catalysts
Article
Mayenite-Based Electride C12A7e−: A Reactivity and
Stability Study
Sebastian Weber 1,†,‡ , Sebastian Schäfer 1, Mattia Saccoccio 1, Nils Ortner 2, Marko Bertmer 3,
Karsten Seidel 4, Stefan Berendts 5, Martin Lerch 5, Roger Gläser 6, Holger Kohlmann 7
and Stephan A. Schunk 1,4,*
Citation: Weber, S.; Schäfer, S.;
Saccoccio, M.; Ortner, N.; Bertmer, M.;
Seidel, K.; Berendts, S.; Lerch, M.;
Gläser, R.; Kohlmann, H.; et al.
Mayenite-Based Electride C12A7e−:
A Reactivity and Stability Study.
Catalysts 2021,11, 334. https://
doi.org/10.3390/catal11030334
Academic Editors:
Alessandro Di Michele
and Carlo Pirola
Received: 19 February 2021
Accepted: 3 March 2021
Published: 5 March 2021
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1hte GmbH, Kurpfalzring 104, 69123 Heidelberg, Germany; [email protected] (S.W.);
2Leibniz Institute for Catalysis (LIKAT Rostock), Albert-Einstein-Straße 29a, 18059 Rostock, Germany;
3Felix-Bloch-Institut für Festkörperphysik, Leipzig University, Linnéstraße 5, 04103 Leipzig, Germany;
4BASF SE, Carl-Bosch-Straße 38, 67056 Ludwigshafen am Rhein, Germany; [email protected]
5
Institute of Chemistry, TU Berlin, Straße des 17. Juni 135, 10623 Berlin, Germany; [email protected] (S.B.);
[email protected] (M.L.)
6Institute of Chemical Technology, Leipzig University, Linnéstraße 3, 04103 Leipzig, Germany;
roger[email protected]
7Institute of Inorganic Chemistry, Leipzig University, Johannisallee 29, 04103 Leipzig, Germany;
holger[email protected]
*Correspondence: stephan.schunk@hte-company.de; Tel.: +49-6221-7497-0
† New address: Institute for Chemical Technology and Polymer Chemistry (ITCP) Engesserstraße 20,
76131 Karlsruhe, Germany.
‡
New address: Institute of Catalysis Research and Technology (IKFT), Karlsruhe Institute of Technology (KIT),
Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.
Abstract:
Ru supported on mayenite electride, [Ca
24
Al
28
O
64
]
4+
(e
−
)
4
a calcium aluminum oxide
denoted as C12A7e
−
, are described in the literature as highly active catalysts for ammonia synthesis,
especially under conditions of low absolute pressure. In this study, we investigated the application of
recently reported plasma arc melting synthesized C12A7e
−
(aluminum solid reductant) as supports
of Ru/C12A7e
−
catalysts in ammonia synthesis up to pressures of 7.6 MPa. Together with the plasma-
arc-melting-based catalyst support, we investigated a similar plasma-synthesized C12A7e
−
(graphite
solid reductant) and a vacuum-sintering-based C12A7e
−
. Complementary to the catalytic tests, we
applied
2
H solid-state NMR spectroscopy, DRUVVis-spectroscopy, thermal analysis and PXRD to
study and characterize the reactivity of different plasma-synthesized and vacuum-sintered C12A7e
−
towards H
2
/D
2
and H
2
O. The catalysts showed an immediate deactivation at
pressures > 1 MPa
,
which can be explained by irreversible hydride formation at higher pressures, as revealed by reactivity
tests of C12A7e
−
towards H
2
/D
2
. The direct formation of C12A7:D from C12A7e
−
is proven. It
can be concluded that the application of Ru/C12A7e
−
catalysts at the industrial scale has limited
prospects due to irreversible hydride formation at relevant pressures > 1 MPa. Furthermore, we
report an in-depth study relating to structural changes in the material in the presence of H2O.
Keywords:
mayenite; electride; calcium aluminates; catalyst support; ruthenium; ammonia synthesis;
catalyst deactivation; hydride ions
1. Introduction
Ammonia synthesis is one of the major processes in the chemical industry and the
established Haber–Bosch process is already more than 100 years old [
1
–
3
]. For indus-
trial applications, Fe-based catalysts are mainly used, which trace back also to the early
studies of Mittasch [
4
]. In industrial conditions, this process is typically carried out in a
Catalysts 2021,11, 334. https://doi.org/10.3390/catal11030334 https://www.mdpi.com/journal/catalysts
Catalysts 2021,11, 334 2 of 18
temperature range of 673 to 773 K and pressures of above 15 MPa [
5
]. Another catalyst
family that has also found industrial use is Ru as an active metal supported on carbon
with promoters: these types of catalyst can be applied and are of general interest as they
are more active compared to Fe-based catalysts at lower temperatures, therefore offer-
ing a process-advantage [
6
]. For Fe-based catalysts, N
2
dissociation is considered as the
rate-determining step of the reaction happening on seven coordinated Fe-sites [
7
–
9
]. For
the Ru-based catalyst, a similar structure sensitivity is reported, and step sites of the Ru
surface are considered as favorable sites while the N
2
dissociation is also assumed as a rate-
determining step [
10
–
14
]. However, Ru-based ammonia synthesis catalysts typically suffer
from poisoning by H
2
limiting the performance at higher pressures, which is explained by
the competitive adsorption of N2and H2on Ru [6,15,16].
Overcoming the aforementioned challenges, and accordingly improving the potentials
of Ru-based catalysts, the group of Hosono reported that Ru supported on a novel support
material, namely, mayenite electrides—[Ca
24
Al
28
O
64
]
4+
(e
−
)
4
—in combination, results in
catalysts that are described as highly active for ammonia synthesis [
17
,
18
]. The electride
support material [Ca
24
Al
28
O
64
]
4+
(e
−
)
4
, denoted as C12A7e
−
, was reported to be stable up
to 673 K under atmospheric conditions. C12A7e
−
is based on the mayenite structure, as first
reported by the group of Hosono [
19
–
21
]. Mayenite is a calcium aluminum oxide, which can
be described by a positively charged framework of 12 cages [Ca
24
Al
28
O
64
]
4+
with anions
distributed inside the cages compensating for the charge, in case of the oxygen mayenite
[Ca
24
Al
28
O
64
]
4+
(O
2−
)
2
, denoted as C12A7 [
22
–
27
]. For the mayenite structure, several
anionic species are reported, e.g., OH
−
[
28
–
30
], H
−
[
19
,
29
,
31
–
34
], N
3−
[
35
], NH
2−
[
35
,
36
]
and NH
2−
[
37
]. Mayenite-based electrides [Ca
24
Al
28
O
64
]
4+
(2
×δ
)e
−
(2
−δ
)O
2−
show
interesting properties depending on the electron concentration N
e
. The material can, in
fact, be an insulator (colorless,
δ
= 0, N
e
= 0 cm
−3
), semi-conductor (green color,
δ
< 1,
Ne< 1 ×1021 cm−3
) or metallic conductor (very dark or black,
δ
> 1, N
e
> 1
×
10
21
cm
−3
)
with a maximum possible N
e
= 2.3
×
10
21
cm
−3
[
20
–
22
,
38
]. The metallic conducting
C12A7e−exhibits a work function of 2.4 eV [39].
Due to the “promoting” properties of the electride support, the group of Hosono
reported that the rate-determining step of ammonia synthesis over Ru/C12A7e
−
was
not the dissociation of N
2
, but rather the subsequent formation of N-H
x
species. At the
same time, it was observed that the nature of the reaction mechanism depends on the
electron concentration of the mayenite electride support [
31
,
40
]. The group concluded that
a metallic conducting electride is required for superior activity, and its action is explained
as an “electronic promoter” for Ru, and that the H
2
poisoning could be overcome by the
reversible incorporation and release of H
−
into the electride structure [
17
,
18
,
31
,
40
]. The
reported turnover frequency (TOF) of 0.98 s
−1
for this catalyst is more than one order
of magnitude higher than that of comparable, conventional Ru-Ba/activated carbon or
Ru-Cs/MgO catalysts [
18
]. However, they reported the catalytic test results only for
pressures up to 1.3 MPa [
17
,
18
], which is considerably lower than industrially relevant
conditions [
5
]. A recent work of Kammert et al. investigated the role of hydrogen for this
catalyst in detail with in situ neutron-scattering techniques and found that the suggested
reversible incorporation and release of H
−
in the mayenite structure does not play a major
role in the mechanism in contrast to the findings of the group of Hosono [
17
,
32
]. They
further applied steady-state isotopic transient kinetic analysis with isotopically labelled
nitrogen to determine the number of reactive nitrogen intermediates on the catalyst surface,
reporting that the promoting effect of the electride support enables much higher coverages
of Ru by adsorbates compared to use of the oxygen mayenite support (coverages 84% vs.
15%, respectively) [
32
]. Thus, the promoting properties of the electride are crucial for the
enhanced performance of the material to prevent poisoning by H
2
and not a reversible
uptake and release of hydrogen in the mayenite structure.
The synthesis of metallic conducting C12A7e−was initially achieved by the reaction
of C12A7 with Ca at 973 K for 240 h in evacuated silica tubes [
21
]. Later, other solid-
reductants or synthesis procedures were applied, as reviewed by Kim et al., Salasin et al.,
Catalysts 2021,11, 334 3 of 18
or Khan et al. [
20
,
22
,
38
]. Potential scalable synthesis procedures of powdered C12A7e
−
,
readily applicable as catalyst supports with high electron concentrations, were only recently
reported. This includes stoichiometric synthesis from CaO, Al
2
O
3
, Al at 1373 K for 8 h
under Ar [
41
], or plasma-arc-melting synthesis with the addition of solid reductants like
Al or graphite, which yielded C12A7e
−
within less than 1 min [
42
]. The synthesis of the
stoichiometric hydride C12A7:H is possible starting from C12A7 by reaction with CaH
2
or TiH
2
at 1073 K for > 120 h in sealed silica tubes [
29
,
34
]. Non-stoichiometric hydrides,
still containing O
2−
, OH
−
or e
−
, were reported for the reaction of C12A7 or C12A7e
−
with gaseous H
2
[
19
,
29
,
31
–
34
]. The hydrogen uptake of Ru/C12A7e
−
and C12A7e
−
by
temperature-programmed absorption, as studied by Kitano et al., showed that, for the
Ru-containing sample, the uptake occurs at lower temperatures, with a maximum of ca.
