Palladium-alumina catalysts prepared from various precursors and by different preparation methods have been studied after calcination by 0, and reduction by H, , using IR spectroscopy of adsorbed CO, EPR and diffuse reflectance measurements. In addition to Pd2+ ions, the unreduced samples contain Pd3+ and Pd+ species.Pd2+ and Pd+ entities are still present on catalysts reduced at 573 K. The influence of the precursor on the metal fraction exposed (MFE) and on the nature of adsorbed CO species is clearly shown. Bridged entities are more sensitive to the MFE value than the linear ones. At 100 Torr [I Torr z (101 325/760) Pa], the bridged entities are compressed, which leads to a CO-(surface Pd) ratio of near unity. The MFE increases when the precursors are taken in the following order nitrate < chloride (impregnated) z acetylacetonate c chloride (exchanged). The sintering brought about by increasing the reduction temperature seems to affect the largest metal particles rather than the small ones which strongly interact with the carrier.

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