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Many catalytic reactions of industrial importance involve
reforming of saturated hydrocarbons to more reactive species. Often the
first step is breaking of a C-H bond, which is a strongly
activated process.
It has been known for a long time that the C-H bond can be
activated already in the adsorbed molecular state.
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The adsorption energy of saturated hydrocarbons is close to the
sublimation energy on most metal surfaces. Therefore these systems have
traditionally been thought of as typical examples of physical
adsorption, where dispersion forces are responsible for the
bonding and there are no significant changes of the electronic
structure upon adsorption. Previous results mainly from
vibrational spectroscopy do, however, show softening of C-H stretch
vibrations. This indicates that there are changes in the
electronic structure upon adsorption.
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Using x-ray emission spectroscopy
(XES) and x-ray absorption spectroscopy (XAS) in combination with
spectrum calculations with density functional theory (DFT) we
have measured both the occupied and unoccupied local electronic structure
of n-octane in an atom specific and symmetry resolved way. |
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XES, which probes the occupied
density of states, reveals new adsorption-induced states, which
we assign to interaction between the mainly the bonding
CH-orbitals and the Cu 3d band. By performing a systematic
investigation of how the XA and XE spectra are influenced by
different structural parameters, we conclude that the geometry
is significantly distorted relative to the gas phase. The
bonding to the surface leads to strengthening of C-C bonds and
weakening of C-H bonds. These changes are interpreted as a
rehybridization of the carbon from sp3 to
sp2.8.
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These results can be useful for the
understanding of the CH bond cleaving mechanism, which is
important in catalysis. Comparison of theoretical spectra
between adsorption of n-octane on Cu and Ni surfaces
show that the main difference is the position of the adsorption
induce occupied states, which follow the position of the metal d
band. On Cu all these states are occupied, whereas on Ni they
cross the Fermi leaving some of them unoccupied, which leads to a
stronger bond. This can explain why Ni is an efficient
dehydrogenation catalyst, whereas Cu is not.
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| References: |
| H. Öström, L. Triguero,
M. Nyberg, H. Ogasawara, L.G.M. Pettersson and
A. Nilsson, Physical Review Letters 91,
046102 (2003) |
| H. Öström, L. Triguero, K. Weiss,
H. Ogasawara, M. G. Garnier, D. Nordlund, M. Nyberg,
L.G.M. Pettersson and
A. Nilsson, Journal of Chemical Physics 118,
3782 (2003) |
| K. Weiss, H. Öström, L. Triguero,
H. Ogasawara, M. G. Garnier, L.G.M. Pettersson and
A. Nilsson, Journal of Electron Spectroscopy and Related Phenomena 128,
179 (2003) |
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