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Reduction of a Ni/Spinel Catalyst for Methane Reforming Jan Kehres, Jens Wenzel Andreasen, Jostein Bø Fløystad, Haihua Liu, Alfons M. Molenbroek, Jon Geest Jakobsen, Ib Chorkendorff, Jane H. Nielsen, Kristin Høydalsvik, Dag W. Breiby, and Tejs Vegge J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp510159m • Publication Date (Web): 17 Dec 2014 Downloaded from http://pubs.acs.org on December 26, 2014

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The Journal of Physical Chemistry

Reduction of a Ni/Spinel Catalyst for Methane Reforming

Jan Kehres,a Jens Wenzel Andreasen,b* Jostein Bø Fløystad,c Haihua Liu,d Alfons Molenbroek,e Jon Geest Jakobsen,e Ib Chorkendorff,a Jane Hvolbæk Nielsen,a Kristin Høydalsvik,c Dag Werner Breiby,c and Tejs Veggeb a

Center for Individual Nanoparticle Functionality, Department of Physics, Technical University of

Denmark, Kgs. Lyngby, Denmark, bDepartment of Energy Conversion and Storage , Technical University of Denmark, Roskilde, Denmark, cDepartment of Physics, Norwegian University of Science and Technology, Trondheim, Norway, dPhysical Biology Center for Ultrafast Science and Technology, Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, USA, eHaldor Topsøe A/S, Kgs. Lyngby, Denmark

E-mail: [email protected]

RECEIVED DATE

ABSTRACT A nickel/spinel (Ni/MgAl2O4) catalyst, w(Ni) = 22 wt%, was investigated in situ during reduction with wide angle X-ray scattering (WAXS) in a laboratory setup and with anomalous small angle Xray scattering (ASAXS) at a synchrotron source. Complementary high resolution transmission

1 ACS Paragon Plus Environment


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electron microscopy (HRTEM) was performed on the fresh catalyst sample. The Ni particles in the fresh catalyst sample were observed to exhibit a Ni/NiO core/shell structure. A decrease of the Ni lattice parameter is observed during the reduction in a temperature interval from 413 – 453 K, which can be related to the reduction of the NiO shell, whereby stress due to the lattice mismatch of Ni and NiO is relieved. KEYWORDS: Nickel/spinel, reduction, methane reforming, in situ, WAXS, ASAXS INTRODUCTION Synthesis gas, a mixture of carbon monoxide and hydrogen, is an important intermediate for various large-scale processes. Methane is conventionally converted into synthesis gas by the steamreforming process, but reforming with carbon dioxide is thought as an alternative.1 Dry reforming with carbon dioxide yields lower H2/CO ratios compared to reforming with steam, which is desirable for the synthesis of synthetic fuels by the Fischer-Tropsch process, methanol and oxoalcohols.2-4 Many transition metals such as Ni, Ru, Pd, Ir and Pt1,5 have been found to show good catalytic activity for the dry methane reforming reaction. Catalysts based on Ni are commercially favorable for this process considering the high cost and limited availability of the noble metals,6 but they have a main drawback in higher carbon deposition rates than most of the noble metals.1 Therefore, it is economically advantageous to modify the Ni-based catalysts to improve their resistance against carbon formation. The rate of carbon formation has been observed to depend on the Ni crystallite size with lower rates observed for smaller Ni crystallite sizes.7,8 Crisafulli et al. reported a lower deactivation rate of bimetallic Ni-Ru catalysts compared to pure Ni during dry reforming experiments and ascribed this to a higher dispersion of the Ni surface enriched bimetallic clusters and thereby formation of a more reactive carbon species compared to pure Ni.9 Lower


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carbon formation rates during steam-reforming of methane with a Ru doped Ni catalyst were also reported by Jeong et al.10 A strong influence of the support material on the activity and carbon formation rates has also been observed. Ni particles supported on MgAl2O4 showed a higher dispersion compared to similarly prepared catalysts supported on γ-Al2O3 and hence higher initial activity6,10 and less deactivation due to lower carbon formation rates.6 This emphasizes the importance of the metal dispersion on the performance of the catalyst: Highly efficient and stable Ni-based catalysts for methane reforming should exhibit a high initial metal dispersion and retain it during the reforming process. Commercially produced Ni based reforming catalysts are usually prepared by incipient wetness impregnation of the support materials with an aqueous solution of a Ni-containing salt and subsequent drying and calcination at high temperatures. The final step of the catalyst preparation is the activation by reduction of NiO with hydrogen. Cyclic oxidation and reduction can also be utilized to regenerate sulphur poisoned Ni-catalysts.11 Several studies of the reduction of unsupported and supported NiO can be found in the literature.6,10,12-17 The reduction temperature of unsupported NiO-nanoparticles has been reported to be independent of the particle size.12 The reduction of NiO initiates after a temperature dependent induction period.13-15 Rodriguez et al. studied the reduction of NiO with X-ray absorption fine structure spectroscopy (XAFS) measurements, time-resolved synchroton X-ray diffraction, photoemission spectroscopy and first principles density functional theory (DFT) calculations and concluded that during the induction, oxygen vacancies are created, which lower the energy barrier for hydrogen bond cleavage.15 An increase of the Ni crystallite size by an order of magnitude during the reduction compared to the initial NiO crystallite size of unsupported NiO was observed.14 Cyclic reduction and oxidation



