High Pressure XPS (X-ray photoelectron spectroscopy) in the millibar range became an in situ tool for investigations of heterogeneous catalytic processes within the last years. The successs of this method is based on the possibility to correlate the catalytic activity of a catalyst with the electronic structure of the working catalyst surface. The application of this technique at a synchrotron radiation facility provides the possibility to measure depth profiles by changing the photon energy and therefore the kinetic energy of the photoelectrons. This kind of measurement allows the identification of subsurface species present below the active surface of a working catalyst. Subsurface species might influence the electronic structre of the active surface and influence the catalytic properties of the catalyst.

Subsurface oxygen was found in polycrystalline Cu foil during the partial oxidation of methanol to formaldehyde. The abundance of this species is correlated to the yield of formaldehyde. Investigations of the same heterogeneous catalytic reaction over Ru(0001) shows that RuO₂ is active in the total oxidation of methanol to CO₂ and H₂O, whereas a RuO_x species catalyses the partial oxidation to formaldehyde.

The selective hydrogenation of pentyne to pentene over polycrystalline Pd foil is characterised by the formation of a subsurface carbon species, which blocks the diffusion of dissolved hydrogen to the surface. Dissolved hydrogen is known to be not selective. In this case only the gas phase hydrogen, which is selective, is available for the active surface. Under reaction conditions favourising the total hydrogenation of pentyne to pentane the carbon layer forms to a much lower extent. These examples show the potential of this in situ technique.

An overview about recent developments in the field of high pressure XPS will be given. The availability of the method for the community will be dicussed.

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