High Performance and Active Sites of a Ceria-Supported Palladium Catalyst for Solvent-Free Chemoselective Hydrogenation of Nitroarenes

Article abstract of DOI:10.1002/cctc.201700631

Cerium oxide-supported palladium catalysts (Pd/CeO₂) prepared by a simple impregnation method exhibit exciting catalytic activity and high chemoselectivity for the solvent-free hydrogenation of a variety of substituted nitroarenes including the reducible functional groups to the corresponding aromatic amines under mild reaction conditions. Taking nitrobenzene as an example, the Pd/CeO₂ catalyst can afford aniline yields of >99 % with turnover frequencies as high as 11 411 h^?1 and 69 824 h^?1 at 40 °C and 100 °C, respectively. Pd²⁺ ion species exist as isolated single atoms with ?Pd²⁺?O^2??Ce⁴⁺? linkages on the surface of Pd_xCe_1?xO_2?σ solid solution and are found to be active sites for the selective hydrogenation of nitroarenes in the absence of solvent. The superior catalytic performance can be attributed to the cooperative effect between Pd²⁺ ions and unique surface sites of CeO₂. A possible mechanism is proposed for the hydrogenation of nitroarenes with H₂ over the Pd/CeO₂. The Pd/CeO₂ catalyst can be recovered easily and reused for at least seven recycling reactions without loss of catalytic properties.

Full text of DOI:10.1002/cctc.201700631

DOI: 10.1002/cctc.201700631
Full Papers
High Performance and Active Sites of a Ceria-Supported
Palladium Catalyst for Solvent-Free Chemoselective
Hydrogenation of Nitroarenes
Xiuxiu Shi,[a] Xueguang Wang,*[a, b] Xingfu Shang,[b] Xiujing Zou,[a] Weizhong Ding,[a, b] and
Xionggang Lu*[a, b]
Cerium oxide-supported palladium catalysts (Pd/CeO2) pre-
pared by a simple impregnation method exhibit exciting cata-
lytic activity and high chemoselectivity for the solvent-free hy-
drogenation of a variety of substituted nitroarenes including
the reducible functional groups to the corresponding aromatic
amines under mild reaction conditions. Taking nitrobenzene as
an example, the Pd/CeO2 catalyst can afford aniline yields of
>99% with turnover frequencies as high as 11411 hÀ1 and
69824 hÀ1 at 408C and 1008C, respectively. Pd2+ ion species
exist as isolated single atoms with ÀPd2+ÀO2ÀÀCe4+À linkages
on the surface of PdxCe1ÀxO2Às solid solution and are found to
be active sites for the selective hydrogenation of nitroarenes in
the absence of solvent. The superior catalytic performance can
be attributed to the cooperative effect between Pd2+ ions and
unique surface sites of CeO2. A possible mechanism is pro-
posed for the hydrogenation of nitroarenes with H2 over the
Pd/CeO2. The Pd/CeO2 catalyst can be recovered easily and
reused for at least seven recycling reactions without loss of
catalytic properties.
