Model Catalytic Studies of Novel Liquid Organic Hydrogen Carriers: Indole, Indoline and Octahydroindole on Pt(111)

Article abstract of DOI:10.1002/chem.201702333

Indole derivatives were recently proposed as potential liquid organic hydrogen carriers (LOHC) for storage of renewable energies. In this work, we have investigated the adsorption, dehydrogenation and degradation mechanisms in the indole/indoline/octahydroindole system on Pt(111). We have combined infrared reflection absorption spectroscopy (IRAS), X-ray photoelectron spectroscopy (XPS) and DFT calculations. Indole multilayers show a crystallization transition at 200 K, in which the molecules adopt a strongly tilted orientation, before the multilayer desorbs at 220 K. For indoline, a less pronounced restructuring transition occurs at 150 K and multilayer desorption is observed at 200 K. Octahydroindole multilayers desorb already at 185 K, without any indication for restructuring. Adsorbed monolayers of all three compounds are stable up to room temperature and undergo deprotonation at the NH bond above 300 K. For indoline, the reaction is followed by partial dehydrogenation at the 5-membered ring, leading to the formation of a flat-lying di-σ-indolide in the temperature range from 330–390 K. Noteworthy, the same surface intermediate is formed from indole. In contrast, the reaction of octahydroindole with Pt(111) leads to the formation of a different intermediate, which originates from partial dehydrogenation of the 6-membered ring. Above 390 K, all three compounds again form the same strongly dehydrogenated and partially decomposed surface species.

Full text of DOI:10.1002/chem.201702333

A Journal of
Accepted Article
Title: Model Catalytic Studies of Novel Liquid Organic Hydrogen
Carriers: Indole, Indoline and Octahydroindole on Pt(111)
Authors: Matthias Schwarz, Philipp Bachmann, Thais Nascimento
Silva, Susanne Mohr, Marlene Scheuermeyer, Florian Späth,
Udo Bauer, Fabian Düll, Johann Steinhauer, Chantal Hohner,
Tibor Döpper, Heshmat Noei, Andreas Stierle, Christian Papp,
Hans-Peter Steinrück, Peter Wasserscheid, Andreas Görling,
and Jörg Libuda
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201702333
Link to VoR: http://dx.doi.org/10.1002/chem.201702333
Supported by

10.1002/chem.201702333
Chemistry - A European Journal
Model Catalytic Studies of
Novel Liquid Organic Hydrogen Carriers:
Indole, Indoline and Octahydroindole on Pt(111)
Matthias Schwarz1,§, Philipp Bachmann1,§, Thais Nascimento Silva1, Susanne Mohr1, Marlene
Scheuermeyer2, Florian Späth1, Udo Bauer1, Fabian Düll1, Johann Steinhauer1, Chantal Hohner1, Tibor
Döpper3, Heshmat Noei4, Andreas Stierle4, Christian Papp1, H.-P. Steinrück1,5,6, Peter Wasserscheid2,5,7
Andreas Görling3,5,6, Jörg Libuda1,5,6*
,
1 Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg,
Egerlandstraße 3, D-91058 Erlangen, Germany
2 Lehrstuhl für Chemische Reaktionstechnik, Friedrich-Alexander-Universität Erlangen-Nürnberg,
Egerlandstraße 3, D-91058 Erlangen, Germany
3 Lehrstuhl für Theoretische Chemie, Friedrich-Alexander-Universität Erlangen-Nürnberg,
Egerlandstraße 3, D-91058 Erlangen, Germany
4 Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, D-22603 Hamburg, Germany,
Fachbereich Physik, Universitꢀt Hamburg, 20355 Hamburg, Germany
5 Erlangen Catalysis Resource Center, Friedrich-Alexander-Universität Erlangen-Nürnberg, 91058
Erlangen, Germany
6 Interdisciplinary Center Interface Controlled Processes, Friedrich-Alexander-Universität Erlangen-
Nürnberg, 91058 Erlangen, Germany
7 Forschungszentrum Jülich GmbH, Helmholtz-Institute Erlangen-Nuremberg for Renewable Energy
(IEK-11), 91058 Erlangen, Germany
Keywords: liquid-organic-hydrogen-carrier, octahydroindole, infrared reflection absorption
spectroscopy, X-ray photoelectron spectroscopy, density functional theory.
§ Matthias Schwarz and Philipp Bachmann contributed equally to this work
*corresponding author: joerg.libuda@fau.de, +49 9131 8527308
1
This article is protected by copyright. All rights reserved.

10.1002/chem.201702333
Chemistry - A European Journal
Abstract
Indole derivatives were recently proposed as potential Liquid Organic Hydrogen Carriers
(LOHC) for storage of renewable energies. In this work, we have investigated the adsorption,
dehydrogenation and degradation mechanisms in the indole/indoline/octahydroindole system on
Pt(111). We have combined infrared reflection absorption spectroscopy (IRAS), X-ray
photoelectron spectroscopy (XPS) and density functional (DF) calculations.
Indole multilayers show a crystallization transition at 200 K, in which the molecules adopt a
strongly tilted orientation, before the multilayer desorbs at 220 K. For indoline, a less
pronounced restructuring transition occurs at 150 K and multilayer desorption is observed 200 K.
Octahydroindole multilayers desorb already at 185 K, without any indication for restructuring.
Adsorbed monolayers of all three compounds are stable up to room temperature and undergo
deprotonation at the NH bond above 300 K. For indoline, the reaction is followed by partial
dehydrogenation at the 5-membered ring, leading to the formation of a flat-lying di--indolide in
the temperature range from 330 to 390 K. Noteworthy, the same surface intermediate is formed
from indole. In contrast, the reaction of octahydroindole with Pt(111) leads to the formation of a
different intermediate, which originates from partial dehydrogenation of the 6-membered ring.
Above 390 K, all three compounds again form the same strongly dehydrogenated and partially
decomposed surface species.
2
This article is protected by copyright. All rights reserved.

