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One-pot synthesis of size- and composition-controlled Ni-rich NiPt alloy nanoparticles in a reverse micro-emulsion system and their application Gregory M Biausque, Paco Laveille, Dalaver H. Anjum, Bei Zhang, Xixiang Zhang, Valérie Caps, and Jean-Marie Basset ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08201 • Publication Date (Web): 16 Aug 2017 Downloaded from http://pubs.acs.org on August 17, 2017

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ACS Applied Materials & Interfaces

One-pot synthesis of size- and compositioncontrolled Ni-rich NiPt alloy nanoparticles in a reverse

micro-emulsion

system

and

their

application Gregory M. Biausque,a,† Paco Laveille,a,†† Dalaver H. Anjum,b Bei Zhang,b Xixiang Zhang,b Valérie Capsa,†††,* a

and Jean-Marie Basseta,*

KAUST Catalysis Center, 4700 King Abdullah University of Science and Technology, Thuwal

23955-6900, Saudi Arabia b

Imaging & Characterization Core Lab, King Abdullah University of Science and Technology,

Thuwal 23955-6900, Saudi Arabia KEYWORDS: Nickel, Platinum, alloy, nanoparticles, micro-emulsion, dry reforming ABSTRACT Bimetallic nanoparticles have been the subject of numerous research studies in the nanotechnology field, in particular for catalytic applications. Control of the size, morphology and composition has become a key challenge due to the relationship between these parameters and the catalytic behavior of the particles in terms of activity, selectivity and stability. Here, we present a one-pot air synthesis of 2 nm Ni9Pt1 nanoparticles with a narrow size distribution.

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Control of the size and composition of the alloy particles is achieved at ambient temperature, in the aqueous phase, by the simultaneous reduction of nickel and platinum precursor with hydrazine, using a reverse micro-emulsion system. After deposition on an alumina support, this Ni-rich nanoalloy exhibits unprecedented stability under the harsh conditions of methane dry reforming. INTRODUCTION The interest in small nanoparticles (NPs) arises from the fact that they present new properties, different from those of atoms and bulk materials. They find applications in many fields, such as electronics,1,2 magnetism,3,4 optics,5 biology6 and catalysis.7 Their properties are size-dependent and a whole new range of reactivity, between that of small molecular clusters and bulk metals, can be uncovered.8,9 These new functionalities may come in some cases from the so-called quantum size effects.10-14 Outside the existence of discrete electronic energy levels and loss of overlapping electronic bands characteristic of bulk metals for the very small particles (< 1-2 nm), these effects owe mostly to the high surface-to-volume ratio.15 In catalysis by metals, this high surface-to-volume ratio maximizes the number of atoms in the outer accessible layer of the metallic particle, thus enhancing the metal utilization, which is economically critical, especially when dealing with noble metals. Furthermore, metallic nanoparticles present polyhedra shapes which allow them to minimize their surface energy during their growth. Decreasing the size of these particles down to about 2 nm results in the preferential exposure of the less-coordinated sites (corners and edges), which might exhibit specific activity.16 It can lead to materials presenting better activity17 or selectivity.18,19


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Nickel nanoparticles are widely used in heterogeneous catalysis for reforming,20-22 dehydrogenation23 and hydrogenation24 reactions, and also in magnetic applications.25 These nanoparticles are typically prepared by bottom-up approaches which generally consist in the low temperature chemical reduction of metal precursors in the presence of surfactant or thermal decomposition of Ni precursors in the presence of alkylamine. Low temperature chemical reduction typically leads to 10-15 nm particles, whether starting from nickel (II) salts (chloride, oxalate) in aqueous solution26-30 or in organic solvents31 or from nickel (II) complexes in organic solvents.24,32-34 The mild decomposition of Ni(0) complexes in the presence of hydrogen and polyvinylpyrrolidone (PVP) leads to small 3-4 nm Ni nanoparticles which however aggregate into 30 nm superstructures.33 The particles obtained in micro-emulsions can be smaller, down to 4-5 nm,35,36 those obtained in the absence of stabilizing agent, in e.g. hydrothermal synthesis, are generally quite large: 2637,38 to 5039 nm. The best size control is obtained by the thermal decomposition of Ni oleylamine complexes in the presence of bulky trioctylphosphine surfactant.40,41 However the mechanism of formation is not clear;42 these syntheses performed under controlled atmosphere can also lead to Ni particles of 15 up to 60 nm,43,44 but particles of 3-4 nm are usually reported,45 especially in the presence of extra reducing agents such as tetrahydridoborate32 or borane tributylamine.23 More recently, 2.5 nm Ni nanoparticles synthesis was reported using surface organometallic chemistry.46 On the other hand, the size of Pt nanoparticles can be more easily controlled between 1-3 nm, whether in aqueous solution,31 in organic media,34 in microemulsions,36 or in hydrothermal synthesis.38



