Excellent catalytic properties over nanocomposite catalysts for selective hydrogenation of halonitrobenzenes

Article abstract of DOI:10.1016/j.jcat.2008.02.025

A partially reduced Pt/γ-Fe₂O₃ magnetic nanocomposite catalyst (Pt/γ-Fe₂O₃-PR) exhibited excellent catalytic properties in the selective hydrogenation of 2, 4-dinitrochlorobenzene and iodonitrobenzenes. The selectivity to 4-chloro-m-phenylenediamine (4-CPDA), meta-iodoaniline (m-IAN), and para-iodoaniline (p-IAN) reached 99.9%, 99.8%, and 99.4%, respectively, at complete conversion of the substrates. The hydrodehalogenation of 4-CPDA and IANs was fully suppressed for the first time over Pt/γ-Fe₂O₃-PR. It was found that CO chemisorption on the Pt nanoparticles deposited on the partially reduced γ-Fe₂O₃ and Fe₃O₄ nanoparticles was very weak, implying a weak tendency of the electronic back-donation from the Pt nanoparticles to the π^* antibonding orbitals of the adsorbed molecules. We believe that this is a cause of the superior selectivity to the haloanilines in the hydrogenation reactions of interest over the Pt/γ-Fe₂O₃-PR catalyst.

Full text of DOI:10.1016/j.jcat.2008.02.025

Journal of Catalysis 255 (2008) 335–342
www.elsevier.com/locate/jcat
Excellent catalytic properties over nanocomposite catalysts
for selective hydrogenation of halonitrobenzenes
Minghui Liang, Xiaodong Wang, Hongquan Liu, Haichao Liu, Yuan Wang ∗
Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species,
College of Chemistry & Molecular Engineering, Peking University, Beijing 100871, PR China
Received 31 October 2007; revised 25 February 2008; accepted 27 February 2008
Abstract
A partially reduced Pt/γ -Fe O magnetic nanocomposite catalyst (Pt/γ -Fe O -PR) exhibited excellent catalytic properties in the selective
2
3
2 3
hydrogenation of 2,4-dinitrochlorobenzene and iodonitrobenzenes. The selectivity to 4-chloro-m-phenylenediamine (4-CPDA), meta-iodoaniline
(m-IAN), and para-iodoaniline (p-IAN) reached 99.9%, 99.8%, and 99.4%, respectively, at complete conversion of the substrates. The hydrode-
halogenation of 4-CPDA and IANs was fully suppressed for the first time over Pt/γ -Fe O -PR. It was found that CO chemisorption on the Pt
2
3
nanoparticles deposited on the partially reduced γ -Fe O and Fe O nanoparticles was very weak, implying a weak tendency of the electronic
2
3
3 4
∗
back-donation from the Pt nanoparticles to the π antibonding orbitals of the adsorbed molecules. We believe that this is a cause of the superior
selectivity to the haloanilines in the hydrogenation reactions of interest over the Pt/γ -Fe O -PR catalyst.
2
3
© 2008 Elsevier Inc. All rights reserved.
Keywords: Platinum nanoclusters; Magnetic nanocomposite catalyst; Hydrogenation; 4-Chloro-m-phenylenediamine; Iodoaniline
1. Introduction
Over the activated Pt/γ -Fe2O3 nanocomposite catalyst, the hy-
drodechlorination of o-CAN was completely inhibited. Ag/SiO2
[6] and Au/SiO2 [7] also have been reported to exhibit very
excellent selectivity to CAN in the hydrogenation of CNB,
although further improvements in the catalytic activity of
Ag/SiO2 and Au/SiO2 seem to be necessary before these cata-
lysts can be used in the industrial production of CAN.
In the field of selective hydrogenation of halonitrobenzenes,
successive research efforts [8–12] have been made for the selec-
tive hydrogenation of chlorine-substituted mononitrobenzenes,
whereas reports on the selective hydrogenation of chlorine-
substituted dinitrobenzenes or iodine-substituted nitrobenzenes
have been very limited [13–16]. It is well known that the Cl–C
bond in chlorophenylenediamines (CPDAs) and the I–C bond
in iodoanilines (IANs) are more susceptible to hydrogenolysis
than the Cl–C bond in chloroanilines (CANs). This is due to
the facts that the second amino group as an electron-donating
group further promotes the hydrogenolysis of the Cl–C bond in
CPDAs [3,13,14], and that the large atomic diameter as well as
low electronegativity of iodine results in the much weaker I–C
bond relative to the Cl–C bond [14–16].
Aromatic haloamines are important organic intermediates in
the synthesis of organic dyes, perfumes, herbicides, pesticides,
preservatives, plant growth regulators, medicines, and light-
sensitive or nonlinear optical materials [1,2]. These widely
applied organic amines are produced mainly by the selec-
tive hydrogenation of the corresponding aromatic halonitro
compounds over metal catalysts, such as noble metals and
Raney nickel; however, hydrodehalogenation of the aromatic
haloamine products often occurs over most metal catalysts,
including platinum, palladium, rhodium, nickel, and copper
chromite, because hydrogenolysis of the carbon–halogen bond
is enhanced by amino substitution in the aromatic rings [3].
Recently, we reported that Ru/SnO2 [4] and magnetic Pt/
γ -Fe2O3 [5] nanocomposite catalysts exhibited excellent cat-
alytic activity and selectivity to ortho-chloroaniline (o-CAN)
in the hydrogenation of ortho-chloronitrobenzene (o-CNB).
*
Corresponding author. Fax: +86 10 6276 5769.
E-mail address: wangy@pku.edu.cn (Y. Wang).
