Pt/Al2O3 coated with N-doped carbon as a highly selective and stable catalyst for catalytic hydrogenation of: P -chloronitrobenzene to p -chloroaniline

Article abstract of DOI:10.1039/d0ra01578d

Selective catalytic hydrogenation of p-chloronitrobenzene on Pt-based catalysts is a green and high-efficient way for p-chloroaniline production. However, supported monometallic Pt catalysts often exhibit undesirable p-chloroaniline selectivity. We herein reported supported Pt catalysts with N-doped carbon (NC) as an overcoating (Pt/Al₂O₃?NC) to overcome the disadvantage. Three Pt/Al₂O₃?NC catalysts with different NC coating amounts were prepared by in situ carbonization of an ionic liquid. For comparison, Al₂O₃ coated by NC and Pt/Al₂O₃ coated by SiO₂ were also prepared. A combination characterization confirmed that the NC overcoating was successfully formed on Pt/Al₂O₃ surface and Pt particles were completely coated by NC layers when ion liquid amount increased to 25 μl per g catalyst. Due to the intimate contact of NC layers and Pt particles Pt-NC heterojunctions were effectively formed on the catalyst surface. For the catalytic hydrogenation of p-chloronitrobenzene, Pt/Al₂O₃?NC with 25 μl ionic liquid as the NC precursor exhibited 100% selectivity to p-chloroaniline at 100% conversion of p-chloronitrobenzene. A lower ionic liquid amount led to decreased selectivity to p-chloroaniline. Furthermore, no deactivation was observed on Pt/Al₂O₃?NC during 5 catalytic cycles. The findings in the study demonstrate that coating noble metal catalysts by N-doped carbon is a promising method to enhance the selectivity and stability for catalytic hydrogenation of p-chloronitrobenzene.

Full text of DOI:10.1039/d0ra01578d

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PAPER
Pt/Al2O3 coated with N-doped carbon as a highly
selective and stable catalyst for catalytic
hydrogenation of p-chloronitrobenzene to p-
chloroaniline†
Cite this: RSC Adv., 2020, 10, 14208
*
Donghong Zhu, Xin Weng, Yuqiong Tang, Jingya Sun,
and Zhaoyi Xu
Shourong Zheng
Selective catalytic hydrogenation of p-chloronitrobenzene on Pt-based catalysts is a green and high-
efficient way for p-chloroaniline production. However, supported monometallic Pt catalysts often exhibit
undesirable p-chloroaniline selectivity. We herein reported supported Pt catalysts with N-doped carbon
(NC) as an overcoating (Pt/Al2O3@NC) to overcome the disadvantage. Three Pt/Al2O3@NC catalysts with
different NC coating amounts were prepared by in situ carbonization of an ionic liquid. For comparison,
Al2O3 coated by NC and Pt/Al2O3 coated by SiO2 were also prepared. A combination characterization
confirmed that the NC overcoating was successfully formed on Pt/Al2O3 surface and Pt particles were
completely coated by NC layers when ion liquid amount increased to 25 ml per g catalyst. Due to the
intimate contact of NC layers and Pt particles Pt-NC heterojunctions were effectively formed on the
catalyst surface. For the catalytic hydrogenation of p-chloronitrobenzene, Pt/Al2O3@NC with 25 ml ionic
liquid as the NC precursor exhibited 100% selectivity to p-chloroaniline at 100% conversion of p-
chloronitrobenzene. A lower ionic liquid amount led to decreased selectivity to p-chloroaniline.
Furthermore, no deactivation was observed on Pt/Al2O3@NC during 5 catalytic cycles. The findings in
the study demonstrate that coating noble metal catalysts by N-doped carbon is a promising method to
enhance the selectivity and stability for catalytic hydrogenation of p-chloronitrobenzene.
Received 19th February 2020
Accepted 12th March 2020
DOI: 10.1039/d0ra01578d
rsc.li/rsc-advances
catalysts were considered as the most selective ones among
supported noble metals for the catalytic conversion of CNBs to
CANs,3,8,9 however, Pt/C catalyst only displayed 75.2% selectivity
1. Introduction
As important chemical raw materials, chloroanilines (CANs) are
widely used as intermediates in pharmaceuticals, herbicides,
pigments and dyes.1–3 Conventionally, CANs could be produced
via reduction of corresponding chloronitrobenzenes (CNBs)
using stoichiometric reductants such as potassium borohydride
and hydrazine hydrate.2–4 However, the use of stoichiometric
reductants would bring large amounts of harmful waste and
severe pollution to the environment. Alternatively, selective
catalytic hydrogenation reduction is an eco-friendly and cost-
effective method for the conversion of CNBs to CANs.
to p-CAN at 100% conversion of p-CNB.10 Additionally, catalyst
deactivation oen occurs as a result of contamination of metal
surface or/and metal leaching during the reaction processes.11,12
For example, Mahata et al.11 found that Pt/CNT deactivated
drastically in the catalytic hydrogenation of o-CNB during the
second and subsequent reaction runs. Hence, exploring effec-
tive strategies to improve the catalytic selectivity and stability of
supported noble metal catalysts for hydrogenation of CNBs to
corresponding CANs is of signicant importance.
