Fe-Containing Magnesium Aluminate Support for Stability and Carbon Control during Methane Reforming

Article abstract of DOI:10.1021/acscatal.8b01039

We report a MgFe_xAl_2-xO₄ synthetic spinel, where x varies from 0 to 0.26, as support for Ni-based catalysts, offering stability and carbon control under various conditions of methane reforming. By incorporation of Fe into a magnesium aluminate spinel, a support is created with redox functionality and high thermal stability, as concluded from temporal analysis of products (TAP) experiments and redox cycling, respectively. A diffusion coefficient of 3 × 10^-17 m² s^-1 was estimated for lattice oxygen at 993 K from TAP experiments. X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) modeling identified that the incorporation of iron occurs as Fe³⁺ in the octahedral sites of the spinel lattice, replacing aluminum. Simulation of the X-ray absorption near edge structure (XANES) spectrum of the reduced support showed that 60 ± 10% of iron was reduced from 3+ to 2+ at 1073 K, while there was no formation of metallic iron. A series of Ni/MgFe_xAl_2-xO₄ catalysts, where x varies from 0 to 0.26, was synthesized and reduced, yielding a supported Ni-Fe alloy. The evolution of the catalyst structure during H₂ temperature-programmed reduction (TPR) and CO₂ temperature-programmed oxidation (TPO) was examined using time-resolved in situ XRD and XANES. During reforming, iron in both the support and alloy keeps control of carbon accumulation, as confirmed by O₂-TPO on the spent catalysts. By fine tuning the amount of Fe in MgFe_xAl_2-xO₄, a supported alloy was obtained with a Ni/Fe molar ratio of ～10, which was active for reforming and stable. By comparison of the performance of Ni-based catalysts with Fe either incorporated into or deposited onto the support, the location of Fe within the support proved crucial for the stability and carbon mitigation under reforming conditions.

Full text of DOI:10.1021/acscatal.8b01039

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Article
Fe-containing magnesium aluminate support for
stability and carbon control during methane reforming
Stavros Alexandros Theofanidis, Vladimir V. Galvita, Hilde Poelman,
Naga Venkata Ranga Aditya Dharanipragada, Alessandro Longo, Maria
Meledina, Gustaaf Van Tendeloo, Christophe Detavernier, and Guy B Marin
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01039 • Publication Date (Web): 22 May 2018
Downloaded from http://pubs.acs.org on May 22, 2018
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obtained with a Ni/Fe molar ratio of ~10, which was active for reforming and stable. By
comparing the performance of Ni-based catalysts with Fe either incorporated into or
deposited onto the support, the location of Fe within the support proved crucial for the
stability and carbon mitigation under reforming conditions.
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Keywords: syngas, Ni-Fe alloy, carbon, synthetic spinel, lattice oxygen, redox properties
1. Introduction
Syngas production from methane has been widely investigated using various reforming
technologies like steam reforming (SMR), dry reforming (DRM), bi-reforming (BIM) 1-2, partial
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oxidation (POM) and autothermal reforming (ARM) over different series of supported
catalysts 4. The used oxidant, energetics and the final H2:CO ratio differ in these processes 5.
To date, only one plant with combination of steam and dry reforming has recently been
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demonstrated by the Japan Oil, Gas and Metals National Corporation . No other industrial
technology for DRM has yet been developed because the selection and design of a suitable
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reforming catalyst remain an important challenge . Nickel-based catalysts are preferred
over noble metals due to their high availability and lower cost. However, they are sensitive
to deactivation by carbon deposition and sintering 8-10. Hence, much research effort is
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focused on improving catalyst activity and stability by the addition of promoters
synthesis of bimetallic catalysts 13-15 and changing the nature of the support 7, 16
,
.
The support plays a crucial role in the activity and stability of a catalyst and therefore it
has been intensely investigated for the aforementioned reforming reactions. The required
properties for a support material are: 1) high temperature resistance, 2) ability to maintain
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the dispersion of active metals during reaction conditions and 3) oxygen storage capacity .
Al2O3 has been widely used as a catalyst support 10, 18, while various other support materials
such as MgO, ZrO2, La2O3, TiO2, SiO2, SiO2- Al2O3 have also been examined 18-27
.
There has recently been a resurgence of research into Fe-modified Ni catalysts because
the addition of Fe provides redox functionality to the catalyst, giving it the ability to restrain
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carbon accumulation 10, 13, 28-30. Ashok and Kawi studied toluene steam reforming over a
Ni-Fe catalyst supported on Fe2O3-Al2O3. They concluded on the existence of a strong metal-
support interaction, which contributed to the elimination of sintering and thus resulted in
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stable catalytic performance. Kim and co-workers
compared bimetallic Ni-Fe catalysts,
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supported on magnesium aluminate hydrotalcite precursors (with a MgxAlyOz matrix), to
monometallic Ni and Fe, under DRM at 923 K. The bimetallic Ni-Fe catalyst, with Ni/(Ni+Fe)
ratio of 0.8, showed the highest activity and stability. These results were in line with the
previous work of Theofanidis and co-workers 13, who attributed the high activity and stability
of Ni-Fe catalysts to the ability of Fe to be oxidized under reforming conditions to FeOx,
which then reacts, via a redox mechanism, with the carbon deposits. The reducibility of Ni is
improved by adding Fe according to Djaidja et al. 31, who synthesized Ni-Fe/MgO and (Ni-Fe-
Mg)2Al catalysts. The activity results showed high performance and a good resistance against
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carbon formation. Buelens and co-workers used Fe2O3 supported on MgAl2O4 as a solid
oxygen carrier material for the super-dry reforming process, where three molecules of CO2
are consumed per one CH4, resulting in an enhanced CO production. On the other hand,
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More and co-workers
used a Ni-Fe alloy as oxygen carrier in chemical looping dry
reforming because the multiple oxidation states of Fe enable tuning of product selectivities
when CH4 is used as a fuel.
Despite years of research, Ni-Fe catalysts still do not provide the required stability. A
major reason is alloy restructuring, induced by changes in the gas phase environment.
During methane reforming, Fe segregates from the alloy 13, 29 due to interaction with the gas
phase oxidizing gas (CO2 or H2O). This Fe segregation process blocks the Ni sites, leading to
catalyst deactivation 34. The present study proposes a strategy for in-situ synthesis of a Ni-Fe
catalyst, supported on an adapted spinel structure, which remains stable under various
methane reforming conditions. Spinels are well-known supports with a variety of
applications and adjustable properties, like element delivery, redox capacity and oxygen
mobility. As such, they can provide catalysts that demonstrate catalytic performances
competitive with noble metals 35. By incorporating Fe into magnesium aluminate, a synthetic
MgFexAl2-xO4 spinel was prepared as support for Ni-Fe for use in reforming reactions. The
location of Fe affects the catalyst properties. To identify the location of Fe, advanced XAS
characterization and modelling were applied, while a transient response technique,
Temporal Analysis of Products (TAP), exemplified the redox properties of the support, based
on oxygen mobility. The resulting catalyst proved stable and capable of removing carbon
while reforming methane.
