Engineering active sites on reduced graphene oxide by hydrogen plasma irradiation: Mimicking bifunctional metal/supported catalysts in hydrogenation reactions

Article abstract of DOI:10.1039/c7gc03397d

H₂ plasma has been used to generate carbon vacancies on reduced graphene oxide to increase its catalytic activity as a hydrogenation catalyst. A relationship between the power of the plasma treatment and the exposure time with the activity of the material was observed for CC double bond hydrogenation. The activity data in the case of 1-octene, showing skeletal isomerization besides hydrogenation, indicate that H₂ plasma treatment can introduce hydrogenating and acid sites rendering a bifunctional catalyst that is reminiscent of the activity of noble metals supported on acid supports.

Full text of DOI:10.1039/c7gc03397d

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Engineering active sites on reduced graphene oxidVeiew Abrticyle Online
DOI: 10.1039/C7GC03397D
hydrogen plasma irradiation: Mimicking bifunctional
metal/supported catalysts in hydrogenation reactions.
Ana Primo,1Antonio Franconetti,2Monica Magureanu,3 Nicolae Bogdan Mandache3, Cristina
Bucur,4 Cristina Rizescu,5 Bogdan Cojocaru,5 Vasile I. Parvulescu,5 Hermenegildo Garcia1
1Instituto Universitario de Tecnología Química, Universitat Politècnica de València-Consejo
Superior de Investigaciones Científicas, Avenida de los Naranjos s/n, 46022 Valencia, Spain
2Departament de Química, Universitat Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Spain
3Department of Plasma Physics and Nuclear Fusion, National Institute for Lasers, Plasma and
Radiation Physics, 077125 Magurele, Romania
4National Institute of Materials Physics, Atomistilor 105b, 077125 Magurele-Ilfov, Romania
5
Department of Organic Chemistry, Biochemistry and Catalysis, Faculty of Chemistry, University
of
Bucharest,
Bdul
Regina
Elisabeta,
4-12,
Bucharest
030016,
Romania;
vasile.parvulescu@chimie.unibuc.ro
KEYWORDS: graphene, hydrogen plasma irradiation, hydrogenation reactions, carbocatalysis
ABSTRACT
H2 plasma has been used to generate carbon vacancies on reduced graphene oxide to increase its
catalytic activity as hydrogenation catalyst. A relationship between the power of the plasma
treatment and the exposure time with the activity of the material was observed for C=C double bond
hydrogenation. The activity data in the case of 1-octene, showing skeletal isomerization besides
hydrogenation, indicates that H2 plasma treatment can introduce hydrogenating and acid sites
rendering a bifunctional catalyst that is reminiscent of the activity of noble metals supported on acid
supports.
INTRODUCTION
The use of carbon-based materials as metal-free catalysts to replace noble or critical metal
catalysts is of a great current interest and is attracting special attention since the last decade.1-4 For
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this purpose, graphenes are among the most widely used materials and have already been investigated
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DOI: 10.1039/C7GC03397D
5-7
as metal-free catalysts in a large range of reactions. Of current importance are the oxygen reduction
reactions,8 the hydrogenation of nitro groups 9-11 or the hydrogenation of alkenes and alkynes.12
One of the key issues in this area is the nature of the active sites involved in the reaction
mechanism.6, 13 Understanding the structure of the active sites can eventually lead to prepare modified
graphenes containing a large density of these centers. There are examples in the literature showing
that the catalytic activity of graphenes can be increased by functionalization of the sheet by several
methods, such as radical reactions, cycloadditions, hydrogenation, oxidations, etc., resulting in an
enlargement of the potential of this type of materials for their use as catalysts in a large variety of
reactions14 or by creating carbon defects in the graphene matrix.15-18 Due to the suitable properties of
graphenes and related materials, the control of the extent and nature of the functional groups by
submitting graphenes to post-modification appear to be essential to achieve more active and stable
graphene catalysts.14
Very recently hydrogen plasma treatment has been proposed as an attractive, scalable
approach for functionalization and patterning of mono- and multilayer graphenes.19, 20 This treatment
+
+
generates H atoms and a mixture of ions (H+, H2 , and H3 ) that are able to react with graphene.21 In
accordance to molecular dynamics simulations, the produced H species present in the plasma are
unable to hydrogenate the graphene basal plane,22-26 but exposure to this plasma can lead to the
generation of holes on the graphene basal plane or to a decrease in the lateral dimensions of the
layer.21 Other calculations aimed at modeling defects on graphene have concluded that carbon
vacancies and holes should indeed play an important role in modulating the electronic properties of
graphene and also in the enhancement of its chemical reactivity.27 Calculations based on models of
these open-shell electronic structures simulating carbon vacancies on graphene have led to the
conclusion that the reactivity of a hydrogen atom with the graphene sheet is significantly increased
in carbon vacancies, suggesting that the creation of these type of defects should be positive to boost
the catalytic activity of graphene for hydrogenation reactions.
