Pyrrolic nitrogen-doped carbon nanotubes: Physicochemical properties, interactions with Pd and their role in the selective hydrogenation of nitrobenzophenone

Article abstract of DOI:10.1039/c4ra12523a

Nitrogen-doped carbon nanotubes (N-CNTs) containing 63%, 73% and 80% pyrrolic-N were synthesized and used to evaluate the influence of pyrrolic nitrogen on the physicochemical properties and catalytic activity of Pd supported on N-CNTs (Pd/N-CNTs). Micrographs of Pd/N-CNTs showed that Pd was located along the defect sites of N-CNTs indicating strong Pd-support interactions. X-ray photoelectron spectroscopy revealed that the abundance of Pd⁰ decreased while that of Pd²⁺ increased as the quantity of pyrrolic nitrogen increased. The Pd²⁺ species were formed as Pd-N coordination complexes, which stabilized Pd²⁺ nanoparticles. Selective hydrogenation of nitrobenzophenone to aminobenzophenone or p-benzylaniline was used to evaluate the catalytic performance of catalysts. Pd/N-CNTs exhibited a higher selectivity towards aminobenzophenone than Pd on carbon nanotubes and Pd on activated carbon. The enhanced selectivity towards nitro-reduction alone, observed with Pd/N-CNTs was attributed to the promoting effect of pyrrolic-N. Hence, Pd/N-CNTs are promising catalysts for the selective reduction of nitro arenes.

Full text of DOI:10.1039/c4ra12523a

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Pyrrolic nitrogen-doped carbon nanotubes: Physicochemical properties, interactions with
Pd and their role in the selective hydrogenation of nitrobenzophenone
Lucy M. Ombaka, Patrick G. Ndungu and Vincent O. Nyamori*
School of Chemistry and Physics, University of KwaZuluꢀNatal, Westville Campus, Private
Bag X54001, Durban, 4000, South Africa
*
Corresponding author. Telephone: +27ꢀ31 260 8256; Fax: +27ꢀ31 260 3091
Eꢀmail address: nyamori@ukzn.ac.za (V. O. Nyamori)
Abstract
Nitrogenꢀdoped carbon nanotubes (NꢀCNTs) containing 63%, 73% and 80% pyrrolicꢀN
were synthesized and used to evaluate the influence of pyrrolic nitrogen on the physicochemical
properties and catalytic activity of Pd supported on NꢀCNTs (Pd/NꢀCNTs). Micrographs of
Pd/NꢀCNTs showed that Pd was located along the defect sites of NꢀCNTs indicating strong Pdꢀ
0
support interactions. Xꢀray photoelectron spectroscopy revealed that the abundance of Pd
2
+
2+
decreased while that of Pd increased as the quantity of pyrrolic nitrogen increased. The Pd
2
+
species were formed as PdꢀN coordination complexes, which stabilized Pd nanoparticles.
Selective hydrogenation of nitrobenzophenone to aminobenzophenone or pꢀbenzylaniline was
used to evaluate the catalytic performance of catalysts. Pd/NꢀCNTs exhibited a higher
selectivity towards aminobenzophenone than Pd on carbon nanotubes and Pd on activated
carbon. The enhanced selectivity towards nitroꢀreduction alone, observed with Pd/NꢀCNTs was
attributed to the promoting effect of pyrrolicꢀN. Hence, Pd/NꢀCNTs are promising catalysts for
the selective reduction of nitro arenes.
1

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1. Introduction
Graphene and carbon nanotubes (CNTs) are carbonaceous materials exhibiting unique
1
, 2
properties, which make them attractive materials for use as metal catalyst supports.
These
3
, 4
3, 5
6, 7
properties include, high mechanical strength, good chemical stability, high surface area,
8
, 9
notable electronic interactions with metal catalysts such as Pt and Ru
and a mesoporous
1
0
11
structure which facilitates favourable reactantꢀproduct mass transport. Due to these unique
properties, CNTs and graphene are classified together with other smart carbonaceous supports
1
2ꢀ14
such as graphene oxides and graphyne, which show novel properties as catalyst supports.
3
When used as metal catalyst supports, CNTs show good mechanical and chemical stability,
15
16
metal dispersion, higher substrate conversion, and an improved regioꢀ and stereoselectivity
compared with traditional supports. This in turn has economic benefits since yields are
generally increased and byꢀproducts or waste is reduced thus upholding green chemistry
principles. Nonetheless, activated carbon (AC) is still a common carbonaceous material used as
a catalyst support in the synthesis of industrial chemicals owing to its high surface area.
1
7
However, AC suffers drawbacks such as faster deactivation rates, poor metalꢀsupport
electronic interactions, lower thermal and mechanical stability, metal catalyst agglomeration
1
8
and leaching. Thus, CNTs, which exhibit better support properties, are potentially the next
generation of preferred carbonaceous supports.
Recent reports indicate that CNTs exhibit superior catalytic activity in industrial
19
2
applications such as hydrogenation and dehydrogenation reactions. To improve the activity of
CNTs, heteroatomꢀcontaining functional groups can be introduced onto the surface of CNTs.
Heteroatoms such as nitrogen, oxygen, phosphorous, sulphur and boron can be used to modify
2
the surface chemistry of CNTs. Incorporation of nitrogen functionalities into the graphene
layers of CNTs to form nitrogenꢀdoped CNTs (NꢀCNTs) improves their utility in catalytic
2
0
reactions. For example, the presence of basic functional groups such as pyrrolic and pyridinic
2
21
nitrogen in NꢀCNTs can enhance the chemical and electronic properties of NꢀCNTs. The
improved chemical and electrical properties of NꢀCNTs result in activation of the metal
22
23
catalyst and enhancement of the binding energy between NꢀCNTs and metal catalysts.
Doping of CNTs with nitrogen also improves the physical properties of CNTs by introducing
surface defects into the graphene structure of NꢀCNTs. Such surface defects promote NꢀCNTs
24
surface wetting resulting in increased catalytic activity.
Depending on the procedure used to synthesize NꢀCNTs, various nitrogen species such as
pyridineꢀlike, pyridineꢀNꢀoxide, pyrroleꢀlike, carbonitrile, graphitic/quaternary nitrogen and
2