100 K lower at about 750 K, compared to the pure C12A7e
−
[
31
]. It should be noted
that, in temperature-programmed desorption experiments, it was found that H
2
was also
released at lower temperatures for the Ru/C12A7e
−
sample compared to C12A7e
−
and
at lower temperatures than the absorption [
31
]. The mayenite materials are typically
hygroscopic, while the reaction of C12A7e
−
with H
2
O was only recently reported by Jiang
et al., showing the formation of Al
2
Ca
4
(OH)
12
and slight amounts of Al(OH)
3
[
43
]. Thus,
potential stability issues of the Ru/C12A7e
−
catalyst towards H
2
O were not frequently
reported. To the best of our knowledge, the only records are a review and a study on a new
potential electride support for ammonia synthesis Y
5
Si
3
—but no detailed information on
the nature of the water instability of the mayenite electride materials is provided in these
publications [17,44].
To date, the studies of the catalytic performance of Ru/C12A7e
−
reported in the
literature were only conducted under model laboratory conditions with low pressures of
less than 1.3 MPa [
17
,
18
,
31
,
32
,
40
]. Studies under relevant industrial operation conditions,
especially higher pressures, of the catalyst in ammonia synthesis, are desirable to evaluate
its potential in industrial applications. In the present study, we first report the application
of Ru/C12A7e
−
catalysts under ammonia synthesis conditions close to atmospheric, and at
elevated pressures, to investigate all industrial-relevant operation conditions. The mayen-
ite electride support materials were obtained by plasma-arc melting of mayenite starting
materials, as reported here [
42
]. We further investigated the C12A7e
−
support material
through reactivity studies towards H
2
/D
2
under atmospheric pressure and higher pressure
to better understand the issues concerning the catalyst stability and potential deactiva-
tion mechanism by hydride formation. The studies are complemented by hydrothermal
reactivity experiments of C12A7 and C12A7e−to assess the stability towards H2O.
2. Results and Discussion
In this study, we investigated the activity, reactivity, and stability of different C12A7e
−
samples prepared by two different synthetic routes. The relevant samples are summarized
in Table 1and can be divided into four groups: 1. Plasma-synthesized C12A7 and C12A7e
−
samples (a)–(d), 2. Vacuum-sintering-based sample (e), 3. Ru supported on C12A7 and
C12A7e
−
(f)–(i) based on samples of the plasma treatment route (I), and 4.
hydride (j)
and deuteride (k) of the vacuum-sintering-based C12A7e
−
. The prepared C12A7e
−
sam-
ples (c)–(e) are of dark green color, which can be attributed to semiconductingclose to
metallicconducting C12A7e
−
samples according to the literature and a previous study on
plasma-treatment-based C12A7e
−
(electron concentration N
e
from approximately 0.1 to
1.2 ·1021 cm−3) [20–22,38,42]
. The plasma-treatment-based samples (a) and (d) are used for
reactivity and stability experiments under hydrothermal conditions. (d) and (e) were tested
regarding their reactivity towards hydrogen and possible hydride/deuteride formation.
Catalytic activity tests were performed for the plasma-based samples (g) and (h), while
sample (i) was used to investigate whether the electride properties are still present after
Ru deposition. The samples (j) and (k) were obtained from high-pressure hydrogenation
experiments in the DSC or autoclave.
Catalysts 2021,11, 334 4 of 18
Table 1. C12A7e−samples studied in this work.
Sample 1Color of Powder Solid Reductant
(a) C12A7 colorless -
(b) C12A7e−(5Al) light green 5 wt.% Al
(c) C12A7e−(20Al) dark green 20 wt.% Al
(d) C12A7e−(3C) dark green 3 wt.% graphite
(e) C12A7e−(SSR) dark green -
(f) Ru/C12A7 gray -
(g) Ru/C12A7e−(5Al) gray 5 wt.% Al
(h) Ru/C12A7e−(20Al) dark gray 20 wt.% Al
(i) Ru/C12A7e−(3C) dark gray 3 wt.% graphite
(j) C12A7:H colorless -
(k) C12A7:D colorless -
1
(a)–(d) are prepared via plasma treatment and (e) is prepared via the solid-state reduction route. Samples (f)–(i)
containing Ru are based on the plasma-synthesized samples (a)–(d). Samples (j) and (k) are the mayenite hydride
and deuteride phases prepared from sample (e), respectively.
In the following sections, we will first discuss the deposition of Ru on plasma-
synthesized C12A7e
−
samples, followed by the catalytic testing of those materials in
ammonia synthesis. This is complemented by reactivity and stability studies of the elec-
tride materials concerning hydrogenation and hydrothermal conditions.
2.1. Ruthenium Deposition
The deposition of Ru was carried out as adapted from the literature procedure of
Kitano et al. [
18
]. Four different Ru-loaded samples (f)–(i), based on C12A7 and C12A7e
−
prepared by plasma synthesis (a)–(d), are discussed in this work. For catalytic tests, the
Al-based samples (g) and (h) were used, while the pure C12A7 (f) and the graphite-based
electride (g) were used to test if the material still qualified as an electride after Ru deposition.
The quantitative Ru deposition on the samples was confirmed by XRF analysis. After Ru
deposition, the color changed from colorless to gray in case of samples (a) and (f), where the
pure oxygen mayenite C12A7 was present. For samples with Ru deposited on C12A7e
−
,
the color changed to dark gray. The obtained powders of samples (f) Ru/C12A7 and
(i) Ru/C12A7e−(3C)
are shown in Figure 1, illustrating the color difference between the
respective supports. Both samples were also studied by STA analysis in synthetic air,
with the DSC and TG curves shown in Figure 1. For both samples, a mass loss can be
observed starting at about 500 K, while the mass loss is greater for the Ru/C12A7 sample
compared to the C12A7e
−
-based one. We expect this mass loss to be caused by either not
fully decomposed Ru
3
(CO)
12
precursors or by the release of adsorbed H
2
O, or both. At
about 900 K, the TG curves for the two samples differ, as the Ru/C12A7e
−
sample shows
a mass uptake with a maximum of about 1150 K. This mass uptake can be assigned to
the transformation of C12A7e
−
to C12A7. The maximum and the temperature region are
well in line with previous studies on plasma-treatment-based C12A7e−and the literature
values [
42
,
45
–
47
], indicating that the C12A7e
−
is still present after Ru deposition. In the
DSC curves, no clear signal for both samples can be observed.
To further investigate the presence of C12A7e
−
after Ru deposition, the samples
(f) Ru/C12A7
and (i) Ru/C12A7e
−
(3C) were characterized by DRUVVis and EPR spec-
troscopy, as shown in Figure 2. The DRUVVis spectra are shown together with the spectra
of the initial C12A7e
−
sample (d). One cannot observe the typical peak at around 2.83 eV
for C12A7e
−
for the Ru-containing sample (i) [
21
]. However, it should be noted that the
Kubelka–Munk intensity for the Ru/C12A7e
−
sample is significantly higher compared to
the Ru/C12A7-based material. Based on the DRUVVis results, we cannot analyze the elec-
tron concentration of the Ru-containing samples and, also from DRUVVis, an unambiguous
presence of C12A7e−after Ru deposition cannot be concluded.
Catalysts 2021,11, 334 5 of 18
Catalysts 2021, 11, x FOR PEER REVIEW 5 of 18
Figure 1. Left top to bottom: photographs of (f) 1.2 wt.%-Ru/C12A7 and (i) Ru/C12A7e− (3C) pow-
ders after Ru deposition. Right: DSC (heat flow, exothermic up, solid) and TG (m/mtotal, dashed)
curves under synthetic air for (f) shown in black and (i) shown in red. 10 mg of sample each was
heated with a ramp of 10 K · min−1 from RT to 1573 K.
To further investigate the presence of C12A7e− after Ru deposition, the samples (f)
Ru/C12A7 and (i) Ru/C12A7e− (3C) were characterized by DRUVVis and EPR spectros-
copy, as shown in Figure 2. The DRUVVis spectra are shown together with the spectra of
the initial C12A7e− sample (d). One cannot observe the typical peak at around 2.83 eV for
C12A7e− for the Ru-containing sample (i) [21]. However, it should be noted that the Ku-
belka–Munk intensity for the Ru/C12A7e− sample is significantly higher compared to the
Ru/C12A7-based material. Based on the DRUVVis results, we cannot analyze the electron
concentration of the Ru-containing samples and, also from DRUVVis, an unambiguous
presence of C12A7e− after Ru deposition cannot be concluded.
The EPR spectra of the samples (f) and (i) are shown, together with the spectra of the
pure C12A7 sample (a). The Ru/C12A7e− exhibits the typical EPR signal for the semicon-
ducting electride at 339.5 mT. The normalized intensity IN of the signal is 410, which is
significantly lower compared to the range from 2500 to 4825, as reported for pure C12A7e−
(3C) samples obtained from plasma treatment [42]. For sample (f) Ru/C12A7, a different
signal at lower magnetic field strength of 334.5 mT is observed, which cannot be attributed
to C12A7e−.
Figure 2. Left: Kubelka–Munk-transformed DRUVVis spectra for Ru-impregnated samples: blue (g) Ru/C12A7, green (i)
Ru/C12A7e− (3C) and dashed (d) C12A7e− (3C). The dotted line at 2.83 eV corresponds to the absorption peak maximum
for Ne = 1 · 1018 cm−3 [21]. The arrow indicates the shift with increasing Ne. Right: first derivative EPR spectra for samples
(a) C12A7 (black), (f) Ru/C12A7 (blue) and (i) Ru/C12A7e− (green) samples, intensities are not normalized.
Based on the results obtained from STA, DRUVVis and EPR analysis, it can be con-
cluded that after Ru deposition on the plasma-synthesized C12A7e− samples, the material
still corresponds to the semiconducting C12A7e−. However, the electron concentration
seems to have decreased, as indicated by the IN of the EPR spectra and the absence of a
clear absorption peak within the DRUVVis experiments. PXRD (Figure S2) of the samples
Figure 1.
(
Left top to bottom
): photographs of (f) 1.2 wt.%-Ru/C12A7 and (i) Ru/C12A7e
−
(3C) powders after Ru
deposition. (
Right
): DSC (heat flow, exothermic up, solid) and TG (m/m
total
, dashed) curves under synthetic air for
(f) shown in black and (i) shown in red. 10 mg of sample each was heated with a ramp of 10 K ·min−1from RT to 1573 K.
