Recent studies show that molecular adsorption on oxide surfaces that are in contact with metals, such as Ag and Pt, is qualitatively different as compared to adsorption on isolated oxide surfaces [1]. For instance, there is a substantial increase in NO2 adsorption energy (in the order of eVs) that is only weakly dependent on the distance between the adsorbate and the metal. This is highly relevant for oxides used in heterogeneous catalysis as the properties of the oxide may be modified with only small amounts of metal dopants [see e.g. 2].
In the present contribution, we report an extensive and systematic first-principles investigation in order to reveal the underling reason for this novel bonding mechanism. We use oxide-metal systems that are commonly used in heterogeneous catalysis, such as, MgO, BaO and Al2O3. The oxides will be in contact with either a metal surface terrace or a small cluster. We focus on the adsorption properties of NO2, a highly relevant molecule in e.g. automotive catalysis. The bonding mechanism is examined by an analysis of structure properties, projected density of states, and charge density differences. For instance, the study reveals that one main component in the bonding mechanism is a charge transfer to the adsorbate.
We find that the novel bonding mechanism is general and is applicable to all oxide-metals under consideration. This result suggests a necessary revision of the common view of an inert oxide support in heterogeneous catalysis.
[1] H. Grönbeck, J. Phys. Chem. 110, 11977, (2006)
[2] Q. Fu, H. Saltsburg, M. Flytzani-Stephanopoulos, Science, 301, 935, (2003)
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