Chemisorption–XRD particle size discrepancy of carbon supported palladium: Carbon decoration of Pd?

• Discrepancy of chemisorption and XRD nanoparticle size measurements explained.
• Cause is decorating layer of carbon on the metal nanoparticle surfaces.
• Decorating layer can be removed with mild oxidation.
• Removal of layer frees metal surface sites and improves activity.
• This oxidation step can be incorporated into chemisorption protocol to regularize results.

A systematic discrepancy is often observed between chemisorption versus X-ray diffraction and/or electron microscopy-determined nanoparticle size estimates for metals supported on carbon. It is hypothesized that lower than expected chemisorption capacity is due to decoration of the metal surface by the carbon support. To test this, a series of carbon supported Pd nanoparticles with different pretreatments was prepared and analyzed using Temperature Programmed Oxidation. Carbon oxidation peaks distinct from high temperature bulk carbon burn off were observed for all samples at around 250 °C and 300 °C. A short oxidation at the latter temperature appears to remove the decorating layer as the TPO peaks disappear and the reduced metal surface chemisorbs at close to the expected capacity.

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