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2. Experimental methods
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2.1 Support and catalyst preparation
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Support preparation
MgFexAl2-xO4 support materials (where x = 0, 0.007, 0.013, 0.04, 0.09, 0.13, 0.18 and
0.26) were prepared by co-precipitation from an aqueous solution of Mg(NO3)2·6H2O (99%,
Sigma-Aldrich®), Al(NO3)3·9H2O (98.5%, Sigma-Aldrich®) and Fe(NO3)3·9H2O (99.99+%, Sigma-
Aldrich®). A precipitating agent, NH4OH (ACS reagent, 28.0-30.0% NH3 basis) was added to
adjust the pH to 10, at 333 K. The produced precipitate was filtered, dried at 393 K for 12 h
and subsequently calcined in air at 1073 K for 4 h. The calcined support samples, named “as-
prepared”, are labeled using their weight percent “z”, as zFe, shown in Table 1.
Catalyst preparation
8wt%Ni catalysts were prepared by incipient wetness impregnation on the support
materials (MgFexAl2-xO4) using an aqueous solution of nitrate Ni(NO3)2·6H2O (99.99+%,
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Sigma-Aldrich®) . The catalysts were dried at 393 K for 12 h and subsequently calcined in
air at 1073 K for 4 h. The calcined samples, named “as-prepared”, are labeled as Ni/zFe
where z is the Fe weight percent of the support. In addition, two bimetallic Ni-Fe catalysts
(8wt%Ni-4wt%Fe and 8wt%Ni-1wt%Fe) were prepared by incipient wetness co-impregnation
on the regular MgAl2O4 support, named as “Ni-4.0Fe” and “Ni-1.0Fe” for comparison
purposes (see § 3.3.1), using an aqueous solution of corresponding nitrate Ni(NO3)2·6H2O
(99.99+%, Sigma-Aldrich®) and Fe(NO3)3·9H2O (99.99+%, Sigma-Aldrich®).
2.2 Support and catalyst characterization
The Brunauer-Emmett-Teller (five point BET) surface area and porosity of each sample
were determined by N2 adsorption at 77 K (Tristar Micromeritics) after outgassing the
sample at 473 K for 2 h. The crystallographic phases of the as-prepared, reduced and re-
oxidized materials were determined by ex-situ XRD measurements (Siemens Diffractometer
Kristalloflex D5000, Cu Kα radiation). The powder patterns were collected in a 2θ range from
10o to 80o with a step of 0.02o and 30 s acquisition per angle. XRD patterns of known
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compounds are referenced by their corresponding number in the Powder Diffraction File
database. By fitting a Gaussian function to a diffraction peak, the crystallite size was
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determined from the peak width via the Scherrer equation , while the peak position gave
information about the lattice spacing based on the Bragg law of diffraction 38.
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The bulk chemical composition of supports and as-prepared catalysts was determined by
means of inductively coupled plasma atomic emission spectroscopy (ICP-AES, ICAP 6500,
Thermo Scientific). The samples were mineralized by acid fusion.
The Fe-K edge XANES (X-ray absorption near edge structure) and EXAFS (Extended X-ray
absorption fine structure) spectra of the support (MgFexAl2-xO4) were collected at the Dutch-
Belgian Beam Line (DUBBLE, BM26A) at the European Synchrotron research Facility (ESRF).
The measurements were carried out in transmission mode using Ar/He filled ionization
chambers at ambient temperature and pressure. The energy of the X-ray beam was tuned
using a double crystal monochromator operating in fixed exit mode using a Si (111) crystal
pair.
In-situ XANES of the support was performed during H2 temperature programmed
reduction (TPR) (0.2 NmL/s of 5%H2/He using a heating ramp of 10 K/min, maximum
temperature = 1073 K) in a capillary reactor with an internal diameter of 1 mm. The capillary
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reactor was implemented in a frame which was connected to gas feed lines
through
Swagelok fittings, while the inlet gas flow rates were maintained by means of calibrated
Brooks mass flow controllers. After reduction, the sample named “reduced” was subjected
to re-oxidation by CO2 without removing it from the reactor, while recording in-situ XANES
(thereafter named “re-oxidized”).
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Theoretical simulations of XANES spectra were performed with the FDMNES software
package. The XANES spectra, three scans per sample, were energy-calibrated, averaged and
analyzed using the multiple scattering theory based on the muffin-tin approximation of the
potential well. The muffin-tin radii were tuned to have a 10% overlap between the different
spherical potentials and the Hedin–Lundqvist exchange potential was applied. Further, a
core-hole broadening of 1.2 eV was used, and the FWHM of the Gaussian used for the
energy resolution was set to 2 eV. The XANES spectrum was simulated considering all atoms
surrounding the Fe absorber within a 7 Å radius sphere. The same approach was applied for
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the as-prepared i.e. non-distorted and reduced i.e. distorted Fe environment. The
approximation of non-excited absorbing atoms was used which better reproduces the
experimental data. The effect of structural disorder in the XANES simulations was not
considered.
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The linear combination fitting (LCF) range was limited to a [-30eV, + 30eV] range around
the edge µ(E) inflection point to limit the metal amount estimation based on pre-edge
features. For the fitting of the Ni-K and Fe-K edges, the standards used were metallic Ni and
Fe, respectively, along with the “as-prepared” material.
EXAFS spectra of the as-prepared support materials were measured on pellets diluted
with a suitable concentration of inert diluent BN to avoid self-absorption. These spectra
were measured in a closed cycle He-cryostat (Oxford Instruments) at 80 K to minimize the
thermal noise contributions from the Debye Waller factors. The EXAFS measurements,
following the H2-TPR (named “reduced”) and CO2-TPO treatment (named “re-oxidized”)
were carried out at room temperature. Three scans per material were measured, energy
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calibrated, averaged and then analyzed using the GNXAS methodology . In this approach,
the local atomic arrangement around the absorbing atom is decomposed into model atomic
configurations containing 2, ..., n atoms. The theoretical EXAFS signal χ(k) is given by the sum
of the n-body contributions γ2, η3, ..., which take into account all possible single and
multiple scattering paths between the n atoms. The fitting of χ(k) to the experimental EXAFS
signal allows to refine the relevant structural parameters of the different coordination shells;
the suitability of the model is evaluated by comparison of the Fourier transformed (FT)
experimental EXAFS signal with the FT of the calculated χ(k) function. The global fit
parameters that were allowed to vary during the fitting procedure were the distance R(Å),
Debye-Waller factor (σ2) and the angles of the η3 contributions, which were defined
according to the spinel structure used in the data analysis. The edge energy E0 was defined
at 7112 eV according to the value of the Fe foil.