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lactams can be present in NꢀCNTs (Fig. 1). The different nitrogen species influence the catalytic
activity of NꢀCNTs in diverse ways. For instance, pyridinic nitrogens were reported to enhance
2
5
the catalytic activity of Pd supported on NꢀCNTs. However, the selective doping of NꢀCNTs
with a single nitrogen species to facilitate structureꢀactivity relationship studies remains underꢀ
researched.
Lactam
H
N
H
O
N
Pyrrolic
H
OH Phenol
N
N
Pyridinic
N
NC
Carbonitrile
O
C
Graphitic
N
O
C
O
Carboxylic anhydride
O
Quinone
HOOC
Carboxylic acid
N+
O
Oꢀ
Ether (pyrone)
N
Pyridinic oxide
O
O
Pyrone
Ether
O
OHC
Aldehyde
O
O
Ester (Lactone)
Fig. 1 Graphical representation of possible nitrogen and oxygen functional groups present in Nꢀ
CNTs.
Oxygen functional groups can be added to the surface of NꢀCNTs via inꢀsitu or exꢀsitu
2
treatment of NꢀCNTs with oxygen containing organic or inorganic compounds. Oxygen
functional groups such as carboxyl, carbonyl, lactone and quinone (Fig. 1) enhance the
wettability of NꢀCNTs, further improving the catalytic performance of NꢀCNTs. The oxygen
functional groups also act as anchoring sites for well dispersed metal nanoparticles supported on
NꢀCNTs. Thus, compared with CNTs, oxygenꢀfunctionalized NꢀCNTs exhibit enhanced metal
nanoparticle properties such as better stability, smaller particle size, better dispersion and
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2
activation, which results in higher catalytic activity and selectivity. As a result, oxygenꢀ
functionalised NꢀCNTs are potentially smart materials for application as catalysts or as metal
catalyst supports used in catalytic reactions.
1
9
The catalytic activity of CNTs in hydrogenation reactions has been considerably studied.
However, limited studies report the catalytic activity of NꢀCNTs in general, and more
specifically pyrrolic containing NꢀCNTs in the selective hydrogenation of nitroꢀarenes.
Selective hydrogenation of nitroꢀarenes to aminoꢀarenes is of immense industrial importance as
aminoꢀarenes are used in pharmaceuticals and in the production of fine chemicals.
Aminobenzophenone is an example of an aminoꢀarene which is industrially important as it is
2
6
used as an intermediate in the synthesis of Schiff bases, and in nonlinear optical materials
2
7, 28
generating blue lasers.
pꢀBenzylaniline is another aminoꢀarene that has the potential to be
used as an intermediate for the synthesis of other Schiff bases. In this study, the selective
hydrogenation of nitrobenzophenone (NBP) to aminobenzophenone was chosen as a model
reaction. This is because NBP contains two reducible functional groups (NO
2
and CO), which
allows for the evaluation of catalysts’ selectivity towards reduction of the nitro group alone.
2
9
Ferrocene is commonly used as a catalyst in the CVD synthesis of NꢀCNTs. To modulate
the level of nitrogenꢀdoping and the nitrogen species incorporated into NꢀCNTs, nitrogenꢀ
30
containing ferrocenyl derivatives can be used as catalysts instead of ferrocene. In this study,
the effect of using 3ꢀferrocenylꢀ2ꢀ(4ꢀcyanophenyl)acrylonitrile as a catalyst for the introduction
of nitrogen species into NꢀCNTs was evaluated. We report the selective synthesis of pyrrolic Nꢀ
CNTs and their physicochemical properties. The influence of pyrrolic nitrogen on the chemical
properties of Pd nanoparticles supported on the pyrrolic NꢀCNTs is also reported. Additionally,
we investigated the role of pyrroleꢀlike nitrogen in the selective hydrogenation of
nitrobenzophenone to aminobenzophenone over Pd supported on NꢀCNTs. For comparison
purposes, the catalytic activity of Pd supported on CNTs and also on AC was evaluated under
similar reactions.
2.
Experimental
2.1
Materials and instrumentation
The reagents 4ꢀnitrobenzophenone (99%), 4ꢀaminobenzophenone (97%) and diethyl ether
(
99.8%) were purchased from Sigma Aldrich, Germany. Palladium acetylacetonate {Pd(acac)2}
synthesis grade) was purchased from Merck, Germany. Pd on activated carbon (5 wt. %) was
(
purchased from SigmaꢀAldrich, Germany. Toluene (99.4%) was purchased from LiChroSolv,
4

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Germany. All reagents and solvents were of analytical grade and used as received from the
suppliers. Pd was loaded onto NꢀCNTs and CNTs in a cylindrical stainless steel reaction
®
chamber (140 mm x 10 mm) sealed at one end with a Swagelok fitting nut. To monitor the
partial pressure used during synthesis of the catalyst, a Thyracont VD84/1 Pirani vacuum gauge
®
was used. Hydrogenation of 4ꢀnitrobenzophenone was conducted in a Parr Instrument Co.
4848 reactor. The catalyst was separated from the reactants by first filtering the mixture through
Whatman ashless, No. 42 filter paper purchased from SigmaꢀAldrich Germany, followed by
filtration through a PVDF 0.45 ꢁm (GVS) membrane syringe filters purchased from Lasec, SA.
2.2
Synthesis of N-CNTs and CNTs
NꢀCNTs were synthesized by use of a chemical vapour deposition (CVD) method similar to
31
32
that reported by Oosthuizen et al. and Koch et al. Briefly, the compound 3ꢀferrocenylꢀ2ꢀ(4ꢀ
cyanophenyl)ꢀacrylonitrile that we previously synthesized and characterized (Supplementary
Information S1), was used as a catalyst, and it was synthesized following the protocol outlined
33
34
by Imrie et al. and Ombaka et al. Acetonitrile was used as a carbon and nitrogen source,
while ethylbenzoate was used as a source of oxygen. The solution used to synthesize NꢀCNTs
was prepared by dissolving 0.25 g of the catalyst in 9.75 g of acetonitrile to make a total of 10 g
of solution (i.e. 2.5 wt. % of catalyst). To introduce 1 wt. % and 2 wt. % of oxygen into the
synthesis precursors, 0.5 and 1.0 g of ethylbenzoate was added to 0.25 g of the catalyst
respectively. Each solution was then prepared to a total mass of 10 g with acetonitrile. The
solution of precursors was injected into a quartz tube (placed inside a tube furnace) at a flow
ꢀ1
rate of 0.8 mL min with a syringe pump. The injected precursors were carried through the
quartz tube by a carrier gas madeꢀup of 10% hydrogen in argon (v/v) which was pumped
ꢀ1
through the system at a rate of 100 mL min and a pressure of 80 kPa. The furnace was set to a
reaction temperature of 850 °C for 30 minutes.
3
1
CNTs were also synthesized by a CVD method as previously detailed by Oosthuizen et al.
For synthesis of CNTs, ferrocene was used as the catalyst while toluene was used as the carbon
source. The solution used to synthesize CNTs was made by dissolving 0.25 g of ferrocene in
9.75 g of toluene to make 10 g of solution. The CNTs were synthesized at a temperature of 850
°C for 30 minutes by use of similar synthesis conditions as those outlined for NꢀCNTs.
The NꢀCNTs and CNTs were collected from the hot zone of the quartz tube upon
completion of the reaction. All products were purified by first calcining the samples at 300 °C
for 3 hours, followed by ultasonication of the samples in 6 M nitric acid for 40 minutes at room
temperature. The sonicated mixture was then refluxed at 100 °C for 24 hours at a constant
5

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stirring rate of 300 rpm. After refluxing, the acid was neutralized with 3 M NaOH and the
mixture sonicated for 40 minutes. Afterwards, the NꢀCNTs or CNTs were separated from the
mixture via filtration and washed with deionized water until a neutral pH was obtained.
2.3
Synthesis of catalysts
Synthesis of the catalysts was achieved by loading Pd onto acid treated NꢀCNTs and CNTs
to yield Pd/NꢀCNTs and Pd/CNTs respectively. Loading of 5 wt. % Pd onto each support was
35
achieved via a metal organicꢀCVD (MOCVD) method as outlined by Suttisawat et al. In
detail, 0.072 g of Pd(acac) was mixed with 0.475 g of acid treated NꢀCNTs or CNTs and
2
ground thoroughly by using a pestle and mortar. The resulting mixture was placed inside a
stainless steel MOCVD reactor and the reactor components sealed. The sealed MOCVD reactor
ꢀ2
was connected to a vacuum pump maintained at a partial pressure of 2.2 x 10 mbar for 45
minutes. After evacuating the MOCVD reactor for 45 minutes, it was placed in the middle of a
muffle furnace operated at 120 °C for 30 minutes. This was followed by increasing the
ꢀ1
temperature to 300 °C at a rate of 2 °C min and thereafter maintaining it at 300 °C for 45
minutes. The system was then allowed to cool to ambient temperature while still under vacuum.
Upon completion of the reaction, the formed catalysts were removed from the reactor by
scraping with a spatula, and then characterized as outlined in Section 2.5.
2.4
Catalytic tests
Hydrogenation of nitrobenzophenone to aminobenzophenone was conducted in a closed
vessel reactor by using a mole ratio of 1: 66 (Pd: nitrobenzophenone). For all catalytic tests,
80.0 mg of the catalyst was put into a Parr reactor stainless steel vessel and mixed with 1250
1
mg of 4ꢀnitrobenzophenone and 100 mL of dry toluene. The reactor vessel and its contents was
sealed and then stirred continuously under hydrogen gas at the desired reaction conditions. A
control experiment without any catalyst was performed under similar conditions. To monitor the
reaction progress, aliquots of 1 mL of the reactant mixture were collected from the reaction
vessel at intervals of 2 hours, quenched with 4 mL of toluene, filtered as described in Section
2.1 and then analysed offꢀline by GCꢀFID or GCꢀMS. Upon completion of the reaction, the
solution in the reaction vessel was cooled to ambient temperature, filtered as described in
Section 2.1 and the filtrate kept in a sealed container for further analysis. The used catalyst was
obtained as a filtration residue, washed with 100 mL of diethylether, dried at 100 °C overnight
in an oven, and then kept in a sealed container for Xꢀray diffraction analysis. The conversion of
6

