Supported Bimetallic NiFe Nanoparticles through Colloid Synthesis for

Aug 29, 2017 - The presence of iron tends to favor migration of nickel into the support; it is possibly associated with its greater propensity to oxid...
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Supported Bimetallic NiFe Nanoparticles through Colloid Synthesis for Improved Dry Reforming Performance Tigran Margossian,† Kim Larmier,† Sung Min Kim,‡ Frank Krumeich,† Christoph Müller,‡ and Christophe Copéret*,† †

Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog-weg 2, Zürich CH-8093, Switzerland Department of Mechanical and Process Engineering, ETH Zürich, Leonhardstrasse 21, Zürich CH-8092, Zürich

‡

S Supporting Information *

ABSTRACT: The conversion of methane and carbon dioxide into a synthesis gas, the so-called dry reforming of methane (DRM), suffers from a stability issue caused by coke formation at the surface of the Ni-based catalysts. Using a colloidal approach, we demonstrate that supported 3−4 nm bimetallic NiFe nanoparticles with a Ni/Fe ratio of 3 have an enhanced stability compared to the corresponding pure Ni-based catalyst and a higher activity compared to conventional NiFe catalysts. The active sites for DRM are associated with Ni0, while FeO, observed by operando XAS under DRM conditions, allows for an effective decoking of the metal centers. KEYWORDS: nickel iron colloids, dry reforming of methane, stability, segregated/alloy metal, XAS operando

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INTRODUCTION Steam methane reforming (SMR, eq 1) produces a hydrogenrich synthesis gas (H2/CO = 3:1), suitable for downstream processes, such as the synthesis of ammonia (Haber process).1 In contrast, dry reforming of methane (DRM, eq 2), where CO2 replaces water, produces a synthesis gas with a equimolar content of hydrogen and carbon monoxide (H2/CO = 1:1). Combining the two processes allows tuning of the H2/CO ratio, which is important for methanol synthesis2 or the Fischer−Tropsch process starting from syngas with a H2/CO ratio lower than 3.3 However, DRM suffers from extensive formation of carbonaceous deposits on the nickel nanoparticle catalyst surface, which causes substantial loss of active sites4−6 as well as severe heat transfer problems during the exothermic regeneration step.7 Steam methane reforming: Δr H298K = +206 kJ·mol−1

CH4 + H 2O = CO + 3H 2 ,

or low-cost metals12,28−31 such as Fe also shows promising results to enhance catalyst stability. In the latter case, the formation of FeO helped to mitigate coke formation, through its reaction with the surface carbon. Herein, we evaluate a combined strategy based on small nanoparticles, Fe promotion and tailored-support, toward the formation of optimal catalysts with high activity and stability using a beneficial small size colloid synthesis.21,23,32,33 Following the synthesis of FeNi colloids, starting from nickel and iron isostructural molecular precursors, the colloids are deposited at the surface of a suitable support for DRM, a Mg, Al mixed oxide,12,31 and evaluated in dry reforming with a detailed analysis of the catalysts by a combination of different spectroscopic methods, for example Fourier transform IR (FTIR) spectroscopy, energy-dispersive X-ray (EDX) spectroscopy, and operando X-ray abosorption spectroscopy (XAS) at the Ni K-edge and the Fe K-edge.

(1)

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Dry reforming of methane: CH4 + CO2 = 2CO + 2H 2 ,

Δr H298K = +247 kJ·mol−1

RESULTS AND DISCUSSION The bimetallic colloids are synthesized by the reaction of two isostructural molecular precursors M[N(SiMe 3 )(2,6iPr2C6H3)]2 (with M = Fe and/or Ni) in a toluene solution under H2 (3 bar) at 55 °C for 48 h in the presence of a long-chain capping agent (0.5 equiv. of hexadecylamine/0.5 equiv. of stearic acid).34 The thus-prepared colloidal solutions in toluene show particle sizes of 3.4 ± 0.7 nm for pure iron colloids (Fe‑col),

(2)