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Bi-metallic Ni-Pt nanoparticles (3-5 nm) with controlled composition and narrow distribution can be synthesized from chemical successive or co-reduction of metals salts in aqueous phase,39,47 by the thermal co-reduction of Ni and Pt complexes in the presence of alkylamines.40,48-50 Smaller PtNi particles (2-4 nm) can be obtained at higher Pt content (> 30 at.%) by the stepwise NaBH4 reduction of Ni and Pt salts in reverse microemulsion,51 by twostep thermal decomposition of Ni and Pt complexes,52 or by a carbonyl complex route followed by hydrogen reduction;53 the Pt content has indeed been shown to influence the particle size.54 Platinum has also been used to catalyse the growth of Ni NP in aqueous media in the presence of hydrazine.55,56 However, only large bimetallic core-shell structures have been obtained so far in those two-step syntheses. Here, we report the room temperature, air synthesis of 2.2 ± 0.5 nm NiPt nanoparticles with a controlled Ni9Pt1 composition by a reverse micro-emulsion method. These are the smallest Nirich NiPt particles ever synthesized by a one-pot chemical, bottom-up approach. Characterization at the atomic level, obtained despite the large amount of organic stabilizer present in the system, highlights the facetted nature of these alloy particles. The key role of the surfactant in particle size control is evidenced by magnetic studies. After deposition on alumina, these nanoalloys further demonstrate catalytic activity in the dry reforming of methane with promising results. EXPERIMENTAL SECTION Chemicals. The reverse micro-emulsion has been prepared by mixing water (MilliQ), cyclohexane (anhydrous, 99.9 Sigma Aldrich) and Polyoxyethylene (5) nonylphenylether (a nonionic surfactant, Igepal®CO520 also denoted NP5). Nickel nitrate hexahydrate (99.999%- Sigma– 4 ACS Paragon Plus Environment

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Aldrich), platinum (IV) chloride (>99.99%, Sigma-Aldrich) were used as metallic precursors, hydrazine monohydrate (N2H4 64-65%, reagent grade 98% Sigma–Aldrich) as reducing agent and ammonium hydroxide solution (28% NH3 in H2O) as precipitated agent. Synthesis of Ni9Pt1 nanoparticles. In a typical synthesis of Ni9Pt1 nanoparticles, two identical batches containing the same amount of cyclohexane (54 mL, continuous phase), 17.5 g of NP5 (nonionic surfactant) were loaded in a flask and stirred (600 rpm) at room temperature in air. In one of the batches, 838 10-6 mol of nickel precursor (244 mg) and 93 10-6 mol of platinum (IV) chloride (31 mg) previously dissolved in 3.6 mL of water (molar ratio 1/5/125 for NP5/water/cyclohexane) is added under high stirring (900 rpm) until optical transparence leading stability of the microemulsion. In the second batch, a aqueous solution containing, 480 µL of hydrazine, 2.1 mL of ammonium hydroxide solution [2 M] and 1 mL of water is added under vigorous stirring until optical transparence too (molar ratio 1/5/125 for NP5/water/cyclohexane). Finally the two stable micro-emulsions (total volume 150 mL) were combined under vigorous stirring (900 rpm) at room temperature under air. A condenser connected to a chiller (15°C) is added on the flask to avoid any evaporation of cyclohexane. The molar ratio between metals/hydrazine/ammonia is fixed to 1/12/5. The transparent greenish micro-emulsion turned quickly to pink color and is let under stirring (450 rpm) at room temperature until turned to black leading to formation of solid particles (5 days). Structural characterization of Ni9Pt1 nanoparticles.