Catalysts 2021, 11, x FOR PEER REVIEW 5 of 18
Figure 1. Left top to bottom: photographs of (f) 1.2 wt.%-Ru/C12A7 and (i) Ru/C12A7e− (3C) pow-
ders after Ru deposition. Right: DSC (heat flow, exothermic up, solid) and TG (m/mtotal, dashed)
curves under synthetic air for (f) shown in black and (i) shown in red. 10 mg of sample each was
heated with a ramp of 10 K · min−1 from RT to 1573 K.
To further investigate the presence of C12A7e− after Ru deposition, the samples (f)
Ru/C12A7 and (i) Ru/C12A7e− (3C) were characterized by DRUVVis and EPR spectros-
copy, as shown in Figure 2. The DRUVVis spectra are shown together with the spectra of
the initial C12A7e− sample (d). One cannot observe the typical peak at around 2.83 eV for
C12A7e− for the Ru-containing sample (i) [21]. However, it should be noted that the Ku-
belka–Munk intensity for the Ru/C12A7e− sample is significantly higher compared to the
Ru/C12A7-based material. Based on the DRUVVis results, we cannot analyze the electron
concentration of the Ru-containing samples and, also from DRUVVis, an unambiguous
presence of C12A7e− after Ru deposition cannot be concluded.
The EPR spectra of the samples (f) and (i) are shown, together with the spectra of the
pure C12A7 sample (a). The Ru/C12A7e− exhibits the typical EPR signal for the semicon-
ducting electride at 339.5 mT. The normalized intensity IN of the signal is 410, which is
significantly lower compared to the range from 2500 to 4825, as reported for pure C12A7e−
(3C) samples obtained from plasma treatment [42]. For sample (f) Ru/C12A7, a different
signal at lower magnetic field strength of 334.5 mT is observed, which cannot be attributed
to C12A7e−.
Figure 2. Left: Kubelka–Munk-transformed DRUVVis spectra for Ru-impregnated samples: blue (g) Ru/C12A7, green (i)
Ru/C12A7e− (3C) and dashed (d) C12A7e− (3C). The dotted line at 2.83 eV corresponds to the absorption peak maximum
for Ne = 1 · 1018 cm−3 [21]. The arrow indicates the shift with increasing Ne. Right: first derivative EPR spectra for samples
(a) C12A7 (black), (f) Ru/C12A7 (blue) and (i) Ru/C12A7e− (green) samples, intensities are not normalized.
Based on the results obtained from STA, DRUVVis and EPR analysis, it can be con-
cluded that after Ru deposition on the plasma-synthesized C12A7e− samples, the material
still corresponds to the semiconducting C12A7e−. However, the electron concentration
seems to have decreased, as indicated by the IN of the EPR spectra and the absence of a
clear absorption peak within the DRUVVis experiments. PXRD (Figure S2) of the samples
Figure 2.
(
Left
): Kubelka–Munk-transformed DRUVVis spectra for Ru-impregnated samples: blue (g) Ru/C12A7, green
(i) Ru/C12A7e−(3C)
and dashed (d) C12A7e
−
(3C). The dotted line at 2.83 eV corresponds to the absorption peak maximum
for N
e
= 1
·
10
18
cm
−3
[
21
]. The arrow indicates the shift with increasing N
e
. (
Right
): first derivative EPR spectra for
samples (a) C12A7 (black), (f) Ru/C12A7 (blue) and (i) Ru/C12A7e−(green) samples, intensities are not normalized.
The EPR spectra of the samples (f) and (i) are shown, together with the spectra of
the pure C12A7 sample (a). The Ru/C12A7e
−
exhibits the typical EPR signal for the
semiconducting electride at 339.5 mT. The normalized intensity I
N
of the signal is 410,
which is significantly lower compared to the range from 2500 to 4825, as reported for pure
C12A7e
−
(3C) samples obtained from plasma treatment [
42
]. For sample (f) Ru/C12A7, a
different signal at lower magnetic field strength of 334.5 mT is observed, which cannot be
attributed to C12A7e−.
Based on the results obtained from STA, DRUVVis and EPR analysis, it can be con-
cluded that after Ru deposition on the plasma-synthesized C12A7e
−
samples, the material
still corresponds to the semiconducting C12A7e
−
. However, the electron concentration
seems to have decreased, as indicated by the I
N
of the EPR spectra and the absence of a clear
absorption peak within the DRUVVis experiments. PXRD (Figure S2) of the samples (g)
and (i) after Ru deposition do not show changes compared to the used C12A7 and C12A7e
−
samples, while no distinct Ru reflections could be observed, indicating X-ray-amorphous
deposition of the Ru on the support material, which could be confirmed as shown in the
TEM image in Figure S1.
2.2. Reactivity Studies of C12A7 and C12A7e−
2.2.1. Catalytic Studies of Ru/C12A7e−
The catalytic activity of the two Ru/C12A7e
−
catalysts (g) and (h) was tested in
ammonia synthesis. Both catalysts are based on plasma-synthesized C12A7e
−
with Al
Catalysts 2021,11, 334 6 of 18
used as a solid reductant. Sample (h) exhibited a higher electron concentration compared
to (g), as indicated by the color difference, while sample (g) is a semiconducting material,
and (h) is expected to be semiconducting close to metallic conduction, as reported in the
literature [
42
,
45
–
47
]. The two samples were tested under different reaction conditions of
ammonia synthesis, denoted as segments, with varied reaction temperatures and pressures,
as listed in Table 2. The tested temperatures ranged from 593 to 673 K, and the pressure
from 0.1 up to 7.6 MPa. The NH
3
formation rate for all testing conditions of the two
catalysts is shown in Figure 3.
Table 2.
Reaction conditions for catalytic experiments of two 1.2 wt.% Ru/C12A7e
−
catalysts based on plasma synthesized
C12A7e−, (g) Ru/C12A7e−(5Al) and (h) Ru/C12A7e−(20Al).
Segment 1 2 3 4 5 6 7 8 9 10 11 12
T/K 673 653 633 623 613 603 593 633 633 633 633 633
p/Mpa 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 1.1 3.1 6.1 7.6
5 g catalyst, 350–500 µm sieve fraction, 5 mL catalyst bed volume, total flow 4.0126 mol ·h−1, 3:1 H2:N2.
Catalysts 2021, 11, x FOR PEER REVIEW 6 of 18
(g) and (i) after Ru deposition do not show changes compared to the used C12A7 and
C12A7e− samples, while no distinct Ru reflections could be observed, indicating X-ray-
amorphous deposition of the Ru on the support material, which could be confirmed as
shown in the TEM image in Figure S1.
2.2. Reactivity Studies of C12A7 and C12A7e−
2.2.1. Catalytic Studies of Ru/C12A7e−
The catalytic activity of the two Ru/C12A7e− catalysts (g) and (h) was tested in am-
monia synthesis. Both catalysts are based on plasma-synthesized C12A7e− with Al used
as a solid reductant. Sample (h) exhibited a higher electron concentration compared to (g),
as indicated by the color difference, while sample (g) is a semiconducting material, and
(h) is expected to be semiconducting close to metallic conduction, as reported in the liter-
ature [42,45–47]. The two samples were tested under different reaction conditions of am-
monia synthesis, denoted as segments, with varied reaction temperatures and pressures,
as listed in Table 2. The tested temperatures ranged from 593 to 673 K, and the pressure
from 0.1 up to 7.6 MPa. The NH3 formation rate for all testing conditions of the two cata-
lysts is shown in Figure 3.
Table 2. Reaction conditions for catalytic experiments of two 1.2 wt.% Ru/C12A7e− catalysts based on plasma synthesized
C12A7e−, (g) Ru/C12A7e− (5Al) and (h) Ru/C12A7e− (20Al).
Segment
1
2
3
4
5
6
7
8
9
10
11
12
T/K
673
653
633
623
613
603
593
633
633
633
633
633
p/MPa
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
1.1
3.1
6.1
7.6
5 g catalyst, 350–500 µm sieve fraction, 5 mL catalyst bed volume, total flow 4.0126 mol ∙ h−1, 3:1 H2:N2.
Figure 3. NH3 formation rate of two different plasma-synthesized 1.2 wt.% Ru/C12A7e− catalysts
in ammonia synthesis, black (g) Ru/C12A7e− (5Al) and red (h) Ru/C12A7e− (20Al). The testing con-
ditions for each segment are summarized in Table 2.
For segments with reaction pressures of 0.1 MPa, both samples are highly active cat-
alysts in ammonia synthesis. However, their specific activity appears to be different.
While the sample with lower electron concentration (g) Ru/C12A7e− (5Al) has its maxi-
mum activity at 673 K, with an NH3 formation rate of 1066 µmol h−1 g−1, the catalyst (h)
exhibits its highest activity at 633 K, with 2493 µmol h−1 g−1. This rate is well in agreement
with the value reported by Kitano et al. of 2730 µmol h−1 g−1 for an Ru/C12A7e− catalyst
with a similar loading of 1.2 wt.% Ru [31]. In the first reported application of the material
for NH3 synthesis, by the group of Hosono et al., they reported a rate of 2760 µmol ∙ h−1 ∙
g−1 for a 1.2 wt.% Ru/C12A7e− sample at 673 K, which is a slightly higher activity than the
Figure 3.
NH
3
formation rate of two different plasma-synthesized 1.2 wt.% Ru/C12A7e
−
catalysts
in ammonia synthesis, black (g) Ru/C12A7e
−
(5Al) and red (h) Ru/C12A7e
−
(20Al). The testing
conditions for each segment are summarized in Table 2.
For segments with reaction pressures of 0.1 Mpa, both samples are highly active cata-
lysts in ammonia synthesis. However, their specific activity appears to be different. While
the sample with lower electron concentration (g) Ru/C12A7e
−
(5Al) has its maximum
activity at 673 K, with an NH
3
formation rate of 1066
µ
mol h
−1
g
−1
, the catalyst (h) exhibits
its highest activity at 633 K, with 2493
µ
mol h
−1
g
−1
. This rate is well in agreement with
the value reported by Kitano et al. of 2730
µ
mol h
−1
g
−1
for an Ru/C12A7e
−
catalyst with
a similar loading of 1.2 wt.% Ru [
31
]. In the first reported application of the material for
NH
3
synthesis, by the group of Hosono et al., they reported a rate of 2760
µ
mol
·
h
−1·
g
−1
for a 1.2 wt.% Ru/C12A7e
−
sample at 673 K, which is a slightly higher activity than
the Ru/C12A7e
−
(20Al) sample discussed here [
18
]. Comparing the Ru/C12A7e
−
(5Al)
and (20Al) we can observe differences in their activity, which we attribute to the different
electron concentration of the material, based on the color appearance of the precursor
C12A7e
−
. This observation is in line with the previous studies of Kanbara et al., who
showed the dependence of the activity of Ru/C12A7e
−
catalysts for NH
3
synthesis on the
electron concentration of the C12A7e−support [40].