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4ꢀnitrobenzophenone (NBP) and selectivity to 4ꢀaminobenzophenone (ABP) was calculated by
using equations A and B, respectively.
Conversion of NBP =ꢀInitialꢀ
ꢁ
NBP
ꢂ
−measuredꢀꢁNBPꢂ
equation (A)
equation (B)
InitialꢀꢁNBPꢂ
ꢃꢄꢅꢆꢇꢈꢄꢉꢀꢁꢊꢋꢌꢂ
Selectivity to ABP =
ꢀ
ꢍꢎꢏꢐꢏꢅꢑꢀꢁꢒꢋꢌꢂꢓꢃꢄꢅꢆꢇꢈꢄꢉꢀꢁꢒꢋꢌꢂ
where, initial [NBP] is the initial concentration of NBP before starting the reaction, measured
NBP] and [ABP] are the concentrations of NBP and ABP at various time intervals as obtained
from GC analysis.
[
2.5
Characterization
Images of NꢀCNTs, CNTs and all catalysts were taken by using a transmission electron
microscope (TEM) (JEOL JEM 1010) and a scanning electron microscope (SEM) (JEOL JSM
100). A high resolutionꢀTEM (JEOL 2100) was used to take higher magnification images. The
6
thermalstability of NꢀCNTs and CNTs was determined by using a Q Series™ Thermal Analyzer
DSC/TGA (Q600). Xꢀray photoelectron spectroscopy (XPS) analysis was performed with a
KRATOS AXIS Ultra DLD equipped with Al Kα (1486 eV) Xꢀrays, an Xꢀray Power of 20 W
and a beam diameter of 100 ꢁm. Fourier transform infrared spectra (FTIR) of supports and
catalysts embedded into KBr pellets were recorded on a Perkin Elmer spectrum RX1 FTIR
2
spectrometer. Temperature programmed reduction (TPR) analysis was conducted in 5% H in
argon by using a Micromeritics Autochem II Chemisorption Analyzer (2920). The graphitic
nature of NꢀCNTs and CNTs was determined by use of a Raman spectrometer (DeltaNu
Advantage 532™). XRD spectra were obtained from a Bruker D8 Advance Xꢀray
diffractometer equipped with a graphite monochromatized highꢀintensity Cu K radiation (λ =
α
0.15406 nm). The Pdꢀloading content was determined by inductively coupled plasmaꢀoptical
emission spectroscopy (ICPꢀOES) (Perkin Elmer Optima 5300 DV). Details for ICPꢀOES
analysis are provided in the Supplementary Information S1. The surface area and nitrogen
adsorptionꢀdesorption isotherms of the catalysts were determined by using a Micromeritics
Tristar II surface area and porosity analyser. The hydrogenation products were characterized by
using GCꢀFID (Shimadzu 2010 gas chromatograph) and GCꢀMS (Shimadzu GCMSꢀQP2010 SE
gas chromatographꢀmass spectrometer). The conditions under which the GCꢀFID and GCꢀMS
analysis were performed are provided in Supplementary Information S1.
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3
3
3
.
Results and discussion
.1
Characterization of N-CNTs and CNTs
.1.1 Morphology of N-CNTs and CNTs
The NꢀCNTs synthesized by using 0, 1 and 2 wt. % oxygen in acetonitrile were coded as Nꢀ
CNTsꢀ0, NꢀCNTsꢀ1 and NꢀCNTsꢀ2 respectively (Fig. 2). The images of NꢀCNTs (Fig. 2B and
3
6
c) showed the presence of bamboo compartments associated with nitrogenꢀdoping of CNTs.
Similar bamboo compartments were absent in the image of CNTs synthesized from ferrocene
and toluene (Fig. 2A). From the TEM images, pristine CNTs exhibited a straight morphology
(Fig. 2A), while NꢀCNTs exhibited a kinked morphology consisting of curved and bent Nꢀ
CNTs (Fig. 2B and C). The observed kinked morphology is caused by introduction of
3
7
pentagonal and heptagonal structures into the graphene layers of NꢀCNTs. Such pentagonal
and heptagonal structures can be induced by pyrrolic and pyridinic nitrogenꢀdoping of NꢀCNTs.
Images of NꢀCNTs synthesized using oxygen showed an increased density of kinked NꢀCNTs,
which is indicative of an increase in pyrrolicꢀN incorporation into NꢀCNTs as discussed later in
Section 3.2.
(A)
(B)
Bamboo
Compartment
(C)
Kinked
Morphology
2
00 nm
200 nm
200 nm
Fig. 2 TEM images of (A) CNTs, (B) NꢀCNTsꢀ0 and (C) NꢀCNTsꢀ2.
All NꢀCNTs exhibited a reduced outer diameters (OD) compared with those of CNTs (Fig. 3).
The reduced OD can be explained by the fact that, nitrogen species dominate tube edges
3
6
impeding further formation of graphitic layers and resulting in closure of the tube. The OD of
NꢀCNTsꢀ1 and NꢀCNTsꢀ2 were observed to be smaller than that of NꢀCNTsꢀ0. Since NꢀCNTsꢀ1
and NꢀCNTsꢀ2 were synthesized using 1 and 2 wt. % oxygen, the further reduction of their OD
can be ascribed to the etching effect of oxygen resulting in thinner tube walls and smaller OD.
All NꢀCNTs exhibited larger inner diameters (ID) compared with those of CNTs (Fig. 3).
This can be attributed to the formation of larger iron catalyst nanoparticles (NPs) in NꢀCNTs
compared to those formed in CNTs during synthesis. It is possible that, the heteroatomꢀ
8