Recently, the development of deactivation-resistant catalysts for DRM has been the center of significant research efforts. Placing metal nanoparticles into a meso-structured material8,9 or onto oxides with a high Tamman temperature is a viable strategy to enhance the stability of catalysts.10−15 Favorable results have also been obtained for supports that can also act as oxygen carriers facilitating the removal of carbon.16−20 In this context, decreasing the size of the catalyst particles has also been shown to improve the performance of the catalysts by avoiding whisker formation.18,21−24 Doping surface metal sites with precious25−27 © 2017 American Chemical Society

Received: June 26, 2017 Revised: August 27, 2017 Published: August 29, 2017 6942

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ACS Catalysis 4.3 ± 1.0 nm for the bimetallic Ni0.5Fe0.5 colloids (Ni0.5Fe0.5‑col), 4.1 ± 1.1 nm for the bimetallic Ni 0.75 Fe 0.25 colloids (Ni0.75Fe0.25‑col), and 3.4 ± 0.7 nm for the pure Ni colloids (Ni‑col). Transmission electron microscopy (TEM) images of the colloids are shown in Figure 1.

8333.0 eV for the Ni K-edge spectra. The spectra could be satisfactorily fit at both edges using a linear combination (LCF) of a metal foil spectrum (Ni or Fe) and a NiFe alloy reference. In both cases, the fits yield a composition of approximately 50% alloy and 50% pure metal (Figure S11). Thus, the analyses performed on the colloids are consistent with the formation of a partial nickel iron alloy having a Ni@NiFe@Fe structure as previously observed on a colloid prepared from Ni(COD)2 and Fe(NSiMe3)2 (Figure S11c).34 The as-deposited samples are calcined at 400 °C (5 h) to remove the capping agents and then treated under a flow of pure hydrogen at two different temperatures (650 and 850 °C, 1h). The thus-reduced catalysts are stored under inert conditions (Ar glovebox). All characteristic data can be found in Table 1. The particle sizes for the sample after reduction at 650 °C are 3.9 ± 2.0 nm for Ni0.5Fe0.5-650, 4.2 ± 2.2 nm for Ni0.75Fe0.25-650, and 3.5 ± 1.5 nm for Ni‑650 similar to the original size measured from the colloidal solution. In the case of the pure iron catalyst, only a few particles are observable after reduction at 850 °C (Fe‑850). Increasing the reduction temperature to 850 °C for all Ni-based catalysts leads to a slight increase of the particle size: 4.5 ± 2.2 nm for Ni0.5Fe0.5-850, 5.2 ± 2.8 nm for Ni0.75Fe0.25-850, and 3.5 ± 2.5 nm for pure Ni‑850. The oxidation state of the metals after the reduction is assessed using in situ XANES under hydrogen flow (20 mL·min−1, see Figure 2). In all Ni-containing catalysts, the Ni K-edge XANES spectra do not show significant white lines, indicating that the metal is essentially reduced. The spectra could be adequately fit using a Ni foil spectrum and a minor fraction (below 10%) of a quaternary oxide (Ni, Fe, Mg, or Al)O as a reference. This latter contribution is probably related to nickel atoms incorporated into the support matrix (Ni@supp), which are hardly reducible, even at 850 °C (see Table 1).12,14 For the iron-containing catalysts (Fe‑650, Ni0.5Fe0.5-650, and Ni0.75Fe0.25-650), a clear white line is observed on the Fe K-edge spectra with an inflection point at 7131.0 eV, showing that iron remains partially oxidized upon H2 treatment. Linear combination fits were required to take three contributions into account: Fe0 from the metal foil, iron oxide (FeO), and a quaternary oxide (Ni, Fe, Mg, or Al)O (Fe@supp). As for nickel, this contribution is related to the iron species, which have likely migrated into the support. The fits show that

Figure 1. TEM images of the different colloids: Fe‑col, Ni0.5Fe0.5‑col, Ni0.75Fe0.25‑col, and Ni‑col.