Many attempts such as addition of a promoting metal or
decorating metal particles with organic modiers have been
made to improve the catalytic selectivity for p-CNB hydrogena-
tion.13–17 For example, Zhu et al.18 prepared a core–shell struc-
tural bimetallic Pt@Cu/TiO2 catalyst with monolayer Cu as the
shell for catalytic hydrogenation of p-CNB and obtained 100%
selectivity to p-CAN. Very recently, coating supported catalysts
have been proved to be an effective approach to enhance cata-
lytic selectivity. Chen et al.14 prepared a Pt-nanowire catalyst
chelated by ethylenediamine on catalyst surface, which exhibi-
ted prominent selectivity for the catalytic production of N-
For catalytic hydrogenation reduction, noble metals such as
Pd and Pt are highly active because of their strong abilities for
H2 activation under mild conditions.5,6 However, supported
noble metal catalysts exhibited low selectivity because dech-
lorinated byproducts were inevitably formed due to their strong
hydrodechlorination capabilities.5,7 For example, supported Pt
State Key Laboratory of Pollution Control and Resource Reuse, School of the
Environment, Nanjing University, Nanjing 210046, China. E-mail: sunsun1118@
126.com; Fax: +86-25-89680596; Tel: +86-25-89680373
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra01578d
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hydroxylanilines from nitrobenzene. Leng et al.19 reported that distilled water at 80 C in a water bath. Aer stirring for 2 h at
ꢀ
controlled and chemoselective hydrogenation of nitrobenzene 80 ꢀC, the mixture was transferred into the autoclave, sealed and
ꢀ
was achieved on a Ru@C60 catalyst. Notably, coating supported kept at 180 C for 72 h in an oven. The resulting mixture was
catalysts was also reported as a prominent strategy to enhance ltered and washed using ethanol for several times and then
catalytic stability through protecting metal particles from oven-dried at 100 ꢀC. Finally, the obtained powder was calcined
leaching and poisoning. For example, Fu et al.20 illustrated that at 550 C in a muffle furnace for 4 h.
ꢀ
a Ni/Al2O3 catalyst coated with carbon nitride layer provided an
Preparation of Pt/Al2O3. Pt/Al2O3 was prepared using
exceptional stability for the catalytic hydrogenation of nitro- a conventional impregnation method with H2PtCl6 as metal
benzene. Li et al.21 developed a Pt catalyst with Pt particles precursor. Briey, 1 g of grinded and sieved Al2O3 was impreg-
embedded in N-doped carbon rods and observed a superior nated with 2.7 ml of 1.9 ꢁ 10ꢂ5 mol mlꢂ1 H2PtCl6 solution. Aer
catalytic stability in the liquid-phase catalytic hydrogenation of stirring for 2 h, the mixture was evaporated under 80 ꢀC in
bromate. Recent years, N-doped carbon (NC) materials have a water bath. The sample was oven-dried at 100 ꢀC overnight
attracted tremendous attentions because of their unique and and calcined in a muffle furnace at 300 ꢀC for 4 h. The nominal
versatile properties. For example, N-doped carbon could trans- content of Pt was 1 wt%.
fer electrons to metal particles or vice versa via metal–NC het-
Preparation of Pt/Al2O3@NC. Pt/Al2O3@NC was prepared by
erojunctions and thus serve as catalytically active sites for the
a
reported method with ion liquid (EMIm-dca) as NC
adsorption and activation of H2 and reactants.20–22 Inspired by precursor.23 Briey, 1 g Pt/Al2O3 and a desired amount of ion
recent advances, it is reasonable to speculate that coating sup- liquid (10, 15 and 25 ml of EMIm-dca, respectively) were added
ported noble metal catalysts with N-doped carbon may probably in a 20 ml of beaker containing 5 ml of distilled water. Aer
improve the catalytic selectivity and stability for hydrogenation mixed well by ultrasonic treatment, the slurry was dried at 70 ꢀC
of CNBs. However, few studies have been conducted for the in a porcelain boat andꢂc1alcined in a tubular furnace with
selective hydrogenation of p-CNB on coated noble metal cata- a heating rate of 5 ꢀC min up to 400 ꢀC and maintained at this
lysts thus far.
