Molecularly Tailored Nickel Precursor and Support Yield a Stable

Article pubs.acs.org/JACS

Molecularly Tailored Nickel Precursor and Support Yield a Stable Methane Dry Reforming Catalyst with Superior Metal Utilization Tigran Margossian,† Kim Larmier,† Sung Min Kim,‡ Frank Krumeich,† Alexey Fedorov,†,‡ Peter Chen,† Christoph R. Müller,‡ and Christophe Copéret*,† †

Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir-Prelog-Weg 1-5, CH-8093 Zürich, Switzerland Department of Mechanical and Process Engineering, ETH Zürich, Leonhardstrasse 21, CH-8092 Zürich, Switzerland

‡

S Supporting Information *

ABSTRACT: Syngas production via the dry reforming of methane (DRM) is a highly endothermic process conducted under harsh conditions; hence, the main difficulty resides in generating stable catalysts. This can, in principle, be achieved by reducing coke formation, sintering, and loss of metal through diffusion in the support. [{Ni(μ2-OCHO)(OCHO)(tmeda)}2(μ2OH2)] (tmeda = tetramethylethylenediamine), readily synthesized and soluble in a broad range of solvents, was developed as a molecular precursor to form 2 nm Ni(0) nanoparticles on alumina, the commonly used support in DRM. While such small nanoparticles prevent coke deposition and increase the initial activity, operando X-ray Absorption Near-Edge Structure (XANES) spectroscopy confirms that deactivation largely occurs through the migration of Ni into the support. However, we show that Ni loss into the support can be mitigated through the Mg-doping of alumina, thereby increasing significantly the stability for DRM. The superior performance of our catalytic system is a direct consequence of the molecular design of the metal precursor and the support, resulting in a maximization of the amount of accessible metallic nickel in the form of small nanoparticles while preventing coke deposition.

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INTRODUCTION Syngas is a key feedstock for the production of hydrogen and hydrocarbons, making reforming an essential process of the petrochemical industry.1,2 Steam reforming of methane (SRM) leads to a syngas with a high content of hydrogen (H2/CO ratio of 3, eq 1). The dry reforming of methane (DRM), in which steam is replaced by carbon dioxide, leads to a H2/CO ratio of 1 (eq 2). This latter process converts CO2 and CH4 into two valuable molecules and provides a way to tune the H2/CO ratio of syngas depending on the final application (Fischer−Tropsch or methanol synthesis for instance).3,4

reverse Boudouard reaction (eq 4), which is particularly severe for large nanoparticles.5,6 Deactivation by coke deposition can be reduced in part by metal doping of the active phase.7−10 An alternative approach is to reduce the size of the nickel nanoparticles down to 2 nm, which also partly alleviates whisker formation or the deposition of carbon while increasing the activity because of a better metal dispersion.5,11 Methane Cracking: CH4 → C + 2H 2 ,

(3)

Reverse Boudouard Reaction:

Steam Reforming of Methane: CH4 + H 2O → CO + 3H 2 ,

ΔH 0 298K = +75 kJ·mol−1

2CO → CO2 + C,

ΔH 0 298K = +207 kJ·mol−1

(4)

However, obtaining small nickel nanoparticles on a suitable support for dry reforming is still challenging.12,13 In particular, while the strong interaction of Ni(0) and alumina support increases the stability of smaller particles toward sintering, it also leads to the formation of inactive nickel aluminate phase during synthesis or under dry reforming conditions via migration of Ni from the particle to the support.14 Tuning the support, for instance by inserting magnesium into the

(1)

Dry Reforming of Methane: CH4 + CO2 → 2CO + 2H 2 , ΔH 0 298K = +247 kJ·mol−1 (2)

Since the DRM is highly endothermic, catalyst stability is important, in addition to low cost and high activity. Aluminasupported nickel nanoparticles are suitable candidates because of their low cost and high initial activity, but they commonly suffer from rapid deactivation. One major issue is carbon deposition (whisker formation) via methane cracking (eq 3) or © 2017 American Chemical Society