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X-ray diffraction pattern (XRD) was collected using a Bruker D8 Advanced A25 diffractometer in Bragg-Brentano geometry fitted with a copper tube. The diffractometer was configured with a 0.18° diverging slit, 3.2° anti-scattering slit, 2.5° Soller slits. Data were acquired in continuous scanning mode (0.04319°) over the 25 to 60° 2θ range which corresponds to the range in which the major reflections relative to the (111) planes of face-centered cubic (FCC) Ni (44.31°) and FCC Pt (39.76°) can be found (Table 1). Analysis was performed by using a silicon wafer in order to avoid background of the holder during the analysis. Given the small size of the particles and the low concentration of the particles in the micro-emulsion, the sample was concentrated prior to XRD analysis. In order to do that, the nanoparticles were first embedded into silica by carrying out the hydrolysis of TEOS within the Igepal® micelle. The solvent was then removed under vacuum (10-3 bar) and the powder containing the nanoparticles confined in the inorganic SiO2 matrix was subsequently prepared as a thin layer on the sample holder for XRD analysis. Raw data are then deconvoluated using the Origin software, following a Gaussian fit (after linear background subtraction). Table 1: Crystal structure data of platinum and nickel

Crystal

Lattice

structure constant (Å)

Space

d-spacing(Å) related to one miller index

Volume

group

(corresponding (2θ) for a 1.5405-CuKα X-Ray)

(Å3)

Miller index

Platinum

FCC

3.92

Fm3m

111

020

202

2.265 (39.76)

1.962 (46.23)

1.183 (81.25)

60.32


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Nickel

FCC

3.52

Fm3m

2.034 (44.31)

1.762 (51.85)

1.246 (76.37)

43.76

Transmission Electron Microscopy (TEM) analyses were performed on a Titan G2 from FEI Company using a Schottky gun (80-300 keV range) and equipped with a EDS (EDAX company), an energy filters (model GIF tridiem from Gatan, Inc) and a 4k x 4k CCD camera (model US4000 from Gatan, Inc). The size distribution of the particles has been obtained by using High Resolution Transmission Electron Microscopy (HRTEM). Scanning Transmission Electron Microscopy (STEM) combined with a High Angle Annular Dark Field detector (HAADF) and EDS has been used to recorded nature and composition. Another microscope, Titan3TM from FEI Company set up with the same equipment than the titan G2 plus a spherical aberration-corrected system (CS-TEM) was used in order to get morphology, composition and elemental analysis of nanoparticles. Morphology was determined by combining CS-HRTEM (to remove electron delocalization) and Electron Energy Loss Spectroscopy (EELS) in zero-loss mode (to improve contrast). Simultaneously, by using the Fast-Fourier Transform (FFT) technique on the micrographs obtained, determination of the spatial frequencies was carried out. From these measurements, dspacing of the phase present (composition) in the sample were correlated to hypothetical one. Elemental analysis was carried out by EDS, using a probe of 0.7 ± 0.03 nm. Due to the presence of the capping agent which leads to a quick carbon contamination of the EDS spectra under the STEM beam (in addition to quick merging of the unveiled particles), results are only qualitative.



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To achieve atomic resolution on these protected 2 nm particles, a specific preparation was developed in order to separate particles from the surfactant, while avoiding coalescence of the particles which are highly sensitive to the beam. First, a solution composed of 0.5 mL of microemulsion, 2 mL of water (destabilizing agent) and 15 mL of absolute ethanol (diluting agent) was prepared. Second, three droplets of 3 µL were successively added on an ultrathin carbon film on holey carbon on 400 mesh copper grid (Ted Pella, inc). Finally, the grid was put into plasma cleaner (Model 1020 Plasma Cleaner; Fischione instruments) before analysis, using a 20 vol.% O2 in Ar as gas mixture for 15 s followed by a second session of 8 s. The magnetic properties of the NiPt nanoparticles were characterized using Quantum Design SQUID-VSM from 5 K to 100 K with applied magnetic field up to 5 Tesla. 150 µl of micro emulsion was loaded into a gelatin capsules (Agar Scientific). In order to extract the contribution from the NiPt nanoparticles, we have measured the control sample (capsule and liquid) as the background. All the data shown have been corrected by subtracting the background.