Our previous studies have shown that defective graphenes exhibit catalytic activity to promote
hydrogenation of C-C multiple bonds and nitro groups,11, 12 but these materials were still much less
efficient than transition metals and, therefore, it is crucial to find ways to improve further their activity
as metal-free catalysts.13 Measurements of the catalytic activity of a series of graphenes for the
fundamental isotopic H-D exchange process, combined with theoretical calculations have led to
predict that one atom carbon vacancy on graphene can act as catalytic center in this process.28
In view of the current state of the art, the present manuscript reports the remarkable
enhancement of the catalytic activity of a reduced graphene oxide (RGO) sample upon modification
by hydrogen plasma treatment, correlating the extent of the graphene sheet exposure with their
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Green Chemistry
performance as catalyst for alkene hydrogenation. Experimental data have been rationalized by
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DOI: 10.1039/C7GC03397D
quantum mechanical calculations on models showing that vacancies of one or two carbon atoms
should be interact strongly with molecular hydrogen, lending support to the existence of an optimal
hydrogen plasma treatment to produce graphene sheets with the highest activity.
EXPERIMENTAL
The RGO sample used in the present study was obtained by Hummers-Offeman oxidation of
graphite, subsequent exfoliation by sonication and hydrothermal reduction, following reported
procedures.29
Plasma treatment of graphenes
The RGO sample was treated in a d.c. glow discharge in hydrogen. A cylindrical plasma
reactor made of glass (inner diameter 65 mm) with two planar electrodes (diameter 40 mm) placed
on the axis of the reactor was used (see Scheme S1). The distance between electrodes was 45 mm.
The reactor chamber was evacuated to a pressure of 3×10-3 mbar, then, hydrogen was continuously
flushed at a flow rate of 5 mL×min-1. The resulting working pressure was 0.1 mbar. Negative d.c.
voltage was applied on one of the electrodes, while the other electrode was grounded. The discharge
voltage was measured using a voltage probe (Tektronix P6015A) connected to an oscilloscope.
Experiments were performed for discharge voltages in the range 500-800 V. The corresponding
discharge power was between 0.4 W and 2 W. The material was placed in the negative glow and the
treatment time was varied from 15 to 60 min.
Samples of graphene submitted to treatments at different powers were prepared. Table 1 lists
the samples prepared in the present study in where the fist number corresponds to the discharge power
and the second one to the treatment time.
Table 1. Samples prepared in the present study and relevant parameters of the plasma treatment.
Sample
G0.4/60
G1.1/15
G1.1/30
G1.1/60
G2/60
Discharge voltage (V) Discharge power (W) Treatment time (min)
500
660
660
660
800
0.4
1.1
1.1
1.1
2.0
60
15
30
60
60
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Characterization of the materials
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Raman spectra were acquired in the extended spectral region from 150 to 4000 cm-1 using a
Horiba JobinYvon - Labram HR UV-Visible-NIR Raman Microscope Spectrophotometer, exciting
with a laser at 633 nm. Diffuse Reflectance Fourier Transform Infrared (DRIFT) spectra were
measured with a Thermo Electron Nicolet 4700 FTIR spectrophotometer adapted with a Smart
Accessory for diffuse reflectance measurement. The IR spectra were scanned in the region of 4000–
400 cm‾1 at the resolution of 4 cm‾1. The final spectra were an average of 400 scans.
Powder X-ray diffraction (XRD) patterns were recorded on a Schimadzu XRD-7000
diffractometer using Cu Kα radiation (λ = 1.5418 Å, 40 kV, 40 mA) in the 5 – 90 degrees 2Θ range.
Pulse chemisorption measurements were performed by an AutoChem II 2920 station from
Micromeritics. The catalyst samples, placed in a U-shaped quartz reactor with an inner diameter of
0.5 cm, were pretreated under He at 100 °C for 1 h. Then, the samples were cooled down to RT under
a helium flow of 50 ml min−1. Pulses of H2 (5% H2 in He) were sent to the sample, until surface
saturation was reached. The amount of adsorbed molecules was obtained by using a calibration plot.
The desorption experiments were carried out using the same station. The H2 desorption experiments
were carried out immediately after chemisorption by heating the sample at a constant heating rate
(10 °C×min-1) in a flow of helium till 400 °C. NH3-TPD measurements were carried out on the same
equipment. Before NH3 adsorption, fresh samples were heated to 120 °C (20 °C min-1) in 30 mL
helium flow. Subsequently, the samples were cooled down to RT also under helium flow. NH3
adsorption was performed under ambient conditions for 120 min in a flow of 10 % NH3 in helium
(30 mL×min-1). Then, the samples were purged with helium to desorb weakly physisorbed ammonia.
Subsequently, thermal desorption of NH3 was carried out heating at a constant rate (10 °C×min-1) in
helium flow till 400 °C.
TEM images were recorded in a JEOL JEM 2100F under accelerating voltage of 200 kV.
Samples were prepared by applying one drop of the suspended material in ethanol onto a carbon-
coated copper TEM grid, and allowing them to dry at room temperature. XPS measurements were
performed at normal angle emission in a Specs set up, using Al Kα monochromated radiation (hν =
1486.7 eV) of an X-ray gun, operating with 300 W (12 kV/25 mA) power. A flood gun with electron
acceleration at 1 eV and electron current of 100 µA was used in order to avoid charging effects. The
energy of photo ejected electrons are measured using a Phoibos 150 analyzer, operating with pass
energy of 30 eV. The XP spectra were fitted using Voigt profiles combined with their primitive
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functions, for inelastic backgrounds. The Gaussian width of all lines and thresholds can be considered
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constant for each spectrum as they should not differ considerably from one spectrum to another, being
always in the range of 2 eV. The spectra were collected in the energy range corresponding to C1s and
O1s species.