temperature for 1 h under N2 atmosphere. For comparison,
The main objective of the present study is to develop highly Al2O3@NC without Pt loading was also prepared. The resulting
selective and stable supported Pt catalysts for the catalytic catalysts were labeled as Pt/Al2O3@NC-X or Al2O3@NC-X, where
hydrogenation of CNBs to CANs. We prepared three Pt/Al2O3 X refers to the amount of ion liquid added. The prepared Pt/
catalysts coated by NC layer with different NC amounts by in situ Al2O3@NC-25 was further calcined at 500 ꢀC in air for 2 h to
carbonization of ion liquid (1-ethyl-3-methylimidazolium remove the NC overcoating to clarify the role of NC. This catalyst
dicyanamide, EMIm-dca). For comparison, Al2O3 coated with was labelled as Pt/Al2O3@NC-25-calcined. All the catalysts were
ꢀ
NC and Pt/Al2O3 coated with SiO2 were also prepared. The pre-reduced in H2 atmosphere at 300 C for 2 h in prior to the
structural properties of the synthesized catalysts were charac- characterization and evaluation of catalytic activity.
terized by a series of techniques and the catalytic performances
of the catalysts for hydrogenation of p-CNB were examined.
Preparation of Pt/Al2O3@SiO2. Pt/Al2O3@SiO2 was prepared
using method reported by Li et al.12 Briey, 1 g of Pt/Al2O3 was
dispersed in 200 ml of disꢂti1lled water, which was stabilized at
80 ꢀC under a 30 ml min N2 ow. Under vigorous stirring,
20 ml of sodium silicate solution (1.0 mol lꢂ1) was added to the
suspension dropwise, and then the pH of the mixture was
2. Experimental
2.1 Catalyst preparation
Chemical and materials. Ion liquid (EMIm-dca) and adjusted to 6.0 using a HCl solution (1 mol lꢂ1). The mixture
hydrogen hexachloroplatinate(IV) hexahydrate (H2PtCl6$6H2O) was further stirred at 80 ꢀC for 30 h. The catalyst was obtained
were purchased from Aldrich. Aluminum isopropoxide, oleyl- aer washed with distilled water for several times and dried at
ꢀ
amine, sodium silicate, ammonium hydroxide (NH3$H2O), 65 C under vacuum for 10 h.
hydrogenchloride (HCl), nitric acid (HNO3), p-CNB, and p-CAN
were obtained from Sinopharm Chemical Reagent Co., Ltd.,
China. All chemicals were used without further purication.
2.2 Catalyst characterization
Preparation of aluminum sol.
A desired amount of Pt contents in the samples were precisely determined by an
aluminum isopropoxide powder was added slowly into 150 ml inductively coupled plasma-atomic emission spectrometer (ICP-
distilled water with a molar ratio of water to aluminum iso- AES) (J-A1100, Jarrell-Ash, USA). The XRD patterns of the
propoxide of 100 : 1 in a three-necked round ask under samples were collected on
a Rigaku D/max-RA powder
vigorous stirring at 80 ꢀC for controlled hydrolysis of aluminum diffraction-meter using a Cu Ka radiation (Rigaku, Tokyo,
ꢀ
isopropoxide. Aer reuxing at 80 C for 1 h, the mixture was Japan). The carbon and nitrogen contents in the catalysts were
ꢀ
further evaporated at 90 C for 1 h to remove formed alcohol. determined using an elementary analyzer CHN–O-rapid appa-
The pH of the mixture was then adjusted to 3.0 using 10% HNO3 ratus (Heraeus, Germany). The Brunauer–Emmett–Teller (BET)
solution. Aluminum sol was obtained aer the mixture further surface areas of the catalysts were measured using a Micro-
ꢀ
reuxed at 80 C for 24 h.
meritics ASAP 2200 instrument (Micromeritics Instrument Co.,
Preparation of Al2O3. Al2O3 was prepared using method re- Norcross, GA). High-angle annular dark eld-scanning trans-
ported by Fu et al.20 Briey, 180 g of alumina sol, 14 g of oleyl- mission electron microscope (HAADF-STEM) and elemental
amine and 10 ml of 25% NH3$H2O was dispersed in 150 ml of mapping images were acquired using an ARM-200F
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transmission electron microscope (JEOL Co., Japan.) equipped
with an X-MAX energy dispersive spectrometer (Oxford, UK). X-
ray photoelectron spectroscopy (XPS) spectra were recorded on
a PHI5000 VersaProbe with a monochromatized Al Ka X-ray
source (hn ¼ 1486.6 eV) (ULVAC-PHI, Japan). The binding
energy was calibrated by measuring the C 1s peak at 284.6 eV.