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containing ferrocenyl derivative used to synthesize NꢀCNTs predominantly forms larger iron
38
catalyst NPs compared to those formed by ferrocene which was used to synthesize CNTs.
Another possible explanation is that the presence of nitrogen and oxygen heteroatoms during
39
synthesis facilities formation of larger iron catalyst NPs. The presence of bigger catalyst NPs
in NꢀCNTs can be related to the increase in residual iron content (CNTs < NꢀCNTsꢀ0 < Nꢀ
CNTsꢀ1 < NꢀCNTsꢀ2) evidenced from the (thermalgravimetric analysis) TGA thermograms of
asꢀsynthesized samples (Fig. 4A). The bigger catalyst particles could have favoured formation
of larger inner diameters.
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
Outer diameter (OD)
Inner diameter (ID)
CNTs
N-CNTs-0 N-CNTs-1 N-CNTs-2
Supports
Fig. 3 Outer diameter and inner diameter of NꢀCNTs and CNTs; for each sample at least 80
tubes were measured.
3.1.2 Thermalstability of N-CNTs and CNTs
The thermal stability and purity of NꢀCNTs and CNTs were determined by the use of TGA.
Fig. 4 represents the thermograms and derivative thermograms (DTG) of asꢀsynthesized and
purified NꢀCNTs and CNTs. From the thermograms and DTG, both the asꢀsynthesized and
purified CNTs were more thermally stable than NꢀCNTs. The decreased thermostability of Nꢀ
CNTs can be credited to the presence of defects induced by nitrogenꢀdoping, which reduces the
crystallinity of NꢀCNTs. Likewise, Raman analysis revealed that the crystallinity of NꢀCNTs
was lower than that of CNTs (Supplementary Information S1). The decreased crystallinity
observed with NꢀCNTs was associated with an increase in defect sites induced by nitrogenꢀ
doping. Such defects perpetuate decomposition at lower temperatures resulting in a lower
40
thermal stability of the whole sample.
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All purified NꢀCNTs were less thermally stable than CNTs implying that the purification
procedure did not severely remove the nitrogen present in NꢀCNTs. The thermal stability of
purified NꢀCNTs decreased in the order NꢀCNTsꢀ2 > NꢀCNTsꢀ1 > NꢀCNTsꢀ0. This decrease in
thermal stability was attributed to an increase in pyrroleꢀlike nitrogen. Purification of NꢀCNTs
and CNTs reduced the iron content in all samples to similar quantities (Fig. 4B). A significant
reduction of the peak associated with amorphous carbons at ca. 400 °C, marked by a dotted
circle was achieved after sample purification (cf. Fig. 4C and D).
1
00
100
80
(
A)
(
B)
8
6
4
2
0
0
0
0
0
#
CNTs
#N-CNTs-0
N-CNTs-1
#N-CNTs-2
*
*
*
*
CNTs
60
N-CNTs-0
N-CNTs-1
N-CNTs-2
#
40
20
0
0
200
400
600
800
0
200
400
600
800
Temperature (°C)
Temperature (°C)
1
1
.4
.2
1
#NꢀCNTsꢀ0
1
0
.2
1
*
CNTs
(D)
(C)
#NꢀCNTsꢀ1
#NꢀCNTsꢀ2
#CNTs
*NꢀCNTsꢀ0
*NꢀCNTsꢀ1
*NꢀCNTsꢀ2
.8
0
0
0
0
.8
.6
.4
.2
0
0.6
.4
0
0.2
0
2
00 300 400 500 600 700
2
00 300 400 500 600 700
Temperature (°C)
Temperature (°C)
Fig. 4. Thermograms of (A) asꢀsynthesized and (B) purified CNTs and NꢀCNTs; derivative
thermograms of (C) asꢀsynthesized and (D) purified CNTs and NꢀCNTs. The symbols
hash (#) and asterisk (*) denote the asꢀsynthesized and purified samples respectively.
3.2
Characterization of catalysts
3.2.1 Surface chemistry
The surface chemistry of Pd/NꢀCNTs and Pd/CNTs was analysed by the use of XPS and
FTIR spectroscopy. From the XPS analysis, the atomic nitrogen percentage of NꢀCNTsꢀ0, Nꢀ
1
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CNTsꢀ1 and NꢀCNTsꢀ2 was 4.4%, 3.5% and 2.5% respectively. The decrease of nitrogen
percentage in NꢀCNTs synthesized by using 1 and 2 wt. % of oxygen was due to the reduction
in the quantity of nitrogen in the precursors used to synthesize these samples (Supplementary
Information S2). A similar observation has been reported wherein the nitrogen percentage in Nꢀ
4
1
CNTs reduced as the amount of nitrogen in the synthesis precursor reduced.
The N 1s spectra of all NꢀCNTs showed a broad peak between 399ꢀ405 eV resulting from
an overlap of different peaks (Fig. 5A). Deconvolution of the broad N 1s peak gave three peaks
at 398.5ꢀ398.4, 400.3ꢀ400.6 and 404.1ꢀ404.7 eV (Supplementary Information S3). The peak at
398.4ꢀ398.5 eV was attributed to pyridinic and carbonitrile nitrogen species and it was coded
4
2, 43
N1.
N2.
The peak at 400.3ꢀ400.6 eV was accredited to pyrrole nitrogen species and it was coded
While the peak at 404.1ꢀ404.7 eV was attributed to nitrogen molecules adsorbed or
43ꢀ45
4
6
encapsulated within the NꢀCNTs, it was coded N3.
(B)
(A)
400.1 399
N3
N2
N1
Pd/N-CNTs-0
Pd/N-CNTs-1
Pd/N-CNTs-2
)
(
410
405
400
395
390
Pd/N-CNTs-0
Pd/N-CNTs-1
Catalyst
Pd/N-CNTs-2
Binding Energy(eV)
Fig. 5 (A) N 1s XPS spectra of NꢀCNTs synthesized by using 0ꢀ2 wt. % oxygen in acetonitrile,
and (B) variations in the nitrogen species present in NꢀCNTs synthesized by using 0ꢀ2
wt. % oxygen.
Increasing the amount of oxygen from 0ꢀ2 wt. % during the synthesis of NꢀCNTs, resulted
in an increase in the amount of N2 and a decrease in the amount of N1 and N3 (Fig. 5B). This
could imply that the presence of oxygen favoured formation of N2 at the expense of N1 and N3.
The decrease in N3 can be due to formation of NO
and nitrogen molecules. The NO molecules exit the reaction chamber as exhaust gases. On the
other hand, the decrease in N1 was accredited to an additional reaction induced by the presence
x
molecules via the reaction between oxygen
x
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1

