Dehydrogenation of dodecahydro-N-ethylcarbazole on Pt(111)

Article abstract of DOI:10.1002/cssc.201300263

Sloshing hydrogen: Liquid organic hydrogen carriers are high-boiling organic molecules, which can be reversibly hydrogenated and dehydrogenated in catalytic processes and are, therefore, a promising chemical hydrogen storage material. One of the promising candidates is the pair N-ethylcarbazole/perhydro- N-ethylcarbazole (NEC/H₁₂-NEC). The dehydrogenation and possible side reactions on a Pt(111) surface are evaluated in unprecedented detail. Copyright

Full text of DOI:10.1002/cssc.201300263

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DOI: 10.1002/cssc.201300263
Dehydrogenation of Dodecahydro-N-ethylcarbazole on
Pt(111)
[
a]
[a]
[a]
[a]
Christoph Gleichweit, Max Amende, Stefan Schernich, Wei Zhao,
Michael P. A. Lorenz, Oliver Hçfert, Nicole Brꢀckner, Peter Wasserscheid,
Jçrg Libuda,
[
a]
[a]
[b]
[b, c]
[a, c]
[a, c]
[a]
Hans-Peter Steinrꢀck,
and Christian Papp*
In the coming decades, the use of renewable energy sources
its toxicity might not allow for large-scale applications. For
metal hydrides the storage capacity is generally low, and for
mixed metal hydrides side reactions can be a serious problem.
One possible solution is “chemical storage” by using liquid or-
will become unavoidable, both to reduce CO emissions for cli-
2
mate reasons and to compensate for declining fossil-fuel re-
sources. Electric energy will mainly be produced from wind or
solar power plants. Due to its dependence on weather condi-
tions, energy availability and demand will be often in a mis-
match time- and location-wise. As a consequence, efficient
energy storage and distribution systems are required. Present-
ly, the most-discussed concepts are the extension of electric
grids and the use of batteries. The first “solution” only im-
proves energy distribution, but not the storage, and the
second is only suitable for short-term storage cycles. For long-
term energy storage (e.g., to compensate for seasonal
changes) or for long-distance energy transport (e.g., to bring
regenerative geothermal energy from Iceland to Western
Europe) chemical energy storage systems are needed. In this
context the use of hydrogen or of hydrogen-carrying chemicals
[6,7]
ganic hydrogen carrier (LOHC) materials.
LOHCs are high-
boiling organic molecules, which can be easily and reversibly
hydrogenated and dehydrogenated in catalytic processes. One
of the promising candidates is N-ethylcarbazole (NEC), which
can be hydrogenated over supported Ru catalysts to dodeca-
hydro-N-ethylcarbazole (H -NEC). In this process, 6 mol H per
12
2
mol NEC are added to the NEC starting material, yielding a H
2
[8,9]
storage capacity of 5.8 wt% in H -NEC.
H -NEC is, in
12
12
a number of practically relevant physicochemical properties
(vapor pressure, flammability, viscosity), similar to today’s com-
monly used diesel fuel. Consequently, the infrastructure for
liquid fuels (such as for example, storage tanks, tank ships, fill-
ing stations) is in principal also useable for storage, transport,
and distribution of LOHCs. Dehydrogenation of H -NEC is typi-
[
1–3]
is most promising.
12
[10–13]
The gravimetric energy storage density of H2 is excellent
cally catalyzed by supported Pt or Pd catalysts.
The dehy-
À1
(
33 kWhkg ). This is in sharp contrast, however, to the volu-
drogenation product NEC can be rehydrogenated in a sustaina-
ble, cyclic process allowing its application as recyclable hydro-
gen carrier.
