Thesis defense: Ultrafast Probing of CO Reactions on Metal
Thursday 30 March 2017
to 13:00 at
Jörgen Gladh (Stockholm University, Department of Physics)
This thesis presents experimental studies of three different chemical reaction steps relevant for heterogeneous catalysis:
dissociation, desorption, and oxidation. CO on single-crystal metal surfaces was chosen as the model systems.
X-ray absorption spectroscopy (XAS) and x-ray emission spectroscopy (XES) provide information about the electronic
structure, and were performed on CO/Fe to measure both a non-dissociative, and a pre-dissociative state. The measurement
on the pre-dissociative state showed a π → π* excitation, which implies a partly broken internal π bond in the molecule.
Ultrafast laser-induced reactions were used to examine the dynamic properties of desorption and oxidation. Here CO/Ru
and CO/O/Ru were used as model systems. Desorption of CO from a Ru surface involve both hot electrons and phonons. In
the case of CO oxidation from CO/O/Ru a pronounced wavelength dependence of the branching ratio between desorption
and oxidation was observed. Excitation with 400 nm showed a factor of 3-4 higher selectivity towards oxidation than 800
nm. This was attributed to coupling to transiently excited, non-thermalized electrons.
Finally, by performing optical pump/x-ray probe XAS and XES changes in the electronic structure during the reaction
could be followed, both for desorption and oxidation. In the CO/Ru experiment, two different transient excitation paths were
observed, one leading to a precursor state, and one where CO moves into a more highly coordinated site. Using selective
excitation in XES, these were shown to coexist on the surface. In the oxidation experiment, probing the reacting species
located near the transition state region in an associative catalytic surface reaction was demonstrated for the very first time.