Zinc-Containing Magnetic Oxides Stabilized by a ... - ACS Publications

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Zinc-containing magnetic oxides stabilized by a polymer: One phase or two? Nicholas Baird, Yaroslav Losovyj, Ekaterina Yu. Yuzik-Klimova, Nina V. Kuchkina, Zinaida B. Shifrina, Maren Pink, Barry D. Stein, David Gene Morgan, Tianhao Wang, Mikhail Rubin, Alexander Sidorov, Esther M. Sulman, and Lyudmila M. Bronstein ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10302 • Publication Date (Web): 16 Dec 2015 Downloaded from http://pubs.acs.org on December 22, 2015

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ACS Applied Materials & Interfaces

Zinc-containing magnetic oxides stabilized by a polymer: One phase or two? Nicholas Baird1, Yaroslav Losovyj1, Ekaterina Yu. Yuzik-Klimova2, Nina V. Kuchkina1, Zinaida B. Shifrina2, Maren Pink1, Barry D. Stein3, David Gene Morgan1, Tianhao Wang4, Mikhail A. Rubin5, Alexander I. Sidorov5, Esther M. Sulman5, Lyudmila M. Bronstein1, 2, 6* 1

Indiana University, Department of Chemistry, Bloomington, IN 47405, USA

2

A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,

28 Vavilov St., Moscow, 119991 Russia 3

Indiana University, Department of Biology, Bloomington, IN 47405, USA

4

Indiana University, Department of Physics, Bloomington, IN 47405

5

Tver State Technical University, Department of Biotechnology and Chemistry, 22 A. Nikitina

St, 170026, Tver, Russia 6

King Abdulaziz University, Faculty of Science, Department of Physics, Jeddah, Saudi Arabia

Key words: zinc oxide, magnetite, nanoparticles, polymer, X-ray photoelectron spectroscopy

ACS Paragon Plus Environment

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ABSTRACT. Here we developed a new family of Zn-containing magnetic oxides of different structures by thermal decomposition of Zn(acac)2 in the reaction solution of preformed magnetite nanoparticles (NPs) stabilized by polyphenylquinoxaline. Upon an increase of the Zn(acac)2 loading from 0.15 to 0.40 mmol (vs 1 mmol of Fe(acac)3), the Zn content increases, while the Zn-containing magnetic oxide NPs preserve a spinel structure of magnetite and an initial, predominantly multicore NP morphology. X-ray photoelectron spectroscopy (XPS) of these samples revealed that the surface of iron oxide NPs is enriched with Zn, although Zn species were also found deeply under the iron oxide NP surface. For all the samples, XPS also demonstrates the atom ratio of Fe3+:Fe2+=2:1, perfectly matching Fe3O4, but not ZnFe2O4, where Fe2+ ions are replaced with Zn2+. The combination of XPS with other physicochemical methods allowed us to propose that ZnO forms an ultrathin amorphous layer on the surface of iron oxide NPs and also diffuses inside the magnetite crystals. At higher Zn(acac)2 loading, cubic ZnO nanocrystals coexist with magnetite NPs, indicating a homogenous nucleation of the former. The catalytic testing in syngas conversion to methanol demonstrated outstanding catalytic properties of Zn-containing magnetic oxides, whose activities are dependent on the Zn loading. Repeat experiments carried out with the best catalyst after magnetic separation showed remarkable catalyst stability even after five consecutive catalytic runs.

1. Introduction Magnetically recoverable catalysts attracted considerable attention due to easy catalyst recovery from reaction mixtures and their repeated use.1-6 This allows more environmentally friendly processes, conservation of energy, and cheaper target products.7-13 ZnO and mixed zinc containing oxides are promising catalysts in transformation of syngas to methanol.14-16 The latter is a source of hydrocarbons of different lengths including gasoline. At


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the same time, syngas can be prepared by pyrolysis of biooil, thus allowing a path from biooil to biofuels. When ZnO is combined with iron oxide, magnetic separation becomes a viable option. In general, there are several approaches to synthesize hybrid ZnO-iron oxide nanoparticles (NPs). Iron oxide NPs can be placed on the surface of ZnO nanorods or spheres resulting in significant changes of magnetic properties compared to pure iron oxide NPs.17-19 Alternatively, ZnO can be placed on the surface of iron oxide.20-21 The third possibility is a mixture of ZnO and iron oxide NPs.22-23 Recently, a synthesis of the ZnO photocatalyst supported on magnetite NPs using surfactants has been reported.24 In the present paper we focus on the formation of Zn-containing magnetic oxide NPs and their structural differences depending on the Zn precursor loading. As a stabilizer we used a thermally stable polyphenylquinoxaline (PPQ), the structure of which is shown in Scheme 1.25-26 When thermally stable polymers are used for catalytic NP formation, the range of possible catalytic reactions is greatly enhanced. The PPQs can withstand temperature of up to 400-500°C9 and high pressures, allowing for the development of exceptionally stable catalysts. The PPQs also possess nitrogen-containing heterocycles in their repeating units that allow coordination with many metals and stabilization of the catalytic species formed.

