Supported Bimetallic NiFe Nanoparticles through Colloid Synthesis for

Aug 29, 2017 - The conversion of methane and carbon dioxide into a synthesis gas, the so-called dry reforming of methane (DRM), suffers from a stabili...
0 downloads 13 Views 6MB Size
Download PDF
Subscriber access provided by University of Texas Libraries

Article

Supported Bimetallic NiFe Nanoparticles Through Colloid Synthesis for Improved Dry Reforming Performance Tigran Margossian, Kim Larmier, Sung Min Kim, Frank Krumeich, Christoph R. Mueller, and Christophe Copéret ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02091 • Publication Date (Web): 29 Aug 2017 Downloaded from http://pubs.acs.org on August 29, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 8

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Supported Bimetallic NiFe Nanoparticles Through Colloid Synthesis for Improved Dry Reforming Performance Tigran Margossian1, Kim Larmier1, Sung Min Kim2, Frank Krumeich1, Christoph Müller2, Christophe Copéret1*‡. 1

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

2

E-mail: [email protected] ABSTRACT: The conversion of methane and carbon dioxide into a synthesis gas, the so called dry reforming of methane (DRM), suffers from 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 ratio Ni/Fe = 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

§

INTRODUCTION

Steam Methane Reforming (SMR, Eq. 1) produce a hydrogen rich synthesis gas (H2/CO ratio of 3:1), suitable for downstream processes, such as the ammonia synthesis (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 the H2/CO ratio, which is important for methanol synthesis2 or the Fischer-Tropsch process starting from syngas with H2/CO ratio lower than 3.3 However, DRM suffers from extensive formation of carbonaceous deposits on the nickel nanoparticles 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: CH4 + H2O = CO+ 3 H2 , ∆rH298K = +206 kJ. mol-1

(1)

Dry Reforming of Methane: CH4 + CO2 = 2 CO+ 2 H2 , ∆rH298K = +247 kJ. mol-1

(2)

Recently, the development of deactivation-resistant catalysts for DRM has been the centre of significant research efforts.

Placing metal nanoparticles into a meso-structured material 8,9 or onto oxides with high Tamman temperature are viable strategies to enhance the stability of catalysts.10-15 Favorable results have been also 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 shown to improve the performance of the catalysts by avoiding whisker formation.18,21-24 Doping surface metal sites with precious25-27 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 surface carbon. Herein, we evaluate a combined strategy based on small nanoparticles, Fe promotion and tailored-support, towards the formation of optimal catalysts with high activity and stability using a beneficial small size colloids 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, e.g. FTIR, EDX and Operando XAS at the Ni K-edge and Fe-Kedge.

§

RESULTS AND DISCUSSIONS

The bimetallic colloids are synthesized by the reaction of two isostructural molecular precursors M[N(SiMe3)(2,6-iPr2C6H3)]2 (with M = Fe and/or Ni) in a toluene solution under H2 (3 bars) at 55 °C for 48 h in the presence of long chain capping agent (0.5 equi. of hexadecylamine/ 0.5 equi. 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), 4.3 ± 1.0 nm for the bimetallic Ni0.5Fe0.5 colloids (Ni0.5Fe0.5-col), 4.1± 1.1 nm for the bimetallic Ni0.75Fe0.25 colloids (Ni0.75Fe0.25-col) and 3.4± 0.7 nm for the pure Ni colloids (Ni-col). The speciation of Ni and Fe in the bimetallic colloids is investigated using EDX and X-Ray Absorption Near-Edge Structure spectroscopies (XANES). Focusing the analysis on the centre of a single particle in Ni0.5Fe0.5-col colloids reveals that nanoparticles have a pure nickel core (Figure S7-8). 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 struc-

