The Effect of Pretreatment on Pd/C Catalysts: I. Adsorption and Absorption Properties

Abstract

Pd catalysts were prepared using a high surface area carbon black which was pretreated at 1223 K under H2 or Ar to remove sulfur contaminants 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 precursor and were characterized by H2, O2, and CO chemisorption, hydrogen titration, X-ray diffraction (XRD) and transmission electron microscopy (TEM). Well-dispersed catalysts were indicated by the TEM and XRD results; however, chemisorption of ail three gases, especially hydrogen, was substantially suppressed on all catalysts. XRD patterns indicated an expansion of the Pd lattice in these catalysts, which is attributed to the presence of carbon atoms within the Pd lattice. This expansion was accompanied by a decreased ability to form the β-phase hydride compared to bulk Pd and Pd dispersed on oxide supports. However, heats of adsorption for H2, O2, and CO on these catalysts were reasonably consistent with previously reported values. The suppressed chemisorption and hydride formation is attributed to carbon contamination of the Pd although sulfur contamination of the Pd on the untreated carbon support also occurred. The ability to form hydrides was partially regained after mild O2 cleaning procedures, but only a limited enhancement in chemisorption was observed. Reduction at 673 K gave normal Pd β-hydride ratios, but chemisorption was still suppressed.