Scheme 1. The structure of the PPQ repeating unit. We determined that differences in the structure of Zn-containing magnetic oxides are due to different nucleation mechanism of Zn species. In the case of solely heterogeneous nucleation at


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the low Zn precursor loading, only a spinel phase is formed. When a homogeneous nucleation also takes place, an additional phase of crystalline ZnO is observed. As a proof of concept, we demonstrate that Zn-containing magnetic oxides display outstanding catalytic properties in the syngas transformation to methanol and high stability of catalytic performance upon magnetic recovery.

2.

Experimental part

2.1. Materials Iron(III) acetylacetonate (99+%) and methanol were purchased from Acros Organics and used as received. Zinc(II) acetylacetonate hydrate (99+%), isopropanol (99.7+%), and benzyl ether (98%) were purchased from Sigma-Aldrich and used as received. Acetone (99.5%) and chloroform (99.8%) were purchased from MACRON Chemicals and used without purification. Ethanol (95%) was purchased from Pharmco-AAPER and used as received.

2.2. PPQ synthesis The monomers for the polymer synthesis were obtained according to procedures described in ref.27 for 3,3',4,4'-tetraaminodiphenyl ether and in ref.

28

for 4,4'-bis(phenylglyoxalyl)diphenyl

ether. The synthesis of the PPQ was carried out according to the procedure reported elsewhere.26 A typical synthesis was carried out in a two-neck round bottom flask equipped with a magnetic stir bar and an argon inlet. For this, 5.0 g (11.5 mmol) of 4,4’-bis(phenylglyoxalyl)diphenyl ether were added to the solution of 2.65 g (11.5 mmol) of 3,3’,4,4’- tetraaminodiphenyl ether in 60 mL of the chloroform/methanol mixture (10/1). The reaction was stirred for 24 h at room temperature, and then the reaction solution was precipitated by ethanol. The polymer was separated by filtration, collected and washed with ethanol in a Soxhlet apparatus. Then the polymer was dried in vacuum at 100°C. The yield of the polymer was 5.65 g (83%). The


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intrinsic viscosity ([η]) was 0.2 dL/g at 25°C in N-methylpyrrolidone and MW was 21,500 by Gel Permeation Chromatography.

2.3. Iron oxide NP synthesis In a typical procedure, the three-neck round-bottom flask (with elongated necks) equipped with a magnetic stir bar, a reflux condenser, and two septa, one of which contained an inserted temperature probe protected with a glass shield and the other had a long needle, was loaded with 0.353 g (1 mmol) of Fe(acac)3, 0.148 g (0.25 mmol) of the PPQ and 7 mL of benzyl ether. The flask was placed in a Glas-Col heating mantle attached to a digital temperature controller which in turn was placed on a magnetic stirrer. The flask was degassed by argon bubbling for 15 min under stirring. Then the temperature was raised to 60°C at 10°/min and kept under stirring at this temperature for 30 min to allow solubilization Then the temperature was increased with a heating rate 10°/min until stabilizing around 283-285°C and heated for 1 h. The flask was then removed from the heating mantle and allowed to cool to room temperature. The reaction solution was precipitated by ethanol, washed several times with ethanol and acetone, until the supernatant was colorless, and then dissolved in chloroform.

2.4. Synthesis of Zn-containing magnetic oxides Syntheses of Zn-containing magnetic oxide NPs in the presence of the PPQ were carried out according to the following protocol. In a typical procedure for the Zn-1 synthesis (Table 1), the three-neck round-bottom flask described above was loaded with 0.353 g (1 mmol) of Fe(acac)3, 0.148 g (0.25 mmol) of the PPQ, and 7 mL of benzyl ether. The flask was degassed by argon bubbling for 15 min under stirring. The temperature was raised to 60°C at 10°/min and kept under stirring at this temperature for 30 min to allow solubilization. Then the temperature was increased with a heating rate 10°/min until stabilizing around 283-285°C (boiling) and was kept heating at this temperature. Meanwhile, a small vial was charged with 0.0396g (0.15mmol) of


