Synthetic ferripyrophyllite: Preparation, characterization and catalytic application

Article abstract of DOI:10.1039/d0dt03125a

Sheet silicates, also known as phyllosilicates, contain parallel sheets of tetrahedral silicate built up by [Si2O5]2- entities connected through intermediate metal-oxygen octahedral layers. The well-known minerals talc and pyrophyllite are belonging to this group based on magnesium and aluminium, respectively. Surprisingly, the ferric analogue rarely occurs in nature and is found in mixtures and conglomerates with other materials only. While partial incorporation of iron into pyrophyllites has been achieved, no synthetic protocol for purely iron-based pyrophyllite has been published yet. Here we report about the first artificial synthesis of ferripyrophyllite under exceptional mild conditions. A similar ultrathin two-dimensional (2D) nanosheet morphology is obtained as in talc or pyrophyllite but with iron(iii) as a central metal. The high surface material exhibits a remarkably high thermostability. It shows some catalytic activity in ammonia synthesis and can serve as catalyst support material for noble metal nanoparticles.

Full text of DOI:10.1039/d0dt03125a

Volume 50
Number 3
21 January 2021
Pages 757-1128
Dalton
Transactions
An international journal of inorganic chemistry
rsc.li/dalton
ISSN 1477-9226
PAPER
Nils Theyssen, Walter Leitner et al.
Synthetic ferripyrophyllite: preparation, characterization and
catalytic application
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Volume

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PAPER
Synthetic ferripyrophyllite: preparation,
characterization and catalytic application†
Cite this: Dalton Trans., 2021, 50,
850
a,b
a
a
b
Yunxiang Qiao,
Claudia Weidenthaler,
Cristina Ochoa-Hernández,
Nils Theyssen, * Bernd Spliethoff, Jan Folke,
a
a
a,c
a,b,d
Wolfgang Schmidt,
Gonzalo Prieto,
Shengfa Ye,
a
a,b
Eckhard Bill,
b
a
b,e
Holger Ruland,
Ferdi Schüth
and Walter Leitner
*
Sheet silicates, also known as phyllosilicates, contain parallel sheets of tetrahedral silicate built up by
2−
[Si
2
O
5
]
entities connected through intermediate metal–oxygen octahedral layers. The well-known min-
erals talc and pyrophyllite are belonging to this group based on magnesium and aluminium, respectively.
Surprisingly, the ferric analogue rarely occurs in nature and is found in mixtures and conglomerates with
other materials only. While partial incorporation of iron into pyrophyllites has been achieved, no synthetic
protocol for purely iron-based pyrophyllite has been published yet. Here we report about the first artificial
synthesis of ferripyrophyllite under exceptional mild conditions. A similar ultrathin two-dimensional (2D)
nanosheet morphology is obtained as in talc or pyrophyllite but with iron(III) as a central metal. The high
surface material exhibits a remarkably high thermostability. It shows some catalytic activity in ammonia
synthesis and can serve as catalyst support material for noble metal nanoparticles.
Received 7th September 2020,
Accepted 5th January 2021
DOI: 10.1039/d0dt03125a
rsc.li/dalton
6
,7
first reported by F.V. Chukhrov et al. in 1978. It is found as a
natural mineral only in mixtures or conglomerates with lots of
Introduction
Phyllosilicates contain parallel sheets of tetrahedral silicate impurities. The structure of ferripyrophyllite is shown in
2
−
1–3
1,5,8,9
built up by [Si
2
O
5
]
entities (Fig. 1). They are an important Fig. 2.
It is a dioctahedral TOT sheet silicate in which
group of minerals that includes the clay minerals as well as the two-thirds of the octahedra are occupied with iron(III) centers,
groups of mica, chlorite, serpentine and talc/pyrophyllite.
Minerals of the latter group contain three layers: silicate
tetrahedral layers are located on the top and on the bottom phyllosilicates is known for cobalt, nickel, cobalt–nickel
with hydroxyl groups attached.
Although the hydrothermal formation of transition metal
1
1
12
1
3
14
(
indicated by [Si O ] in the formula), which are connected via mixtures and copper, no synthetic protocol for iron-based
4 10
a metal–oxygen octahedral layer. Materials with such t
hedral/octrahedral/tetrahderal arrangements are classified as yet. Prominent iron silicate minerals are fayalite (Fe
TOT sheet silicates.