For the analysis of the “as-prepared” and “re-oxidized” support samples, one γ2 term,
representing the six fold Fe-O distance (R1) at ~1.97 Å in the octahedral spinel site, as well as
two three-body configurations η3 involving the Fe-O-M (M = Al, Mg) triangular arrangement
(Figure S1) and their multiple scattering effects were taken into account. In this case the
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coordination numbers were released as free parameters. The obtained values are reported
in Table 2.
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For the reduced support sample, two different Fe-O distances can be detected in the
EXAFS data which can be identified as a close to unmodified Fe3+ (non-distorted) and a
distorted Fe2+ local environment. The existence of two different local Fe environments is in
good agreement with XANES. Therefore, the Fe local environment has been modeled
considering two γ2 terms, corresponding to Fe-O distances of ~1.92 Å and ~2.10 Å for non-
distorted and distorted Fe sites respectively, to account for partial reduction. Accordingly,
four η3 three-body configurations, involving the Fe-O-M (M = Al, Mg) were used. During the
reduction treatment, Fe3+ reduces to Fe2+ which has a larger ionic radius 42. Thus, an increase
of the average Fe-O distance is expected upon reduction.
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In order to carry out a quantitative assessment of the two phases, the total coordination
number of the first shell was constrained to its fully coordinated value ± 1 (coordination
number Fe-O equal to 6) and the respective contributions were parameterized as a function
of the total Fe amount (parameter β, Table 2). The main reason to fix the coordination
numbers is that the error in this parameter is higher than the number of possible oxygen
vacancies confined to the Fe neighborhood, which can be present in the reduced structure.
Because the disorder term of the first shell is correlated to the ones of the higher shells,
whose distance is correlated to the first shell, the Debye Waller factor was fixed to the value
as obtained from the single shell analysis.
The suitability of the model was evaluated by comparing the Fourier transform (FT) of
experiment and model data to refine parameters such as coordination numbers, bond
distances and Debye-Waller factors from the two-body and three-body configurations, which
were defined according to the crystallographic structures used in the model.
2.3 In-situ quick-XAS (QXAS) of Ni catalysts
An in-situ QXAS study was performed for Ni catalysts supported on MgFexAl2-xO4 during
H2-TPR (0.2 NmL/s of 5%H2/He using a heating ramp of 10 K/min, up to 1073 K). QXAS was
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measured in transmission at the ROCK beam line of the SOLEIL synchrotron using one
oscillating monochromator over both Fe-K and Ni-K edges (7112 and 8331 eV), by means of a
macro for fast switching between edges. Approximately 5mg of as-prepared material, 50%
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diluted by boron nitride, was inserted into a 2 mm quartz capillary reactor and fixed by
quartz wool plugs. The capillary reactor was implemented in a frame which was connected
to gas feed lines through Swagelok fittings while the inlet gas flow rates were maintained by
means of calibrated Brooks mass flow controllers.
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2.4 In-situ time resolved XRD
In-situ XRD measurements were performed in a reactor with Kapton foil window for X-ray
transmission inside a Bruker-AXS D8 Discover apparatus (Cu Kα radiation of 0.154 nm). The
setup is equipped with a linear detector covering a 2θ range of 20o with an angular
resolution of 0.1o. Patterns were acquired in 10 s time and temperatures were measured
with a calibrated K-type thermocouple. For each sample, approximately 10 mg of powdered
sample was evenly spread on a single crystal Si wafer. No interaction of the catalyst material
with the Si wafer was observed. Before each experiment the reactor chamber was evacuated
to a base pressure of 4 Pa by a rotation pump. Gases were supplied to the reactor chamber
with calibrated mass-flow controllers. He (10 NmL/s) was flowing for 10 min before the flow
was switched to 10 NmL/s of 5%H2/He or CO2 for TPR and TPO experiments (up to 1123 K,
heating rate of 20 K/min), respectively. Peaks in the in-situ XRD patterns appeared at slightly
shifted angular positions compared to both full scans and tabulated values due to
temperature-induced lattice expansion and different sample heights. These positions shifts,
not related to underlying physicochemical processes, were taken into account during peak
assignment.
2.5 Isothermal Temporal Analysis of Products (TAP) experiments
Transient measurements were performed in a TAP-3E reactor (Mithra Technologies, St.
Louis, USA) equipped with an Extrel Quadrupole Mass Spectrometer (QMS). The details of
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TAP experiments can be found in . For the experiments, 20 mg (250<d<500 μm catalyst
fraction) of the calcined catalyst was placed in a quartz microreactor (I.D. = 4 mm and ~2 mm
bed length), which was located between two inert beds of quartz particles with the same
sieved fraction. The temperature of the catalyst was measured by a K-type thermocouple
housed inside the catalytic zone. Prior to the experiments, the catalyst was reduced in a 1
NmL/s flow of 30%H2/Ar at 1123 K (heating rate of 20 K/min) and atmospheric pressure. A
series of CH4 pulses (~10-7 mol/pulse) were fed isothermally at 993 K. For every 1 second,
data were recorded with millisecond time resolution in each pulse. CO2, CO, H2, H2O, CH4
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and Ar (internal standard) responses were monitored at amu signals of 44, 28, 2, 18, 16 and
40, respectively.
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2.6 Catalytic activity
Activity and stability measurements at 1023 K and 111.3 kPa were performed in a quartz
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fixed bed reactor with an internal diameter of 9 mm, which was housed inside an electric
furnace. A coated (by amorphous silicon) incoloy alloy 800-HT reactor of the same
dimensions was used for the high pressure experiments (911.7 kPa). The activity of the
reactor was tested in a blank experiment at reaction conditions, before each measurement.
The temperature of the catalyst bed was measured with K-type thermocouples touching the
outside and inside of the reactor at the position of the catalyst bed. The inlet gas flow rates
were maintained by means of calibrated Bronkhorst mass flow controllers. A sample with
particle size fraction of ~50 μm was diluted with inert α-Al2O3 for improved heat conductivity
(ratio catalyst/inert ~1/70) and packed between quartz wool plugs, resulting in
approximately 10-2 m of catalyst bed length.
Prior to each experiment, the as-prepared sample was reduced in a 1 NmL/s flow of
30%H2/Ar at 1123 K (heating rate of 20 K/min) for 20 minutes (named “reduced”) and then
the flow was switched to 1 NmL/s of Ar for 10 minutes. A mixture of CH4, CO2 and Ar (total
flow of 105 NmL/min, volumetric ratio CH4:CO2:Ar = 1:1:0.5, Ar internal standard) was used
for DRM and a mixture of CH4, CO2, H2O and Ar (total flow of 300 NmL/min, 15% CH4, 11%
CO2, 4% H2O and 70% Ar, Ar internal standard) for BIM, while the produced CO, H2 and
unconverted CH4 and CO2 were detected at the outlet using a calibrated OmniStar Pfeiffer
mass spectrometer (MS). MS signals were recorded for all major fragments. For
quantification of reactants and products, the MS was focused to different amu signals. H2
was monitored at 2, CH4 at 16, H2O at 18, CO at 28, Ar at 40 and CO2 at 44 amu. A correction
was applied to remove contributions from unavoidable interference with fragmentation
peaks of other gases. The carbon balance was systematically verified based on an internal
standard method and a maximum deviation of 6% was obtained. The latter is originating
from experimental error rather than from deposited carbon. To evaluate the significance of
external and internal mass transfer limitations, the criteria of Carberry number 45 and Weisz-
Prater 46 were applied, while for heat transport limitations the diagnostic criteria reported by
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Mears
were employed. A radial temperature gradient of ~29 K was calculated. This
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temperature gradient was taken into account for the calculation of the equilibrium
conversion. The same conversion (XCH4 ~ 70%) was achieved, with ~100% selectivity to CO,
for all of the investigated samples, by varying the amount of catalyst (WNi:F0CH4 = 0.024-0.08
kgNi·s·molCH4-1). It always remained below the equilibrium conversion (Xeq,CH4 ~ 80% at 1023 K
and 111.3 kPa considering the above mentioned radial temperature gradient). In view of
these high conversions, the reactor is not considered differential and therefore the space
time yield is reported.