Deposition of Ni9Pt1 nanoparticles on γ-Al2O3. The mass of alumina is determined according to the final metal loading targeted, taking into account the quantity of water physisorbed on the raw material (19% w/w in our case, as determined by TGA). The volume of ethanol needed to destabilize the micro-emulsion is set to 4 times the volume of micro-emulsion used for deposition. Typically, for a metal loading of 5% w/w, 595 mg boehmite is stirred in 230 mL of ethanol. Then 75 mL of micro-emulsion, prepared as described above (25 mg of reduced metal), is added drop-wise to the ethanol/bohemite mixture. Once all micro-emulsion has been added, the mixture is heated to reflux at 80°C and 8 ACS Paragon Plus Environment

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kept under stirring for 12 h. The supported catalyst is recovered by centrifugation (10 min at 10,000 g), washed 3 times with ethanol and dried at 90°C over night. After drying the material is thermally treated under dynamic conditions (100 mL min-1) with a mixture of 5%H2/2%NO in N2. Temperature program starts from room temperature to 120°C at 5°C min-1, 1 h isothermal at 120°C, then up to 220°C at 5°C min-1, staying 1 h at 220°C, and finally up to 700°C at 6°C min-1 and staying 2 h at 700°C. This thermal treatment allows the solid-solid phase transition of boehmite to γ-Al2O3. Catalytic tests under dry reforming of methane. Supported catalysts have been tested under typical conditions for CO2 reforming of CH4 (DRM) in a PID Eng & Tech micro-activity dynamic reactor. 50 mg of Ni9Pt1 supported on Al2O3, prepared as described above, is diluted with 150 mg SiC and loaded in a quartz reactor (4 mm ID, 30.5 cm length). The system is heated up to 700°C under 80 mL min-1 N2. A prereduction step is performed at this temperature for 1 h at 20% H2/N2 (80 mL min-1). Then the catalyst is purged again at 80 mL min-1 N2 before introducing the reactant gas with 1/1/8 of CH4/CO2/N2 into the reactor to start the DRM reaction at WHSV = 120 L h-1 g-1. These conditions allow the reaction to be under kinetic regime; therefore any loss of activity observed can be attributed to the catalyst deactivation. RESULTS AND DISCUSSION In a typical synthesis, a NP5/water/cyclohexane reverse micro-emulsion (1/5/12.5 molar ratio) containing hydrazine (480 µL), ammonia (2 M, 2.1 mL) and 1 mL of water was added under vigorous stirring (900 rpm) at room temperature in air to a NP5/water/cyclohexane reverse micro-emulsion (1/5/12.5 molar ratio) containing nickel nitrate (244 mg / 838 µmol) and 9 ACS Paragon Plus Environment


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platinum (IV) chloride (31 mg / 93 µmol). Both micro-emulsions use polyoxyethylene (5) nonylphenylether (NP5), a nonionic surfactant also referred to as Igepal®CO520. They are stirred independently until optical transparency indicating formation and stability of the microemulsion is reached. After mixing, the transparent greenish micro-emulsion contains a metals/hydrazine/ammonia molar ratio of 1/12/5. It quickly turns pink within minutes and is let under mild stirring (450 rpm) at room temperature. After about 5 days, the micro-emulsion turns black indicating the formation of solid particles.


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Figure 1. Ni9Pt1 nanoparticles structure. Bright-field TEM (inset: particle size distribution) (a) and HAADF-STEM (b) images of Igepal®-stabilized Ni9Pt1 microemulsion. (c,d) CS-HRTEM images of purified Ni9Pt1 nanoparticles. Inset: Theoretical model of a 2.2 nm cubo-octahedron particle.



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Figure 1 shows images of the micro-emulsion obtained by Transmission Electron Microscopy (TEM, Figure 1a) and by Scanning Transmission Electron Microscopy (STEM, Figure 1b) combined with a High Angle Annular Dark Field detector (HAADF). Particles of about 1-2 nm can be seen. They are embedded in an amorphous/organic matrix, which stems from the presence of the capping agent. The sample is clearly reacting due to the electron beam and coalescence of the particles can be observed, leading to 3-4 nm particles within seconds. The statistical analysis of the particle diameters, performed on TEM images obtained in the minimum amount of time, gives a narrow distribution (inset of Figure 1a) centered at 2.2 nm, associated with a standard deviation of 0.5 nm. The homogeneity in size (STD < 14%) is typical of surfactant-assisted syntheses35,36,40,41 and shows that the Igepal® surfactant, normally used for the synthesis of SiO2 spheres in water-in-oil microemulsions57-60 or for the synthesis of silica coatings around colloidal particles,40,61,62 is an efficient metal stabilizer, just like polyoxyethylene lauryl ether (Brij30).63,64 Atomic resolution of the particles was achieved by Aberration-Corrected High Resolution Transmission Electron Microscopy (CS-HRTEM, Figure 1 c,d) after the heavily protected particles were submitted to a suitable plasma treatment, which was developed to remove most of the organic component whist minimizing coalescence of the bimetallic particles.


