CATALYTIC ACTIVITY OF HIGHLY DISPERSED PALLADIUM .1. THE MECHANISM OF CYCLOHEXENE HYDROGENATION AND THE ROLE OF ZRO2 SUPPORT

The hydrogenation of cyclohexene vapor on dispersed palldaium was studied in the pressure range of 10-40 Torr at temperatures between 283 and 323 K. Palladium was impregnated on ZrO//2 and alpha -Al//2O//3 over the wide range of 0. 03 to 2. 1 wt%, and the percentage exposed was varied from 13 to 96% for Pd/ZrO//2 and from 2 to 15% for Pd/ alpha -Al//2O//3. The hydrogenation activity, V//g, per unit of weight of the Pd dispersed on ZrO//2 , exhibited a sharp maximum at 0. 05 wt%, but the turn-over frequency, V//s, per exposed atom around this maximum remained almost contant, which shows the reaction to be structure-insensitive. The value of V//s for Pd/ZrO//2 at 301 K was larger than those for Pd/ alpha -Al//2O//3 and Pd bulk metal by one order of magnitude. The ZrO//2 carrier affected the catalytic behavior of palladium; the reaction orders with respect to the hydrogen and cyclohexene pressures changed to 0. 73 plus or minus 0. 04 and 0. 64 plus or minus 0. 04 respectively from the corresponding values, 0. 97 plus or minus 0. 03 and 0. 07 plus or minus 0. 04 for Pd / alpha -Al//2O//3 and Pd metal.