̲e tra- pyrophyllite has been reported to the best of our knowledge
1
0
15
̲
̲
2 4
SiO ),
4
,5
16–18
Pyrophyllite is aluminum-based ferrosilite (FeSiO3 or Fe Si O ),
iscorite (Fe(II) Fe
2
2
6
5
1
9,20
21,22
(Al
2
[Si
4
O
10](OH)
2
) and talc is magnesium-based (Mg
3
[Si
4
O
10
]
(III)
2
SiO10
)
2 5 4
and greenalite ([Fe(II), Fe(III)]2-3 Si O (OH) ).
(
OH)
2
). The ferric analogue of pyrophyllite, known as ferripyro- They are either formed under rather severe reaction conditions
phyllite, has the idealized formula Fe [Si O ](OH) and was (>1000 °C, high pressure) or only found in nature.
2
4
10
2
Herein, we describe the discovery of Ferripyrophyllite for-
mation at mild temperatures and almost ambient pressure.
a
Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim
an der Ruhr, Germany. E-mail: theyssen@kofo.mpg.de
b
Max Planck Institute for Chemical Energy Conversion, Stiftstr. 34-36,
4
5470 Mülheim an der Ruhr, Germany. E-mail: walter.leitner@cec.mpg.de
c
ITQ Instituto de Tecnología Química, Universitat Politècnica de València-Consejo
Superior de Investigaciones Científicas (UPV-CSIC), Avenida de los Naranjos s/n,
4
6022 Valencia, Spain
d
Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
4
57 Zhongshan Road, 116023 Dalian, PR China
e
Institut für Technische und Makromolekulare Chemie, RWTH Aachen University,
Worringerweg 1, 52074 Aachen, Germany
Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0dt03125a
†
2
−
Fig. 1 [Si
2
O
5
]
as basic structure units of sheet silicates (top view).
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The oxidation of metallic iron to Fe(III) centers during the
synthesis is accompanied by proton reduction according to eqn
(
1). The evolution of hydrogen is indicated by a distinct pressure
increase during the reaction and was confirmed via GC analysis
−1
of the gas phase. The presence of about 4.89 mmol g hydroxyl
groups in the iron silicate (8.31 wt%, after drying at 100 °C for
Fig. 2 Two unit cells of Ferripyrophyllite – the proposed structure of 12
h to remove physisorbed water) as determined by
the iron silicate described herein (side view).
24,25
−
titration
also accounts for the OH formation.
0
3þ
ꢀ
2
Fe þ 6 H2O ! 2 Fe þ 6 OH þ 3 H2
ð1Þ
The initial isolation resulted from serendipitous observations
carrying out catalytic reactions with SBA-15 material in
aqueous media in steel reactors. Based on this initial finding,
a reproducible synthetic protocol was developed that allows
the usage of different silica sources (including for example
also MCM-41, AlSBA-15, and aerosil) in combination with
simple Fe powder in water. The preparation is typically per-
formed at 100 °C in sealed stainless steel autoclaves (with
glass inlet) under initial ambient pressure of air (see ESI for
experimental details and S1–S11 for TEM and EDX analyses†).
X-ray photoelectron spectroscopy (XPS) revealed an atomic
ratio of Fe to Si of exactly 1 : 2 on the surface of the material.
The XPS spectrum of Si 2p shows a single peak at 103.1 eV
(
Fig. S14†), which is well within the reported values for sili-
26,27
cates (101.6–103.8 eV).
The spectra of Fe 2p1/2 and Fe 2p3/
2
show characteristic peaks at 725.6 and 712.0 eV, together
with associated satellite peaks (Fig. S15†). Combined with the
single peak of Fe 3p at 56.9 eV, a clear dominance of Fe(III)
28,29
centers can be deduced from these data.
5
7
The Fe(III) oxidation state is further confirmed by Fe
3
0–34
Mössbauer spectroscopy (Fig. S16†).