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studies of a 2 wt% Ni/Al2O3 catalyst did not show any loss in surface area and it was concluded that this was due to the large interparticle distance, but an influence of the reduction temperature on the crystallite size was noted.16 Jeangros et al. studied the reduction kinetics of NiO with environmental transmission electron microscopy (ETEM) and electron energy loss spectroscopy (EELS) and found that the reduction kinetics were best described by Avrami nucleation models.17 Since the activity and resistance against carbon deposition of Ni-based methane reforming catalysts are correlated with the Ni metal dispersion, it is crucial to follow and understand dynamical processes during the reduction and reforming reactions to improve start-up processes and to identify the optimum operating conditions. Because of their penetrating properties, X-rays with an energy >5 keV are suitable for studying catalyst materials under working conditions at temperatures and pressures comparable to large scale processes. WAXS can be applied to extract information about phase transitions, phase composition and crystallite growth. Small angle X-ray scattering (SAXS) is sensitive to the electron density inhomogeneities and is therefore a powerful tool to study the nanostructure of heterogeneous catalysts. Combinations of both techniques have been applied to investigate nanostructured catalysts.18,19 From SAXS it is not necessarily clear which electron density difference is dominant, i.e. between pores and support, pores and particles or particles and support. Characterization of the sample with complimentary techniques, such as HRTEM is therefore indispensable for the analysis. A proper data analysis of the SAXS is impossible if the characteristic lengths scales of the probed electron density differences are too similar or if the catalyst composition is complex. However, as demonstrated,20,21 this problem can be overcome by measuring ASAXS (anomalous SAXS) to distinguish the SAXS for different contributing phases by contrast variation.


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In this paper, we describe the in situ reduction SAXS/WAXS experiment of a pre-reduced and reoxidized 22 wt% Ni/MgAl2O4 catalyst in pure hydrogen to follow the changes during the reduction of NiO. In situ ASAXS experiments were performed to resolve the morphology of the fresh and reduced catalyst particles.

EXPERIMENTAL SECTIONIN SITU REDUCTION EXPERIMENTS The catalyst preparation of the 22 wt% Ni/MgAl2O4 was reported in detail by Sehested et al.22 The catalyst sample was crushed and sieved. Samples with a particle size of 65 - 125 µm were loaded in 0.7 mm quartz capillaries (Mark tube from Hilgenberg GmbH) with a catalyst bed length of approximately 10 mm, embedded between glass wool plugs and mounted gas tight with Swagelok® fittings using graphite ferrules. The reduction experiments were performed in a flow cell custom designed for in situ studies at the B1 beamline at HASYLAB in Hamburg, Germany,23 in a constant gas flow of 2 ml/min hydrogen (alphagaz, Air Liquide, > 99.999 %) at standard temperature and pressure (STP). The in situ cell has been described in detail by Andreasen et al.24 Constant gas flows were applied to the sample using a gas control system including a mass flow controller. The flow scheme is illustrated in Figure 1.



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Figure 1. Flow scheme of the in situ cell. 2D ASAXS patterns of the catalyst at RT, prior to reduction, and of the reduced catalyst at 773 K were captured with a Pilatus 300k detector (Dectris) at sample-to-detector distances of 1313 and 3564 mm. Five patterns were recorded in an X-ray energy range from 8113 to 8326 eV, at 7, 17, 39, 91 and 220 eV below the Ni-K-edge at 8333 eV. During the heating from RT to 773 K with a ramp of 10 K min-1 SAXS patterns were acquired continuously with exposure times of 120 s at a fixed sample detector distance of 3564 mm and at an energy of 8112 eV. Completeness of the reduction was verified by X-ray absorption scans around the Ni-K-edge. The WAXS experiments were performed using a combined SAXS/WAXS pinhole laboratory setup, placed in the X-ray center at DTU Campus Risø. X-rays were produced with a Rigaku RU300 rotating anode generator operating in fine focus mode at 40 kV and 60 mA, equipped with a Mo-target. The sample cell was adapted to the laboratory pinhole SAXS camera with modified Xray Kapton® windows to cover the wide angle scattering and a heater permitting sample temperatures in an interval of 298 - 1073 K.18 WAXS patterns were recorded with an image intensified Gemstar 125 CCD camera mounted 30° off-axis to the direct beam. The camera with a CCD chip of 1040 × 1392 pixels and a nominal pixel size of 109 × 109 µm2 covers a q-range from 2.0 – 6.5 Å-1 by using Mo Kα radiation with λ = 0.71 Å. The instrumental resolution of the CCD


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camera was determined from powder diffraction of a silicon standard (NIST SRM 640c) and is on the order of ∆q / q = 4 ×10-3. Three WAXS patterns were acquired at each of the temperatures 298, 373, 473, 573, 673, 773 and 873 K.

