Core-Shell Fe-Pt Nanoparticles in Ionic Liquids: Magnetic and Catalytic Properties

The reaction of Fe(CO)(5) and Pt-2(dba)(3) in 1-n-butyl-methylimidazolium tetrafluoroborate (BMIm.BF4), hexafluorophosphate (BMIm.PF6), and bis(trifluoromethanesulfonyl)imide (BMIm.NTf2) under hydrogen affords stable magnetic colloidal core-shell nanoparticles (NPs). The thickness of the Pt shell layer has a direct correlation with the water stability of the anion and increases in the order of PF6 > BF4 > NTf2, yielding the metal compositions Pt4Fe1, Pt3Fe2, and Pt1Fe1, respectively. Magnetic measurements give evidence of a strongly enhanced Pauli paramagnetism of the Pt shell and a partially disordered iron-oxide core with diminished saturation magnetization. The obtained Pauli paramagnetism of the Pt shell is 2 orders of magnitude higher than that of bulk Pt, owing to symmetry breaking at the surface and interface, resulting in a strong increase in the density of states at the Fermi level, and thus to enhanced Pauli susceptibility. Moreover, these ultrasmall NPs showed efficient catalytic activity for the direct production of selective short-chain hydrocarbons (C-1-C-6) by the Fischer-Tropsch synthesis with efficient conversion (18-34%) and selectivity (69-90%, C-2-C-4). The selectivity and activity were dependent on the Fe-oxides@Pt particle size. The catalytic activity decreased from 34 to 18% as the NP size increased from 1.7 to 2.5 nm at 15 bar and 300 degrees C.