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Zn(acac)2 and 1 mL of benzyl ether and then sonicated for 10-15 min. This solution was injected via a syringe 1 h after the beginning of the Fe(acac)3 reaction solution boiling. After the injection, the temperature was allowed to stabilize around 283-285°C. Following stabilization, the flask was allowed to reflux for 1 hr. The flask was then removed from the heating mantle and allowed to cool to room temperature. The reaction solution was precipitated by ethanol, washed several times with ethanol and acetone, until the supernatant was colorless, and then dissolved in chloroform. Syntheses of other samples listed in Table 1 were carried out in the same manner adding different amounts of Zn(acac)2. In the case of Zn-5, three portions of the Zn(acac)2 solution each containing 0.0659 g (0.25 mmol) of Zn(acac)2 in 1 mL of benzyl ether were prepared and injected consecutively. The first portion was injected similar to the procedure described above, while two other portions were injected with a 30 min time interval at the reaction temperature. The compositions of Zn-containing magnetic oxides are presented in Table 1. Table 1. Compositions of Zn-containing magnetic oxides stabilized by PPQ. Sample notation

Zn(acac)2 loading, mmola)

Zn-1

a)

Content by XPS, at.%

Fe:Zn atomic ratio

Fe

Zn

0.15

58.8

41.2

1.43:1

Zn-2

0.25

45.8

54.2

0.82:1

Zn-3

0.40

38.4

61.6

0.63:1

Zn-4

1

25.2

74.8

0.34:1

Zn-5

0.75 (0.25×3)

11.3

88.7

0.13:1

In all syntheses 1 mmol of Fe(acac)3 was used.


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2.5. Characterization Electron-transparent NP specimens for TEM were prepared by placing a drop of a diluted solution onto a carbon-coated Cu grid. Images were acquired at an accelerating voltage of 80 kV on a JEOL JEM1010 transmission electron microscope. Images were analyzed with the National Institute of Health developed image-processing package ImageJ to estimate NP diameters. Between 150 and 300 NPs were used for this analysis. High resolution TEM (HRTEM) images and scanning TEM (STEM) energy dispersive X-ray spectra (EDS) were acquired at an accelerating voltage of 300 kV on a JEOL 3200FS transmission electron microscope equipped with an Oxford Instruments INCA EDS system. The same TEM grids were used for all analyses. X-ray powder diffraction (XRD) patterns were collected on an Empyrean from PANalytical. X-rays were generated from a copper target with a scattering wavelength of 1.54 Å. The stepsize of the experiment was 0.02. X-ray photoelectron spectroscopy (XPS) experiments were performed using PHI Versa Probe II instrument equipped with a monochromatic Al K(alpha) source. The X-ray power of 25 W at 15 kV was used for a 200 micron beam size. The instrument work function was calibrated to give a binding energy (BE) of 84.0 eV for the Au 4f7/2 line for metallic gold and the spectrometer dispersion was adjusted to give BEs of 284.8 eV, 932.7 eV and of 368.3 eV for the C 1s line of adventitious (aliphatic) carbon present on the non-sputtered samples, Cu 2p3/2 and Ag 3d5/2 photoemission lines, respectively. The PHI dual charge compensation system was used on all samples. The ultimate Versa Probe II instrumental resolution was determined to be better than 0.125 eV using the Fermi edge of the valence band for metallic silver. XPS spectra with the energy step of 0.1 eV were recorded using SmartSoft–XPS v2.0 and processed with PHI MultiPack v9.0 software at pass energies of 46.95 eV, and 23.5 eV for Fe 2p and Zn 2p, 23.5 eV for C 1s, 3p of Fe and Zn. Peaks were fitted using GL line shapes and/or asymmetric shapes,


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i.e., a combination of Gaussians and Lorentzians with 10-50% of Lorentzian contents. Shirley background was used for curve-fitting. NP samples for XPS were prepared by drop casting from solution onto the native surface of a Si(111) wafer. Magnetic measurements were performed on a Quantum Design MPMS XL magnetometer using the systems DC measurement capabilities. Milligram quantities of the sample were placed in a standard gelatin capsule. For zero-field cooling (ZFC) curves, the sample was cooled in a null field (below 0.4 Oe) to 4.5 K. A 50 Oe field was then applied, and measurements were taken at regular temperature increments up to 300 K. The sample was then cooled in the 50 Oe field, and the measurements were repeated at the same temperature increments for the field cooling (FC) curves. These ZFC/FC curves were used to establish the blocking temperature.