metric energy storage density, which is only 3 Wh per liter of
gaseous H under ambient pressure. In existing technical appli-
2
cations H is, therefore, either stored in its gaseous state under
Presently, the details of the dehydrogenation reaction from
H -NEC to NEC are not fully understood. First efforts to study
2
very high pressures (up to 700 bar) or in the liquid state (at
À2538C). Both scenarios are unfavorable for large-scale practi-
cal use and complicate storage, transport, and distribution
12
this reaction under model conditions have been undertaken
[14]
on Pd particles. However, the size of the molecules (with, for
technologies of molecular H . Reforming of hydrocarbons or al-
example, 40 atoms in the case of H -NEC) makes these LOHC
2
12
cohols and the application of metal hydrides have also been
systems difficult to address by means of surface science meth-
ods. Furthermore, most spectroscopic properties are unknown,
especially when adsorbed on surfaces. Thus, these systems
represent a major challenge for detailed surface science and
model catalytic studies, which have proven to work well for
much smaller molecules such as hydrocarbons up to the size
of benzene and with high symmetry; for examples, see Refer-
ences [15–17].
[
4,5]
proposed as possible storage technologies.
However, re-
forming suffers from a high CO content of the hydrogen gas
flow, which poses a problem for fuel cells; in case of methanol,
[
a] C. Gleichweit, M. Amende, S. Schernich, W. Zhao, M. P. A. Lorenz, O. Hçfert,
Prof. J. Libuda, Prof. H.-P. Steinrꢀck, Dr. C. Papp
Lehrstuhl fꢀr Physikalische Chemie II
Friedrich-Alexander-Universitꢁt Erlangen-Nꢀrnberg
Egerlandstrasse 3, 91058 Erlangen (Germany)
E-mail: christian.papp@chemie.uni-erlangen.de
Herein, we present an in situ study of H -NEC on a Pt(111)
12
single-crystal surface as model for the (111) facets of real cata-
lyst particles and address the mechanism of dehydrogenation
and side reactions on the molecular scale. We demonstrate
that synchrotron radiation-based in situ high resolution X-ray
photoelectron spectroscopy (HR-XPS) is an excellent method
to investigate the thermally induced dehydrogenation, dealky-
lation, and decomposition of H -NEC. These reactions can be
[
b] N. Brꢀckner, Prof. P. Wasserscheid
Lehrstuhl fꢀr Chemische Reaktionstechnik
Friedrich-Alexander-Universitꢁt Erlangen-Nꢀrnberg
Egerlandstrasse 3, 91058 Erlangen (Germany)
[
c] Prof. P. Wasserscheid, Prof. J. Libuda, Prof. H.-P. Steinrꢀck
Erlangen Catalysis Resource Center
Friedrich-Alexander-Universitꢁt Erlangen-Nꢀrnberg
Egerlandstr. 3, 91058 Erlangen (Germany)
12
followed in great detail through careful and quantitative analy-
sis, and individual reaction steps can be identified. These in-
clude the desired dehydrogenation of H -NEC to NEC, but also
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201300263.
12
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The small differences between
the coverage values as derived
from the N1s and the C1s re-
gions arise from differences in
photoelectron diffraction at the
low kinetic energy of the photo-
electrons (ꢀ100 eV).
Next, the thermal evolution
of the adsorbed H NEC layer is
12
followed by temperature-pro-
grammed XPS (TPXPS), that is,
by heating the sample with
À1
a constant rate of 0.5 Ks while
continuously measuring XP
spectra. Selected N1s and C1s
spectra are shown in Figure 1e
and f, the color-coded density
plots of the full data sets in Fig-
ure 1c and d, and the corre-
sponding quantitative analyses
in Figure 2c and d, respectively.
The starting point of these ex-
periments is the end point of
the
adsorption
experiment
shown in Figure 1a and b. The
difference between the end of
the adsorption experiment and
the beginning of the heating
experiment in the N1s region is
due to postadsorption of H -
Figure 1. XP spectra of adsorption and reaction of H12-NEC on Pt(111); on the left hand side in (a) and (c) the N1s
spectra of the adsorption and reaction of H12-NEC, and on the right hand side in (b) and (d) the respective C1s
spectra are shown as a color-coded density plot. The lines in (a)–(d) indicate the temperatures/exposures of the
spectra shown in the upper panels (e) and (f).
1
2
NEC after closing the valve.
the unwanted dealkylation of NEC to carbazole and the forma-
tion of elemental carbon on the surface. Controlling the latter
two processes is important to prevent loss of NEC in the stor-
age cycle and catalyst deactivation.
