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Facile fabrication of functionalized core-shell Fe3O4@SiO2@Pd microspheres by urea-assisted hydrothermal route and their application in the reduction of nitro compounds

Gil, J.C, Ferreira, L.F., Silva, V.C., Oliveira, A.C., de Oliveira, R.R., Jacinto, M.J.
Environmental nanotechnology, monitoring & management 2019 v.11 pp. 100220
catalysts, catalytic activity, hydrogen, hydrolysis, magnetism, magnetite, microparticles, nanoparticles, nitrates, p-nitrophenol, pH, reducing agents, silica, sodium hydroxide, urea
Fe3O4@SiO2 microspheres were fabricated by a simple urea-assisted hydrothermal route. The spheres have a diameter of about 400 nm containing a dense magnetic core (150 nm) composed of individual magnetite nanoparticles (12 nm). The material was functionalized with different organosilanes containing electron-donor groups (APTES and APTDS). The materials were further used to stabilize Pd nanoparticles deposited on the microspheres surface, obtained by the in-situ reduction of Pd(NO3)2. The catalysts were applied in the reduction of 4-nitrophenol using benign molecular H2 as a reducing agent under mild reaction conditions (75 °C, PH2 = 6 atm). The core-shell structures, dubbed Fe3O4@SiO2@-APTES-Pd and Fe3O4@SiO2@-APTDES-Pd were able to reduce p-nitrophenol to p-aminophenol in less than 7 min, and TOF as high as 533 h−1 was achieved. A single portion of each catalyst was also able to be reused several times in successive reaction batches, and the functionalization was crucial to obtain high catalytic activity. When comparing the activity of both catalysts, a single portion of Fe3O4@SiO2@-APTES-Pd could be used up to 22 times giving a total turnover number (TON) of 385 which is more than doubled compared to that obtained using the Fe3O4@SiO2@-APTDES-Pd catalyst. Another important result achieved in this study is the morphology of the core-shell (Fe3O4@SiO2) microspheres (420 nm) obtained using a new coating methodology. The synthetic protocol uses urea as pH regulator to promote the hydrolysis of the shell precursor (TEOS) to SiO2. We found out that using urea instead of unfriendly alkaline reactants leads to the formation of regular core-shell Fe3O4@SiO2 microspheres whereas the use of NaOH results in the formation of a random silica matrix containing magnetite nanoparticles.