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of oxygen that favoured formation of N2. The possibility of an additional reaction initiated by
oxygen was deduced from evaluating the C 1s and O 1s spectra of NꢀCNTs and CNTs.
Deconvolution of the C 1s spectra of NꢀCNTs and CNTs gave two common carbon species
(C1 and C2) in all NꢀCNTs and CNTs. A third carbon species (C3) was only present in Nꢀ
CNTsꢀ1 and NꢀCNTsꢀ2, while the fourth carbon species (C4) was only present in CNTs and Nꢀ
CNTsꢀ0 (Table 1 and Supplementary Information S3). The lowest energy peak obtained was at
4
2, 47
284.4ꢀ284.7 eV (C1); this was assigned to graphitic carbons.
A second peak was obtained at
4
8, 49
285.5ꢀ285.9 eV (C2); this peak was assigned to CꢀN and CꢀO species.
The third peak was
obtained at 287.3ꢀ287.7 eV (C3); this was attributed to carbonyls in quinone, lactones and
4
9, 50
carboxyl groups.
carbonyls in carboxyl groups.
in the aromatic rings were observed at 291.1ꢀ291.6 eV.
The fourth peak was obtained at 288.2ꢀ288.8 eV (C4); this was ascribed to
4
9, 50
Resonance shakeꢀup satellite peaks caused by πꢀ πꢀtransitions
50
Table 1 Summary of C 1s XPS spectra of Pd/NꢀCNTs and Pd/CNTs.
Sample
C at. %
C1
O at. %
O Total
10.4
C2
C3
ꢀ
C4
8.9
ꢀ
Pd/NꢀCNTsꢀ0
Pd/NꢀCNTsꢀ1
Pd/NꢀCNTsꢀ2
Pd/CNTs
52.7
45.9
42.4
57.4
33.8
28.6
27.3
25.1
21.7
23.5
ꢀ
8.3
ꢀ
13.5
10.0
6.6
All NꢀCNTs exhibited lower quantities of C1 (graphitic carbon) compared with CNTs
implying that, nitrogenꢀdoping reduced the graphitic nature of CNTs. Additionally, the amount
of graphitic carbon in NꢀCNTs further reduced as the percentage of pyrrolic groups increased
(cf. Table 1 and Fig. 5B). Since the quantity of graphitic carbon in NꢀCNTs did not show a
positive correlation with the total nitrogen percentage, the decrease in graphitic carbon was
ascribed to formation of more pyrrole groups. An increase in pyrrolic groups enhanced the
disorder in the NꢀCNTs, accounting for the reduced thermal stability of NꢀCNTsꢀ1 and Nꢀ
CNTsꢀ2 observed from the thermograms and DTG of these samples. Similarly, the decrease in
graphitic carbon resulted in decreased crystallinity of the NꢀCNTs as observed from Raman
51
analysis (Supplementary Information S1).
Based on the C 1s and O 1s spectra of NꢀCNTs, we propose that during the synthesis of Nꢀ
52
CNTsꢀ1 and NꢀCNTsꢀ2 lactones are formed as explained in the Supplementary Information
S4. The formed lactones react with ammonia obtained from the reaction of hydrogen and
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53
acetonitrile to form lactams. The lactams are then converted to pyrrole groups by the removal
4
4
of a ꢀCO group (Scheme 1). As more oxygen is added into the synthesis precursor, more
lactones are formed resulting in an increased formation of pyrrole groups at the expense of
pyridinic and carbonitrile groups. Hence, lactones provided an additional route to that already
present in NꢀCNTsꢀ0, by which pyrrole groups are formed, thus, increasing selectivity towards
pyrrole formation.
O
O
C
O
NH3
C
N
ꢀCO
CX HY
+ OX
N
H
H
lactone
lactam
pyrrole
Scheme 1. Plausible synthetic route for pyrrole formation.
Further identification of the functional groups present in Pd/NꢀCNTs and Pd/CNTs was
accomplished by the use of FTIR. From the FTIR spectra of all samples, a clear distinction was
ꢀ1
observed between the functional groups present in NꢀCNTs and CNTs (Fig. 6). At ≈2373 cm a
5
4,
peak caused by an overlap between carbonitriles and CꢀO stretching vibrations was observed.
5
5
The intensity of this peak was strong in Pd/NꢀCNTs but very weak in Pd/CNTs. Hence, in
ꢀ1
Pd/NꢀCNTs the peak at ≈2373 cm was majorly from carbonitrile groups, further supporting the
presence of carbonitriles in NꢀCNTs as observed from XPS analysis. Since the peak at ≈2373
ꢀ1
cm was weak in Pd/CNTs, it is possible that in Pd/CNTs this peak originated only from CꢀO
stretching vibrations.
ꢀ1
55, 56
Another important peak was that at 1466 cm assigned to C=N in pyridinic groups.
The
intensity of this peak was quite weak in all Pd/NꢀCNTs but completely absent in Pd/CNTs. The
ꢀ1 57, 58
transmittance peak of C=C in the graphene layers of CNTs was observed at 1653 cm ,
ꢀ1 55, 59
while the peak assigned to CꢀN groups was observed at ≈1570 cm .
In Pd/CNTs, the peaks
attributed to CꢀN and carbonitrile groups were absent but they were clearly visible in Pd/Nꢀ
CNTs. The absence of these peaks indicated that no nitrogenꢀdoping was present in Pd/CNTs as
also observed from XPS analysis (Supplementary Information S3). It was interesting to note
that for NꢀCNTsꢀ0 the C=C and CꢀN peaks exhibited almost the same intensity while in Nꢀ
CNTsꢀ1 and NꢀCNTsꢀ2 the intensity of the C=C peak was significantly lower than that of CꢀN.
This was related to the presence of more pyrrole groups in NꢀCNTsꢀ1 and NꢀCNTsꢀ2 compared
with NꢀCNTsꢀ0.
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C-O
C-O(H)
C=N
C-O
C-N
C-O, CN
C=C
C-O
CH2
OH
Pd/N-CNTs-2
Pd/N-CNTs-1
Pd/N-CNTs-0
Pd/CNTs
3
500 3000 2500 2000 1500 1000 500
-
1
W avenumber
(
cm
)
Fig. 6 FTIR spectra of Pd/CNTs and Pd/NꢀCNTs.
ꢀ1
60, 61
The peak at 3420 cm , attributed to hydrogen bonded –OH groups,
was present in all
ꢀ1
61
samples. Peaks at 1393 and 1258 cm allotted to stretching of the CꢀO and C=C in the tubes
ꢀ1
were stronger in Pd/CNTs than in Pd/NꢀCNTs. While the peaks at 1089 and 790 cm , assigned
57, 58
to CꢀO vibrations and stretching modes,
were only present in Pd/CNTs. The carbonyl peak
ꢀ1
57, 58
appearing at 1706 cm was very weak in Pd/CNTs but slightly stronger in Pd/NꢀCNTs.
This could imply that the hydrogen bonded –OH groups in Pd/CNTs were mainly from phenol
groups and adsorbed water, while the –OH groups in Pd/NꢀCNTs are mainly due to carboxylic
ꢀ1
groups and adsorbed water. The peak at 2937 cm , assigned to CH groups, was present in all
2
55, 57
samples.
3.2.2 Textural properties
The surface area and pore size of CNTs and all NꢀCNTs increased after acid treatment
(Supplementary Information S4). This implied that the acid treatment effectively diꢀbundled the
tubes and opened their closed ends. The textural properties of Pd/NꢀCNTs and Pd/CNTs are
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presented in Table 2. Doping of CNTs with 4.4 at. % nitrogen containing 63% pyrrolic groups
2
ꢀ1
increased the surface area of Pd/CNTs from 52 to 55 m g . The surface area of Pd/NꢀCNTs
2
ꢀ1
further increased to 89 m g as the nitrogenꢀcontent dropped to 3.5 at. % and the percentage of
pyrrolic groups increased to 73%. A further drop in the nitrogen content to 2.5 at. %
accompanied by an increase in the pyrrole content to 80% decreased the surface area of Pd/Nꢀ
2
ꢀ1
CNTs from 89 to 38 m g . Thus, no clear relationship was observed between surface area and
the nitrogenꢀdoping levels as Pd/CNTs had a higher surface area than Pd/NꢀCNTsꢀ2. Hence, the
cause of surface area variation remained unclear.
Table 2 Textural properties of Pd/CNTs and Pd/NꢀCNTs.
Pyrrole
composition (%)
Surface area of Pore size
catalyst (m g ) distribution (nm)
Catalyst
at. % N
2 ꢀ1
Pd/CNTs
0
0
52
55
89
38
4.1
4.5
4.1
4.1
Pd/NꢀCNTsꢀ0
Pd/NꢀCNTsꢀ1
Pd/NꢀCNTs ꢀ2
4.4
3.5
2.5
63
73
80
All supports were classified as type IV mesoporous materials, exhibiting a hysteresis loop
6
2
between 0.5ꢀ1.0 P/P
0
(Fig. 7, Supplementary Information S4). Pd/CNTs and Pd/NꢀCNTsꢀ0
which appeared very similar to a H type
hysteresis loop. Such a hysteresis loop is intrinsic of independent regular cylindrical and
showed a hysteresis loop between 0.7ꢀ1.0 P/P
0
1
6
3
64
spherical shaped pores. In addition, Pd/NꢀCNTsꢀ0 showed a more open hysteresis loop than
Pd/CNTs (Supplementary Information S4), indicating that the pore size of CNTs increased upon
doping with nitrogen (Table 2). The increase in pore size observed with Pd/NꢀCNTsꢀ0 was
associated with the increase of ID observed from TEM analysis (Fig. 3).
Pd/NꢀCNTsꢀ1 and Pd/NꢀCNTsꢀ2 showed an almost horizontal hysteresis loop between 0.5ꢀ
63
0
.9 P/P with a cavitation at ca. 0.5 P/P that is associated with a H type hysteresis loop. The
0 0 3
H
3
type hysteresis loop could be due to disordered pores forming the network of a porous
6
5
matrix, or large pores between entangled nanotubes. The disordered pores could be due to
extensive structural disorders dominated with pyrrole groups, which are five membered rings
that distort the graphene structure of CNTs. Alternatively, the disordered pores could result from
oxygen etching some of the formed regular shaped pores, making them irregularly shaped.
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3
3
2
2
1
1
50
(B)
(
b)
Pd/N-CNTs-1
2
1
1
00
50
00
Pd/CNTs
(A)
00
50
00
50
00
Cavitation
5
0
0
5
0
0
0
.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/Po)
Relative Pressure (P/Po)
2
Fig. 7 Representative N adsorption–desorption isotherms of Pd/CNTs and Pd/NꢀCNTs.
3.2.3 Interactions of pyrrole N-CNTs with Pd nanoparticles
The influence of pyrrolic nitrogen on the interactions between NꢀCNTs and Pd NPs was
inferred from XPS, FTIR, TPR, TEM and XRD analysis. The Pd 3d XPS spectra of Pd/NꢀCNTs
and Pd/CNTs exhibited four overlapping peaks (Fig. 8). Deconvolution of these peaks
0
2+
(
Supplementary Information S3) revealed the presence of Pd and Pd species as summarized
in Table 3.
0
Pd 3d
3
/2
0
Pd 3d
2
+
2+
5
/2
Pd 3d
Pd 3d
3
/2
5/2
Pd/CNTs
Pd/N-CNTs-2
Pd/N-CNTs-1
Pd/N-CNTs-0
3
48
344
340
336
332
Binding Energy (eV)
Fig. 8. Pd 3d XPS spectra of Pd/CNTs and Pd/NꢀCNTs synthesized by using 0ꢀ2% oxygen
in acetonitrile.
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Table 3 Pd species and atomic % of Pd in NꢀCNTs and CNTs.
Pd 3d5/2
0Pd 3d3/2
0
2+
2+
Pd
BE (eV) at%
Pd
BE (eV) at%
Pd
Pd
Sample
BE (eV)
340.8
340.8
340.3
340.9
at%
24.8
23.1
11.3
BE (eV) at%
342.8 18.1
342.8 22.7
342.4 29.3
Pd/NꢀCNTsꢀ0 335.6
Pd/NꢀCNTsꢀ1 335.4
Pd/NCNTsꢀ2 335.2
36.5 337.7
20.5
34.1
45.2
11.5
20.0 337.3
8.16 337.0
44.11 337.5
Pd/CNTs
335.6
44.11 342.9 10.62
0
As shown in Table 3 the binding energies at ca. 335.6 and 340.8 eV were assigned to Pd
47
2+
66ꢀ69
2+
species while, those at ca. 337.7 and 342.8 eV were assigned to Pd species.
The Pd
0
species in Pd/CNTs could be PdO formed via an oxidation reaction between Pd and the surface
2
+
oxygen groups in CNTs. However, the Pd species in NꢀCNTs were not entirely from PdO as
2+
no linear correlation was observed between the oxygen percentage and the percentage of Pd
(cf. Table 1 and 3). It was noted that as the amount of pyrrolic nitrogen increased from 0 (in
0
2+
CNTs) to 80% (in NꢀCNTsꢀ2), the quantity of Pd reduced while that of Pd increased (Table 3
0
and Fig. 8). This indicated that as the percentage of pyrrolic nitrogen increased, Pd species
2
derived from Pd(acac) were progressively being bonded to the pyrrolic nitrogen to form PdꢀN
complexes. This interaction could occur during synthesis of the catalyst following the reaction
0
depicted in Scheme 2. Such a reaction could be due to the interaction of Pd with neighbouring
66, 70
2+
pyrrolic cations.
The covalently bonded Pd could be stable, preventing further reduction
2
+
0
of Pd to Pd during synthesis.
Pd0
+
Pd2+
NH
NH
Pyrrole
2
2
Scheme 2. Possible interactions between pyrrolic nitrogens and Pd nanoparticles.
To probe further, the existence of an interaction between Pd NPs and NꢀCNTs, the FTIR
spectrum of NꢀCNTsꢀ2 was compared with that of Pd/NꢀCNTsꢀ2 (Fig. 9A). In NꢀCNTsꢀ2 the
ꢀ1
peak assigned to CꢀN groups appeared at ca. 1567 cm , while in Pd/NꢀCNTsꢀ2 this peak
ꢀ1 59
appears at around 1552 cm . The shift of the CꢀN peak to lower wavenumbers in the presence
70, 71
of Pd NPs is indicative of interactions between pyrrolic nitrogen and Pd NPs.
The interaction between Pd NPs and the surface of different supports was also studied by
using TPR and the profiles obtained are presented in Fig. 9B. From the TPR profiles, a broad
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7