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Figure 2. Composition of Ni9Pt1 nanoparticles. (a) XRD pattern of the evaporated microemulsion system in the 2θ range of 25 to 60°. (b) EDS spectrum of the Igepal®-stabilized suspension of Ni9Pt1 nanoparticles. (c,d) Elemental mapping of two different particles obtained by STEM-EDS. STEM-EDS analysis of individual particles using a probe size (FWHM) of 1 nm shows the presence of both platinum and nickel in all particles (Figure 2a). The actual Ni/Pt ratio found is close to the expected 9:1 composition in most cases (>98%). Traces of two other Pt-enriched compositions, NiPt and Ni3Pt1, are also detected. These are well-known, thermodynamically stable phases of the binary Ni-Pt phase diagram,65 which could result from coalescence upon beam exposure. Hence, the confined co-reduction of nickel and platinum precursors within the micelle leads to bimetallic Ni-Pt nanoparticles with a homogeneous composition controlled by the initial Ni/Pt molar ratio in the micelle. Powder XRD analysis concentration of the concentrated bimetallic particles was carried out over the 25 to 60° 2θ range which should contain the major reflections relative to the (111) planes of face-centered cubic (FCC) Ni (44.31°, ICDS), of FCC Pt (39.76°, ICDS) and of any Ni-Pt alloy. As expected from the small size of the particles, diffraction is weak and the signal obtained is broad. However, one peak is observed (Figure 2b). It can be deconvoluated into one single Gaussian peak at 43.96°. The width at half maximum (FWHM = 13°) is consistent with the 2 nm crystallite size of the particles, according to the Scherrer law. According to Vegard’s law, the (111) reflection for a Ni9Pt1 crystal should be at 43.86°. The slight shift observed can be attributed to the approximation used for the fitting, which takes into account only the (111) reflection of the alloy and omits the second main reflection of the (200) plane, theoretically at


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51.29°, and thus comprised within the broad signal observed. Minor contributions of peaks at 42.04° and 43.17°, corresponding to the (111) reflections of the minor populations of NiPt and Ni3Pt1 respectively, could also account for the difference. However, a slightly different spatial configuration of Ni and Pt atoms, leading to a slightly different Miller index which allows to minimize surface energy in very small (< 4 nm) alloy particles,49 or lattice contraction,66 cannot be excluded. The presence of a NiPt alloy is consistent with the d-spacing measured on the CS-HRTEM atomically resolved images of the plasma-treated bimetallic particles (Figure 1 c,d). The measured value of 0.21 nm is indeed an intermediate distance between the d-spacing of the (111) planes reported for FCC Ni (0.2034 nm) and FCC Pt (0.2265 nm). It is thus consistent with the d-spacing of the (111) planes of an FCC Ni-rich Ni-Pt solid solution. The bimetallic Ni9Pt1 nanoparticles produced at ambient temperature in air in the reverse micro-emulsion thus consist in a solid solution of Ni and Pt atoms. Furthermore, elemental analysis of the 2.2 nm particles shows that there is no significant segregation of Pt in the bimetallic particle. For this, the STEM beam was decreased as much as possible to a FWHM diameter of 0.7 nm and EDS was used as the analyzer. At this level of analysis, the beam spreading (b) was determined to be 0.03 nm, using the single scattering model ,

by Goldstein et al.67 = 8 × 10 (/ )( )

where b and t (thickness) are in m, E0 is

in keV and Nv is the number of atoms/m3. Elemental analysis of three parts of the particles alongside the cross-section was thus carried out with the 7.0 ± 0.3 Å spot-size. Spectra acquired on the three spots generally exhibit the same apparent ratio between nickel and platinum, whether in the core or on the external shell of the particle (Figure 2c). A few particles however