Catalytic tests for cyclohexene hydrogenation and 1-octene hydroisomerization
The catalytic tests were carried out with suspended RGO and hydrogen plasma-treated RGO
samples (Table 1). For comparison, a commercial zeolite Beta (PQ Industries, Si/Al ratio of 18) and
physical mixtures of RGO and zeolite Beta have been investigated as well. The physical mixtures
were prepared by 30 min sonication of RGO/zeolite Beta mixtures in ratio 1:1.
The catalytic tests were performed under stirring conditions in a batch autoclave from Hell
o
using 10 mg of catalyst and 3 ml of substrate. The experiments were carried out at 120 C for
cyclohexene (3 ml) and 80 oC for 1-octene (3 ml), under 30 bar H2 pressure, using 3 ml of n-heptane
as a solvent and reaction times in a range of 1.5 – 4 h. All the investigated compounds were
purchased from Sigma-Aldrich. Before pressurization, the autoclave was flushed several times with
H2. The reactants and products were analyzed using a GC-MS apparatus (Thermo Electron Trace
GC Ultra/Trace DSQ) equipped with a non-polar GC capillary column (TG-5SilMS, 30 m × 0.25
mm × 0.25 µm) operated with He as carrier gas. Temperature profile was set to 2 min dwell at 60 °C,
5 ºC×min ramp up to 170 °C, followed by a 5 min dwell. Mass spectra were recorded for positive
ions and full scan mode in the range m/z = 50−300.
Computational details
Theoretical calculations were carried out by means of the Gaussian 09 program.30 Geometry
optimizations were performed at two different levels of theory and basis sets. Geometrical
calculations were initially made using the PBE0 hybrid functional at 6-31G(d). A 4 × 4 graphene
sheet terminated by H atoms containing one or two carbon vacancies in its structure was used as
model for activation of a H2 molecule. For all optimizations, frequencies analyses were applied to
characterize the graphene flakes as minimum, transition state or as metastable state. In addition, the
role of different spin multiplicity (from singlet to triplet) on the graphene geometry as well as on the
energy of the system was studied.
The formation energies (Ef, in eV) of vacancies in graphene are defined as follows31:
Ef = Ed + nEc – Ep
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In which Ed and Ep are the energies of optimized structures for defective and perfect
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graphenes, respectively. Ec is the energy for the removed carbon atom whereas n is 1 or 2 depending
on the number of removed atoms for building vacancies.
Moreover, adsorption energies (eV and kcal mol-1) between defective graphenes and
molecular hydrogen H2 at PBE0/6-31G(d) were calculated as follows:
∆Eadsorption = Ecomplex – Egraphene – EH2
in which Ecomplex is the corresponding energy of defective graphene···H2 complexes where
Egraphene and EH2 are the optimized energies for single defective graphenes and H2 molecule,
respectively.
RESULTS AND DISCUSSION
Characterization of hydrogen plasma treated graphenes
Table 2, Figure S1 and Figure S2A compile the H2 chemisorption and desorption
measurements for the hydrogen plasma-treated graphene samples under study. These measurements
can serve to determine the adsorption capacity and strength of the interaction of the different samples
with H2 as a function of the hydrogen plasma treatment. These values are relevant in the context of
the use of these hydrogen plasma-treated graphenes as hydrogenation catalysts. As it can be seen in
Table 2, plasma exposure of RGO enhanced H2-adsorption, increasing along the power and duration
of the plasma treatment, there being an abrupt increase for long exposure times (60 min) and high
powers (1.1 or 2.0 W). However, the hydrogen desorption measurements up to 400 ºC do not follow
the same trend, those being a maximum for G1.1/30. These differences can be attributed to a spillover
of hydrogen on the graphene sheet, becoming chemically bound for those graphene samples
submitted to long plasma exposure times. Finally the chemisorbed hydrogen corresponding to the
difference between adsorbed and desorbed amounts was recovered for temperatures higher that 450
oC. Note that according to Table 2, light to moderate hydrogen plasma treatments increases equally
the amount of sorbed and desorbed hydrogen indicating that under these conditions plasma treatment
results almost exclusively on physisorbed hydrogen. In contrast, harsh plasma treatment results in a
notable increase of chemisorbed H2 that is notably more strongly bound to graphene, indicating that
other types of centers are created under these conditions. According to our theoretical calculations
the differences in the chemisorbed and desorbed hydrogen can be an indication of the ratio between
one carbon atom vacancy (ie strongly interacting H2) and double carbon vacancy (ie weaker
interacting H2). Since one H2 molecule can occupy one vacancy, according to Table 2 and Figure S1
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the population of one atom vacancies is increasing with the power and time of the plasma treatment.
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It can be calculated as the difference between the chemisorbed and desorbed hydrogen.
Table 2. Summary of H2 adsorption and desorption measurements for the samples under study.
Sample
H2 adsorption
H2 desorption
Molecules×1020/g
up to 400 ºC
Molecules×1020/g
RGO
7.63
7.77
4.27
7.10
G0.4/60
G1.1/15
G1.1/30
G1.1/60
G2/60
10.06
10.78
40.50
45.60
8.49
10.12
6.32
3.61
TPD-NH3 measurements show clearly an increase of the number of moderate strength acidic
sites along with the plasma power and irradiation time (figure S2B), except G1.1/30 which is the most
acidic in the series.