The small isomer
shift of about 0.36 mms− at room temperature indicates Fe(III)
high spin centers with six hard ligands, which are expected to
be oxygen. A defined coordination environment is difficult to
deduce, however, because the quadrupole splitting is quite
1
Results and discussion
Material characterization
The standard synthetic method produces a beige homo-
geneous Fe-silicate material that is typically contaminated with
small amounts of SiO . The morphology is best described as a
2
−1
large (about 0.79 mm s ), showing deviations from cubic sym-
metry. Furthermore, the high experimental line width of about
continuous two-dimensional (2D) nanosheet structure (Fig. 3),
−1
0
.74 mm s , which could be fitted with a Gaussian distri-
2
−1
resulting in a relatively large BET surface area of 273 m g
bution of Lorentzian lines, points to heterogeneities and an
overlap of several similar lattice sites. Almost 100% Fe(III)
species were found to be present, a minor subspectrum (max.
3
−1
and a total pore volume of 0.568 cm
g
. Its CO
2
sorption
3
−1
capacity at 273 K was measured to be 5 cm g at 100 mbar
and 20 cm g at 1000 mbar (Fig. S13†).
3
−1
23
7%) indicates the presence of only traces of Fe(II) species. A
According to TEM analysis, the apparent thickness of the
nanosheets were in a range between 0.5 and 0.8 nm, which is
consistent with the theoretical value of 0.7 nm (Fig. S12†).
While parallel formation of a thinner TO sheet silicate cannot
be ruled out completely, we have no evidence from any of our
analytical techniques.
broad peak in the EPR spectrum (Fig. S17†) is typical of ferro-
magnetic resonances of small magnetic domains.
35
Magnetization curves under zero-field-cooled (ZFC) and field-
cooled (FC) conditions confirm that this iron silicate is para-
magnetic or superparamagnetic (Fig. S18†).
Composition analysis with a combination of Energy-
Dispersive X-ray spectroscopy (EDX) and Scanning
Transmission Electron Microscopy (STEM) revealed a very
uniform distribution of the elements Fe, Si and O in the
nanosheets (Fig. 4), measuring an average composition of
FeSi2.0O8.0 for the specific sample. EDX bulk analysis of iron sili-
cates from five different batches gave an average composition of
FeSi2.0O6.4. Another 76 EDX-analyses in combination with high
resolution TEM allowed sampling directly at the nanosheet
structures. These measurements resulted in an average compo-
sition of FeSi1.7±0.2
O5.2±0.4 (confidence level P = 95%).
The silicon-to-iron ratio of 2 : 1, together with clearly domi-
nating Fe(III) centers are consistent with double chain silicates
of the amphibole subgroup such as Fe
sheet silicates of the serpentine subgroup of phyllosilicates
such as Fe [Si 10](OH)
2 4
[Si O11] as well as with
2
4
O
2
.
The already mentioned content of hydroxyl groups
Fig. 3 TEM image of iron silicate nanosheets synthesized from iron
powder and SBA-15 at 100 °C in water.
(8.31 wt%) measured via titration fits well with the chemical
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Fig. 6 ATR-IR spectra of SBA-15 and the obtained iron silicate after
hydrothermal synthesis.
Fig. 4 STEM-EDX mapping of the synthetic iron silicate nanosheets.
2 4 2
formula Fe [Si O10](OH) , corresponding to 8.13 wt% hydroxyl
groups theoretically. Further evidence for this composition
results from the hydrogenative decomposition of the material
in a temperature window between room temperature and
2 3
of Fe O and SBA-15 showed only the additive signals of the
two components (Fig. S21†), distinct differences are evident
for the new material (Fig. 6 and also Fig. S22†). The character-
5
00 °C over 13 h. A total amount of 6.1 mmol water was
−1
istic absorbance of silica at 1057 cm (asymmetric Si–O–Si
formed from a sample of 1.458 mmol iron silicate as measured
via integration of a calibrated online IR-detector signal
−1
stretching) was shifted to the lower frequency at 1023 cm
,
with concomitant inversion of the transmission ratio to the
(Fig. S19†), matching the stoichiometry for a sheet silicate in
−1
absorbance at 445 cm (O–Si–O bending). Together with the
eqn (3) more closely than for eqn (2).