ΔH 0 298K = −172 kJ·mol−1

Received: February 15, 2017 Published: April 26, 2017 6919

DOI: 10.1021/jacs.7b01625 J. Am. Chem. Soc. 2017, 139, 6919−6927

Article

Journal of the American Chemical Society

Information, Figure S1). In case of Al2O3−500, the surface OH density was reported previously to be 1.1 mmolOH/gsupport.32 [{Ni(μ2-OCHO)(OCHO)(tmeda)}2(μ2-OH2)] (1). NiCO3 (4.0 g, 34 mmol) was vigorously stirred with 100 mL of deionized water. An excess of formic acid (10 mL, 277 mmol) was added to the mixture. After 1 h, a pale-green solution was formed. The solvent was removed under reduced pressure and the reaction mixture was dried (40 mbar), treated with 100 mL of absolute ethanol and dried again under vacuum (10−2 mbar) for 16 h, yielding 6.1 g (95%) of Ni(OCHO)2·2H2O. Ni(OCHO)2·2H2O (4.0 g, 22 mmol) was stirred in 150 mL of deionized water for 40 min at 40 °C. Tetramethylethylenediamine (3.8 mL, 25 mmol, 1.14 equiv) was quickly added to the resulting pale green homogeneous solution. The solution became dark blue and a precipitate formed. After 15 min, the reaction mixture was dried under reduced pressure (40 mbar). Ethanol (100 mL) was added to the palegreen solid, the resulting reaction mixture stirred for 10 min and the solvent evaporated to dryness under reduced pressure. The residue was then extracted in 200 mL of toluene and filtered through Celite to remove unreacted Ni(OCHO)2·2H2O. Concentrating the filtrate and storing at −38 °C gave large turquoise crystals of 1 suitable for X-ray crystallography. CCDC 1522535 contains the supplementary crystallographic data, and the CIF file is provided as Supporting Information. Two successive recrystallizations from toluene yielded 5.1 g of 1 (82%). Anal. Calcd (%) for C16H38Ni2N4O9: C = 35.08%, H = 6.99%, N = 10.23%. Found: C = 35.18%, H = 7.03%, N = 10.12%. IR (KBr): 3020, 2986, 2889, 2843, 2817, 2791, 2780, 2733, 2696, 2087,1647, 1561, 1464, 1366 cm−1. 1H NMR (250 MHz, THF-d8): δ = 27.0, 74.3, 79.9, 88.8, 104.0 ppm. Catalyst Synthesis. Impregnation (I) of Ni(NO3)2 on Al2O3 in Water: NiNO3/Al2O3(IH2O). γ-Al2O3 (2 g) was treated by IWI with 1.2 mL of an aqueous solution of Ni(NO3)2 (0.426 M). The material was dried at 120 °C (1 °C·min−1) for 12 h in a flow of synthetic air (80 mL·min−1). The nominal nickel loading is 1.0 wt%. Impregnation of 1 on Al2O3 in Water: 1/Al2O3(IH2O). γ-Al2O3 (2 g) was treated by IWI with 1.2 mL of an aqueous solution of 1 (0.426 M) that was preheated to 50 °C for 40 min to increase the solubility. The material was dried at 120 °C (1 °C·min−1) for 12 h in a flow of synthetic air (80 mL·min−1). The nominal nickel loading is 1.0 wt%. Impregnation of 1 on Al2O3−500 in THF: 1/Al2O3(ITHF). γ-Al2O3−500 (2 g) was treated by IWI with 1.2 mL of solution of 1 (0.426 M) in THF. The material was dried under high vacuum (10−5 mbar) at room temperature for 16 h. The nominal nickel loading is 1.0 wt%. Specific Adsorption of 1 on Al2O3−500 from THF: 1/Al2O3(ATHF). Complex 1 (65 mg, 0.12 mmol, 0.11 equiv) was dissolved in THF (20 mL) and contacted with 1.0 g of Al2O3−500 (1.1 mmol AlOH, 1 equiv). The reaction mixture was gently stirred for 2 h at room temperature. The solid was washed three times with 20 mL of THF and dried under vacuum (10−5 mbar) for 16 h to yield a light green solid. IR: 3669, 3548, 2980, 2881, 2809, 2791,1662, 1610, 1470, 1394, 1368 cm−1. Elemental analysis: Ni = 1.40%, C = 2.01%, H = 0.71%, N = 0.51%, which corresponds to the molar ratio of N/Ni = 1.4 and C/ Ni = 7. Specific Adsorption of 1 on Mg@Al2O3−500 from THF: 1/Mg@ Al2O3(ATHF). The material was prepared as described above but using Mg@Al2O3−500 as a support instead of Al2O3−500 and 74 mg of 1 (0.14 mmol). A light green solid was obtained. Nominal loading: Ni = 1.6 wt%. IR: 3539, 3018, 2979, 2878, 2850, 2806, 2730, 1609, 1469, 1371 cm−1. Reduction of the Impregnated Materials To Give Supported Nanoparticles. The dried samples were reduced under a flow of pure hydrogen at 650 °C (1 °C·min−1) for 8 h. After outgassing under high vacuum (10−5 mbar), the catalysts were stored in an Ar atmosphere in a solvent-free glovebox. The code 2 in the numbering below, refers to the reduced supported nanoparticles prepared from complex 1. Reference Catalyst, Niref/Al2O3. The material was prepared by IWI using 1.2 mL of aqueous solution of Ni(NO3)2·6H2O (1.58 M). The as-synthesized material with a nominal 10 wt% Ni loading was dried at 110 °C overnight (ca. 12 h under static air) followed by calcination at

alumina lattice, can prevent the formation of mixed nickel and aluminum oxide.8,15−18 Thus, one challenge in designing better DRM catalysts is the generation of small Ni(0) nanoparticles without losing nickel into the support. The most common preparation methods to Ni DRM catalysts are based on the impregnation of alumina with nickel salts in aqueous media: conditions under which Ni2+ ions react with a hydrated alumina surface to form, after calcination, a poorly reducible hydrotalcite phase.19,20 The detrimental formation of mixed nickel− aluminum oxide can be partially mitigated by utilizing specific Ni precursors, such as ethylenediamine Ni complex on γalumina.19−21 However, this method is not general. Surface organometallic chemistry (SOMC) has emerged as a powerful approach to generate single site catalysts.22 SOMC can also be used to prepare supported nanoparticles through grafting tailored molecular precursors in a dry organic solvent followed by a subsequent treatment under H2, yielding small supported particles for a broad range of metals and supports.23−29 SOMC could thus be ideal to generate small Ni nanoparticles from tailored molecular precursors and supports for the preparation of dry reforming catalysts with minimal loss of Ni and improved catalytic performances. Herein we describe the development of an air-stable molecular precursor, easily prepared on a gram scale and soluble in a broad range of solvents (water, THF, toluene, and pentane), [{Ni(μ2-OCHO)(OCHO)(tmeda)}2(μ2-OH2)] (1, tmeda = tetramethylethylenediamine), which can be used to easily generate small, narrowly dispersed (2.0 ± 1.0 nm) Ni(0) nanoparticles on various supports such as γ-Al2O3 or Mg-doped alumina, providing highly active and stable catalysts. Operando X-ray Absorption Spectroscopy (XAS) demonstrates that this increase of catalytic performance of Ni supported on Mg-doped alumina arises from the formation of small Ni nanoparticles without loss of Ni in the support, neither during the preparation nor under dry-reforming conditions, thanks to the tailored supports and the use of a practical molecular complex.