2.6. Catalytic studies Catalytic experiments were carried out in a batch autoclave reactor with a stirrer. The stirring rate was 1500 rpm to avoid diffusion limitations. To provide a good interphase contact, the reactor was equipped with an internal flow mixer. In a typical procedure, the reactor was charged with 150 mL of isopropanol and 50-60 mg of the catalyst. The reactor was purged three times with syngas of the molar composition CO:H2 = 1:4 at room temperature. Then the temperature was raised to 150 °C within 15 min and the pressure was set at 1.6 MPa. Reaction was carried out for 6 hours. After that the reactor was allowed to cool to room temperature and the products were analyzed by gas chromatography (GC). In the case of the catalyst repeated use, the catalyst was separated from a liquid with a rare earth magnet, washed with isopropanol three times and dried at 100 °C for 3 hours.


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3.

Results and Discussion

3.1. Iron oxide NP formation For magnetically recoverable catalysts, the catalytic species should be placed on top of a magnetic support (magnetic NPs) to ensure access for reacting molecules. In the case of Zncontaining magnetic oxides, this can be accomplished using heterogeneous nucleation of Zn species on iron oxide NPs. To better understand the changes in the morphology of iron oxide NPs after deposition of Zn species, we first carried out a synthesis of iron oxide NPs. These NPs were formed by thermal decomposition of Fe(acac)3 in the presence of the PPQ as capping molecules. Figure 1a shows TEM and HRTEM images of these NPs. They mainly consist of multicore NPs with a mean size of about 25 nm and a smaller fraction of quasi-spherical singlecore NPs with diameters in the range of 8-13 nm.

Figure 1. TEM image (a) and XRD pattern (b) of iron oxide NPs stabilized by the PPQ. Inset in (a) shows a HRTEM image and Fast Fourier Transforms (FFT) of multicore NPs indicated by white rectangles. Red arrows in (a) show multicore NPs, while the blue arrows show single core particles. The crystallite size from XRD (using the Scherrer equation) is 11.9±0.3 nm.


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In our preceding work,29-30 we reported on multicore iron oxide NPs stabilized by polyphenylenepyridyl dendrons and dendrimers, whose formation due to oriented attachment of single cores resulted in mesocrystals, where individual nanocrystals were aligned in a common crystallographic fashion without coalescence.31 In the case of the PPQ, FFTs of single multicore NPs in the HRTEM image (inset in Figure 1a) clearly show that these particles are polycrystalline, i.e., the oriented attachment does not take place. The positions and intensity of the Bragg reflections in the XRD pattern of this sample (Figure 1b) are typical for those of magnetite,32 however, considering similarity of the XRD patterns of magnetite (Fe3O4) and maghemite (γ-Fe2O3) NPs due to line broadening, this is a tentative assignment and based on the reaction conditions (argon atmosphere) when the oxidation should be prevented. In the following sections the XPS data confirm that magnetite NPs are formed. Magnetic properties of these NPs are presented in Figure S1 (the Supporting Information, SI). The hysteresis curves show no remanence or coercivity at 300 K, demonstrating superparamagnetic behavior, unlike ferrimagnetic behavior of mesocrystals.29-30 Figure S1b (SI) shows zero-field cooling (ZFC) and field cooling (FC) susceptibility curves which allow one to obtain the blocking temperature, TB, i.e., the point where the two curves merge.33 The blocking temperature for this sample is about 200 K which is typical for superparamagnetic iron oxide NPs of a comparable size.34 It is noteworthy that for mesocrystals of the same size studied in our preceding work, the blocking temperature was above 300 K.29-30

3.2. Zn-containing magnetic oxides: TEM, STEM EDS, and XRD To form Zn-containing magnetic oxides in one-pot reaction, first iron oxide NPs were formed and then a benzyl ether solution of Zn(acac)2 was injected into the reaction solution of iron oxide NPs at the reaction temperature. Figure S2 (SI) displays TEM images of Zn-containing magnetic oxides with different fractions of Zn species. The figure shows that at low Zn(acac)2 loadings


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(between 0.15 and 0.40 mmol), the morphologies of the Zn-containing iron oxides are similar to that of Fe3O4 stabilized by the PPQ (Fig. 1). At the Zn(acac)2 loading of 1 mmol (Zn:Fe=1:1 atom), cubic NPs appear along with multicore NPs and single core spherical NPs discussed above. To assess the composition of NPs at low and high Zn(acac)2 loadings, we carried out STEM EDS mapping.

