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

Article Cite This: J. Phys. Chem. C 2018, 122, 4641−4650

pubs.acs.org/JPCC

Core−Shell Fe−Pt Nanoparticles in Ionic Liquids: Magnetic and Catalytic Properties Janice Adamski,† Muhammad I. Qadir,† Jilder P. Serna,⊥ Fabiano Bernardi,‡ Daniel L. Baptista,‡ Benjamin R. Salles,⊥ Miguel A. Novak,*,⊥ Giovanna Machado,§ and Jairton Dupont*,† †

Institute of Chemistry and ‡Institute of Physics, UFRGS, Av. Bento Gonçalves, 9500, Porto Alegre 91501-970, Rio Grande do Sul, Brazil ⊥ Centro de Tecnologias Estratégicas do Nordeste (CETENE), Recife 50740-545, Brazil § Institute of Physics Goma Universidade Federal do Rio de Janeiro, Rio de Janeiro 21941-972, Brazil S Supporting Information *

ABSTRACT: The reaction of Fe(CO)5 and Pt2(dba)3 in 1-n-butylmethylimidazolium tetrafluoroborate (BMIm.BF4), hexafluorophosphate (BMIm.PF 6 ), 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 (C1−C6) by the Fischer−Tropsch synthesis with efficient conversion (18− 34%) and selectivity (69−90%, C2−C4). 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 °C. Fischer−Tropsch synthesis.10 The formation of such metal oxides is almost unavoidable due to the reactivity of reduced Fe or Co toward oxidation under FTS conditions. The use of bimetallic catalysts facilitates the reduction of metal oxides to active species under FTS reaction conditions, and it is one of the solutions to the problem that is currently under investigation.11−13 The synthesis of the NPs generally involves the use of Fe(CO)5 and Pt salt precursors in the presence of surfactants.14 This approach has been used successfully for the generation of various kinds of surfactant capped core−shell Fe/ Pt NPs, but it is not a viable approach to the production of “naked” NPs. Therefore, the preparation of stable colloidal naked Fe−Pt core−shell NPs is still a challenge, and the influence of the choice of surfactants on the properties of these materials has not yet been fully assessed. In this respect, ionic liquids (ILs), particularly 1,3-dialkyl imidazolium salts, provide one of the most versatile liquid platforms/templates for the generation of a plethora of stable and naked colloidal mono-

1. INTRODUCTION Transition metals containing core−shell nanoparticles (NPs) can have multiple functions that do not exist in singlecomponent compounds as well as unique properties that exist only in nanometer (nm)-sized materials.1 Indeed, the development of core−shell NPs has attracted a lot of interest because their biomedical,2,3 magnetic,4,5 and catalytic properties6,7 that can be tailored using two disparate components. In particular, Pt/Fe-oxide NPs containing a variable atomic percentage of Fe and Pt are an important class of magnetic nanomaterials.8 The magnetic properties of each core/shell nanoparticle can be tuned by varying the thickness of the Fe3O4@Pt shell, for example. Such magnetic core−shell NPs represent a novel class of nanostructured magnetic materials. Precise engineering of their magnetic properties may be possible through selective tuning of anisotropy, magnetization, and the dimensions of the core and shell. This would facilitate their use in the fabrication of novel nanomagnetic devices.9 Controlling the size and shell thickness as well as the ratio of the soft and hard components is key to modulating the magnetic properties of core−shell NPs. Moreover, the presence of Pt can facilitate the reduction of inactive metal oxides formed during catalysis, such as in © 2018 American Chemical Society

Received: December 12, 2017 Revised: January 30, 2018 Published: January 30, 2018 4641