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EXPERIMENTAL SECTION

General. Nickel nitrate (99.5%), aluminum nitrate (>98.5%), magnesium nitrate (99%), nickel carbonate (99.995%), and formic acid (≥96%) were purchased from Sigma-Aldrich. Deionized water was purified using a Purilab instrument (>10 MΩ·cm). THF and benzene were distilled from sodium under Ar (benzophenone used as an indicator of dryness). Impregnation of aqueous solutions was carried out in air. Impregnation or specific adsorption using dry organic solvents were carried out using a Schlenk line with Ar (grade 4.5) and 10−2 mbar vacuum. H2 was purified over activated R3-11 BASF catalyst and activated 4 Å molecular sieves (MS) prior to use. γ‑Alumina was obtained from Sasol (Puralox SBA 200, SBET = 243 m2· g−1, Vpore = 0.6 mL·g−1). For water-based impregnation routes, the supports were exposed to ambient air. All other catalysts were prepared under air-free conditions using standard Schlenk line techniques. Materials. Magnesium-doped alumina (Mg@Al2O3) was prepared via an incipient wetness impregnation (IWI) method using Mg(NO3)2·6H2O ([Mg2+] = 5.5 M). After calcination at 650 °C (5 °C·min−1) for 5 h, Mg@Al2O3 had a BET surface area of 199 m2· g−1, corresponding to a coverage of 20 Mg·nm−2.30,31 After exposure to air, all supports were dehydroxylated at 500 °C (5 °C·min−1 ramp) for 12 h in a flow of synthetic air (100 mL·min−1), degassed under high vacuum (10−5 mbar) for 1 h and then stored in a solvent-free glovebox. These supports, referred to as Mg@Al2O3−500, MgO500, and Al2O3−500, were also characterized by powder XRD (Supporting 6920


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Journal of the American Chemical Society 800 °C (5 °C·min−1) for 2 h. The catalyst was reduced at 850 °C in the fixed bed reactor for 1 h prior to the DRM test.33,34 Catalytic Dry Reforming of Methane. The DRM reaction was carried out in a fixed-bed quartz reactor (400 mm length, 12.6 mm internal diameter). A typical experiment was performed with 50 mg of catalyst. The catalysts were first reduced in situ in 10 vol% H2/N2 at 650 °C for 1 h. Subsequently, the bed was purged with N2 (100 mL·min−1) for 10 min, and then the feed gas was introduced. The total flow rate of the feed gas was 100 sccm (GHSV of 108 000 mL·gcat−1·h−1; composition: 45% CH4, 45% CO2 and 10% N2). The composition of the off-gas at the outlet was analyzed with a micro-GC (Thermo Scientific) identifying the following products: CH4, CO2, H2, CO and N2 (thermal conductivity detector (TCD); 5 Å MS and U‑plot column cartridges). Physicochemical Characterization. Elemental Analysis. The metal content in each catalyst was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES). The ICP-OES measurement were carried out by Pascher laboratories. Scanning Transmission Electron Microscopy (TEM/STEM). Prior to TEM analysis, all samples were oxidized by the slow diffusion of air to the catalysts under argon. After oxidation, materials were dispersed in ethanol and a droplet of the suspension was deposited on a lacey carbon foil supported on a copper grid. Dark-field TEM images were recorded on a Philips CM12 microscope operated at 120 kV or on a Jeol 2100F microscope operated at 200 kV. Particle size distribution (PSD) was determined by analyzing 100 nanoparticles and fitting a normal distribution (particle size = mean value ± standard deviation). Scanning transmission electron microscopy images were recorded on an aberration-corrected Hitachi HD2700CS microscope with a highangle annular dark field detector (HAADF STEM). Energy-Dispersive X-ray (EDX) spectra were measured with a detector attached to the HD2700CS. PSDs and EDX spectra are shown in the Supporting Information (SI). The Ni nanoparticles appear bright in the HAADFSTEM images as Ni has the highest atomic number (Z contrast) among the elements present, which was also confirmed by EDX spectroscopy. N2 Adsorption. Nitrogen adsorption isotherms at −196 °C were recorded on a Bel-Mini apparatus (Bel-Japan). Prior to the measurement, the samples were outgassed under vacuum (ca. 10−3 mbar) at 350 °C for 2 h. The BET method was applied to calculate the total surface area.35 H2 Chemisorption. Chemisorption experiments were carried out on a BELSORB-max apparatus (Bel-Japan). Approximately 200 mg of reduced catalyst was loaded in an airtight cell inside a glovebox which was then mounted on the apparatus. The sample was outgassed at 350 °C for 3 h at 10−6 mbar. The chemisorption measurements were performed at 25 °C, the pressures at equilibrium were recorded when the pressure variation was below 0.01% per minute. The uptake of hydrogen was calculated by fitting the isotherm with the Langmuir isotherm model. Temperature-Programmed Reduction (TPR) Studies. TPR experiments were carried out using a BELCAT-B apparatus (Bel Japan) equipped with a homemade cell allowing the transfer of the sample from a glovebox to the instrument without exposure to air. The dried sample (250 mg) was introduced into the cell inside a glovebox. Prior to opening the cell mounted in the apparatus, the inlet and outlet were purged with argon for 30 min using a bypass system. The measurement was performed from 25 to 900 °C in the gas flow consisting of 6% H2/94% He using a ramp of 1 °C·min−1. The gas released was analyzed using a calibrated TCD detector and a massspectrometer (Bel-Mass). CO Adsorption Monitored by FTIR Spectroscopy. The experiment was conducted in a 200 mL glass reactor with IR-transparent CaF2 windows. A thin pellet of the sample was pressed in a glovebox and loaded to the reactor using a sample holder. A spectrum was recorded in an argon atmosphere. The reactor was outgassed (10−5 mbar) for 5 min at room temperature. Subsequently, 100 mbar of CO were introduced to the reactor and a second spectrum was recorded. The reactor was outgassed and a third spectrum was recorded under vacuum (10−5 mbar). Spectra were recorded in a transmission mode