EX SITU WIDE ANGLE X-RAY SCATTERING WAXS pattern with higher resolution compared to those acquired in the SAXS/WAXS pinhole laboratory setup were recorded with a “standard” Bruker D8 Bragg-Brentano diffractometer using Cu Kα radiation with λ = 1.542 Å at 40 kV and 40 mA. The diffraction patterns with higher resolution were recorded at ambient conditions in air and in a q-range from 1.1 - 6.5 Å-1 with step sizes of ∆q = 1.4·10-3 Å-1. TRANSMISSION ELECTRON MICROSCOPY Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were acquired with a JEOL 3000 F microscope with an acceleration voltage of 300 kV. The Ni/MgAl2O4 catalyst was dispersed in ethanol with a dilution of 1:1500 using ultrasonic treatment. 10 µl of this dispersion was coated on a 400 mesh holey carbon grid. DATA ANALYSIS In order to transform the WAXS data to q-space, a silicon standard (NIST SRM 640c) was measured in the sample position before the experiments. From a fit of the silicon powder rings the camera position and orientation relative to the sample was determined and subsequently applied for binning the 2D powder pattern of the Ni/MgAl2O4 catalyst. The 2D ASAXS acquired at B1 at HASYLAB (Hamburg, Germany) was processed and transformed using the Matlab scripts provided at the beamline.



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WAXS

Rietveld refinement of the WAXS data was performed to obtain the lattice parameters and particle size broadenings from the structural data for Ni,25 NiO,26 MgAl2O427 and α-Al2O328 using the Rietica software29 and assuming Voigt functions for the peak profiles. The instrumental peak broadening was determined from refinement of a silicon reference (NIST SRM 640c). ASAXS In the vicinity of element-specific absorption edges, the atomic form factor is significantly modified, which can be utilized to separate small angle scattering cross sections from different components in a mixture. The atomic form factor can be expressed as

() = + ′ () + ′′ () ,

(1)

where E is the energy of the incident X-rays, the non-resonant term being the atomic form factor which, at small scattering angles, equals the number of electrons of the element, and () and ′′ () the real and imaginary part of the so-called dispersion corrections. The dispersion correction for Ni estimated from the Cromer & Liebermann parameterization,30 together with the energies at which ASAXS were acquired, are shown in Figure 2. The scattered intensity close to the element specific absorption edge can be described by:31

( , ) = ( ) + ( ) () + ( )[ () + () ] ,

(2)

where Inon(q) is the non-resonant scattering as observed far away from the absorption edge, Icross(q) is a cross term describing scattering between the probed element and the matrix and Ires(q) is the resonant scattering. The contributions of Inon(q), Icross(q) and Ires(q) can be obtained by having at least three scattering curves acquired at different energies close to an absorption edge and solving


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the set of linear equations.31 We used singular value decomposition to obtain the scattering contribution of the Ni catalyst particles.

Figure 2. Calculated dispersion corrections for the Ni form factor with Cromer & Lieberman parameterization in electron units (e.u.). The energies at which ASAXS data were acquired are marked (o). The analysis of the separated Ni small angle scattering cross sections were performed with the SASfit software,32 using a model for spherical core/shell particles ( , , Δ, ) = ()() [ !( , , Δ) " !( , , Δ(1 " )] d ,

(3)

where the particle size distribution is N(r), the particle volume V(r), the scattering length density (SLD) difference between the shell and the matrix ∆η, the ratio between radius of the core and overall radius ξ, and the SLD difference between the core and the matrix relative to the shell ν. The form factor F(q,r,∆η) for a homogeneous sphere is given by ! ( , , Δ) = Δ

%[&'(())*) +,& ())] ())-

(4)

The Ni/NiO particle size distribution N(r) was assumed to follow a lognormal distribution,



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( ) =

.

/ √1

exp 5"

[6(()*7]8 / 8

9,

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(5)

with the mean, µ, standard deviation, σ and the particle radius r. RESULTS AND DISCUSSION CHARACTERIZATION OF THE FRESH SAMPLE TEM TEM micrographs of the fresh catalyst sample were acquired to get an estimate of the particle shape and radius of the NiO/Ni catalyst, see Figure 3. The Ni/NiO particles exhibit irregular shapes, and as the particle shapes are random and do not exhibit strong anisotropy, we concluded that they can be approximated with spherical symmetry for the ASAXS data analysis.

Figure 3. TEM of a fresh catalyst sample.


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The TEM micrographs were utilized to achieve an estimate of the Ni/NiO particle size distribution (PSD), evaluated from measuring 125 individual particles. From a least square fit of equation 5 to the PSD we obtained for the mean, µ = 0.99 and standard deviation, σ = 0.58 corresponding to a mean particle radius of 3.2 ± 1.9 nm. The relative abundance of the particles is well described by the fit, see Figure 4. The parameters refined from TEM have a limited significance due to the small number of measured particles for the analysis and the detection of only a few larger particles would change the size distribution drastically. A significant mean particle radius is determined utilizing ASAXS, where a many orders of magnitude larger number of particles contribute to the scattering pattern. The parameters of the PSD refined from the TEM are however a reasonable starting guess for the ASAXS analysis.