DOI: 10.1021/acs.jpcc.7b12219 J. Phys. Chem. C 2018, 122, 4641−4650

Article

The Journal of Physical Chemistry C

film. Particle size distributions were calculated from HAADFSTEM images. TGA analysis was performed on a TA Instruments SDT Q600 using aluminum pans. Temperatureprogrammed reduction (TPR) analyses were performed in a quartz reactor (Micro Reactor CatLab-Hidden Analytical), connected to a mass spectrometer (QIC 20-Hidden Analytical) through a melt silica cannula. All of the experiments were performed with 10 mg of the material at 40 mL min−1 flux of 7% hydrogen in argon. The conversion was monitored by the drop in pressure. GC analyses were performed using an Agilent 6820 GC System. The gaseous products were analyzed using an Agilent Micro-GC System 3000A. CO2 selectivity was not measured. Product selectivity was calculated as the equivalent amount of the desired hydrocarbon with respect to the total amount of hydrocarbons produced. 2.1. Synthesis of Core−Shell Fe-Oxide@Pt Bimetallic NPs. In a 50 mL Parr reactor, equimolar amounts of [Fe(CO)5] (0.6 mmol, 80 μL) and [Pt2(dba)3] complexes (0.3 mmol, 327.9 mg) were added into ILs (3 mL) under argon atmosphere. The reactor was charged with hydrogen (2 × 10 bar), released to remove argon, and filled with 30 bar of H2. The reaction was conducted at a temperature of 120 °C, using a heating mantle and hard stirring. After 18 h, the gas was released at room temperature, and the resulting black suspension containing FexOy@Pt NPs in IL was placed under vacuum in order to remove volatiles. The obtained NPs were collected by centrifugation, washed with dichloromethane (5 × 10 mL), and dried under vacuum. The NPs were stored under argon prior to use. 2.2. Selective Fischer−Tropsch Synthesis. FTS reactions were performed in a DRIFT cell that was charged with 10 mg of isolated NPs. The cell was placed under vacuum to remove atmospheric oxygen, then filled with 15 bar of syngas (CO/H2= 1/2), and the reaction was allowed at the chosen temperature for 16 h. Then, the DRIFT cell was cooled at room temperature and connected to the Micro GC to analyze the gaseous phase.

and bimetallic nanoparticles.5,15−20 Appropriate choice of the anion and the alkyl side chain of the cation can regulate the size and distribution of metal NPs. For example, smaller (2.3 nm) Pt NPs are obtained by the simple decomposition of a Pt(0) organometallic precursor in hydrophobic 1-n-butyl-3-methylimdazolium hexafluorophosphate IL, whereas larger Pt NPs (3.4 nm) are produced in the hydrophilic tetrafluoroborate IL analogue under the same reaction conditions.21 Transmission electron microscopy, X-ray photoemission spectroscopy (XPS), and small-angle X-ray scattering (SAXS) analysis clearly show the interactions of the IL with the metal surface, demonstrating the formation of a semiorganized IL protective layer surrounding the platinum nanoparticles.21,22 Different types of iron-oxide and Fe(0) NPs can be also prepared in ILs by the simple decomposition of Fe(0) organometallic precursors and reduction of Fe salts.23−27 Alloys and core−shell Co/Pt,5 Ru/ Pt,16 Pd/Pt,28 and Ru/Fe29 bimetallic NPs are also easily accessible via simple decomposition of their organometallic complexes in ionic liquids. Consequently, ILs may provide adequate templates for the generation of core−shell Fe−Pt NPs. Recently, the preparation and magnetic properties of carbonsupported Fe@Pt NPs have been reported with different concentrations of the Pt skin, but this approach is limited to carbon supports.30 We report herein that the simple reaction of Fe(CO)5 with [Pt2(dba)3] (dba = dibenzylideneacetone) in imidazolium ILs provides Fe-oxides@Pt NPs, in which the metal composition can be tuned using hydrophobic or hydrophilic anions. We characterized these NPs using powder X-ray diffraction (XRD), Rutherford backscattering spectroscopy, scanning transmission electron microscopy (STEM), X-ray photoemission spectroscopy (XPS), and small-angle X-ray scattering (SAXS) analyses. Moreover, we also report magnetization measurements as a function of the temperature and field of these naked core−shell nanoparticles. Finally, the catalytic properties of these Feoxides@Pt NPs have been probed in the selective production of hydrocarbons (C1−C6) by FTS.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of Fe-Oxides@Pt NPs. Bimetallic core−shell Fe-oxides@Pt nanoparticles were prepared by a one-step reaction of [Fe(CO)5] and [Pt2(dba)3] complexes (equimolar amounts) in BMIm.NTf2, BMIm.PF6, and BMIm.BF4 ILs (Scheme 1). The Pt/Fe atomic composition

