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Saturated Hydrocarbons

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.
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.
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.
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.
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.
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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