Bimetallic Au-Pd catalysts for selective hydrogenation of butadiene: Influence of the preparation method on catalytic properties
Résumé
Supported Au catalysts modified by the addition of small amount of Pd (Au/Pd atomic ratio (10) were prepared according to three different methods, co-deposition-precipitation with urea (DPU), deposition of colloids stabilised either by polyvinyl alcohol (PVA) or by tetrakis (hydroxypropyl) phosphonium chloride (THPC) on two oxide supports, alumina and titania. In all cases, after thermal treatments (under H2 at 300 C for DPU samples and under air then H2 at 500 C for PVA and THPC samples), the metal particles were smaller than 5 nm. Characterisation by DRIFTS coupled with CO adsorption revealed that the particles are bimetallic and that Pd is present on the particle surface as isolated Pd by Au atoms and as Pd dimers. In addition, the surface composition depends on the preparation method and the Au/Pd ratio and evolves under CO exposure towards a gradual enrichment in palladium. AuPd catalysts showed improved catalytic properties compared to Au catalysts in selective hydrogenation of butadiene in presence of excess of propene. Activity was higher and directly related to the Pd content. The nature of the support has no influence on the catalytic properties. An optimised selectivity to butenes with poor formation of alkanes (propane and butane) could be achieved for an atomic Au/Pd ratio of 20. THPC-based catalysts showed higher activity than DPU samples in spite of slightly larger metal particles, and this was attributed to the presence of residual phosphorus after catalyst activation. PVA-based catalysts, which contained slightly larger metal particles, were slightly less active, but were more selective to butenes. This was related to the absence of Pd dimers, responsible for overhydrogenation.