2. EXPERIMENTAL SECTION All of the ILs (BMIm.NTf2, BMIm.PF6, and BMIm.BF4) were prepared by well-established methods31 and dried under vacuum at 70 °C for 48 h prior to use. [Fe(CO)5] was purchased from Sigma and [tris(dibenzylideneacetone) bisplatinum(0)] [Pt2(dba)3] complexes were prepared by wellestablished methods.32 H2 (>99.99%) gas was purchased from White-Martin Ltd., Brazil. X-ray powder diffraction (XRD) experiments were conducted in a D/max-3B diffractometer with Cu Kα radiation. The scans were recorded in the 2θ range of 0−6° with a scan rate of 0.5°/min (low-angle diffraction) and in the 2θ range of 20−90° with a scan rate of 10°/min (wide-angle diffraction). STEM-HAADF analysis was performed using XFEG Cs-corrected FEI Titan 80/300 microscopy at INMETRO, operated at 300 kV. High Z-contrast images were acquired through STEM using a high-angle annular dark field detector (HAADF) and a semiconvergence angle of 27.4 mrad. The typical resolution was greater than 0.01 nm. The images were processed with high-frequency FETfilters to reduce noise. Energy dispersive X-ray spectroscopy (EDS) analysis was performed in STEM mode, allowing nanometer-scale spatial resolution. For STEM analysis, 2−3 mg of isolated bimetallic NPs were dispersed in isopropanol and deposited on a copper grid (300 mesh) coated with carbon

Scheme 1. Synthesis of Core−Shell FexOy@Pt NPs in ILs with Different Anionsa

a

The Pt shell increases with the water stability of the anion of IL.

of 4:1 (Pt4Fe1), 3:2 (Pt3Fe2), and 1:1 (Pt1Fe1) of the prepared NPs in BMIm.PF 6 , BMIm.BF 4 , and BMIm.NTf 2 ILs, respectively, was determined by Rutherford backscattering spectroscopy (further details are provided in the SI, Figure S1). The Pt content decreases with the hydrophobicity of the IL. BMIm.BF4 is soluble in water at room temperature, whereas BMIm.PF6 has low water solubility and BMIm.NTf2 is only 4642


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Figure 1. HAADF−TEM images (a−c) and XRD Rietveld refinement (d−f) of FexOy@Pt NPs prepared in BMIm.NTf2, BMIm.PF6, and BMIm.BF4 ILs, respectively.

marginally soluble in water. The obtained NPs were characterized by STEM-HAADF analysis, which showed that NPs prepared in the BMIm.NTf2 IL have a diameter of 1.7 ± 0.2 nm, whereas the diameters of those prepared in BMIm.PF6 and BMIm.BF4 IL are 1.8 ± 0.3 and 2.5 ± 0.4 nm, respectively (Figure 1a−c). Rietveld refinement analysis of the XRD patterns of NPs prepared in BMIm.NTf2 IL (Figure 1d) suggests that the NPs are in accordance with the FePt3 phase with an average crystalline grain size of 1.88 ± 0.05 nm, corroborating the STEM results. The phase was identified as a face-centered cubic ( fcc) structure, with broad peaks at 39.72, 46.20, 67.40, 81.18, and 85.62°, which were indexed as the (111), (200), (220), (311), and (222) peaks, respectively, of the lattice planes of FePt3 (JCPDS, No. 89−2050), with a cell parameter of 3.9296 Å. On the other hand, NPs prepared in BMIm.PF6 IL have two phases, in which the main phase (84.1%) is of a well-defined face-centered cubic (fcc) FePt3 structure (JCPDS, No. 89−2050), with the space group Pm3m and a cell parameter of 3.9421 Å, implying that the sample presents a FePt3 lattice structure with a crystalline grain size of 2.55 ± 0.04 nm (Figure 1e). The minor phase (15.5%) presents a tetragonal phase of FeF2 with the space group P 42/m n m (JCPDS, No. 45−1062) and cell parameters a = 4.7101 Å and b = 3.4468 Å. Meanwhile, NPs prepared in BMIm.BF4 IL (Figure 1f) have an average crystalline grain size of 3.18 ± 0.06 nm and also consist mainly of the FePt3 phase (87.5%; JCPDS, No. 89− 2050), with a cell parameter of 3.9216 Å and a minor phase (12.5%) of FeF2 (JCPDS, No. 45−1062). Since Fe (0) species are not observed in XPS (Figure 2b,d), the formation of FePt alloys in our NPs is excluded.33

