Alkyl chain length-dependent surface reaction of dodecahydro-N-alkylcarbazoles on Pt model catalysts
Christoph Gleichweit, Max Amende, Udo Bauer, Stefan Schernich, Oliver Höfert, Michael P. A. Lorenz, Wei Zhao , Michael Müller, Marcus Koch, Philipp Bachmann, Peter Wasserscheid, Jörg Libuda, Hans-Peter Steinrück, and Christian Papp
Citation: The Journal of Chemical Physics 140, 204711 (2014); doi: 10.1063/1.4875921
View online: /10.1063/1.4875921
View Table of Contents: /content/aip/journal/jcp/140/20?ver=pdfcov
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THE JOURNAL OF CHEMICAL PHYSICS 140,204711(2014)
Alkyl chain length-dependent surface reaction of
dodecahydro-N -alkylcarbazoles on Pt model catalysts
Christoph Gleichweit,1,a)Max Amende,1,a)Udo Bauer,1Stefan Schernich,1Oliver Höfert,1Michael P .A.Lorenz,1Wei Zhao,1Michael Müller,2Marcus Koch,2Philipp Bachmann,1Peter Wasserscheid,2,3Jörg Libuda,1,3Hans-Peter Steinrück,1,3and Christian Papp 1,b)
1
Lehrstuhl für Physikalische Chemie II,Friedrich-Alexander-Universität Erlangen-Nürnberg,Egerlandstrasse 3,91058Erlangen,Germany 2
Lehrstuhl für Chemische Reaktionstechnik,Friedrich-Alexander-Universität Erlangen-Nürnberg,Egerlandstrasse 3,91058Erlangen,Germany 3
Erlangen Catalysis Resource Center,Friedrich-Alexander-Universität Erlangen-Nürnberg,Egerlandstrasse 3,91058Erlangen,Germany
(Received 11February 2014;accepted 21April 2014;published online 28May 2014)
The concept of liquid organic hydrogen carriers (LOHC)holds the potential for large scale chem-ical storage of hydrogen at ambient conditions.Herein,we compare the dehydrogenation and decomposition of three alkylated carbazole-based LOHCs,dodecahydro-N -ethylcarbazole (H 12-NEC),
dodecahydro-N -propylcarbazole (H 12-NPC),and dodecahydro-N -butylcarbazole (H 12-NBC),on Pt(111)and on Al 2O 3-supported Pt nanoparticles.We follow the thermal evolution of these systems quantitatively by in situ high-resolution X-ray photoelectron spectroscopy.We show that on Pt(111)the relevant reaction steps are not affected by the different alkyl substituents:for all LOHCs,stepwise dehydrogenation to NEC,NPC,and NBC is followed by cleavage of the C–N bond of the alkyl chain starting at 380–390K.On Pt/Al 2O 3,we discern dealkylation on defect sites already at 350K,and on ordered,(111)-like facets at 390K.The dealkylation process at the defects is most pronounced for NEC and least pronounced for NBC.©2014AIP Publishing LLC .[/10.1063/1.4875921]
INTRODUCTION
Recent developments in energy politics show that in the near future renewable energy sources,such as wind or solar energy,will play an increasingly important role for electricity production.Unfortunately,these two very impor-tant technologies for a sustainable energy production are characterized by an unsteady energy output with fluctua-tions caused by seasonal,climatic,and weather effects.An increasing share of unsteady energy electricity output cre-ates a demand for efficient energy distribution and stor-age systems.1Focusing on future energy storage systems,hydrogen is one of the most promising candidates.How-ever,practical applications are hampe
red by the unfavor-able physico-chemical properties of molecular hydrogen re-quiring high pressures (up to 700bars)or cryogenic tem-peratures (typically −253◦C)for storage at reasonable en-ergy densities.Note that the volumetric energy capacity of gaseous hydrogen at standard conditions is as low as 3Wh/l (compared to ca.10.000Wh for a liter of diesel).2–4
One possible way to overcome these challenges is to store hydrogen chemically in an organic compound that is liquid under ambient conditions.Among the different concepts of chemical energy storage,the use of so-called liquid organic hydrogen carriers (LOHCs)is particularly attractive as no ex-tra gas component is necessary (in ,to methanol
a)C.Gleichweit and M.Amende contributed equally to this work.
b)Author to whom correspondence should be addressed.Electronic mail:
christian.papp@fau.de
or formic acid production by CO 2hydrogenation).Hydro-gen storage in LOHC systems includes the reversible catalytic dehydrogenation and hydrogenation of alicyclic/aromatic or-ganic compounds.The latter are typically characterized by diesel-like physico-chemical properties to ensure their op-erability i
n traditional infrastructures built for liquid energy carriers.3,5,6Establishing a suitable storage system based on this concept requires the optimization of both the LOHC and the catalyst,which is used for the loading and unloading cy-cles.Regarding the catalyst,a balance between price and ef-ficiency has to be found for a competitive storage technology.In this context,the evaluation of the performance of nano-sized catalysts with a large surface/bulk ratio is important,in particular in view of the fact that the presently used catalysts contain the precious metals Pt and Pd.