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2+
peak was observed between 60ꢀ150 °C; this peak was associated with reduction of Pd species,
7
2
2+
possibly in PdO. The Pd reduction peak was observed to gradually broaden towards higher
temperatures as the quantity of pyrrolic groups increased in NꢀCNTsꢀ0 and NꢀCNTsꢀ2. This
shift towards higher temperatures is indicative of formation of stronger metalꢀsupport
2
+
interactions as the number of pyrrolic nitrogen groups increased. However, the Pd reduction
peak of Pd/NꢀCNTsꢀ1 occurred at a lower temperature than expected.
(
B)
(
A)
Pd/N-CNTS-2
Pd/N-CNTS-1
Pd/N-CNTs-2
N-CNTs-2
1
552
Pd/N-CNTS-0
Pd/CNTs
1
567
3
200
2400
1600
800
-
1
Wavenumber (cm )
100
200
300
400
C)
500
Temperature (
°
Fig. 9 (A) FTIR spectra of NꢀCNTsꢀ2 and Pd/NꢀCNTsꢀ2, and (B) TPR profiles of Pd/CNTs and
Pd/NꢀCNTs.
2
+
TPR profiles showed that the intensity of the Pd reduction peak did not increase in a
manner similar to that observed from the Pd 3d XPS spectra (Fig. 8). This implied that the
2
+
increased quantity of Pd observed from the Pd 3d XPS spectra was not entirely due to the
formation of more PdO. This observation further supported our hypothesis that the increase in
2
+
Pd species in Pd/NꢀCNTs was due to interactions between Pd NPs and pyrrolic nitrogen
groups. A weak desorption peak was observed at ca. 50 °C in the TPR profile of Pd/CNTs
73
alone. This peak was ascribed to decomposition of Pdꢀ
β
ꢀhydride and its absence in all the
other profiles implied that no Pdꢀ
CNTs.
βꢀhydride was formed when Pd NPs were supported on Nꢀ
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The TEM images of Pd/CNTs and Pd/NꢀCNTs are presented in Fig. 10. Pd NPs supported
on NꢀCNTsꢀ0, NꢀCNTsꢀ1 and NꢀCNTsꢀ2 exhibited a more even particle dispersion compared
with Pd NPs supported on CNTs (cf. Fig. 10A, 10B, 10C and 10D). Pd NPs were randomly
distributed on CNTs, while on NꢀCNTsꢀ1 and NꢀCNTsꢀ2 Pd NPs were typically located along
the defect sites. These defect sites consist of pentagons and heptagons induced by nitrogenꢀ
36
doping. This indicated that electronic charges are localized on the pentagons and heptagons,
which enhance electronic interactions between Pd NPs and defect sites, leading to well
7
4, 75
dispersed Pd NPs.
Interestingly, hexagonal Pd NPs were only observed in Pd/NꢀCNTsꢀ2 as
shown in Fig. 10E which is a higher magnification of Fig. 10D. This could imply that the high
composition of pyrrolic nitrogen (80%) provided a sufficient quantity of molecules that acted as
surface capping agents, also facilitated formation of hexagonal Pd NPs.
(A)
(B)
(
E)
Hexagonal Pd
NPs
2
00 nm
200 nm
(D)
(C)
100 nm
0
2
00 nm
200 nm
Fig. 10 TEM images of (A) Pd/CNTs, (B) Pd/NꢀCNTsꢀ0, (C) Pd/NꢀCNTsꢀ1, (D) Pd/NꢀCNTsꢀ2
and (E) a higher magnification image of Pd/NꢀCNTsꢀ2. Inset histograms in images
a, b, c and d are the corresponding Pd NPs size distributions.
Representative images of supported Pd NPs analysed by HRꢀTEM are shown in Fig. 11.
From the HRꢀTEM images, Pd NPs supported on NꢀCNTsꢀ0 (Fig. 11A) appeared spherical
while Pd NPs supported on NꢀCNTsꢀ2 (Fig. 11B) appeared hexagonal. The hexagonal Pd NPs
similar to those observed in the lower magnification images of Pd/NꢀCNTsꢀ2 (Fig. 10E).
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(A)
(B)
Tiny Pd
Fig. 11 TEM images of (A) Pd/CNTsꢀ0 and (B) Pd/NꢀCNTsꢀ2,
High magnification images of Pd/NꢀCNTsꢀ0 showed the presence of tiny Pd NPs forming
nanoclusters and larger Pd NPs (Fig. 11A). Such tiny particles were mainly present in Pd/Nꢀ
CNTsꢀ0 containing the highest level of nitrogenꢀdoping. Consequently, formation of
nanoclusters was ascribed to greater nitrogenꢀdoping levels and the presence of more
pyridinic/carbonitrile groups. Generally, higher nitrogenꢀdoping levels favoured the formation
of tiny Pd NPs, while higher levels of pyrrolic nitrogen favoured formation of hexagonal Pd
NPs.
The XRD diffractograms of various catalysts are presented in Fig. 12 where Pd/AC was
used as an external standard. The peak at 26.4° was indexed as the C(002) reflection of the
hexagonal graphite in CNTs and NꢀCNTs. The peaks at 40° and 45° present in Pd/AC, Pd/CNTs
0
76
and all Pd/NꢀCNTs were indexed as the Pd (111) and Pd (200) reflections of Pd . Pd/CNTs
showed an extra peak at 67° that was indexed as the Pd (220) reflection. All catalysts had
similar Pd loading levels as obtained from ICPꢀOES analysis (Table 4). Therefore, the absence
of the Pd (220) reflection in Pd/NꢀCNTs was related to differences in Pd NP sizes. A further
evaluation of the Pd NP sizes was performed by using the Pd (111) diffraction peak and TEM
image analysis.
The Pd NP sizes were estimated from the Pd (111) diffraction peak, by applying the Debyeꢀ
Scherrer equation given by:
0
.89
ꢖꢗꢘꢙꢚ
= 0.154 nm which is the wavelength of the Xꢀrays, θ is
the diffraction angle of the Pd (111) reflection and β is the full width at half maximum of the Pd
111) reflection.
λ
ꢔ ꢕ
where D is the crystal size of Pd NPs,
λ
(
2
0