0021-9517/$ – see front matter © 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2008.02.025

336
M. Liang et al. / Journal of Catalysis 255 (2008) 335–342
Winans reported that in the hydrogenation of 2,4-dinitro-
2. Experimental
chlorobenzene (2,4-DNCB) and o-iodonitrobenzene (o-INB)
over a nickel catalyst, 91% of 2,4-DNCB was transformed into
the dechlorinated byproduct m-phenylenediamine (m-PDA),
and the yield of o-iodoaniline (o-IAN) was only 23% [14].
Krishnan determined that the yield of 4-CPDA in the hydro-
genation of 2,4-DNCB over a Pt/C catalyst was about 89%, and
the byproducts were mainly the dechlorinated compounds [17].
Earlier strategies for minimizing such hydrodehalogenation in
the hydrogenation of DNCBs and INBs involved mainly ei-
ther alloying the catalytic metals or introducing dehalogenation
inhibitors into the reaction media. For example, Maleski and
Mullins reported that over Raney nickel catalysts alloyed with
Al, Mo, or Co, the highest yield of 4-CPDA in the hydro-
genation of 2,4-DNCB could be improved to ca. 92.5% [18].
For this reaction, Baumeister et al. decreased the yield of the
dechlorinated byproducts to 3% over Raney nickel by intro-
ducing formamidine acetate as an inhibitor [13]. It also has
been reported that in the hydrogenation of m-INB, introduc-
ing phosphorous acid as an inhibitor into the reaction system
could increase the yield of m-IAN to 93% over a Pt/C cat-
alyst [16]. But these methods still cannot provide complete
suppression of the hydrodehalogenation of aromatic haloaniline
products. Moreover, addition of the dehalogenation inhibitors
into the reaction systems often restrains the hydrogenation rates
of halonitrobenzenes and leads to difficulty in separating the
products.
Consequently, it remains a challenge to completely inhibit
the hydrodehalogenation of CPDAs and IANs in the hydrogena-
tion of DNCBs and INBs and maintain the high catalytic hy-
drogenation rates of the corresponding halonitrobenzenes over
noble metal catalysts. The production of CPDAs and IANs of
high purity by efficient hydrogenation of the corresponding
halonitrobenzenes is not only of commercial interest, but also of
significance in fundamental scientific research. In this paper, we
report the excellent catalytic properties of a partially reduced
Pt/γ -Fe2O3 nanocomposite catalyst (Pt/γ -Fe2O3-PR) for the
selective hydrogenation of 2,4-DNCB and INBs. Over this cat-
alyst, the reactions of interest could be accomplished with high
rates and superior selectivities to the target products 4-CPDA,
m-IAN, and p-IAN. The hydrodehalogenation of CPDA and
IANs was fully inhibited for the first time.
2.1. Reagents
Hydrogen (99.999%) was supplied by Beijing Gases Com-
pany. Pt/C (5 wt%) was purchased from ACROS. 2,4-DNCB
(ACROS, 99%), 2-chloro-5-nitroaniline (ACROS, 99%),
4-chloro-3-nitroaniline (ACROS, 97%), 4-CPDA (TCI, 98%),
m-INB (Alfa, 99%), p-INB (Alfa, 98%), m-IAN (Alfa, 98%),
p-IAN (Alfa, 99%), biphenyl (Alfa, 99%) and methanol (Fisher,
HPLC grade, 99.9%) were used without further purification.
All other reagents used in this work were of analytical grade.
2.2. Catalyst preparation
The Pt/γ -Fe2O3 nanocomposite catalyst was prepared as de-
scribed previously [5], with some modifications. In a typical
experiment, an aqueous solution of ammonia (10%) was added
dropwise to a solution of FeCl3 in 100 ml of water (4%) to
adjust the pH to about 7.5, producing a precipitate that was sep-
arated by a filter, washed with water, and peptized in 30 ml
of aqueous solution of FeCl3 (1.2%), resulting in a transpar-
ent colloidal solution of ferric hydroxide. The ferric hydroxide
colloidal solution was mixed with 20% volume of ethylene gly-
col and a colloidal solution of Pt nanoclusters (dav = 2.2 nm)
in ethylene glycol [20] at the required ratio. The mixture was
heated in a Teflon-lined autoclave at 353 K for 3 days to pro-
duce a magnetic precipitate, which was separated by an applied
magnetic field, washed with water and methanol, and dried at
353 K in air, giving a brownish-red solid of Pt/γ -Fe2O3 (1 wt%
Pt).
To study the IR spectra of CO adsorbed on the catalysts,
four samples were prepared involving Pt/γ -Fe2O3-PR, Pt/γ -
Fe2O3-PR-O, Pt/γ -Fe2O3-R, and Pt/Fe3O4-imp. Pt/γ -Fe2O3-
PR (the activated catalyst) was obtained by reducing fresh
Pt/γ -Fe2O3 (1 wt% Pt) in methanol at 303 K with hydrogen
under atmospheric pressure for 45 min and drying under vac-
uum. Pt/γ -Fe2O3-PR-O was obtained through the oxidation of
Pt/γ -Fe2O3-PR at 353 K for 2 days in air. Pt/γ -Fe2O3-R was
prepared by reducing fresh Pt/γ -Fe2O3 under flowing hydro-
gen at 373 K for 30 min in a tubular furnace. Pt/Fe3O4-imp (Pt:
3 wt%) was prepared by the conventional impregnation method
with H2PtCl6·6H2O and γ -Fe2O3 as the initial materials.
The perfect selectivity to haloanilines over our Pt/γ -Fe2O3-
PR nanocomposite catalyst in the hydrogenation of haloni-
trobenzenes, including CNB [5], bromonitrobenzene (BNB)
[19], DNCB, and INB reflects some specific interactions
between the nanoparticles of Pt and iron oxides in Pt/
γ -Fe2O3-PR.