−1
shoulder at 678 cm (Si–O–Fe symmetric stretching mode),
this provides strong evidence for the extensive incorporation of
iron atoms in the silica framework. The absorbance at
Fe2½Si4O11ꢁ þ 3H2 ! 2 Fe þ 4 SiO2 þ 3H2O
ð2Þ
ð3Þ
443 cm− can be assigned to FeOOH units,
1
44,45
correlating
1
Fe2½Si4O10ꢁðOHÞ þ 3H2 ! 2 Fe þ 4SiO2 þ 4H2O
2
with its presence from PDF analysis. The broad band around
3350 cm− and the peak at 1635 cm are assigned to O–H
1
−1
The absence of characteristic features of FeSiO and Fe O
2 3
4
in pair distribution function (PDF) analysis further confirms a stretching (involving hydrogen bond interactions) and bending
4
6
unique structure rather than a mixture of these two known modes of adsorbed water, respectively.
3
6,37
materials (Fig. S20†).
Comparing the main atom distances
Upon heating the material at 450 °C for 4 h under vacuum
as derived from the peak maxima in the iron silicate with to initiate dehydration, the OH stretching vibration region
other known materials indicates a coordination of iron with showed a sharp band at 3745 cm− (in the absorbance mode
bridged oxygen atoms and hydroxyl groups (labelled as FeO only) ascribed to the presence of the terminal silanol groups
1
(
(
OH)). Such entities are in fact also present in Ferripyrophylite (Si–OH) (Fig. S23a†) The acidity of this material was assessed
Fig. 2), and the measured data correlates nicely with simu- by pyridine adsorption/desorption. After pyridine desorption
lated data of this structure (Fig. 5).
at 150 °C (Fig. S23b†), the iron silicate exhibits two bands at
The existence of Fe–O–Si units in the material was evi- 1610 and 1450 cm− associated with pyridine coordinated
1
3
8–43
−1
denced by ATR-IR spectroscopy.
While physical mixtures Lewis acid sites. Very weak bands at 1637 and 1545 cm
+
characteristic of pyridinium ions (PyH ) indicate the presence
4
7,48
of low amounts of Brønsted acid sites.
When desorption
temperature increases up to 250 °C, the band intensities drop,
indicating the presence of acid sites with different strengths.
A series of NH -TPD measurements have been performed in
3
order to evaluate the surface acidity of the iron silicate
material relative to standard silica–alumina solid acids
(
Fig. S24–S27†). The acid site density (per mass of the sample)
and acid strength were found to be comparable with that of
amorphous SiO –Al materials.
2
2 3
O
The iron(III) silicate is chemically quite stable in weak
bases. In fact, it was purified from a 6 M NH3 solution.
However, it is not stable towards strong bases like diluted
NaOH solutions. Moreover, it decomposes in acidic media,
like diluted mineral acids such as HCl, H SO , and HF (1 M).
Fig. 5 Pair distribution function (PDF) analysis of the iron silicate com-
pared to simulated PDFs for Ferripyrophylite and FeO(OH).
2
4
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5
3
Thermostability of the iron silicate is very high, as evi- netic or thermal characteristics. A variety of applications has
5
4–56
denced by temperature-dependent X-ray diffraction (XRD) emerged, covering the areas of catalysis
measurements (Fig. S28†). Only broad peaks were detected due storage devices, sensors, gas storage materials and others.
to the thin sheet structutre. Interestingly, the pattern is not Due to the good thermal stability and high surface area, the
reported in any of the screened databases. Slight peak shifts iron silicate was used as a support material for Pd catalysts
as well as energy
5
7
58
were observed after calcination under synthetic air at 300 °C synthesized by chemical fluid reactive deposition. The Pd-
(
Fig. S29†), consistent with reported dehydration temperatures decorated material was tested for hydrogenation of unsatu-
4
9–52
of FeO(OH) units between 250 and 300 °C.
