The effect of pretreatment on the adsorption, absorption and catalytic behavior of carbon-supported palladium

A high surface area carbon black was treated under H{dollar}sb2{dollar} or Ar at high temperatures to remove sulfur and oxygen-containing functional groups, and one sample was oxidized by HNO{dollar}sb3{dollar} to create new surface oxygen functional groups. Pd was dispersed on these carbons using a Pd acetylacetonate precursor and an anaerobic incipient wetness technique. These catalysts and supports were characterized by H{dollar}sb2{dollar}, O{dollar}sb2{dollar} and CO chemisorption, H{dollar}sb2{dollar} titration, x-ray diffraction, transmission electron microscopy and calorimetry. TEM and XRD measurements indicated well dispersed Pd irrespective of carbon pretreatment; however, hydride formation and chemisorption of gases, especially hydrogen, was suppressed on these catalysts after reduction at 573 K. The heats of H{dollar}sb2{dollar}, O{dollar}sb2{dollar} and CO adsorption for Pd/C were fairly consistent with reported values for bulk and supported Pd, although Q{dollar}sb{lcub}rm ad{rcub}{dollar} values were somewhat higher for H{dollar}sb2{dollar} and lower for CO. The decreased ability to form the {dollar}beta{dollar}-phase hydride was accompanied by lattice expansion of Pd as seen from XRD. Cleaning pretreatments with O{dollar}sb2{dollar} slightly contracted the Pd lattice and enhanced hydride formation and chemisorption. After a high temperature reduction at 673 K, normal hydride ratios and Pd lattice parameters were obtained although chemisorption was still suppressed. Pd lattice expansion and suppression of chemisorption and hydride formation are attributed to carbon atoms present on the Pd surface and in the bulk; additional contamination due to sulfur can occur with Pd supported on the untreated carbon. The catalytic performance of these Pd/C catalysts in three probe reactions was consistent with the model of carbon contamination of the Pd surface and bulk. Sulfur contamination of Pd on the untreated carbon significantly lowered its activity for benzene hydrogenation. Residual sulfur in the high temperature treated carbons did not affect the Pd activity, and TOFs were similar to those obtained with Pd/oxide supports. However, markedly lower activity was seen in CO methanation, no activity was detectable below 633 K, and apparent activation energies varied over a wide range. Under the low temperature conditions used in CO oxidation, Pd/C catalysts had TOFs similar to or greater than {dollar}rm Pd/Alsb2Osb3{dollar}, and similar activation energies.