Chemistry Department Seminar

The directed development of new catalysts necessitates the understanding of the structure – performance relation. Such relations can only be derived when the structure of a catalyst is determined under catalytically relevant conditions, using, for example, synchrotron-based XRD, XAS, and XES to determine the electronic and geometric structures in combination with a quadrupole mass spectrometer or gas chromatograph to determine the performance. Identifying the relevant structure, e.g. that of the reactive intermediate and / or a surface species poses a real challenge. Many important structural changes in catalysts, such as oxidation-reduction and structural re-ordering, occur in the sub-second to minute range. In this talk, I will show how sub-second XAS, as installed at the SuperXAs beam line at the Swiss Light Source, enables to identify the relevant structure of a catalyst and to identify active surface species.
The first example is that of one of the most studied reactions in catalysis: carbon monoxide oxidation on the surface of a supported Pt catalyst. Using sub-second, spatially-resolved XAS, we followed the oscillating CO oxidation at different positions in a plug-flow reactor. We identified a short lived intermediate, when switching between the non active, surface poisoned metallic phase and the active partial oxidic phase that shines new light on the oxidation mechanism in CO oxidation in a packed-bed reactor.
The second example deals with the application of modulation spectroscopy to XAS to identify small changes in catalyst structure which belong to the 'active' surface atoms of the catalyst. In modulation spectroscopy, the catalyst’s structure is modulated between two states by periodically changing, for example, the gas composition and collecting XAS spectra with a high repetition rate. After demodulation, spectra are obtained that are filtered from all contributions that do not change with the modulation frequency, which ar