This interpret- rated compounds, and high conversions and yields were
ation is supported by in situ diffuse reflectance infrared Fourier obtained under standard screening conditions (Table 1). In
transform spectroscopy (DRIFT) indicating concomitant dis- comparison with 3 wt% Pd/SBA-15 and Pd/C under otherwise
−1
appearance of the absorbance at 1440 cm
(Fig. S30†). identical reaction conditions (not shown here), deoxyfunctio-
Fe Mössbauer spectroscopy suggests the preservation of the nalization is much less pronounced. Instead, the formation of
Fe-silicate structure at this stage (Fig. S31 and S32†). benzylic alcohols is preferred (entries 2–5). Interesting is also
5
7
No further changes were observed in the XRD pattern upon the relatively high selectivity for hydrogenation of the aromatic
heating up to 800 °C in synthetic air (Fig. S28†). The BET ring in entry 6, while the benzene ring of benzylidene acetone
2
−1
surface area remained high at about 182 m g (Fig. S33†). remained intact (entry 7). Furthermore, 3% Pd/iron silicate
TEM investigations showed that the ultrathin iron silicate showed good stability and reusability in the hydrogenation of
sheets remained intact even after three hours at 600 °C this substrate as shown in the ESI in Table S1.† The reduced
(
Fig. 7). EDX analysis on seven different locations revealed an conversion from run 5 onwards seems to be caused mainly by
average composition of FeSi2.0O5.6 fully in line with the native incomplete catalyst recovery rather than deactivation. So
material. further studies in particular for hydrogenation of multifunc-
Only at temperatures around 1000 °C, the material converts tional compounds seem promising.
to SiO2 and Fe O according to TEM and XRD analyses Finally, the iron silicate was also tested as a catalyst precur-
5
9,60
2
3
6
1–66
(
Fig. S34 and S35†). Exchanging the oxidative atmosphere with sor in ammonia synthesis (Fig. S40†).
The comparison of
6
1
pure nitrogen, temperature-dependent XRD measurements the activity with a model catalyst (MgFe
2 4
O ), and a state-of-
show phase transitions already between 550 and 600 °C. The the-art industrial catalyst at different temperatures is shown in
new phases remain stable up to 850 °C and prevail also after
cooling to room temperature (Fig. S36†). The new reflections
could be assigned to Fayalite and Fe O (Fig. S37†). Under Table 1 Catalytic data of hydrogenation using 3 wt% Pd/iron silicate as
3
4
catalyst
inert atmosphere, the reduction is accompanied with oxygen
evolution. TG-DSC-MS analysis under argon revealed the loss
Entry
Substrate
Conv. [%]
>99
Product
Yield [%]
>99
of molecular O in addition to water. In contrast, only water
2
a
was detected under air, explaining the higher temperature
stability of the iron silicate under air as compared to inert
atmospheres (Fig. S38 and S39†).
1
2
90
76
Ultrathin 2D-nanostructures are finding great attention due
to a broad spectrum of interesting properties related to their
chemical, electrochemical, catalytic, electronic, optical, mag-
3
4
5
>99
>99
78
93
85
66
6
99
64
2
3
9
7
>99
92
5
Fig. 7 TEM images of iron silicate after heating the sample to 600 °C Reaction conditions: 3% Pd/iron silicate (10 mg), substrate (5 mmol),
a
for 3 h under synthetic air.
no solvent, H
2
(20 bar), 120 °C, 3 h. Reaction was performed at 30 °C.
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(
100 mg), Fe powder (64.5 mg, nFe : nSi = 1 : 1.4) and deionized
water (5.0 mL) were mixed inside the autoclave (equipped with
a glass inlet). The autoclave was tightened under air atmo-
sphere and then heated. The starting point of the reaction
time was defined for the moment at which the inner tempera-
ture of the autoclave reached 100 °C. After continuous stirring
(
400–450 rpm) for 24 h the autoclave was cooled down to room
temperature in a controlled manner using an ice water bath.
Excess Fe powder was separated without further treatment
because it adsorbs on the surface of the magnetic stirring bar.
The solid material was separated by centrifugation (6 K,
20 min), then washed with a mixture of H
2
O and acetone ((5 +
5) mL × 2) and acetone (10 mL × 1) which were also separated
Fig. 8 Catalytic activity of the iron silicate in ammonia synthesis in
by centrifugation (6 K, 20 min). Beige colored powders
were obtained after drying at room temperature in the air for
several minutes. The obtained powder was further purified
from amorphous silica and unconverted SBA-15 via double
treatment with an ammonia solution (6 M, 10 mL ammonia
solution for 100 mg powder) at 40 °C for 16 h followed by cen-
trifugation, washing and drying. The degree of purity is esti-
mated to be around 90–95% judged by at least 100 TEM
images.
comparison with an industrial catalyst and a model catalyst.
