The value of X-ray photoelectron spectroscopy (XPS) resides in its capacity to supply information on the surface elementary composition (a few nm) and the chemical environment of the atoms probed, something that is particularly useful for characterizing the active sites of catalysts.
Until recently, this characterization method was limited to operation under ultra highvacuum conditions, which meant that it was not possible to monitor the surface evolution of the catalysts in reactional medium.
A dedicated system (NAP-XPSa) now allows us to work in environmental mode at pressures of around few millibars. Combined with the high brillance of a synchrotron radiation, which is energy-adjustable, this system is much more efficient than a laboratory spectrometer. The possibility of introducing gas into the analysis chamber means that it is now possible to monitor the evolution of the species formed under activation or reactive conditions.
In this way, a model catalyst, based on cobalt oxide supported on silica, has been characterized on the TEMPO beamline of the SOLEIL synchrotron.
Catalyst activation was performed in two stages: first, partial reduction of the cobalt at 400°C under 1 mbar of hydrogen, then introduction of a gas mixture (2 H2 + CO) at 230°C in order to simulate the reaction conditions of the Fischer-Tropsch process.
Recording of the spectra for cobalt, at an incident energy of 1,100 eV, demonstrated the evolution at 400°C of the oxide form towards a more reduced form, characterized by a lower binding energy. Reduction of the catalyst continues slightly in the presence of the reactional mixture at 230°C.
These first tests on a catalyst are promising and demonstrated that it is possible to overcome the analytical problems related to the insulating character of the support, thereby enabling surface modifications to be monitored in the reactional atmosphere.
a - Near Ambient Pressure XPS.