ACS Paragon Plus Environment

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

tures 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 spectroscopy after dispersion on Mg(Al)O support.12,35 XANES spectra of the asdeposited bimetallic equimolar colloids onto Mg(Al)O – Ni0.5Fe0.5-col/Mg(Al)O – show nickel and iron in metallic state with a first inflection point at 7112.0 eV for Fe K-edge and of 8333.0 eV for Ni K-edge spectra. The spectra could be satisfactorily fitted 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 fittings yield a composition of approximately 50% alloy and 50 % of pure metal (Figure S11). Thus, the analyses performed on the colloids is consistent with the formation of a partial nickel iron alloy having a Ni@NiFe@Fe structure as previously observed on colloid prepared from Ni(COD)2 and Fe(NSiMe3)2 (Figure S11c).34

Page 2 of 8

inflection point at 7131 eV, showing that iron remains partially oxidized upon H2 treatment. Linear Combination Fittings required to take 3 contributions into account: Fe0 from the metal foil, iron oxide (FeO) and a quaternary oxide (Ni,Fe,Mg,Al)O (Fe@Supp). As for nickel, this contribution is related to iron species, which have likely migrated into the support. The fits show that 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 non-reduced fraction is a mixture of FeO and Fe@Supp.

Figure 1: TEM pictures of the different colloids. Fe-col, Ni0.5Fe0.5-col, Ni0.75Fe0.25-col, Ni-col.

The as-deposited samples are then 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 oC and 850 o 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 oC 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 from the original size measured from the colloidal solution. In the case of pure iron catalyst, only few particles are observable after reduction at 850 oC (Fe-850). Increasing the reduction temperature to 850 °C for all Ni-based catalysts leads to a slight increase of the particle size, with 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 fitted using a Ni foil spectrum and a minor fraction (below 10 %) of a quaternary oxide (Ni,Fe,Mg,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, Ni0.75Fe0.25-650), a clear white line is observed on the Fe K-edge spectra with an

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 B) at the Fe Kedge.

The results are consistent with Temperature Programmed Reduction, which shows that pure nickel is fully reduced at 650 oC whereas the complete reduction of the pure iron sample requires much higher temperatures (>900oC, details in Figure S28). After reduction at 650 oC, the Ni/Fe ratio determined by EDX spectroscopy focused on the centre of 10 single nanoparticles is 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 oC to 850 o C decreases the Ni/Fe ratio 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 (Figure S29-32). Therefore, increasing the temperature from 650 oC to 850 oC induces further reduction and migration of iron atoms to the nickel nanoparticles.

ACS Paragon Plus Environment

Page 3 of 8

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Table 1: Physical features prior to and after reaction for the different Nix/(x+y)Fy/(x+y )-Reduction temperature catalysts with x,y molar quantity of Ni and Fe, respectively. a: Determined by elemental analysis (Table S1); b: H2 uptake determined by hydrogen chemisorption at 25 oC (calculation details in Figure S34); c: determined from Linear Combination Fittings of Ni K-edge spectra; d: determined from Linear Combination Fittings of Fe K-edge spectra e: particle size determined by electron microscopy; f: Ni/Fe ratio determined by EDX spectroscopy focused on the centre of 10 single nanoparticles; g: Coke quantity determined by TPO experiment. Features Ni (wt%)a Fe(wt %)

a

Fe-650

Ni0.5Fe0.5-650

Ni0.5Fe0.5-850

Ni0.75Fe0.25-650

Ni0.75Fe0.25-850

Ni-650

Ni-850

0

0.37

0.37

0.53

0.53

0.80

0.80

0.80

0.40

0.40

0.17

0.17

0

0

Ni/(Ni+Fe) (mol/mol)

0

0.47

0.47

0.75

0.75

1

1

H2 Uptake b(mmolH2.molNi-1)

0

50±10

20±10

90±15

30±10

110±20

80±15

Ni Dispersion b (%)

0

10±2

4±2

18±3

6±2

22±4

16±3

-

92/8

-

92/8

91/9

98/2

-

-

25/31/44

-

0/54/46

19/37/44

-

-

-

3.9 ± 2.0

4.5 ± 2.2

4.2 ± 2.2

5.2 ± 2.8

3.5 ± 1.5

3.5 ± 2.5

Final dp (nm)