Notably, the presence of FeF2 (JCPDS, No. 45−1062) as a byproduct may originate from the decomposition of the IL counteranions (PF6 and BF4). Indeed, both anions can easily undergo hydrolysis and generate HF that could react with iron (0) precursors and/or Fe (0) NPs to produce FeF2. This behavior of ILs containing BF4 and PF6 anions has been observed several times, for example, in the use of BMIm.BF4 IL for the preparation of Fe NPs using Fe(CO)5.34 Thermogravimetric measurements (Figure S2) of isolated NPs show weight loss proceeding in two steps. The first step occurred over a range of 100−250 °C due to adsorbed water from the air, whereas the second step in weight loss up to 450 °C is due to the decomposition of residual IL at the surface of the nanoparticles,35 typically observed for NPs prepared in these ILs.36 H2-temperature-programmed reduction (TPR) profiles showed distinct peaks at 433, 481, and 484 K for Fe1Pt1, Fe1Pt4, and Fe2Pt3 NPs (Figure S3), respectively, which probably correspond to the reduction of Pt oxides to Pt (0).37−39 The absence of peaks in the range >300 °C confirms the absence of FexOy on the surface of nanoparticles.40 These observations strongly suggest that these bimetallic NPs are probably rich in Pt at the surface. Iron-oxide phases were also not detected in XRD of NPs based on the Rietveld refinement. In order to obtain information about the chemical states that exist at the surface of the bimetallic nanoparticles, XPS measurements were performed using synchrotron radiation with a photon energy of 1840 eV using the SXS beamline at the Brazilian Synchrotron Light Laboratory (LNLS). Figure 2a,b shows a comparison between the Pt 4f and Fe 2p3/2 XPS 4643


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Fe3+ chemical components, respectively.41 The Fe2+ component varied in its energy binding position by 0.1 eV from sample to sample. However, the Fe3+ component varied by a larger amount, 0.3 eV, from sample to sample, indicating that the Fe3+ component is associated with the different nature of the IL used in each case, while the Fe2+ component is probably associated with the bonding of the ions with oxygen atoms. For all of the synthesized samples, the Fe3+/Fe2+ ratio is constant and equal to about four. It is interesting to note that the component fractions presented in Table 1 for the Pt 4f electronic region do not change when the probed depth is increased. On the other hand, an increase in the Fe2+ component relative to the Fe3+ component is observed when probing deeper regions for all of the synthesized NPs (see Supporting Information). The explanation is directly related to the atomic arrangement that exists in the bimetallic NPs. The atomic arrangements of the NPs were studied by means of XPS measurements with variable incident photon energy (3000 eV, Figures 2c,d).42 The Pt 4f/Fe 2p3/2 ratios, normalized by the incident photon flux and the differential cross section factors, were calculated43 and are shown in Table 2. A strong decrease in the Pt 4f/Fe 2p3/2 normalized ratio is Table 2. Pt 4f/Fe 2p3/2 Normalized Ratio as a Function of the Incident Photon Energy

spectra of the different synthesized nanoparticles. Table 1 shows a quantification of the components calculated from the Table 1. XPS Quantification of the Relative Percentages of the Different Components Found in Fe-Oxide@Pt NPs by XPS Measurements at hv = 1840 eV IL

NPs

Pt0 (%)

Pt2+ (%)

Pt4+ (%)

Fe2+ (%)

Fe3+ (%)

1 2 3

BMIm.NTf2 BMIm.PF6 BMIm.BF4

Fe1Pt1 Fe1Pt4 Fe2Pt3

28.7 49.5 46.5

61.2 48.0 46.7

10.1 2.5 6.8

19.6 21.0 19.7

80.4 79.0 80.3

NPs

hv = 1840 eV

hv = 3000 eV

1 2 3


0.99 4.91 1.37

0.09 0.36 0.10

clearly observed when the probed depth increases. It can be interpreted as a result of the existence of a Pt-rich shell and Ferich core regions in Fe1Pt1, Fe1Pt4, and Fe2Pt3 NPs. In fact, it is also consistent with the changes in the chemical components observed for the Pt 4f and Fe 2p regions. The Pt0, Pt2+, and Pt4+ amounts are constant at both probed depths. Since the Pt atoms are present in the shell region, the full shell thickness is probed, and there is no change in the chemical components at either photon energy. On the other hand, when tuning the photon energy in the Fe 2p3/2 XPS measurements, the outer and inner regions at the core of the nanoparticle are probed, and a change in the chemical components at Fe 2p3/2 is observed. In conclusion, the XPS measurements suggest a FexOy@Pt structure for all of the NPs prepared in ILs. A singleparticle STEM−EDS profile of FexOy@Pt NPs prepared in BMIm.BF4 IL, which was taken as an example, suggests the presence of a Pt shell with a Fe rich core (Figure S6). Extended X-ray Absorption Fine Structure (EXAFS) measurements were also performed using the XAFS1 beam line at synchrotron LNLS in order to probe the local atomic order around the Pt and Fe atoms of the NPs. Figure 3 shows (i) the EXAFS signal χ(k) and (ii) the corresponding FT at the (a) Pt L3 edge (11.6 eV) and (b) Fe K edge (7.1 eV) of all the nanoparticles studied. The gray line represents the best fit found in each case. There is a strong dumping on the EXAFS oscillations when comparing the signal from the nanoparticles and the standard (bulk Pt, not shown here), which is explained by the small size of the synthesized NPs. This can also be observed by comparing the amplitude of the first peak in the FT data. The FT of the EXAFS oscillations at the Fe K edge was adjusted by considering an Fe−O scattering path, which is

