THE EFFECT OF PRETREATMENT ON PD C CATALYSTS .1. ADSORPTION AND ABSORPTION PROPERTIES/

Pd catalysts were prepared using a high surface area carbon black whic h was pretreated at 1223 K under H-2 or Ar to remove sulfur contaminan ts and oxygen-containing functional groups on the surface. One carbon sample was oxidized by HNO3 to create new oxygen functional groups on its surface. Pd catalysts were prepared using a Pd acetylacetonate pre cursor and were characterized by H-2, O-2, and CO chemisorption, hydro gen titration, X-ray diffraction (XRD) and transmission electron micro scopy (TEM). Well-dispersed catalysts were indicated by the TEM and XR D results; however, chemisorption of ail three gases, especially hydro gen, was substantially suppressed on all catalysts. XRD patterns indic ated an expansion of the Pd lattice in these catalysts, which is attri buted to the presence of carbon atoms within the Pd lattice. This expa nsion was accompanied by a decreased ability to form the beta-phase hy dride compared to bulk Pd and Pd dispersed on oxide supports. However, heats of adsorption for H-2, O-2, and CO on these catalysts were reas onably consistent with previously reported values. The suppressed chem isorption and hydride formation is attributed to carbon contamination of the Pd although sulfur contamination of the Pd on the untreated car bon support also occurred. The ability to form hydrides was partially regained after mild O-2 cleaning procedures, but only a limited enhanc ement in chemisorption was observed. Reduction at 673 K gave normal Pd beta-hydride ratios, but chemisorption was still suppressed. (C) 1995 Academic Press, Inc.