As the N1s and the C1s spectra were obtained in independent
At first we address the adsorption of the hydrogen-loaded
H -NEC molecule on Pt(111) at 140 K, which is the starting
12
point of the subsequent unloading of hydrogen, that is, the
dehydrogenation reaction. Both, the N1s and C1s regions
were measured continuously during exposure to H -NEC; se-
12
lected individual spectra are shown in Figure 1e and f, and the
respective complete data sets are depicted in color-coded den-
sity plots in Figure 1a and b. Molecular adsorption is verified
by the growing peaks at 401.3 eV in the N1s region and at
284.5 eV in the C1s region at 2 langmuir (L). At higher expo-
sures, the C1s peak increases in width and shifts towards
higher binding energies, whereas in the N1s region a second
signal develops at approximately 399.5 eV. These changes are
attributed to the formation of the second H -NEC layer (and
12
further layers, subsequently). The layer-wise growth is in line
[
15,18]
with previous studies on small molecules.
The quantitative nature of XPS allows us to determine the
surface coverage from the N1s and C1s spectra as a function
of exposure, as shown in Figure 2a and b, respectively. The
completed first H -NEC layer (monolayer) corresponds to ap-
Figure 2. Quantitative analysis of the XPS experiments with H12-NEC shown
in Figure 1. In (a) and (c) the analysis of the N1s spectra of the adsorption
and reaction, and in (b) and (d) the respective analysis of the C1s region are
shown.
12
proximately 0.07 monolayers (ML) (molecules per substrate
atom) or approximately 1.0 C atom per Pt atom on the surface.
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experiments, also the final coverage achieved after adsorption
and b- carbon atoms) does not change as the other carbon
atoms in the six-membered carbon rings are successively dehy-
drogenated to finally yield NEC. Direct evidence for NEC as the
final product at 380 K comes from a comparison of the C1s
spectra from the heating series of H -NEC to spectra obtained
(and thus before the TPXPS measurement) is not identical,
leading to a slightly different damping, that is, intensity loss of
the monolayer by the multilayer on top of it. The small differ-
ences in the coverage of different intermediates in the quanti-
tative analysis in Figure 2 are assigned to photoelectron dif-
fraction, and/or uncertainties in the peak fitting, in particular in
the C1s region.
12
for the direct adsorption of NEC on Pt(111) at low tempera-
tures (see the Supporting Information). These spectra are in
very good agreement, within the margin of error, indicating
that this species is indeed NEC.
In the N1s region, shown in Figure 1c, a pronounced de-
crease of the peak at 399.5 eV is observed up to 240 K, indicat-
ing desorption of the physisorbed H -NEC multilayer. Simulta-
Starting at 390 K, further pronounced changes are observed.
In the N1s region a new peak develops at 398.1 eV, at the cost
of the peak at 399.5 eV, and in the C1s region a more gradual
shift from 284.5 to 283.8 eV is observed. These changes are as-
signed to the unwanted dealkylation of NEC to carbazole,
which in a technical process would modify the properties of
the LOHC system in an undesired way, for example, due to the
higher melting point of carbazole in comparison to NEC. While
the N1s peak shows a strong increase in intensity reaching
a maximum at approximately 450 K, the corresponding C1s
peak intensity increases at a much slower pace. This is attribut-
ed to the fact that the dealkylation directly influences the ni-
trogen atom, whereas the carbon atoms in the benzene rings
are affected to a much lesser extent. As there only is one nitro-
gen atom, the signal directly reflects the chemical environ-
ment. In contrast, there is a large number of non-equivalent
carbon atoms. Thus only gradual shifts and a significant peak
broadening are observed in the transition region, making the
assignment of signals to surface species difficult. At even
higher temperatures, up to 1000 K, further dehydrogenation
and the formation of carbon and carbon fragments is occur-
ring (data not shown).
12
neously, the peak of the H -NEC monolayer at 401.6 eV in-
12
creases, due to reduced damping by the multilayer on top.
In the C1s region, shown in Figure 1d, the broad peak due to
the multilayer/monolayer centered at 285.3 eV decreases in
height and width and slightly shifts to lower binding energy
(
285.2 eV) due to desorption of the H -NEC multilayer. The re-
12
maining monolayer intensity has also increased in height due
to the reduced damping.