Figure 2. Pt 4f and Fe 2p XPS spectra of the FexOy@Pt NPs with hν = 1840 eV (a,b) and hν = 3000 eV (c,d). Black points represent the experimental data, the dashed black lines represent the Shirley background used, the red solid lines represent the performed fit, and the blue, green, and magenta solid lines represent the Pt0, Pt2+, and Pt4+ (a,b) and Fe2+ and Fe3+ (c,d) components, respectively.

entry

entry

fitting procedure (see SI). The Pt 4f region presents three distinct components associated with Pt0 (at 71.5 eV), Pt2+ (at 73.0 eV), and Pt4+ (at 75.3 eV).16 These values varied by up to 0.2 eV from sample to sample. In all cases, the Pt4+ fraction was small, whereas the amounts of Pt0 and Pt2+ were essentially equal for FexOy@Pt NPs prepared in BMIm.BF4 and BMIm.PF6 ILs (Table 1, entries 2 and 3). The same cannot be said in the case of Fe1Pt1 NPs, where the amount of Pt2+ was around three times higher than the amount of Pt0. The Fe 2p3/2 XPS region shows the presence of two distinct components at 710.8 and 712.3 eV, associated with Fe2+ and 4644


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Figure 4. M(H) isotherms (a) at 2 K and (b) at 180 K. The solid lines correspond to the fit with eq 1 (the sum of a Langevin function and a linear function); (c) shows curves of (b) after subtraction of the linear contribution obtained from the fit.

tions: one being superparamagnetic and the other paramagnetic, the latter being linear at high temperature. We observe that at temperatures as low as 2 K no curves saturate up to 60 kOe. The Langevin function is associated with the FexOy core, whereas the paramagnetic signal may come from frustrated or magnetically isolated Fe atoms, either from the core−shell interface or the disordered core,44,45 and from the Pt shell.46 M(H) curves at 180 K still display a narrow irreversibility that is minimized, and thus excluded from our analysis, by averaging the increasing and the decreasing field branches. This allowed us to assume that at 180 K all macromoments are in the superparamagnetic regime and to fit the high-temperature M(H) curves with the expression (eq 1)44,45,47,48

Figure 3. Comparison of (i) the EXAFS signals χ(k) and (ii) the corresponding Fourier transforms at (a) the Pt L3 edge and (b) the Fe K edge of the synthesized sample. The black points and the gray lines represent the experimental data and the best fit found, respectively.

consistent with the oxidation states found for the Fe atoms by XPS measurements at both probed depths. The FT of the EXAFS oscillations at the Pt L3 edge was adjusted by considering Pt−Pt and Pt−Fe scattering paths. It is important to stress here the fundamental role played by the Pt−Fe scattering path in the FT adjustment, since the fit quality decreased significantly without this scattering path. This result, at first sight, contradicts the results found for the EXAFS measurements at the Fe K edge, since no Fe−Pt scattering path was included in that case. These results can be explained by considering the shell region as being of small thickness and with a relatively large core radius. If so, considerable fraction of Pt atoms is at the interface between the core and shell regions, which have Fe atoms as neighbors. On the other hand, only a small fraction of the Fe atoms is present at this interface (large core) and the Fe−Pt scattering path contribution thus becomes negligible. Table S1 shows the structural parameters obtained from the FT fitting. 3.2. Magnetic Properties of Fe-Oxides@Pt NPs. Magnetic measurements of NPs give additional information about particle size, magnetic (dis)order, interparticle and intraparticle interactions, and the surface/interface contribution to the magnetic anisotropy. Pt and FexOy have distinct magnetic properties: bulk Pt is a Pauli paramagnet, and FexOy is a ferrimagnet with high sensitivity to the parameter x and structural disorder. Based on the XPS results, we assume that metallic phases like the FePt alloy and Fe are not present. Figure 4a,b shows the magnetization versus magnetic field [M(H)] curves at temperatures of 2 and 180 K, respectively. The magnetization seems to be composed by two contribu-