Figure 4. Particle size distribution of the Ni/NiO of a fresh 22 wt% Ni/MgAl2O4 catalyst sample determined from (HR)TEM (red bars) and fit with a lognormal probability density function (blue line). WAXS The crystalline phases of the fresh catalyst were determined from ex situ powder diffraction performed at room temperature using the standard X-ray powder diffractometer, see Figure 5. The



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catalyst support was found to consist of MgAl2O4 (spinel; space group #227: Fd-3m) and α-Al2O3 (space group #167: R-3c) and the catalyst particles of Ni (space group #225: Fm-3m) and NiO (space group #225: Fm-3m).

Figure 5. High resolution powder diffraction pattern of a fresh catalyst at room temperature. The phase content of elemental Ni in the fresh catalyst sample was identified by observation of the Ni (200), (220), (311), and (222)-peaks and by a shoulder on the (111)-reflection on the low qside of the MgAl2O4 (400)-peak. The presence of NiO in the catalyst sample was indicated by the weak, broad NiO(220) reflection at q = 4.16 Å-1, see Figure 5. The analysis of the in situ WAXS data, acquired with the CCD detector, was more complicated than the analysis of the pattern recorded with the standard X-ray powder diffractometer. The lower resolution of the CCD compared to the standard X-ray powder diffractometer in the present setup resulted in an overlapping of most of the relevant Ni- and NiO-reflections with reflections from the catalyst support, which complicated the analysis of the diffraction pattern. To overcome this problem, we performed Rietveld refinement on a pattern recorded with the standard powder diffractometer setup and applied it to refine the patterns acquired with the CCD camera taking the different instrumental resolution refined with a Si standard into account. An example of a Rietveld


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refinement from a WAXS acquired with the CCD camera of the fresh catalyst sample at 298 K is given in Figure 6.

Figure 6. Rietveld refinement of a fresh Ni/spinel catalyst sample at 298 K acquired with the standard powder diffractometer setup (a) and with the in situ setup (b). Refined sample phases (o) from top to bottom, Ni, α-Al2O3, MgAl2O4 and NiO. The experimentally observed and the simulated intensities show good agreement for both patterns recorded in a standard X-ray powder diffractometer and with the CCD camera; the results refined from WAXS recorded at 298 K are shown in Table 1. The uncertainties listed in Table 1 were estimated by reference to the standard deviations reported for Rietveld refinement Round-Robin studies of the domain size from ceria sample33 and for phase quantities refined from mixtures with different corundum, fluorite and zincite concentrations34. The estimated uncertainties for the refined crystallite sizes and phase quantities for the patterns acquired with the CCD camera are most likely higher because of the low instrumental resolution in the experimental geometry.



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Crystallite size

Weight fraction

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Phase

Std. Powd. Diff.

CCD

MgAl2O4

41 (1) nm

-*

α-Al2O3

95 (1) nm

-*

Ni

13 (1) nm

9 (1) nm

NiO

3 (1) nm

2 (1) nm

MgAl2O4

67 (4) % wt

67 (4) % wt

α-Al2O3

7 (2) % wt

7 (2) % wt

Ni

14 (2) % wt

11 (1) % wt

NiO

12 (1) % wt

15 (2) % wt

Table 1. Crystallite sizes from line broadening and weight fraction of a fresh catalyst sample at 298 K determined with Rietveld refinement of patterns recorded with the CCD camera and a standard powder diffractometer. *) Not determined, because the instrumental broadening is huge compared to the size broadening expected for the MgAl2O4 and α-Al2O3 crystallites. Crystallite sizes determined from the Rietveld refinement of Ni and NiO from data acquired with both setups agree well, although the crystallite sizes refined from the patterns recorded with the CCD camera are about 30 % smaller compared to those observed with the standard X-ray powder 14 ACS Paragon Plus Environment

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diffractometer, which might be explained by differences in the instrumental resolution determined for both setups from a standard reference material. For comparison with the TEM data the volume averaged diameter was determined from the distances measured in the TEM micrographs, assuming the particles exhibit spherical particle morphology and neglecting one very large particle. The volume averaged diameter from TEM corresponds to dV = 12 nm and this in good agreement with the average Ni-crystallite size of 13 nm, determined by Rietveld refinement of the patterns recorded in the standard powder diffractometer and with the reported crystallite size of 12 nm by Sehested et al.,22 determined from size broadening of the Ni(200) reflection. The relative abundance of the different phases refined from patterns in both setups show good agreement. Crystallite sizes for MgAl2O4 and α-Al2O3 could only be refined from the standard powder diffractometer data, since the maximum resolvable size with the CCD camera in the current setup is around 18 nm. Our primary interest in this study is to resolve the processes of the Ni- and NiO-particles in situ during sample annealing in a hydrogen atmosphere and alterations of the support particles in the measured time frame and temperature interval are not expected.

IN SITU REDUCTION The initiation of the reduction could be observed in the WAXS pattern as a decrease in relative intensity of the (220)-NiO reflection and an increase in the intensity of the Ni peaks between 373 and 473 K. The WAXS pattern of the fresh catalyst sample at 298 K and the first pattern at 473 K, combined with the simulated contributions of the Ni- and NiO-phase using the parameters determined from Rietveld refinement, are shown in Figure 7.



