochemical and catalytic properties of barium-modified platinum/alumina catalysts

Addition of barium (from 2 to 16 wt.-%) to alumina was performed by impregnation of the support with barium nitrate and by a sol-gel procedure using alkoxide precursors. After calcination at 1073 K and for high barium loading, the formation of barium monoaluminate was shown by X-ray measurements. The barium hexaaluminate phase was observed after calcination at 1473 K. The solids prepared by the sol-gel method showed a very high specific area which was not preserved for a calcination temperature exceeding 1473 K. Platinum was introduced onto the barium-modified alumina using hexachloroplatinic acid. The properties of the metallic phase were strongly dependent on the nature of the solvent used for H2PtCl6 (water or ethanol). In the case of an aqueous solution, the BaAl2O4 phase disappeared during the impregnation procedure because of an acidic attack and the metallic area accessible to gases was low. On the other hand, when H2PtCl6 dissolved in ethanol was used, the barium monoaluminate phase was still present and the chemisorptive properties of platinum were less affected by comparison with impregnation with aqueous solutions. A Fourier transform IR study of carbon monoxide adsorption showed that the surface properties of the platinum particles were strongly modified by the presence of barium additive; the changes in surface properties were less marked when platinum was introduced by ethanolic solutions. It was postulated that the dissolution of barium aluminate in aqueous medium led to the formation of barium containing compounds onto the metal particles or close to the platinum crystallites. The influence of the medium used for the impregnation by the platinum precursor was also shown in cyclohexane dehydrogenation: the catalytic activity was less affected by barium addition when platinum was introduced by using ethanolic solutions.