-

4.2 ± 1.9

4.2 ± 1.8

4.3 ± 1.6

4.2 ± 2.8

3.3 ± 1.8

5.8 ± 3.6

Initial Ni/Fe ratiof

-

5.0 ± 1.2

1.6 ± 0.6

7.6 ± 2.1

3.4 ± 1.0

-

-

Final Ni/Fe ratiof

-

7.0 ± 3.0

6.2 ± 1.5

5.7 ± 2.3

4.9 ± 2.8

-

-

-4

-4

-4

-4

0

Initial Ni /Ni@Sup content (%)c Initial Fe0/FeO/Fe@Sup content (%)d Initial dpe (nm) e

g

-1

Coke (molC.molCConv )

0.89.10

1.25.10

0.53.10

1.11.10

2.62.10

-4

3.29.10-4

Figure 3: Structure investigation of the bimetallic material Ni0.75Fe0.25 reduced at different temperature at 650 oC (bottom) and at 850 oC (top) A) HAADFSTEM images with EDX spectra of the outlined areas; B) supported nanoparticle scheme; C) CO adsorption at 25 oC.

We probed the potential surface sites by adsorption of hydrogen and carbon monoxide. Hydrogen chemisorption at 25 oC allows for quantifying metal surface sites having the ability to dissociate hydrogen. Since metallic iron does not readily chemisorbed hydrogen at 25 oC,12 chemisorption likely titrates selectively the surface nickel sites. Except for the pure iron sample (Fe-650), materials reduced at 650 oC 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, 110 mmolH2.molNi-1 for Ni-650, considering a stoichiometry 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 (100 x nNisurface/ntotal metal, Figure S33-

34) 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 oC affects significantly the final hydrogen uptake for sample Ni0.5Fe0.5-850, Ni0.75Fe0.25-850 while keeping particle size almost constant. This is possibly due to the formation of a surface Ni-Fe alloy as observed by XAS, which likely does adsorb less H2 than Ni surface site. CO adsorption tracked by Fourier Transformed Infrared (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 acidic site of spinel support (Figure S35-43).14 The materials with pure nickel (Ni-650 and Ni-850) show similar spectra with vibrations at 2084 cm-1, 2047 cm-1 and 1955 cm-1

ACS Paragon Plus Environment

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

and are related to the different CO vibration onto nickel nanoparticles.14,38 The pure iron catalysts (Fe-650 and Fe-850) shows four different features with three (at 2108 cm-1, 2058 cm-1 and 1995 ± 1 cm-1 ) related to CO adsorbed on Fe0 site and the most blue shifted at 2142 cm-1 assigned to Fe(II) surface site.39 Contributions of the Fe0 sites are becoming relatively more intense when increasing the reduction temperature from 650 to 850 oC as iron gets more efficiently reduced, in line with TPR and XAS results (Figure 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. Ni0.5Fe0.5-650 shows in addition a major peak at 1975 cm-1 that does not match the stretching frequencies of CO adsorbed on pure nickel nor on pure iron. This band also appears for both bimetallic systems reduced at 850 oC (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 alloying for these samples, we propose to assign the band around 1975 cm-1 to CO adsorbed on surface Ni-Fe alloy. Thus, the two adsorption experiments confirm the presence of a nickel rich surface when the reduction is set at 650 oC but a partial metal alloying when the reduction is set at 850 oC (see scheme Figure 3B).

Page 4 of 8

tively 45 % and 20%. After a 3 days test, the activity of Ni0.75Fe0.25-650 remained high, i.e. 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 oC (Ni0.5Fe1-850, Ni0.5Fe1-850) decreases the initial activity from 1.30 to 0.80 molCH4.molmetal1 -1 .s for the Ni0.5Fe0.5 system (Ni0.5Fe0.5-650, Ni0.5Fe0.5-850) and from 2.18 to 1.80 molCH4.molmetal-1.s-1 for the Ni0.75Fe0.25 system (Ni0.75Fe0.25-650, Ni0.75Fe0.25-850), but 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 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 (Figure S51-52).