The speciation of Ni and Fe in the bimetallic colloids is investigated using EDX and X-ray absorption near-edge structure (XANES) spectroscopies. Focusing the analysis on the center of a single particle in Ni0.5Fe0.5‑col colloids reveals that nanoparticles have a pure nickel core (Figures S7 and S8). However, focusing the analysis on a larger area of the sample containing several particles decreases the Ni/Fe ratio to about 2 (Figure S10). This is possibly due to the formation of core−shell structures with a nickel core and an iron shell. However, the presence of small individual iron nanoparticles cannot be excluded. The colloids are also studied by XAS after dispersion on a Mg(Al)O support.12,35 XANES spectra of the as-deposited bimetallic equimolar colloids onto Mg(Al)O (Ni0.5Fe0.5‑col/Mg(Al)O) show nickel and iron in metallic states with a first inflection point at 7112.0 eV for the Fe K-edge and at

Table 1. Physical Features Prior to and after Reaction for the Different Nix/(x+y)Fey/(x+y)‑reduction temperature Catalysts with x and y Molar Quantities of Ni and Fe, Respectively Ni0.5Fe0.5-850

Ni0.75Fe0.25-650

Ni‑650

Ni‑850

Nia (wt %) Fea (wt %) Ni/(Ni + Fe) (mol/mol) H2 uptakeb(mmolH2·molNi−1)

feature

Fe‑650 0 0.80 0 0

Ni0.5Fe0.5-650 0.37 0.40 0.47 50 ± 10

0.37 0.40 0.47 20 ± 10

0.53 0.17 0.75 90 ± 15

Ni0.75Fe0.25-850 0.53 0.17 0.75 30 ± 10

0.80 0 1 110 ± 20

0.80 0 1 80 ± 15

Ni dispersionb (%) initial Ni0/Ni@sup contentc (%) initial Fe0/FeO/Fe@sup contentd (%) initial dpe (nm) final dpe (nm) initial Ni/Fe ratiof final Ni/Fe ratiof cokeg (molC·molCconv−1)

0 -

10 ± 2 92/8 25/31/44 3.9 ± 2.0 4.2 ± 1.9 5.0 ± 1.2 7.0 ± 3.0 0.89.10−4

4±2 4.5 ± 2.2 4.2 ± 1.8 1.6 ± 0.6 6.2 ± 1.5 1.25.10−4

18 ± 3 92/8 0/54/46 4.2 ± 2.2 4.3 ± 1.6 7.6 ± 2.1 5.7 ± 2.3 0.53.10−4

6±2 91/9 19/37/44 5.2 ± 2.8 4.2 ± 2.8 3.4 ± 1.0 4.9 ± 2.8 1.11.10−4

22 ± 4 98/2 3.5 ± 1.5 3.3 ± 1.8 2.62.10−4

16 ± 3 3.5 ± 2.5 5.8 ± 3.6 3.29.10−4

Determined by elemental analysis (Table S1). bH2 uptake determined by hydrogen chemisorption at 25 °C (calculation details in Figure S34). Determined from linear combination fits of Ni K-edge spectra. dDetermined from linear combination fits of Fe K-edge spectra. eParticle size determined by electron microscopy. fNi/Fe ratio determined by EDX spectroscopy focused on the center of 10 single nanoparticles. gCoke quantity determined by a TPO experiment. a c

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The results are consistent with temperature-programmed reduction, which shows that pure nickel is fully reduced at 650 °C whereas the complete reduction of the pure iron sample requires much higher temperatures (>900 °C, details in Figure S28). After reduction at 650 °C, the Ni/Fe ratios determined by EDX spectroscopy focused on the center of 10 single nanoparticles are 5.0 ± 1.2 in Ni0.5Fe0.5-650 and 7.6 ± 2.0 in Ni0.75Fe0.25-650, in both cases significantly higher than the expected stoichiometry. Hence, the particles are mainly composed of nickel. However, increasing the reduction temperature from 650 to 850 °C decreases the Ni/Fe ratios on the particles to 1.5 ± 0.6 for the Ni0.5Fe0.5-850 system and to 3.4 ± 1.0 for the Ni0.75Fe0.25-850 system, close to the nominal composition (Figures S29−32). Therefore, increasing the temperature from 650 to 850 °C induces further reduction and migration of iron atoms to the nickel nanoparticles. We probed the potential surface sites by adsorption of hydrogen and carbon monoxide. Hydrogen chemisorption at 25 °C allows for quantifying metal surface sites having the ability to dissociate hydrogen. Since metallic iron does not readily chemisorb hydrogen at 25 °C,12 chemisorption likely titrates selectively the surface nickel sites. Except for the pure iron sample (Fe-650), materials reduced at 650 °C show a practically constant hydrogen uptake normalized per mole of nickel with 100 mmolH2·molNi−1 for Ni0.5Fe0.5-650, 120 mmolH2·molNi−1 for Ni0.75Fe0.25-650, and 10 mmolH2·molNi−1 for Ni-650, considering a stoichiometry of 1 H/1 Nisurface as for pure Ni nanoparticles.36 This is in line with the microscopic and XAS observations showing particles of similar sizes, mainly composed of nickel. Therefore, increasing the quantity of nickel yields a linear augmentation of the Ni dispersion (100nNi surface/ntotal metal, Figures S33 and S34) from 10% for Ni0.5Fe0.5-650 to 18% for Ni0.75Fe0.25-650 and 22% for Ni‑650. In the case of bimetallic samples, increasing the reduction temperature up to 850 °C significantly affects the final hydrogen uptake for samples Ni0.5Fe0.5-850 and Ni0.75Fe0.25-850, while keeping the particle size almost constant. This is possibly due to the formation of a surface