Figure 1 presents the XRD patterns of RGO and the H2-plasma treated samples. All of these
o
present the typical diffraction line of the graphitic stacking of reduced sheets at around 2Θ 24.5 .
The layer spacing calculated with the aid of Bragg equation (λ=2dsin(Ɵ), where λ is the wavelength
of the X-ray beam, d the distance between the adjacent sheets or layers, and Ɵ the diffraction angle)
corresponds 0.364, 0.358, 0.361, 0.356, 0.359 and 0.358 nm for RGO, G0.4/60, G1.1/15, G1.1/30,
G1.1/60, and G2/60, respectively, which is an evidence of the stability of RGO towards plasma
treatment against delamination. The broadness of this line is associated to two factors: i) the small
sheet size (1 μm and below) and, ii) a relatively short domain order or turbostratic arrangement of
RGO stacked sheets.32 Beside this broad line, RGO presents two additional narrow and sharp, less-
o
intense lines at 2Θ 21.4 and 23.7 , respectively. These can be attributed to residual graphite and
correspond to an interlayer space of 0.414 and 0.375 nm, respectively. In addition, G1.1/15 and
G1.1/30 present a broad line at about 2Θ 43.5 o, assigned to the facet (100) of carbon with an interlayer
spacing of 0.208 nm and associated with a partial reduction.
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f
e
d
c
b
a
10 20 30 40 50 60 70 80 90
2Θ, o
Figure 1. X-Ray diffractograms of a) RGO, b) G0.4/60, c) G1.1/15, d) G1.1/30, e) G1.1/60,
f) G2/60
Raman spectroscopy of graphenes is sensitive to defects, excess charge, strain or the atomic
arrangement of the edges. The increase of the disorder in graphene parallels the D/G intensity ratio.
A larger density of defects creates a more elastic scattering (the as named regime of „low” defect
density). An increase of the defect’s density after a certain level results in a more amorphous carbon
structure which attenuates the Raman peaks (the as named regime of “high” defect density). This
corresponds to a decrease of the D/G intensity ratio.33 On the other side, broader and overlapped G
and D lines correspond to a more oxidized graphene oxide, while a high D/G intensity ratio and the
narrow lines indicate a decrease in the average size of the sp2 domains in a reduced GO34. The D/G
intensity ratio (Figure 2) increases in the following order G1.1/30 < G1.1/60 ~ G2/60 < G1.1/15 <
G0.4/60. A shift of the D band to higher wavenumbers can be observed for the G1.1/15, G1.1/30 and
G1.1/60 samples. This is accompanied by a broadening of the G and D bands (overlapping) which
can be attributed to a higher disorder and correlates to XRD data showing a peculiar pattern for
G1.1/15 and G1.1/30. These changes stress the importance of the irradiation time.
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G D
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G+D
2D D+D''
E
e
d
c
b
a
4000 3500 3000 2500 2000 1500 1000 500
-1
Raman Shift (cm )
Figure 2. Raman spectra of a) G0.4/60, b) G1.1/15, c) G1.1/30, d) G1.1/60, e) G2/60
DRIFT spectroscopy was also used to determine any possible influence of the graphene sheet
in the H2 chemisorption and desorption experiments, particularly the possible formation of C-H
bonds. The band associated with hydroxyl groups in the 3200-3500 cm-1 (range not shown) was
absent in all the collected spectra. RGO and G0.4/60 present similar DRIFT spectra (Figure 3 A)
although the intensity of the bands is much lower for RGO. The band at about 785 cm-1 can be
attributed to C-O in ethers,35 while that at 1010-1090 cm-1 to C-O in C-O-C epoxy,36 C-OH or
37
COOH,35 the band at 1550 cm-1 is assigned to C=O or to in-plane vibrations of sp2-hybridized
C=C,35 while that at 1735 cm-1 to carboxyl groups.
Treating RGO at higher plasma power led to a modification of the samples which was
reflected in the DRIFT spectra. The band at 785 cm-1 attributed to C-O shifted to 955 cm-1. The new
bands at about 1080 and 1150 cm-1 are attributed to COOH groups,35 while that at 1385 cm-1 to
organic carbonates. The new bands at 2887 and 2979 cm-1 confirm the aromatic structure of
graphene.38
The exposure to H2 does not change the band positions (Figure 3 B). Only a slight increase of
the intensity was observed after the samples were exposed to H2. This provides a nice evidence of the
stability of these hydrogen plasma irradiated RGO samples under the conditions required in the
hydrogenation reaction.
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0.5
0.4
0.3
0.2
0.1
0.0
B
A
f
e
d
b
a
c
b
a
2000 1500 1000 5000 2000 1500 1000 500
Wavenumber (cm-1)
Wavenumber (cm-1)
Figure 3. A. DRIFT spectra of the samples: a) RGO, b) G0.4/60, c) G1.1/15, d) G1.1/30, e) G1.1/60,
f) G2/60 (bands in blue were detected in all the samples; those in red for the high energy irradiated
samples; and those in green for RGO and G0.4/60); B. DRIFT spectra of G1.1/60 a) fresh and b)
after TPD-H2 experiments
Figures 4 and 5 depict the XP spectra in the binding energy range corresponding to the C1s
and O1s levels. The deconvolution of the bands assigned to these levels revealed the presence of the
multiple functionalities (C-C, C=C, C-O, C-OH and C=O). Their intensity in the separate spectra of
C1s and O1s correlates very well. Also, the intensity of the deconvoluted bands was directly
dependent on the H2-plasma irradiation conditions (see also Table S1). Thus, the H2 plasma irradiation
treatment increases the population of the OH groups that is also described by the O/C ratio (Figure
S1) and the C-O/C=O ratio (Figure S2). These changes also correlate to the formation of defects.