A remarkably different behavior of the Ru/C12A7e
−
(5Al) and (20Al) samples com-
pared to studies of the group of Hosono can be observed for tests at higher pressures. In
the available reports, test results of the materials were only reported for pressures up to
Catalysts 2021,11, 334 7 of 18
1.3 Mpa
at 673 K [
17
,
18
]. As soon as Ru/C12A7e
−
(5Al) and (20Al) were tested at 633 K and
higher pressures of >1.0 Mpa, an immediate decrease in the NH
3
formation is observed,
as shown in Figure 3, for both catalysts. Kitano et al. reported an increase in activity
by increasing the pressure from 0.1 to 1.0 Mpa at 673 K [
18
]. From the catalytic testing
results, we assume that the immediate deactivation is caused by the transformation of
an mayenite electride to a mayenite-based hydride phase (C12A7:H), where, instead of
electrons, the cages are occupied with H
−
ions. This behavior contrasts with the studies of
the group of Hosono at low pressures. The group investigated the H
2
uptake of the catalyst
material by reaction with H
2
(H
2
(75 kPa), Ar(25 kPa), 633 K) and during reaction conditions
(
H2(75 kPa)
, Ar(25 kPa), 633 K), which suggested that only 1% of the electrons react with
hydrogen to H
−
under reaction conditions, while under H
2
/Ar conditions 50% of the
electrons are exchanged after 40 h [
31
]. With H
2
uptake experiments and kinetic studies,
they concluded that H
2
is reversibly incorporated into the cages of the mayenite structure
and prevents the poisoning of Ru with H
2
under these reaction conditions, concluding that
the Ru/C12A7e
−
catalyst is stable for NH
3
synthesis [
17
,
18
,
31
]. Kammert et al. studied the
H
2
and D
2
uptake of Ru/C12A7e
−
by in situ neutron diffraction and steady-state isotopic
transient kinetic analysis. They concluded that H
2
and D
2
in the cages are not likely to
participate in the reaction mechanism, as only a minor exchange is observed, and a rather
high possible coverage of adsorbed species on Ru is the reason for the poisoning resistance
and enhanced activity [
32
]. However, their experiments were not carried out at pressures
higher than 0.1 Mpa. In combination with these findings, we tend to explain the immediate
loss of activity upon higher pressures as caused by the irreversible transformation of the
mayenite electride into a mayenite-based hydride C12A7:H, causing a loss of the promoting
properties of the electride for Ru, and thus causing an irreversible H
2
poisoning in the
reaction. Based on these findings, we conclude that, despite Ru/C12A7e
−
showing a
promising and remarkable performance at ambient pressure for ammonia synthesis, as
pioneered by the group of Hosono, for application under industrial-relevant conditions
of higher pressures of >1.0 Mpa, no sustained activity retention is observed. The catalyst
exhibits stability issues due to bulk structural transformation, and thus the material is prob-
ably not suitable for application in the established ammonia synthesis process. To further
understand the reactivity of the C12A7e
−
support, and to obtain a better understanding of
the stability issues from a materials basis, in the following sections we discuss reactivity
studies on H2and H2O.
2.2.2. Reactivity towards Forming Gas and Hydrogen
The reactivity of C12A7e
−
towards hydrogen was tested in a broad range of tem-
peratures (i) in a tube furnace system in the presence of forming gas at 0.1 Mpa; (ii) by
simultaneous thermal analysis (STA) in the presence of forming gas at 0.1 Mpa; (iii) in a
DSC experiment under H
2
up to 6.7 Mpa; (iv) by autoclave reaction with deuterium gas at
6 Mpa and 773 K.
Initial reactivity tests of C12A7e
−
towards hydrogen were performed in a tube furnace
system at temperatures up to 723 K and 0.1 Mpa in a 95% N
2
/5% H
2
atmosphere. The
experiments were carried out with the plasma-synthesized C12A7e
−
(d), and the resulting
pictures of the powder after treatment at different temperatures are shown in Figure 4.
Starting from 623 K, one can observe a decolorization of the powder from dark green to
grayish, which becomes more significant at higher temperatures. A similar experiment was
carried out for the C12A7e
−
(d) sample by heating in N
2
instead of N
2
/H
2
atmosphere,
where no detectable decolorization compared to the N
2
/H
2
atmosphere could be observed.
A similar grayish color for C12A7:H is reported by Jiang et al. [
33
], while an onset of
H
2
consumption in temperature-programmed absorption studies is observed at similar
temperatures by Kitano et al. [
31
], as indicated by the color change in Figure 4for the
present study.
Catalysts 2021,11, 334 8 of 18
Catalysts 2021, 11, x FOR PEER REVIEW 8 of 18
grayish, which becomes more significant at higher temperatures. A similar experiment
was carried out for the C12A7e− (d) sample by heating in N2 instead of N2/H2 atmosphere,
where no detectable decolorization compared to the N2/H2 atmosphere could be observed.
A similar grayish color for C12A7:H is reported by Jiang et al. [33], while an onset of H2
consumption in temperature-programmed absorption studies is observed at similar tem-
peratures by Kitano et al. [31], as indicated by the color change in Figure 4 for the present
study.
Figure 4. Reactivity test series of (d) C12A7e− (3C) under forming gas (95% N2/5% H2) atmosphere inside the quartz-glass
tube. From left to right: pictures of the sample after heating for each 1 h at the respective temperature.
The reactivity of the plasma synthesized C12A7 (a) and C12A7e− (d) was further in-
vestigated by simultaneous thermal analysis (STA). STA experiments were carried out
under a forming gas atmosphere (95% N2/5% H2). The results are shown in Figure 5 (left).
For both samples, a mass uptake in the TG could be observed, though the uptake is larger
for the C12A7e− sample. The difference between both samples is pronounced in the region
where the color change, as shown in Figure 4, takes place for the C12A7e− sample. The
DSC curves for both samples do not show any significant signals in the region from 300
to 800 K. From the TG results, we conclude that the reaction of C12A7e− with H2 starts at
600 K. Thus, the STA results support the findings of the initial reactivity studies.
The samples studied in the tube furnace under N2/H2 and N2 atmosphere were char-
acterized by DRUVVis, as shown in Figure 5 (right). For the sample which was treated in
N2/H2 atmosphere, no clear absorption band of around 2.8 eV can be identified. This indi-
cates a strong decrease in the electron concentration and loss of the electride properties.
The sample treated under N2 atmosphere still exhibits the absorption band around 2.8 eV,
as in the initial C12A7e−; however, the intensity is decreased, and the band is not as pro-
nounced as before. The intensity decrease indicates a reduction in the electron concentra-
tion, as reported by Matsuishi et al. [21].
Figure 5. Left: DSC (heat flow, exothermic up, solid) and TG (m/mtotal, dashed) curves under forming gas (95% N2/5% H2)
for (d) C12A7e− (3C) shown in red and (a) C12A7 shown in black. 10 mg of sample each were heated with a rate of 10 K ·
min−1 from RT to 1173 K. The dotted blue lines indicate the region of the color change for C12A7e− as shown in Figure 4.
Right: Kubelka–Munk-transformed DRUVVis spectra for (d) C12A7e− (3C) shown in black and the resulting powders after
reactivity tests at 723 K under forming gas (blue) and N2 (green). The dotted line at 2.83 eV corresponds to the absorption
peak maximum for Ne = 1 · 1018 cm−3 [21]. The arrow indicates the shift with increasing Ne.
Figure 4.
Reactivity test series of (d) C12A7e
−
(3C) under forming gas (95% N
2
/5% H
2
) atmosphere inside the quartz-glass
tube. From left to right: pictures of the sample after heating for each 1 h at the respective temperature.
The reactivity of the plasma synthesized C12A7 (a) and C12A7e
−
(d) was further
investigated by simultaneous thermal analysis (STA). STA experiments were carried out
under a forming gas atmosphere (95% N
2
/5% H
2
). The results are shown in Figure 5(left).
For both samples, a mass uptake in the TG could be observed, though the uptake is larger
for the C12A7e
−
sample. The difference between both samples is pronounced in the region
where the color change, as shown in Figure 4, takes place for the C12A7e
−
sample. The
DSC curves for both samples do not show any significant signals in the region from 300 to
800 K. From the TG results, we conclude that the reaction of C12A7e
−
with H
2
starts at
600 K. Thus, the STA results support the findings of the initial reactivity studies.
Catalysts 2021, 11, x FOR PEER REVIEW 8 of 18
grayish, which becomes more significant at higher temperatures. A similar experiment
was carried out for the C12A7e− (d) sample by heating in N2 instead of N2/H2 atmosphere,
where no detectable decolorization compared to the N2/H2 atmosphere could be observed.
A similar grayish color for C12A7:H is reported by Jiang et al. [33], while an onset of H2
consumption in temperature-programmed absorption studies is observed at similar tem-
peratures by Kitano et al. [31], as indicated by the color change in Figure 4 for the present
study.
Figure 4. Reactivity test series of (d) C12A7e− (3C) under forming gas (95% N2/5% H2) atmosphere inside the quartz-glass
tube. From left to right: pictures of the sample after heating for each 1 h at the respective temperature.
The reactivity of the plasma synthesized C12A7 (a) and C12A7e− (d) was further in-
vestigated by simultaneous thermal analysis (STA). STA experiments were carried out
under a forming gas atmosphere (95% N2/5% H2). The results are shown in Figure 5 (left).
For both samples, a mass uptake in the TG could be observed, though the uptake is larger
for the C12A7e− sample. The difference between both samples is pronounced in the region
where the color change, as shown in Figure 4, takes place for the C12A7e− sample. The
DSC curves for both samples do not show any significant signals in the region from 300
to 800 K. From the TG results, we conclude that the reaction of C12A7e− with H2 starts at
600 K. Thus, the STA results support the findings of the initial reactivity studies.
The samples studied in the tube furnace under N2/H2 and N2 atmosphere were char-
acterized by DRUVVis, as shown in Figure 5 (right). For the sample which was treated in
N2/H2 atmosphere, no clear absorption band of around 2.8 eV can be identified. This indi-
cates a strong decrease in the electron concentration and loss of the electride properties.
The sample treated under N2 atmosphere still exhibits the absorption band around 2.8 eV,
as in the initial C12A7e−; however, the intensity is decreased, and the band is not as pro-
nounced as before. The intensity decrease indicates a reduction in the electron concentra-
tion, as reported by Matsuishi et al. [21].