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present a Ni-rich core (Figure 2d), which is consistent with the presence of a few Pt-enriched alloy NP that may have been formed upon beam exposure. Pt-surface segregation has indeed been reported for thermally treated Ni-Pt alloys68 and Pt-rich Ni-Pt nanoalloys.69 However, in most cases, Pt and Ni atoms seem randomly located inside the particle, in a real Ni-Pt solid solution, despite the higher surface energy and smaller atom size of Ni. This suggests that the Ni is in close interaction with the surfactant and that the Ni-Igepal interaction is more favourable than the Pt-Igepal interaction (higher bond strength in Ni-Igepal).70 Depending on their size and composition, particles may exhibit different morphologies. The irregular hexagonal shape observed in the 2D Cs-corrected HRTEM images (Figure 1 c,d) is typical of the cross-section of a cubo-octahedron,71 which is generally observed for Pt-rich Ni-Pt nanoparticles (> 500 atoms).72 Using this model,73,74 the total number of atoms in the 2.2 nm Ni9Pt1 cluster can be calculated to be about 450, taking into account a 3 atom-edge (m) of the truncated octahedron with excedent atoms located on the facets (inset of Figure 1d). 500 atoms can be found from an extension of the spherical cluster approximation, which consists in determining the number of atoms by estimate a ratio between the cluster volume Vc and an effective atomic volume Va*6. These numbers, which do not exactly match the “magic number” in pure nickel75 and pure platinum76 cluster models (simulations at 0 K), suggest that the growth of the clusters in the constrained environment of the micro-emulsion is controlled by the interaction between the organic component and the surface metal atoms. Indeed, the thermodynamically stable shape of free nickel clusters (UHV conditions) smaller than 1200 atoms and of free Pt clusters smaller than 100 atoms is reported to be the icosahedron.77-80 Hence, it is interesting that the 10 at.% Pt content of the bimetallic Ni9Pt1 particles (ca. 50 Pt atoms) can direct, in the presence of the Igepal® surfactant, the growth of the 500 atom-particle 16 ACS Paragon Plus Environment

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towards the cubo-octahedron vs. the icosahedron shape. It is also interesting that the growth stops at 2.2 nm in the chemical medium, at which point 50% of the atoms in the particle are on the outer surface of the particle. Particles with a larger diameter than 2.2 nm can be observed upon extended beam exposure. Those have a more spherical shape and less sharp edges, which is typical for particles growing by atom migration or coalescence of smaller facetted crystallites. It can be observed for facetted particles treated at elevated temperatures.71

Figure 3. Magnetic properties of Ni9Pt1 nanoparticles. (a) Temperature dependent magnetization measured with zero-field cooled (ZFC) and field cooled (FC) process under a magnetic field of 100 Oe. Insert: ZFC magnetization hysteresis loop obtained at 5 K. (b) Hysteresis loops measured at 5 K and 100 K during the temperature-dependant magnetization of Ni9Pt1 nanoparticles. Figure 3a shows the temperature dependent magnetization of the nanoparticles measured with zero-field cooled (ZFC) and field cooled (FC) process under a low magnetic field of 100 Oe. It is clear that a narrow peak appears in the ZFC curve, which corresponds to the blocking of the magnetic moments of the nanoparticles. The narrow peak is strongly indicative of a narrow particle size distribution,4 which confirms the TEM observation (Figure 1). The peak



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temperature that represents the average blocking temperature, TB = 10.5 K, is governed by the magnetic anisotropy and volume of the particles, i.e. 25kBTB = KV, where K is the effective anisotropy constant, V the volume of the nanoparticles and kB the Boltzmann constant. By taking the diameter of 2.2 nm and TB = 10.5 K, we calculated the magnetic anisotropy K = 7.47 x 106 erg/cm3, which is much larger than K = 5 x 104 erg/cm3 at 4.2 K for bulk Ni.81 The much larger anisotropy constant in the bimetallic NiPt nanoparticles is attributed to the high surface-tovolume ratio (50% dispersion), interface anisotropy (interaction of the surface atoms with surfactant) and shape anisotropy.3 Alloying of Ni with Pt atoms may also largely contribute to the high anisotropy observed, in line with the observation that the coercive field is 320 Oe at 5 K for NiPt (1/1) nanoparticles.54 Above the blocking temperature, both ZFC and FC follow the Curie-Weiss law. Shown in Figure 3b are the hysteresis loops measured at 5 K and 100 K. The hysteresis loops measured at 5 K were corrected by removing the diamagnetic contribution from diamagnetic solution and capsule (used as sample holder). It is interesting that it is hard to saturate the Ni nanoparticles even with magnetic fields of 5 T. This could be due to the large percentage of surface spins (ca. 50%) in 2.2 nm nanoparticles. The surface spins usually form a spin glass phase3,82 and/or behave paramagnetically. The well-defined magnetic hysteresis loop (inset of Figure 3a) with a moderate coercive field, Hc = 200 Oe, indicates that the crystallization of the particles is very good, and that non-spin glass phase was formed. If the spin glass phase was formed, the coercive field would increase to several kOe.82,83 Such spin glass phase is generally caused by the poor crystallization of the surface atoms that lose some of their neighbors. The good crystallization of surface atoms in our particles (Figure 1 c,d) could be due to interaction/bonding of surface atoms (Ni and Pt) with the surfactant. However, the surface 18 ACS Paragon Plus Environment

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spin, particularly at the edges and corners, might behave para-magnetically, which may partially lead to the un-saturated magnetization even at 5 K. The contribution of isolated Ni ions may however not be excluded. It is clear that at 100 K, the magnetization is dominated by the diamagnetic solution and capsule, because the paramagnetic contribution from quasisuperparamagnetic NiPt particles is negligible.