Table 2: Initial DRM performances (after 1 h TOS) for the different catalysts reduced at 650 oC.

Features

Fe

Ni0.5Fe0.5

Ni0.75Fe0.25

Ni

Initial XCH4

0

11.1

17.3

20

Initial XCO2

0

23.4

31.0

34

Initial H2/CO ratio

0

0.54

0.62

0.67

Initial CH4 rate (molCH4.molmetal-1.s-1)

0

1.3

2.1

2.4

The DRM performances of the catalysts are finally examined in a fixed-bed flow reactor. The tests are performed at 650 °C under 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.moltotalmetal-1.s-1 for Ni0.5Fe0.5-650 , 2.18 molCH4.moltotalmetal1 -1 .s for Ni0.75Fe0.25-650 and 2.37 molCH4.moltotalmetal-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 of 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). In contrast, the bimetallic (Ni0.5Fe0.5-650, Ni0.75Fe0.25-650) systems deactivate slowly and reach after 30 h 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 respec-

Figure 4: A) Dry Reforming Performance at 650oC under 100mL.min-1 flow of CO2/CH4/N2 (0.45/0.45/0.1). Methane rate consumption in molCH4.mole-1metal.s-1. GHSV=2.7x105 mL.h-1.gCat-1 for all catalysts. B) Rate of consumption of methane per molCH4.mole-1metal.s-1 after 1 h TOS vs the initial Nickel dispersion D (%) for catalyst reduced at 650 oC.

4

ACS Paragon Plus Environment

Page 5 of 8

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

TPO experiments of the different spent catalyst allows 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 molCoke.molCarbonConverted-1 for Ni-650 and 3.29 10-4 molCoke.molCarbonConverted-1 for Ni-850. For the two bimetallic systems reduced at 650 oC, the rate of coke formation decreases significantly with only 0.53 and 0.89 10-4 molCoke.molCarbonConverted-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.molCarbonConverted-1 for Ni0.5Fe0.5-850 and from 0.53 to 1.11 molCoke.molCarbonConverted-1 (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.

The Ni/Fe ratio of the metal particles for spent Ni0.5Fe0.5-650 and spent Ni0.75Fe0.25-650 remains within the same range with 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 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 to Ni0 and FeO. 12

We further assessed the state of the metals under reaction conditions by operando XANES experiments. After reduction under hydrogen, DRM is conducted at 650 oC 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 bimetallic systems. The trends in activity observed in operando mode (see Figure S53) are qualitatively similar to what is observed in 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, Ni650) (see Figure S20-23). The pure nickel catalyst (Ni-650), undergoes a slight decrease in the quantity of Ni0 from 98 % to 90 % after 4 hours under reaction condition, 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 TGA data. For the bimetallic systems, similar trends are observed as for Ni, although the Ni0 content is decreasing down to approximately 80 % for catalysts Ni0.75Fe0.25-650, Ni0.75Fe0.25-850 and to 70 % for Ni0.5Fe0.5-650 after 5h on stream. The presence of iron tends to favour 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 Fe K-edge 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 fittings (Fe0, FeO and Fe@Supp) as shown on Figure 5B. For Ni0.75Fe0.25-650 reduced at 650 oC, 54 % of iron is FeO and the complementary portion is incorporated in the support (vide supra). After 1 hour reaction, the FeO is levelling off at 25 %; concomitantly, the content of iron inside the support is increasing up to 75 %. Ni0.75Fe0.25-850 entails after reduction 19 % of Fe0, 37 % of 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 5h TOS, the quantity of Fe0 has decreased to 10 %. The spent catalysis was also analysed by EDX spectroscopy focused on the centre of the nanoparticles (Figure S90-93). 5

Figure 5: Operando XAS Dry Reforming experiment at 650 oC under 23mL.min-1 flow of CO2/CH4/N2 (0.45/0.45/0.1). GHSV=1.5x105 mL.h1 .gCat-1 for different catalysts. A) Spectra at Ni K-edge for catalyst Ni0.75Fe0.25-650, B) Spectra at Fe K-edge for catalyst Ni0.75Fe0.25-650. and Ni0.75Fe0.25-850 after 5h TOS C) Fe content (%) function of reaction time for catalyst Ni0.75Fe0.25-650 (full lines) and Ni0.75Fe0.25-850 (dotted lines); Black lines corresponds to LCF at Fe K-edge value for Fe@Sup or (Ni,Fe,Mg,Al)O; Blue lines corresponds to LCF at Fe K-edge value for FeO; Red lines corresponds to LCF at Fe K-edge value for FeO.