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Figure 4. XP spectra of the H2-plasma treated samples in the region of the C1s level.
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Figure 5. XP spectra of the H2-plasma treated samples in the region of the O1s level.
The expected effect of hydrogen-plasma treatment producing carbon vacancies and holes as
reported in the literature 18, 21 was addressed by transmission electron microscopy (TEM) of the G2/60
that corresponds to the sample submitted to the highest plasma power for the longest time. In the
literature it has been observed that plasma treatment can results in the generation of holes of about 15
nm diameters. No such holes were observed in the present case, even at the harsher conditions
corresponding to G2/60. Thus, in our cases, the defects are more likely to consist in changes in the
oxygenated functional groups and carbon vacancies that are more difficult to be seen by electron
microscopy due to their much smaller size and the low contrast of light atoms. Figure 6 presents
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representative TEM images of this sample, showing that at the highest magnification the atom
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alignment exhibits a considerable density of white spots that can be attributed to atom vacancy in this
sample. These subnanometric white spots observed at the highest resolution in Figure 6 are proposed
to correspond to the defects generated by the H2 plasma treatment. Figure S3 in the supplementary
information provides an additional series of TEM images corresponding to the samples G1.1/15,
G1.1/60 and G2/60, where an increase in the density of the white spots on the graphene sheet
attributed to defects can be seen.
Figure 6. TEM images taken for G2/60 at different magnifications, showing the characteristic
morphology of few layers graphene with a light contrast and the presence of wrinkles. At the highest
magnification, the white spots in the lines corresponding to the hexagonal atom arrangement are
proposed to be the defects consisting in carbon atom vacancies originated by hydrogen plasma
treatment.
Catalytic activity
As commented in the introduction, the purpose of the present study is to show the possibility
to increase the activity of graphenes as hydrogenation catalysts by generating defects on the sheet
that can act as active sites to promote hydrogenations.13, 28
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The beneficial effect of H2-plasma irradiation on the activity as hydrogenation catalysts can
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be clearly seen in Figure 7, where the time conversion plots for the liquid-phase cyclohexene
hydrogenation promoted by the various graphene samples under study are presented. As expected in
view of the existing precedents, the treated graphene samples were able to promote cyclohexene
hydrogenation to cyclohexane as the only product observed,11, 12 increasing the catalytic activity along
the power and duration of the plasma treatment.
100
GO
G0.4/60
90
G1.1/15
G1.1/30
80
70
60
50
40
30
20
10
0
G1.1/60
G2/60
1.5
2.0
2.5
3.0
3.5
4.0
Time (h)
Figure 7. Time-conversion plots for hydrogenation of cyclohexene catalyzed by hydrogen
plasma-treated graphene catalysts. Reaction conditions: cyclohexene (3 ml), n-heptane as solvent
(3 ml), catalyst 10 mg, initial H2 pressure 30 bar, 80 oC, magnetic stirring in autoclave.
Similar relative catalytic order as that commented for cyclohexene was also observed using
1-octene as substrate, although in this case the product distribution includes positional and skeletal
isomers of 1-octene as well as octane, as indicated in Scheme 1. Thus, Figure 8 presents the time-
conversion plots for 1-octene conversion, showing that the activity of the graphene catalysts increases
with the power and time of H2-plasma irradiation.
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Scheme 1. Hydroisomeration and hydrogenation products formed from 1-octene.
100
80
60
40
20
0
GO
G0.4/60
G1.1/15
G1.1/30
G1.1/60
G2/60
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
Time (h)
Figure 8. Time-conversion plots for 1-octene transformation under H2 catalyzed by hydrogen
plasma-treated graphene catalysts. Reaction conditions: 1-octene (3 ml), n-heptane as solvent (3 ml),
catalyst 10 mg, initial H2 pressure 30 bar, 120 oC, magnetic stirring in autoclave.
One difference between the activity data for cyclohexene and 1-octene is the negligible
catalytic activity observed for the parent reduced graphene oxide in the 1-octene conversion.
However, the most salient feature of 1-octene as substrate is that it allows a better understanding of
the effect of the changes produced in reduced graphene oxide by plasma irradiation with hydrogen
by analysis of the selectivity of the reaction, particularly from the appearance of skeletal isomer
2-methyl-1-heptene. Figure 9 presents product selectivity for the reaction of 1-octene as a function of
the reaction time in the presence of the various graphene catalysts used in the present study.
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DOI: 10.1039/C7GC03397D
Figure 9. The variation of the selectivity to 2-octene, 2-methyl-1-heptene and octane using different
graphene samples as catalyst as a function of the reaction time. Reaction conditions: 1-octene (3 ml),
o
n-heptane as solvent (3 ml), catalyst 10 mg, initial H2 pressure 30 bar, 120 C, magnetic stirring in
autoclave.