Figure 5. Left: DSC (heat flow, exothermic up, solid) and TG (m/mtotal, dashed) curves under forming gas (95% N2/5% H2)
for (d) C12A7e− (3C) shown in red and (a) C12A7 shown in black. 10 mg of sample each were heated with a rate of 10 K ·
min−1 from RT to 1173 K. The dotted blue lines indicate the region of the color change for C12A7e− as shown in Figure 4.
Right: Kubelka–Munk-transformed DRUVVis spectra for (d) C12A7e− (3C) shown in black and the resulting powders after
reactivity tests at 723 K under forming gas (blue) and N2 (green). The dotted line at 2.83 eV corresponds to the absorption
peak maximum for Ne = 1 · 1018 cm−3 [21]. The arrow indicates the shift with increasing Ne.
Figure 5.
(
Left
): DSC (heat flow, exothermic up, solid) and TG (m/m
total
, dashed) curves under forming gas
(
95% N2
/
5% H2
) for (d) C12A7e
−
(3C) shown in red and (a) C12A7 shown in black. 10 mg of sample each were heated
with a rate of
10 K ·min−1
from RT to 1173 K. The dotted blue lines indicate the region of the color change for C12A7e
−
as shown in Figure 4. (
Right
): Kubelka–Munk-transformed DRUVVis spectra for (d) C12A7e
−
(3C) shown in black and
the resulting powders after reactivity tests at 723 K under forming gas (blue) and N
2
(green). The dotted line at 2.83 eV
corresponds to the absorption peak maximum for N
e
= 1
·
10
18
cm
−3
[
21
]. The arrow indicates the shift with increasing N
e
.
The samples studied in the tube furnace under N
2
/H
2
and N
2
atmosphere were
characterized by DRUVVis, as shown in Figure 5(right). For the sample which was
treated in N
2
/H
2
atmosphere, no clear absorption band of around
2.8 eV
can be identified.
This indicates a strong decrease in the electron concentration and loss of the electride
properties. The sample treated under N
2
atmosphere still exhibits the absorption band
around
2.8 eV
, as in the initial C12A7e
−
; however, the intensity is decreased, and the band
is not as pronounced as before. The intensity decrease indicates a reduction in the electron
concentration, as reported by Matsuishi et al. [21].
Complementary to the initial reactivity tests with plasma-synthesized C12A7e
−
(d),
the reactivity towards H
2
was tested for the vacuum-sintering-based C12A7e
−
(e) sample
at higher pressures. For this purpose, initial tests were carried out in a DSC experiment
under H
2
pressure (5.0 to 6.7 MPa, up to 700 K), while the C12A7:D sample was prepared
in an autoclave synthesis (6.0 MPa, 773 K). The hydrogenation in the DSC apparatus did
not show any thermal effects, similar to the results we obtained for the plasma-synthesized
samples, as shown in Figure 5(left). However, the color change from green vacuum-
sintered (e) C12A7e
−
to colorless (j) C12A7:H clearly indicates a reaction during the DSC
Catalysts 2021,11, 334 9 of 18
experiment. The reaction of green vacuum-sintered C12A7e
−
with deuterium led to a
small change in the lattice parameter from a = 11.9790(2) Å for sample (e) C12A7e
−
to
a = 11.9874(1) Å
for the colorless sample (k) C12A7:D (Figures S3 and S4) used in
2
H solid-
state NMR spectroscopy. The color change directly indicates the reaction of the C12A7e
−
with deuterium gas, which is accompanied by the loss of the electride properties.
To clarify the presence of deuteride anions within C12A7:D,
2
H NMR studies were
performed. The
2
H solid-state NMR spectrum of C12A7:D in Figure 6shows a quadrupolar
line shape with an isotropic chemical shift of 4.8 ppm, a quadrupolar coupling C
Q
of
16 kHz
and an asymmetry parameter
η
of 0. The quadrupolar parameters indicate a
covalent character of the deuteride, while the coupling strength is rather weak. The latter
suggests a non-cubic symmetry around the hydride (deuteride) ion, giving rise to the
quadrupolar coupling. There are indications for a minor site with a large quadrupolar
coupling of 145 kHz and an isotropic chemical shift of
−
1 ppm, which we assign to an OD
−
group, such as mayenite hydroxide. The observed chemical shifts compare well with the
literature data [
29
]. Therefore, we assume the sample to be mayenite hydride (deuteride)
with some minor amount of OD
−
groups in the clathrate cages. Mayenite hydride may
thus be synthesized not only by the known routes, from C12A7/C12A7e
−
and CaH
2
[
34
] or
C12A7 and TiH
2
[
29
], but also by direct hydrogenation of C12A7e
−
at elevated hydrogen
gas pressures.
Catalysts 2021, 11, x FOR PEER REVIEW 9 of 18
Complementary to the initial reactivity tests with plasma-synthesized C12A7e− (d),
the reactivity towards H2 was tested for the vacuum-sintering-based C12A7e− (e) sample
at higher pressures. For this purpose, initial tests were carried out in a DSC experiment
under H2 pressure (5.0 to 6.7 MPa, up to 700 K), while the C12A7:D sample was prepared
in an autoclave synthesis (6.0 MPa, 773 K). The hydrogenation in the DSC apparatus did
not show any thermal effects, similar to the results we obtained for the plasma-synthe-
sized samples, as shown in Figure 5 (left). However, the color change from green vacuum-
sintered (e) C12A7e− to colorless (j) C12A7:H clearly indicates a reaction during the DSC
experiment. The reaction of green vacuum-sintered C12A7e− with deuterium led to a small
change in the lattice parameter from a = 11.9790(2) Å for sample (e) C12A7e− to a =
11.9874(1) Å for the colorless sample (k) C12A7:D (Figures S3 and S4) used in 2H solid-
state NMR spectroscopy. The color change directly indicates the reaction of the C12A7e−
with deuterium gas, which is accompanied by the loss of the electride properties.
To clarify the presence of deuteride anions within C12A7:D, 2H NMR studies were
performed. The 2H solid-state NMR spectrum of C12A7:D in Figure 6 shows a quadrupo-
lar line shape with an isotropic chemical shift of 4.8 ppm, a quadrupolar coupling CQ of
16 kHz and an asymmetry parameter η of 0. The quadrupolar parameters indicate a cova-
lent character of the deuteride, while the coupling strength is rather weak. The latter sug-
gests a non-cubic symmetry around the hydride (deuteride) ion, giving rise to the quad-
rupolar coupling. There are indications for a minor site with a large quadrupolar coupling
of 145 kHz and an isotropic chemical shift of −1 ppm, which we assign to an OD− group,
such as mayenite hydroxide. The observed chemical shifts compare well with the litera-
ture data [29]. Therefore, we assume the sample to be mayenite hydride (deuteride) with
some minor amount of OD− groups in the clathrate cages. Mayenite hydride may thus be
synthesized not only by the known routes, from C12A7/C12A7e− and CaH2 [34] or C12A7
and TiH2 [29], but also by direct hydrogenation of C12A7e− at elevated hydrogen gas pres-
sures.
Figure 6. 2H-NMR spectrum of the deuteride sample from the hydrogenation experiment in the autoclave (k) C12A7:D
(blue) together with the fitted spectrum (red).
Kammert et al. investigated the reaction of Ru/C12A7e− with H2/D2 at atmospheric
pressure, but showed no complete exchange of e− by H− [32]. They instead found that,
Figure 6. 2
H-NMR spectrum of the deuteride sample from the hydrogenation experiment in the autoclave (k) C12A7:D
(blue) together with the fitted spectrum (red).
Kammert et al. investigated the reaction of Ru/C12A7e
−
with H
2
/D
2
at atmospheric
pressure, but showed no complete exchange of e
−
by H
−
[
32
]. They instead found that,
under atmospheric pressure, the amount of H
−
incorporated in C12A7e
−
increases by
raising the temperature from 673 to 873 K, concluding that, under reaction conditions,
the electride properties are maintained, as the C12A7e
−
does not completely react with
H
2
[
32
]. The group of Hosono proposed that H
−
is incorporated in small amounts into
the cages at the initial stages of ammonia synthesis, but the cage H
−
ions are readily
released by the following reaction: H
−→
H
0
+ e
−
[
17
,
18
]. However, our findings from the
catalytic activity tests and the reactivity studies towards H
2
at higher pressures support
the initial assumption that the catalyst deactivation is caused by the formation of C12A7:H
Catalysts 2021,11, 334 10 of 18
under ammonia synthesis conditions at higher pressures. The results from the catalytic
activity tests and reactivity studies are schematically shown in Scheme 1. If ammonia
synthesis is carried out at mild pressures below 1 MPa, no bulk transformation of C12A7e
−
to C12A7:H takes place. An increase in pressure leads to the formation of bulk C12A7:H,
which is accompanied by a loss of the electride properties and, thus, loss of performance in
ammonia synthesis.
Catalysts 2021, 11, x FOR PEER REVIEW 10 of 18
under atmospheric pressure, the amount of H− incorporated in C12A7e− increases by rais-
ing the temperature from 673 to 873 K, concluding that, under reaction conditions, the
electride properties are maintained, as the C12A7e− does not completely react with H2 [32].
The group of Hosono proposed that H− is incorporated in small amounts into the cages at
the initial stages of ammonia synthesis, but the cage H− ions are readily released by the
following reaction: H− H0 + e− [17,18]. However, our findings from the catalytic activity
tests and the reactivity studies towards H2 at higher pressures support the initial assump-
tion that the catalyst deactivation is caused by the formation of C12A7:H under ammonia
synthesis conditions at higher pressures. The results from the catalytic activity tests and
reactivity studies are schematically shown in Scheme 1. If ammonia synthesis is carried
out at mild pressures below 1 MPa, no bulk transformation of C12A7e− to C12A7:H takes
place. An increase in pressure leads to the formation of bulk C12A7:H, which is accompa-
nied by a loss of the electride properties and, thus, loss of performance in ammonia syn-
thesis.
It can be speculated that, at low pressures, the mechanism proposed by the group of
Hosono [17,18,31,40], involving a partial and dynamic exchange of hydrogen between ac-
tive metal and support, leads to an activity enhancement, and follows only a partial and
still-reversible exchange between mayenite electride and hydride in the upper surface lay-
ers. Once the pressure is increased and the bulk phase is transformed into the hydride
phase, the beneficial promoting properties are lost. With the loss of the promoting prop-
erties, the activity of the catalyst is strongly reduced, which was also found by Kammert
et al. [32] and the group of Hosono [17,18,40], with mayenite type supports with no- or
low-electron concentrations.