ACS Paragon Plus Environment

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Combining different spectroscopic techniques, we show that the pre-treatment temperature drastically influences the structure of the bimetallic catalysts. Increasing the reduction temperature from 650 oC to 850 oC changes the population of surface sites from Ni0/FeO to Ni-Fe/FeO but under DRM reaction, the metal species evolve towards a similar speciation. Fe0 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 one hour on stream is almost independent on the reduction temperature. It has previously been shown that FeO is effective to reduce coke formation for large nickel nanoparticle under DRM.12 Thus, independent of the reduction temperature, the deactivation rate is similar, but the initial activity is diminished when Ni-Fe alloy is present (in Ni0.75Fe0.25-850 for instance).

§

§

CONCLUSION

Bimetallic nanoparticles of size 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 generate 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 co-precipitation. However, despite their small size, the nanoparticles undergo fast deactivation via coke formation. Optimizing the ratio 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 o C) decreases the activity, which is due to the presence of a significant amount of Fe0 surface sites at the surface of the particle.

§

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental section, the powder X-ray diffractograms, FTIR of the reduced sample, HAADF-STEM-EDX results, H2 chemisorption isotherm, TPR data, XANES data, FTIR spectra for CO adsorption experiment.

AUTHOR INFORMATION

Corresponding Author * C.C.: tel, +41 44 633 93 94; e-mail, [email protected]

Funding Sources This research was funded by ETH Zürich and the Swiss National Funding (SNF) in relation with Swiss Competence Centers for Energy Research (SCCER Heat and Electricity Storage).

Notes The authors declare no competing financial interest.

§

AKNOWLEDGMENT

The authors thank the SCCER Heat and Energy Storage and ETH Zürich (ETH-57_12-2) for financial supports. 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 oC for Ni0.75Fe0.25-650 and Ni0.75Fe0.25-850