Fig. 8. The iron silicate shows indeed activity for ammonia syn-
thesis with very similar activities as the model catalyst in the
kinetic regime between 325 and 425 °C. STEM analysis of the
iron silicate (Fig. S41†) shows the expected decomposition of
the structure, resulting in a mixture of elemental Fe and SiO .
2
Accordingly, the BET surface area decreased to ca. 76 m g . It
2
−1
(
b) Scale-up production of iron silicate. We have scaled up
is noteworthy that the resulting iron shows a distinct activity
already with SiO as the only additional component.
2
the synthesis of the iron silicate using a 500 mL autoclave with
6
1
g SBA-15, 3.84 g iron powder and 300 mL water reacting at
00 °C. TEM and SEM-EDX mapping (Fig. S3 and S4†) show
nearly the same morphology and composition. However, a
higher content of SiO impurity was obtained compared with
the batches on a smaller scale. After reaction at 100 °C for
4 h, the powder was separated by centrifugation and further
purified by double treatment of 6 M ammonia solution at
0 °C for 16 h followed by centrifugation, washing and drying.
c) Production of iron silicates with other silica sources.
Production of iron silicates with other silica sources were done
similarly. Other silica sources than SBA-15 can be used for the
formation of iron silicates. For example, MCM-41 can
smoothly be converted under the same reaction conditions
Conclusion
2
In summary, Ferripyrophyllite, an iron silicate with a Fe–Si-
ratio of 1 : 2 and 2D nanosheet morphology, was prepared
from Fe powder and various silica sources under surprisingly
mild conditions. While iron silicates are generally available
only from natural sources or hydrothermal synthesis under
extreme conditions, the developed protocol uses aqueous solu-
tions at 100 °C in standard autoclaves or even in simple glass
flasks. In-depth analytical characterizations are most consist-
ent with a dioctahedral TOT sheet silicate of the serpentine
subgroup with Fe [Si O ](OH) entities as represented sche-
2
4
(
(
Fig. S5 and S6†). Similar morphology can be obtained when
2
4
10
2
Al (6 wt%) incorporated SBA-15 was used as silica source. One
can clearly see that all four elements are very well distributed
matically in Fig. 2. The obtained ferripyrophyllite has relatively
high surface area and shows very high thermostability.
Preliminary catalytic tests for hydrogenation of organic sub-
strates with supported palladium nanoparticles and ammonia
synthesis with the pristine material showed promising results.
The straightforward preparation protocol provides flexible
access to synthetic materials of this class of compounds,
making them available for systematic evaluation of their pro-
perties and potential applications.
(
Fig. S7 and S8†). Nanosheets could also be achieved when
aerosil was used, but with lots of unreacted SiO (Fig. S9†).
2
Due to the mild reaction conditions, even production in
simple glass flasks is possible.
(
d) Preparation of 3 wt% Pd/iron silicate. Preparation of
3
wt% Pd/iron silicate using Chemical Fluid Deposition
5
8
method was similar as preparation of 3 wt% Pd/SBA-15,
except the support material was iron silicate instead of SBA-15.
Catalytic reactions
Experimental section
Material synthesis
(
a) Hydrogenation of organic compounds. Hydrogenations
were performed in a 28 mL autoclave made of hastelloy
(a) Synthesis of iron silicates (please see ESI for further (material number: 2.4819). Under argon flushing, a glass inlet
experimental details and Fig. S1–S11 for iron silicate character- with the weighted catalyst (10 mg) and the organic substrate
ization by TEM and EDX analysis)†. The synthesis was per- (5.0 mmol) was placed inside the autoclave. After tightening,
formed in a 28 mL autoclave (material No. 2.4819). SBA-15 the autoclave was pressurized with 20 bar H at room tempera-
2
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ture. The starting point of the reaction time was defined for tion (PDF) analysis. W. Schmidt performed TG-DSC-MS
the moment at which the inner temperature of the autoclave measurement. G. Prieto performed NH -TPD measurement,
3
reached 120 °C (about 15 min). After 3 hours reaction time, and C. Ochoa-Hernández made Pyridine-FTIR and DRIFT
the autoclave was cooled down to room temperature in a con- analysis. E. Bill and S.F. Ye analyzed the material with
trolled manner using an ice water bath. The product mixture Mössbauer and electron paramagnetic resonance (EPR) spec-
was diluted with acetone, then filtrated over RC 4 syringe filter troscopy. They determined the magnetic properties as well.
and further analyzed via GC analysis.