10.1002/chem.201702333
Chemistry - A European Journal
1. Introduction
Many sources of renewable energy, in particular sunlight and wind, are highly intermittent. With
an increasing share of renewables in our energy mix, this intermittency leads to either energy
shortage or substantial energy overproduction depending on weather conditions. As a
consequence, it becomes increasingly important to store excess energy at “energy rich” times to
bridge the “energy-lean” periods. For energy storage at the large scale, chemical approaches are
superior, with hydrogen being the most efficient carrier from the gravimetric point of view.[1]
However, cryogenic temperatures or high pressures are required to achieve sufficiently high
volumetric densities in physical hydrogen storage of elemental hydrogen.[1-2]
A promising alternative concept is hydrogen storage in Liquid Organic Hydrogen Carrier
(LOHC) systems, i.e. chemical storage of hydrogen via fully reversible hydrogenation and
dehydrogenation of liquid organic compounds.[3] Depending on the carrier molecule used,
hydrogen contents up to 7.2 wt%, high reversibility, and moderate dehydrogenation temperatures
can be realized. Furthermore, the LOHC technology is compatible with the existing
infrastructure for the transportation of liquid fuels. Therefore, LOHC is a promising technology
for both mobile and stationary large scale applications.[3b, 4]
Ideally, an LOHC system should provide practically perfect reversibility to allow for a maximum
number of charging and discharging cycles. Other physical and chemical requirements include
favorable melting temperatures and hydrogenation thermodynamics, high thermal stability, low
toxicity, technical availability and low price.[3a-c, 4a, 5] A particular promising group of LOHC
4a, 5]
compounds are N-heterocyclic hydrocarbons.[3e,
This is due to their favorable
thermodynamics, which arises from the fact that fewer hydrogen equivalents have to be released
to obtain aromatic stabilization.[6] Among the most popular heterocyclic LOHC compounds is N-
ethylcarbazole, which takes up 5.8 % hydrogen upon hydrogenation to dodecahydro-N-
ethylcarbazole.[3b] Previously, a large number of model and real catalytic studies were performed
on this system.[3d, 7] Typical catalysts for the dehydrogenation reaction are Pd/Al2O3 or Pt/Al2O3
with similar performance in practical applications. Note that after the catalyst life cycle, the
noble metal in these materials can be recycled almost completely.
3
This article is protected by copyright. All rights reserved.

10.1002/chem.201702333
Chemistry - A European Journal
Recently, structurally related indole derivatives were proposed as alternative LOHC systems.[8]
For instance, N-ethylindole[8a] and 2-methylindole[8b] have hydrogen storage capacities of 5.23
wt% and 5.76 % and full hydrogenation was achieved at 160 °C over 5 wt% Ru/Al2O3 within 80
and 40 min and at 90 and 70 bar H2, respectively. During hydrogenation, 2H- and 4H-indole
derivatives are formed as intermediates.[8] For 2-methylindole, it was found, that hydrogenation
[8b]
starts at the six-membered ring
similar as had been found for N-ethylcarbazole [9]. A
particularly appealing feature of this system is that dehydrogenation of indoline is
thermodynamically more favorable than for most other hydrogen-rich LOHC compounds. As a
result, hydrogen can be released at temperatures as low as 100° C, i.e. close to the operation
conditions of low temperature PEM fuel cells.[10]
Main goals of LOHC catalysis are to achieve maximum selectivity (to allow for a high number
of storage cycles) and highest activity (to minimize size of the converter units and the noble
metal content). Still, the knowledge-driven development of improved LOHC catalysis is
hampered by our limited understanding of the (de)hydrogenation mechanism and the competing
side reactions that lead to degradation of the carrier. Such mechanistic insights are available,
however, from surface science and model catalytic studies.[7a, 11] For the case of perhydro-N-
ethylcarbazole/N-ethylcarbazole, we previously studied the dehydrogenation using a broad
spectrum of surface science methods. Specifically, we identified the activation mechanism,
intermediates, decomposition mechanisms both for Pt and Pd model catalysts.[7a, 7d-i] For the
indole/indoline/octahydroindole system, no comparable work has been published to date. Here,
we present the first comprehensive study on the adsorption and reaction of indole, indoline, and
octahydroindole on Pt(111). We combine infrared reflection absorption spectroscopy (IRAS),
high resolution X-ray photoelectron spectroscopy (HR-XPS) and density functional theory
(DFT) to identify the most abundant reaction intermediates and investigate the related reaction
mechanisms.
2. Results and Discussion
2.1. Vibrational Spectra of Indole, Indoline, and Octahydroindole in the Multilayer
Regime: ATR, IRAS, and DFT
4
This article is protected by copyright. All rights reserved.

10.1002/chem.201702333
Chemistry - A European Journal
As a basis for our vibrational spectroscopy studies, we investigated the multilayer IR spectra of
indole (see Figure 1), indoline (see Figure 2) and octahydroindole (see Figure 3). For each
compound, we compared the bulk IR spectrum obtained in attenuated total reflection (ATR)
spectrum, the IRAS multilayer spectrum after deposition of a multilayer of onto Pt(111) recorded
at low temperature, and a calculated gas phase spectrum from DFT. The calculated spectra are
shown with a Lorentzian broadening (full width at half maximum, FWHM), of 8 cm-1 to
facilitate comparison with the experiment.
All bands found in ATR-IR, IRAS and the DFT calculations are listed in Table 1 for indole, in
Table 2 for indoline and in Table 3 for octahydroindole, together with their assignments.
Starting with indole (see Figure 1, Table 1), we note that in ATR-IR and IRAS all bands show
nearly identical positions whereas the peak ratios differ to some extent. We attribute the latter
effect to the preferential orientation in the frozen molecular layer for the case of the IRAS
experiment. Here, the band intensity depends on the orientation of the dynamic dipole moment
relative to the surface normal. As a result of the metal surface selection rule (MSSR), only the
components of the dynamic dipole moments perpendicular to the surface give rise to absorption
bands in IRAS.[12] To assist the band assignment, we simulated the IR spectra on the basis of DF
calculations of indole in the gas phase. The nature, symmetry and polarization of the
corresponding modes were analyzed using the visualization software QVibePlot.[13] Selected
visualizations are displayed in Figure 1. Briefly, stretching movements between two atoms are
shown as blue and red lines, deformations of bond angles between three atoms as blue and red
arcs, and torsions between four atoms as dark and light green Bezier curves. The same/different
color indicates an in-phase/out-of-phase movement and the thickness or size represents the
[13]
amplitude (see
for details). Briefly, the band at 3581 (3408) cm-1 is assigned to the NH
stretching mode, (NH) (the experimental values from IRAS are given in brackets). The weak
band at 3191 (3105) cm-1 is assigned to CH stretching vibrations of the five-membered ring,
(C2,3H). The features at 3125 and 3113 cm-1 (3056) cm-1 are assigned to CH stretching modes
of the aromatic six-membered ring, (CH)ar. The dynamic dipole moments of all CH stretching
bands of indole are oriented within the molecular plane. The bands at 1444 (1457), 1415 (1417),
1354 (1354), 1338 (1338), 1271 (1279), and 1231 (1246) cm-1 are assigned to combinations of
CC stretching and in plane CH deformation modes, (CH). All these modes are polarized within
5
This article is protected by copyright. All rights reserved.