Page 6 of 8

REFERENCES

(1) Erisman, J. W.; Sutton, M. A.; Galloway, J.; Klimont, Z.; Winiwarter, W. Nature Geosci 2008, 1, 636. (2) Olah, G. A. G., Alain; Prakash,G. K. Surya 2nd ed. ed.; Wiley-VCH: Weinheim, 2011. (3) Arpe, H.-J.; Wiley-VCH: Weinheim, 2010. (4) Bitter, J. H.; Seshan, K.; Lercher, J. A. J. Catal. 1999, 183, 336. (5) Ginsburg, J. M.; Piña, J.; El Solh, T.; de Lasa, H. I. Ind. Eng. Chem. Res. 2005, 44, 4846. (6) Schulz, L. A.; Kahle, L. C. S.; Delgado, K. H.; Schunk, S. A.; Jentys, A.; Deutschmann, O.; Lercher, J. A. Appl. Catal. A 2015, 504, 599. (7) Rostrup-Nielsen, J. R. Catal. Today 1997, 37, 225. (8) Xie, T.; Shi, L.; Zhang, J.; Zhang, D. Chem. Comm. 2014, 50, 7250. (9) Cao, Y.; Lu, M.; Fang, J.; Shi, L.; Zhang, D. Chem. Comm. 2017, 53, 7549. (10) Guo, J.; Lou, H.; Zhao, H.; Chai, D.; Zheng, X. Appl. Catal. A 2004, 273, 75. (11) Karthikeyan, J.; Song, H.; Olsbye, U.; Fjellvåg, H.; Sjåstad, A. O. Top. Catal. 2015, 58, 877. (12) Kim, S. M.; Abdala, P. M.; Margossian, T.; Hosseini, D.; Foppa, L.; Armutlulu, A.; van Beek, W.; Comas-Vives, A.; Copéret, C.; Mueller, C. J. Am. Chem. Soc. 2017, 139, 1937. (13) Abdel-Mageed, A. M.; Eckle, S.; Behm, R. J. J. Am. Chem. Soc. 2015, 137, 8672. (14) Mette, K.; Kühl, S.; Tarasov, A.; Willinger, M. G.; Kröhnert, J.; Wrabetz, S.; Trunschke, A.; Scherzer, M.; Girgsdies, F.; Düdder, H.; Kähler, K.; Ortega, K. F.; Muhler, M.; Schlögl, R.; Behrens, M.; Lunkenbein, T. ACS Catal. 2016, 6, 7238. (15) Perez-Lopez, O. W.; Senger, A.; Marcilio, N. R.; Lansarin, M. A. Appl. Catal. A 2006, 303, 234. (16) Wolfbeisser, A.; Sophiphun, O.; Bernardi, J.; Wittayakun, J.; Föttinger, K.; Rupprechter, G. Catal. Today 2016, 277, Part 2, 234. (17) Xu, P.; Zhou, Z.; Zhao, C.; Cheng, Z. Catal. Today 2016, 259, Part 2, 347. (18) Baudouin, D.; Margossian, T.; Rodemerck, U.; Webb, P. B.; Veyre, L.; Krumeich, F.; Candy, J.-P.; Thieuleux, C.; Copéret, C. ChemCatChem 2017, 9, 586. (19) Lercher, J.; Jentys, A.; Steib, M.; Lou, Y. ChemCatChem 2017, DOI: 10.1002/cctc.201700686.

6

ACS Paragon Plus Environment

Page 7 of 8

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(20) Li, X.; Li, D.; Tian, H.; Zeng, L.; Zhao, Z.-J.; Gong, J. Appl. Catal. B 2017, 202, 683. (21) Baudouin, D.; Rodemerck, U.; Krumeich, F.; Mallmann, A. d.; Szeto, K. C.; Ménard, H.; Veyre, L.; Candy, J.-P.; Webb, P. B.; Thieuleux, C.; Copéret, C. J. Catal. 2013, 297, 27. (22) Gonzalez-Delacruz, V. M.; Pereñiguez, R.; Ternero, F.; Holgado, J. P.; Caballero, A. ACS Catal. 2011, 1, 82. (23) Abba, M. O.; Gonzalez-DelaCruz, V. M.; Colón, G.; Sebti, S.; Caballero, A. Appl. Catal. B 2014, 150–151, 459. (24) Margossian, T.; Larmier, K.; Kim, S. M.; Krumeich, F.; Fedorov, A.; Chen, P.; Müller, C. R.; Copéret, C. J. Am. Chem. Soc. 2017, 139, 6919. (25) Crisafulli, C.; Scirè, S.; Maggiore, R.; Minicò, S.; Galvagno, S. Catal. Lett. 1999, 59, 21. (26) Crisafulli, C.; Scirè, S.; Minicò, S.; Solarino, L. Appl. Catal. A 2002, 225, 1. (27) Pakhare, D.; Spivey, J. Chem. Soc. Rev. 2014, 43, 7813. (28) Benrabaa, R.; Boukhlouf, H.; Löfberg, A.; Rubbens, A.; Vannier, R.-N.; Bordes-Richard, E.; Barama, A. J. Nat. Gas Chem. 2012, 21, 595. (29) Benrabaa, R.; Löfberg, A.; Rubbens, A.; Bordes-Richard, E.; Vannier, R. N.; Barama, A. Catal. Today 2013, 203, 188. (30) Theofanidis, S. A.; Galvita, V. V.; Sabbe, M.; Poelman, H.; Detavernier, C.; Marin, G. B. Appl. Catal. B 2017, 209, 405. (31) Theofanidis, S. A.; Galvita, V. V.; Poelman, H.; Marin, G. B. ACS Catal. 2015, 5, 3028. (32) Bai, L.; Wang, X.; Chen, Q.; Ye, Y.; Zheng, H.; Guo, J.; Yin, Y.; Gao, C. Angew. Chem., Int. Ed. Engl. 2016, 55, 15656. (33) Cargnello, M.; Doan-Nguyen, V. V. T.; Gordon, T. R.; Diaz, R. E.; Stach, E. A.; Gorte, R. J.; Fornasiero, P.; Murray, C. B. Science 2013, 341, 771. (34) Margeat, O.; Ciuculescu, D.; Lecante, P.; Respaud, M.; Amiens, C.; Chaudret, B. Small 2007, 3, 451. (35) Broda, M.; Kierzkowska, A. M.; Baudouin, D.; Imtiaz, Q.; Copéret, C.; Müller, C. R. ACS Catal. 2012, 2, 1635. (36) Sinfelt, J. H.; Taylor, W. F.; Yates, D. J. C. J. Phys. Chem. 1965, 69, 95. (37) Busca, G.; Lorenzelli, V. Mater. Chem. 1982, 7, 89. (38) Poncelet, G.; Centeno, M. A.; Molina, R. Appl. Catal. A 2005, 288, 232. (39) Mihaylov, M.; Ivanova, E.; Chakarova, K.; Novachka, P.; Hadjiivanov, K. Appl. Catal. A 2011, 391, 3. (40) Foppa, L.; Silaghi, M.-C.; Larmier, K.; Comas-Vives, A. J. Catal. 2016, 343, 196.

