Characterization of Pd nanoparticles and of silica-supported Pd-catalysts for the Suzuki-Miyaura reaction.
Hanif, Mohammad Abu
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The importance of palladium (Pd) as a catalyst in organic coupling reactions is undeniable. Mesoporous silica is widely used as the support of Pd catalysts. But the porous structure of the mesoporous silica limits complete characterization. As a result, heterogeneous Pd catalysts are not fully understood. Model Suzuki-Miyaura reaction catalysts have been made by immobilizing Pd on a mercaptopropyltrimethoxysilane (MPTMS) functionalized Si substrate. Two types of Pd species were found on the fresh catalysts that may be attributed to a S-bound Pd (II) species and Pd nanoparticles. A sulfur species that has not been previously reported on this class of catalysts has also been observed. A systematic study of various palladium/sulfur complexes using XPS was carried out to identify this species, which may be assigned to high oxidation state sulfur formed by oxidation of thiol during the reduction of the Pd(OAc)2 used to load the catalyst with Pd. Shifts in binding energy observed for both Pd and S spectra of the used catalysts were examined in order to probe the change of electronic environment of reactive palladium center and the thiol ligand during the reaction. Electron and atomic force microscopic imaging of the surfaces demonstrates the formation of Pd nanoparticles on fresh catalysts and subsequent size reduction of the Pd nano-particles following reaction. Nanoparticles have gained increased attention due to their unique properties different from the bulk materials. Polyvinyl pyrrolidone (PVP) and dodecyl sulfide stabilized palladium nanoparticles have been synthesized with a size range of 3 to 10 nm and 2 to 5 nm respectively. XPS confirmed the conversion of Pd (II) to Pd (0) and presence of stabilizing agent on nanoparticles surface by observing Pd 3d, N1s (for PVP stabilized) and S 2p (for dodecyl sulfide stabilized) high resolution spectra. Dispersion and size of the nanoparticles were measured by TEM. It was observed that the shift of binding energy of the nanoparticles is strongly size dependent. Finally, the ability of N-heterocyclic carbenes (NHCs) to substitute the stabilizing agent of Pd nanoparticles was investigated. The XPS studies suggest the substitution of dodecyl sulfide from the Pd nanoparticles by NHCs.