Figure 2. STEM dark-field image (a) and EDS maps of Fe (b) and Zn (c) and their superposition (d) for the Zn-containing magnetic oxide prepared with 0.15 mmol of Zn(acac)2. Figure 2 demonstrates that Fe and Zn maps have the same shape as the NPs in the dark-field image, revealing that the Zn species are located on or in iron oxide NPs (oxygen map is not shown but is similar to that of Fe). The superposition of Zn and Fe maps (Figure 2d) shows a


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good fit, except for the fact that the NPs in the Zn map appear larger than those in the Fe map. This can be due to two factors. First, a fuzzier Zn image can be due to the lower Zn content and the lower signal-to-noise ratio than those for Fe. The other possibility is that the ZnO phase is formed on top of the iron oxide phase leading to a core-shell NP.

Figure 3. HRTEM images of the Zn-containing magnetic oxides prepared with 0.15 mmol (a) and 0.25 mmol (b) of Zn(acac)2. Dark insets show FFTs of the corresponding multicore NPs, identified by white rectangles. To answer this question, we carried out HRTEM of the samples prepared with different amounts of Zn(acac)2. The HRTEM images of the samples prepared with 0.15 mmol and 0.25 mmol of Zn(acac)2 (Figure 3) show no crystalline ZnO shell on the iron oxide NPs, however, a thin amorphous shell might be invisible. We reported a similar case when a gold layer was not identified by HRTEM on the surface of iron oxide NPs, while it was well characterized by XPS and the NPs were stabilized by thiols.35 For the sample prepared with 0.15 mmol of Zn(acac)2, the FFT image of a single multicore particle shows that it is polycrystalline. On the other hand, for the sample prepared with 0.25 mmol of Zn(acac)2, some multicore particles are


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polycrystalline (for example, upper right with the upper FFT image), while some multicore NPs appear as single crystals (or mesocrystals). The example is the lower left particle and the lower FFT image. This indicates that incorporation of the ZnO species facilitates recrystallization or reattachment of single cores. The other way to prove or disprove the presence of a separate crystalline ZnO phase along with the spinel structure of magnetite is XRD. Figure 4 displays the XRD patterns of the samples prepared with 0.15, 0.40, and 1.0 mmol of Zn(acac)2. The samples prepared with 0.15 mmol and 0.40 mmol of the Zn species show the XRD patterns (Fig. 4a and b) whose reflections are largely identical to those of the Fe3O4 NPs (Fig. 1b), i.e., the spinel structure is fully preserved. It is worth noting however that characteristic diffraction peaks of magnetite (RRUFF Database No. R080025) or zinc ferrite (RRUFF Database No. R070137) are the same as is discussed in ref.36 Alternatively, when the Zn loading is equal to that of Fe, the XRD pattern is very different (Fig. 4c). It contains both reflections from spinel and additional signals, indicated by red font. For comparison, we synthesized ZnO NPs by thermal decomposition of Zn(acac)2 in the presence of the PPQ in the conditions similar to those developed for the Zn-containing magnetic oxides but with no added Fe(acac)3. The TEM image and the XRD pattern of this sample are shown in Figure S3 (SI). The XRD pattern contains mainly reflections from ZnO and weak reflections which can be ascribed to Zn(0).37-38 Thus, the reflections indicated in red in Figure 4c can be assigned to the ZnO hexagonal phase (JCPDS File no. 89-1397). STEM EDS of this sample presented in Figure 5 shows cubic NPs consisting solely of ZnO, multi-core iron oxide NPs with hardly any Zn and multicore/single core NPs containing both Zn and Fe species. These data are in a good agreement with XRD.


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(440) (511)

(220) (400)

2000

(422)

6000 4000

(220)

(400)

2000

(440) (511)

30

40

50

60

70

2 Theta degrees

80

90

1000 0

-2000

20

2000

(422)

0

0

3000

(440)

4000

8000

(112)

X-ray intensity

6000

(311)

4000

10000

(100)

8000

(400) (102) (422) (511)/(110)

(311)

(220)

12000

X-ray intensity

(311)

(101)

b

b

a X-ray intensity

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20

40

60

80

2 Theta degrees

20

30

40

50

60

70

80

90

2 Theta degrees

Figure 4. XRD patterns of Zn-containing magnetic oxides prepared with 0.15 mmol (a), 0.40 mmol (b), and 1.0 mmol (c) of Zn(acac)2 towards Fe(acac)3. Red font in (c) shows reflections from ZnO. The crystallite sizes obtained from Scherrer equation are 11.6±0.4 nm in (a) and 11.0±0.2 nm in (b). In (c) the crystallite size for Fe3O4 is 12.8±1.2 nm, while for ZnO, it is 8.0±0.9 nm.


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It is worth noting that Zn-containing magnetic oxides with the Zn(acac)2 loading between 0.15 and 0.40 mmol are easily separated with a rare earth magnet. SI shows that for the samples containing also ZnO NPs along with Fe3O4 NPs, the separation is very different.