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Figure 7. WAXS pattern of a fresh catalyst at 298 K (black) and after initiation of the reduction at 473 K (red) and the simulated contributions of the NiO phase at 298 K (blue) and 473 K (green). The SAXS patterns, acquired during heating with 10 K min-1 in H2, exhibit the first changes at high q at temperatures above 393 K, which can be attributed to the reduction of NiO. Continuous changes in the SAXS were observed up to the highest sample temperatures of 773 K, but they were most pronounced at a temperature up to 493 K.

Figure 8. SAXS data at high q during acquired during heating of the sample with 10 K min-1. Kinetic studies of reduction of pure NiO have revealed that the reduction occurs at temperatures as low as 428 K.13 Temperature programmed reduction (TPR) of pure NiO with similar heating 16 ACS Paragon Plus Environment

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ramps to the presented experiment showed maximum hydrogen uptake at temperatures TM between 493 – 640 K.14,35-36 Significantly higher TM of 778 – 1073 K6,37 were observed for the reduction of NiO supported on MgAl2O4 catalysts,6,37 and the reducibility was observed to increase with metal loading. It was assumed that higher Ni loading results in larger NiO particles, which are more readily reduced than finely dispersed NiO.6 A similar trend has been reported for NiO supported on γ-Al2O3 and it was assumed that with increasing Ni loading a higher amount of non-interacting bulk NiO on the surface is formed.35 TPR of a physical mixture of NiO and MgO calcinated at 673 K showed a similar reduction behavior as pure NiO. With increasing calcination temperature, partial interaction between NiO and MgO was observed.36 The herein presented experiments were performed with a pre-reduced catalyst, re-oxidized at low temperature, and the most pronounced changes in the SAXS were observed at temperatures comparable to those reported for unsupported NiO. The reducibility of the Ni/NiO particles on MgAl2O4 at temperatures of bulk NiO may be explained by the fact that already during the first pre-reduction larger Ni particles were formed which are more readily reduced than finely dispersed NiO formed after incipient wetness impregnation of the MgAl2O4 support and calcination. Rietveld refinement was performed on all the powder diffraction patterns acquired during the in situ reduction, but the complete set of sample parameters was only refined at 298 K. At higher temperatures, only the scaling factors for the phases present in the sample, the background, the lattice parameter of MgAl2O4 and Ni and the size broadening of Ni and NiO were refined. Lattice parameters of α-Al2O3 and NiO were fixed during the refinement to improve convergence. Refinement of the lattice parameters of α-Al2O3 or NiO was observed to be unstable due to the minor phase contribution of α-Al2O3 and the small crystallite size of NiO resulting in broad peaks. The influence of the sample temperature on the lattice parameter was corrected by taking the linear thermal expansion for α-Al2O3 (6.5 × 10-6 K-1) and NiO (14.1 × 10-6 K-1) into account. The relative



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weight fractions were obtained from the Rietveld refinement phase scale factors. Figure 9 illustrates the molar Ni- and NiO-content as a function of time and sample temperature.

Figure 9. The molar fraction x(i) = n(i) / n(total) of Ni(black) and NiO(blue) and the applied temperature (red) as a function of time during the in situ reduction. Quantitative phase analysis suggests that the reduction of NiO is a function of both sample temperatures and time, which is consistent with observations reported in the literature.13-15 Below sample temperatures of 473 K, the Ni and NiO fractions seem to be unaltered. Small fluctuations of the Ni and NiO fraction can be explained by uncertainties in the Rietveld refinement. An increase of the molar Ni fraction and a corresponding decrease of the molar NiO fraction were noted at temperatures of 473 K and above, indicating the onset of the reduction. To verify the significance of the refined phase quantification, we summed the molar fractions of Ni and NiO from each refined pattern because the sum of both fractions (the amount of Ni) should remain constant. Overall, a decrease of the summed molar fractions from 0.43 at 298 K to 0.39 at 873K was noted. This would correspond to a mass fraction of w(Ni) = 24 % ± 1 % at the start and w(Ni) = 21 ± 1 % at the end of the experiment, if all NiO is assumed to be reduced to Ni. The results of the quantification from


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Rietveld refinement are in good agreement with the actual concentration of the catalyst of w(Ni) = 22 %. Deviations of the determined molar fractions during the reduction can most probably be related to too small refined NiO crystallite sizes. The quality of the refinement of the NiO crystallite sizes was limited due to strong overlap of the broad and weak NiO reflections with those of other sample phases (see Figure 7). Figure 10 illustrates the refined Ni lattice parameter as a function of the sample temperature during sample heating in hydrogen.