It was found for the first time that CO could hardly be
chemisorbed on the Pt nanoparticles deposited on the par-
tially reduced γ -Fe2O3 nanoparticles under our measurement
conditions, implying a weak tendency of the d-electron back-
donation from the Pt nanoparticles to the π∗ antibonding or-
bitals of the adsorbed molecules. We believe that this character-
istic is a reason for the superior selectivity to the haloanilines
in the hydrogenation reactions of interest over the partially re-
duced Pt/γ -Fe2O3 catalyst (Pt/γ -Fe2O3-PR).
2.3. Catalytic reactions
Hydrogenation of 2,4-DNCB, m-INB, or p-INB was carried
out in a 50-ml reactor under magnetic stirring at 303 K and at-
mospheric pressure. Before the reaction, air in the system was
replaced by hydrogen 15 times, and the catalyst, Pt/γ -Fe2O3
(1 wt%, 0.05 g) or Pt/C (5 wt%, 0.01 g), dispersed in 10 ml
methanol, was activated under hydrogen for 30 min. Then a
methanol solution of 2,4-DNCB, m-INB, or p-INB was added
to the reactor to start the reaction. The hydrogen uptake was
monitored by a constant pressure gas burette. The products
were identified by GC-MS coupling (Perkin-XL) and quantita-
tively analyzed by gas chromatography (Shimadzu GC-2010),

M. Liang et al. / Journal of Catalysis 255 (2008) 335–342
337
using a flame ionization detector and a capillary column Rtx-5
MS (∅0.25 mm × 60 m). Biphenyl as an internal standard sub-
stance was added into the samples for GC measurement rather
than into the reaction system, to avoid its influence on the reac-
tion. The semiquantitative analysis of trace amounts (<0.1%)
of aniline (AN) or m-PDA in the reaction mixture was con-
ducted by comparing the peak area of AN or m-PDA in the GC
analyses with that of a standard solution with an AN or m-PDA
content of 0.1%.
2.4. Characterization of the catalyst
2.4.1. IR-CO probe characterization of the catalysts
Five samples were tested involving fresh Pt/γ -Fe2O3
(25 mg), Pt/γ -Fe2O3-PR (25 mg), Pt/γ -Fe2O3-PR-O (25 mg),
Pt/γ -Fe2O3-R (25 mg), and Pt/Fe3O4-imp (15 mg). In a typ-
ical experiment, the Pt/γ -Fe2O3 catalyst was ground under
air and pressed into a self-supporting wafer with a diameter
of 1.5 cm, which was treated in the IR cell under vacuum of
1 ∼ 3 × 10−5 Torr at 373 K for 2 h to remove adsorbed water
and impurities on the sample. After the sample was cooled to
303 K, 5 kPa of CO was charged into the IR cell to achieve the
chemisorption equilibrium. The gaseous CO was removed by
evacuation, and the spectrum of CO adsorbed on the catalyst
was recorded on a FTIR spectrometer (Tensor 27, Bruker) with
a resolution of 4 cm−1. The IR spectrum of the sample before
contacting CO was used as a subtraction background.
Fig. 1. STEM image of Pt nanoparticles dispersed on the iron oxide support.
The small bright spots were Pt nanoparticles which deposited on the nanoparti-
cles of iron oxide (see Ref. [5]).
In another experiment, a self-supporting wafer of the Pt/
γ -Fe2O3-PR catalyst (25 mg) was first treated under flowing
hydrogen at 323 K for 2 h, and then heated at 573 K in vac-
uum for 3 h in the IR cell. IR-CO probe measurements on this
sample were conducted in the same manner and conditions as
applied in the typical experiment.
Fig. 2. Cumulative hydrogen uptake of 2,4-DNCB hydrogenation over Pt/
γ -Fe O -PR and Pt/C catalysts. Reaction conditions: temperature, 303 K; hy-
2
3
drogen pressure, 0.1 MPa; substrate, 0.5 mmol; solvent, 20 ml methanol; load-
ing of Pt, 0.5 mg.
2.4.2. CO chemisorption on the catalysts
In the chemisorption experiments on the fresh and partially
reduced Pt/γ -Fe2O3 catalysts, the CO/Pt ratio was determined
on an ASAP 2010C. In the analysis of the reduced sample,
0.05 g of Pt/γ -Fe2O3 (3 wt%) was loaded into a U-type tube
and treated in the following steps: evacuation at 308 K for
0.5 h and then at 473 K for 1 h, reduction by flowing hydro-
gen at 383 K for 1 h, and evacuation at 573 K for 2 h and at
308 K for 0.5 h. The CO adsorption was measured at 308 K
and at 4–25 kPa of CO. The fresh catalyst was treated and
analyzed as described previously, except for the hydrogen re-
duction process.
to the analysis chamber after the pretreatment chamber was
flushed with argon and evacuated.
3. Results and discussion
The Pt/γ -Fe2O3 (1 wt%) nanocomposite catalyst used in this
work had a specific surface area of 79 m2/g. The Pt nano-
clusters, with an average diameter of 2.2 nm as measured by
scanning transmission electron microscopy (STEM), were well
dispersed in the matrix composed of γ -Fe2O3 nanoparticles, as
shown in Fig. 1. During activation of the Pt/γ -Fe2O3 catalyst
in methanol under 0.1 MPa of hydrogen pressure at 303 K, a
color change from brownish-red to black was observed. XRD
measurements revealed that γ -Fe2O3 nanoparticles in the cat-
alyst were partially reduced by hydrogen, due to the catalytic
function of the attached Pt nanoclusters [19]; therefore, the ac-
tual catalyst in the present catalytic reactions was the partially
reduced Pt/γ -Fe2O3 nanocomposite (Pt/γ -Fe2O3-PR).