on a Nicolet 6700 FTIR spectrophotometer using 64 scans at a resolution of 2 cm−1. In Situ Deposition of Complex 1 Followed by 1H NMR Analysis. The spectra were recorded on a 250 MHz Bruker spectrometer. Al2O3−500 (85 mg) was introduced to a J Young tube. A solution containing complex 1 (8 mg, 15 μmol) and FeCp2 (6 mg, 33 μmol, internal standard) in 0.4 mL of THF-d8 was prepared in a J Young tube inside a glovebox. Spectra were recorded using the following settings: 256 scans, d1 = 0.05 s, bandwidth = 300 ppm for paramagnetic samples and 16 scans, d1 = 20 s, bandwidth = 25 ppm for diamagnetic samples. Ultraviolet−Visible (UV−vis) Spectroscopy. UV−vis spectra were recorded with a resolution of 1 nm in transmission mode on an Agilent Technologies, Cary Series spectrometer using the corresponding solvent as a reference. Diffuse reflectance UV−vis spectra of the solids were recorded with a resolution of 1 nm on the same spectrometer, using KBr as a reference. A sample was loaded to an airtight cell inside a glovebox. To convert the reflectance data to a signal proportional to the absorption, the diffuse reflectance spectra were processed using Kubelka−Munk theory. Operando Dry Reforming Quick X-ray Absorption Spectroscopy (qXAS). qXAS at the Ni K-edge was measured at the SuperXAS beamline at the Swiss Light Source (SLS; Paul Scherrer Institute, Villigen, Switzerland). The SLS is a third-generation synchrotron operating at a 2.4-GeV electron energy and a current of 400 mA. The incident beam was collimated by a Si-coated mirror at 2.8 mrad, monochromatized using a double crystal Si(111) monochromator, and focused with an Rh-coated toroidal mirror (at 2.8 mrad) down to 100 × 100 μm with a beam intensity of (4−5) × 1011 ph/s. XAS spectra were collected in the transmission mode using ion chambers filled with He−N2 gas mixtures. Multiple X-ray adsorption scans (8000−9200 eV) were averaged with a scan time of 5 min. The DRM reaction was carried out in a fixed-bed capillary quartz reactor (40 mm length, 3 mm internal diameter). A typical experiment was performed with 10 mg of catalyst. The catalysts were first reduced in situ in 5 vol % H2/N2 at 650 °C for 1 h. Subsequently, the feed gas was introduced with a total flow rate of 28 sccm (GHSV of 151200 mL·gcat−1·h−1; composition: 45% CH4, 45% CO2 and 10% N2). Water was removed from the outlet flow using a membrane separator (Genie 170 technologies). The composition of the off-gas at the outlet was analyzed with a micro-GC (Agilent Technologies) identifying the following products: CH4, CO2, H2, CO and N2 (thermal conductivity detector (TCD); 5 Å MS and U-plot column cartridges). Scans were recorded every 30 min during the reduction and the catalytic reaction. Reference NiAl2O4 and NiO oxides were pressed into wafers and sealed in Kapton tape. All spectra were calibrated with respect to the spectrum of a Ni reference foil recorded simultaneously by setting the first inflection point to 8333 eV (Ni K-edge). X-ray absorption near edge structure (XANES) spectra (8320−8420 eV) of the supported nanoparticles were fitted using a linear combination fitting of Ni foil and NiAl2O4 references. Extended X-ray absorption fine structure (EXAFS) data were fitted in the R-space (1−4 Å) after a Fourier transform (k = 1−11 Å−1) using a k weight of 1, 2, and 3 for complex 1 and using a k weight of 1, 2, and 3 for the specifically adsorbed species. Powder X-ray Diffraction (PXRD). The PXRD measurements were performed using a Bruker-AXS “D8 Advance” diffractometer using Cu Kα monochromatic radiation (λ = 1.5418 Å). The XRD patterns were recorded between 10 and 80° (2θ with a step size of 0.033°). Single-Crystal X-ray Diffraction (XRD). The data were collected on a Bruker D8 Venture diffractometer equipped with a CCD area detector using Mo Kα radiation. The crystal was placed in Paratone and mounted in the beam under a flow of nitrogen at 100 K. An empirical absorption correction was performed with SADABS-2008/1 (Bruker). The structure was solved with SHELXL36 using intrinsic phasing followed by a least-squares refinement (SHELXL-97) using the OLEX 2−1.2 suite of programs.37 The non-hydrogen atoms were refined anisotropically. The hydrogen atoms were placed at the calculated positions (see the Supporting Information for details). 6921


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RESULTS AND DISCUSSION Synthesis of [{Ni(μ2-OCHO)(OCHO)(tmeda)}2(μ2-OH2)], 1. Complex 1 was synthesized in 82% yield by reacting ca. 1.1 equiv of tetramethylethylenediamine (tmeda) in water with nickel formate, prepared from NiCO3 (see Experimental Section for details). The synthesis can be carried out on a multigram scale to give pure crystalline complex 1 that is very soluble in a broad range of solvents: 15 g·L−1 in water, 115 g· L−1 in toluene, and 170 g·L−1 in THF at 25 °C. Complex 1 was characterized by X-ray crystallography (Figure 1). It crystallizes