Figure 10. Ni lattice parameter obtained from Rietveld refinement during a heating experiment in steps of 100 K (o) and additional acquired data at intermediate temperatures (:) on another similarly prepared sample to resolve the discontinuity between 373 and 473 K. During heating experiments in hydrogen, a linear increase of the Ni lattice parameter was observed at temperatures below 373 K and above 473 K. This can be related to thermal expansion of the Ni crystal lattice. However, with initiation of the reduction, in the temperature interval 373 473 K, a decrease of the Ni lattice parameter was noted. An additional experiment with the same catalyst sample was performed at temperatures of 393 K, 413 K, 433 K, 453 K, and 533 K to resolve the interval where the lattice parameter changes and confirmed the observation, localizing it



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to temperatures between 413 and 453 K. Heating experiments performed in helium did not show a decrease of the Ni lattice parameter at any temperature and we therefore conclude that it is related to the initiation of the reduction. The decrease of the lattice parameter at the initiation of the reduction can be explained by the following observations. Ni nanoparticles are observed to form a passivating NiO surface layer of around 2 nm, irrespective of their grain size.38-

40

Phase identification (see Figure 5) of a fresh

catalyst sample and the good agreement of the simulated and experimental data during the Rietveld refinement (see Figure 6) show that Ni and NiO are both in the face centred cubic phases (space group #225: Fm-3m). The lattice parameter of Ni (3.52 Å25) is only 19 % smaller than the lattice parameter of NiO (4.18 Å) at ambient temperature.26 On this account, the lattice parameter of the Ni core relaxes due to interfacial stress between the metallic core and the oxide shell;41 this effect was reported to be particle size dependent. With decreasing particle sizes, increasing Ni lattice parameters were noted.41,42 The decrease of the lattice parameter during the in situ reduction was observed in a temperature interval where the reduction initiates, indicated by a decreasing intensity of the NiO(220)-reflection in the WAXS pattern. Therefore, we conclude that the decrease of the Ni lattice parameter can be directly related to the reduction of the NiO shell, whereby stress due to the lattice mismatch is alleviated.


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Figure 11. Ni/NiO core shell particle on MgAl2O4-support.

Ni/NiO core-shell particles were also observed in the HRTEM micrographs. Figure 11 shows a Ni/NiO core-shell particle on a MgAl2O4 substrate. The Ni core shows characteristic lattice fringes for the Ni(200) planes. No evidence of grain boundaries from crystallite domains were noted, indicating that this core consists of single crystalline Ni. However, other Ni particles have been observed to be polycrystalline, and this is consistent with observations of Sehested et al.22 Lattice distortions of the Ni lattice were noted close to the Ni/NiO interface, indicating stress due to the lattice mismatch of Ni and NiO. In contrast to the observed core, the shell is polycrystalline, with several domains with fringes comparable to the NiO(111) and NiO(200) plane being observed. The SAXS patterns of the Ni/MgAl2O4 catalyst prior to reduction measured at 220, 91, 39, 17 and 7 eV below the Ni-K-absorption edge at 8333 eV are shown Error! Reference source not found.. The difference of the SAXS is caused by the energy dependent changes of the dispersion corrections, see Figure 2.



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Figure 12. Small angle X-ray scattering cross sections of the Ni/MgAl2O4 catalyst prior to reduction, multiplied by q4 to emphasize the intensity variation at the five energies measured below the Ni-absorption edge at 8333 eV. Separation of the small angle X-ray scattering cross section of the Ni/NiO particles was performed by singular value decomposition. The separated scattering cross sections for the fresh and reduced catalyst particles were analysed by least square fitting of a model for a spherical core/shell particle, described in equations 3 and 4, assuming a lognormal particle size distribution. The separated SAXS curves for the fresh Ni/NiO particles and for the same particles after reduction at 773 K are shown together with the best fits in Figure 13.


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Figure 13. Separated Ni small angle scattering cross section of the fresh (o) and reduced (;) catalyst particles and the best fits obtained from a fit of polydisperse spherical core/shell particles (red curves). The parameters obtained from the least square fit indicate that the fresh Ni/NiO core shell particles have a mean radius of rASAXS = 3.7 ± 2.0 nm, and this is in good agreement with the mean radius obtained from analysis of the TEM micrographs of rTEM = 3.2 ± 1.9 nm. For the ratio between core and overall size we obtained ν = 0.6, which would correspond to a core radius of 2.2 nm and a shell thickness of 1.5 nm. Analysis of the separated small angle scattering cross section of the Ni particles at 773 K resulted in a ratio between core and overall size of ν = 1, indicating that the particles exhibit a homogeneous electron density. The same result was obtained when fitting the separated scattering cross section with a fixed ratio between core and overall size of ν = 0.6, taking the SLD difference between the core and the matrix relative to the shell ξ as a free parameter. The fit converged to a value of ξ = 1, supporting that the particles had a homogeneous electron density. We conclude that the NiO shell had been completely reduced and that the particles consisted of pure Ni under the experimental conditions. The simulated scattering cross section of the Ni/NiO and Ni particles is plotted together with the SAXS acquired at an energy of 8112 eV before and after reduction in Figure 14. The scattering cross sections were simulated using the parameters obtained



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from the least square fit of the separated cross sections from the ASAXS data. The plot shows that the changes of the SAXS at high q can be attributed to the reduction of the NiO shell. SAXS data acquired simultaneously with the in situ WAXS in the home laboratory shows similar behaviour at high q (see supporting information).

