The structure of a recently developed Pd catalyst, named sulfur-modified gold-supported palladium (SAPd), has been determined to be composed of multi-layered Pd nanoparticles. SAPd is easily prepared by selfassembly on a sulfur-modified gold surface, and near-edge X-ray absorption fine structure (NEXAFS) analysis at the Pd K-edge determined that the Pd in SAPd is zero-valence analogous to metallic Pd.However, transmission electron microscopy (TEM) analyses and extended X-ray absorption fine structure (EXAFS) analysis clarified that SAPd has approximately 10 layers and consists of nanoparticles with a diameter less than 5 nm. High-density Pd nanoparticles were embedded without condensation. NEXAFS analysis at the S and C K-edge revealed that the organic matter containing sulfate and xylene as a major ingredient is distributed between Pd nanoparticles, and it seems to prevent condensation. These findings suggest that a highly efficient cross-coupling reaction, which was reported in earlier works, has been achieved by the high-density Pd nanoparticles.

Abstract:We have developed a conceptually and methodologically novel self-assembled multilayer nickel nanoparticle (NP) catalyst -sulfur-modified gold-supported Ni NPs (SANi) -for organic synthesis. The SANi catalyst was easily prepared through a three-step procedure involving simultaneous in situ metal NP and nanospace organization. This unique method does not require any conventional preformed template for immobilizing and stabilizing NPs. SANi catalyzes carbon-carbon bond-forming cross-coupling, Kumada coupling, and Negishi coupling reactions under ligand-free conditions and can be used repeatedly for these reactions. Physical analysis of SANi showed that the active species in these reactions are self-assembled multilayer zerovalent Ni NPs with a size of~3 nm.

An improved process for the preparation of sulfur-modified gold-supported palladium material [SAPd, second generation] is presented. The developed preparation method is safer and generates less heat (aqueous Na2S2O8 and H2SO4) for sulfur fixation on a gold surface, and it is superior to the previous method of preparing SAPd (first generation), which requires the use of the more heat-generating and dangerous piranha solution (concentrated H2SO4 and 35% H2O2) in the sulfur fixation step. This safer and improved preparation method is particularly important for the mass production of SAPd (second generation) for which the catalytic activity was examined in ligand-free Buchwald-Hartwig cross-coupling reactions. The catalytic activities were the same between the first and second generation SAPds in aromatic aminations, but the lower palladium leaching properties and safer preparative method of second generation SAPd are a significant improvement over the first generation SAPd.

The density functional calculation is performed to determine the structure of the reused palladium catalyst on sulphur terminated GaAs(001) surface for the Heck reaction. The DFT calculation has been done for S‐terminated GaAs(001) surface with and without As of the topmost layer. The difference the two model of the S‐terminated GaAs(001) is very small. Since, for both case, the binding energy of palladium atom is stronger due to the co‐adsorption of sulphur, the role of sulphur is to make the binding of palladium stronger. Moreover, the heights of the position of the adsorbed palladium atom for both model cases are lower than sulphur atom on the surface. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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