Figure 1. Crystal structure of complex 1. Thermal ellipsoids were set at 50% probability level and except for μ-H2O, hydrogen atoms and the co-crystallized water molecule are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ni1−O1 = 2.049(2), Ni1−O2 = 2.099(1), Ni1−O3 = 2.097(2), Ni1−O4 = 2.022(2), Ni1−N1 = 2.168(2), Ni1−N2 = 2.148(2), Ni2−O7 = 2.030(2), Ni2−N4 = 2.178(2), Ni1−O2−Ni2 = 117.15(8), O1−Ni1−O2 = 89.29(8).

as a dimer with octahedral Ni centers connected by three μ2 bridges (two formate anions and a water molecule). Each nickel atom is independently connected to another κ1 formate ligand and one κ2 tmeda. Preparation and Characterization of Supported Nickel Nanoparticles. Using a conventional and prototypical preparation route, i.e., incipient wetness impregnation (IWI) of an aqueous solution of nickel nitrate followed by reduction at 650 °C in a hydrogen flow, yields nanoparticles with an average diameter of 4 nm and a comparatively broad size distribution of ±2.5 nm (Figure 2a). In contrast, the size of the nanoparticles is smaller (3.4 ± 1.5 nm) when using precursor 1 (IWI of aqueous solution, Figure 2b). Changing from water to THF during the deposition step via IWI allows for the formation of even smaller nanoparticles with a narrower distribution (2.9 ± 1.2 nm, Figure 2c). Hence, avoiding water clearly improves the PSD of the nickel particles obtained after reduction. Finally, we performed the specific adsorption of complex 1 in a THF solution on γ-alumina. The nanoparticle size distribution obtained with this method was further improved to 2.0 ± 1.0 nm according to the dark-field STEM images (Figure 2d). Thus, tuning the precursor, the conditions and the deposition methods allows for controlling the particle size and distribution. The XRD patterns of the materials do not show any specific signature due to the small Ni nanoparticle size, consistent with the STEM results (Figure S1). Transmission IR spectra of the reduced nanoparticles (Figure S2) contain no C−H frequencies indicating that the Ni surface is free from remaining organic ligands. The metal dispersion was further determined by hydrogen chemisorption at 25 °C (Table 1 and isotherm in Figure S3). We observe that a decrease in the particle size of Ni, as determined by microscopy, correlates with an increase in the hydrogen uptake from 1.3 ± 0.1 mmol H2·g Ni−1 for NiNO3/Al2O3(IH2O) to 2.8 ± 0.2 mmol H2·g Ni−1 for 2/Al2O3(ATHF). Accordingly, the metal dispersion increases as

Figure 2. Particle size distribution (upper panels) as determined by HAADF-STEM (lower panels) for alumina-supported catalysts: (a) NiNO3/Al2O3(IH2O), (b) 2/Al2O3(IH2O), (c) 2/Al2O3(ITHF), and (d) 2/Al2O3(ATHF).

the metal particle size decrease from D = 15% for NiNO3/Al2O3(IH2O) to D = 33% for 2/Al2O3(ATHF) (Table 1). The fraction of Ni(0) in the reduced samples 2/Al2O3(ATHF) and NiNO3/Al2O3(IH2O) was further assessed by in situ XANES spectroscopy at 650 °C under H2 flow (Figure 3). The XANES spectrum of NiNO3/Al2O3(IH2O) contains a white line with a normalized adsorption value that is 20% higher than that of the Ni foil reference, indicative of the presence of partially oxidized nickel species. This is consistent with the high-temperature peak (960 °C) in the TPR spectrum of NiNO3/Al2O3(IH2O) due to the formation of the mixed metal aluminate compounds, which are difficult to reduce even under harsh conditions (Figure S8). Linear combination fitting of the XANES data for NiNO3/Al2O3(IH2O) using the Ni foil and NiAl2O4 references (Figure S9) shows that only 60% of total nickel was reduced at 650 °C under an H2 flow. In contrast, the XANES spectrum of 2/Al2O3(ATHF) (Figure 3, b) is very similar to that of the Ni foil reference, displaying identical pre-edge (8333 eV) and edge (8345 eV) positions.38 Linear combination fitting of the XANES spectrum of 2/Al2O3(ATHF) (Figure S9) indicates that >97% of nickel is reduced to Ni(0) in this sample. Next, the reduced Ni surface was probed by CO adsorption.39 The IR spectra plotted in Figure S13 were obtained by subtracting the spectrum of the as-reduced catalyst (a, b) or a respective 6922


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Journal of the American Chemical Society Table 1. Characterization of the As-Prepared Supported Nanoparticle Catalysts entry

impregnated material

1 2

Ni(NO3)2/Al2O3 (ref) Ni(NO3)2/Al2O3(IH2O)

3

1/Al2O3(IH2O)

4 5 6

1/Al2O3(ITHF) 1/Al2O3(ATHF) 1/Mg@Al2O3(ATHF)

reduced material

support

method

nickelc (wt%)

particle sized (nm)

H2 uptakee (mmol H2·g Ni−1)

Df (%)

γ-Al2O3 γ-Al2O3

IWI IWI

10 0.92

15.0 ± 7.0 4.0 ± 2.5

0.8 ± 0.1 1.3 ± 0.1

9 15

2/Al2O3(IH2O)

γ-Al2O3

IWI

1.10

3.5 ± 1.5

2.0 ± 0.1

24

2/Al2O3(ITHF) 2/Al2O3(ATHF) 2/Mg@Al2O3(ATHF)

γ-Al2O3a γ-Al2O3a Mg@Al2O3a

IWI S.Ads.b S.Ads.b

1.05 1.40 1.60

2.9 ± 1.2 2.0 ± 1.0 1.3 ± 0.7

2.2 ± 0.2 2.8 ± 0.2 3.0 ± 0.2

26 33 36

ref

Ni /Al2O3 NiNO3/Al2O3(IH2O)