There are numerous requirements for an ideal LOHC compound,including storage density,liquid range,ther-mal stability,toxicity/ecotoxicity,and large-scale availabil-ity.Among the feasible systems,the most widely stud-ied molecule is N -ethylcarbazole (NEC),which is able to store 5.8wt.%hydrogen (H 2)when fully hydrogenated to dodecahydro-N -ethylcarbazole (H 12-NEC).7,8However,NEC has a relatively high melting point of about 68◦C,which is one of the shortcomings of this carrier.The melting point of the unloaded carrier can be lowered by using a longer alkyl chain,due to entropic reasons:by replacing the ethyl entity of NEC by an n -propyl group,one obtains N -propylcarbazole (NPC),with the melting point lowered to 48◦C.The use of eutectic mixtures of NEC and NPC promises even lower
0021-9606/2014/140(20)/204711/9/$30.00
©2014AIP Publishing LLC
140,
204711-1
H -N-ethylcarbazole
12H -N-propylcarbazole
12H -N-butylcarbazole
12Sin g le crystal
Oxide-supported nanoparticles
LOHC Molecules Pt(111)
storage capacity 5.8 w t%
storage capacity 5.4 w t%
storage capacity 5.1 w t%
FIG.1.Schematic overview of the investigated molecules and model catalyst surfaces.
melting points.9However,as the propyl chain does not partic-ipate in the dehydrogenation/hydrogenatio
n cycle such struc-tural modification reduces the gravimetric storage density of the system to 5.4%.On the other hand,NPC shows a lower activation barrier for full hydrogenation on Ru catalysts and thereby has an improved applicability.9
One important goal of fundamental studies concerning the applicability of LOHCs is the evaluation of the ele-mentary steps of the involved surface reactions.Concerning the dehydrogenation reaction,one specifically relevant as-pect is the avoidance of unwanted side reactions during un-loading,such as dealkylation of the N -alkyl substituent that would lead to an unfavorable change in the liquidus range of the LOHC system.In this context,we extensively stud-ied H 12-NEC regarding its dehydrogenation mechanism on various model catalysts including Pt(111),10Pd(111),11and on both Al 2O 3-supported Pt 12and Pd 13nanoparticles in ultra-high vacuum (UHV)using high-resolution X-ray photoelec-tron spectroscopy (HR-XPS),infrared reflection-absorption spectroscopy (IRAS),and temperature-programmed molecu-lar beam methods (TPMB).The N 1s spectrum of the nitro-gen atom of NEC and H 12-NEC is very suitable to monitor the C–N bond scission in HR-XPS.In fact,this reaction is observed to occur both on industrial catalysts (<2%dealky-lation after heating NEC for 72h at 543K in contact with a commercial Pt on AlOx catalyst 14)and on model catalytic surfaces.Dealkylation of H 12-NEC on a Pt(111)single crystal is observed starting at 390K,10,15while on Pd(111)the onset of deal
kylation is found already at 350K.11The same reac-tion was found to take place on supported Pt and Pd model systems,where the activation barrier of C–N bond scission is reduced as compared to single crystals.12
The aim of this work is to study modified LOHCs based on the carbazole system.Herein,we discuss the effect of the length of the alkyl chain on the dehydrogenation reac-tion and on the side reactions of the respective LOHC.We present a comparison between the reaction of dodecahydro-N -
ethylcarbazole (H 12-NEC),dodecahydro-N -propylcarbazole (H 12-NPC),and dodecahydro-N -butylcarbazole (H 12-NBC),on both Pt(111)and supported Pt nanoparticles under UHV conditions studied by in situ high-resolution X-ray photoelec-tron spectroscopy (HR-XPS);a schematic overview of the in-vestigated LOHCs and substrates is given in Figure 1.On the single crystal surface,we obtain a detailed quantitative analysis of the reaction due to the highly defined nature of the substrate.10,11,15On the other hand,the investigation of supported model catalysts provides a more realistic picture of reactions taking place on the complex surface of an in-dustrial heterogeneous catalyst.Note that for the hydrogen-lean compounds,the abbreviations NEC,NPC,and NBC will be used (equivalent to H 0-NEC,H 0-NPC,H 0-NBC,respec-tively).When addressing all three carriers,hereinafter,the terms NXC and H 12-NXC are used.EXPERIMENTAL
The HR-XPS experiments were conducted at the beam-line U49/2-PGM1of the 3rd generation synchrotron BESSY II of Helmholtz Zentrum Berlin using a transportable ultra-high vacuum (UHV)setup consisting of two chambers.16The preparation chamber provides typical surface-science instru-ments such as low energy electron diffraction (LEED),elec-tron beam evaporators for metal deposition (Focus EFM 3),and a sputter gun (Specs IQE 11/35).The measurement cham-ber is equipped with an electron energy analyzer (Omicron EA 125HR U7)and is directly connected to a supersonic molecular beam and an evaporator unit for LOHC deposition,which allows us to dose substances while taking XP spectra.The differentially pumped evaporator is separated from the main chamber by a gate valve and can be baked out sepa-rately.A filament situated at the back of the sample allows us to heat without disturbing the emitted electrons by elec-tric or magnetic stray fields.Measurement times of ∼10s per spectra were used.
