Preparation of carbon supported platinum catalysts:: role of π sites on carbon support surface

The role of the pi sites in the adsorption of anionic platinum precursor (PtCl62-) on the carbon surface was studied. A set of carbon supports was prepared by the oxidative treatments of a commercial carbon black in liquid or thermal treatment in a nitrogen gas flow. The supports obtained had similar physical structure, but were different in their surface chemistry, e.g., the types and the amount of the surface functional groups. The Pt/C catalysts were prepared by the impregnation of the supports with an aqueous solution of hexachloroplatinic acid. N-2 adsorption, acid-base titration, X-ray photoelectron spectrometry (XPS), ICP, and TEM were used to characterize the supports and the catalysts. It was found that the absence of the surface acidic groups led to a higher platinum loading amount on the supports; and platinum loading amount would decrease when the higher H2PtCl6 impregnating solution concentration was used. The 71 sites in the basal planes play a more important role than the surface acidic groups in PtCl62- ions adsorption on the carbon surface. Upon impregnation, the delocalized pi electrons form coordination bonds with the platinum precursors, and further cause the reduction of Pt(lV) complex to Pt(11) complex, which was strongly chemisorbed on the surface of the supports. PtCl62- ions adsorbing at the surface acidic groups would easily desorb during the washing process. However, when the impregnating solution was used at a higher concentration, a large amount of H3O+ was firstly adsorbed in the pi sites, leading to a decrease of the amount of strong PtCl62- ions adsorption on the supports.