10.1002/chem.201702333
Chemistry - A European Journal
the plane of the molecule. The band at 1082 (1095) cm-1 is assigned to the (N1C2) stretching
vibration coupled to CH deformations in the same range. This mode is also polarized within the
molecular plane. The bands at 716 (771) and 691 (752) cm-1 are assigned to CH out of plane
modes, which are polarized perpendicular to the molecular plane. They serve as a spectroscopic
indicator for the orientation of the heterocyclic system. All further assignments are listed in
Table 1.
The corresponding data for indoline is given in Figure 2 and Table 2. Regarding the assignment,
we focus on the most prominent bands only. The band at 3494 (3349 and 3245) cm-1 is assigned
to the NH stretching vibration, (NH), with the splitting and broadening attributed to H-bonding
in the multilayer. For indoline, H-bonding is more favorable than for indole, as the lone pair on
the nitrogen is not as strongly embedded into the conjugated -system. The overlapping bands
around 3110 (3050) cm-1 are assigned to CH stretching modes,(CH)ar, of the six-membered
aromatic ring. The peaks at 3017 and 2948 (2956) cm-1 are assigned to CH stretching modes,
(C3H2), located at the C3 atom of indoline. The most red-shifted CH band at 2903 (2850) cm-1
is attributed to the (C2H2) mode in -position to the nitrogen. The feature at 1609 (1610) cm-1
is associated with a CC stretching and CH in-plane deformation mode localized on the six-
membered aromatic ring. It is polarized within the C6 ring plane. The bands at 1481 (1490) and
1455 (1466) cm-1 are assigned to C2H2 scissoring modes combined with CC stretching and in-
plane CH deformation modes in the aromatic C6 ring. The peaks at 1350 (1328) and 1295
(1315) cm-1 are attributed to C2H2 wagging modes combined with NH deformation, CC
stretching and in-plane CH deformation modes of the C6 ring. The band at 1252 (1248) cm-1 is
associated with CH2 twisting, C7aN1 stretching and in-plane CH deformation in the aromatic C6
ring. The features at 1043 (1058) and 1022 (1026) cm-1 originate from the asymmetric N1C2C3
stretching mode mixed with CH2, NH and CH deformation. Finally, the band at 725 (757) cm-1 is
assigned to a CH out-of-plane deformation mode, (CH)oop. The latter is polarized perpendicular
to the aromatic ring plane which allows us to use this feature for determining the molecular
orientation.
Finally, the corresponding data for octahydroindole is shown in Figure 3 and Table 3. Again, we
list the most prominent bands only. The band at 3443 (3224) cm-1 is assigned to the NH
stretching mode, (NH). Its calculated intensity is very low, whereas the band is intense and
6
This article is protected by copyright. All rights reserved.

10.1002/chem.201702333
Chemistry - A European Journal
strongly red-shifted in IRAS. These effects are attributed to hydrogen bonding which is not
accounted for in the DF calculation of the isolated molecule in the gas phase. The bands between
and 2950 (2929) cm-1 are CH stretching bands, (CH), of the CH2 and CH units. Specifically, the
red-shifted CH bands at 2916 and 2786 (2853) cm-1 are assigned to CH stretching modes in the
direct periphery of the nitrogen atom, i.e. (C2H) and (C7aH), respectively. The band at 1436
(1448) cm-1 is attributed to CH2 scissoring modes, (CH2)scissoring, whereas the features between
1300 and 1080 cm-1 are assigned to CH and NH deformation vibrations, partly combined with
CN stretching modes. The band at 1015 (1025) cm-1 is an asymmetric stretching mode of the
neighboring CN and CC bond at the C2 atom, namely as(N1C2C3). Finally, the bands at 974
(984), 907 (915), 859 (873) 817 (856), and 781 (795) cm-1 are assigned to combinations of CC
stretching, CN stretching, CC deformation and CH deformation motives.
2.2. Adsorption and Desorption of Indole, Indoline and Octahydroindole Multilayers
To analyze the desorption behavior of indole, indoline, and octahydroindole in the multilayer
regime, we performed identical temperature-programmed IRAS experiments for all three
compounds. To this end, IR reference spectra were recorded on clean Pt(111), before the
multilayer films were deposited at a low temperature (see Figures 4 to 6). Subsequently, the
films were heated with a rate of 2 K/min while continuously recording IR spectra.
In Figure 4, the resulting data for indole is shown as a color plot for the temperature region from
120 to 280 K together with selected spectra in the same region. In addition, we show the total
integrated IR intensity over all bands in the spectral region from 700 to 3500 cm-1 as a function
of temperature (see Supporting Information for the complete set of data). Hardly any spectral
changes are observed in the beginning of the heating ramp. At 202 K (± 5 K), however, a sudden
decrease in intensity of several bands is observed. In particular, the NH stretching band at
 cm-1 and the CH out-of-plane bands at 705 and 757 cm-1 are strongly attenuated. The strong
drop in the overall integrated IR intensity is mainly due to these bands. Noteworthy some other
features behave differently. In particular, the CH stretching bands at 3050 and 2956 cm-1 as well
as the (CC) and (CH) bands at 1417 and 1338 cm-1 show an increasing intensity at the same
temperature. We attribute these changes to a restructuring transition in the multilayer, i.e. the
7
This article is protected by copyright. All rights reserved.