While RGO merely produces hydrogenation of 1-octene to octane (Figure 5) and, in a lesser
extent isomerization to 2-octene, the changes in the surface functionalities, including generation of
mildly acidic OH groups, due to plasma irradiation, allowed skeletal isomerisation resulting in the
formation of 2-methyl-1-heptene (Scheme 1) which is present in the rest of the graphene catalysts. It
is well-established that the formation of this product requires a bifunctional acidic and hydrogenating
catalyst and this process is typically observed, for instance, with catalysts containing noble metal
nanoparticles supported on a solid acid.39, 40 Moreover, the selectivity towards 2-methyl-1-heptene
reaches a maximum of 15.5 % for the G1.1/30 sample. The total selectivity to the isomerization
products on this G1.1/30 catalyst was about 31%. By correlation to the H2 chemisorption, TPD-H2,
XPS and DRIFT spectroscopic characterization, the behavior of G1.1/30 seems to correspond to an
intermediate population of acidic groups and an intermediate level density of defects. This
interpretation agrees with acidity characterization data for the various graphene samples under study
determined by NH3 TPD that are provided in supporting information (Figure S4). NH3 TPD
measurements show that the acidity changes with the hydrogen plasma treatment and that the most
acidic samples are those submitted to low energy plasma and short irradiation times, particularly,
G1.1/30 and G1.1/15, decreasing as the time or the power increase further. These acidity data from
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NH3 TPD agree well with the assumption that the skeletal isomerization leading to 2-methyl-1-
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DOI: 10.1039/C7GC03397D
heptane requires a balanced contribution of acidity and hydrogenating activity.
To further confirm the role of acidity in the skeletal hydroisomerization of 1-octene, the
catalytic behavior of a zeolite Beta and the 1:1 physical mixture with RGO was also evaluated. The
results are presented in Figures 10 and 11, which show the time-conversion plot for 1-octene and the
product selectivity at three different reaction times for zeolite Beta, RGO and the 1:1 physical
mixture. As can be seen in these Figures, due to its known acidity, Beta zeolite also afforded the
skeletal hydroisomerization of 1-octene in similar extent as the irradiated G1.1/30 sample (compare
selectivity data from Figures 9 and 10), while this product is not observed in the RGO test. Likewise,
in terms of total isomerization activity the catalytic behavior of zeolite Beta was similar as that of the
plasma irradiated G1.1/30 sample (ca 34%), exhibiting a maximum selectivity to octane of 72% after
4 h reaction time. In other words, the combination of zeolite Beta and RGO results in a catalytic
behavior similar to that of G1.1/30, supporting our assumption that the activity of G1.1/30 for the
transformation of 1-octene can be rationalized as arising from the hydrogenating activity similar to
RGO plus acidity that could be introduced alternatively by zeolite Beta.
15
GO
Zeolite
GO+Zeolite
10
5
0
1.5
2.0
2.5
3.0
3.5
4.0
Reaction time (h)
Figure 10. Time-conversion plot for the reaction of 1-octene with H2 in the presence of the series of
catalysts under study. Reaction conditions: 1-octene (3 ml), n-heptane as solvent (3 ml), catalyst 10
mg, initial H2 pressure 30 bar, 120 oC, magnetic stirring in autoclave.
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2-methyl-1-heptene
octane
1-octene
100
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DOI: 10.1039/C7GC03397D
90
80
70
60
50
40
30
20
10
0
1.5
3
4
1.5
3
4
1.5
Zeolite+GO
3
4
Time, h
Zeolite
GO
Figure 11. Variation of the conversion of 1-octene vs. time on RGO, Zeolite Beta and physical
mixtures of two.
We also compared these catalysts with recently reported 3D graphene-like microporous
carbons synthesized in a zeolite template41 and with 0.5wt% Pt and Pd deposited onto the GO support.
Compared to the 3D graphenes, plasma RGO catalysts showed an enhanced activity. Thus, after 4h
G2/60 afforded a conversion of cyclohexene higher than 90% compared to 64% for the β-3D
graphene and 49% for the Y-3D graphene. Both 0.5wt% Pt and Pdwt%/GO catalysts produced an
almost complete reduction of cyclohexene but with the additional price of a 0.5wt% noble metal
loading.
Superior differences were also observed for the hydroisomeration and hydrogenation of 1-
octene. In this case, after 4 the conversion on G2/60 was of almost 60% compared to 41% for the β-
3D graphene and 26% for the Y-3D graphene. However, the selectivity to octane was superior on
G2/60 that is an additional proof of the suppressing of the acidic functions during the H2-plasma
treatment. 0.5wt% Pt and Pdwt%/GO provided very close selectivities to the two 3-D graphenes
confirming thus the preservation of the acidic functions of the GO.
Theoretical calculations.
To provide some rationalization in support of the role of holes on the graphene sheet on their
activity as hydrogenation catalysts, quantum chemical studies were performed. Theoretical
calculations by using graphene sheet models of one or double carbon vacancies were applied to study
the activation of H2 on the defective graphene surface. Prior to these calculations, preliminary
calculations were carried out to establish the possible influence of the number of C atoms in the model
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on the geometry. Models of graphene sheets (from 8 × 8 up to 4 × 4 carbons were used) determine
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DOI: 10.1039/C7GC03397D
their geometry. All optimizations of defect-free graphenes afforded similar minimum structures and,
for that reason, the simplest 4 × 4 graphene sheet model was chosen for further studies. This graphene
sheet is characterised by C(sp2)−C(sp2) bond lengths between 1.41 and 1.43 Å, being significantly
shorter near to the graphene edge (ca. 1.38 Å).