Figure 2. XANES spectra recorded during an in situ reduction experiment of Ni0.75Fe0.25-650 and Ni0.75Fe0.25-850 (A) at the Ni K-edge and (B) at the Fe K-edge.

only a small fraction of the iron is reduced (below 25%). Increasing the reduction temperature to 850 °C or the Fe/Ni ratio increases the fraction of reduced iron. The nonreduced fraction is a mixture of FeO and Fe@supp.

Figure 3. Structural investigation of the bimetallic material Ni0.75Fe0.25 reduced at different temperatures at 650 °C (bottom) and at 850 °C (top): (A) high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images with EDX spectra of the outlined areas; (B) supported nanoparticle scheme; (C) CO adsorption at 25 °C. 6944

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ACS Catalysis Table 2. Initial DRM Performances (after 1 h TOS) for the Different Catalysts Reduced at 650 °C Fe

Ni0.5Fe0.5

Ni0.75Fe0.25

initial XCH4

feature

0

11.1

17.3

20

initial XCO2

0

23.4

31.0

34

initial H2/CO ratio initial CH4 rate (molCH4·molmetal−1·s−1)

0 0

0.54 1.3

Ni−Fe alloy as observed by XAS, which likely does adsorb less H2 than the Ni surface site. CO adsorption tracked by FTIR spectroscopy was conducted on the reduced catalysts.37 For all systems, there is a common contribution at approximately 2198 cm−1, specific of stretching frequencies of CO adsorbed on an acidic site of a spinel support (Figures S35−43).14 The materials with pure nickel (Ni‑650 and Ni‑850) show similar spectra with vibrations at 2084, 2047, and 1955 cm−1; the vibrations are related to the different CO vibrations onto the nickel nanoparticles.14,38 The pure iron catalysts (Fe‑650 and Fe‑850) show four different features with three (at 2108, 2058, and 1995 ± 1 cm−1) related to CO adsorbed on the Fe0 site and the most blue-shifted at 2142 cm−1 assigned to the Fe(II) surface site.39 The contributions of the Fe0 sites are becoming relatively more intense when increasing the reduction temperature from 650 to 850 °C as iron gets more efficiently reduced, in line with temperature-programmed reduction (TPR) and XAS results (Figures S20−26). Ni0.75Fe0.25-650 shows very similar features compared to the Ni spectra; thus, mostly Ni surface sites are exposed in this sample. In addition, Ni0.5Fe0.5-650 shows a major peak at 1975 cm−1 that does not match the stretching frequencies of CO adsorbed on pure nickel or on pure iron. This band also appears for both bimetallic systems reduced at 850 °C (1970 cm−1 for Ni0.5Fe0.5-850 and 1986 cm−1 for Ni0.75Fe0.25-850). In view of the presence of reduced iron and the formation of a partial Ni−Fe alloy for these samples, we propose to assign the band around 1975 cm−1 to CO adsorbed on the surface Ni−Fe alloy. Thus, the two adsorption experiments confirm the presence of a nickel-rich surface when the reduction is set at 650 °C but a partial metal alloying when the reduction is set at 850 °C (see Figure 3B). The DRM performances of the catalysts are finally examined in a fixed-bed flow reactor. The tests are performed at 650 °C under the kinetic regime, far from the equilibrium conversion (thermodynamic XCH4 = 57%) with methane conversion below 20% (Table 2). The DRM activity is only discussed in the context of the rate of methane consumption as the reaction is thermodynamically equilibrated by the RWGS for most systems.21,24,40 No DRM activity is observed for the pure iron catalyst (Fe‑650). The initial methane rate consumption increases linearly with the Ni dispersion with a rate of 1.25 molCH4·moltotal metal−1·s−1 for Ni0.5Fe0.5-650, 2.18 mol CH 4 ·mol total metal −1 ·s −1 for Ni 0.75 Fe 0.25-650 , and 2.37 molCH4·moltotal metal−1·s−1 for Ni‑650 (Figure 4B), confirming that surface Ni sites are the active sites for DRM under these conditions. The determined TOF is 10 s−1, which is the same order of magnitude as reported in previous work conducted under similar conditions.12,24 After 30 h of time on stream, the pure nickel-based catalysts suffer from deactivation and only display an activity of 0.95 molCH4·molmetal−1·s−1 for Ni‑650 (62% of deactivation) and of 0.26 molCH4·molmetal−1·s−1 for Ni-850 (89% of deactivation).

