Metal oxide-supported nanoparticles of the platinum group metals Pt, Rh and Pd were studied at ambient temperature and atmosphere using Kelvin probe force microscopy. In all cases, the results reveal electron transfer from the metal to the oxide support which decreases in the order TiO 2 > CeO 2 >> Al 2 O 3 , leading to charge polarization at the Schottky type interfaces analogous to that of a parallel plane capacitor. This polarization cancels out to a large extent for the Kelvin signal. On top of this there is a much smaller number of positive charges at the outer catalyst particle surface, compensated by negative charges near the oxide surface. They show the same trend over the different oxides. These charges determine the constant electrical potential of the metal and are suggested to be the important component of the electronic catalytic metal-support interaction which are known to be much stronger for reducible than for non-reducible oxides.KEYWORDS: Noble metal catalysis; metal oxide support; catalyst-support effect; charge polarization; Kelvin probe microscopy 2

Existing models for the analysis of Kelvin probe microscopy experiments are extended and used to analyze the experimental electrical potential profiles for a Pt/TiO 2 model nanoparticle.The derived model reproduces in detail the Kelvin probe image that reveals a characteristic ring-shaped negative charge zone at the surface around the particle: A planar negative charge zone at the surface of the support extends beyond the diameter of the Pt particle. It is compensated mostly by a planar layer of positive charges in the metal across the interface, and by a smaller number of positive charges at the metal-air interface. These latter charges determine the positive electrical potential of the metal particle, and they are likely responsible for the extent of the metal-support interaction in catalytic reactions.

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