Introduction
As important organic intermediates, aromatic amines are
widely used for the synthesis of dyes, agrochemicals, polymers,
pharmaceuticals, herbicides, and pigments.[1–3] The selective re-
duction of nitro aromatics is an important route for the pro-
duction of aromatic amines. Nowadays, the conventional re-
duction of nitro aromatics with stoichiometric reducing agents
such as Fe[4] and Zn[5] has become unacceptable because this
process produces large amounts of waste acids or bases and
residues, leading to serious environmental problems. In this
context, recyclable heterogeneous transition-metal catalysts
have extensively been investigated for the selective hydroge-
nation of nitro compounds to the corresponding amines by
using molecular hydrogen (H2) as an environmentally benign
and cost-effective reducing agent.[6–19] These catalytic process-
es frequently require the use of organic solvents and the aid of
additives (base and metal salts) in liquid-phase reaction sys-
tems, causing various secondary pollution troubles. On the
other hand, in most cases, Ni or Pt catalysts can afford high
catalytic activity for the hydrogenation of nitro aromatics with
H2, but they lack the necessary chemoselectivity when other
reducible functional groups such as alkene, nitrile, carbonyl,
hydroxyl, and halides are present in the same molecule;[20–22]
Au and Ag catalysts exhibit excellent chemoselectivity, but
poor catalytic activity owing to their low ability for H2 activa-
tion.[23–25] Therefore, there is a strong incentive to develop
highly active and recyclable heterogeneous catalysts that do
not require solvents and additives, and can quantitatively hy-
drogenate functional nitroarenes to the corresponding aromat-
ic amines with H2.[26,27]
Pd-based catalysts have been reported to be active and ex-
hibit a certain chemoselectivity for the hydrogenation of sub-
stituted nitro aromatics to aromatic amines.[28–33] The product
selectivity can be improved by means of modifying the proper-
ties of metal and supports, catalyst preparation process, and
selected reaction conditions, etc. Wang et al.[34] reported that
the presence of PdO could significantly improve the catalytic
performance of Pd catalysts for the reduction of 4-nitrophenol
with NaBH4, which exhibited much higher catalytic activity
than single metal Pd. Zhang et al.[35] reported that Pd clusters
supported on porous nanorods of CeO2 with a high surface
Ce3+ fraction and a large concentration of oxygen vacancies
showed high catalytic activity and chemoselectivity for the hy-
drogenation of various nitroarenes at 808C.
[a] Dr. X. Shi, Prof. X. Wang, X. Zou, Prof. W. Ding, Prof. X. Lu
State Key Laboratory of Advanced Special Steel
School of Materials Science and Engineering
Shanghai University
Shanghai 200072 (P.R. China)
Fax: (+86)21-56338244
E-mail: wxg228@shu.edu.cn
luxg@shu.edu.cn
CeO2 has attracted much attention in the fields of catalyst
and biological chemistry owing to its redox and unique acid–
base bifunctional properties,[36–38] which strongly affect the
properties of metal species and the adsorption characteristics
of the catalyst surface. However, these properties of CeO2 are
generally employed at high temperature (>473 K) in catalysis
[b] Prof. X. Wang, X. Shang, Prof. W. Ding, Prof. X. Lu
Key Laboratory of Advanced Ferrometallurgy
Shanghai University
Shanghai (P.R. China)
Supporting information for this article can be found under:
https://doi.org/10.1002/cctc.201700631.
ChemCatChem 2017, 9, 1 – 10
1
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

Full Papers
such as the conversion of exhaust gases, CO oxidation, CO/NO
reaction, water–gas shift reaction, and the oxidative coupling
of methane.[39,40] It has been reported that the acid–base bi-
functionality of CeO2 at low temperature (ꢀ373 K) is correlated
with high catalytic activity in liquid-phase organic reac-
tions.[41,42] However, the redox property of CeO2 at low temper-
ature (ꢀ373 K) in organic reactions has been rarely reported.
Recently, some direct observations have indicated that the
oxygen vacancies on CeO2 are mobile at 303 K, which is associ-
ated with the presence of surface sites with redox ability at
low temperature.[43–45] Tamura et al.[46] first exhibited the redox
property of CeO2 in the direct synthesis of imines from alcohol
and amine at a low temperature of 303 K.
Herein, we demonstrate for the first time that cerium oxide-
supported Pd catalysts (Pd/CeO2) exhibit high catalytic activity
and chemoselectivity for the hydrogenation of a variety of sub-
stituted nitroarenes to the corresponding aromatic amines in
the absence of solvent and additives at low temperatures. Pd
element was highly dispersed on the surface of the CeO2 sup-
port in the form of Pd2+ ions with ÀPd2+ÀO2ÀÀCe4+À linkages,
which served as active sites for the selective hydrogenation of
nitro aromatics with H2. A possible hydrogenation pathway is
proposed over the Pd/CeO2 catalyst for the hydrogenation of
nitroarenes with H2 in the absence of solvent.