2.4.3. XPS measurement of the catalysts
XPS measurements were carried out with an Axis Ultra
photoelectron spectrometer using an AlKα (1486.7 eV) X-ray
source, with the pressure of the measuring chamber set at
5 × 10−9 Torr. The binding energy scales for the samples were
referenced by setting the C 1s binding energy of contamina-
tion carbon to 284.8 eV. The Pt/γ -Fe2O3-PR catalyst for the
XPS measurement was prepared by reducing fresh Pt/γ -Fe2O3
(Pt: 1 wt%) with hydrogen at 303 K for 1 h in the pretreat-
ment chamber of the XPS spectrometer. The sample was moved
Fig. 2 shows the cumulative hydrogen uptake curves in the
hydrogenation of 2,4-DNCB over Pt/γ -Fe2O3-PR and Pt/C cat-
alysts. Over the Pt/γ -Fe2O3-PR nanocomposite catalyst, the

338
M. Liang et al. / Journal of Catalysis 255 (2008) 335–342
Table 1
Catalytic properties of Pt/γ -Fe O -PR and Pt/C for the hydrogenation of
2
3
a
m-INB
Catalyst
Reaction
time (min)
Initial H uptake Yield of products (mol%)
2
b
−2
rate (×10
)
m-IAN
AN
c
Pt/C
75
160
46
42
35
23
51
58
d
Pt/C
Pt/C
66
30
300
d
Pt/γ -Fe O -PR 38
99.8
99.8
0.2
0.2
2
3
Pt/γ -Fe O -PR 300
2
3
a
Reaction conditions: temperature, 303 K; hydrogen pressure, 0.1 MPa; sub-
strate, 0.375 mmol; solvent, 30 ml methanol; loading of Pt, 0.5 mg.
b
mol
/(mol s).
Pt
Hydrogen
c
The reaction was not complete.
Reaction time for the complete conversion of the substrate and all interme-
d
diates.
Fig. 3. Composition of the reaction mixture of 2,4-DNCB hydrogenation over
the Pt/γ -Fe O -PR catalyst as a function of time. Reaction conditions were
2
3
ing the reaction time to 5 h caused no detectable increase in the
m-PDA content. On the contrary, as shown in Fig. 4, over the
Pt/C catalyst, the highest yield of 4-CPDA in the entire hydro-
genation process was only 65%. When the reaction was con-
ducted for 50 min, the yield of 4-CPDA decreased to 30%, and
the content of m-PDA in the reaction mixture increased to 62%.
The appearance, increased and decreased content, and
disappearance of the 4-chloro-3-nitroaniline (4-CNAN) and
2-chloro-5-nitroaniline (2-CNAN) intermediates also were ob-
served over both the Pt/γ -Fe2O3-PR and Pt/C catalysts. In
addition, the formation of the coupling intermediates, such as
3,3ꢀ-diaminoazo- and azoxychlorobenzenes, was detected in
the reaction process by GC-MS analysis. The further increase
in yield of 4-CPDA over the Pt/γ -Fe2O3-PR catalyst after the
complete exhaustion of the substrate as well as 4-CNAN and
2-CNAN intermediates should be derived from the transforma-
tion of the coupling intermediates.
similar to those in Fig. 2.
The results for m-INB hydrogenation over the Pt/γ -Fe2O3-
PR and Pt/C catalysts are summarized in Table 1. Over the Pt/C
catalyst, the reaction time for the complete conversion of the
substrate and all of the intermediates was 160 min under the
specified conditions. When the substrate and intermediate prod-
ucts were exhausted, the selectivity to m-IAN, AN, and other
byproducts was 42%, 51%, and 7%, respectively. The highest
yield of m-IAN obtained over the Pt/C catalyst was only 46%
during the entire course of the reaction. But excellent catalytic
properties for the hydrogenation of m-INB were achieved over
the Pt/γ -Fe2O3-PR nanocomposite catalyst. Over this catalyst,
the reaction time for the complete conversion of the substrate
and all of the intermediates was only 38 min, with an average
product formation rate of 6.4 × 10−2 molm-IAN/(molPt s). Ob-
viously the average formation rate of m-IAN over Pt/γ -Fe2O3-
PR was much higher than that over the Pt/C catalyst, although
the initial rate of hydrogen uptake over the Pt/C catalyst was
2.2 times that over Pt/γ -Fe2O3-PR. The m-IAN selectivity of
99.8% was obtained at complete conversion of the substrate and
intermediates over Pt/γ -Fe2O3-PR, and the content of AN in
the final products was only about 0.2%. Moreover, extending
the reaction time after the exhaustion of the substrate and all in-
termediates caused no detectable increase in the content of the
deiodination byproduct over the Pt/γ -Fe2O3-PR catalyst.
Fig. 4. Composition of the reaction mixture of 2,4-DNCB hydrogenation over
the Pt/C catalyst as a function of time. Reaction conditions were similar to those
in Fig. 2.
hydrogen uptake reached the theoretical amount for the com-
plete transformation of 2,4-DNCB into 4-CPDA within 35 min.
Subsequently, no further increase in the hydrogen uptake was
detected during the extended reaction time. Quite different fea-
tures of the hydrogen uptake curve were observed over the
Pt/C catalyst, which was characterized by a fast initial rate of
hydrogen consumption, with a further increase in hydrogen up-
take after the stoichiometric hydrogen for producing 4-CPDA
was consumed. A similar hydrogen uptake discrepancy over the
Pt/γ -Fe2O3-PR and Pt/C catalysts was observed in the hydro-
genation of m-INB and p-INB.
