terium chemisorption. The catalysts were prereduced in D2 for 3-4 h at room temperature in a standard gas titration apparatus. They were then evacuated under vacuum (1CT1 234 mbar) overnight at 270 °C to yield the zero-valent clean metal surface. Deuterium was chosen over hydrogen to avoid the ,3-hydride phase formation and the deuterium titrations were performed with the catalyst heated to 100 °C as a further measure to avoid deuterium absorption in to the Pd bulk.24 Electron Microscopy Studies. The powdered Pd/CaC03 catalysts were crushed by an agate mortar and dispersed into suspension by shaking in ethanol. A drop of suspension was placed on a holey-carbon grid and allowed to dry. These specimens were examined in a JEOL JEM-200CX high resolution as well as in an analytical transmission electron microscope.Auger Electron Spectroscopy. The Auger electron spectroscopic studies were obtained from the Charles Evans & Associates Materials Characterization Laboratories in Redwood City.General Hydrogenation Procedure. Hydrogenations were carried out at ambient conditions in a three-necked round-bottom flask connected to a hydrogen reservoir maintained at normal pressure. The catalysts were placed in the flask and flushed (24) Benson,

The effect of the Lindlar treatment in the preparation of Pd catalysts for the cis-selective hydrogenation of alkynes is investigated with hydrogenation studies on 2-hexyne. The study emphasizes the effect of lead on the hydrogenation selectivity of Pd catalysts. The role of the additives in the catalysts is examined by transmission electron microscopy. The effect of lead deposition on the catalytic activity and selectivity of Pd surfaces is studied with lead-coated Pd foils. No selectivity enhancing effects of the deposition of lead on the Pd particles could be identified.Registry No. MeC=C(CH2)2Me, 764-35-2; Pd, 7440-05-3; Pb(OAc)2, 301-04-2.

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