Figure 1. XRD patterns of the xPd/CeO2 catalysts calcined at 3508C.
ture of CeO2 (JCPDS card no. 81–0792). The apparent sizes of
the CeO2 crystallites were estimated to range from 15 to
19 nm by using the full-widths at half maximum (FWHM) of
the CeO2 (111) peaks at 2q=28.58 by using the Scherrer equa-
tion. No diffraction peaks associated with Pd species were
found, even for the 5.0%Pd/CeO2 sample. This result indicated
that the Pd species were highly dispersed in the CeO2 matrices,
which was further confirmed by the representative transmis-
sion electron microscopy (TEM) images of the 2.5%Pd/CeO2
and 5.0%%Pd/CeO2 samples (Figure S1b–d in the Supporting
Information). In the TEM image, there was not any crystallite of
Pd species observed, demonstrating that the Pd species exist-
ed exclusively as isolated single atoms, or sub-nanometer clus-
ters on the catalyst surface or in matrices of CeO2 likely owing
to the strong interaction between the CeO2 and Pd species.
X-ray photoelectron spectra (XPS) were used to analyze the
oxidation states of Pd species on the surfaces of the xPd/CeO2
catalysts, and the results are illustrated in Figure 2. The Pd3d
spectra for the 1.0%Pd/CeO2 exhibited two symmetric peaks
positioned at 337.6 eV and 342.9 eV, respectively, which were
attributed to Pd2+ ions on the surface and sub-surface of
PdxCe1ÀxO2Às solid solution with ÀPd2+ÀO2ÀÀCe4+À linkag-
es.[47–51] With increasing the Pd content, the Pd3d spectra
could be clearly deconvoluted into two-group curves: one cor-
responded to Pd2+ ions on the surface of PdxCe1ÀxO2Às; and
the other peaks at 336.6 eV and 341.9 eV could be assigned to
PdOx species located on the catalyst surface.[51] The relative in-
tensity of PdOx to Pd2+ ions was clearly strengthened with the
Pd content in the xPd/CeO2 catalysts, indicating the increase in
the amount of PdOx on the catalyst surface. Table 1 lists the
molar ratios of [PdOx]/[PdOx +Pd2+] in the xPd/CeO2 catalysts,
estimated by the peak areas of the Pd3d spectra in Figure 2. It
could be seen that the [PdOx]/[PdOx +Pd2+] ratios increased to
56% if the Pd content was raised to 5.0%. Notably, the
amount of Pd2+ ions remained almost unchanged on the basis
Results and Discussion
Physical and chemical properties of Pd/CeO2 catalysts
The CeO2 support was prepared by the precipitation method
and calcined in air at 5008C for 5 h, and the Brunauer–
Emmett–Teller (BET) specific surface area of CeO2 was mea-
sured to be 111 m2 gÀ1. The xPd/CeO2 (x=1.0%, 2.0%, 2.5%,
3.0%, 5.0%) catalysts were prepared by the impregnation
method, followed by calcination at 3508C in air for 5 h. Table 1
summarizes the actual contents of Pd in the xPd/CeO2 cata-
lysts, which were determined by inductively coupled plasma
atomic emission spectrometry (ICP-AES), and were in agree-
ment with the nominal Pd contents in the xPd/CeO2 catalysts.
Powder X-ray diffraction (XRD) was first used to examine the
crystalline phases of the prepared materials. Figure 1 illustrates
the XRD patterns of the xPd/CeO2 catalysts. All the xPd/CeO2
catalyst showed strong diffraction peaks around 2q=28.58,
33.08, 47.48, and 56.38, corresponding to cubic fluorite struc-
Table 1. Actual contents of Pd and [PdOx]/[PdOx +Pd2+] molar ratios in
the xPd/CeO2.
[b]
Catalyst
Pd content[a]
[PdOx]/[PdOx +Pd2+
]
I(1171+597)/I462
[%]
[%]
CeO2
–
–
<1
8
16
27
56
4.8
5.9
9.8
10.7
15.4
18.8
1.0%Pd/CeO2
2.0%Pd/CeO2
2.5%Pd/CeO2
3.0%Pd/CeO2
5.0%Pd/CeO2
0.87
1.73
2.35
2.73
4.55
[a] from ICP. [b] from XPS.