Temperature programmed oxidation (TPO) was conducted
on an AMI-300 setup (Altamira Instruments, USA) with a gas
analysis system (Thermo Star, Germany). Typically, 50 mg of
catalyst was loaded in a U-shaped quartz tube, which was pre-
treated at 120 ꢀC for 1 h using a 25 ml minꢂ1 of He ow in
a heating furnace. Aer cooling down, the catalyst was in situ
oxidized with a 5% O2/He ow (25 ml minꢂ1) from ambient
temperature to 600 ꢀC at a rate of 5 ꢀC minꢂ1. The evolved gases
were monitored with intensities of m/z 17 (NH3), 28 (CO), 30
(NO), 44 (CO2) and 46 (NO2), respectively.
2.3 Catalytic hydrogenation of p-chloronitrobenzene
The catalytic hydrogenation of p-CNB over the catalysts was
conducted in a high pressure stainless steel autoclave with
a controlling system. Briey, 20 mg catalyst, 100 mg p-CNB and
20 ml ethanol was added to the autoclave. Before hydrogenation
reaction, the autoclave was ushed with N2 for 5 times to
replace air in the reactor under ambient condition. The reaction
was conducted under 1 MPa pressure of H2 at 80 ꢀC with stirring
rate xing at 1000 rpm. Samples were taken periodically and
detected by a 6980 gas chromatograph equipped with a FID
detector and a DB-5 capillary column (Agilent Technologies,
US), and n-hexane was used as the inner standard substance.
For catalyst reuse, the used catalyst was prepared under the
same reaction conditions but without sampling. Aer reaction
for 180 min, the catalyst was collected by ltration, rinsing with
ꢀ
ethanol and distilled water and drying at 60 C for 12 h. The
catalyst was then subjected to the subsequent reaction test.
The conversion of p-CNB and selectivity to p-CAN and aniline
were calculated following equations below:
m
ðp-CNBÞ0 ꢂ m
ðp-CNBÞt
Conversion ¼
Selectivityðp-CANÞ
SelectivityðanilineÞ
ꢁ 100%
ꢁ 100%
(1)
(2)
(3)
m
ðp-CNBÞ0
m
ðp-CANÞt
¼
¼
m
þ m
ðp-CANÞt
ðanilineÞt
m
ðanilineÞt
ꢁ 100%
m
ðp-CANÞt þ m
ðanilineÞt
where m refers to the molar amount.
3. Results and discussion
3.1 Catalyst characterization
Table 1 shows Pt content of the catalysts determined by ICP-
AES. For Pt/Al2O3, Pt content was determined to be 0.96 wt%.
As for Pt/Al2O3@NC, Pt content were in the range of 0.71–
0.78 wt%, which were lower than that of Pt/Al2O3 due to
successful loading of NC overcoating.
The compositions of C and N elements in the catalysts were
determined by elemental analysis and the results are
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Fig. 1 XRD patterns of the catalysts.
Fig. 2 (a) HAADF-STEM image of Pt/Al2O3@NC-25 and the corre-
sponding EDS elemental mapping of (b) the overlapping of Pt, C, and N
element, (c) Pt element (red), (d) N element (blue), and (e) C element
(purple).
summarized in Table 1. For Pt/Al2O3@NC-10, Pt/Al2O3@NC-15
and Pt/Al2O3@NC-25, the C contents were 0.33%, 0.63% and
0.76% respectively, exhibiting increased C content with the
increase of ion liquid loading amount from 10 ml to 25 ml.
Consistently, N contents in Pt/Al2O3@NC catalysts increased
from 0.17% to 0.39% with ion liquid loading. For Al2O3@NC-25,
the C and N content were determined to be 0.58% and 0.24%
respectively, lower than those of Pt/Al2O3@NC-25, probably
because the presence of Pt particles on Al2O3 surface stabilized
the NC matrix formed by pyrolysis of ion liquid via evoking
interactions with NC matrix. The molar N/C ratios in the cata-
lysts were in the range of 0.35–0.44 which was lower than 0.63 of
ion liquid, this can be ascribed to more prominent loss of
Results are listed in Table 1. The average sizes of Pt particles
of Pt/Al2O3, Pt/Al2O3@NC-10, Pt/Al2O3@NC-15 and Pt/
Al2O3@NC-25 were 4.0, 3.9, 4.0 and 4.1 nm, respectively, indi-
cating that NC coating did not affect the structure of Pt/Al2O3.