J. Folke and H. Ruland performed the catalytic ammonia syn-
(b) Ammonia synthesis. The ammonia synthesis tests were thesis. Y.X. Qiao wrote the initial manuscript with input from
performed in a commercial flow set-up equipped a plug flow all authors and N. Theyssen and W. Leitner modified the
reactor as a synthesis reactor as well as an online IR-detector paper in its present form. All authors discussed the results
(
Emerson X-stream) for quantitative gas analysis of NH and and edited the manuscript before submission.
3
2
H O. 0.838 g of the catalyst precursor (grain fraction of
2
1
50–425 µm) was diluted with ∼1 g SiC (average grain size
54 µm). Beyond and below the catalyst bed, a bed of pure SiC
Conflicts of interest
was placed, held in position by a glass wool plug at the
entrance and exhaust of the reactor. The sample was reduced There are no conflicts to declare.
by heating it in a gas flow of 440 mL min (75% H , 25% N )
with a heating rate of 1 K min up to 500 °C. Afterwards, the
conditions were kept constant for around 88 h. The pressure
was not elevated. The resulting pressure in the reactor (due to
−
1
n
2
2
−1
Acknowledgements
pressure drop) was in the range of 3–4 bar. For high pressure The authors gratefully acknowledge the following people for
activity measurements, the temperature was kept at 500 °C, support with analytical measurements and data analysis:
and the pressure was increased up to 90 bar. During the Hans-Josef Bongard (SEM-EDX), Silvia Palm (EDX bulk),
pressure increasing, the total flow was also increased to accel- Adrian Schlüter (TEM), Norbert Pfänder (STEM), Jan
erate this procedure. After 90 bar was reached, the total flow Ternieden and Jan Nicolas Büscher (XRD and XPS), Prof.
−
1
was adjusted to 200 mL min (75% H , 25% N ). The temp- Dr Osamu Terasaki and Dr Yanghang Ma (3D electron diffrac-
n
2
2
erature was kept constant for 48 h, and afterwards, tempera- tion tomography: failed due to the poor crystallinity and stabi-
ture steps at 475, 450, 425, 400, 375, 350, 325, 350, 375, 400, lity under strong beam irradiation), Dr Nicolas Duyckaerts
4
25, 450, 475 and 500 °C were adjusted. Heating rates between (NH -TPD measurements), Kai Jeske (GC gas analysis),
3
−
1
the temperature steps were set to be 1 K min . All tempera- Dr Yuxiao Ding (ATR-IR) and Dr Zhengwen Cao (titration). The
tures of the temperature steps were kept constant for 155 min authors also would like to thank Prof. Dr Robert Schlögl,
Materials characterization (please see ESI for further details Dr Thomas Lunkenbein, Fabian Pienkoß and Dr Gaetano
on the characterization methods†). Obtained composite Calvaruso for helpful and enthusiastic discussions, as well as
materials were characterized by elemental analysis, trans- Niklas Fuhrmann and Lars Winkel for technical support. The
mission electron microscopy (TEM), energy-dispersive X-ray studies were carried out as part of our activities in the Cluster
spectroscopy (EDX) and scanning transmission electron of Excellence “Tailor-Made Fuels from Biomass” (EXC 236)
microscopy (STEM), Mössbauer spectroscopy, electron para- and “The Fuel Science Center” funded by the Deutsche
magnetic resonance (EPR), pair distribution function (PDF) Forschungsgemeinschaft (DFG, German Research Foundation)
analysis, Fourier-transform infrared spectroscopy (FTIR), under Germany’s Excellence Strategy – Exzellenzcluster 2186
in situ diffuse reflectance infrared fourier spectroscopy “The Fuel Science Center” ID: 390919832. Open Access
(
DRIFT), SQUID measurement, thermogravimetric analysis– funding provided by the Max Planck Society.
mass spectrometry (TG–MS), sorption isotherms (N -BET),
X-ray diffraction (XRD) and X-ray photoelectron spectroscopy
XPS).
2
(
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