7

ACS Paragon Plus Environment

EDS Quantitative Results Element Wt% At% FeK 32.84 33.96 NiK 67.16 66.04

ACS Catalysis TOC: 20.00nm

ZC

500000x Matrix: 512x400 Data Type: ZC(ADC) Magnification: 1000000x Image Size: 0.0001x0.0001mm Reso:134.3 Amp.T:102.4 kV: 200.0 16:16:00 Tilt: 0

C:\USERS\FRANKK\28JAN16\TM345BIS_ZC+EDXS-4A.SPC kV:200.0

Dry Reforming of Methane Ni-Fe Colloids

Tilt:0.00

FS : 248

2.5

Ni

0.7 F 5 e 0.2 5

2.0

0.5

alys

t

R

Embrace stability High activity Ni reference catalyst

0.0 0

10 20 30 Time on stream (h)

Fe Ni

R

Det:SUTW

Characterization

Prst:None

28-Jan-2016

HAADF STEM with EDX Ni Ni/Fe = 3.4 ± 1! Ni Fe Fe

At 850oC

R

Ni

R

10.00nm

ZC

Fe

Fe

Cu

C:\EDAX32\GENESIS\GENMAPS.SPC

C

R

kV:200.0

Tilt:0.00

FS : 209

R

Ni c ata lyst

1.5 1.0

cat

R

Tkoff:3.62

LSec : 53.9

Cu Ni

Tkoff:3.62

LSec : 110.1

Det:SUTW

Prst:None

Reso:134.3

28-Jan-2016

Amp.T:102.4

14:14:23

Cu Cu

O Al Mg Ni 0.80 Cu

Si

10 nm 2.40

1.60

3.20

4.00

4.80

5.60

6.40

7.20

keV

keV 6.4 7.2. keV

6.40 7.20

De-alloyed Ni/Fe = 7.6 ± 2! Ni High Ni(0) dispersion and FeO

At 650oC Normalized µ(E)

-1. -1

CH4 consumption(molCH4.molmetal s )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 8

O

C

Ni

FeO

Mg(Al)O

0.80

Ni

1.5 Al

Mg

Time

1.0 0.5 10 nm 1.60

2.40

0.0 8320

3.20

4.00

Fe Fe 4.80

Cu Cu

Operando XAS keV 6.40 7.20 5.60

6.40

7.20

8340 8360 8380 Energy (eV)

keV

8400

8

ACS Paragon Plus Environment