However, NC overcoating layer was not clearly visible in TEM
images, probably because of the very low NC contents (as
indicated by elemental analysis results). To verify the presence
of NC overcoating on the catalysts, EDS mapping analysis was
conducted on Pt/Al2O3@NC-25 and the images are presented in
Fig. 2. In Fig. 2a light particles with diameter around 5–7 nm
could be clearly observed as marked by red arrows. Consistently,
Pt signal was very dense in the same area in the mapping image
in Fig. 2c, reecting that the lightness variation was due to the
presence of Pt particles on Al2O3 support. The EDS mapping of
elemental C and N in Pt/Al2O3@NC-25 (see Fig. 2d and e)
showed that elemental C and N were evenly distributed on Al2O3
surface. The clear spatial correlation among elemental Al, C
and N was indicative of a close contact of NC with Al2O3
support, verifying the presence of NC overcoating on Al2O3
surface.
unstable
N
species upon carbonization at high
temperatures.24,25
Fig. 1 shows the XRD patterns of the catalysts in the range of
20–80ꢀ. Diffraction peaks centered at 37.3ꢀ, 39.5ꢀ, 45.7ꢀ, and
67.1ꢀ indexed to the g-Al2O3 phase, were clearly observed on all
the catalysts. No diffraction peaks characteristic of Pt were
found in the XRD patterns probably due to low metal contents
of the catalysts.26 Notably, in the range of 15–25ꢀ, a weak and
broad peak centered at 19.5ꢀ were observed on Pt/Al2O3@NC
catalysts. The very weak peak was ascribed to amorphous N-
doped carbon, which was lower than that of amorphous
carbon (23.5ꢀ) due to the increase of inter-layer distance of
carbon upon N doping.27 As for the XRD pattern of Pt/Al2O3@-
SiO2, the broad peak centered at 23.1ꢀ was assigned to amor-
phous silica. In comparison with Pt/Al2O3@NC, Pt/Al2O3@SiO2
had markedly weakened intensities of peaks of g-Al2O3 phase,
likely resulting from the thicker SiO2 overcoating than NC
overcoating.
To further investigate the composition of Pt/Al2O3@NC-25,
temperature-programed oxidation of Pt/Al2O3@NC-25 was
Fig. S1, ESI† shows the HAADF-STEM images of the catalysts.
Al2O3 had uniform rhombus morphology with diameter in the
range of 30–60 nm. The TEM images showed that Pt particles
were spherical and were well dispersed on Al2O3 surface. The
average Pt particle diameter was measured based on the surface
area weighted diameter:28,29
.
X
X
ds ¼
nidi3
nidi2
(4)
where ni is the number of selected Pt particles having diameter
P
of di and the selected total particle number ( ni) exceeds 100.
Fig. 3 Temperature programmed oxidation profile of Pt/Al2O3@NC-
25.
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conducted, and the dependence of gas release on oxidation (see Table 3). Because calcination led to the oxidation of NC
temperature are shown in Fig. 3. In Fig. 3, CO and CO2 exhibited matrix of the catalyst and thus resulted in the exposure of
very similar evolving proles, in which a strong and broad peak blocked Pt sites for CO chemisorption. Notably, the CO chem-
with two shoulders presented in the range of 200–550 ꢀC. In isorption amount of Pt/Al2O3@NC-25-calcined was lower than
parallel, marked release of NH3 and NO was also observed in the that of Pt/Al2O3, probably due to incomplete oxidation of NC
ꢀ
range of 180–530 C. The presence of CO2, CO, NH3 and NO2 overcoating upon calcination.
could be ascribed to the oxidation of C and N containing
Fig. 4 shows the XPS spectra of the catalysts. Because Pt 4f
species, conrming the presence of NC overcoating on the region was overlapped by the strong Al 3p peak, the surface Pt
catalyst surface.30 Notably, the oxidation temperature of N compositions of the catalysts were analyzed by XPS spectra in Pt
species was slightly lower than that of C species, reecting 4d region. In Fig. 4a, two peaks around 315.2 and 332.7 eV were
that N species were more prone to lose than C species upon high found for Pt/Al2O3, characteristic of Pt 4d5/2 and 4d3/2, respec-
temperature, in line with elemental analysis results.