M(H , T ) =

NMsπρ 6

∫0

∞

⎛ πM D3H ⎞ ⎟f (D)dD + χp H D3 3⎜ s ⎝ 6kBT ⎠ (1)

where N is the number of particles, ρ is an average FeOx density, L(x) is the Langevin function, f(D) is the log-normal distribution of size, kB is the Boltzmann constant, Ms is the saturation magnetization of the superparamagnetic phase (emu/cc), and χp is the paramagnetic contribution. By calculating the amount of iron oxide per particle from the chemical composition determined by RBS and XPS analyses and knowing the mean particle size,49 we deduced the mean core diameter ⟨Dεcore⟩ and fixed it for each sample in the analysis. The fit parameters, i.e., MS, the number of particles N, the log-normal distribution width WD , and the linear susceptibility χpara, are presented in Table 3. We obtained Table 3. Fit Parameters of Magnetization Curves at 180 K Considering Equation 1

4645

entry

NPs

⟨Dεcore⟩ [nm]

MS [emu/ g]

WD

χpara [10−5 emu/ g]

1 2 3


1.4 1.2 2.0

11 39 32

0.7 ± 0.01 0.6 ± 0.03 0.5 ± 0.02

1.22 3.28 1.38


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The Journal of Physical Chemistry C saturation magnetization values of 55 emu/cm3 (11 emu/g), 200 emu/cm3 (39 emu/g), and 165 emu/cm3 (32 emu/g) for Fe1Pt1, Fe1Pt4, and Fe2Pt3 NPs prepared in BMIm.NTf2, BMIm.PF6, and BMIm.BF4 ILs, respectively. Bulk Fe3O4 has a saturation magnetization of about 93 emu/ g. We infer that the diminished magnetic moment per NP reveals a strong structural disorder in the iron-oxide NP core.47 We observed that the saturation magnetization tends to be lower for samples with a higher proportion of iron oxide. A thin, magnetically dead layer at the interface between the Pt shell and the iron-oxide core is a possibility but is difficult to confirm. Regarding the distribution widths WD, they are larger than the log-normal distribution widths obtained from TEM, as they are widened by structural disorder. At 2 K, the coercive fields Hc have the values of 590, 380, and 170 Oe for Fe1Pt1, Fe1Pt4, and Fe2Pt3 NPs, respectively. It is interesting to note that Hc is larger for smaller particles, suggesting that surface anisotropy plays a major role, as it is greater for particles with a smaller magnetic core. This assumption is based on the already observed anisotropy enhancement of Fe atoms at the Fe/Pt interface, due to high Pt spin−orbit coupling and to symmetry breaking at the interface.50,51 Note that even pure Pt NPs may exhibit ferromagnetism.52,53 However, spontaneous Pt magnetization, if it exists in these NPs, may be disregarded, as its amplitude is orders of magnitude lower than that of spontaneous FexOy magnetization. The zero-field cooled (ZFC) and field cooled (FC) magnetization curves with a magnetic field of 100 Oe are shown in Figure 5. In a first approximation, the maxima of the

anisotropy and may also be related to interparticle interactions. The monotonic increases in the FC curves, without saturation, as the temperature decreases down to 2.2 K (considerably lower than any ZFC maximum temperature), are not expected for a pure SP system. Additionally, we observe an increase in the ZFC curve at temperatures below its maximum for Fe1Pt1 and Fe2Pt3 NPs. Fe1Pt4 NPs show a rather different behavior, as a second maximum seems to be present in the ZFC curve, at a temperature slightly lower than its main maximum. We infer that this second peak may also exist for Fe1Pt1 or Fe2Pt3 NPs at temperatures lower than 2.2 K. However, the rise of the ZFC curve at temperatures below its maximum for Fe1Pt1 and Fe1Pt4 NPs could also be due to paramagnetic moments distributed in the disordered core. In order to gain more insight into the paramagnetic Pt shell and the SP regime of the FexOy core, we plotted χT versus temperature (inset of Figure 5). Above a certain temperature, which is different for each sample, the χT curves increase linearly without intercepting the origin and thus may be fit by the simple model T χT = C + TχPauli

where, χTPauli is the temperature independent Pauli susceptibility, and C is the Curie constant (see Table 4). As we attribute the Table 4. Estimated TB and Fit Parameters of χT at High Temperatures with a Simple Model χT = C + TχTPaulia entry

NPs

TB [K]

1 2 3


25.9 14.5 13

χTPauli [10−7 emu]

Pt shell χTPauli [10−5 emu/g]

C [10−3 emu·K/g]

2.5 ± 0.1 1.1 ± 0.1 3.9 ± 0.1

2.8 1.1 4.8

7.6 ± 0.2 5.2 ± 0.1 4.7 ± 0.1

a

We used the mass percentage of Pt per inorganic material in order to determine χTpara in emu/g.