Figure 5. STEM dark-field image (a) and EDS maps of Fe (b) and Zn (c) and their superposition (d) for the Zn-containing magnetic oxide prepared with 1.0 mmol of Zn(acac)2.


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The possible reason for the ZnO nanocube formation is a homogenous nucleation of ZnO species instead of a heterogeneous nucleation. When a large amount of Zn(acac)2 is injected in the reaction solution of iron oxide NPs, according to the Lamer mechanism, a monomer concentration in solution is exceeded, leading to homogenous nucleation.39 To avoid homogenous nucleation and the formation of ZnO NPs as a separate phase while, at the same time, boosting the Zn content, we added Zn(acac)2 in several small portions. As we discussed above, an addition of 0.15-0.40 mmol of Zn(acac)2 results in a single spinel phase. Figure S5 (SI) shows the TEM image and the XRD pattern of the sample prepared by the addition of three portions of 0.25 mmol of Zn(acac)2 with 30 minute intervals between additions at the reaction temperature. The TEM image of this sample shows a smaller fraction of ZnO nanocubes than the sample prepared by simultaneous addition of 1.0 mmol of Zn(acac)2 (Figure 5). The fraction of the ZnO crystalline phase in the XRD pattern (Figure S5b, SI) is also smaller. Moreover, STEM EDS (Figure S6, SI) reveals that the ZnO NPs coexist with NPs containing both Zn and Fe species. Thus, the stepwise addition of Zn(acac)2 allows us to diminish homogeneous nucleation, however, it does not completely prevent it. This can be due to the fact that when a heterogeneous nucleation takes place and Zn species form on the surface of iron oxide NPs, there are still Zn species in solution whose concentration is below the critical monomer concentration. The addition of the next portion of Zn(acac)2 may briefly increase the concentration above the critical monomer concentration, leading to a separate ZnO phase formation.

3.3. Zn-containing magnetic oxides: XPS The data in the previous section show that at low Zn(acac)2 loadings (0.15-0.40 mmol), a single spinel phase exists. Then, it might be reasonable to expect the ZnFe2O4 structure. The zinc


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ferrite formation was reported in literature when iron oxide was doped with Zn species,40-41 however, ZnFe2O4 is not known to be a catalyst in the reaction studied and not desired. To clarify the composition of Zn-containing magnetic oxides, we carried out XPS studies. The survey XPS spectrum of the Zn-containing magnetic oxide prepared with 0.25 mmol of Zn(acac)2 is presented in Figure S7 (SI). Analogous survey spectra were obtained for the rest of the samples studied. High resolution (HR) Fe 2p and Zn 2p XPS spectra of the samples obtained at different Zn(acac)2 loadings are presented in Figure 6.

Figure 6. Normalized HR XPS spectra in Fe 2p (a) and Zn 2p (b) regions of the Zn-containing magnetic oxides prepared with different amounts of Zn(acac)2. From the bottom to the top: Zn-1, Zn-2, Zn-3, and Zn-4 (Table 1). In all the samples the HR Zn 2p3 XPS shows BE of 1021.9 eV which is typical for Zn2+.42 The HR Fe 2p XPS spectra are also very similar and show a main peak with a binding energy (BE) of 711.4 eV which is typical for iron oxides. A weak satellite structure normally observed at a BE value of 8 eV higher than the main peak, is absent. This satellite indicates the excess of the Fe3+ species beyond the Fe3+:Fe2+=2:1 ratio of magnetite.43-45

In the case of magnetite, the


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combination of the Fe3+ and Fe2+ satellites leads to a flat plateau between the Fe 2p3/2 and Fe 2p1/2 peaks,46 as is observed in our case. The deconvolution of the XPS spectra of the sample prepared with 0.25 mmol of Zn(acac)2 (Figures S8 and S9 (SI)) demonstrates a consistently good Fe3+:Fe2+=2:1 ratio in the Fe 2p and Fe 3p regions. If Zn2+ would replace Fe2+ in magnetite forming ZnFe2O4, it would result in the above mentioned satellite structure.42,