10.1002/chem.201702333
Chemistry - A European Journal
formation of a crystalline film, in which the molecules have a defined orientation with respect to
the surface. Such transitions are often observed for aromatic systems upon heating.[14] The
decrease of the (CH)oop bands, which are polarized perpendicular to the ring plane, together
with the simultaneous increase of the (CH), (CC), and (CH) peaks, which are polarized in
plane, indicate that the indole molecules adopt an orientation in which the molecular plane is
strongly tilted with respect to the surface. In the TP-IRAS experiment, the crystallization
transition is followed by multilayer desorption at a temperature of 221 K (± 5 K). Multilayer
desorption is indicated by a drop of the total band intensity to nearly zero. Note that the residual
monolayer signals are too weak to be detected in the TR-IRAS experiment. Instead, the
monolayer species will be identified in isothermal IRAS experiments below.
The results of the corresponding experiment for indoline are shown in Figure 5 (see Supporting
Information for the complete set of data). Between 120 and 153 K (± 5 K), the IR spectra of the
indoline multilayer remain practically unchanged. At 153 K (± 5 K), a steplike change in total
intensity is observed which is, however, much less pronounced than the one observed for indole
(see Figure 5). Specifically, we find a slight decrease in intensity and a change of shape in the
range of the NH stretching modes. We associate these changes to minor structural
rearrangements in the multilayer which are mainly driven by an increase in hydrogen bonding. In
contrast to indole, however, no clear change in molecular orientation is observed. We attribute
the differences in the thermal behavior between the two molecules to the differences in
molecular shape. Whereas -interactions between the flat indole entities drive the formation of
crystalline phases with a well-defined orientation, this is the case to a much lesser extent for the
more irregular shaped indoline. At 203 K (± 5 K), we observe a steplike decrease of the
integrated band intensity to a value near zero. Again, we attribute this change to desorption of the
multilayer. Note that the multilayer desorption temperature is approximately 20 K lower
compared to indole. Again, this can be attributed to the weaker intermolecular interactions for
indoline, the structure of which does not allow for stacking and strong --interactions.
The corresponding TP-IRAS experiment for octahydroindole is shown in Figure 6 (compare to
the experiments for indole and indoline, Figures 4 and 5; see Supporting Information for the
complete set of data). The octahydroindole multilayer does not show any spectral changes up to
around 170 K. At this temperature minor traces of co-deposited water desorb, as indicated by the
8
This article is protected by copyright. All rights reserved.

10.1002/chem.201702333
Chemistry - A European Journal
decrease of the OH band between 3450 and 3200 cm-1. Water desorption occurs at temperatures
only slightly below desorption of the octahydroindole multilayer at 186 K (± 5 K).
Octahydroindole shows the lowest desorption temperature of the three compounds under
investigation. These differences are typical when comparing fully saturated cycloalkanes to their
un-saturated (aromatic) counterparts (see e.g. cyclohexane[15] and benzene[16]) and can be
attributed to the stronger intermolecular interactions between the -systems. Note that small
features in the CH region are still observable after multilayer desorption which indicate the
presence of chemisorbed species in the monolayer. These monolayer features will be discussed
in detail below.
2.3. Temperature Dependent Reaction of Indole on Pt(111)
Next we focus on the dehydrogenation and decomposition of indole, indoline, and
octahydroindole on Pt(111). To this end, we carried out identical isothermal reaction experiments
under steady state conditions at various temperatures above the multilayer desorption threshold.
We applied an experimental procedure in which the surface was continuously exposed to the
respective molecule (at a pressure of 1x10-7 mbar) and IR spectra were recorded after the steady
state had been established.
Figure 7 shows the corresponding series of isothermal deposition experiments for indole. In
addition, integrated areas of selected bands are given as function of reaction temperature. The
first spectrum of the series at 210 K was recorded close to the temperature of multilayer
desorption. The spectra are dominated by the CH out-of-plane bands at 768 and 746 cm-1. The
feature at 3411 cm-1 is assigned to the NH stretch. At 3060 cm-1, the CH stretching band of the
aromatic system is observed. For assignment of the other bands, we refer to Table 1, as they are
basically identical to those of the multilayer. The only additional band observed is the small
feature centered at 825 cm-1. We assign it to a CH out-of-plane band of the molecules that
interact more strongly with the surface. Similar shifts have been observed previously for benzene
on Pt(111)[17]. In general the IR spectra indicate that at low temperature indole adsorbs
molecularly, even in the monolayer region. As both in-plane and out-of-plane modes are
observed, on average the molecules must adopt a tilted orientation with respect to the surface.
9
This article is protected by copyright. All rights reserved.