Pauli paramagnetism to the Pt shell, we normalized it by the estimated shell mass (see Supporting Information) and obtained surprisingly high values, 2 orders of magnitude larger than those for bulk Pt.54 Recall that Pt is a transition metal near the onset of ferromagnetism that exhibits strong exchangeenhanced Pauli paramagnetism. The observed high value of Pauli susceptibility may be interpreted as a narrowing of valence states due 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 the enhanced Pauli susceptibility.55 The Curie constants C obtained from χT shown in Table 4 can be compared to the ones deduced from the saturation magnetization in Figure 4c, which are 5.3 × 10−8, 1.1 × 10−8, and 1.4 × 10−6 emu·K for Fe1Pt1, Fe2Pt3, and Fe1Pt4 NPs, respectively. We observe that M(H) leads to a Curie constant (CM(H)) that is systematically smaller than the one obtained with χT(T), the relative difference being greater for the smaller particles. This may be explained by the fact that on the one hand CM(H) is proportional to the squared macromoment (superparamagnetic contribution), and on the other hand, C is proportional to all paramagnetic (Curie) and superparamagnetic moments present in the samples. Indeed, it is to be expected that NPs with a low core saturation magnetization are more disordered, and therefore will have a larger amount of frustrated or magnetically isolated Fe atoms; these atoms contribute to the Curie

Figure 5. ZFC (open symbols) and FC (full symbols) magnetization versus temperature measured at 100 Oe for Fe1Pt1, Fe1Pt4, and Fe2Pt3 NPs. The inset shows χT versus temperature obtained from FC with H = 100 Oe. The solid lines correspond to the linear fit χT(T) above 200 K.

ZFC curves may be considered as the average blocking temperatures TB, which are 26 K for Fe1Pt1, 14.5 K for Fe1Pt4, and 13 K for Fe2Pt3 NPs. As expected, the highest (lowest) values for the coercive field correspond to the highest (lowest) blocking temperatures. The ZFC and FC curves show several indications that FexOy@Pt NPs do not behave as a noninteracting superparamagnetic (SP) system. In a SP system, the temperature derivative of the difference between the FC and ZFC curves versus temperature is directly correlated to the blocking-temperature distribution and is proportional to the NP size distribution when the anisotropy constant is independent of size. This correspondence between size and blocking-temperature distributions is not observed in our case (not shown), and may be attributed to surface or interface 4646


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The Journal of Physical Chemistry C constants C obtained from χT but are not taken into account in the Curie constants CM(H) obtained from the Langevin fit. 3.3. Catalytic Properties. We investigated the catalytic performance of the as-prepared isolated FexOy@Pt NPs in the gas-phase FTS, using syngas (H2/CO= 2:1) in a DRIFT cell at 230 and 300 °C for 16 h (Table 5). In the absence of syngas, no

absorbed H atoms. CO molecules inserted to a Pt−H bonds are the first initiation step to form a surface aldehyde species that can be hydrogenated by nearby hydrogen atoms to CH3. Subsequently, CO can be inserted into metal−carbon resulting in an enol, which can be hydrogenated, forming H2O and a CH2 group. The formation of these CH2 groups is proposed as the chain growth step. It may be assumed that these CH2 (CH and CH3) species are more mobile and are able to move over the catalyst surface.60 It is important to note that the catalytic activity and selectivity toward higher HCs (C2−C6) displays a strong correlation with the size of our catalysts. As the particle size decreases, conversion toward HCs increases (Figure 6), which

Table 5. Catalytic Testsa product selectivity (% C)d entry

NPsb

size (nm)c T (°C)

1 2

Ptd Fe1Pt1

1.88

3

Fe1Pt4

2.55

4

Fe2Pt3

3.18

5

Fe1Pt1e

230 230 300 230 300 230 300 230

conv. (%)

CH4

C2− C4

C5 − C6

3 29 34 19 23 15 18 -

2 1 5 6 30 17 -

92 90 95 80 59 69 -

7 5 5 14 11 14 -

a

Tests were performed with 15 bar of H2/CO (2/1) for 16 h. The product mixture that was analyzed consisted of C1 to C6 HCs. bCat. (10 mg). cSizes of NPs were determined from PXRD dSelectivities were calculated on the basis of total hydrocarbons produced. eWithout syngas.