Removal of one carbon atom (colored as red in Figure 8) generates a monovacancy that leads
to a metastable graphene in where the carbon vacancy causes a restructuration of the sheet appearing
a 5- and another 9- membered ring, in agreement also with previous DFT calculations.42 In order to
characterize theoretically the model for a carbon vacancy, geometry optimization was performed
using two different calculations. It was estimated that bond lengths and dihedral angles remained
unchanged regardless the different PBE0/6-31G(d) or PBE0/6-31+G(d,p) basis sets used, thus,
increasing the confidence on the calculations with regard to the geometry of the optimized model of
monovacant graphene. The most notable outcome from these calculations is a large C(sp3)−C(sp3)
distance about 1.73 Å calculated for the 5-membered ring appearing as consequence of the carbon
vacancy and asymmetric bond length distribution around the hole of the vacancy displaying values
in the range between 1.35 and 1.49 Å (Figure 11).
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DOI: 10.1039/C7GC03397D
Figure 12. Generation of the carbon vacancy on the 4 × 4 graphene model by removal of the carbon
atom indicated in red in part a. Then, the geometry of the resulting model was optimized at PBE0/6-
31G(d) level of theory (b). Dissociative H2 adsorption at a monovancancy hole presented from two
different perspectives (c and d) calculated at the same level of theory. Peripheral hydrogen atoms are
omitted.
In order to achieve activation of H2 inside the cavity of the monovacant graphene sheet, the
starting structure for the calculation was built by placing H2 molecule inside the vacancy. It was
realized that the distance between the centroid of C(sp3)−C(sp3) and the opposite edge of the vacancy
(2.48 Å, Figure 12) is sufficiently long to accommodate a H2 molecule having a bond length of 0.74
Å [at PBE0/6-31G(d) level of theory] and an estimated total length of 1.04 Å. Then, the system
constituted by the model of a single carbon atom vacancy and a H2 molecule was allowed to relax to
the minimum energy.
After the optimization process, the outcome showed a stable (−2.4 eV) dissociative absorption
of H2 at the vacancy site, affording two new C−H bonds as presented in models c and d of Figure 12.
This estimation is in accordance with previous calculations for other different graphene models that
also concluded the formation of C-H bonds.43 Indeed, the formation of these new C-H bonds provokes
a symmetrical reconstruction of the vacancy hole in which all carbon around the edge own a sp2
character. On the other hand, the H···H bond distance becomes dramatically increased by the
chemisorption from the 0.74 Å length for isolated H2 molecule to 1.94 Å when interacting with the
graphene model with a single carbon atom vacancy, indicating the occurrence of H-H bond
dissociation. Planarity of the model is also disturbed by this H2 chemisorption, two new C−H bonds
appearing above and below the average graphene plane (Figure 12, drawing d).
A different situation emerged when double carbon vacancy on the 4 × 4 graphene model was
considered (Figure 13). Similarly to the monovacancy model, optimization of the geometry in the
divacancy models leads to a reconstruction around the defect resulting in the appearance of three new
rings of 5, 8 and 5 members. The divacancy model continues being completely planar after geometry
optimization, with similar average C(sp2)−C(sp2) bond lengths (ca. 1.43 Å). However, a significant
difference was the C(sp3)−C(sp3) bond lengths that were 1.73 and 1.56 Å for mono- and divacancy
models, respectively. Moreover, an additional stabilization of −0.3 eV was achieved in the
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calculations of the graphene model for divacancy, when a triplet spin multiplicity rather than a singlet
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DOI: 10.1039/C7GC03397D
configuration was considered.
Figure 13. (a, b) Formation of two carbon vacancies in a 4 × 4 graphene model by removal of atoms
indicated in red. The resulting model for divacant graphene was, then, optimized at PBE0/6-31G(d)
level of theory. (c, d) Top and side views of H2 position upon optimization at the same level of theory.
Pheripheral hydrogen atoms are omitted.
One important feature that arises from the geometry optimization for the double vacancy
graphene sheet model is that the distance between the two opposite 5-membered rings edges was 3.27
Å (Figure 13), even larger than that of the graphene model for monovacancy. It is expected that the
dimensions of the hole will play a large role in H2 activation, since the current state of the art on
frustrated Lewis acid-base pairs is based on opposite sites at a short distance commensurate with the
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H2 bond length, but sufficiently long to avoid the collapse of the acid and base sites.44, 45 Therefore,
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DOI: 10.1039/C7GC03397D
it appears that there is narrow distance range in which the H2 molecule can be activated. Similarly
here, vacancies of adequate dimensions should be necessary to produce efficient H2 activation.