Starting at 200 K, new N1s and C1s peaks develop at 399.5
and 284.5 eV, respectively, indicating the beginning of the de-
hydrogenation of H -NEC (note that the N1s peak has the
1
2
same binding energy as the multilayer peak, and consequently
the separation is somewhat ambiguous in the transition region
between 200 and 260 K). At 300 K, these peaks start to strong-
ly increase at the cost of the H -NEC monolayer peaks at 401.6
12
and 285.2 eV, respectively. The large binding energy difference
of 2.1 eV for the N1s peak, as compared to 0.7 eV for the C1s
peak, suggests that the initial dehydrogenation is accompa-
nied by a major change of the bonding of the nitrogen atom
to the surface. This is in line with a previously suggested reac-
tion mechanism on Pd, in which the first dehydrogenation
occurs at the carbon atoms next to the nitrogen atom, that is,
In conclusion, we have derived a detailed understanding of
the reaction behavior of a large liquid organic hydrogen carrier
molecule on an atomically well-defined Pt(111) single crystal
surface, which serves as model for the (111) facets of catalyst
particles. In situ XPS during adsorption and while heating
yields quantitative insights into the surface reaction in an un-
precedented fashion, as is summarized in Scheme 1: As first re-
action step, desorption of the H -NEC multilayers is observed,
[
13,14]
the a-carbon atoms of the pyrrole ring.
Between 330 and
3
3
90 K, the binding energy and the intensity of the N1s peak at
99.5 eV remain unchanged, whereas the corresponding C1s
peak shows a further increase and a decreasing high-energy
shoulder; this behavior is indicative of a further stepwise dehy-
drogenation in that temperature range. The fact that this fur-
ther progressing reaction does not lead to changes in the N1s
region indicates that the pyrrole-ring-structure in NEC is not af-
fected. This can be understood best, if also the b-hydrogen
atoms of the pyrrole ring are rapidly abstracted in the initial
12
leaving a monolayer of H -NEC. H -NEC starts to dehydrogen-
12
12
ate at 200 K, forming H -NEC as the first stable surface inter-
8
mediate at approximately 330 K. The latter dehydrogenates to
dehydrogenation phase, leading to H -NEC as a first observable
8
intermediate. H -NEC was also found as stable intermediate in
8
[
19–21]
other dehydrogenation experiments.
The further stepwise
dehydrogenation is supported by the fact that the H -NEC
12
signal vanishes already at 350 K in the N1s region, whereas it
is observed up to 380 K in the C1s region as the mentioned
high-energy shoulder. This slower decrease of the “H -NEC”
12
C1s peak at 285.2 eV can be understood if one assigns this
peak mainly to the CH subunits in H -NEC and the peak at
2
x
2
84.5 eV mainly to CH subunits of H -NEC. Then the observed
x
behavior is due to the progressive dehydrogenation of H -NEC
8
to NEC. As mentioned above, the absence of changes in the
N1s signal between 330 and 390 K indicates that the local ge-
ometry of the pyrrole subunit (with the dehydrogenated a-
Scheme 1. Reaction Scheme of H12-NEC on a Pt(111) surface.
ChemSusChem 2013, 6, 974 – 977 976
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NEC until 380 K, whereas the detrimental reactions that form
either carbazole or carbon fragments on the surface occur at
temperatures above 390 K. The unloading reaction starts with
the hydrogen atoms at the a- and b-carbon atoms in the pyr-
role ring, whereas the outer benzene entities lose their hydro-
gen atoms at higher temperatures. Our study illustrates that in
a certain temperature range H -NEC indeed is a regenerable
beamtime. P.W. acknowledges the European Research Council
(ERC) for supporting his fundamental work on the catalytic dehy-
drogenation of LOHC systems.
Keywords: dehydrogenation
· hydrogen storage · liquid
organic hydrogen carriers · photoelectron spectroscopy ·
surface reactions
12
hydrogen carrier molecule, with an optimal dehydrogenation
temperature around 380 K under the conditions of a surface
science experiment. We expect that the obtained mechanistic
understanding of our model system will be extremely benefi-
cial for the design and understanding of catalysts for LOHCs
on a knowledge-driven basis.