47

Thus, the

absence of this satellite unambiguously proves the absence of zinc ferrite. A further confirmation was obtained from a comparison of the Fe and Zn contents. The Fe and Zn contents obtained from XPS (Table 1 and Figure S10, SI) show that in the sample obtained with 0.15 mmol of Zn(acac)2 the atomic ratio of Fe:Zn is 1.43:1 which is lower than that anticipated for ZnFe2O4 (2:1). For the samples prepared with 0.25 and 0.40 mmol of Zn(acac)2, the Zn content on the NP surface further increases compared to that expected for ZnFe2O4, while the XRD diffraction patterns still show only the spinel phase. These facts reveal that ZnO might form an amorphous layer on the iron oxide NP surface. In the case of the Zn-containing magnetic oxide prepared with 1.0 mmol of Zn(acac)2, the Fe:Zn atomic ratio is 0.34:1 (Figure S10, SI), demonstrating a significant fraction of Zn in the sample. This comes as no surprise because both XRD and EDS confirm a substantial fraction of ZnO NPs. Surprisingly, however, when Zn(acac)2 was added stepwise in three portions of 0.25 mmol each, the Zn content on the NP surface is the highest (Table 1), while the XRD shows a comparatively small fraction of the separate ZnO phase. This indicates that the stepwise addition of Zn(acac)2 favors heterogeneous nucleation and formation of ZnO on iron oxide NPs vs formation of a separate ZnO crystalline phase. To elucidate whether ZnO is indeed located only on the surface of iron oxide NPs, we applied the angular dependent XPS. For this, the measurements were carried out at 15 and 45°. These


18

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measurements allowed us to determine a surface vs bulk ratio of the Zn and Fe species across the particle.48 While in general this method was developed for the thin film analysis,49 the formation of a NP film makes it also applicable in our case with the assumption that the NP films are sufficiently flat. The data for the Fe 2p transition (Figure S11 and Table S3, SI) indicate only a slightly higher Zn content (by 1.6 at.%) at the more surface sensitive geometry. On the other hand, for the 3p transitions, the Zn contents are much lower (by ~20 at.%). These observations are consistent with the kinetic energies of 2p and 3p electrons. The kinetic energy of 3p photoelectrons is significantly larger, thus, these electrons probe deepest layers versus those producing 2p electrons.50-51 Therefore, the higher Zn contents for 2p transitions and the lower ones for 3p transitions indicate surface enrichment with ZnO species. Thus, the most likely scenario for the Zn-containing magnetic oxide formation is that amorphous ZnO forms an ultrathin layer on the iron oxide NP surface, the thickness of which depends on the Zn(acac)2 loading. In addition, ZnO diffuses under the surface of Fe3O4 NPs without substitution of Fe2+ with Zn2+ which, otherwise, would lead to zinc ferrite. The mixing of ZnO and iron oxide was reported elsewhere as an intermediate phase before formation of zinc ferrite.42 Thus, the final material composition is ZnO/Fe3O4/PPQ. Both an ultrathin layer and diffused ZnO species create the best conditions for being in close proximity to iron oxide, the beneficial environments for catalysts.

3.4. Catalysis: Proof of concept Zn-containing magnetic oxides developed in this work were tested in a liquid-phase transformation of syngas (CO+H2) to methanol.52-54 The advantages of the liquid-phase process over a gas-phase one are lower temperatures and pressures, however, the catalyst separation is needed in a number of reaction setups. From this point of view, magnetically recoverable catalysts are especially beneficial. In addition, when catalytic NPs (often stabilized by polymers)


19


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are used, higher activity and selectivity compared to traditional catalysts can be achieved due to high catalytic NP surface areas and numerous active sites.55-57 Table 2 displays the methanol accumulation rates in the methanol synthesis for the catalysts prepared with 0.15, 0.25, 0.40, and 1.0 mmol of Zn(acac)2. The two control experiments were carried out with Fe3O4 NPs containing no Zn and ZnO NPs without iron oxide NPs, stabilized by the PPQ in both cases. The data show that iron oxide NPs do not catalyze the methanol synthesis. Alternatively, ZnO NPs lead to the methanol accumulation rate of 94 g meth/kg cat ×h. For the Zn-containing magnetic oxides, the methanol accumulation rates are comparable and vary between 73 and 107 g meth/kg cat ×h. They seem to be dependent on the Zn(acac)2 loading with the exception of Zn-4. However, the Zn contents in these catalysts vary between 2.4 wt.% for Zn-1 and 29.3 wt.% for ZnO-PPQ, i.e., by more than an order of a magnitude. Considering that the Zn species are the only catalytic species in these materials, the methanol accumulation rates should be recalculated per the amount of Zn (not the total catalyst amount). This recalculation allowed us to observe remarkable dependences. Table 2. Methanol accumulation rates for Zn-containing magnetic oxides. Sample notation Zn(acac)2 loading, mmol a)

Zn content Methanol from the accumulation rate, elemental analysis, wt.% g meth./kg cat ×h