tively (see Fig. 4a).32,33 Upon NC coating, slightly lowered
The effective decoration of NC overcoating on Pt particle binding energies of Pt 4d were observed. For example, the
could be further veried by CO chemisorption (see Table 1). The binding energies of Pt 4d5/2 was 315.0, 314.8 and 314.7 eV for Pt/
CO adsorption amount on Pt/Al2O3 was determined to be 12.0 Al2O3@NC-10, Pt/Al2O3@NC-15 and Pt/Al2O3@NC-25 respec-
mmol gꢂ1. Accordingly, the average diameter of Pt particles of Pt/ tively. The gradual red shi of the binding energy of Pt 4d5/2
Al2O3 could be calculated by CO chemisorption amount with with NC coating amount in the catalysts may be ascribed to the
a chemisorption stoichiometry of CO/Pt ¼ 1. The diameter was electron transfer from electronic-enriched N sites to Pt due to
calculated to be 4.3 nm, in good agreement with that measured their strong interactions.34,35 Fig. 4b shows XPS spectra of the
by TEM. Aer NC coating, the CO adsorption amount markedly catalysts in the N 1s region. Deconvolution of the N 1s peak gave
decreased. For example, CO adsorption amount of Pt/ rise to two peaks located at 398.6 and 400.2 eV, characteristic of
Al2O3@NC-10 and Pt/Al2O3@NC-15 was 3.7 mmol gꢂ1 and 0.5 pyridinic N and pyrrole N respectively, further conrming the
mmol gꢂ1 respectively, much lower than that of Pt/Al2O3. successful formation of NC overcoating layers. The ratio of
Because CO could chemically adsorb only on exposed Pt sites of pyridinic N to pyrrole N of the catalysts were calculated and the
the catalyst, the decreased chemisorption amount of CO clearly results are summarized in Table 1. Notably, the ratio of
reected lowered exposure of Pt sites of the catalysts as a result pyridinic N (N1) to pyrrole N (N2) markedly differed with
of NC overcoating.31 Further increasing NC coating amount samples. For Al2O3@NC-25, the ratio of N1/N2 was 0.47, which
even led to the absence of CO adsorption on Pt/Al2O3@NC-25, was much lower than that of Pt/Al2O3@NC-25 (0.71). In
indicating that Pt sites were completely blocked by NC over- comparison with Pt/Al2O3@NC-25, Pt/Al2O3@NC-15 with
ꢀ
coating in the catalyst. For Pt/Al2O3@NC-25 calcined at 500 C
in air, the CO chemisorption amount increased to 7.0 mmol gꢂ1
Fig. 5 Catalytic hydrogenation of p-CNB on the catalysts. Reaction
conditions: catalyst dosage: 20 mg, alcohol: 20 ml, p-NCB amount:
Fig. 4 XPS spectra of catalysts in the regions of (a) Pt 4d and (b) N 1s. 100 mg, reaction temperature: 80 ꢀC, H2 pressure: 1 MPa.
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a lower NC content has a higher N1/N2 ratio of 0.92. Notably, be reduced on NC matrix on catalyst surface. Considering the
binding energy in N 1s region characteristic of covalently bound negligible catalytic activity of Al2O3@NC-25, a synergistic effect
N–Pt species was found to be 398.6 eV, which was identical to between Pt and NC overcoating could be deduced. Previous
the binding energy of pyridinic N.36 Hence, the much higher N1/ reports proved that supported metal catalysts with carbona-
N2 ratio in Pt/Al2O3@NC-25 than that of Al2O3@NC-25 could be ceous materials as the overcoating could catalyze hydrogenation
attributed to the formation of additional N–Pt species when ion reactions.20,21,37 It was proposed that intimate contact between
liquid was pyrolyzed on catalyst surface. The results clearly metal and carbon matrix would cause a metal–carbon matrix
conrmed strong interactions between Pt and N atoms in NC heterojunction, through which H2 could be adsorbed and acti-
matrix as also indicated by elemental analysis results. Consis- vated and electrons could be transferred to reactants. Accord-
tently, the higher N1/N2 ratio of Pt/Al2O3@NC-15 can be ingly, the mechanism well explained the hydrogenation of p-
explained in terms of the formation of more Pt–N species as CNB on Pt/Al2O3@NC-25. XPS results conrmed strong inter-
a result of the higher Pt : N ratio in Pt/Al2O3@NC-15 than that of actions between Pt particles and NC matrix, forming Pt-NC
Pt/Al2O3@NC-25 assuming identical Pt sites in Pt/Al2O3@NC-15 heterojunction. Furthermore, the catalytic performance of the
to those in Pt/Al2O3@NC-25.
catalyst with non-conductive overcoating also conrmed this
mechanism. For example, Pt/Al2O3 coated by SiO2 exhibited
negligible catalytic activity for hydrogenation of p-CNB (1.1% p-
CNB conversion in 180 min reaction time) (see Table 2). This
was because that SiO2 is an inert insulator, on which electron
transfer was substantially suppressed, thus resulting in a negli-
gible catalytic activity than that of Pt/Al2O3@NC-25. Similarly, Li
et al.12 observed a much higher catalytic activity of Pd/CNT with
N-doped carbon as overcoating than that of SiO2 coated Pd/CNT
for catalytic reduction of Cr (VI) in liquid phase.