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C002
Pd/CNTs
Pd/NꢀCNTsꢀ1
Pd/NꢀCNTsꢀ0
Pd/NꢀCNTꢀ2
Pd/AC
10000
Pd111
Pd200
Pd220
0
15
20
30
40
50
60
70
80
Fig. 12 XRD diffractograms of Pd/CNTs, Pd/NꢀCNTs and Pd/AC.
In addition, the particle size of supported Pd NPs was determined by measuring the size of
at least 200 Pd NPs from the TEM images (Fig. 10). Both the TEM and XRD analyses of Pd NP
sizes showed a comparable trend in the particle size variation (Table 4). Based on the XRD
analysis, Pd NPs supported on CNTs exhibited larger particle sizes than Pd NPs supported on
NꢀCNTs. Pd NPs supported on NꢀCNTsꢀ0 showed the smallest particle size. The Pd NP sizes
decreased as the total percentage nitrogen in NꢀCNTs increased. This indicated that increasing
nitrogenꢀdoping levels in NꢀCNTs introduced more functional groups which adequately
anchored the Pd NPs. The wellꢀanchored Pd NPs were less prone to NP sintering and
2
agglomeration during the cooling stages of catalyst synthesis resulting in smaller particle sizes.
Table 4 Summary of Pd wt. %, Pd NP size and Xꢀray structural parameters of catalysts.
Pd loading Ave. Pd NP Ave. Pd NP
wt. % size (nm) diameter (nm) Pd(111)
ICPꢀOES) TEM
2θ of
FWHM Inter layer
of Pd(111) spacing
peak (°) (Å)
Catalyst
(
XRD
4.5
peak (°)
39.429
39.938
39.924
39.892
40.278
Pd/CNTs
4.9
4.5
8
1.867
2.254
2.11
2.28347
2.25554
2.25631
2.25807
2.23728
Pd/NꢀCNTsꢀ0
5
3.8
Pd/NꢀCNTs ꢀ1 5.0
7
4.0
Pd/NꢀCNTsꢀ2
Pd/AC
4.9
5
4.1
2.085
0.793
$
5.0
12
10.7
$
NP: Nanoparticle, As determined by the manufacturer, Ave: Average, FWHM: Full width at
half maximum
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3.3
Catalytic activity tests
The catalytic activity of Pd/NꢀCNTs, Pd/CNTs, NꢀCNTsꢀ2 and Pd/AC was evaluated with
the aim of determining the influence of pyrrolic nitrogen on the selective nitroꢀreduction of
NBP. Since NBP has three reducible groups vis. NO , CHO and C=C, selectivity towards ABP
2
was more vital than the conversion of NBP because this selectivity is an indication of the
catalysts’ efficiency in the formation of substituted anilines. Fig. 13 and Table 5 presents the
catalytic activity of these catalysts. All Pd/NꢀCNTs showed a greater conversion of NBP than
Pd/CNTs after 24 hours of the reaction. This indicated that doping of CNTs with nitrogen
enhanced the activity of Pd NPs (Fig. 13A).
Pd/NꢀCNTsꢀ1 and Pd/NꢀCNTsꢀ2 with a smaller at. % N content were more active than
Pd/NꢀCNTsꢀ0 which had the highest at. % N. Since Pd/NꢀCNTsꢀ1 and Pd/NꢀCNTsꢀ2 had a
higher pyrrolic N content than Pd/NꢀCNTsꢀ0, it is plausible that pyrrolic nitrogen enhances
activity. The pyrrolic nitrogen groups could have promoted Pdꢀsupport interactions by forming
77
PdꢀN coordination complexes. Such complexes can alter the electronic and structural
76
properties of Pd NPs as explained in Section 3.2.3 resulting in improved activities. The
improved activity of Pd/NꢀCNTs could also be due to the stronger metalꢀsupport interactions as
observed from TPR analysis. Such strong metalꢀsupport interactions can enhance Pd NP
dispersion, reduce Pd NP agglomeration and sintering during reactions, resulting in a higher
2
5
activity.
(B)
(A)
100
1
00
8
0
8
6
4
2
0
0
0
0
No catalysts
N-CNTs-2
Pd/NCNTs-2
Pd/CNTs
60
40
Pd/NCNTs-0
Pd/NCNTs-1
Pd/NCNTs-2
Pd/AC
20
2
4
6
8
10
0
5
10
15
20
25
Time (Hours)
Time (Hours)
Fig. 13 (A) Conversion of NBP over Pd/NꢀCNTs, Pd/CNTs, Pd/AC and (B) conversion of NBP over
Pd/NꢀCNTsꢀ2 and NꢀCNTsꢀ2.
Pd/NꢀCNTsꢀ2 with the highest percentage of pyrrolic nitrogen and the lowest surface area
showed an activity higher than that of Pd/NꢀCNTꢀ0 and Pd/CNTs, but slightly lower than that of
Pd/NꢀCNTsꢀ1. To further probe the cause of the enhanced activity over Pd/NꢀCNTsꢀ2, the
2
2

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activity of purified NꢀCNTsꢀ2 was evaluated. NꢀCNTsꢀ2 exhibited a higher activity than Pd/Nꢀ
CNTsꢀ2 (Fig. 13A). A possible explanation for the lower activity observed with Pd/NꢀCNTsꢀ2
is that some of the pyrrolic groups were bound to Pd NPs, thus reducing the quantity of pyrrolic
groups available for NBP conversion. This implied that the pyrrolic groups in NꢀCNTsꢀ2 largely
promoted the catalytic activity of Pd/NꢀCNTsꢀ2. Thus, the enhanced activity of Pd/NꢀCNTsꢀ2
despite having a low surface area, could be due to the promotin effect of pyrroclic groups. The
availabiliy of more pyrrolic groups also resulted in more disorders in Pd/NꢀCNTsꢀ2 compared
with other catalysts as evidenced form the XPS and TGA analysis. Such a high content of
disorded sites are related to nitrogenꢀinduced defects which can lower the work function of
7
8
materials resulting in increased catalyst activity. Pd/AC showed the highest activity, and this
2
ꢀ1
was attributed to its high surface area of 890 m g (Supplementary Information S5). No
correlation was observed between Pd NP size and catalytic activity.
Hydrogenation of NBP over Pd/CNTs, Pd/NꢀCNTsꢀ1 and Pd/NꢀCNTsꢀ2 yielded only ABP,
while hydrogenation of NBP over Pd/NꢀCNTsꢀ0 and Pd/AC yielded ABP and pꢀbenzylaniline
(Scheme 3). Although, Pd/AC showed the highest convertion of NBP, its selectivity to either
ABP or pꢀbenzylaniline was poor compared to that of Pd/CNTs and Pd/NꢀCNTs. Up to 10 hours
of reaction, all Pd/NꢀCNTs showed greater selectivity towards ABP than Pd/CNTs (Fig. 14A
and Table 5). Thereafter selectivity over Pd/NꢀCNTsꢀ2 increased, and that of Pd/NꢀCNTsꢀ1 and
Pd/CNTs were almost constant while that of Pd/NꢀCNTsꢀ0 sharply dropped. Pd/AC showed the
lowest selectivity to ABP throughout the whole reaction period. Given that Pd/ꢀNꢀCNTsꢀ1 and
Pd/NꢀCNTsꢀ2 showed a greater selectivty than Pd/CNTs, we deduced that nitrogenꢀdoping of
7
9
CNTs enhanced the selective reduction of the nitro group to an amine group. As the amount of
pyrrolic nitrogen increased, selectivity towards ABP also increased for Pd/NꢀCNTsꢀ1 and Pd/Nꢀ
CNTsꢀ2, while no correlation was observed between the selectivities and the total N%. This
implied that selectivity was influenced more by the presence of pyrrolic nitrogen than the total
N%. Other studies have shown that the species of nitrogen and not just the nitrogen content in
NꢀCNTs play a significant role in influencing the catalytic properties of NꢀCNTs as as metal
80
catalyst supports.
NꢀCNTsꢀ2 showed a much lower selectivity towards ABP compared with Pd/NꢀCNTsꢀ2
Fig. 14B), impying that Pd NPs were responsible for the selectivty observed with Pd/NꢀCNTsꢀ
(
2. Therefore, the enhanced selectivity over Pd/NꢀCNTs was majorly attributed to the presence
of PdꢀN complexes. Such complexes minimise Pd NP sintering and agglomeration during the
7
6
reaction. This increases the stability and dispersion of Pd NPs while minimizing the formation
of Pd black which is catalytically inactive. The increased stability and dispersion of Pd NPs
2
3