Methanol accumulation rate, g meth./kg Zn ×h

Fe3O4-PPQ

0

0

0

0

ZnO-PPQ

1.0

29.3

94

321

Zn-1

0.15

2.4

73

3042

Zn-2

0.25

2.8

90

3214

Zn-2 second use 0.25

2.8

89

3179

Zn-2 fifth use

2.8

88

3143

0.25


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Zn-3

0.40

5.4

107

1982

Zn-4

1.0

9.9

101

1020

Cu/ZnO/Al2O358 5 wt.% of Cu and Cu:Zn=1:1 (mol)

-

61b)

Cu/ZnO/Al2O353 ZnO:Cu=2:1 (mol)

-

199c)

-

a)

For all the samples except for ZnO-PPQ, 1.0 mmol of Fe(acac)3 was used to form magnetic NPs; b) calculated per a sum of Cu and Zn; c) calculated only per Cu content, while Zn is the catalyst as well. First, the methanol accumulation rate is much higher for all Zn-containing magnetic oxides that that of ZnO-PPQ. This can be explained by comparatively large ZnO NPs and their aggregation in the case of ZnO-PPQ (Figure S3, SI). Alternatively, the ZnO species in Zncontaining magnetic oxides are apparently very small (not observable by TEM), except for Zn-4, where cubic ZnO NPs are observed. For other samples, a methanol accumulation rate first increases when the Zn(acac)2 loading increases from 0.15 to 0.25 mmol and then decreases with the further increase of the Zn(acac)2 loading to 0.40 mmol. We assume that in all three samples (Zn-1, Zn-2, and Zn-3) where only a spinel phase was observed, the ZnO species are distributed as a very thin layer on top of iron oxide NPs along with diffused species in the magnetite crystal. Given that this amount is moderate for Zn-1 and Zn-2, the highest influence by the iron oxide species is realized. In this case, the iron oxide species can behave as a dopant increasing Zn oxygen vacancies or as an electron reservoir as was reported by us59-60 and others.61 When the Zn loading further increases (for Zn-3), a thicker ZnO layer might form on the iron oxide NPs, thus, the influence of the latter on some ZnO species is less pronounced. Finally, formation of a separate ZnO crystalline phase further decreases the methanol accumulation rate, which however, remains higher by a factor of three compared to that of ZnO-PPQ due to smaller ZnO


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NPs in Zn-4 and possible influence of iron oxide. It is noteworthy that the traditional Cu/ZnO catalysts used for liquid phase methanol synthesis display much lower activities (Table 2) than those reported in our work.53, 58 Repeat experiments carried out with Zn-2 after magnetic separation from the reaction mixture to recover the used catalyst (Table 2) showed remarkable catalyst stability even after five consecutive catalytic runs. This makes the Zn-containing magnetic oxides a catalyst of choice for future industrial applications.

4. Conclusion We developed a family of Zn-containing magnetic oxides of different structures and compositions by thermal decomposition of Zn(acac)2 in the reaction solution of preformed magnetite NPs stabilized by PPQ. At the Zn(acac)2 loading in the range 0.15-0.40 mmol, the samples contain mainly multicore NPs along with quasi-spherical NPs and possess a spinel structure which is typical for both magnetite and zinc ferrite. XPS revealed that for all the samples the Zn content is higher than that for ZnFe2O4 and no substitution of Fe2+ ions with Zn2+ ions takes place. Moreover, a comparison of 2p and 3p transitions for Fe and Zn shows that although the NP surface is enriched with Zn, the significant amount of Zn species are also found deeply under the iron oxide NP surface. These findings make us to believe that a thin layer of amorphous ZnO, undetectable by HRTEM, is formed on the iron oxide NP surface. At the same time, ZnO species diffuse inside the magnetite crystals without substitution of Fe2+ and formation of ZnFe2O4. The intimate contact between ZnO and Fe3O4 species ensures outstanding catalytic performance of these catalysts in the transformation of syngas to methanol and their high stability in the repeated use.


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ASSOCIATED CONTENT Supporting Information. Magnetic characteristics, TEM images, XRD patterns, and XPS data. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions The manuscript was written through contributions of all authors. ACKNOWLEDGMENT The research leading to these results has received funding from the European Community’s Seventh Framework Programme [FP7/2007-2013] under grant agreement no. 604296. E.Yu.-K. thanks the Russian Foundation for Basic Research (14-03-31669) and Z.S. thanks the Program of Presidium of the Russian Academy of Sciences (P8). M.R., A.S., and E.S. thank Russian Science Foundation (project 15-13-20015). We also thank the Indiana University Nanoscale Characterization Facility for access to the instrumentation.

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