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

The role of the π 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. N₂ 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 π 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 π electrons form coordination bonds with the platinum precursors, and further cause the reduction of Pt(IV) complex to Pt(II) 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 π sites, leading to a decrease of the amount of strong PtCl₆²⁻ ions adsorption on the supports.