Scheme 1. Illustration of the pressure-dependent behavior of the Ru/C12A7e− catalyst during am-
monia synthesis (top part) and the C12A7e− during hydrogenation experiments (bottom part). For
graphical illustration, each cage contains an electron or hydride/deuteride, while maximum 1/3 of
the cages are occupied in the stoichiometric compounds.
2.2.3. Hydrothermal Reactivity Tests
The hydrothermal reactivity was tested for plasma-synthesized samples (a) C12A7
and (d) C12A7e−. To study the general stability of the mayenite structure, the Ar plasma-
synthesized C12A7 sample (a) was exposed to H2O, as described in the experimental sec-
tion. The PXRD of the resulting powder after each exposure time is shown in Figure 7.
Even after only one hour of exposure to H2O at ambient conditions, we could observe
structural changes in the material by the formation of a secondary phase identified as
Al2Ca4(OH)12CO3 · 5H2O (orange dashed–dotted lines in Figure 7) and an unidentified
Scheme 1.
Illustration of the pressure-dependent behavior of the Ru/C12A7e
−
catalyst during ammonia synthesis
(top part)
and the C12A7e
−
during hydrogenation experiments (bottom part). For graphical illustration, each cage contains an electron
or hydride/deuteride, while maximum 1/3 of the cages are occupied in the stoichiometric compounds.
It can be speculated that, at low pressures, the mechanism proposed by the group
of Hosono [
17
,
18
,
31
,
40
], involving a partial and dynamic exchange of hydrogen between
active metal and support, leads to an activity enhancement, and follows only a partial
and still-reversible exchange between mayenite electride and hydride in the upper sur-
face layers. Once the pressure is increased and the bulk phase is transformed into the
hydride phase, the beneficial promoting properties are lost. With the loss of the promot-
ing properties, the activity of the catalyst is strongly reduced, which was also found by
Kammert et al. [32]
and the group of Hosono [
17
,
18
,
40
], with mayenite type supports with
no- or low-electron concentrations.
2.2.3. Hydrothermal Reactivity Tests
The hydrothermal reactivity was tested for plasma-synthesized samples (a) C12A7
and (d) C12A7e
−
. To study the general stability of the mayenite structure, the Ar plasma-
synthesized C12A7 sample (a) was exposed to H
2
O, as described in the experimental
section. The PXRD of the resulting powder after each exposure time is shown in
Figure 7
.
Even after only one hour of exposure to H
2
O at ambient conditions, we could observe
structural changes in the material by the formation of a secondary phase identified as
Al
2
Ca
4
(OH)
12
CO
3·
5H
2
O (orange dashed–dotted lines in Figure 7) and an unidentified
phase (red dotted lines in Figure 7), accompanied by a reduction in the intensity of the
C12A7 reflections. After 5 h exposure to H
2
O, we could observe a complete decomposition
of the C12A7 structure in the PXRD patterns by the absence of C12A7 reflections, while the
reflections of the unidentified phase (red dotted lines) disappear in conjunction with the
occurrence of another unidentified phase. Figure 7also shows that, when the C12A7e
−
sample was exposed to H
2
O for 16 h, the color of the sample changed from dark green to
gray, which indicates a loss of electride properties.
Catalysts 2021,11, 334 11 of 18
Catalysts 2021, 11, x FOR PEER REVIEW 11 of 18
phase (red dotted lines in Figure 7), accompanied by a reduction in the intensity of the
C12A7 reflections. After 5 h exposure to H2O, we could observe a complete decomposition
of the C12A7 structure in the PXRD patterns by the absence of C12A7 reflections, while
the reflections of the unidentified phase (red dotted lines) disappear in conjunction with
the occurrence of another unidentified phase. Figure 7 also shows that, when the C12A7e−
sample was exposed to H2O for 16 h, the color of the sample changed from dark green to
gray, which indicates a loss of electride properties.
Figure 7. Left: PXRDs for the reactivity test series of sample (a) C12A7 in H2O under ambient con-
ditions for different reaction times after drying at 80 °C for 30 min. The diffraction pattern before
hydrothermal treatment (grey) shows only reflections of C12A7; Al2Ca4(OH)12CO3 · 5H2O (orange
dashed–dotted), unidentified phase (green dashed), and unidentified phase (red dotted lines) are
present until a reaction time of 5 h. Right: Photographs of the resulting samples from the reactivity
test experiment for (d) C12A7e− (3C) under H2O at room temperature. Initial C12A7e− powder
(bottom) and reacted powder (top) after 16 h under H2O.
To exclude the potential influence of the atmosphere and for a more defined mixing,
the stability experiments were additionally carried out under more controlled conditions.
For this, the C12A7e− (d) sample was suspended in tetrahydrofuran (THF) under Ar at-
mosphere. After suspending the material for 5 min in THF, the color of the powder was
still dark green and did not change compared to the initial C12A7e− sample. After the
stepwise addition of H2O, the sample changed its color to white, which may indicate a
decomposition of the C12A7e−. In the PXRD shown in Figure 8, a complete decomposition
of the C12A7 structure and the formation of mainly katoite Al2Ca3(OH)12 and some
Al2Ca4(OH)12CO3 ∙ 5H2O, after reaction of the C12A7e− with H2O in THF, can be observed.
These results are similar to those found by Jiang et al. for hydrothermal reactivity experi-
ments. They found Al2Ca3(OH)12 and slight amounts of Al(OH)3 by immersing the sample
in H2O for 72 h or storing it under 100% relative humidity for 1 week. [43]. As we found
additional aluminum calcium hydroxide phases in the experiments under ambient condi-
tions, it could be possible that different decomposition pathways or stages of the C12A7
structure might exist under hydrothermal conditions. Hayashi et al. reported the for-
mation of C12A7:OH upon reaction of C12A7 in a wet atmosphere without structural de-
composition [28]. With respect to a potential broader application of the material in heter-
ogeneous catalysis, the hydrothermal stability of the material might be crucial, especially
for reactions where H2O is present as solvent or is produced as a reaction product. For the
Ru/C12A7e− catalyst, the stability issues towards water have only been reported as an
aside in a study of a water-stable electride, Y5Si3 [17,44].
The results of the hydrothermal reactivity tests are summarized in Scheme 2. Upon
contact with liquid H2O and 100% relative humidity, the material exhibits a structural
decomposition to calcium aluminum hydroxide phases, as shown by Jiang et al. and the
results in the present study [43]. Based on the literature knowledge and our study, we
Figure 7.
(
Left
): PXRDs for the reactivity test series of sample (a) C12A7 in H
2
O under ambient conditions for different
reaction times after drying at 80
◦
C for 30 min. The diffraction pattern before hydrothermal treatment (grey) shows
only reflections of C12A7; Al
2
Ca
4
(OH)
12
CO
3·
5H
2
O (orange dashed–dotted), unidentified phase (green dashed), and
unidentified phase (red dotted lines) are present until a reaction time of 5 h. (
Right
): Photographs of the resulting samples
from the reactivity test experiment for (d) C12A7e
−
(3C) under H
2
O at room temperature. Initial C12A7e
−
powder (bottom)
and reacted powder (top) after 16 h under H2O.
To exclude the potential influence of the atmosphere and for a more defined mixing,
the stability experiments were additionally carried out under more controlled conditions.
For this, the C12A7e
−
(d) sample was suspended in tetrahydrofuran (THF) under Ar
atmosphere. After suspending the material for 5 min in THF, the color of the powder
was still dark green and did not change compared to the initial C12A7e
−
sample. After
the stepwise addition of H
2
O, the sample changed its color to white, which may indicate
a decomposition of the C12A7e
−
. In the PXRD shown in Figure 8, a complete decom-
position of the C12A7 structure and the formation of mainly katoite Al
2
Ca
3
(OH)
12
and
some Al
2
Ca
4
(OH)
12
CO
3·
5H
2
O, after reaction of the C12A7e
−
with H
2
O in THF, can be
observed. These results are similar to those found by Jiang et al. for hydrothermal reactivity
experiments. They found Al
2
Ca
3
(OH)
12
and slight amounts of Al(OH)
3
by immersing the
sample in H
2
O for 72 h or storing it under 100% relative humidity for 1 week. [
43
]. As we
found additional aluminum calcium hydroxide phases in the experiments under ambient
conditions, it could be possible that different decomposition pathways or stages of the
C12A7 structure might exist under hydrothermal conditions. Hayashi et al. reported the
formation of C12A7:OH upon reaction of C12A7 in a wet atmosphere without structural
decomposition [
28
]. With respect to a potential broader application of the material in het-
erogeneous catalysis, the hydrothermal stability of the material might be crucial, especially
for reactions where H
2
O is present as solvent or is produced as a reaction product. For the
Ru/C12A7e
−
catalyst, the stability issues towards water have only been reported as an
aside in a study of a water-stable electride, Y5Si3[17,44].
The results of the hydrothermal reactivity tests are summarized in Scheme 2. Upon
contact with liquid H
2
O and 100% relative humidity, the material exhibits a structural
decomposition to calcium aluminum hydroxide phases, as shown by Jiang et al. and the
results in the present study [
43
]. Based on the literature knowledge and our study, we
conclude that an initial deactivation might be caused by an exchange of the electrons inside
the cage and the formation of OH
−
while, during longer exposures to H
2
O, structural
decomposition takes place. A more detailed investigation of the decomposition pathways
of C12A7 by reaction with H
2
O might be carried out by in situ PXRD studies for a more
holistic picture and detailed understanding of the structural decomposition pathway.
Catalysts 2021,11, 334 12 of 18
Catalysts 2021, 11, x FOR PEER REVIEW 12 of 18
conclude that an initial deactivation might be caused by an exchange of the electrons in-
side the cage and the formation of OH− while, during longer exposures to H2O, structural
decomposition takes place. A more detailed investigation of the decomposition pathways
of C12A7 by reaction with H2O might be carried out by in situ PXRD studies for a more
holistic picture and detailed understanding of the structural decomposition pathway.
Figure 8. PXRDs for the reactivity test of (d) C12A7e− (3C) in THF with 5 mL H2O addition after 72
h and following drying under vacuum. Initial diffraction pattern (black) shows C12A7 and minor
krotite (CA) reflections. The PXRD after the experiment (red) shows mainly Al2Ca3(OH)12 reflec-
tions and minor Al2Ca4(OH)12CO3 · 5H2O.