3.2 Catalytic hydrogenation of p-CNB
The catalytic performances of the catalysts for hydrogenation of
p-CNB are presented in Fig. 5 and the reaction constants are
compared in Table 2. Aniline and p-CAN were identied as
products in all the reactions. Control experiment showed that
Al2O3@NC-25 exhibited very low activity for catalytic hydroge-
nation of p-CNB with p-CNB conversion of 0.6% within 180 min
of reaction time, reecting the very weak capability of NC
overcoating and Al2O3 for H2 activation under reaction condi-
tions. In contrast, Pt/Al2O3 exhibited a much higher catalytic
activity with 100% of p-CNB conversion within 30 min indi-
cating the high activity of Pt particles for H2 activation and p-
CNB reduction. Upon NC coating, the conversion of p-CNB over
Pt/Al2O3@NC gradually decreased with NC coating amount. For
example, p-CNB was completely converted at 50, 90, and
150 min on Pt/Al2O3@NC-10, Pt/Al2O3@NC-15 and Pt/
Al2O3@NC-25 respectively (see Table 2). The deteriorated cata-
lytic activity of Pt/Al2O3@NC was likely due to blocking Pt sites
by NC overcoating from p-CNB as evidenced by CO chemi-
sorption results. Accordingly, decreased catalytic activity as
a result of coating catalyst surface had been reported previ-
ously.37,38 Notably, effective hydrogenation of p-CNB was found
on Pt/Al2O3@NC-25 although Pt sites were completely blocked
by NC matrix (see CO chemisorption results). This results sug-
gested that H2 was capable of being activated and p-CNB could
Besides catalytic activity coating Pt/Al2O3 substantially
impacted the selectivity in catalytic hydrogenation of the p-CNB.
For example, Pt/Al2O3 displayed a catalytic selectivity of 81.8%
to p-CAN at 100% of p-CNB conversion, suggesting that Pt sites
were active for both reduction of NO2 group and activation of C–
Cl bond in p-CNB. In contrast, Pt/Al2O3@NC exhibited signi-
cantly improved p-CAN selectivity, and the selectivity to p-CAN
was strongly dependent on NC overcoating amount. Speci-
cally, increasing ion liquid amount to 25 ml per g catalyst
resulted in p-CAN selectivity increasing monotonically from
81.8% to 100% at 100% conversion of p-CNB, indicating effec-
tive inhibition of hydrodechlorination owing to NC coated on
the surface of Pt particles. Notably, because Cl group in p-CNB
would increase the electron density of NO2 group, the reduction
of NO2 group in p-CNB was favored by the presence of Cl
group.3,39 Accordingly, p-CAN is the main production in the
hydrogenation of p-CNB. Aer p-CNB was 100% converted,
however, the catalytic selectivity to p-CAN markedly decreased
on all catalysts except Pt/Al2O3@NC-25. For example, the
selectivity to p-CAN on Pt/Al2O3, Pt/Al2O3@NC-10 and Pt/
Al2O3@NC-15 decreased to 54.2%, 92.8% and 95.9% aer
180 min of reaction time, respectively. In contrast, Pt/
Al2O3@NC-25 maintained 100% selectivity to p-CAN throughout
the catalytic hydrogenation process even when only p-CAN le
in the reaction system, clearly indicating that the catalytic
dechlorination of p-CAN was completely inhibited on Pt/
Al2O3@NC-25. In parallel, CO chemisorption showed that the
coverage of Pt surface were 69%, 96% and 100% for Pt/
Al2O3@NC-10, Pt/Al2O3@NC-15 and Pt/Al2O3@NC-25 respec-
tively (see Table 1), conrming that C–Cl bond could only be
cleaved on exposed Pt sites and NC overcoating was incapable of
catalytic hydrodechlorination. Accordingly, the catalytic hydro-
genation of p-CNB on Pt/Al2O3 and Pt/Al2O3@NC-25 were
depicted in Scheme 1.
