No abstract

A detailed analysis of the system Pd + Pb on CaCO, is presented. This system known as Lindlar catalyst is used for selective hydrogenations of C-C bonds to C=C bonds. It was found to consist of seven distinct chemical phases. Essential ingredients are solid solutions of hydrogen and oxygen in Pd. The latter phase enables the metal particles to be attached to the support aia an orientational relationship calcite (113) 1) Pd + 0 (111). The addition of Pb neither changes the micromorphology of the disk-shaped Pd particles nor does it modify the electronic structure of Pd as studied by chemisorption experiments. The addition of Pb seems to block certain active sites and, thus, to enhance the selectivity. The support was found to be a multi-domain mixture of calcite and aragonite with domain sites of several hundred A. The free surface of the pore-free solid is covered by a calcium-hydroxide carbonate. This phase as well as several other Pb phases seem to represent only spectator materials. Verification of this as well as finding the correlation between structure and function of the various components requires a kinetic analysis which is outside the scope of this paper. It is demonstrated that a variety of analytical techniques is required to unravel the complex nature of such a 'simple catalyst' as it is Pd on CaCO?. 1.Introduction. -Pd supported on various materials is widely used as hydrogenation cataylst. A special variety, known as Lindlar catalyst, consists of Pd supported on CaCO,, modified with Pb and further poisoned by organic bases such as quinoline. It is highly selective for the reduction of triple bonds to the (Z)-configurated double bonds [l] and has been the subject of various investigations. Fukuda and Kusama have studied the influence of the Pb concentration [2] and of the support material [3] on the hydrogenation of 1,4-butynediol and attribute to Pb a not further specified changing influence on the catalyst surface. One of the objectives of the newer investigations was to determine how the addition of Pb causes the high selectivity of typically over 95 %. A recent publication

No abstract

The detailed analysis of the Pd + Pb/CaCO3 system reveals the presence of seven distinct chemical phases, e.g. solid solutions of hydrogen and oxygen in Pd.