Figure 14. SAXS acquired at 8112 eV of the fresh (black) and reduced (red) w(Ni) = 22wt% Ni/MgAl2O4 catalyst and simulated scattering cross section for the Ni/NiO core shell (blue) and pure Ni particles (magenta).

CONCLUSION In this study, in situ WAXS and ASAXS were utilized to follow the changes of a 22 wt% Ni/MgAl2O4 catalyst during reduction in hydrogen. Quantification of the relative abundance of Ni and NiO phases reveals that the reduction initiates in a temperature interval from 373 - 473 K and completes at temperatures above 673 K. A decrease of the Ni lattice parameter in a temperature range from 413 - 453 K was observed and indicates the reduction of the NiO shell. This is also consistent with ASAXS experiments confirming that the initial particles show a Ni/NiO core/shell structure. The decrease of the Ni lattice parameter can be explained by contraction of the Ni lattice


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due to release of interfacial stress during the reduction of the NiO shell. HRTEM confirmed the presence of Ni/NiO core shell particles in the initial catalyst sample.

ACKNOWLEDGEMENTS We thank Steen Bang, Ove Rasmussen, Torben Kjær and Jens Borchsenius for maintenance of the equipment and technical support during the implementation of the in situ laboratory setup, Hjalte Sylvest Jacobsen for improvement of the WAXS 1D data transformation, Henning Engelbrecht for guidance regarding LabVIEW® and data acquisition during the implementation of the sample gas system, beamline scientist Ulla Vainio from B1 at HASYLAB for all the help provided during our beam time and the Danish National Research Foundation's Center for Individual Nanoparticle Functionality (DNRF54),

the Copenhagen Graduate School for

Nanoscience and Nanotechnology (C:O:N:T), the Danish Council for Independent Research | Natural Sciences (DANSCATT) and Haldor Topsøe A/S for financial support. The Norwegian Research Council is acknowledged for financial funding through the SYNKROTRON program. CINF is funded by the Danish National Research Foundation.

ASSOCIATED CONTENT Supporting Information In situ SAXS acquired simultaneously with the WAXS in our home laboratory SAXS/WAXS setup and illustration of the separation of the Ni small angle scattering cross section from ASAXS



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measurements with a singular value decomposition based approach. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES 1. Rostrup-Nielsen, J. R.; Bak Hansen, J.-H. CO2-Reforming of Methane over Transition Metals J. Catal. 1993, 144, 38-49. 2. Gadalla, A. M.; Bower, B. The Role of Catalyst Support on the Activity of Nickel for Reforming Methane with CO2 Chem. Eng. Sci. 1988, 1988, 3049 – 3062. 3. Holm-Larsen, H. CO2 Reforming for Large Scale Methanol Plants – an Actual Case Stud. Surf. Sci. Catal. 2001, 136, 441-446. 4. Noweck, K.; Grafahrend, W. Fatty Alcohols - Ullmann's Encyclopedia of Industrial Chemistry - online edition, 2006, 117 - 141. 5. Wei, J. M.; Iglesia, E. Isotopic and Kinetic Assessment of the Mechanism of Reactions of CH4 with CO2 or H2O to Form Synthesis Gas and Carbon on Nickel catalysts J. Catal. 2004, 224, 370 – 383. 6. Guo, J.; Lou, H.; Zhao, H.; Chai, D.; Zheng, X. Dry Reforming of Methane over Nickel Catalysts Supported on Magnesium Aluminate Spinels Appl. Catal. A 2004, 273, 75 – 82. 7. Borowieki, T. Nickel Catalysts for Steam Reforming of Hydrocarbons; Size of the Crystallites and Resistance to Coking Appl. Catal. 1982, 4, 223 – 231.


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17. Jeangros, Q, Hansen T. W. , Wagner, J. B., Damsgaard, C. D., Dunin-Borkowski, R. E., Hébert, C., Van herle, J., Hessler-Wyser, Reduction of Nickel Oxide Particles by Hydrogen Studied in an Environmental TEM J. Mater. Sci. 2013, 48, 2893 – 2907. 18. Kehres, J.; Andreasen, J. W.; Krebs, F. C.; Molenbroek, A. M.; Chorkendorff, I.; Vegge, T. Combined In Situ Small- and Wide-angle X-ray Scattering Studies of TiO2 Nanoparticle Annealing to 1023 K J. Appl. Cryst. 2010, 43, 1400 – 1408. 19. Kehres, J.; Jakobsen, J. G.; Andreasen, J. W.; Wagner, J. B.; Liu, H.; Molenbroek, A.; Sehested, J.; Chorkendorff, I.; Vegge, T. Dynamical Properties of a Ru/MgAl2O4 Catalyst during Reduction and Dry Methane Reforming J. Phys. Chem. C 2012, 116, 21407 – 21415. 20. Andreasen, J. W.; Rasmussen, F. B.; Helveg, S.; Molenbroek, A.; Ståhl, K.; Nielsen, M. M.; Feidenhans’l, R. Activation of a Cu/ZnO Catalyst for Methanol Synthesis J. Appl. Crys. 2006, 39, 209 – 221. 21. Høydalsvik, K.; Fløystad, J. B.; Voronov, A.; Voss, G. J. B.; Esmaeili, M.; Kehres, J.; Granlund, H.; Vainio, U.; Andreasen, J. W.; Rønning, M. & Breiby, D. W. Morphology Changes of Co Catalyst Nanoparticles at the Onset of the Fischer-Tropsch Synthesis. J. Phys. Chem. C 2014, 118, 2399 – 2407. 22. Sehested, J.; Carlsson, A.; Janssens, T. V. W.; Hansen, P. L.; Datye, A. K. Sintering of Nickel Steam-Reforming Catalysts on MgAl2O4 Spinel Supports J. Catal. 2001, 197, 200 – 209. 23. Haubold, H. G; Gruenhagen, K.; Wagener, M.; Jungbluth, H.; Heer, H.; Pfeil, A.; Rongen, H.; Brandenberg, G.; Moeller, R.; Matzerath et al. H. JUSIFA - A New User‐dedicated ASAXS Beamline for Materials Science Rev. Sci. Instrum. 1989, 60, 1943 – 1946. 28 ACS Paragon Plus Environment