Dehydroxylated at 500 °C. b“S.Ads.” stands for specific adsorption. cNickel loading determined by elemental analysis. dMeasured by HAADF/ STEM. eH2 chemisorption uptake at 25 °C using a Langmuir model. fDispersion = surface Ni/total Ni assuming that one H-atom adsorbs on each surface Ni (details in Supporting Information). The amount of total Ni was determined by elemental analysis. a

Figure 4. Schematic drawing of samples: (a) NiNO3/Al2O3(IH2O) and (b) 2/Al2O3(ATHF).

assess the coordination of the Ni2+ center of 1 on the alumina surface (Figure 5). The UV−vis spectrum of 1/Al2O3(IH2O) is Figure 3. In situ XANES spectrum at the Ni K-edge under hydrogen flow (650 °C, 1 bar): (a) NiNO3/Al2O3(IH2O), (b) 2/Al2O3(ATHF), (c) Ni foil reference, and (d) NiAl2O4 reference.

reference materials (c, d) from the spectrum recorded in a CO atmosphere. The alumina support and NiAl2O4 reference material bind CO (frequency at ν = 2209 cm−1 for Al2O3−700 and at ν = 2173 cm−1 for NiAl2O4). The former band is also present in NiNO3/Al2O3(IH2O) and 2/Al2O3(ATHF). Multiple bands in the range 1740 cm−1 to 2120 cm−1 are assigned to different coordination sites and vibration modes of CO on metallic Ni nanoparticles.40,41 Assuming that the contribution of the bands that are due to the binding of CO on alumina is constant for different catalysts, the fraction of Ni(0) bands increases with decreasing particle size (Table S2). No characteristic blue-shifted band due to the Ni(II)-CO stretching vibration around 2173 cm−1 is observed for NiNO3/Al2O3(IH2O) as might be anticipated from the incomplete reduction of this material, suggesting migration of Ni into the bulk of the support and in line with its low reducibility. Thus, the analyses presented above demonstrate that deposition of the precursor using organic solvents leads to nanoparticles with a decreased mean size and narrow PSD down to 2.0 ± 1.0 nm (2/Al2O3(ATHF)). The specific adsorption route also mitigates the migration of nickel into the alumina matrix and the formation of nonreducible NiAl2O4 species, affording >95% of reduced Ni(0) (Figure 4). In general, the method discussed here gives smaller and more narrowly dispersed nanoparticles than those reported previously from a strongly adsorbed nickel ethylenediamine complex (2−7 nm).21,42 Investigation of the Specific Adsorption of Ni Complex 1. Since the preparation route has a large influence on the size of the particles, we investigated the interaction of 1 with the surface of γ-alumina. UV−vis spectroscopy was used to

Figure 5. Background-subtracted UV−vis spectra of materials 1/Al2O3(IH2O) (a), 1/Al2O3(ATHF) (b), and 1 in THF solution (c).

very similar to that of the molecular complex 1 showing bands at 388 and 665 nm, that can be assigned to the d-d transition of the octahedral Ni2+(d8) (Figure 5).43 In contrast, the specific adsorption of complex 1 onto alumina from dry THF, (b) on Figure 5, produces material with bands at 404 and 703 nm. While Ni2+ sites remain octahedral, the bands are red-shifted compared to the bands in 1. This can be explained by a change in the nickel environment for a weaker field ligand such as Al‑OH.21 Fits of the EXAFS spectra of 1/Al2O3(ATHF) presented in Figure S19 confirm the presence of the octahedral Ni2+ center. A nickel−nickel path allows fitting the feature centered at 3.10 Å thereby demonstrating that 1 retains its dimeric structure at the surface of alumina (Figure 6). Complete structure investigation of 1/Al2O3(ATHF) is described in the SI (Figures S14−S20 and Table S3). 6923


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Dry Reforming of Methane. The performances of the catalysts prepared under DRM conditions were examined in a fixed-bed flow reactor. The tests were performed at 650 °C under kinetic regime, far from the equilibrium conversion (thermodynamic XCH4 = 57%) with methane conversion below 40% and far from a diffusion regime. Figure 7A shows the rate of methane consumption (in molCH4·molNi−1·s−1 (rCH4)) after 0.5 and 20 h on stream. With a dispersion of 10% and 15%, Niref/Al2O3 and NiNO3/Al2O3(IH2O) have an initial activity of 0.68 and 0.94 molCH4·molNi−1·s−1. Further increasing the dispersion enhances the initial activity to 1.45 molCH4·molNi−1·s−1 for 2/Al2O3(IH2O) (D = 24%) and 1.40 molCH4·molNi−1·s−1 for 2/Al2O3(ITHF) (D = 25%). The smallest supported nickel nanoparticles have the greatest initial activity of 1.79 molCH4· molNi−1·s−1 for 2/Al2O3(ATHF) (D = 33%). Thus, we observe that the initial rate of consumption of methane increases linearly with the nickel dispersion (Figure 8). This clearly points to Ni(0) being the active phase in DRM in line with our

Figure 6. Proposed structure of a surface-coordinated complex of 1 on Al2O3−500.

Overall, the spectroscopic data is consistent with a ligand exchange reaction that replaces tmeda in 1 by two Al−OH and/or Al−O−Al weak-field ligands and retaining the dimeric structure and the octahedral geometry of Ni centers. On the other hand, impregnation procedures lead mostly to the precipitation of 1 on the support.