In view of these considerations, a H2 molecule was initially placed at the center inside the hole
defined by the divacancy, parallel to the graphene plane, before starting the optimization of the
system. Upon optimization, the results show a relocation of H2 molecule ending up aligned at the top
of the vacancy hole (Figure 13) at an optimal distance RH2···centroid of 3.10 Å.46 In this case, stabilization
involves a weak binding energy of −0.1 eV (∆Ebind = −1.2 kcal mol-1), such value being in the range
expected for van der Waals interactions. Even though the H2 molecule was not bonded to graphene,
the non-covalent contact caused a graphene plane distortion from ϕ = 179.4º to 1.1º. With regard to
the H−H bond length, RH-H, after optimization of the system, it was calculated that it remains
unchanged (0.74 Å) respect the value corresponding to isolated H2 molecules. However, a significant
polarization of this H2 bond as consequence of the interaction with the graphene sheet was the most
notable outcome of the calculation. Thus, upon optimization at the PBE0/6-31G(d) level of theory a
slight charge density Hδ+−Hδ- (Mulliken charge: +0.07 and -0.012, respectively), as well as a dipole
moment (µ = 0.3 D) through the double vacancy hole were calculated, as consequence of the
interaction with the surface of the divacant graphene model. Obviously, this bond polarization does
not occur in gas-phase H2 molecules and should be considered as H2 preactivation.
Therefore, the outcome of the present calculations with models of graphene sheets with one or
two carbon atom vacancies indicate that there is a considerable restructuring of the carbon atoms
around the vacancies and that this rearrangement notably affects the interaction with H2. One carbon
atom vacancy appears to interact too strongly with H2 as consequence of the better match between
distances in the vacancy hole and the size of the H2 molecule, but this can result in the creation of C-
H bonds that will be undesirable from the catalytic point of view, if their formation were not
reversible. Moreover, no spectroscopic evidence for the creation of these C-H bonds has been
obtained by DRIFT spectroscopy. However, although the interaction of double vacancy with H2
molecules is weaker due to its larger dimensions and no H2 bond length increase is predicted,
adsorption on the divacancy would lead to polarization of the H atoms that can result in some H2
preactivation towards hydrogenation of C=C bonds. However, the small adsorption energy and slight
polarization appear to be still too weak to cause activation of H2 molecules. Therefore, it seems that
the exact nature of the active sites for H2 activation still remains open and it could probably involve
the presence of some heteroatom at the site.
General assessment
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Previous studied combining DFT calculations and H/D isotopic exchange demonstrated that
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DOI: 10.1039/C7GC03397D
H2 activation can take place at carbon atom vacancies, the activation energy depending on the way in
which H2 approaches the site and the steric encumbrance for the allocation of C-H bonds.47 In the
case of GO acidic properties were dominant and, on this basis, these materials can catalyze transfer
hydrogenation reactions.48 For the catalysts prepared by the hydrogen plasma treated GO two kinds
of active sites are generated, as also confirmed by XPS, DRIFTs, H2 chemisorption and TPD-H2.
They correspond to a combination of network vacancies responsible for the hydrogenation reactions,
and functionalities like OH and COOH produced via a disproportionation process, responsible for the
acidic behavior. DFT calculations made in this study gave theoretical support to these
characterization. On this basis the hydrogenation activity can be attributed to the network vacancies,
while the isomerization to the generated acidic sites.
Conclusions.
Precedents in the literature have shown that hydrogen plasma treatment is a useful technique
to produce carbon vacancies and holes on graphene sheets. The purpose of the present study was to
show that the generation of these holes can increase and tune the catalytic activity of graphene as
metal-free hydrogenation catalyst, a reaction of large importance from the synthetic and industrial
point of view. The characterization data obtained in the present study agrees with the expected
creation of defects and holes by plasma irradiation of graphene and, in accordance with our initial
hypothesis, the resulting treated graphenes exhibit higher hydrogen adsorption capacity and higher
hydrogenation activity. Notably the observation of some activity typical of bifunctional
hydrogenation/acidic catalysts has been observed for the case of 1-octene hydrogenation by the
appearence of skeletal isomer. Theoretical calculations with models of mono- and divacancies
indicate a large difference in their interaction with H2, calculations with the two models still provide
an unsatisfactory rationalization of the H2 activation observed in catalysis. Thus, it seems that the
issue of the exact structure of the hydrogenation sites remains still open and could probably involve
oxygen atoms. Overall, the present study illustrates the possibility to engineer active sites on graphene
by available treatments, tuning their catalytic activity and the need to combine experimental data and
theory to gain a deep understanding on the catalytic process.
Indeed, the hydrogen plasma treatment emerges as a very green procedure to modify
graphenes. The energetics of this treatment is much favorable to plasma. In the investigated
experiments the maximal consumption was of 2 W. This consumption for the time of the experiments
corresponds to 3.78 oC that is completely insignificant. Noteworthy, to produce some non-selective
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chemical changes in these graphenes in the way have been produced by plasma, temperatures higher
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DOI: 10.1039/C7GC03397D
than 500 oC are necessary.
Acknowledgements
Financial support by the Spanish Ministry of Economy and Competitiveness (Severo Ochoa,
CTQ2015-69563-CO2-R1 and Grapas) is gratefully acknowledged. AP thanks the Ministry for a
Ramón y Cajal research associate contract. AFG thank the Center of Supercomputing of Galicia
(CESGA) for the computational facilities. MM acknowledges financial support from the
PN 16 47 01 04 project. VIP kindly acknowledges UEFISCDI for financial support (project PN-III-
P4-ID-PCE-2016-0146, Nr. 121/2017).
Supporting information.
Scheme of the plasma reactor used for treating graphene; Plots correlating H2 adsorption data and
XPS parameters; Thermoprogrammed desorption plots for H2 and NH3; XPS C-O/C=O ratio; TEM
images, Summary of the binding energy and intensity of the main components.
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