The reaction pathways in the hydrogenation of halonitroben-
zenes have been intensively investigated [3,4,19]. The evolution
of the detectable reaction compounds measured by GC during
the hydrogenation of 2,4-DNCB over Pt/γ -Fe2O3-PR and Pt/C
is characterized in Figs. 3 and 4. It can be seen from Fig. 3 that
over the Pt/γ -Fe2O3-PR catalyst, the yield of the desired prod-
uct 4-CPDA increased monotonously, reaching 99.9% when
the reaction was accomplished. The content of dechlorinated
byproduct m-PDA in the final products was <0.1%. Extend-

M. Liang et al. / Journal of Catalysis 255 (2008) 335–342
339
In the case of p-INB hydrogenation, the highest yield of
p-IAN obtained over the Pt/C catalyst was 43%. When the sub-
strate and intermediates were completely exhausted, the p-IAN
yield decreased to 33%. Sequentially extending the reaction
time under the reaction conditions would cause further deio-
dination of p-IAN. When the reaction was conducted for
300 min, the content of the desired product, p-IAN, was only
9% over the Pt/C catalyst, whereas the content of AN in the
reaction mixture increased to 83% (Table 2). In contrast, over
Pt/γ -Fe2O3-PR, the selectivity to p-IAN was as high as 99.4%.
This high selectivity could be maintained even when the re-
action time was extended by several hours after p-INB was
exhausted. Similar to the case of m-INB hydrogenation, al-
though the initial rate of hydrogen uptake over the Pt/C catalyst
was 2.5 times that over Pt/γ -Fe2O3-PR, the average formation
rate of p-IAN was much higher over Pt/γ -Fe2O3-PR than over
the Pt/C catalyst. This finding may be related to the polariza-
tion effect of the iron cations in the catalyst surface on the –NO
groups in iodonitrosobenzene [19].
Reaction conditions have been reported to have certain ef-
fects on the hydrogenation rate and catalytic selectivity in the
hydrogenation of CNBs over some supported metal catalysts
[16,21]. In the present work, we investigated the influence of re-
action temperature and hydrogen pressure on the hydrogenation
of p-INB over the Pt/γ -Fe2O3-PR catalyst; the results, summa-
rized in Table 3, show that the hydrogenation rate of p-INB over
the Pt/γ -Fe2O3-PR catalyst reached a maximum value with in-
creasing reaction temperature, with the optimum temperature
about 313 K. At temperatures above 323 K, the hydrogena-
tion rate decreased significantly, and the selectivity to p-IAN
declined slightly. The reaction rate was enhanced 12-fold with
increasing hydrogen pressure from 0.1 to 8.0 MPa; an increased
hydrogen pressure also had a positive effect on catalyst selec-
tivity.
We also investigated the stability of the Pt/γ -Fe2O3-PR cata-
lyst at 293 K and 3.0 MPa of hydrogen. In the hydrogenation of
m-INB with a substrate/Pt molar ratio of 50,000 and a substrate
concentration of 0.5 M, a selectivity to m-IAN of 99.9% and an
average catalytic activity of 1.85 molm-INB/(molPt s) were ob-
tained over Pt/γ -Fe2O3-PR. Moreover, the recovered catalyst
could be reused for recycling experiments, and a total turnover
number (TON) of >100,000 over Pt/γ -Fe2O3-PR for m-INB
hydrogenation could be achieved with no detectable loss of the
superior selectivity to m-IAN. These results suggest that the
Pt/γ -Fe2O3-PR catalyst is very stable and suitable for practical
applications.
To verify the suppression capability of the partially re-
duced γ -Fe2O3 nanoparticles in Pt/γ -Fe2O3-PR for the hy-
drodehalogenation of haloanilines over the Pt nanoparticles,
we compared the hydrogenolysis activity of carbon-halogen
bonds in 4-CPDA and p-IAN over the Pt/γ -Fe2O3-PR, fresh Pt/
γ -Fe2O3, and Pt/C catalysts. As shown in Table 4, over the Pt/C
catalyst, in a 5-h hydrogenolysis of p-IAN, 97% of p-IAN was
consumed and converted into 93% of AN and 4% of other prod-
ucts. In contrast, no dehalogenated products were detected over
Table 3
Effects of reaction temperature and hydrogen pressure on the hydrogenation of
a
Table 2
p-INB over the Pt/γ -Fe O -PR catalyst
Catalytic properties of Pt/γ -Fe O -PR and Pt/C for the hydrogenation of
2
3
2
3
a
p-INB
Reaction
tempera-
ture (K)
P
Loading
of Pt
(mg)
Reaction
Yield of products (mol%)
H
2
b
(MPa)
rate
Catalyst
Reaction
time (min)
Initial H uptake Yield of products (mol%)
p-IAN
AN
2
−2
b
−2
(×10
)
rate (×10
)
p-IAN
AN
c
303
313
323
333
303
303
303
303
0.1
0.1
0.1
0.1
1.0
2.0
4.0
8.0
0.5
0.5
0.5
0.5
0.1
0.1
0.1
0.1
6.8
8.1
6.6
3.4
44
64
76
81
99.4
99.4
99.3
99.2
99.5
99.6
99.7
99.8
0.6
0.6
0.7
0.8
0.5
0.4
0.3
0.2
Pt/C
45
90
43
33
9
99.4
99.4
25
59
83
0.6
0.6
d
Pt/C
Pt/C
90
36
300
d
Pt/γ -Fe O -PR 36
Pt/γ -Fe O -PR 300
2
3
2
3
a
b
c
d
Reaction conditions are similar to those in Table 1.
mol /(mol s).
The reaction was not complete.
Reaction time for the complete conversion of the substrate and all interme-
Pt
Hydrogen
a
Reaction conditions: p-INB, 0.375 mmol; solvent, 30 ml methanol.
b
Average rate, mol
/(mol s).
Pt
p-INB
diates.