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could increase the hydrogen chemisorption capacity of Pd NPs hence increasing the selectivity.
The presence of more disordered pores in NꢀCNTs could also facilitate a faster mass transfer of
2
ABP from the catalyst surface back into the solution. This could prevent a further reduction of
ABP to pꢀbenzylaniline resulting in an increased selectivity. Corma et al. compared the
selective nitroꢀreduction of nitrobenzaldehyde to aminobenzaldehyde over supported Au and
81
Pd. Their findings shows a lower selectivity over Pd compared with Au. Hence, the enhanced
selectivity towards ABP observed with Pd/NCNTs can be due to the promoting effect of Nꢀ
CNTs supports.
O
O
Pd/CNTs or
Pd/NꢀCNTsꢀ1 and ꢀ2
H N
0
2
O N
H 1 bar, 50 C
2
2
(70%ꢀ89%)
Nitrobenzophenone (NBP)
Aminobenzophenone (ABP)
O
O
Pd/AC or
Pd/NꢀCNTsꢀ0
+
H N
0
H N
2
H 1 bar, 50 C
2
(20ꢀ24%)
pꢀBenzylaniline
O N
2
(37%ꢀ74%)
2
Nitrobenzophenone
Aminobenzophenone
Scheme 3. Hydrogenation of NBP over various catalysts.
90
75
60
45
30
15
8
7
6
0
0
0
(B)
(
A)
No catalyst
N-CNTs
Pd/NCNTs-2
Pd/CNTs
50
40
Pd/NCNTs-0
Pd/NCNTs-1
Pd/NCNTs-2
Pd/AC
3
2
0
0
10
0
2
4
6
8
10
0
5
10
15
20
25
Time (Hours)
Time (Hours)
Fig. 14 (A) Selectivity towards ABP over Pd/NꢀCNTs, Pd/CNTs, Pd/AC and (B) selectivity towards
ABP over Pd/NꢀCNTsꢀ2 and NꢀCNTsꢀ2.
Selectivity over Pd/NꢀCNTsꢀ0 dropped after 10 hours implying that Pd/NꢀCNTsꢀ0 possess a
different catalytic pathway compared with Pd/CNTs, Pd/NꢀCNTsꢀ1 and Pd/NꢀCNTsꢀ2. The
2
4

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different catalytic pathway in Pd/NꢀCNTsꢀ0 was accredited to the existence of tiny Pd NPs
forming nanoclusters. A similar drop in selectivity was observed over Pd/AC which also had
tiny Pd NPs in the form of clusters (Supplementary Information S6). Such Pd nanoclusters may
82
possess a different reaction mechanism from the larger Pd NPs. This different reaction
mechanism could have favoured the further reduction of ABP to pꢀbenzylaniline accounting for
the drop in selectivity (Supplementary Information S6 and Table 5).
Table 5 Summary of the catalytic performance of catalysts after 24 hours of the reaction.
Conversion of Selectivity Selectivity to
*
Catalyst
Other products
NBP (%)
to ABP (%) pꢀbenzylaniline (%)
morpholineꢀ4ꢀ
carbal, 2ꢀ
Pd/CNTs
85
70
ꢀ
phenylethanol
Pd/NꢀCNTsꢀ0
Pd/NꢀCNTsꢀ1
93
63
83
89
24
6.3
ꢀ
morpholineꢀ4ꢀcarbal
morpholineꢀ4ꢀcarbal
morpholineꢀ4ꢀcarbal
benzophenone
100
Pd/NꢀCNTs ꢀ2 98
ꢀ
Pd/AC
100
24
*
Other hydrogenation products detected from the GCꢀMS
Since all catalysts had a similar quantity of residual iron catalyst (Fig. 4B) the possibility of
the residual iron catalyst influencing the observed catalytic trend was ruled out. From the TEM
analysis no iron particles were observed on the surface of the supports. This implied that the
residual iron was encapsulated inside the CNTs and NꢀCNTs and hence not available for surface
catalysis.
Pd/NꢀCNTsꢀ2, which showed the highest selectivity after 24 hours, was used to evaluate the
effect of pressure on the catalytic activity and selectivity. It was observed that as the pressure
increases the catalytic activity and selectivity over Pd/NꢀCNTsꢀ2 reduced (Supplementary
Information S7). Thus, the optimum pressure for the hydrogenation of NBP was 1 bar.
3.4
Stability of Pd NPs after hydrogenation reactions
The stability of Pd NPs on NꢀCNTsꢀ0, NꢀCNTsꢀ1 and NꢀCNTsꢀ2 after use was analysed by
XRD. From the XRD diffractograms, the Pd NP size of used catalysts was determined in a
similar manner to that outlined in Section 3.2.3. Used Pd/CNTs was not analysed because it
strongly adhered to the filter paper resulting in a low catalyst recovery, which was insufficient
for XRD analysis. Table 6 shows the average diameters of the Pd NPs before and after reaction.
2
5

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The size of the Pd NPs in NꢀCNTsꢀ0 and NꢀCNTsꢀ1 increased after the reaction while in Nꢀ
CNTsꢀ2 the size of the Pd NPs did not change. This implied that Pd sintering and agglomeration
may have occurred only in Pd/NꢀCNTsꢀ0 and Pd/NꢀCNTsꢀ1. Thus, the Pd NPs in NꢀCNTsꢀ2
were considered very stable; owing to the formation of more PdꢀN complexes in Pd/NꢀCNTsꢀ2
than in Pd/NꢀCNTsꢀ0 and Pd/NꢀCNTsꢀ1. In a similar manner, Chen et al. showed that pyridinic
3
+
3+
nitrogen can complex with Ru as a ligand to form stable Ru particles, which were resilient to
20
metal sintering and agglomeration.
Table 6 Average Pd NP sizes of various catalysts before and after a single reaction.
#
$
Catalyst
Ave. Pd NP diameter (nm)
Ave. Pd NP diameter (nm)
Pd/NꢀCNTsꢀ0
Pd/NꢀCNTsꢀ1
Pd/NꢀCNTsꢀ2
3.8
4.0
4.1
5.0
5.8
4.1
#
$
Ave: average, NP: nanoparticle, before hydrogenation reaction, after hydrogenation
reaction
The diffractograms of the used catalysts showed an increase in the intensity of the Pd (111)
reflection compared with those of the fresh catalyst (Supplementary Information S8). This
0
increase in the Pd (111) reflection indicated that more Pd were formed via the hydrogen
2
+
reduction of Pd during the reaction.
4
. Conclusions
Pyrrolic NꢀCNTs containing 2.5ꢀ4.4 at. % of nitrogen were successfully synthesized by
means of CVD method. Selectivity towards formation of pyrrolic nitrogen was promoted by the
incorporation of oxygen derived from ethylbenzoate during synthesis of the NꢀCNTs. Increasing
the amounts of pyrrolic groups in NꢀCNTs significantly altered the physicochemical properties
of NꢀCNTs such as thermal stability, textural properties and surface wetness. Also, an increase
in the pyrrolic percentage improved the interactions between Pd and NꢀCNTs resulting in better
dispersion and more stable Pd NPs. The improved dispersion and stability of Pd NPs was
associated with a strong interaction between the pyrrolic nitrogen and Pd NPs. The catalytic
activity and selectivity of Pd/NꢀCNTs in the hydrogenation of NBP was dependent on the
percentage of pyrrolic nitrogen and not on the total nitrogen content. Pd/NꢀCNTs showed an
enhanced selectivity compared with Pd/CNTs and Pd/AC. Thus, Pd supported on pyrrolic Nꢀ
2
6

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CNTs is a promising catalyst for the selective reduction of NBP and other industrially important
nitro arenes such as nitrobenzene derivatives.
Acknowledgements
The authors wish to thank the University of KwaZuluꢀNatal and the National Research
Foundation (NRF) for financial support. We are grateful to Professor Bice Martincigh and Dr.
R. S. Mwakubambanya for their insightful suggestions during the manuscript development and
their assistance with proofꢀreading.
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