Table 2 Catalytic performances of the catalystsa
Reaction
time
p-CAN
selectivity
(%)
Aniline
selectivity
(%)
Conversion
(%)
Catalysts
(min)
Pt/Al2O3
30
50
90
150
180
180
100.0
100.0
100.0
100.0
0.6
81.8
97.1
98.6
100.0
100.0
100.0
18.2
2.9
1.4
0.0
0.0
0.0
Pt/Al2O3@NC-10
Pt/Al2O3@NC-15
Pt/Al2O3@NC-25
Al2O3@NC-25
Pt/Al2O3@SiO2
1.1
a
Reaction conditions: catalyst dosage: 20 mg, alcohol: 20 ml, p-
nitrochlorobenzene: 100 mg, reaction temperature: 80 ꢀC, reaction
time: 60 min; H2 pressure: 1 MPa.
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residual NC matrix impacted the catalytic selectivity of Pt/
Al2O3@NC-25-calcined. XPS results suggested that strong
interactions and electron transfer existed between NC over-
coating and Pt, causing negatively charged Pt sites. Because
NH2 groups in p-CAN tended to donate electron to Cl groups,
which would give rise to electrostatic repulsion between Cl
group and negatively charged Pt sites and thus suppressed the
cleavage of C–Cl bond.10,40,41 Similarly, Shi et al.35 reported that
Pt supported on N-doped carbon nanotube exhibited high
selectivities to haloanilines for catalytic hydrogenation of hal-
onitrobenzenes because of the electron-rich chemical state of Pt
particles.
Scheme 1 Schematic illustration of catalytic hydrogenation of p-CNB
on Pt/Al2O3 and Pt/Al2O3@NC-25.
To further explore the effect of NC overcoating on the cata-
lytic performances of the coated catalysts. Pt/Al2O3@NC-25 was
ꢀ
calcined at 500 C in air for 2 h to remove NC coating and the
The stability of catalyst is of important signicance for
supported noble metal catalysts. For stability test, the consec-
utive catalytic hydrogenation of p-CNB was examined on Pt/
Al2O3@NC-25 and results are shown in Fig. 7. It was very
interesting to nd that in the initial two cycles markedly
increased activity of Pt/Al2O3@NC-25 from 47.3% to 100% at
30 min reaction time was observed, likely due to the presence of
exposed Pt sites as a result of partial elimination of NC over-
coating via washing catalyst surface by reaction solution.
Accordingly, previous studies indicated that surface coke of
used catalysts could be partially removed via washing by organic
solvents.42 Consistently, characterization results showed that
CO chemisorption amount of Pt/Al2O3@NC-25 was 4.3 mmol gꢂ1
aer 5 catalytic cycles (see Table 3), reecting a portion of Pt
sites were again exposed. Aer 5 catalytic cycles, the catalytic
activities remained constant and no catalyst deactivation was
observed indicative of a very high catalytic stability of Pt/
Al2O3@NC-25. HAADF-STEM and ICP results showed that the
catalytic hydrogenation of p-CNB was conducted on the calcined
catalyst. The results are presented in Fig. 6. In comparison with
Pt/Al2O3@NC-25, Pt/Al2O3@NC-25-calcined exhibited a signi-
cantly enhanced catalytic activity, which was very close to that of
Pt/Al2O3. The increased catalytic activity of Pt/Al2O3@NC-25-
calcined can be explained by the removal of NC overcoating
on catalyst surface via calcination as reected by TPO and CO
chemisorption results. However, the catalytic selectivity of Pt/
Al2O3@NC-25-calcined was still much higher than that of Pt/
Al2O3, with a slightly decreased selectivity of 97.9% aer
180 min reaction time in comparison with Pt/Al2O3@NC-25. CO
chemisorption suggested that partial NC matrix remained on
Pt/Al2O3@NC-25-calcined surface. Hence, the high selectivity of
Pt/Al2O3@NC-25-calcined was probably related with NC matrix
le on catalyst surface. On one hand, the remaining NC matrix
would block partial Pt sites from catalytic hydrodechlorination.
On the other hand, the interactions between Pt sites and
Fig.
6 Catalytic hydrogenation of p-CNB on Pt/Al2O3@NC-25- Fig. 7 The reuse of Pt/Al2O3@NC-25 for catalytic hydrogenation of p-
calcined in comparison with Pt/Al2O3@NC-25 and Pt/Al2O3. Reaction CNB. Reaction conditions: catalyst dosage: 20 mg, alcohol: 20 ml, p-
conditions: catalyst dosage: 20 mg, alcohol: 20 ml, p-NCB amount: NCB amount: 100 mg, reaction temperature: 80 ꢀC, H2 pressure:
100 mg, reaction temperature: 80 ꢀC, H2 pressure: 1 MPa.
1 MPa.
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Table 3 Structural properties of the catalysts
Notes and references
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