Abstract
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.
Highlights
CO oxidation, where a CO molecule interacts with an oxygen molecule to form CO2, is one of many reactions that occur in an automotive catalyst to clean the exhaust gases from the toxic CO molecules
The planar laser-induced fluorescence (PLIF) images of CO and CO2 display a boundary layer with a spherical shape around the surface when the mass transfer limited (MTL) is reached during CO oxidation
This boundary layer has a significantly different gas composition than that measured at the chamber gas inlet or outlet, with a CO concentration below the detection limit of the ambient pressure X-ray photoelectron spectroscopy (AP-XPS)
Summary
CO oxidation, where a CO molecule interacts with an oxygen molecule to form CO2, is one of many reactions that occur in an automotive catalyst to clean the exhaust gases from the toxic CO molecules. The reaction process is well-known under ultra-high vacuum (UHV) conditions but less is known about the surface structure and reaction mechanism under more realistic operating conditions for the catalyst. More realistic conditions in turn involve higher pressure, which implies that the number of molecules interacting with the surface increases significantly. The gas molecules interacting with the catalyst surface are essential for the surface structure and a change in the gas composition close to the surface may result in a change of the surface composition [3, 4]. It is important to perform catalysis experiments in situ where knowledge of the gas molecules interacting with the surface can be achieved and used to understand the active surface structure of a catalyst in a better way [5]
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