Table 4
a
Hydrodehalogenation of 4-CPDA and p-IAN over the Pt-based catalysts
b
c
Catalyst
Hydrodechlorination of 4-CPDA
Hydrodeiodination of p-IAN
Reaction
time (h)
Composition (mol%)
4-CPDA
Reaction
time (h)
Composition (mol%)
m-PDA
p-IAN
AN
d
Pt/γ -Fe O
1
15
–
99.7
>99.9
–
0.3
<0.1
–
–
5
5
–
–
2
3
e
Pt/γ -Fe O -PR
>99.9
<0.1
93
2
3
Pt/C
3
a
Reaction conditions: temperature, 303 K; hydrogen pressure, 0.1 MPa; loading of Pt, 0.5 mg.
0.50 mmol of 4-CPDA in 20 ml methanol.
0.375 mmol of p-IAN in 30 ml methanol.
Catalyst was used without the activation process.
Catalyst was activated at 303 K and 0.1 MPa of hydrogen pressure for 30 min before used for the reaction.
b
c
d
e

340
M. Liang et al. / Journal of Catalysis 255 (2008) 335–342
Pt/γ -Fe2O3-PR during the reaction process of either p-IAN
or 4-CPDA, further confirming that the hydrodehalogenation
of the haloanilines was fully inhibited over Pt/γ -Fe2O3-PR.
The trace of m-PDA or AN produced in the hydrogenation of
2,4-DNCB or INBs over this catalyst was derived from other
hydrogenolysis processes [19]. Moreover, over a Pt/Fe3O4 cat-
alyst (Pt: 3 wt%) prepared by the impregnation method, the
same selectivity to 4-CPDA in the hydrogenation of 2,4-DNCB
was achieved, and the hydrodehalogenation reaction of the
product was also completely suppressed. But over fresh Pt/
γ -Fe2O3 without activation with H2 before the reaction, a slow
dehalogenation process was observed in the hydrogenolysis of
4-CPDA. These experimental findings indicate that the struc-
ture of the actual catalytic site that exhibits excellent selectivity
to the aromatic haloamines is characterized by Pt nanoparticles
deposited on the partially reduced γ -Fe2O3 or Fe3O4 particles.
Regarding the mechanism of the hydrodehalogenation of
haloanilines over metal catalysts, most researchers agree that
it involves an electrophilic attack of cleaved hydrogen on
the carbon–halogen bond [22–24]. We have previously re-
ported that the electronic back-donation from Pt nanoclusters
to the aromatic ring in the produced haloanilines favors the
hydrogenolysis reaction, because such a back-donation effect
weakens the p–π conjugation between the halogen atom and
the benzene ring [5].
To investigate the electronic back-donation effect of the Pt
particles in the Pt/γ -Fe2O3-PR catalyst, we conducted IR-CO
probe and CO chemisorption experiments on the catalysts, be-
cause the chemisorption of CO on iron oxide is negligible at
room temperature [25,26]. The chemisorption of CO on the sur-
face of Pt nanoparticles occurs mainly in a linear form [27].
The extent of electronic back-donation from the d orbitals of
Pt to the 2π∗ orbital of CO plays an important role in the
stability of Pt–CO coordination [26]. If the extent of the back-
donation from Pt particles is weakened, then the adsorption
strength of CO on the Pt particles decreases, and the amount
of CO chemisorbed on the Pt-based catalysts is decreased ac-
cordingly.
Fig. 5a shows the IR spectrum of CO adsorbed on the fresh
Pt/γ -Fe2O3 catalyst. In this spectrum, a strong peak centered
at 2070 cm−1 can be attributed to the signal of CO adsorbed
on the on-top site in Pt surface, whereas a broad band in the
range of 1730–1870 cm−1 may be ascribed to the signals of
CO adsorbed on the bridge and 3-fold sites on the Pt surface
[26,28]. However, the typical signals of CO adsorbed on Pt
were not detected in the IR-CO probe measurements on Pt/
γ -Fe2O3-PR (Fig. 5b), suggesting that the chemisorption of CO
on the Pt nanoparticles in the Pt/γ -Fe2O3-PR was very weak
and unstable at room temperature. The magnetic and brownish-
red Pt/γ -Fe2O3-PR-O catalyst was prepared by the oxidation of
Pt/γ -Fe2O3-PR in air at 353 K for 2 days. The typical signals of
CO adsorbed on Pt particles were clearly detected in the IR-CO
probe measurements on Pt/γ -Fe2O3-PR-O (Fig. 5c), confirm-
ing that CO can be stably adsorbed on the Pt/γ -Fe2O3 catalysts.
To avoid the possible influence of the solvent or impurities
on the IR-CO probe measurements on Pt/γ -Fe2O3-PR, fresh
Pt/γ -Fe2O3 was partly reduced with hydrogenation at 373 K in
Fig. 5. Infrared spectra of CO adsorbed on (a) fresh Pt/γ -Fe O , (b) Pt/γ -
2
3
Fe O -PR prepared by reducing Pt/γ -Fe O in methanol with hydrogen at
2
3
2 3
303 K for 45 min, (c) Pt/γ -Fe O -PR-O prepared by oxidizing Pt/γ -Fe O -PR
2
3
2 3
at 353 K for 2 days under air, (d) Pt/γ -Fe O -R prepared by reducing Pt/
2
3
γ -Fe O by flowing hydrogen at 373 K for 30 min, (e) Pt/Fe O -imp prepared
2
3
3 4
by the impregnation method, and (f) Pt/γ -Fe O -PR-HT prepared by treating
2
3
Pt/γ -Fe O -PR with hydrogen in the IR cell at 323 K and heating in vacuum
2
3
at 573 K for 3 h.
a tubular furnace for 30 min. The results of IR-CO probe ex-
periments on the Pt/γ -Fe2O3-R sample (Fig. 5d) were identical
to those on Pt/γ -Fe2O3-PR. In addition, the signals of CO ad-
sorbed on the Pt particles in the Pt/Fe3O4 catalyst prepared by
the impregnation method also were not detectable in our exper-
iments (Fig. 5e).
