Core–Shell Fe–Pt Nanoparticles in Ionic\nLiquids: Magnetic and Catalytic Properties

Abstract

The\nreaction of Fe­(CO)₅ and Pt₂(dba)₃ in 1-<i>n</i>-butyl-methylimidazolium tetrafluoroborate\n(BMIm.BF₄), hexafluorophosphate (BMIm.PF₆),\nand bis­(trifluoromethanesulfonyl)­imide (BMIm.NTf₂) under\nhydrogen affords stable magnetic colloidal core–shell nanoparticles\n(NPs). The thickness of the Pt shell layer has a direct correlation\nwith the water stability of the anion and increases in the order of\nPF₆ > BF₄ > NTf₂, yielding\nthe metal\ncompositions Pt₄Fe₁, Pt₃Fe₂, and Pt₁Fe₁, respectively. Magnetic measurements\ngive evidence of a strongly enhanced Pauli paramagnetism of the Pt\nshell and a partially disordered iron-oxide core with diminished saturation\nmagnetization. The obtained Pauli paramagnetism of the Pt shell is\n2 orders of magnitude higher than that of bulk Pt, owing to symmetry\nbreaking at the surface and interface, resulting in a strong increase\nin the density of states at the Fermi level, and thus to enhanced\nPauli susceptibility. Moreover, these ultrasmall NPs showed efficient\ncatalytic activity for the direct production of selective short-chain\nhydrocarbons (C₁–C₆) by the Fischer–Tropsch\nsynthesis with efficient conversion (18–34%) and selectivity\n(69–90%, C₂–C₄). The selectivity\nand activity were dependent on the Fe-oxides@Pt particle size. The\ncatalytic activity decreased from 34 to 18% as the NP size increased\nfrom 1.7 to 2.5 nm at 15 bar and 300 °C.