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The DRM activity of the catalysts was screened in a short term experiment, with time-on-
stream (TOS) = 2 h. The best candidates of the activity tests were further tested for longer
TOS (stability) during DRM at 1023 K and 111.3 kPa. The space time values (WNi:F0CH4) of the
samples used during the stability tests are in the range [0.023 - 0.025] kgNi·s·mol-1 for
CH4
Ni/0.5Fe, Ni/1.0Fe and Ni/2.5Fe (initial XCH4~ 70%). One stability test was also performed for
Ni/0.5Fe at lower initial CH4 conversion value (~54%), using a WNi:F0CH4 of 0.011 kgNi·s·mol-
1
.
CH4
Two samples (Ni/0Fe and Ni/1.0Fe) were examined under BIM at 1023 K and 911.7 kPa.
The space time values (WNi:F0CH4) of these samples used during the activity tests were in the
range [0.003 - 0.004] kgNi·s·mol-1
.
CH4
The following expressions are used to determine the activity of different catalysts. The
percent conversion for a reactant is calculated as:
ꢄ
ꢂ ꢅꢂ
ꢃ
ꢀꢁ =
ꢃ · 100%
(1)
ꢄ
ꢂ
ꢃ
where ꢆꢇ and ꢆ are the inlet and outlet molar flow rates of gas i (mol·s-1).
ꢁ
ꢁ
The CH4 consumption rate (mol·s-1·kgNi-1, Eq. 2) was calculated from the difference
between the inlet and outlet molar flow rates, as measured relative to an internal standard
(Ar), while the space time yield (STY, mol·s-1·kgNi-1, Eq. 3) was calculated for the products:
ꢄ
|ꢂ ꢅꢂ |
ꢃ
ꢃ
CH4 consumption rate =
(2)
ꢈ
ꢉꢃ
ꢂ
ꢃ
STYi = ꢈ
(3)
ꢉꢃ
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where ꢆꢇand ꢆ are the inlet and outlet molar flow rates of gas i (mol·s-1), and mNi the
ꢁ
ꢁ
4
5
amount of Ni (kg) present in the catalyst.
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7
8
9
To determine the amount of deposited carbon after the stability tests, TPO
measurements were performed in the same experimental setup at atmospheric pressure,
under a flow of diluted O2 (1 NmL/s of 10% O2/He). Once the reactor was cooled down to
room temperature, the spent catalyst was subjected to oxidation with a heating rate of 10
K/min up to 1173 K. Assuming that all carbon underwent complete combustion in the
presence of oxygen, the CO2 peaks measured by MS allow calculation of the amount of
carbon that was present on the spent catalyst. TPO was carried out on all studied samples
after each stability test.
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2.7 Mechanical mixture
A mechanical mixture of graphite and 10Fe support (100 mg graphite - 300 mg support)
was used to investigate the support oxygen transfer properties in a temperature
programmed experiment up to 1173 K, using a heating rate of 10 K/min, under an inert
environment (1 NmL·s-1 Ar flow). The reactor configuration for this experiment is shown in
Figure S2. A CO or CO2 signal in the reactor outlet then indicates graphite oxidation by the
support.
Another mechanical mixture of 7.5Fe support and Ni/0Fe was used in a reduction
experiment, in order to investigate the Fe migration process from the iron containing
support towards supported Ni. The mechanical mixture was reduced under 10 NmL—s-1 of
5%H2/He at a pressure of 101.3 kPa and up to 1073 K, using in-situ XRD.
3. Results
3.1 Support Characterization
The metal content and textural properties for the support materials are reported in Table
1. The surface area values are in accordance with literature reports 17, 48. The 0Fe support
showed the highest surface area and the largest pore volume, while the addition of Fe
resulted in surface area decrease for the support materials. A slight decrease in pore volume
was observed upon adding Fe (Table 1), while the pore size increased, as the peak maximum
shifted to higher pore width values (Figure S3). The crystallite sizes of the “as-prepared”
support materials were determined from XRD and a slight increase was calculated upon Fe
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addition, from 7.7±0.5 nm when x = 0, to 21±0.8 nm when x = 0.26, in agreement with
literature reported values 48.
6
7
Table 1: Support properties. Metal loading from ICP-AES, BET and pore volume from N2 adsorption.
8
9
Metal loading (wt.%)
Pore
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Label
“zFe”
BET
Catalyst
volume
(cm³/gcat)
Mg
Al
Fe
(m2/gcat)
0Fe
MgAl2O4
14.35
15.90
15.50
14.12
12.76
12.66
11.90
11.78
32.91
-
81.0 ± 4.1 0.23 ± 0.03
78.1 ± 1.9 0.21 ± 0.05
79.3 ± 0.8 0.20 ± 0.02
77.1 ± 1.9 0.18 ± 0.01
63.7 ± 3.5 0.17 ± 0.01
57.9 ± 2.1 0.20 ± 0.01
50.1 ± 2.2 0.18 ± 0.01
31.1 ± 0.3 0.13 ± 0.00
0.5Fe
1.0Fe
2.5Fe
5.0Fe
7.5Fe
10Fe
15Fe
MgFe0.007Al1.993O4
MgFe0.013Al1.987O4
MgFe0.04Al1.96O4
MgFe0.09Al1.91O4
MgFe0.13Al1.87O4
MgFe0.18Al1.82O4
MgFe0.26Al1.74O4
36.00 0.51
35.80 1.01
30.85 2.87
29.29 5.76
28.22 8.88
26.80 12.18
26.50 20.20
In the as-prepared state, the 5.0Fe support lattice contains randomly distributed iron as
shown by High Angle Annular Dark Field Scanning Transmission Electron Microscopy
(HAADF-STEM) combined with Energy-Dispersive X-ray spectroscopy (EDX) (Figure 1(A-C)).
Figure 1: EDX elemental maps of as-prepared 5.0Fe support. (A): Al, (B): Mg and (C): Fe.