Scheme 2. Illustration of the hydrothermal reactivity of C12A7e− as reported in the literature by
Jiang et al. [43], Hayashi et al. [28], Lu et al. [44], Hara et al. [17] and the results from the present
study. Ambient condition denoted as a.c. The material can either react by formation of OH− species
in the cages or structural decomposition (blue boxes).
3. Materials and Methods
3.1. Synthesis
3.1.1. Synthesis of Electrides and Mayenite Hydrides
The electride samples used in this study were synthesized via two different proce-
dures: (I) plasma synthesis and (II) solid-state reduction in a vacuum-sintering furnace.
Pathway (I), thoroughly detailed in a previous publication [42], consisted of mixing
reagents to prepare the precursor, which was then calcined and mixed with a reductant
prior to plasma treatment. The oxygen-mayenite precursor (C12A7) was prepared as re-
ported earlier by mixing 56.91 g of Disperal P2 with 43.65 g of CaO and 150 mL of deion-
ized H2O in a ball mill. The three reagents were mixed at 600 rpm four consecutive times,
Figure 8.
PXRDs for the reactivity test of (d) C12A7e
−
(3C) in THF with 5 mL H
2
O addition after
72 h
and following drying under vacuum. Initial diffraction pattern (black) shows C12A7 and minor
krotite (CA) reflections. The PXRD after the experiment (red) shows mainly Al
2
Ca
3
(OH)
12
reflections
and minor Al2Ca4(OH)12CO3·5H2O.
Catalysts 2021, 11, x FOR PEER REVIEW 12 of 18
conclude that an initial deactivation might be caused by an exchange of the electrons in-
side the cage and the formation of OH− while, during longer exposures to H2O, structural
decomposition takes place. A more detailed investigation of the decomposition pathways
of C12A7 by reaction with H2O might be carried out by in situ PXRD studies for a more
holistic picture and detailed understanding of the structural decomposition pathway.
Figure 8. PXRDs for the reactivity test of (d) C12A7e− (3C) in THF with 5 mL H2O addition after 72
h and following drying under vacuum. Initial diffraction pattern (black) shows C12A7 and minor
krotite (CA) reflections. The PXRD after the experiment (red) shows mainly Al2Ca3(OH)12 reflec-
tions and minor Al2Ca4(OH)12CO3 · 5H2O.
Scheme 2. Illustration of the hydrothermal reactivity of C12A7e− as reported in the literature by
Jiang et al. [43], Hayashi et al. [28], Lu et al. [44], Hara et al. [17] and the results from the present
study. Ambient condition denoted as a.c. The material can either react by formation of OH− species
in the cages or structural decomposition (blue boxes).
3. Materials and Methods
3.1. Synthesis
3.1.1. Synthesis of Electrides and Mayenite Hydrides
The electride samples used in this study were synthesized via two different proce-
dures: (I) plasma synthesis and (II) solid-state reduction in a vacuum-sintering furnace.
Pathway (I), thoroughly detailed in a previous publication [42], consisted of mixing
reagents to prepare the precursor, which was then calcined and mixed with a reductant
prior to plasma treatment. The oxygen-mayenite precursor (C12A7) was prepared as re-
ported earlier by mixing 56.91 g of Disperal P2 with 43.65 g of CaO and 150 mL of deion-
ized H2O in a ball mill. The three reagents were mixed at 600 rpm four consecutive times,
Scheme 2.
Illustration of the hydrothermal reactivity of C12A7e
−
as reported in the literature by
Jiang et al. [
43
], Hayashi et al. [
28
], Lu et al. [
44
], Hara et al. [
17
] and the results from the present
study. Ambient condition denoted as a.c. The material can either react by formation of OH
−
species
in the cages or structural decomposition (blue boxes).
3. Materials and Methods
3.1. Synthesis
3.1.1. Synthesis of Electrides and Mayenite Hydrides
The electride samples used in this study were synthesized via two different procedures:
(I) plasma synthesis and (II) solid-state reduction in a vacuum-sintering furnace.
Pathway (I), thoroughly detailed in a previous publication [
42
], consisted of mixing
reagents to prepare the precursor, which was then calcined and mixed with a reductant prior
to plasma treatment. The oxygen-mayenite precursor (C12A7) was prepared as reported
earlier by mixing 56.91 g of Disperal P2 with 43.65 g of CaO and 150 mL of deionized
H
2
O in a ball mill. The three reagents were mixed at 600 rpm four consecutive times, with
alternating directions of rotation, each for 10 min and with a 5 min break in between. The
obtained paste was calcined for 8 h at 1373 K with 10 K
·
min
−1
in a muffle furnace and
a constant, clean dry-air flow. After the calcination step, the product was ground and
characterized by powder X-ray diffraction (PXRD). For batches containing krotite (CA)
and tricalcium aluminate (C3A) as the secondary phase, the product was calcined again to
complete the reaction. The obtained C12A7 was mixed with a solid reductant and pressed
Catalysts 2021,11, 334 13 of 18
into 13 mm pellets (about 0.5 g) with 10 t using a motorized pellet press. As solid reductants,
Al powder (99.5% metal basis by Alfa Aesar) and graphite powder (C) (general purpose
grade by Fischer Chemical) were used. Four different samples were prepared via plasma
treatment of the following mixtures: (a) pure C12A7, (b) C12A7e
−
(5 wt.% Al), (c) C12A7e
−
(20 wt.% Al) and (d) C12A7e
−
(3 wt.% C). Plasma treatment was carried out in an arc
furnace “Compact Arc Melter MAM-1” (Edmund Bühler GmbH) with a plasma intensity
level of 5; see [
42
] for details regarding the plasma intensity. Before plasma treatment,
the furnace chamber was flushed and evacuated three times with the desired gas mixture.
Sample (a) was treated under Ar atmosphere, while samples (b)–(d) were treated under
95% Ar/5% H
2
with a chamber pressure of 0.07 MPa for both atmospheres. Sample (a)
was plasma-treated three times from the top side and once from the bottom side for about
60 s each, until a homogeneous melting could be observed. Samples (b)–(d) were treated
similarly, but only once from the top and bottom side, for about 60 s.
The electrides obtained from the vacuum sintering pathway (II) were synthesized
as follows. C12A7 was prepared by a solid-state reaction of stoichiometric amounts of
CaCO
3
(99%, ChemPur GmbH, Karlsruhe, Germany) and Al
2
O
3
(99.99%, ChemPur GmbH,
Karlsruhe, Germany). The powders were first mixed with an agate mortar and pistil and
afterwards pressed into pellets by a uniaxial hand-press (Perkin Elmer, 25 t). The pellets
were heated in a chamber furnace (Nabertherm GmbH, Lilienthal, Germany) under air
conditions for 16 h at 1573 K. Afterwards, they were transferred to a vacuum-sintering
furnace (FCT-Anlagenbau, Sonneberg, Germany). Here, the pellets were treated under
argon atmosphere at 1523 K for 4 h. The furnace chamber was heated through graphite
electrical resistance, which caused the oxygen partial pressure to be extremely low, resulting
in a reduction in the mayenite to the electride by loss of oxygen.
All samples obtained from plasma treatment and vacuum sintering are summarized
in Table 1.
3.1.2. Ruthenium Deposition
Deposition of catalytic active Ru on C12A7 or (d) graphite-based C12A7e
−
obtained
by pathway (I) was carried out with a chemical vapor deposition method by adapting
the literature procedure [
18
]. As a ruthenium precursor, triruthenium dodecacarbonyl
(Ru
3
(CO)
12
) (99%, Sigma Aldrich) was used. For impregnation, the precursors were
transferred into an Ar-filled glove box. The plasma-treated C12A7 and C12A7e
−
powders
were mixed with the desired amount of the ruthenium precursor in an agate mortar. The
mixtures were then transferred into the TFM
TM
-PTFE inserts of DAB-3 autoclaves (Berghof
Products + Instruments GmbH). The autoclaves were tightened by hand inside the glove
box and closed outside the glove box with a torque of 40 Nm. The autoclaves were placed
into a muffle furnace and heated with the following temperature program: heating to
313 K
(1 h, 2 K
·
min
−1
), 343 K (1 h, 0.25 K
·
min
−1
), 393 K (2 h, 0.4 K
·
min
−1
) and finally 523 K
(
3 h, 0.9 K ·mi
n
−1
). After heating, the autoclave screws were loosened and the autoclaves
were transferred into the glove box. The desired ruthenium content for all samples was
1.2 wt.%. The obtained samples are summarized in Table 1.
3.2. Characterization
Powder X-ray diffraction (PXRD) characterization was performed with a Bruker D8
Advance diffractometer with Bragg–Brentano geometry, a Lynxeye XE 1D-detector, a
Cu-X-ray-tube
with K
α1λ
= 154.06 pm and K
α2λ
= 154.44 pm and a ratio of
Kα1:Kα2= 2
.
The samples were thoroughly ground in an agate mortar and were measured on flat
poly(methyl methacrylate) sample holders. PXRD were measured in a range of 2
θ
= 10–90
◦
with a step size of 0.0205
◦
and measuring time of 0.2 s
·
step
−1
. Qualitative phase analysis
was carried out using Match!—Version2 software (Crystal Impact GbR) with entries from
the Crystallography Open Database (COD) [
48
] and Inorganic Crystal Structure Database
(ICSD) [
49
]. For the Rietveld refinement of selected samples, PXRD was collected on
Huber G670 Guinier diffractometers with either Cu-K
α1
or Mo-K
α1
radiation in the range
Catalysts 2021,11, 334 14 of 18
4◦≤2θ≤100◦
. Samples were enclosed between kapton
®
foils in apiezon
®
grease. Lattice
parameters were determined by Rietveld refinement using the software FullProf [50].
Simultaneous thermal analysis (STA) of plasma-synthesized samples was performed
using a STA 449 F3 Jupiter
®
(NETZSCH), with a combination of thermogravimetric analysis
(TG) and differential scanning calorimetry (DSC). STA experiments were carried out under
nitrogen or synthetic air up to 1573 K and forming gas (95% N
2
/5% H
2
) up to 1273 K.
The heating rate was generally 10 K
·
min
−1
. Typically, 10 mg sample were measured in
corundum crucibles.
EPR spectra were recorded with a MS 100 X-Band-EPR-Spectrometer by Magnettech.