Figure 7. (A) Dry reforming performance of the catalysts prepared at 650 °C using 100 mL·min−1 of CO2/CH4/N2 (0.45/0.45/0.1), 1 μmol of Ni (25−50 mg) of catalyst, GHSV = 98 400 mL·h−1·gcat−1. Rate of methane consumption in molCH4·molNi−1·s−1 for 0.5 and 20 h TOS (left axis) and coke formed·(carbon converted−1) in % for the different catalysts (right axis, blue column). (B) Operando XANES dry reforming experiment: (1) XANES spectra recorded during the operando experiment for 2/Al2O3(ATHF). (2) Performance of the catalysts prepared at 650 °C using 23 mL·min−1 of CO2/CH4/N2 (0.45/0.45/0.1), 0.25 Ni μmol (5−10 mg) of catalyst, GHSV = 98 400 mL·h−1·gcat−1. Rate of methane consumption in molCH4·molNi−1·s−1 for the line/markers (left axis) and Ni(0) content (%) for the line and dotted line (right axis). (3) XANES spectra recorded during the operando experiment for 2/Mg@Al2O3(ATHF). 6924


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determined by linear combination fitting of the XANES Ni K‑edge spectra (using Ni foil and NiAl2O4 references, Figure S27) and plotted against time on stream (TOS) (Figure 7B2). After reduction (t = 0 h), 2/Al2O3(ATHF) have a Ni(0) content above 95%. In an early stage of the reaction (t = 0.17 h) the XANES spectrum of 2/Al2O3(ATHF) shows already some change with a white line intensity increasing by 10% attributed to the formation of NiAl2O4. The loss in activity clearly parallels the decrease in Ni(0) content with TOS as it is converted into NiAl2O4. These data are also consistent with Ni(0) being the active species in DRM. The main deactivation mechanism for this sample is thus the oxidation/migration of Ni(0) into the alumina lattice. Similar experiment was performed on NiNO3/Al2O3(IH2O), which shows that in this case NiAl2O4 is initially present after reduction and the Ni(0) content does not significantly diminish over the course of the reaction (Figures S28 and S29) while the activity drops. Thus, in this case, deactivation is mostly linked to the coke formation. Overall, two distinct deactivation mechanisms come into play in this set of samples. Larger nickel particles deactivate mostly by coke formation (NiNO3/Al2O3(IH2O)). Smaller nickel nanoparticles (2/Al2O3(ATHF)), while allowing for an increased initial reaction rate and lower coking, deactivate due to the strong interactions between the metal and the support, leading to the migration of Ni into the support to generate nickel aluminate. Indeed, it was possible to regenerate after 10 h on stream the activity of the samples with a large particle size by an oxidative/reductive treatment. This is expected as the main deactivation mechanism is coke formation. However, this regeneration treatment is not successful for the highly dispersed sample (2/Al2O3(ATHF)); it decreases further the activity, possibly due to the increased formation of nickel-aluminate-like species (Figure S22). Tailoring the Support and the Catalyst Performance. In order to prevent the migration of the metal into the support deactivating the “smaller” nanoparticles, we introduced Mg2+ ions in the lattice of alumina by impregnating magnesium nitrate solution on γ-alumina followed by calcination.30,31 After calcination, the diffraction pattern resembles that of the starting γ-Al2O3, although the peaks are shifted by 1° toward lower angles with main peaks at 36.5°, 45.4°, and 66.2° (Figure S1). These data indicate an expansion of the lattice, likely due to the insertion of Mg2+ ions into the structure.45 No segregation of extended MgO domains could be found neither by TEM mapping (Figure S30) nor by PXRD (Figure S1); hence this procedure likely generates a homogeneous MgAl2O4-like layer on the alumina. The deposition of 1 on Mg@Al2O3 pretreated at 500 °C under synthetic airflow was carried out by the specific adsorption procedure, leading to a similar surface coordinated complex as on the alumina surface (Figures S31−S33). After reduction of the Ni2+ surface species under a flow of hydrogen, 1.3 ± 0.8 nm nickel nanoparticles were observed on Mg@Al2O3 surfaces (Figure S34, metal loading of 1.6%, respectively) containing 94% of Ni(0) according to XANES (Figure S35). The thus-obtained 2/Mg@Al2O3(ATHF) displayed an initial activity of 1.92 molCH4·molNi−1·s−1 slightly higher than its pure aluminum counterpart, 2/Al2O3(ATHF), in line with the slightly higher metal dispersion (see Figure 8). More importantly, the magnesium-doped alumina supported

Figure 8. Rate of consumption of methane per molCH4·molNi−1·s−1 after 0.5 h TOS vs the initial metal dispersion D (%).