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Figure 4. Properties of γ-Al2O3-supported Ni9Pt1 nanoparticles. TEM (a) and STEM (b) images. (c) Catalytic CO2 () reforming of CH4 () (750°C, 1 bar, 1/1/8 CO2/CH4/N2, GHSV = 120 L h-1g-1, 50 mg 5wt.% Ni9Pt1/ γ-Al2O3). In order to convert this colloidal nanomaterial into a functional catalyst, we have developed a suitable deposition method. One of the drawbacks of metal nanoparticles produced by reverse phase micro-emulsion is the difficulty to perform a proper deposition of the catalyst on an oxide support.84 Final result, in term of yield and dispersion, strongly depends on the selected supports and methods.85,86 Addition of solvent such as ethanol, which is soluble in both the organic solvent and the water contained in the micro-emulsion, is usually needed to destabilize the micelles surrounding and protecting the nanoparticles and force them to deposit on the surface of the support. In our experiments, direct deposition of 2.2 nm Ni9Pt1 contained in the microemulsion on γ-alumina was not feasible due to low affinity of metal nanoparticles with the surface of γ-alumina. In these cases, only a small amount of Ni9Pt1 contained in the microemulsion could be deposited which makes the final loading of the material unpredictable. On the other hand, when boehmite, the oxyhydroxide form of alumina, was used as initial support, all the metal present in the micro-emulsion could be deposited. This is attributed to the highest affinity of the water droplets with the more hydrophilic surface of boehmite compared to γalumina.87 The stable γ form of alumina could then be obtained by thermally treating the material at 800°C under a mixture of H2 and NO. Such use of NO is to limit sintering due to exothermicity of organic decomposition.88 Upon such treatment, only moderate sintering of the nanoparticles can be observed. The average particle size increases from 2.2 to only 4 nm (Figure


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4 a,b), despite the high temperature used, and thus remains in the state-of-the-art sizes for supported Ni particles,89,90 likely due to surface stabilization.91 As a proof of concept, we have tested this supported Ni9Pt1/ γ-Al2O3 catalyst under harsh CO2 dry reforming of methane (DRM). The economic and environmental interest of this reaction has been reviewed many times.92-95 The high temperatures involved lead to undesired phenomena, namely coke formation by secondary reactions (methane decomposition, CO disproportionation) and sintering of the metallic particles. Supported metal catalysts from groups VIII (nickel, cobalt, ruthenium, rhodium, iridium, palladium and platinum) are common catalysts for the DRM reaction. Catalysts based on noble metals (Rh, Ru, Pt, Pd and Ir) are reported to be more active and less sensitive to coking than base metals (nickel and cobalt), Rh and Ru being the less sensitive to coke deposition.94 However noble metals are costly and have limited availability. Hence, Ni-based catalysts would be a strategic choice of catalytic material for commercialized reforming reactions at an industrial scale. Many researches now focus on the synthesis of non-noble metal-based alloys with enhanced coke resistance properties compared to monometallic catalyst. Garcia-Dieguez et al. have demonstrated that NiPt alloys prepared by incipient wetness impregnation had higher activity under DRM conditions associated with lower production of coke than pure Ni.96 Liu et al. also reported the beneficial effect of Pt addition on the Ni/MCM-41 with improved coke resistance for CO2 reforming.97 They detected an enhanced interaction between Pt and Ni, but no alloy formation was mentioned. Guczi et al. added 0.5 wt.% gold to the Ni/MgAl2O4 catalyst and found that the catalytic activity and the stability were improved and the carbon formation was inhibited for CO2 reforming.98 In the absence of gold, graphite-like carbon deposits and carbon nanotubes were generated, leading to deactivation of the catalyst. On the catalyst containing gold, neither graphite nor carbon nanotubes were 21 ACS Paragon Plus Environment