The structure of 5.0Fe was further examined during reduction/oxidation by means of in-
situ X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine
Structure (EXAFS). XANES spectra (Figure 2) give insight in electronic changes around the
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absorber atom and its local symmetry 49-51, while EXAFS allows for detailed identification of
the neighboring shells. The XANES region of 5.0Fe shows reduction through a shift of edge
energy position towards lower values. Reduction to reference FeO (red line) and metallic Fe
(black line) is not observed implying that Fe is not segregated from the support lattice. The
EXAFS analysis of as-prepared 5.0Fe identified an AB2O4 spinel structure with Mg2+ in the
tetrahedral A site, along with Fe3+ and Al3+ in the octahedral B sites, showing Fe
incorporation into 5.0Fe (Figure S4). The location of Fe in octahedral position of the 5.0Fe
lattice can also be inferred from the coordination number in the “as-prepared” state, which
is equal to 6 ± 0.5 (Table 2).
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Figure 2: (A): XANES spectra of 5.0Fe support at the Fe-K edge during H2-TPR (0.2 NmL/s of
5%H2/He using a heating ramp of 10 K/min up to 1073 K). (1): FeO reference; (2): metallic Fe foil.
(B): experimental and simulated XANES spectra for 5.0Fe after H2-TPR with (C): contributions from
two different Fe sites, non-distorted (bottom), and reduced and distorted (top). (
)
simulated, (
octahedron.
) experimental, (
) distorted octahedron, (
) non-distorted
During temperature-programmed reduction (TPR) in hydrogen, Fe3+ partially transforms
to Fe2+, while remaining in the lattice, leading to an increase of the average Fe-O bond
42
distance, due to the larger ionic radius of Fe2+
(Table 2). The XANES spectrum after
reduction (Figure 2(B)) was simulated using two octahedral Fe sites: reduced Fe with strong
distortions and adjusted Fe-O distance, and non-distorted and fully oxidized Fe, i.e. identical
to the as-prepared support. Approximately 60% ± 10% of Fe in 5.0Fe appeared in a distorted
environment (Table 2 and Figure S5), while there was no segregation of Fe from the support
lattice, in alignment with Dharanipragada and co-workers 52. Re-oxidation using CO2 restored
the as-prepared state of fully oxidized spinel, as indicated by the coordination number and
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the Fe-O bond distance. This implies that lattice oxygen can be easily regenerated by
oxidizing agents like (CO2 or H2O) making MgFexAl2-xO4 an active support, with redox
properties based on oxygen mobility.
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9
Table 2: The results from the EXAFS model data of the as prepared, reduced and re-oxidized
support 5.0Fe. N is the coordination number, σ² the Debye Waller factor, R the radial distance from
the central Fe absorber, θ the angle in the Fe-O-M three-body configuration. In reduced state, two
Fe contributions are considered: 1: non-distorted, 2: reduced/distorted octahedron. β represents
the corresponding fractions. Bold values were fitted, non-bold values were calculated.
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Reduced
As-prepared
Re-oxidized
Non-distorted
40
Distorted
60
β (%)
N1
100
100
6.0 ± 0.5
2.8 ± 0.5
3.0 ± 0.5
5.9 ± 0.9
σ2
0.0037 ± 0.0007
1.97 ± 0.02
0.0190 ± 0.0008
1.92 ± 0.02
0.0190 ± 0.0008
2.08 ±0.02
0.0180 ± 0.0007
1.96 ± 0.02
1
R1Fe-O (Å)
N2
5.3 ± 0.5
0.0031 ± 0.0007
2.95 ± 0.05
95ο ± 5
2.4 ± 0.5
0.0030 ± 0.001
2.90 ± 0.05
98ο ± 5
3.2 ± 0.5
0.0032 ± 0.0008
2.94 ± 0.05
95ο ± 5
5.1 ± 0.5
0.0032 ± 0.0008
2.89 ± 0.05
95ο ± 5
σ2
2
R2Fe-O-Al (Å)
θ
N3
4.4 ± 0.5
0.0051 ± 0.0007
3.41 ± 0.05
116ο ± 5
2.2 ± 0.5
0.0052 ± 0.0008
3.42 ± 0.05
118ο ± 5
1.0 ± 0.5
0.0052 ± 0.0008
3.50 ± 0.05
116ο ± 5
4.0 ± 0.5
0.0052 ± 0.0008
3.43 ± 0.05
116ο ± 5
σ2
3
R3Fe-O-Mg (Å)
θ
To confirm the redox functionality of the MgFexAl2-xO4 support, a mechanical mixture of
10Fe with commercial graphite (weight ratio 3:1) was exposed to a temperature
programmed treatment in an inert environment (Ar). The oxidation of graphite by lattice
oxygen gave rise to CO formation, starting at 1000 K and up to 1173 K, and in this process 5
-1
mmolO·kgcat were consumed. This was further exemplified by TAP experiments using CH4
pulses at 993 K over an “as-prepared” 5.0Fe support. The inset of Figure 3 shows the catalyst
bed configuration during these experiments. CO2 was produced (Figure 3), implying that CH4
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was decomposed on 5.0Fe (CH4(g) → C (s) + 2H2(g)), forming H2 (not shown). The deposited
4
5
-1
carbon was subsequently oxidized by lattice oxygen, producing CO2. 15.6 mmolO·kgsupport
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were consumed during the CH4 pulses, corresponding to 30% of the maximum calculated
oxygen storage capacity, when assuming complete reduction of Fe3+ to Fe2+. Reduction to
Fe0 cannot be achieved, even at high temperature (1073 K), as shown previously (Table 2
and Figure 2(A)) 52. Such amount of consumed lattice oxygen is originating from exposed
surface and sub-surface, 2 to 4 layers deep depending on the crystallite size of the MgFexAl2-
xO4 support (see §3.1). The diffusion coefficient of the lattice oxygen can be assessed by
applying the Einstein approximation equation (Eq. 4):
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D= x2/t
(4),
where x is the mean traveled distance of the lattice oxygen species (m), t is the time of
occurrence of the CO2 product (s) after pulsing CH4, i.e. 0.22 s (Figure 3), and D is the
diffusion coefficient (m2—s−1). Based on XRD, a lattice parameter of 8.1 Å was found for 5.0Fe,
52
in agreement with previous Rietveld analysis on a similar sample . Taking into account 2-4
sublayers, an average distance of 24.3 Å needs to be bridged by lattice oxygen to reach the
surface sites. Then, for oxygen diffusion through a MgFexAl2-xO4 support lattice at 993 K, a
diffusion coefficient of approximately 3—10-17 m2—s−1 was calculated according to Eq. 4.
To verify that all MgFexAl2-xO4 (0.007≤ x ≤0.26) support materials with Fe modification
acquire redox functionality, a similar experiment (not shown) was performed over 1.0Fe (x=
0.013), containing five times less Fe than the 5.0Fe support. Now, 2.1 mmolO·kgsupport-1 were
consumed during CH4 pulses at 993 K, corresponding to 20% of the maximum calculated
oxygen storage capacity. This implies that the incorporation of Fe into magnesium
aluminate, even at low concentrations, will provide redox properties to the support.