10.1002/chem.201702333
Chemistry - A European Journal
At up to 240 K, all bands decrease in intensity but no new bands are observed. Noteworthy, the
NH band at 3411cm-1 disappears nearly completely. This vanishing NH signal may be either due
to deprotonation or due to the NH bond being oriented parallel to the surface. Note that other IR
studies of amines on Pt(111) also showed the absence of NH stretching bands for
non-dissociatively adsorbed layers.[18]
In order to obtain additional information on the chemical state of the indole, we conducted high
resolution XPS experiments using synchrotron radiation. Selected N 1s spectra are displayed in
Figure 8. All spectra were recorded after adsorption of indole at 140 K and subsequent annealing
to the temperatures given in the Figure. After annealing to approximately 230 K, we observe a
single N 1s component centered at 399.0 eV. Upon heating to 380 K, the peak shifts by 1.0 eV to
lower binding energy (398.0 eV). We attribute this binding energy shift to the deprotonation of
nitrogen. This assignment is in accordance with previous studies of bond breaking at aminic
nitrogen, where a shift of 1 eV or more was observed.[7h, 11b, 19] We conclude that the indole
adsorbs molecularly with an intact NH bond at low temperature. It adopts a flat-lying adsorption
geometry in which the NH bond is aligned parallel to the surface.
At higher temperatures, the IR spectra change drastically while little change is observed in the
N 1s XP spectra. Between 270 and 300 K, the band at 825 cm-1 increases in intensity and a
second feature in the CH out-of-plane region is observed at 699 cm-1. Simultaneously, the in-
plane bands at higher wavenumber disappear completely, indicating a flat-lying adsorption
geometry. Surprisingly, the disappearance of the aromatic CH stretching bands at 3060 cm-1 is
accompanied by the appearance of a new band at 2949 cm-1, which suggests the formation sp3-
type CH bonds. The latter observation and the flat-lying orientation of the aromatic ring suggest
the formation of a flat-lying di--indolide species, with sp3-type carbon at C2 and C3 positions
(compare Scheme 1). The proposed di--indolide is the equivalent to the di--ethylene formed
from ethylene on Pt(111),[20] which also shows a CH stretching frequency in the aliphatic range
(2908 cm-1,[20b]). We assign the observation that the di- species is not formed below room
temperature to the fact that the molecule has to undergo deprotonation at the NH group first.
For the structurally related molecules pyrrole[21] and pyridine[22], upright standing surface species
have been observed on Pt(111), previously. The latter forms a Pt-bound -pyridyl. The
10
This article is protected by copyright. All rights reserved.

10.1002/chem.201702333
Chemistry - A European Journal
equivalent for indole would be a -indolinidylidene (see Scheme 1). To form this species,
hydrogen migration from C2 to C3 would be required, also yielding a sp3-hybridized carbon.
Previous DF calculations support the possibility of such a reaction.[23] It was shown that the
3H-indole tautomer is comparably stable, even without a mediating surface. Also, the calculated
CH2 stretching of 2929 cm-1 is close to our observations. This species is considered to be less
likely, however, on the basis of the XPS data of the C1s region (not shown).
Proceeding to reaction temperatures between 330 and 390 K, we observe that the CH out-of-
plane band at 699 cm-1 vanishes again. Instead, the new CH stretching band at 2949 cm-1 and the
CH out-of-plane band at 825 cm-1 are the dominating features. Based on the above discussion,
we assign these bands to the flat-lying di--indolide species or, less likely, to a -
indolinidylidene (see Scheme 1).
At temperatures above 420 K, the spectra change drastically again. Specifically, the out-of-plane
band at 825 cm-1 and CH stretching feature at 2949 cm-1 vanish, while new bands appear at 1284,
1425, 1560 and 3060 cm-1. The latter is assigned to CH stretching bands of unsaturated or
aromatic hydrocarbon species. The other bands are assigned to CN and CC stretching modes.
The width of these features indicates some degree of heterogeneity on the surface. We assign the
new bands to further dehydrogenation and decomposition products of indole. Similar bands were
found for pyrrole on Pd(111),[24] where a broad band at 1540 cm-1 (191 meV) observed by
electron energy loss spectroscopy (EELS), was also assigned to a CN vibration.
2.4. Temperature Dependent Reaction of Indoline with Pt(111)
Next, we turn to the thermal behavior of indoline. The results of the corresponding isothermal
IRAS experiment are shown in Figure . Note that the measurements were performed under
conditions identical to those applied for the indole (compare Figure 7). At 210 K, the IR
spectrum is very similar to the one observed in the multilayer regime and we refer to Section 3.1
and
11
This article is protected by copyright. All rights reserved.

10.1002/chem.201702333
Chemistry - A European Journal
Table 2 for details of the band assignments. Briefly, the CH out-of-plane band at 747 cm-1 is the
dominating feature and two broad features at 3358 and 3204 cm-1 are identified as the NH
stretching bands. The broad band at 3060 cm-1 is attributed to the CH stretching mode of the
aromatic six-membered ring. The CH stretching bands of the CH2 groups are centered at 2962
and 2852 cm-1. Based on the DF calculations, we assign the band at lower frequency to a mode,
(C2H2), which is primarily localized at the C2 position. Various small bands between 1610 and
843 cm-1 are assigned to (CH), (CC), (NH), and (CN) modes, which are polarized mainly
within the molecular plane. Among these, the (CC)ar band at 1610 cm-1 is particular important
as it is exclusively localized at the six-membered ring. The fact that most characteristic bands of
the multilayer are also observed in the monolayer suggests that indoline adsorbs molecularly at
210 K. When comparing to the intensities of the multilayer spectrum, the relative intensities of
the in-plane modes are low in relation to the CH out-of-plane mode at 747 cm-1. This suggests
that the indoline adsorbs molecularly without deprotonation and adopts a rather flat-lying
orientation. These findings are in accordance with our XPS data (see Figure 8). In the N 1s
region we observe a single component at a binding energy of 399.6 eV after annealing to
approximately 230 K. This component is attributed to the protonated aminic N in the intact
indoline.
Only upon annealing to higher temperatures (380 K), the N 1s signal shifts to 398.0 eV. The
larger shift of 1.6 eV as compared the indole is associated with both the deprotonation of the NH
group and the dehydrogenation of the five-membered ring system. The same processes can be
observed in more detail in the IR spectra in Figure 9. At temperatures between 240 and 300 K,
we first detect the disappearance of the NH stretching band. In parallel, the aromatic CH
stretching band at 3060 cm-1 decreases in intensity, while the CH stretching bands from the CH2
groups remain visible. Whereas the (C3H2) band at 2962 cm-1 shows little change, the (C2H2)
band splits into features at 2823 and 2877 cm-1. These changes indicate that the five-membered
ring interacts with the surface, however, without clear indication of dehydrogenation up to
300 K. Further changes are observed in the region of the in-plane modes, where the (CC)ar band
of the aromatic ring at 1610 cm-1 vanishes and new features arise at 1167, 1273, 1297 and
1417 cm-1. In the CH out-of-plane region, the two new bands at 827 and 867 cm-1 indicate the
interaction of the aromatic ring with the surface (oop(CH)surf).[17] Clear indications for
12
This article is protected by copyright. All rights reserved.

