Shape dependent catalytic activity of nanoflowers and nanospheres of Pd_4S generated via one pot synthesis and grafted on graphene oxide for Suzuki coupling?

摘要

Nanoflowers and nanospheres of Pd_4S have been prepared for the first time from a single source precursor complex, [PdCl_2(PhS-CH_2CH_2CH_(2-)NH_2)] (1), by its one pot thermolysis at 195 °C. In oleylamine, flower shaped nanoparticles of Pd_4S were formed but in an oleic acid (OA) and octadecene (ODE) mixture (1: 1) the product was nanospheres of Pd_4S (size in the range ～23–38 nm and 15–28 nm, respectively). These nanoparticles (NPs) were grafted on graphene oxide (GO) at room temperature to prepare nanocomposites, GO–Pd_4S. HRTEM, powder X-ray diffraction (PXRD) and TEM-EDX have been used to authenticate the nanoparticles and their composites. XPS of Pd_4S NPs indicates the oxidation states of Pd and S are both zero with a Pd: S ratio ～4.1: 0.9. For the catalysis of Suzuki–Miyaura coupling reactions the nanoparticles individually and in the form of composites with GO were explored. The flower shaped NPs are superior than the spherical ones for this catalysis in aqueous ethanol and the catalytic efficiency increases on grafting the nanoflowers/spheres onto GO. The conversion was ～99% (in 5 h; at 80 °C) for the composite of graphene oxide (GO) with the Pd_4S nanoflowers (Pd: 0.2 mol%). The catalytic efficiency follows the order GO–Pd_4S-nanoflowers > GO–Pd_4S-nanospheres > Pd_4S nanoflowers > Pd_4S nanospheres. The recyclability of the GO–Pd_4S nanoflower catalyst was examined for the coupling reaction and conversion was found to be ～46% in the fourth run even after increasing the reaction time to 12 h. To understand whether the catalytic process with the GO–Pd_4S nanoflowers was homogeneous or heterogeneous mercury poisoning, triphenylphosphine and three phase tests were carried out. They suggest that active Pd leached from GO–Pd_4S nanoflowers does the catalysis significantly in a homogeneous fashion. Overall the catalysis appears to be a cocktail of homogeneous and some heterogeneous nature.