The Flexible
Surface Revisited: Adsorbate-Induced
Reconstruction, Homocoupling, and Sonogashira Cross-Coupling on the
Au(100) Surface
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Abstract
Phenylacetylene (PA) and iodobenzene
(IB) are prototypical reactants
in Sonogashira cross-coupling. Their adsorption behavior and reactivity
on the Au(100) surface were studied by STM, temperature-programmed
desorption and reaction, and DFT calculations that included the effect
of dispersion forces. The two species exhibited very different behavior.
Thus, even at 200 K, PA rearranged Au surface atoms so as to lift
the hex reconstruction and adsorb in 4-fold-symmetric islands on the
unreconstructed 100 surface. On the other hand, IB adsorbed on the
reconstructed hex surface, again as islands, forming three different
coexisting close-packed structures. The DFT results are in good accord
with these findings, demonstrating the strong preference of PA and
IB for the (100) and hex surfaces, respectively. Moreover, the calculated
adsorption energies were in satisfactory agreement with values estimated
from the desorption data. Adsorbed separately, both PA and IB underwent
homocoupling, yielding diphenyl diacetylene and biphenyl, respectively;
in the former case, reaction appeared to originate at island boundaries.
On the well-annealed surface, coadsorbed PA and IB behaved independently,
generating only products of homocoupling. However, on the Ar<sup>+</sup> roughened surface, Sonogashira cross-coupling also occurred, yielding
diphenyl acetylene. These findings are discussed in terms of the island-forming
propensity of the reactants, amplified by the labile nature of the
Au 100 surface under adsorption and the marked preference of the two
reactants for different substrate structures, factors that act to
inhibit the formation of a mixed adlayer and suppress reactivity.
The implications for the behavior of practical Au nanoparticle catalysts
are considered