The Flexible Surface Revisited: Adsorbate-Induced Reconstruction, Homocoupling, and Sonogashira Cross-Coupling on the Au(100) Surface

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⁺ 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

Theoretical Study of the Interaction of CO on TiC(001) and Au Nanoparticles Supported on TiC(001): Probing the Nature of the Au/TiC Interface

The interaction of CO with the bare TiC(001) surface and with Au_<i>n</i> (<i>n</i> = 4, 9, 13) nanoparticles supported on the same TiC(001) surface has been studied by means of periodic density functional theory (DFT) based calculations with large supercell slab models. CO adsorption on the bare TiC(001) surface involves the direct interaction with a C surface atom and leads to a significant deformation of the underlying substrate. Because of this feature the calculated adsorption energy significantly varies with coverage. A comparison with available experimental data shows that this system is more complex than expected. The interaction of CO with the Au nanoparticles involves preferential bonding to low coordinated Au atoms. However, although the supported Au nanoparticles bind CO well, the adsorption energy of the molecule on the admetal is somewhat smaller than the one corresponding to the naked carbide surface and decreases with increasing the particle size, which is also consistent with a rather small red shift of the vibrational frequency of the adsorbed CO molecule that also decreased with increasing particle size. Implications for the use of Au/TiC systems in catalytic reactions involving CO are also discussed