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Figure 3: Support oxygen mobility: CO2 molar flow rate during CH4 pulses over 5.0Fe at 993 K. Inset:
Catalyst bed configuration during pulses in TAP reactor.
Apart from the structure evolution during reduction and oxidation, the stability of the
support during alternating reducing/oxidizing environment at high temperature (1023 K) was
investigated during redox cycles of H2 and CO2 (Figure S6). The 5.0Fe support remained
stable for 60 cycles, producing approximately 0.015 molCO—kgsupport-1. This implies that adding
Fe to MgAl2O4 makes a stable and active support, MgFexAl2-xO4 with 0.007 ≤ x ≤ 0.26, that
also provides redox capacity.
3.2 Catalyst Characterization
Using these new supports, 8wt%Ni-based catalysts were synthesized. Table 3 displays
their properties. The Ni catalysts have lower surface area and pore volume than the
corresponding supports, mainly due to surface covering and pore blockage, similar to what
has been reported about the effect of different additives on the support properties 13, 53-54
.
Their crystallographic structure was characterized in as-prepared, reduced and CO2 re-
oxidized state, using XRD. Figure 4 displays the full scan XRD patterns of pure 7.5Fe support,
together with as-prepared, reduced and reoxidized Ni/7.5Fe catalyst (Ni/7.5Fe is displayed
for better signal quality). MgFexAl2-xO4 spinel (31.3o,37o,45o, 55.5o, 59o, 65o, Powder
Diffraction File (PDF) card number: 01-071-1238) was the predominant phase for the
catalysts in each state. NiO peaks (37.3o, 43.3o, 63o, PDF: 01-089-5881), present for the as-
prepared sample, disappeared upon reduction.
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Table 3: Catalyst properties. Metal loading from ICP-AES, BET and pore volume from N2 adsorption.
Metal loading
Label
Fe/Ni ratio
BET
Pore volume
Catalyst
(wt.%)
Fe
“Ni/zFe”
(mol
—
mol-1) (m2/gcat)
(cm³/gcat)
Ni
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60
Ni/0Fe
8%Ni/MgAl2O4
8.51
-
0
62.8 ± 2.9
55.0 ± 4.0
63.0 ± 3.0
54.4 ± 2.3
45.1 ± 2.7
41.1 ± 2.2
35.4 ± 1.3
21.8 ± 1.7
0.17 ± 0.01
0.16 ± 0.05
0.19 ± 0.04
0.14 ± 0.01
0.13 ± 0.01
0.15 ± 0.01
0.14 ± 0.00
0.10 ± 0.01
Ni/0.5Fe 8%Ni/MgFe0.007Al1.993O4 9.10
Ni/1.0Fe 8%Ni/MgFe0.013Al1.987O4 9.08
0.48
0.92
2.52
5.13
7.95
10.94
17.40
0.05
0.11
0.32
0.65
1.04
1.43
1.98
Ni/2.5Fe 8%Ni/MgFe0.04Al1.96O4
Ni/5.0Fe 8%Ni/MgFe0.09Al1.91O4
Ni/7.5Fe 8%Ni/MgFe0.13Al1.87O4
8.30
8.29
8.05
7.99
9.25
Ni/10Fe
Ni/15Fe
8%Ni/MgFe0.18Al1.82O4
8%Ni/MgFe0.26Al1.74O4
The diffraction pattern of the reduced catalyst shows peaks at lower angles than metallic
10, 13,
Ni. This shift can be attributed to Ni-Fe alloy formation as observed in previous reports
15, 55, implying that upon catalyst reduction metallic Fe can migrate from the support towards
Ni and form an alloy. Hence, Ni induces deeper reduction of Fe in the support lattice than
was observed for the pure support, allowing it to alloy (§3.1). Upon temperature
programmed CO2 re-oxidation of this reduced catalyst, the same crystalline phases were
observed in XRD as for the as-prepared sample (Figure 4). This was unexpected since
13
oxidation of Ni by CO2 at these conditions is not favorable, as established previously for
Ni/MgAl2O4. CO2 however decomposes the alloy and simultaneously oxidizes Fe in the
support, which in turn oxidizes Ni using lattice oxygen. Hence, the redox activity of the
MgFexAl2-xO4 support unexpectedly enables Ni re-oxidation.
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ꢀ MgFe0.13Al1.87O4 ꢀ NiO − Ni-Fe
5
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Ni/7.5Fe
after CO2-TPO
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Ni/7.5Fe
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after H2-TPR
Ni/7.5Fe
as-prepared
7.5Fe
25
30
35
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45
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60
65
70
2θ (°)
Figure 4: Full XRD scans of 7.5Fe support and “as-prepared”, “reduced” (10 NmL/s of 10%H2/He
mixture at a total pressure of 101.3 kPa and 1123K) and “re-oxidized” (10 NmL/s of CO2 mixture at
a total pressure of 101.3 kPa and 1123K) Ni/7.5Fe.
The elemental distribution of a reduced catalyst with lower Fe loading in the support,
Ni/1.0Fe, was investigated using HAADF-STEM combined with EDX (Figure 5). In the as-
prepared sample (Figure S7), Ni (green) is evenly distributed on the support without
formation of NiO clusters as previously observed over MgAl2O4 34. Upon reduction, Ni forms
small clusters of approximately 9 nm (Figure 5), with Fe segregated from the support,
decorating the Ni particles (Figure S8), forming a Ni-rich alloy phase due to the low Fe
concentration in Ni/1.0Fe. Samples containing higher Fe concentration showed similar
pictures with more Fe segregated from the support lattice (not shown).
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Figure 5: (A): HAADF-STEM image of the mapped area of reduced Ni/1.0Fe together with the EDX
elemental map (B). Inset of Figure 5(A): Ni-Fe particle size distribution as calculated from Figure
5(A).
3.3 Temperature Programmed Reduction (TPR)
3.3.1 In-situ XAS
To understand the mechanism under which Ni enhances the reducibility of Fe and hence
enables its mobility from the support lattice towards Ni to alloy, an in-situ XAS experiment
was performed on Ni/5.0Fe. The XANES spectra were recorded quasi-simultaneously at the
Fe-K and Ni-K edges (Figure S9) and indicate that the addition of Ni onto 5.0Fe causes deeper
reduction of Fe compared to the pure support (Figure 2(A)), in agreement with the XRD
experiments (Figure 4).
56-57
The reduction of Fe occurs via hydrogen spillover
from the Ni surface towards
MgFexAl2-xO4 support, as established in a reduction experiment of a mechanical mixture of
Ni/0Fe and 7.5Fe, followed using in-situ XRD (Figure S10). The deeper reduction of Fe in
Ni/MgFexAl2-xO4 is crucial for Fe to become mobile. So, hydrogen spillover induces deep
reduction, steering the migration of Fe to Ni and the formation of a Ni-Fe alloy.