10.1002/chem.201702333
Chemistry - A European Journal
dehydrogenation are, however, observed between 330 or 360 K. Here two distinct bands at 2953
and 827 cm-1 dominate the spectra. These bands are identical to those observed for indole at the
same temperatures, suggesting that identical surface intermediates are formed for the two
reactants. This finding is corroborated by our XPS data, which shows identical N 1s spectra for
indole and indoline after annealing to 380 K (see Figure 8).
Following the discussion on indole (see above), we propose that deprotonation of the NH group
is followed by dehydrogenation at C2 and C3 position, leading to the formation of a di--
indolide species, i.e. the same entity that is found for indole. Alternatively, dehydrogenation at
C2 only could lead to formation of -indolinidylidene. Again, this species is considered to be
less likely on the basis of the XPS data of the C 1s region (not shown). Once the same
intermediate has been formed at 360 K either from indole or from indoline, the surface chemistry
of both reactants is identical also at higher temperature. Above 390 K, appearance of the bands at
1284, 1425, 1560, and 3060 cm-1 indicates further dehydrogenation and decomposition.
2.5. Temperature Dependent Interaction of Octahydroindole with Pt(111)
In Figure , we show the corresponding dataset for the fully hydrogenated molecule
octahydroindole. Again, the spectrum at 210 K is reminiscent of the one for the multilayer
(compare Section 3.1 and
13
This article is protected by copyright. All rights reserved.

10.1002/chem.201702333
Chemistry - A European Journal
Table 3). The dominating CH bands at 2936 and 2859 cm-1 indicate the presence of the CH2
groups. The most obvious difference to the multilayer spectrum is the absence of the NH
stretching band at 3450 cm-1. Similar as for indole and indoline, however, XPS suggests that
deprotonation does not occur below 300 K. The XP spectra show a single signal at 399.6 eV (see
Figure 8), which is assigned to the protonated aminic nitrogen (see Section 2.4). Therefore, we
attribute the absence of the NH band to a molecular orientation in which the NH bond is oriented
parallel to the surface. In addition, the formation of H bridges can lead to a broadening of the NH
band, making it more difficult to detect in the IR spectrum. All bands in the fingerprint region
(~1650 to ~700 cm-1) are rather weak, but those which are observed correspond to molecular
octahydroindole (see Figure 3). Specifically, we note the presence of the (N1C2C3) stretching
band at 1025 cm-1 which indicates that the five-membered ring remains intact. From these
findings, we conclude that octahydroindole adsorbs molecularly and without deprotonation at
210 K.
Above 240 K, strong changes are observed in the CH stretching region. First, the band at
2936 cm-1 transforms into a doublet at 2938 and 2927 cm-1 and, subsequently, develops an
additional component at 2967 cm-1. Above 300 K, the (C2H2) band at 2859 cm-1 also starts to
decrease in intensity, while the (N1C2C3) stretching band shows hardly any change. These
observations suggest that deprotonation of octahydroindole starts already at 270 K. Noteworthy,
the fact that the (N1C2C3) band remains unchanged up to 360 K suggests that dehydrogenation
does not involve the C2 and C3 positions. This observation implies that dehydrogenation does
not start at the five-membered ring but rather at the six-membered ring. Noteworthy, this finding
is in contrast to our previous studies on N-ethylcarbazole, where the N-containing five-
membered ring was found to dehydrogenate first.[7g] For cyclohexane, on the other hand, it has
been shown that dehydrogenation starts at temperatures as low as 300 K,[25] demonstrating that
the reaction is indeed feasible in this temperature range. The XPS results (see Figure 8) are in
agreement with this interpretation. In contrast to indole and indoline, a second smaller peak at
398.8 eV develops for octahydroindole upon annealing to 380 K. The additional component
indicates a reaction pathway which is different from that of indole or indoline and would, indeed,
be compatible with partial dehydrogenation of the six-membered ring of octahydroindole.
14
This article is protected by copyright. All rights reserved.

10.1002/chem.201702333
Chemistry - A European Journal
In the IR spectra at 390 K, we observe that the CH band at 2927 cm-1 shifts back to 2939 cm-1,
the feature at 2967 cm-1 decreases in intensity and a new, strongly red-shifted CH band appears
at 2731 cm-1. The latter band is quite sharp (FWHM < 20 cm-1), suggesting that the red-shift does
not originate from an agostic interaction (see e.g.[26]). Our DF calculations predict a strongly red-
shifted CH band for the CH stretching mode at the C7a position of octahydroindoline, (C7aH)
(2786 cm-1, see Figure 3). Previously, bands in the same frequency region were found for
methylene groups next to conjugated -systems, for instance in 5-cycloheptadienyl on
Pt(111).[27] Accordingly, we attribute the band to the (C7aH) mode of a partially
dehydrogenated flat-lying intermediate in which the C7a__H bond is oriented perpendicular to the
surface.
Above 390 K, all saturated CH bands vanish and the band of the aromatic system appears at
3060 cm-1, indicating further dehydrogenation. In addition, we observe additional bands at 1560,
1424, and 1284 cm-1, which are identical to those obtained for indole and indoline at the same
conditions. We conclude that all three molecules form the same dehydrogenated and partly
decomposed surface species at temperatures of 420 K and above.
3. Conclusion
We investigated the adsorption behavior and surface chemistry of indole, indoline, and
octahydroindole on Pt(111) using time-resolved and temperature programmed IRAS, HR-XPS
using synchrotron radiation, and DF calculations. All compounds were deposited by PVD under
UHV conditions. The main findings are summarized in the following:
(1) Indole: Indole multilayers deposited at low temperature show a crystallization transition at
202 K ± 5 K, leading to the formation of an ordered phase in which the indole molecules are
strongly tilted with respect to the surface. The indole multilayer desorbs at 221 K ± 5 K.
Below 300 K, indole adsorbs molecularly, with slightly tilted orientation in the monolayer.
Between 300 to 390 K, the indoline undergoes deprotonation at the NH groups. The reaction
results in the formation of a flat-lying di--indolide and, possibly, other intermediates which
are stable up to 390 K. Above this temperature, further dehydrogenation and decomposition
is observed.
15
This article is protected by copyright. All rights reserved.