hydrocarbons (HCs) were detected under standard conditions. No significant activity (Table 5, entry 1) was observed with pure Pt NPs because Pt-based catalysts are not active catalysts for CO hydrogenation but are good catalysts for the water−gas shift (WGS) reaction,56 whereas Fe-based catalysts are active catalysts for the production of HCs by FTS. Interestingly, on incorporation of iron with platinum (Fe/Pt = 1:1, 1:4, 2:3), the conversion of syngas to low-molecular-weight HCs was observed. The highest activity (conv. = 29%) was observed using the Fe1Pt1 catalyst (Table 5, entry 2) at 230 °C with the highest selectivity (98%) for low-weight HCs (C2−C6). The Fe2Pt3 catalyst displayed a higher selectivity for methane (30%) that is not desirable for gas-phase FTS (Table 5, entry 4). It is important to note that the absence of short-chained olefins may be correlated with the presence of Pt, which is an active hydrogenation catalyst.57 It is recognized that iron-based catalysts become Fischer− Tropsch active only after reducing and carburizing in the presence of H2/CO at low temperatures (90% toward lower-molecular-weight C2−C6 HCs, while directing almost no carbon to methane (Table 5, entries 2 and 3). Fe2Pt3 NPs also demonstrated good activity for C2−C4 HCs, but the methane product fraction was 30%. With an increase in temperature to 300 °C, a slight increase in conversion toward HCs was observed, reaching 34, 23, and 18% for Fe1Pt1, Fe2Pt3, and Fe1Pt4 NPs, respectively, with no significant effect on HC selectivity. It is widely accepted that on iron carbide particles, CH4 formation takes place at highly active low-coordination sites residing at corners and edges, while lower HCs (C2−C4) are produced at terrace sites.62 Note that the best catalytic FTS performance for monometallic NPs is usually observed when using NPs of around 8 nm.63

4. CONCLUSION The simple codecomposition of Fe(0) and Pt(0) organometallic precursors in ILs generates core−shell Fe-oxides@Pt NPs, for which the thickness of the Pt shell layer is related to the water stability of the anions of the ionic liquid. The thickness of the Pt shell increases in the order PF6 > BF4 > NTf2, yielding the metal compositions Pt4Fe1, Pt3Fe2, and Pt1Fe1, respectively. Small NPs are formed in hydrophobic ILs (BMIm.PF6 and BMIm.NTf2), whereas larger 2.5 nm NPs were produced in the hydrophilic IL BMIm.BF4. XPS and XAS measurements of the NPs are consistent with the model of a FexOy@Pt atomic structure. The FexOy core shows superparamagnetic behavior at high temperatures, with a saturation magnetization much lower than that of bulk magnetite. The obtained Pauli paramagnetism of the Pt shell is 2 orders of

Scheme 2. Schematic Drawing Illustrating the Reaction Active Species over FexOy@Pt NPs during FTS

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magnitude higher than that of bulk Pt and represents one of the highest values for confined Pt Pauli susceptibility reported until now. A possible explanation is the narrowing of valence states due to symmetry breaking at the surface and interface, which results in a strong increase of the density of states at the Fermi level, and hence to the enhanced Pauli susceptibility observed. Moreover, the size of these core−shell NPs is strongly correlated with the catalytic activity and selectivity toward hydrocarbon production in FTS. As the NP size increases from 1.7 to 2.5 nm, not only does the production of higher HCs decrease but also the activity toward CO hydrogenation decreases.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b12219. Rutherford backscattering spectrometry (RBS) measurements; X-ray photoelectron spectroscopy (XPS) measurements; X-ray absorption spectroscopy (XAS) measurements; data analysis for XPS and EXAFS; magnetic measurements; RBS analysis of the NPs; thermogravimetric analysis of the NPs; H2-temperature-programmed reduction (TPR); histograms of FexOy@Pt NPs; longscan XPS measurements of core shell Fe−Pt NPs; STEM−EDS profile and EDS of Fe1Pt4 NPs; EXAFS results of Fe−Pt NPs; magnetic analysis of the NPs: discussion regarding the core and shell dimensions (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.D.) *E-mail: [email protected] (M.A.N.) ORCID

Fabiano Bernardi: 0000-0001-6817-6860 Jairton Dupont: 0000-0003-3237-0770 Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS XPS and XAS measurements were performed at LNLS, and the RBS measurements were performed in the Ion Implantation Laboratory (IF-UFRGS). The authors are thankful to CAPES, FAPERGS, INCT-Catal., CNPq, and FAPERJ for financial support.

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REFERENCES

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

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