0.62 2.1

Ni

0.67 2.4

Figure 4. (A) Dry reforming performance at 650 °C under 100 mL· min−1 flow of CO2/CH4/N2 (0.45/0.45/0.1). Methane rate consumption in molCH4·mol−1metal·s−1. GHSV = 2.7 × 105 mL·h−1·gCat−1 for all catalysts. (B) Rate of consumption of methane per molCH4·mol−1metal· s−1 after 1 h TOS vs the initial nickel dispersion D (%) for catalyst reduced at 650 °C.

In contrast, the bimetallic (Ni0.5Fe0.5-650, Ni0.75Fe0.25-650) systems deactivate slowly and after 30 h reach an activity of 0.52 molCH4·molmetal−1·s−1 for Ni0.5Fe0.5-650 and 1.51 molCH4·molmetal−1·s−1 for Ni0.75Fe0.25-650 giving an approximate deactivation of 45% and 20%, respectively. After a 3 day test, the activity of Ni0.75Fe0.25-650 remained high, that is, 1.37 molCH4·molmetal−1·s−1 whereas Ni‑650 has a final methane consumption of 0.90 molCH4·molmetal−1·s−1. Increasing the reduction temperature of the two bimetallic systems up to 850 °C (Ni0.5Fe1-850, Ni0.5Fe1-850) decreases the initial activity from 1.30 to 0.80 molCH4·molmetal−1·s−1 for the Ni0.5Fe0.5 system (Ni0.5Fe0.5-650, Ni0.5Fe0.5-850) and from 2.18 to 1.80 molCH4· 6945