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produced. Damyanova et al. confirmed that the addition of a small amount of palladium (Pd, 0.5%) to the MCM-41 supported Ni catalysts led to improved catalytic activity and stability for DRM, compared to monometallic Ni catalysts, although some filamentous carbon species were observed.99 No evidence of alloy formation was reported. Nowosielska et al. compared Al2O3 and SiO2-supported Ni and Ni-Rh catalysts and found that the activity and coke resistance of NiRh catalysts are much higher than Ni catalysts.100 Additionally, many experimental and theoretical studies confirmed that the size of Ni particle had a significant effect on the coke formation.101 Bengaard et al. reported a good comparison between nickel catalysts with different mean particle sizes.102 The thermo-gravimetric results clearly showed that the majority of carbon was deposited on large particles rather than on small particles. One explanation for the different behaviors of carbon deposition is that carbon growth requires a nucleus of a minimum size and that such process is facilitated by crystal facets, which are more numerous on larger, rather than on small, metal nanoparticles.73 These composition and size effects are not limited to dry reforming of methane as deactivation by coke and sintering is very common in high temperature hydrocarbon conversion technologies.103,104 Therefore, identifying chemical protocols, such as the method presented herein, that lead to the synthesis of nano-sized particles of defined composition and morphology, is of interest. In this regard, Ni9Pt1/γ-Al2O3 (5% w/w), has been catalytically tested under typical DRM conditions (750°C, 1 bar, 1/1/8 CH4/CO2/N2, SV=120 L h-1 g-1). Conversion was set to be in the range of 50-60%, i.e. below thermodynamic equilibrium of the reaction under these conditions. The reaction is thus under kinetic control and any loss of activity can be directly attributed to deactivation of the catalyst. Results presented in Figure 4c show very stable CH4 and CO2 conversions for over 100 h. H2/CO also remains stable, around 0.85-0.90, during the whole test. 22 ACS Paragon Plus Environment

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This suggests that the catalytically active phase derived from the micro-emulsion method remains stable under the reaction conditions used. Besides, the H2/CO ratio is superior to what has been previously reported for PtNi-catalyzed DRM.96 Post-run characterization reveals that carbon deposition is significantly inhibited as compared with a Ni/γ-Al2O3 reference catalyst (Figure S1). Besides, the carbon structures grown over Ni9Pt1/γ-Al2O3 are significantly different from those observed over the Ni reference (Figure S2) and other supported NiPt catalysts.96,105 Post-run TEM indeed shows the presence of graphitic shells around the metallic particles and the absence of any 2D, filamentous carbon structures (Figure S2). Finally, the Ni/Pt ratio determined by EDS over single particles is unchanged (Figure S3): it remains similar to the nominal Ni/Pt ratio of 9. This suggests the absence of nickel oxidation during the reaction. CONCLUSIONS In summary, a reverse micro-emulsion method has been successfully used to prepare 2 nm Ni9Pt1 alloy nanoparticles. This is to our knowledge the smallest Ni-rich nanoparticles ever synthesized. Deep structural characterization shows cubo-octahedral crystalline nanoparticles, with no obvious Pt segregation. The material could be deposited on boehmite, further recovered and thermally treated to get Ni9Pt1/ γ-Al2O3 solid powder. Catalytic activity under the harsh conditions of the dry reforming of methane is characterized by stable conversion for 100 h under kinetic control associated with a high H2/CO ratio of 0.85. Hence, size- and compositioncontrolled metal nanoparticles with promising catalytic performances can be synthesized by this 20 nm

alternative, non-air sensitive method.



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ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Post-run analyses of Ni9Pt1/γ-Al2O3, including TGA, TEM, STEM and EDS (PDF).

AUTHOR INFORMATION Corresponding authors: *E-mail: [email protected] *E-mail: [email protected] ORCID Xixiang Zhang: 0000-0002-3478-6414 Valérie Caps: 0000-0001-9330-2566 Jean-Marie Basset: 0000-0003-3166-8882 Present address: †

SABIC, PO Box 4700, Thuwal, 23955-6900, KSA

††

Takreer Research Center, PO box 3593, Abu Dhabi, United Arab Emirates

†††

ICPEES (Institut de Chimie et Procédés pour l'Energie, l'Environnement et la Santé),

University of Strasbourg / CNRS UMR 7515, 25 rue Becquerel, 67087 Strasbourg Cedex 02, France 24 ACS Paragon Plus Environment

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Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT We thank Saudi Basic Industries Corporation (SABIC) for its financial support and scientific contribution as well as KAUST for exceptional facilities. REFERENCES (1)

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