Book of Abstracts Book of Abstracts - Universität Konstanz

The Bonding of CO with Au8: Evidence of Cluster Charging
M. A. Röttgen 1 , A. S. Wörz 1 , K. Judai 1,3 , S. Abbet 1 , J. M. Antonietti 1 , U. Heiz 1 , B. Yoon 2 , and
U. Landman 2
1 Institute of Surface Chemistry and Catalysis, Universität Ulm, 89069 Ulm, Germany
2 Georgia Institute of Technology, School of Physics, Atlanta, Georgia 30332-0430, USA
3 Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan
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While Gold is inert as bulk material, it becomes catalytically active as nanoscale particles or
clusters supported onto oxide support materials. In the last few years, effort was made to get a
better insight into the relevant reaction mechanisms and to understand this astonishing
phenomenon on a molecular level. For large gold particles a few nanometers in size, it is
discussed on the one hand whether the catalytic activity can be explained by the presence of
active sites on the cluster-support interface or by low-coordinated, highly reactive sites. On the
other hand, for small clusters, the gold octamers bound to F-centers of an MgO surface are the
smallest known gold heterogeneous catalysts able to transform CO into CO2. The key for the
low-temperature conversion on Au8 is the binding of the molecular oxygen and the concomitant
O-O bond activation to a peroxo-like adsorbate state. This is possible as the cluster’s electronic
states show resonance with the 2π* molecular states of oxygen and as these antibonding states
are pushed below the Fermi level of the system upon interaction with the cluster. Interestingly,
the same cluster bound to a terrace site of an MgO surface is inert for the combustion reaction.
Extensive ab initio calculations predicted that charging from the F-center into the cluster is
essential for the activation of gold octamers.
In this contribution, we present new findings showing experimental evidence for this prediction.
The measured CO stretch frequencies, ν(CO), under reaction conditions are red-shifted by about
30-50 cm-1 for the octamer bound to the F-center (Au8/CO/O2/MgO(FC)) in comparison to the
octamer bound to the five-coordinated terrace sites (Au8/CO/O2/MgO(O5c)). Extensive ab
initio calculations reveal that this shift is caused by enhanced backdonation into the antibonding
2π* orbital of CO adsorbed on the Au8/O2/MgO(FC) complex and reveal important details of
the bonding of CO for understanding the backdonation. In addition, calculations on free
Au8/O2/CO complexes indeed show this effect to be related to a substantial change in charging
of the cluster [1].
References
[1] B. Yoon et al., Accepted for Publication in Science (2004).
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