Reduction and alloying do not consume all Fe from the support (Figure 6(A)). Linear
Combination Fitting of the reduced Fe-K XANES spectrum indicates that approximately 50%
of Fe stays incorporated in the support. This is beneficial as the support preserves oxygen
mobility, and therefore remains reducible. Similarly, only 59% of Ni reached the metallic
state after reduction (Figure 6(A)), while no NiAl2O4 spinel structure was detected in XRD
(Figure 4). On the other hand, deeper reduction of Ni was achieved in the co-impregnated
sample, Ni-4.0Fe, with 82% reaching the metallic state (Figure 6(A) and S.12(B)), while the
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remainder formed a NiAl2O4 spinel phase, as previously identified by XRD 13. All these results
indicate the existence of strong metal-support interactions (SMSI) 58-59, in case of Ni
supported on MgFexAl2-xO4. SMSI effects have previously been reported for Ni impregnated
on spinel supports like Ca-MgAl2O4 60. SMSI comprises a number of effects occurring at the
interface between active metal and its support. One such effect is enhancing the metal
dispersion 61-62, which is illustrated by the active phase crystallite size determined from XRD
(8.8 ± 0.6 nm for Ni/7.5Fe, compared to 10.5 ± 1.0 nm for Ni/0Fe). SMSI can also inhibit the
formation of carbon (Figure S11) 62-63. Further, SMSI in Ni/MgFexAl2-xO4 ensures that Ni at the
interface with the support remains oxidized, as thin NiOx layer, even under reducing
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conditions 64, . A schematic representation of Ni-Fe alloy formation on Ni/MgFexAl2-xO4
upon Fe migration from the support during H2-TPR is illustrated in Figure 6(B).
To investigate the effect of Fe location on its reducibility, a catalyst prepared by co-
impregnation of Ni and Fe on MgAl2O4 (“Ni-4.0Fe” reference) i.e. a support without
incorporated Fe, was examined during reduction using in-situ XANES. The Fe-K edge XANES
spectrum shows deeper reduction of Fe, compared to Ni/5.0Fe, as 90% of it reaches the
metallic state (Figure 6(A) and S.12(A)). This implies that by incorporating Fe in the
magnesium aluminate lattice, it is stabilized in the lattice even under reducing conditions.
Figure 6: (A): Linear Combination Fitting analysis of Fe- and Ni-K edge XANES data. Calculated total
molar fraction of metallic Ni and Fe in the catalyst after H2-TPR (0.2 NmL/s of 5%H2/He using a
heating ramp of 10 K/min) as a function of support material and Fe location, respectively.
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MgFexAl2-xO4 and Ni/MgFexAl2-xO4 corresponds to 5.0Fe and Ni/5.0Fe, Ni-Fe/MgAl2O4 to co-
impregnated Ni-4.0Fe reference material. (B): Schematic representation of Ni-Fe alloy formation on
Ni/MgFexAl2-xO4 upon Fe migration from the support during H2-TPR. Green represents NiO, grey Ni,
black Ni-Fe alloy while the arrows indicate the migration of Fe from the support towards Ni.
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3.4 Activity and stability tests
The catalytic activity for all Ni/MgFexAl2-xO4 samples during methane dry reforming (fixed-
bed reactor, 1023 K, 111.3 kPa, CH4:CO2 = 1, XCH4 = 70 ± 5%) is a strong function of Fe
concentration or Fe/Ni ratio. The CO and H2 space time yields (STYs) after 2 h time-on-
stream (TOS) as a function of Fe/Ni ratio (Figure 7(A)) can be divided into two parts: the first
where the addition of Fe to the Ni catalyst is not affecting the activity (Fe/Ni ≤ 0.32) and the
second where it has a negative effect (Fe/Ni > 0.32). However, a positive effect of Fe
addition is found in stability tests (Figure 7(B)) and in the overall amount of carbon that is
deposited (Table S.1). Ni/0.5Fe and Ni/1.0Fe were the most stable samples, using a space
time WNi:F0 of 0.024 kgNi·s·molCH4-1, as they lost only 18% of their initial activity after 50
CH4
-
and 65 h TOS, respectively. By decreasing the space time for Ni/0.5Fe to 0.011 kgNi·s·molCH4
1, the catalyst remained stable, as its activity decreased by 19.5% after 35 h TOS, implying
high stability of the Ni/MgFexAl2-xO4 catalysts with Fe/Ni < 0.32.
Ni/2.5Fe (Fe/Ni = 0.32) exhibited high activity during the 2h TOS experiment (Figure 7(A)),
while it was deactivated over 50% after 55 h TOS (Figure 7(B)). Carbon formation was
examined as a possible reason for the observed deactivation, however, even after 55 h TOS
there was no deposited carbon (below detection limits). The origin of the observed
deactivation thus originates from sintering and/or decomposition of the Ni-Fe surface alloy
34. Similarly, the Ni/1.0Fe sample did not show carbon deposition after 65 h TOS, whereas
-1
0.4·102 molc·kgcat had been deposited on the Ni/0Fe sample after only 12 h TOS at 111.3
kPa. Co-impregnation of 1.0wt% Fe with the same amount of Ni on a MgAl2O4 support also
led to carbon formation, with 0.4·102 molc·kgcat deposited after 40 h TOS. Clearly, the
-1
location of Fe affects the catalyst carbon resistance and Fe is required to be part of the
support lattice (Table S.1).
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Figure 7: Activity and stability during methane reforming at 1023 K. (A): STY for CO and H2 after 2 h
TOS under DRM at 111.3 kPa as a function of Fe/Ni ratio for all studied Ni/MgFexAl2-xO4 catalysts
(Table 3). Dashed lines are guides to the eye. (B): CH4 consumption rate during DRM at 111.3 kPa:
-1
●: Ni/2.5Fe, ♦: Ni/1.0Fe,
▲
: Ni/0.5Fe at WNi:F0 = 0.024 kgNi∙s∙molCH4
,
▲
: Ni/0.5Fe at WNi:F0
=
CH4
CH4
0.011 kgNi∙s∙molCH4-1. Error bars: twice the standard deviation from three independent experiments.
The activity of the Ni/1.0Fe catalyst was also examined during steam-dry reforming (bi-
reforming) of methane at 1023 K and at elevated reaction pressures (911.7 kPa, Figure S13).
As expected, more carbon is deposited when increasing the reaction pressure. Figure 8
shows a comparison of deposited carbon over Ni/1.0Fe and Ni/0Fe as a function of total
pressure. Three times less carbon was deposited on Ni/1.0Fe compared to Ni/0Fe (0.62·102
to 1.84·102 molc·kgcat-1, respectively), implying the ability of the 1.0Fe support to suppress
the carbon deposition. The carbon formation can be further eliminated by increasing the
partial pressure of the oxidizing gases, CO2 or H2O. From an economic point of view, less CO2
or H2O will be required for Ni/1.0Fe than for Ni/0Fe in order to remove all carbon.