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ACS Catalysis molmetal−1·s−1 for the Ni0.75Fe0.25 system (Ni0.75Fe0.25-650, Ni0.75Fe0.25-850); however, the deactivation rate is not too impacted. The lower initial activity is likely due to a lower concentration of pure Ni surface sites decreasing upon reduction. The particle sizes are also evaluated after the catalytic test for the different samples; they are 4.2 ± 1.9 nm for Ni0.5Fe0.5-650, 4.2 ± 1.8 nm for Ni0.5Fe0.5-850, 4.3 ± 1.6 nm for Ni0.75Fe0.25-650, 4.2 ± 2.8 nm for Ni0.75Fe0.25-650, 3.3 ± 1.8 nm for Ni‑650, and 5.8 ± 3.6 nm for Ni‑850. Except for Ni‑850, the particle size is not affected by the DRM reaction, which shows that sintering phenomena are not the origin of deactivation. In the case of the pure nickel catalysts (Ni), carbon whiskers are also observed by electron microscopy of the spent catalyst (Figures S51 and S52). Temperature programmed oxidation (TPO) experiments of the different spent catalysts allow the determination of the quantity of coke formed during the reaction. As expected, the Ni catalysts entail the largest quantity of coke formed with 2.62 × 10 −4 mol coke ·mol carbon converted −1 for Ni ‑650 and 3.29 × 10−4 molcoke·molcarbon converted−1 for Ni‑850. For the two bimetallic systems reduced at 650 °C, the rate of coke formation decreases significantly with only 0.53 and 0.89 × 10−4 molcoke·molcarbon converted−1 for catalysts Ni0.5Fe0.5-650 and Ni0.75Fe0.25-650, respectively. Reducing the catalysts at higher temperature yields an increased quantity of coke, from 0.89 to 1.25 molcoke·molcarbon converted−1 for Ni0.5Fe0.5-850 and from 0.53 to 1.11 molcoke·molcarbon converted−1 for Ni0.75Fe0.25-850. Thus, reduction at a higher temperature leads to a higher coke deposition, which indicates an ineffective role of the iron doping. Thus, Ni0.75Fe0.25-650 appears as the optimal catalyst as it allows for the best compromise between activity and stability. We further assessed the state of the metals under reaction conditions by operando XANES experiments. After reduction under hydrogen, DRM is conducted at 650 °C under a flow of CH4/CO2/N2 (20 mL·min−1). The outlet gas composition is monitored by gas chromatography while recording XANES spectra at the Ni K-edge and Fe K-edge for the bimetallic systems. The trends in activity observed in the operando mode (see Figure S53) are qualitatively similar to what is observed in the flow reactor (Figure 4A). As mentioned above, spectra at the Ni K-edge show that, after the reduction treatment, above 90% of the nickel is reduced for all Ni-containing catalysts (Ni0.5Fe0.5-650, Ni0.75Fe0.25-650, Ni‑650) (see Figures S20−23). The pure nickel catalyst (Ni‑650), undergoes a slight decrease in the quantity of Ni0 from 98% to 90% after 4 h under the reaction conditions, as shown by the appearance of a white line in the spectra. This is assigned to a partial migration of nickel into the support. Nonetheless, the extent of this phenomenon does not parallel the strong deactivation, showing that most of the activity loss of Ni‑650 is caused by coke deposition, consistent with thermogravimetric analysis data. For the bimetallic systems, similar trends are observed as for Ni, although the Ni0 content is decreasing down to approximately 80% for catalysts Ni0.75 Fe 0.25-650 and Ni0.75Fe0.25-850 and to 70% for Ni0.5Fe0.5-650 after 5 h on stream. The presence of iron tends to favor migration of nickel into the support; it is possibly associated with its greater propensity to oxide Ni, which can more easily migrate inside the support. The evolution of the state of Fe was also examined by operando XAS. LCF analysis of the XANES spectra at the Fe Kedge recorded under DRM conditions was carried out for Ni0.75Fe0.25-650 and Ni0.75Fe0.25-850. As mentioned above, three contributions are required to achieve satisfactorily fits (Fe0, FeO, and Fe@Ssupp) as shown in Figure 5B. For Ni0.75Fe0.25-650

Figure 5. Operando XAS dry reforming experiment at 650 °C under 23 mL·min−1 flow of CO2/CH4/N2 (0.45/0.45/0.1). GHSV = 1.5 × 105 mL·h−1·gcat−1 for different catalysts. (A) Spectra at the Ni K-edge for catalyst Ni0.75Fe0.25-650; (B) spectra at the Fe K-edge for catalyst Ni0.75Fe0.25-650 and Ni0.75Fe0.25-850 after 5 h TOS; (C) Fe content (%) function of reaction time for catalysts Ni0.75Fe0.25-650 (full lines) and Ni0.75Fe0.25-850 (dotted lines); black lines correspond to LCF at the Fe Kedge value for Fe@sup or (Ni, Fe, Mg, or Al)O; blue lines correspond to LCF at the Fe K-edge value for FeO; Red lines corresponds to LCF at Fe K-edge value for FeO.

reduced at 650 °C, 54% of iron is FeO, and the complementary portion is incorporated in the support (vide supra). After 1 h reaction, the FeO is leveling off at 25%; concomitantly, the content of iron inside the support is increasing up to 75%. Ni0.75Fe0.25-850 entails after reduction 19% Fe0, 37% FeO, and 54% in the support. Similarly to Ni0.75Fe0.25-650, FeO content is dropping to 25%, and iron migrating inside the support increases up to 65%. After 5 h TOS, the quantity of Fe0 has decreased to 10%. 6946

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°C) decreases the activity, which is due to the presence of a significant amount of Fe0 surface sites at the surface of the particle.

