Regensburg 2010 – scientific programme

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O: Fachverband Oberflächenphysik

O 49: Surface chemical reactions I

O 49.7: Talk

Wednesday, March 24, 2010, 12:00–12:15, H42

Material and Orientation dependent activity for heterogeneously catalyzed carbon-bromine bond homolysis — •Hermann Walch, Rico Gutzler, Thomas Sirtl, Georg Eder, and Markus Lackinger — LMU Munich, Section Crystallography

Adsorption of the organic molecule 1,3,5-tris(4-bromophenyl)benzene on different metallic substrates, namely Cu(111), Ag(111) and Ag(110) has been studied by variable temperature Scanning Tunneling Microscopy (STM). Depending on substrate temperature, material and orientation, we observe a surface-catalyzed dehalogenation reaction. Deposition onto the catalytically active substrates Cu(111) and Ag(110) held at room temperature leads to cleavage of the carbon-bromine bonds and subsequent formation of protopolymers, i.e radical metal coordination complexes. However upon deposition on Ag(111) no such reaction has been observed. Instead, various self-assembled ordered structures based on intact molecules could be identified. Also sublimation onto either substrate held at 80 K did not result in any dehalogenation, thereby exemplifying that the dehalogenation reaction is thermally activated. We explain the differences in catalytic activity by charge transfer into unoccupied molecular orbitals and subsequent destabilization of the C-Br bond, whereby enhanced molecule-substrate interaction leads to an increasing magnitude of charge transfer. The interaction strength follows the general reactivity order Cu > Ag > Au for (111) faces and is generally enhanced on higher corrugated surfaces as the (110) facet in case of fcc substrates.