10.1002/chem.201702333
Chemistry - A European Journal
(2) Indoline: Indoline multilayer films deposited at low temperature show weak structural
rearrangements around 153 K ± 5 K, mainly driven by hydrogen bonding. The indoline
multilayer desorbs at 203 K ± 5 K. Below 300 K, indoline adsorbs molecularly, with slightly
tilted orientation in the monolayer. Between 330 and 390 K, the indoline undergoes
deprotonation at the NH group and dehydrogenation at the 5-membered ring. The reaction
leads to the same surface intermediate as formed from indole in this temperature range, i.e. a
flat-lying di--indolide. Above 390 K, the same dehydrogenation and decomposition
behavior is observed as for indole.
(3) Octahydroindole: Octahydroindole multilayers deposited at low temperature show no
restructuring transition below multilayer desorption, which is observed at 186 K ± 5 K.
Below 300 K, octahydroindole adsorbs molecularly in the monolayer. Above this
temperature, deprotonation of the NH group and dehydrogenation is observed. At
temperatures up to 390 K, dehydrogenation leads to products that are different from those
observed for indole and indoline. Specifically, we observe products in which the six-
membered ring is partially dehydrogenated. Above 420 K, the same dehydrogenated and
partly decomposed surface species are formed as for indole and indoline.
16
This article is protected by copyright. All rights reserved.

10.1002/chem.201702333
Chemistry - A European Journal
4. Experimental
IRAS experiments: All IRAS experiments were performed in a UHV system described in detail
elsewhere (base pressure 1×10-10 mbar)[28]. In short, the system is equipped with a vacuum
Fourier-transform infrared (FTIR) spectrometer (Bruker Vertex 80v), several evaporator sources,
two quadrupole mass spectrometers, and all required sample preparation and characterization
methods. The IR spectra were acquired at a spectral resolution of 4 cm-1. Previous to the
deposition, a reference spectrum of the clean sample at the respective adsorption temperature
was recorded. For the IR spectra of the multilayer, acquisition times of 60 s were used. For the
temperature-programmed (TP-)IRAS experiments, the sample was heated at a rate of 2 K/min,
while IR spectra were recorded with an acquisition time of 60 s. The TP-IRAS spectra are
referred to a reference spectrum taken on the clean sample before uptake. The monolayer spectra
were acquired under isothermal and steady state conditions. The molecules were continuously
dosed at pressures of 1x10-7 mbar for 30 min. During the deposition three spectra of 10 min were
acquired. These three spectra were identical for all experiments, confirming that the steady state
was reached.
XPS experiments: The XPS experiments were performed at beamline UE56-2 PGM-2 of
Helmholtz Zentrum Berlin using a transportable UHV-setup, described in detail in [29]. In the
preparation chamber, typical preparation and surface science characterization tools like a dosing
system, LEED and a sputter gun are mounted. The main chamber is equipped with an electron
analyzer and additional gas dosing facilities. The manipulator is cooled with liquid nitrogen and
allows for reaching temperatures as low as 100 K on the sample. Direct resistive heating of the
crystal enables heating up to 1300 K. Peak fitting of the N 1s spectra was done using Doniach-
[30]
Šunjić-profiles
convoluted with a Gaussian function. The N 1s region was measured at 500
eV with a resolution of 150 meV at an emission angle of 0° relative to the surface normal. Each
spectrum is referenced to the fermi edge of Pt(111) taken prior to each spectrum at 500 eV.
Sample preparation: The Pt(111) crystal (MaTecK) was cleaned by ion bombardment (Ar+, 1.50
keV, 300 K) and subsequent annealing to 1000 K in UHV for 3 min. To clean the surface from
carbon contaminations it was annealed in O2 (1x10-7 mbar). Long range order and cleanliness of
the surface was checked by LEED and by IRAS using CO (20 L (Langmuir), 1 L = 10-6 Torr.s,
sample temperature 140 K) as a probe molecule.[31] Indole (99%) and indoline (99%) were
17
This article is protected by copyright. All rights reserved.