The spent catalysis was also analyzed by EDX spectroscopy focused on the center of the nanoparticles (Figures S90−93). The Ni/Fe ratios of the metal particles for spent Ni0.5Fe0.5-650 and spent Ni0.75Fe0.25-650 remain within the same ranges of 7.0 ± 3.0 and 5.7 ± 2.3, respectively. For spent Ni0.5Fe0.5−-850 and Ni0.75Fe0.25-850, the nickel content in the particles increases with a Ni/Fe ratio of from 1.6 ± 0.6 to 6.2 ± 1.5 and 3.4 ± 1.0 to 4.9 ± 2.8, respectively. Taken together with the loss of Fe0 in these samples, as shown by XAS, the results indicate that Ni−Fe dealloy under reaction conditions, as Fe0 is oxidized and migrates to the support. This is similar to what is observed for larger nickel iron nanoparticles.12,30,31 Initially these nickel−iron particles have an alloy structure, and upon exposure to reaction conditions, the metal segregates resulting in Ni0 and FeO.12 Combining different spectroscopic techniques, we show that the pretreatment temperature drastically influences the structure of the bimetallic catalysts. Increasing the reduction temperature from 650 to 850 °C changes the population of surface sites from Ni0/FeO to Ni−Fe/FeO, but under the DRM reaction, the metal species evolve toward a similar speciation (see Figure 6). Fe0

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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/acscatal.7b02091. Experimental section, TEM images, scheme of the Ni0.5Fe0.5‑col structure, dispersion data, dry reforming performance data, the powder X-ray diffractograms, FTIR of the reduced sample, HAADF-STEM-EDX results, H2 chemisorption isotherm, TPR data, XANES data, and FTIR spectra for CO adsorption experiment (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; phone: +41 44 633 93 94. ORCID

Kim Larmier: 0000-0002-5199-1516 Sung Min Kim: 0000-0001-6602-1320 Christoph Müller: 0000-0003-2234-6902 Christophe Copéret: 0000-0001-9660-3890 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by ETH Zürich (ETH-57_12-2) and the Swiss National Funding (SNF) in relation with Swiss Competence Centers for Energy Research (SCCER Heat and Electricity Storage). We acknowledge the Paul Scherrer Institut, Villigen, Switzerland for provision of synchrotron radiation beam time at beamline Super XAS (20160169) of the SLS and would like to thank Dr. Olga V. Safonova and Dr. Maarten Nachtegaal for assistance. We thank ScopeM (ETH Zürich) for providing STEM measuring time.

Figure 6. Scheme of the structure evolution during DRM at 650 °C for Ni0.75Fe0.25-650 and Ni0.75Fe0.25-850.

tends to be oxidized to either FeO or Fe2+ located in the support, and FeO itself migrates into the support matrix to some extent for Ni0.75Fe0.25-650. The content of FeO after 1 h on stream is almost independent of the reduction temperature. It has previously been shown that FeO is effective to reduce coke formation for large nickel nanoparticles under DRM.12 Thus, independent of the reduction temperature, the deactivation rate is similar, but the initial activity is diminished when the Ni−Fe alloy is present (in Ni0.75Fe0.25-850 for instance).

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REFERENCES

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CONCLUSION Bimetallic nanoparticles of sizes 3−4 nm are synthesized via a colloidal synthesis approach using two isostructural molecular precursors under H2. The controlled deposition of the nanoparticles onto a suitable Mg(Al)O periclase material generates a series of small size bimetallic supported nanoparticles. While the pure iron catalyst is completely inactive for DRM, the pure nickel catalyst shows very high initial activity with a rate of methane consumption 10 times higher than the reference catalyst prepared by coprecipitation. However, despite their small size, the nanoparticles undergo fast deactivation via coke formation. Optimizing the ratio of nickel to iron to 3:1 generates a catalyst with a high content of Ni0 active sites and enhanced stability for catalysts prepared at 650 °C, as evidenced by operando XAS. However, a high reduction temperature (850 6947

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Research Article

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DOI: 10.1021/acscatal.7b02091 ACS Catal. 2017, 7, 6942−6948