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24. Andreasen, J. W.; Rasmussen, O.; Feidenhans’l, R.; Rasmussen, F. B.; Christensen, R.; Molenbroek, A. M.; Goerigk, G. An In Situ Cell for Small-angle Scattering Experiments on Nano-structured Catalysts J. Appl. Cryst. 2003, 36, 812-813. 25. Owen, E. A.; Yates, E. L. X-ray Measurements of the Thermal Expansion of Pure Nickel Philos. Mag. 1936, 21, 809 - 819 26. Schmahl N. G.; Eikerling, G. F. Über Kryptomodifikationen des Cu(II)-Oxids Z. phys. Chem. 1968, 62, 268 – 279. 27. Levy, D.; Pavese, A.; Hanfland, M. Synthetic MgAl2O4 (spinel) at High-pressure Conditions (0.0001 – 30 GPa): a Synchrotron X-ray Powder Diffraction Study Am. Miner. 2003, 88, 93 - 98 . 28. Pillet, S.; Souhassou, M.; Lecmonte, C.; Schwarz, K.; Blaha, K.; Rerat, M.; Lichanot, A.; Roversi, P. Recovering Experimental and Theoretical Electron Densities in Corundum Using the Multipolar Model: IUCr Multipole Refinement Project Acta Cryst. A 2001, 57, 290 – 303. 29. Hunter B. Rietica – A Visual Rietveld Program Int. Union Cryst. Newsletter 1998, 20, 21. 30. Cromer, D. T.; Liberman, D. Relativistic Calculation of Anomalous Scattering Factors for X Rays J. Chem. Phys. 1970, 53, 1891 – 1898. 31. Stuhrmann, H. Resonance Scattering in Macromolecular Structure Research Adv. Polym. Sci. 1985, 67, 123-163. 32. Kohlbrecher, J.; Bressler, I., Software Package SASfit for Fitting Small-angle Scattering Curves, https://kur.web.psi.ch/sans1/SANSSoft/sasfit.html.



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TABLE OF FIGURES Figure 1. Flow scheme of the in situ cell. Figure 2. Calculated dispersion corrections for the Ni form factor with Cromer & Lieberman parameterization in electron units (e.u.). The energies at which ASAXS data were acquired are marked (o). Figure 3. TEM of a fresh catalyst sample. Figure 4. Particle size distribution of the Ni/NiO of a fresh 22 wt% Ni/MgAl2O4 catalyst sample determined from (HR)TEM (red bars) and fit with a lognormal probability density function (blue line). Figure 5. High resolution powder diffraction pattern of a fresh catalyst at room temperature.



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Figure 6. Rietveld refinement of a fresh Ni/spinel catalyst sample at 298 K acquired with the standard powder diffractometer setup (a) and with the in situ setup (b). Refined sample phases (o) from top to bottom, Ni, α-Al2O3, MgAl2O4 and NiO. Figure 7. WAXS pattern of a fresh catalyst at 298 K (black) and after initiation of the reduction at 473 K (red) and the simulated contributions of the NiO phase at 298 K (blue) and 473 K (green). Figure 8. SAXS data at high q during acquired during heating of the sample with 10 K min-1. Figure 9. The molar fraction x(i) = n(i) / n(total) of Ni(black) and NiO(blue) and the applied temperature (red) as a function of time during the in situ reduction. Figure 10. Ni lattice parameter obtained from Rietveld refinement during a heating experiment in steps of 100 K (o) and additional acquired data at intermediate temperatures (∇) on another similarly prepared sample to resolve the discontinuity between 373 and 473 K. Figure 11. Ni/NiO core shell particle on MgAl2O4-support. Figure 12. Separated Ni small angle scattering cross section of the fresh (o) and reduced (;) catalyst particles and the best fits obtained from a fit of polydisperse spherical core/shell particles (red curves). Figure 13. SAXS acquired at 8112 eV of the fresh (black) and reduced (red) w(Ni) = 22wt% Ni/MgAl2O4 catalyst and simulated scattering cross section for the Ni/NiO core shell (blue) and pure Ni particles (magenta).


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