The Pt(111)single crystal was cleaned by Ar+bom-
bardment(1kV,5×10−6mbar)and annealing in vacuum
prior to each experiment.The cleanliness of the surface was
checked by XPS.For preparation of the supported catalyst,
a NiAl(110)single crystal was cleaned by several cycles of
Ar+bombardment(1kV,5×10−6mbar)and annealing in
vacuum.On top of the clean substrate,the Al2O3film was
prepared by at least two cycles of O2exposure(550K,7×10−6mbar)and subsequent annealing to1150K.17Theflux of the metal evaporator,which was equipped with a Pt wire
(MaTecK,99.95%),was calibrated using a quartz microbal-
ance,yielding Pt deposition rates around0.05Å/min.While
evaporating a total amount of1ÅPt,the sample was kept at
150K to obtain defect-rich Pt aggregates.Based on an esti-
mate of the densities in the order of1013particles cm−1,we
roughly estimate an average value of70atoms per particle at
a nominal Pt coverage of0.1nm(6.8×1014Pt atoms cm−2).
H12-NEC,H12-NPC,and H12-NBC were deposited by
physical vapor deposition(PVD).During H12-NXC evapora-
tion,the background pressure in the analysis chamber rises to
approximately1.3×10−8mbar.
In order to avoid radiation induced damage of the surface,
the X-ray spot on the sample was shifted prior to each mea-
surement.The N1s(C1s)spectra were acquired at a photon
energy of500eV(380eV)with an overall resolution of ap-
proximately300meV(180meV)at an emission angle of the
photoelectrons of0◦with respect to the sample normal.The
linear heating rate during TPXPS experiments was0.5K/s.
For peakfitting we used Doniach-Sunjic functions to repre-
sent the peak shape after subtraction of the background.In the
C1s spectra two Doniach-Sunjic functions were used as en-
velopes to represent the surface species H12-NXC,NXC,and
carbazole.These two contributions were used solely to repre-
sent the envelope and do not have a physical meaning.In the
N1s region,one Doniach-Sunjic was used(see,for example,
the supplementary material of Ref.10).
NPC and NBC were produced by alkylation18of car-
bazole using the precursors propyl iodide and butyl bromide,
respectively.NEC was bought from Hydrogenius Technolo-
gies GmbH(www.hydrogenious)in sulfur-free quality.
Hydrogenation was performed in cyclohexane at150◦C at
a hydrogen pressure of50bars using a Ru/AlO x catalyst
(5wt.%Ru on AlO x).Purity was ensured through
vacuum distillation and consequent checks using gas
chromatography–mass spectrometry(GC–MS).
RESULTS AND DISCUSSION
Adsorption and thermal evolution on Pt(111)
As afirst step,the adsorption of H12-NEC,H12-NPC,
and H12-NBC on the Pt(111)single crystal surface at low
temperature(140K)was investigated by continuously mea-
suring C1s and N1s spectra during adsorption(data not
shown).This allows us to compare the spectra of the fully
hydrogenated molecules at similar coverage prior to any re-
action.The curves with open symbols in Figure2show spec-
tra taken slightly above the monolayer coverage.For all three
molecules,two signals are found in the N1s region,which are assigned to the chemisorbed monolayer at a binding en-
ergy of401.3eV and the physisorbed multilayer at399.5eV.