The spectra were acquired at room temperature with B
0
= 340.0 mT, B
0sweep
= 49.9 mT,
sweep time t
sweep
= 500 s, 4096 steps, modulation amplitude MA = 0.1 mT, a microwave
attenuation MWA = 10 dB and a microwave power MWP = 10 mW. The gain (GA) was
adjusted for each sample. Samples were measured in 50
µ
L Duran
®
glass micro pipettes by
Hirschmann Laborgeräte GmbH, and were closed with wax. A normalized intensity I
N
was derived for all spectra, as described in the Supplementary Information, to compare
spectra of different samples.
The
2
H solid-state NMR spectrum was recorded on a Bruker Avance 750 spectrometer
(magnetic field 17.6 T) at a frequency of 114.94 MHz. In the one-pulse experiment, a pulse
length of 2
µ
s and a recycle delay of 500 s were used. The MAS spinning frequency was
2240 Hz.
Diffuse reflectance UV-Vis (DRUVVis) spectroscopy was performed using a PerkinElmer
Lambda 950 spectrometer with a 150 mm Ulbricht-sphere by Labsphere. A Spectralon
®
white standard by Labsphere was used as a reference. The spectra were recorded in a range
from 200 to 2500 nm, with 250 nm
·
min
−1
, an integration time of 0.2 s and data interval of
1 nm at room temperature. Details regarding the analysis and derivation of the electron
concentration Nefrom DRUVVis spectra are described in the ESI.
Transmission electron microscopy (TEM) measurements were done with a FEI Osiris
TEM. A 200 kV acceleration voltage was applied. The samples were prepared by dispersing
the sample with EtOH between two glass object slides. A graphite-covered Cu net was
used as a sample holder and was dipped into the dispersion.
X-ray fluorescence (XRF) analysis was carried out with a Bruker Tornado EDXRF
with a rhodium X-ray tube. The determination of the sample composition was done by
semi-quantitative phase analysis of the obtained spectra using the software QUANTAX
Version 1.3.
3.3. Reactivity Studies
3.3.1. Catalytic Experiments
Catalytic experiments in ammonia synthesis were carried out for two different plasma-
synthesized Ru/C12A7e
−
samples. For this purpose, 5 g (350 to 500
µ
m sieve fraction,
~5 mL
catalyst bed volume) of 1.2 wt.% Ru/C12A7e
−
based on samples (b) C12A7e
−
(5Al)
and (c) C12A7e
−
(20Al) were used. The tests were performed in a fixed-bed reactor with
an inner diameter of 5 mm. A total flow of 4.0126 mol
·
h
−1
of a 3:1 mixture of H
2
:N
2
was applied in the catalytic tests. The performance of the materials was tested at different
temperatures and under different pressures as listed in Table 2.
3.3.2. Reactivity of Electrides towards Hydrogen
The reactivity of the plasma-synthesized sample (d) C12A7e
−
(3C) was initially tested
in an in-house constructed tube furnace system. A total of 0.5 g of the sample (d) C12A7e
−
(3C) was placed in a quartz glass reactor (diameter = 2 cm) with a porous silica frit. The
sample and reactor were flushed for 15 min with a forming gas mixture (95%N
2
/5%H
2
,
0.1 MPa, GHSV = 5000 h
−1
). In the following, the sample was heated with 5 K
·
min
−1
to
the desired temperature ranging from 373 to 723 K, as summarized in Figure 4, and kept
for 1 h before cooling to room temperature. After each of these steps, the sample inside
the quatz tube was photographically documented. Before continuing the experiment, the
Catalysts 2021,11, 334 15 of 18
sample was flushed again for 15 min with the forming-gas stream. In comparison to the
forming-gas atmosphere, 0.5 g of the sample (d) C12A7e
−
(3C) was heated under N
2
to
723 K
with 5 K
·
min
−1
for 1 h. Afterwards the samples were ground and analyzed by
PXRD, DRUVVis and EPR spectroscopy.
Hydrogenation experiments of the vacuum sintering-based samples were carried
out in a differential scanning calorimeter (DSC) Q1000 from TA Instruments (New Castle,
DE, USA) equipped with a gas-pressure cell. A powdered sample of 8.9 mg well-ground
dark-green vacuum sintered mayenite electride was loaded into an aluminum pan. The
experiments were performed under isochoric conditions with a heating rate of 10 K
·
min
−1
under 5.0 MPa hydrogen (99.999% Air Liquide) at 298 K, which increased to 6.7 MPa at
the final temperature of 700 K. After the experiment, the sample (Table 1, (j) C12A7:H)
was colorless.
Green vacuum-sintered mayenite electride from the synthesis pathway (II) was ground
in an agate mortar and reacted with 6.0 MPa deuterium gas (99.8%, isotopic purity, Air
Liquide) pressure at 773 K for 48 h in an autoclave made of Inconel alloy (Böhler L718V).
The reaction yielded a colorless powder (Table 1, (k) C12A7:D).
3.3.3. Hydrothermal Reactivity of C12A7 and C12A7e−
For hydrothermal reactivity studies of the C12A7 material (a) based on plasma synthe-
sis under Ar atmosphere, 0.4 g each of (a) were suspended in 4 mL deionized H
2
O and
stirred in a 10 mL glass vial with a magnetic stirrer bar for 1, 2, 5, 20 and 28 h at ambient
conditions, respectively. H
2
O was decanted and the sample was dried for 30 min at 353 K
in a compartment dryer. The dried samples were ground in an agate mortar and analyzed
by PXRD. Similar 0.4 g of the plasma-synthesized sample (d) C12A7e
−
(3C) sample was
suspended and stirred in H
2
O for 16 h. To exclude a potential influence of air on the
hydrothermal stability, 0.5 g of sample (d) C12A7e
−
(3C) was suspended in 15 mL dry THF
in a 50 mL Schlenk flask under Ar atmosphere at room temperature. The suspension was
stirred for 5 min before deionized H
2
O was added in 0.5 mL steps every 5 min until a total
volume of 5 mL H
2
O was added. The suspension was stirred for 72 h before THF and H
2
O
were removed under reduced pressure. The resultant solid was dried carefully using a heat
gun. The obtained powder was ground and analyzed by PXRD.
4. Conclusions
Plasma-synthesized C12A7e
−
were successfully applied as support materials for
ammonia synthesis over Ru/C12A7e
−
catalysts. The electride properties of C12A7e
−
were preserved after Ru deposition. At pressures of 0.1 MPa, the Ru/C12A7e
−
catalysts
showed comparable performance to the literature-reported activity data. However, we
found an immediate deactivation at higher absolute pressures (>1.0 MPa). By reactivity
studies towards H
2
/D
2
we could elaborate the reason for this deactivation. The irreversible
formation of C12A7:H, accompanied by loss of the electride properties, was identified as the
reason for the deactivation of the catalysts at higher pressures. The C12A7:D phase could be
directly obtained by high-pressure autoclave synthesis from C12A7e
−
with deuterium gas.
Complementary hydrothermal reactivity experiments revealed the general stability issues
of the C12A7 structure and C12A7e
−
towards H
2
O by the formation of different aluminum
calcium hydroxide phases. From our results, we conclude that the application potential of
mayenite electrides under industrially relevant conditions is limited due to the sensitivity
to water and hydrogen at elevated pressures; however, we believe that it is important
to explore alternative catalyst materials with electride-like properties under industrially
relevant conditions, going beyond model laboratory conditions. Model conditions, far from
being industrially relevant, are suitable for fundamental studies and understanding of the
catalyst, which, indeed, can lead to remarkable findings about the catalytic properties of
the Ru/C12A7e
−
catalyst. However, an application in the established ammonia process is
impeded by the observed stability limitations. Therefore, the future challenge is to better
understand the nature of the promoting mechanism of electride materials and to identify
Catalysts 2021,11, 334 16 of 18
alternative material classes with similar promoting properties to the C12A7e
−
electride
support, but with increased stability towards the found deactivation mechanisms, i.e.,
irreversible hydride transformation of the bulk phase at higher hydrogen pressures and
improved hydrothermal stability.
5. Patents
G. Kolios, T. Mattke, J. M. Mormul, A. N. Parvulescu, F. Rosowski, S. Schäfer, S.
A. Schunk, WO002018189216A1
G. Kolios, T. Mattke, J. M. Mormul, A. N. Parvulescu, F. Rosowski, S. Schäfer, S.
A. Schunk, WO002018189218A1
Supplementary Materials:
The following are available online at https://www.mdpi.com/2073-4
344/11/3/334/s1: Figure S1. Bright-field TEM measurement of the sample Ru/C12A7e
−
(20Al).
Figure S2
. PXRD after Ru deposition on plasma synthesized C12A7e
−
(3C) (red) and C12A7 (blue)
samples. Figure S2. Rietveld refinement of the crystal structure of mayenite electride (vacuum
sintering-based C12A7e
−
(e) sample. Figure S3. Rietveld refinement of the crystal structure of the
deuteride of mayenite electride (C12A7:D (k) sample).
Author Contributions:
Conceptualization, S.W., S.S., M.S., H.K. and S.A.S.; methodology, S.W.,
S.S., M.S., H.K. and S.A.S.; validation, S.W., S.S., M.S., M.B., K.S., R.G., H.K. and S.A.S.; formal
analysis, S.W., N.O., M.B. and H.K.; investigation, S.W., M.S., N.O., M.B., S.B. and H.K.; resources,
M.L., H.K., S.A.S.; data curation, S.W., H.K. and S.A.S.; writing—original draft preparation, S.S.,
S.B., H.K.; writing—review and editing, S.W., S.S., M.S., N.O., M.B., K.S., S.B., M.L., R.G., H.K. and
S.A.S.; visualization, S.W. and H.K.; supervision, S.S., M.S., M.L., R.G., H.K. and S.A.S.; project
administration, S.S., M.S., M.L., H.K. and S.A.S.; funding acquisition, H.K. and S.A.S. All authors
have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Data Availability Statement:
The data presented in this study are available on request from the
corresponding author.
Acknowledgments:
Thanks to Stephan Arnold, Tamara Gabriel and Sertac Altay (all hte GmbH,
Heidelberg) for support in measuring PXRDs. Thanks to Armin Bader and Ulrich Flörchinger (all
BASF SE, Ludwigshafen) for supporting with DRUV/Vis-spectroscopy and TEM measurements,
respectively. Thanks to Jamal N. M. Aman and Jörn Schmedt auf der Günne (both University of
Siegen) for help and conceptual discussion of EPR experiments. Thanks to Mert Özen and Andreas
Kugler (hte GmbH, Heidelberg) for initial studies on the plasma synthesis. Thanks to Raphael Finger
for supporting the hydrogenation studies. Funding by BASF SE is gratefully acknowledged.
Conflicts of Interest: The authors declare no conflict of interest.
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