The Effect of Pretreatment on Pd/C Catalysts: II. Catalytic Behavior

Abstract

Pd catalysts were anaerobically prepared using a Pd acetylacetonate precursor and a high surface area turbostratic carbon black which had been pretreated to change its surface functionalities and remove sulfur impurities. These Pd/C catalysts exhibited suppressed chemisorption and hydride formation, which was attributed to the presence of carbon atoms on the surface and in the bulk of Pd particles. The effect of these carbon atoms on three probe reactions-benzene hydrogenation, CO hydrogenation, and CO oxidation-was examined. If sulfur were not removed from the carbon, it contaminated the Pd surface and caused a drastic decrease in benzene hydrogenation activity and a decrease in both activity and activation energy in CO hydrogenation. Coverage of the Pd surface by carbon atoms had different effects on the three probe reactions. The turnover frequency (TOF) in benzene hydrogenation, a structure-insensitive reaction, was not affected for Pd supported on the clean carbon blacks, although specific activity was lower than expected based on crystallite size, and the apparent activation energies (8.5-10 kcal/mole) were slightly lower than anticipated. In contrast, the TOFs for methanation were substantially decreased, and the apparent activation energies varied over a wide range (5-30 kcal/mole). Under an oxidizing environment, these Pd/C catalysts had TOFs for CO oxidation similar to or higher than those for Pd/Al2O3 catalysts, and similar activation energies near 24 kcal/mole were obtained, thus indicating that the Pd surfaces present were alike.