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TOS=10 h
TOS=12 h
TOS=20 h
0
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400
600
800
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Pressure (kPa)
Figure 8: Comparison of deposited carbon (molC∙kgNi-1) during methane steam-dry reforming
(15%CH4, 11% CO2, 4% H2O and 70% Ar) at 111.3 and 911.7 kPa. ♦: Ni/1.0Fe, x: Ni/0Fe. Dashed lines
are guides to the eye. Error bars: twice the standard deviation from three independent
experiments.
4. Discussion
It is well known that one of the main challenges of reforming processes over Ni is the
catalyst deactivation due to carbon deposition, especially under reforming conditions using a
CH4:CO2 (or H2O) mixture with ratio near unity, which is thermodynamically considered to be
a carbon-rich area 66. Bimetallic Ni-Fe catalysts have proven to suppress carbon formation 10,
13, 29, 34 compared to monometallic Ni, but they do not offer the required stability.
13, 28
After reduction, Ni and Fe elements are uniformly distributed
in an alloy structure.
34
However, our previous work showed that the Ni-Fe alloy rearranges under methane dry
reforming conditions (Figure 9). This redistribution of elements results in Fe species located
on top of alloy particles, in alignment with Wang and co-workers, who observed Fe species
enriched in the outer layers of alloy particles 15. This alloy restructuring process under
reaction conditions cannot be avoided and its rate is controlled by the ratio between
reducing and oxidizing gases 34, 67. For high Fe content, Fe/Ni > 0.32, this causes deactivation
(Figure 7(A)) as large patches of Fe cover Ni. Previously, noble metals were used to stabilize
the surface Ni-Fe alloy during reforming 34. However, by using Fe-modified magnesium
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aluminate as support for Ni, high stability can also be obtained without the use of noble
metals.
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Figure 9: Catalyst deactivation during reforming reactions due to Fe segregation from the Ni-Fe
alloy for high (Fe/Ni > 1:3), low Fe content (Fe/Ni ≤ 1:9).
The catalyst activity and stability during reforming reactions, are sensitive to the Fe
content of the support (Figure 7(A) and (B)). By including only a small amount of Fe inside
the magnesium aluminate support, Fe/Ni ≤ 0.32, the Ni-Fe alloy composition is such that it
rearranges into an active phase (Figure 9), achieving high catalyst activity and stability,
together with efficient carbon removal. CH4 dry or steam reforming on Ni-Fe/MgFexAl2-xO4
proceeds with CH4 reacting on Ni sites to form H2 and surface carbon 13, 29, 68. The carbon
removal from the Ni-Fe alloy surface during reaction depends on Fe segregation from the
alloy and concomitant formation of FeOx species due to the presence of the oxidizing gas
13
(CO2 or H2O) . The role of FeOx on the surface of Ni-Fe alloy catalysts is to provide lattice
oxygen for carbon removal, as previously demonstrated in several papers 13, 29, 55. It should
be mentioned that direct interaction of carbon with gas phase CO2 has not been observed up
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to 1173 K and therefore its contribution is neglected. However, to make both a carbon-
resistant and stable Ni-Fe catalyst, Fe also requires to be part of the support lattice (Table
S.1), providing an extra pathway for carbon removal, namely through lattice oxygen from the
support, thereby reducing the incorporated Fe from 3+ to 2+. The support oxygen can be
refilled by the oxidizing gas (CO2, H2O) 48, 52. As such, oxidized iron, in the support and at the
alloy surface, eliminates carbon accumulation, operating as scavenger.
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A novel spinel support, MgFexAl2-xO4, was synthesized by incorporating Fe in a magnesium
aluminate lattice. The incorporation of Fe occurs in the octahedral sites of the spinel lattice,
replacing Al and providing redox functionality to the support, while at the same time
preserving the high thermal stability that magnesium aluminate offers. By applying CH4
pulses in TAP, the amount of mobile lattice oxygen was quantified and it varied with the Fe
content. This lattice oxygen originates from the exposed surface and the sub-surface layers,
2 to 4 depending on the crystallite size of the MgFexAl2-xO4 support. A diffusion coefficient of
3—10-17 m2—s-1 was estimated for the lattice oxygen at 993 K from TAP experiments.
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The addition of Ni onto MgFexAl2-xO4 steers the formation of a surface Ni-Fe alloy via
migration of Fe due to hydrogen spillover, during the reduction process. 50% of Fe remains
in the support lattice after reduction, maintaining the spinel structure and its redox
properties. The activity and stability of Ni/MgFexAl2-xO4 were tested under various conditions
and strongly depend on the amount of incorporated Fe. By carefully down tuning the
amount of Fe in MgFexAl2-xO4, an active Ni-Fe surface alloy is obtained (Fe/Ni molar ratio
~1/10), which, in combination with the MgFexAl2-xO4 support, shows promising stability and
fully mitigates carbon formation at atmospheric conditions during reforming reactions. The
location of Fe plays a key role for carbon mitigation. By comparing the same Fe loading, but
in different locations, incorporated into the support and deposited onto the support, it was
concluded that Fe is required to be part of the support lattice. At higher pressure, 911.7 kPa,
carbon deposition was three times less for 1.0Fe compared to conventional 0Fe. The stability
offered by the Ni/MgFexAl2-xO4 catalyst makes it an ideal candidate for all reforming
processes.
Supporting information
The supporting Information is available free of charge on the ACS Publication website.
Representation of the structure used in the modelled EXAFS signal, catalyst bed
configuration with mechanical mixture, pore size distribution for MgFexAl2-xO4 support
materials, representation of the local environment of randomly dispersed Fe, a FT figure of
the EXAFS signals, CO yield figure during redox cycles, EDX elemental mapping, HAADF-STEM
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images, XANES data during H2-TPR, X-ray diffractograms, schematic representation of carbon
formation over benchmark Ni/MgAl2O4 and Ni/MgFexAl2-xO4, steam-dry reforming
performance data.
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Acknowledgments
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This work was supported by the FAST industrialization by Catalyst Research and
Development (FASTCARD) project, which is a Large Scale Collaborative Project supported by
the European Commission in the 7th Framework Programme (GA no 604277), the “Long Term
Structural Methusalem Funding by the Flemish Government”, the Interuniversity Attraction
Poles Programme, IAP7/5, Belgian State – Belgian Science Policy, and the Fund for Scientific
Research Flanders (FWO-Vlaanderen) in supplying financing of travel costs and beam time at
the DUBBLE beam line of the ESRF.
The authors acknowledge the assistance from the DUBBLE (ESRF, XAS campaign 26-01-1048)
and ROCK staff (SOLEIL, proposal 201502561). The authors equally acknowledge support
from a public grant overseen by the French National Research Agency (ANR) as part of the
“Investissements d’Avenir” program (reference: ANR-10-EQPX-45) for the ROCK beam line
and from Lukas Buelens and Rakesh Batchu (Laboratory for Chemical Technology, Ghent
University) for the STEM measurements and TAP experiments, respectively.
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