In the C1s region,one peak is observed at approximately
284.7eV.With increasing coverage(not shown here),this
peak shifts to higher binding energies and a broadening of
the signal is observed,indicating multilayer adsorption.From
these observations,we conclude that the behavior of H12-NPC
and H12-NBC during adsorption is similar to the one of H12-
NEC that was already addressed in great detail in our previous
studies.10–13,15
Next,we discuss the reactions occurring during heating
of the adsorbed layers.To eliminate any contributions of the
multilayer signal,which are difficult to separate from peaks
of the chemisorbed species,for these experiments adsorption
was performed at∼250K(except for the C1s experiment for
H12-NPC,where the molecule was adsorbed at140K and then
heated to250K).After saturation of the monolayer signal(at ∼4L),the crystal was heated linearly at a rate of0.5K/s.The spectra were taken continuously(Temperature Programmed
XPS,TPXPS)to monitor the surface reaction in situ.Selected
spectra at the denoted temperatures are displayed as curves
(solid symbols)in Figure2.
First,we briefly discuss the thermal evolution of H12-
NEC.At250K,the N1s peak at399.5eV is caused by
partial dehydrogenation already during the adsorption exper-
iment(note that it has the same binding energy as the multi-
layer,by coincidence).In the initial reaction step,H12-NEC
partly dehydrogenates to H8-NEC by abstraction of the four
hydrogen atoms in the nitrogen-containing heterocycle.The
nitrogen atom is strongly affected by these changes,which
leads to the strong shift to lower binding energy from401.3
to399.5eV.In the C1s region,partial dehydrogenation is
reflected by a lower binding energy shoulder appearing at
284.1eV.At temperatures above300K this shoulder shifts
to slightly higher energy(284.4eV)and strongly increases
at the expense of the signal at285.2eV due to dehydrogena-
tion to H8-NEC.This process continues,even when the N1s
signal at401.3eV has vanished at330K,indicating further
dehydrogenation.Finally,it yields NEC at380K,as is evident
from the quantitative analysis in Figures3(a)and3(d).Note
that the N1s signal of H12-NEC already vanishes at330K,
since the nitrogen atom in the pyrrole ring is not affected by
the further dehydrogenation from H8-NEC to NEC.
The data for H12-NPC and H12-NBC show a comparable
behavior,with some differences,which are evident from the
quantitative analysis of both the C1s and N1s levels for all
three molecules in Figure3.At250K,for all three LOHCs,
comparable coverages of H12-NXC and NXC are deduced
from the N1s spectra,indicating that dehydrogenation has
occurred at this temperature to about the same extent.Also in
the C1s data,a very similar behavior is observed for the three
LOHCs.One difference to the N1s data is that in the C1s
spectra the H12-NXC signals at250K are in all cases about
a factor of2.5larger than the NXC signals(Figure3).This
atfirst sight puzzling behavior is due to the fact that in the
N1s spectra only the chemical surrounding of the nitrogen
atom is ,the dehydrogenation at the pyrrole ring,
which results in a clear chemical shift;on the other hand,the
C1s peaks contain information on all carbon atoms in the
N 1s
C 1s
I n t e n s i t y [a r b .u .]
Bindin g ener g y [eV]
Bindin g ener g y [eV]
reaction to a book or an articleFIG.2.Selected XP spectra of H 12-NEC,H 12-NPC,and H 12-NPC on Pt(111)at different temperatures for both the (a)–(c)C 1s and (d)–(f)N 1s core levels.
The bottom spectra (open symbols)are taken during adsorption at 140K at similar coverage,the other spectra (filled symbols)were recorded after adsorption at 250K until saturation (a)and (c)–(f)and during subsequent heating to 600K;in (b)heating was performed after adsorption at 140K.
respective molecules,and due to the stepwise reaction the C 1s peak at 285.2eV assigned to “H 12-NXC”also contains in-tensity from the CH 2subunits in H y -NXC (y ≤8).As a con-sequence,the amount of H 12-NXC is overestimated at cover-ages,where H 12-NXC and H y -NXC coexist on the surface,as is the case at 250K.
Overall,the thermal evolution shows the same behavior for all LOHCs.Upon heating,in both the N 1s and the C 1s regions,the H 12-NXC signals decrease and,simultaneously,the signals for the dehydroge
nated species increase (note that the increase is less pronounced for the N 1s region of NPC).In the N 1s region,starting from 330K the NXC signal is the only peak.As the nitrogen atom is again mainly influ-enced by the nearby carbon atoms,only the dehydrogenation of the pyrrole ring,but not of the two six-membered rings,is reflected by the N 1s binding energy.The C 1s region,however,is sensitive to the latter process and,indeed,the signals of H 12-NXC vanish only at 375±10K,where the dehydrogenated species (NXC)reach their maximum inten-sities.This is deduced from a comparison of the C 1s sig-nal of the dehydrogenated carrier molecule and the surface species at 380K and additionally no further decrease of the
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