To further validate the weak chemisorption of CO on Pt/
γ -Fe2O3-PR, we measured the amounts of CO chemisorbed
on the Pt nanoparticles in the Pt/γ -Fe2O3 and Pt/γ -Fe2O3-
PR catalysts. For the fresh Pt/γ -Fe2O3 catalyst evacuated at
573 K, the CO/Pt ratio in the CO chemisorption experiment
was determined to be 15.6%; in contrast, for the partially re-
duced catalyst Pt/γ -Fe2O3-PR treated under the same condi-
tions, the measured CO/Pt ratio was as low as 0.02%. Because
Pt/γ -Fe2O3-PR is a very active catalyst for the hydrogenation
of aromatic halonitrobenzenes, and because the signals of CO
adsorbed on the Pt particles in Pt/γ -Fe2O3 could be clearly
detected in the IR-CO probe measurements, the possibility of
complete encapsulation of the Pt nanoparticles by iron oxides
in Pt/γ -Fe2O3-PR can be ruled out. The CO chemisorption re-
sults did confirm the finding that chemisorption of CO on Pt
nanoparticles in the Pt/γ -Fe2O3-PR catalyst was very weak.
It should be mentioned that during the preparation of the
Pt/γ -Fe2O3 catalyst and the IR samples, oxygen is adsorbed
on the Pt particles, and thus a thin Pt-oxide skin possibly
could form on the Pt particles. But it is known that the Pt-
oxide skin can be reduced by CO at room temperature [29,30].
Moreover, Solomennikov and Davydov [31] reported that in
an IR-CO probe measurement at 298 K on a Pt/Al2O3 cata-
lyst that had been treated in oxygen at 673 K for 1 h, a signal
at 2090 cm−1, attributed to CO adsorbed on metallic Pt, was
clearly detected [31]. These previous findings account for the
finding of clearly detectable signals of CO adsorbed on the Pt
particles in our Pt/γ -Fe2O3 catalyst.

M. Liang et al. / Journal of Catalysis 255 (2008) 335–342
341
To avoid the possible influence of the adsorbed oxygen or Pt-
oxide skin on the IR-CO probe measurements on Pt/γ -Fe2O3-
PR, we first treated the IR sample of Pt/γ -Fe2O3-PR under
flowing hydrogen at 323 K for 2 h in the IR cell before the IR-
CO probe measurements (see the Experimental section). The
signal of CO adsorbed on the Pt particles in this sample could
not be detected in our experiments, as shown in Fig. 5f.
XPS measurements were conducted on the Pt/γ -Fe2O3-
PR catalyst and Pt nanoparticles with the same particle size
protected by poly(N-vinyl-2-pyrrolidone) (PVP). The PVP-
protected Pt particles were prepared by adding the colloidal
solution of Pt nanoclusters applied in the catalyst preparation
to an ethanol solution containing PVP. The colloidal solution
thus obtained was stirred in a hydrogen atmosphere at room
temperature for 1 h, after which acetone was added to the col-
loidal solution to form a precipitate of the PVP-protected Pt
nanoparticles (PVP/Pt weight ratio = 9). The binding energy of
the Pt 4f7/2 core-level in Pt/γ -Fe2O3-PR had a value of 71.3 eV,
0.5 eV higher than that of the Pt particles protected by PVP
(70.8 eV). This provides evidence of an electron transfer from
the Pt nanoparticles to iron cations or the iron oxide particles in
Pt/γ -Fe2O3-PR.
Based on the foregoing results of hydrodehalogenation of
haloanilines and the adsorptive behavior of CO on the different
catalysts, a close relationship between the CO adsorption prop-
erties and the hydrodehalogenation activity over the catalysts
can be inferred. The fresh Pt/γ -Fe2O3 catalyst on which CO can
be stably adsorbed exhibited catalytic activity for the dechlo-
rination of 4-CPDA. In contrast, Pt/γ -Fe2O3-PR, on which
CO cannot be chemically adsorbed at room temperature, did
not catalyze the hydrodehalogenation of haloanilines. More-
over, some other Pt-based catalysts with a strong capability to
chemically adsorb CO, such as Pt/C [40], Pt/Al2O3 [41] and
Pt/TiO2 [42], can smoothly catalyze the hydrodehalogenation
of haloanilines [19,22].
The electron feedback from Pt to haloanilines adsorbed
on Pt/C, Pt/γ -Fe2O3, Pt/Al2O3, and Pt/TiO2 would make the
carbon-halogen bonds in the haloanilines more susceptible to an
electrophilic attack of the adsorbed hydrogen atoms. The weak
tendency of electronic back-donation from the Pt nanoparticles
in Pt/γ -Fe2O3-PR to the π∗ antibonding orbitals of adsorbed
haloanilines would not obviously weaken the p–π conjugation
between the halogen atom and the benzene ring. This is a key
factor in the extremely high selectivity to haloanilines in the
hydrogenation of halonitrobenzenes over Pt/γ -Fe2O3-PR.
Some iron oxides have been reported to be excellent cata-
lysts for the H-transfer reduction of monosubstituted nitroben-
zenes to corresponding anilines with hydrazine hydrate, and it
has been found that azo groups in the substrates cannot be re-
duced by this process in the absence of metal particles [43,44].