[
[
[
[
[
[
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1
Experimental Section
2
The experiments were conducted at the synchrotron BESSY II at
beamline U 49/2 PGM 1 using a transportable two chamber ultra-
2
[22]
high vacuum (UHV) setup. The preparation chamber houses typi-
cal surface science preparation and analysis tools such as a sputter
gun, low energy electron diffraction (LEED), and dosing facilities.
The main chamber is equipped with the electron energy analyzer
and several dosing facilities, including a supersonic molecular
beam and a dedicated dosing unit for the adsorption of H -NEC
2
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12
[
9] K. M. Eblagon, K. Tam, K. M. K. Yu, S.-L. Zhao, X.-Q. Gong, H. He, L. Ye, L.-
and NEC. The manipulator allows to cool the Pt(111) crystal to
about 120 K by using liquid nitrogen; through direct heating
sample temperatures of up to 1400 K can be reached. Additionally
a filament is installed in the back of the sample that allows heating
the sample while measuring without any disturbing effects. During
adsorption or heating the sample, XP spectra in the region of inter-
est can be obtained continuously, with typically 10 s per spectrum.
The coverage was calibrated by comparison to known surface cov-
erage, that is, the c(4ꢂ2) CO superstructure on the Pt(111) surface
C. Wang, A. J. Ramirez-Cuesta, S. C. E. Tsang, J. Phys. Chem. C 2010, 114,
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scheid, H.-P. Steinrꢀck, A. Gçrling, J. Libuda, Chem. Eur. J. 2011, 17,
[
[23]
for carbon atoms. For peak fitting in the N1s spectra we used
a Doniach–Sunjic function to represent the peak shape after sub-
traction of the background. In the C1s spectra we used envelopes
that represent the surface species; see for example Reference [24].
These envelopes consisted of two Doniach–Sunjic functions for
one species, such as H -NEC and NEC, and of only one for CO and
11542–11552.
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18] N. Luckas, K. Gotterbarm, R. Streber, M. P. A. Lorenz, O. Hçfert, F. Vines,
12
carbon fragments at higher temperatures, see the Supporting In-
formation. Please note that the amount of CO on the surface
during the experiments was below 0.1 ML. CO did not participate
in the reaction and desorbed until 410 K, see Figure 2.
[
C. Papp, A. Gçrling, H.-P. Steinrꢀck, Phys. Chem. Chem. Phys. 2011, 13,
1
6227–16235 .
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7–72.
20] Z. Wang, I. Tonks, J. Belli, C. M. Jensen, J. Organomet. Chem. 2009, 694,
854–2857.
NEC was purchased in a sulfur-free quality (S-content<50 ppm)
from Hydrogenious Technologies GmbH (www.hydrogenious.net),
whereas H -NEC was synthesized by hydrogenation of NEC using
[
[
6
1
2
a Ru/AlOx catalyst (Alfa Aesar, 5 wt% Ru on support, 1 mol%) by
reacting NEC (0.1 mol, 19.5 g) in cyclohexane (150 mL) under
2
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[23]
5
0 bar hydrogen and 1508C for 2 h.
12839–12845.
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9
Acknowledgements
[23] J. S. McEwen, S. H. Payne, H. J. Kreuzer, M. Kinne, R. Denecke, H. P. Stein-
rꢀck, Surf. Sci. 2003, 545, 47–69.
[
24] C. Papp, B. Trꢄnkenschuh, R. Streber, T. Fuhrmann, R. Denecke, H. P.
We acknowledge the Cluster of Excellence “Engineering of Ad-
vanced Materials” and the BMW group for financial support.
S.S. gratefully acknowledges a grant of the Fonds der Chemi-
schen Industrie. BESSY staff is acknowledged for support during
Steinrꢀck, J. Phys. Chem. C 2007, 111, 2177–2184.
Received: March 25, 2013
Published online on May 14, 2013
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2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 2013, 6, 974 – 977 977