Low-loaded Pd-Pb/alpha-Al2O3 catalysts: Effect of alloying in the hydrogenation of buta-1,3-diene and hydrogenation and isomerization of butenes

Butadiene hydrogenation, butene isomerization, and hydrogenation reactions were studied on low-loaded Pd-Pb/alpha -Al2O3 catalysts and the results wer e compared with those previously obtained on Pd/alpha -Al2O3 catalysts (Goe tz et at, J. Catal, 164, 369 (1996)). A deuterium tracer study was used to investigate the reaction mechanisms. The studied catalysts exhibited differ ent catalytic properties which were related to physicochemical characterist ics already described (Goetz et at, J. Catal. 167, 314 (1997)). Catalysts w ith a low metal content (bi-or monometallic) containing particles in strong interactions with the support promoted mainly the isomerization reaction, occurring by intramolecular hydrogen shift, with a rate proportional to the number of Pd surface atoms. The low hydrogenation activity observed on the se particles was not affected by lead addition. In contrast, the addition o f lead to Pd/alpha -Al2O3 with a high metal content, containing rugged part icles noninteractive with the support, induced a decrease in hydrogenation and an increase in isomerization rates. These observations are well explain ed assuming a decrease in hydrogen coverage due to an alloying effect. The Pd-Pb alloy would also be responsible for the superior semihydrogenation se lectivity observed at high conversion in butadiene hydrogenation. (C) 2001 Academic Press.