But over the present Pt/γ -Fe2O3-PR catalyst, the azo interme-
diates in the hydrogenation of CNB, BNB, and INB could be
easily reduced to the corresponding haloanilines; indicating that
over this catalyst, the hydrogenation of halonitrobenzenes oc-
curred mainly on the Pt nanoclusters deposited on the partly
reduced γ -Fe2O3 particles. The possible contribution of the
H-transfer reduction process to the reaction of interest over the
Pt/γ -Fe2O3-PR catalyst remains an open question.
The aforementioned results of IR-CO probe and CO chem-
isorption experiments definitely revealed that CO was hardly
chemisorbed on the Pt nanoparticles in the Pt/γ -Fe2O3-PR and
Pt/Fe3O4 catalysts under the measurement conditions. Thus
suggests a very weak tendency of electronic back-donation
from Pt nanoparticles in both catalysts to the adsorbed mole-
cules. This weak tendency is obviously related to the structures
of the iron oxide particles. γ -Fe2O3 and Fe3O4 have very sim-
ilar cubic inverse spinel crystal structures, in which the oxy-
gen anions (O2−) form a closely packed face-centered cubic
(fcc) sublattice with iron cations located in interstitial sites. In
γ -Fe2O3, all of the iron cations are Fe3+, whereas in Fe3O4,
there are two different kinds of cation sites: tetrahedrally co-
ordinated sites occupied by Fe3+ and octahedrally coordinated
sites occupied by Fe3+ and Fe2+ ions in equal numbers. The
Fe2+ cation can be considered to be Fe3+ plus an “extra” elec-
tron, hopping freely between the octahedral sites above the Ver-
wey transition temperature [32].
It is known that γ -Fe2O3 can be reduced to Fe3O4 with hy-
drogen at about 250 ◦C [33]. However, as mentioned above, by
virtue of the catalytic function of the Pt nanoparticles, γ -Fe2O3
nanoparticles in the Pt/γ -Fe2O3 nanocomposite were partially
reduced by H2 during the activation process at room tem-
perature; that is, part of the Fe3+ cations in the Pt/γ -Fe2O3
nanocomposite were reduced to Fe2+ cations during the mild
activation process. XRD studies have shown that the crystalline
structure of the partially reduced γ -Fe2O3 nanoparticles is quite
close to that of Fe3O4 [19]. Because the catalytic reduction of
the γ -Fe2O3 nanoparticles should start on their surfaces, it is
reasonable to deduce that the surface structure of the partially
reduced γ -Fe2O3 nanoparticles resembles that of Fe3O4 parti-
cles.
The (111) surface structure of Fe3O4 has been intensively in-
vestigated [25,34–39]. Under oxidation conditions at high tem-
perature, the exposed surface of Fe3O4(111) is terminated with
oxygen atoms [36–38]; however, under more reducing condi-
tions (i.e., annealing in vacuum without oxygen), the surface
of Fe3O4(111) is terminated by iron cations [39]. It has been
shown that Fe3O4(111) surface is terminated by 1/2 ML of
iron, with the outermost 1/4 ML consisting of octahedral Fe2+
cations situated above 1/4 ML of tetrahedral Fe3+ ions [25]. We
believe that during the partial reduction of γ -Fe2O3 nanoparti-
cles in the Pt/γ -Fe2O3 by H2, catalyzed by the deposited Pt
nanoparticles, some of the surface O2− anions in the γ -Fe2O3
nanoparticles will be removed by the formation of H2O, and
the Pt nanoparticles will directly contact the outermost layer of
iron cations. The positive electric fields of these cations or the
electron transfer from Pt nanoparticles to the cations may de-
crease the electronic b∗ack-donation from the d orbitals of the Pt
nanoparticles to the π anti-orbitals of the adsorbed molecules.
4. Conclusion
The Pt/γ -Fe2O3-PR nanocomposite catalyst exhibited very
excellent catalytic properties for the selective hydrogenation of
2,4-DNCB, m-INB, and p-INB. The formation of 4-CPDA,

342
M. Liang et al. / Journal of Catalysis 255 (2008) 335–342
m-IAN, and p-IAN was much higher over the Pt/γ -Fe2O3-PR
catalyst than over the Pt/C catalyst. The high catalytic selectiv-
ity (i.e., >99.9% to 4-CPDA, 99.4% to m-IAN, and 99.8% to
p-IAN) could be readily achieved at complete conversion of
the substrates. Moreover, the undesired hydrodehalogenation
of 4-CPDA and IANs over the Pt/γ -Fe2O3-PR nanocompos-
ite catalyst was fully suppressed for the first time. Increasing
the hydrogen pressure from 0.1 to 8.0 MPa not only increased
the hydrogenation rate of p-INB over the Pt/γ -Fe2O3-PR cat-
alyst by one order of magnitude, but also increased the p-IAN
selectivity to 99.8%. The Pt/γ -Fe2O3-PR catalyst also exhib-
ited excellent stability. The total TON for m-INB hydrogena-
tion exceeded 100,000 with an m-IAN selectivity of 99.9% at
293 K and 3.0 MPa H2. IR-CO probe and CO chemisorption
experiments revealed that CO was hardly chemisorbed on Pt
nanoparticles in the Pt/γ -Fe2O3-PR catalyst under the exper-
imental conditions, suggesting very week back-donation of d
electrons from the Pt nanoparticles to the adsorbed molecules.
This is considered to play an important role in the complete
suppression of hydrodehalogenation of haloanilines in the hy-
drogenation of halonitrobenzenes over this catalyst.
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Acknowledgments
This work was jointly supported by NSFC (grants 20573005,
50521201, 90206011, 20433010), Chinese Ministry of Science
and Technology (NKBRSF 2006CB806102), and the Ministry
of Education of China (RFDP). The authors thank Professor
Jinglin Xie, College of Chemistry and Molecular Engineering,
Peking University for his assistance with measuring the XPS
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