Comparison of AP-XPS and PLIF Measurements during CO Oxidation over Pd Single Crystals

The interaction between the gas-phase molecules and a catalyst surface is crucial for the surface structure and are therefore important to consider when the active phase of a catalyst is studied. In this study we have used two different techniques to study the gas phase during CO oxidation over Pd single crystals. Gas-phase imaging by planar laser-induced fluorescence (PLIF) shows that a spherical boundary layer with a decreasing gradient of CO₂ concentration out from the surface, is present close to the surface when the Pd crystal is highly active. Within this boundary layer the gas composition is completely different than that detected at the outlet of the chamber. The PLIF images of the gas-phase distribution are used to achieve a better understanding of the gas composition between the surface and the detector of a set-up for ambient pressure X-ray photoelectron spectroscopy (AP-XPS), a common technique for surface structure determination of model catalysts. The results show that also the gas-phase peaks present in the AP-XPS spectra truly represent the gas closest to the surface, which facilitates the interpretation of the AP-XPS spectra and thereby also the understanding of the mechanism behind the reaction process.