recent study. 10 These data contradict recent proposal describing four coordinated Ni2+ as active sites in DRM.44 From the slope of the plot rCH4 = f(D), the initial turnover frequency for the consumption of methane per nickel surface atom is estimated to 5.5 s−1 (at 650 °C). Thus, increasing the dispersion clearly enhances the (initial) rate of DRM per gNi. In this respect the catalyst with the higher dispersion prepared by the specific adsorption route from THF 2/Al2O3(ATHF) is the most efficient. However, this sample undergoes a rapid deactivation after several hours on stream and displays very poor performance after 20 h on stream with an activity loss of 95% (0.08 molCH4·molNi−1·s−1). Similarly, Niref/Al2O3 and NiNO3/Al2O3(IH2O) undergo a significant deactivation with a performance loss of 75%, yielding an activity of 0.25 molCH4·molNil−1·s−1 after 20 h. Nickel catalysts with an intermediate dispersion2/Al 2 O 3 (I H 2 O ) and 2/Al2O3(ITHF)show an improved performance after 20 h with an activity of approximately 0.70 molCH4·molNi−1·s−1. We therefore further investigated the possible deactivation mechanisms by examining the formation of coke and the oxidation state of the metal under reaction condition. Investigation of Deactivation Mechanisms. Imaging of the spent catalyst allows a better understanding of the different deactivation pathways. Carbon whiskers and sintered metal nanoparticles (>10 nm) are only observed for the spent NiNO3/Al2O3(IH2O) and Niref/Al2O3 while 2/Al2O3(ATHF) still have supported particles with 2 nm size (Figures S24−S26) with no obvious carbon contamination. Quantification of the carbon content by TGA indicated that 3 × 10−4 mol of coke formed per mol of carbon converted after 20 h on stream for the high dispersion samples (D > 26%) and a 7 × 10−4% for NiNO3/Al2O3(IH2O). Thus, larger nickel nanoparticles have a higher tendency to form coke than smaller nanoparticles, in line with previous studies conducted on silica.5 Decreasing the nickel nanoparticle size allows reducing coke formation while improving activity. Thus, the strong deactivation of 2/Al2O3(ATHF) cannot be attributed to the formation of carbon, since the samples 2/Al2O3(IH2O) and 2/Al2O3(ITHF), although forming more coke according to TGA analysis, deactivate much less over 20 h (by about 50% each). We then examined the evolution of the oxidation state of nickel by operando XAS under DRM conditions. Ni(0) content was 6925


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utilization and maintains high specific activity over several hours on stream thanks to a molecular understanding of the catalyst at every step of the preparation and during the reaction conditions by operando XANES spectroscopy.

catalysts retained a relatively high activity after 20 h TOS with a rate of methane consumption of 1.17 molCH4·molNi−1·s−1 (about 40% of deactivation), in sharp contrast to what is observed on pure alumina. Operando XANES analysis shows no significant change in the oxidation state of this sample, with only a slight decrease in the Ni(0) content from 94% to about 90%, which is substantially smaller than the observed catalyst deactivation (Figure 7B3). Thus, modifying γ-alumina by inserting Mg2+ ions in the structure significantly reduces the deactivation of small nickel nanoparticles by migration of Ni into the support matrix, possibly due to a decreased quantity of defects, thereby helping maintaining high activity over 20 h of TOS. Catalyst 2/Mg@Al2O3(ATHF) still undergoes modest deactivation that is not related to carbon formation, since no significant coke formation is observed (TGA and TEM) but is attributed to a partial sintering of the particles during reaction leading to a particle size of 2.7 ± 2.5 nm after 20 h on stream (Figures S36 and S37). This is consistent with the behavior of small Ni particles,5 and is also supported by the fact that regeneration attempts by oxidative treatment had no effect on the activity (Figure S22). Finally, we assessed the activity of the best performing catalyst 2/Mg@Al2O3(ATHF) at a higher reaction temperature (900 °C, Figure S23). The conversion was kept below 40% to ensure operation in the kinetic regime (space velocity was increased to GHSV = 1 350 000 mL h1 gcat−1). The catalyst deactivates by about 50% within the first 5 h on stream, but then retains a stable activity of 7.6 molCH4 molNi−1 s−1 over more than 15 h. Hence, the synthesis method proposed here allows for the preparation of an efficient catalyst that can operate even under harsh reaction conditions.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b01625. Single-crystal XRD of complex 1, PXRD patterns, FTIR of the reduced sample, HAADF-STEM-EDX results, H2 chemisorption isotherm, TPR data, XANES data, FTIR spectra for CO adsorption experiment, 1H NMR spectra, and selectivity data for DRM tests, including Figures S1− S38 and Tables S1−S4 (PDF) X-ray crystallographic data for 1 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Sung Min Kim: 0000-0001-6602-1320 Alexey Fedorov: 0000-0001-9814-6726 Christoph R. 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 and the Swiss National Funding (SNF) in relation with Swiss Competence Centers for Energy Research (SCCER Heat and Electricity Storage). The authors thank the SCCER Heat and Energy Storage and ETH Zürich (ETH-57_12-2) for financial support. 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. We also would like to thank Florian Allouche (ETHZ) for his kind help with the single crystal X-ray diffraction experiment, and P. Trüssel and R. Mäder (ETH Zürich) for their contribution building the operando reactor setup.

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CONCLUSION We have shown how mechanistic insight into catalyst deactivation allows designing a very active and stable nickel catalyst on alumina-based supports for dry reforming by increasing metal dispersion and preventing deactivation pathways (coke formation and metal migration to the support). This improvement was achieved by tailoring the nickel precursor, the deposition method and the support. Nickel dispersions as high as 33% could be obtained using an easily accessible Ni(II) precursor, namely [{Ni(μ2-OCHO)(OCHO)(tmeda)}2(μ2-OH2)], soluble in a broad range of solvents, by depositing this precursor on the surface of the support by a specific adsorption procedure in an organic solvent. After reduction, small and narrowly dispersed supported nanoparticles are obtained (2.0 ± 1.0 nm). We have shown that a higher degree of dispersion increases the initial DR rate. Using a combination of operando spectroscopy and adsorption techniques we demonstrate that Ni(0) is the active phase for DRM. Small Ni particles not only allow for an increased activity, but they also strongly reduce coke formation. Nevertheless, such small particles dispersed on γ-alumina deactivate very quickly by migrating into the alumina support forming NiAl2O4, according to operando XAS experiments. Incorporating magnesium into the alumina lattice suppresses the latter deactivation mechanism occurring on small size nanoparticles and enables high and stable activity over 20 h on stream. As a result, a straightforward route to prepare highly dispersed supported nickel nanoparticles for DRM by rational tailoring of the metal precursor and the support was demonstrated. The resulting catalyst has an optimal metal

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Molecularly Tailored Nickel Precursor and Support Yield a Stable

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