10.1002/chem.201702333
Chemistry - A European Journal
purchased from Sigma-Aldrich. Octahydroindole (96%) was used as synthesized (compare
below). During the deposition, minor traces of water were observed, which could not be avoided.
All three substances were dosed from a quartz glass reservoir via a leak valve and a doser
directed towards the sample. Prior to each experiment, indole was pumped for 5 min over a
separate pump line. Indoline and octahydroindole were purified by repeated freeze-pump-thaw
cycles.
Synthesis of octahydroindole: The octahydroindole was obtained by hydrogenation of 22.01 g
(0.188 mol) indole (99%, Sigma Aldrich) dissolved in 100 mL cyclohexane with a 5 mass%
Rh/AlOx catalyst (0.438 g from Sigma Aldrich) in a 250 mL steel autoclave. After sealing the
reactor, the gas phase was purged three times with helium and then heated to 100 °C, and
pressurized with 30 bar of H2. After 20 hours of reaction, the mixture war filtered and the solvent
was removed with a rotary evaporator. The resulting brownish oil was distilled at 30 °C and 0.5
mbar to give pure, colourless octahydroindole (16.9 g .0.135 mol) in good yield 71.8 %. The
1
product was stored under inert gas atmosphere at 8 °C. H-NMR (δ, 293 K, CDCl3): multiplet
13
1.07-1.96 ppm (13 H); multiplet 2.72-2.82 ppm (2 H); C: 21.56 ppm (1 C), 23.49 ppm (1 C),
26.99 ppm (1 C); 27.62 ppm (1 C), 30.71 ppm (1 C), 38.07 ppm (1 C), 43.96 ppm (1 C), 57.39
ppm (1 C)). In addition, the identity and purity of the compound was determined with gas
chromatography and NMR. The 1H and 13C NMR spectra were recorded on a Jeol ECX-400. The
chemical shifts are in ppm relative to CDCl3 (1H: 7.26 ppm; 13C: 77.16 ppm). For gas
chromatography, a Varian 3900 gas chromatograph with a Factor FourTM capillary column VF-
1ms, 15 m, 0.25 mm, 0.25 µm was used. The temperature of the injector was 280 °C The heating
ramp started at 60 °C and was heated to 150 °C with a heating rate of 10 K min-1. Then the
temperature was further raised to 280 °C with a heating rate of 20 K min-1.The final temperature
was hold for 5 minutes. The detector was a FID at 280 °C. All starting materials were used as
received, without further treatment.
Computational details: For identification of our IR signals we performed DF calculations using
the Turbomole program suite.[32] We employed the exchange correlation functional of Perdew,
Becke and Enzershof (PBE)[33] in combination with the def2-TZVP basis set of Wiegand and
[34]
Ahlrichs
as well as the Grimme D3 dispersion correction[35]. The calculations utilized the
18
This article is protected by copyright. All rights reserved.

10.1002/chem.201702333
Chemistry - A European Journal
RI-J[36] and MARI-J[37] approximations. The frequencies were calculated within the harmonic
approximation.
19
This article is protected by copyright. All rights reserved.

10.1002/chem.201702333
Chemistry - A European Journal
Acknowledgements:
This project was financially supported by the “Deutsche Forschungsgemeinschaft” (DFG) within
the Excellence Cluster “Engineering of Advanced Materials” (www.eam.fau.de) in the
framework of the excellence initiative and by the Clariant Produkte (Deutschland) GmbH. The
authors thank the BESSY staff for support during the beamtime and the Helmholtz-Zentrum
Berlin for travel support and the allocation of synchrotron beamtime.
20
This article is protected by copyright. All rights reserved.

10.1002/chem.201702333
Chemistry - A European Journal
Figure 1: Left: Comparison of infrared spectra of indole: (top) ATR spectrum of solid indole,
(middle) IR reflection absorption spectrum of an indole multilayer on Pt(111), (bottom)
calculated IR spectrum from DFT (shown with a Lorentzian broadening of 8 cm-1 FWHM).
Right: visualization of selected vibrational modes of indole (see text for details).
21
This article is protected by copyright. All rights reserved.

10.1002/chem.201702333
Chemistry - A European Journal
Figure 2: Left: Comparison of infrared spectra of indoline: (top) ATR spectrum of solid
indoline, (middle) IR reflection absorption spectrum of an indoline multilayer on Pt(111),
(bottom) calculated IR spectrum from DFT (shown with a Lorentzian broadening of 8 cm-1
FWHM). Right: visualization of selected vibrational modes of indoline (see text for details).
22
This article is protected by copyright. All rights reserved.

10.1002/chem.201702333
Chemistry - A European Journal
Figure 3: Left: Comparison of infrared spectra of octahydroindole: (top) ATR spectrum of solid
octahydroindole, (middle) IR reflection absorption spectrum of a octahydroindole multilayer on
Pt(111), (bottom) calculated IR spectrum from DFT (shown with a Lorentzian broadening of 8
cm-1 FWHM). Right: visualization of selected vibrational modes of octahydroindole (see text for
details).
23
This article is protected by copyright. All rights reserved.

10.1002/chem.201702333
Chemistry - A European Journal
Figure 4 Top: Temperature-programmed IRAS series of indole multilayer shown as a contour
plot. In addition, selected spectra plotted at their respective temperatures. Bottom: Integrated
peak intensity in the spectral region from 698 to 3473 cm-1 shown as a function of temperature.
24
This article is protected by copyright. All rights reserved.

10.1002/chem.201702333
Chemistry - A European Journal
.
Figure 5: Top: Temperature-programmed IRAS series of indoline multilayer shown as a contour
plot. In addition, selected spectra plotted at their respective temperatures. Bottom: Integrated
peak intensity in the spectral region from 698 to 3473 cm-1 shown as a function of temperature.
25
This article is protected by copyright. All rights reserved.

10.1002/chem.201702333
Chemistry - A European Journal
Figure 6: Top: Temperature-programmed IRAS series of octahydroindole multilayer shown as a
contour plot. In addition, selected spectra plotted at their respective temperatures. Bottom:
Integrated peak intensity in the spectral region from 698 to 3473 cm-1 shown as a function of
temperature.
26
This article is protected by copyright. All rights reserved.

10.1002/chem.201702333
Chemistry - A European Journal
Figure 7: Left: IR spectra taken during exposure of Pt(111) to indole at the temperatures
indicated. Right: Integrated intensity of selected peaks as a function of the surface temperature
(see text for details).
27
This article is protected by copyright. All rights reserved.

10.1002/chem.201702333
Chemistry - A European Journal
c)
430 K
380 K
232 K
b)
435 K
381 K
226 K
a)
426 K
383 K
225 K
401.0
400.0
399.0
398.0
397.0
Binding energy [eV]
Figure 8: N 1s XP spectra of a) indole, b) indoline and c) octahydroindole on Pt(111) taken after
deposition of the compound at 140 K and subsequent annealing to different temperatures (hν =
500 eV; normal emission).
28
This article is protected by copyright. All rights reserved.

10.1002/chem.201702333
Chemistry - A European Journal
Figure 9: Left: IR spectra taken during exposure of Pt(111) to indoline at the temperatures
indicated. Right: Integrated intensity of selected peaks as a function of the surface temperature
(see text for details).
29
This article is protected by copyright. All rights reserved.