&
ChemCatChem 2017, 9, 1 – 10
www.chemcatchem.org
2
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Papers
peaks at 265, 594, and 1170 cmÀ1 were attributed to the
second-order transverse acoustic mode, the defect-induced
mode, and the second-order longitudinal optical mode, respec-
tively.[51,55,56] For the xPd/CeO2 samples, two new peaks were
observed in the Raman spectra. The peak at approximately
649 cmÀ1 could be assigned to the vibration mode of the PdÀ
O bond, and the peak at 835 cmÀ1 could be ascribed to the
formation of PdxCe1ÀxO2Às solid solution with ÀPd2+ÀO2ÀÀCe4+
À bonds.[47,51] The peak intensity for the PdÀO bond always in-
creased with the Pd content, whereas the intensities for
PdxCe1ÀxO2Às remained almost unchanged if the Pd content
was more than 2.5%, which were consistent with the XPS re-
sults in Figure 2 and Table 1. On the other hand, after loading
Pd, the intensity ratio of I(1171+597)/I462, which reflected the intrin-
sic defect concentration,[51,55] was clearly increased in the
Raman spectra of the xPd/CeO2 catalysts (Table 1). These re-
sults demonstrated that the presence of Pd species could
create more oxygen vacancies on the surface of CeO2.
Hydrogenation of nitrobenzene
Figure 2. Pd3d XPS spectra of the xPd/CeO2 catalysts calcined at 3508C.
The hydrogenation of nitrobenzene was performed as a model
reaction to compare the catalytic activities of various support-
ed noble metal catalysts in the absence of solvent at 408C,
and the results are summarized in Table 2. It was clearly dem-
onstrated that the natures of supports and metals strongly af-
fected the catalytic activities of Pd catalysts. The 2.5%Pd/CeO2
catalyst exhibited a much higher catalytic activity (25.2% nitro-
benzene conversion) than all the 2.5%Pd catalysts on other
supports, such as Al2O3, CeO2-C, La2O3, SiO2, TiO2, and ZrO2, for
of actual Pd loadings in the xPd/CeO2 catalysts if the nominal
Pd content was higher than 2.5%. This result indicated that
the Pd2+ ions on the surface of PdxCe1ÀxO2Às might have
reached saturation at the Pd content of 2.5 wt%. It could be
reasonably speculated that the Pd2+ ion species existed as iso-
lated single atoms and the PdOx species were dispersed as
sub-clusters on the catalyst surface.
Figure 3 presents the Raman spectra of CeO2 and xPd/CeO2
catalysts. In the case of pure CeO2, four bands were observed:
a strong band peaked at 462 cmÀ1 and three weak bands
peaked at 265, 594, and 1170 cmÀ1. The strong peak corre-
sponded to the F2g mode vibration of cubic fluorite. The weak
Table 2. Reaction results for the hydrogenation of nitrobenzene over var-
ious heterogeneous catalysts.[a]
Entry
Catalyst
Conv.
[%]
TOF
[hÀ1
]
1
2
3
4
5
6
7
8
CeO2
0
0.6
25.2
12.1
0.2
4.3
0.3
3.5
<0.1
<0.1
0.9
<0.1
<0.1
7.4
15.3
26.1
26.4
0
2.5%Pd/Al2O3
2.5%Pd/CeO2
2.5%Pd/CeO2-C
2.5%Pd/La2O3
2.5%Pd/SiO2
2.5%Pd/TiO2
2.5%Pd/ZrO2
2.5%Ag/CeO2
2.5%Au/CeO2
2.5%Pt/CeO2
2.5%Rh/CeO2
2.5%Ru/CeO2
1.0%Pd/CeO2
2.0%Pd/CeO2
3.0%Pd/CeO2
5.0%Pd/CeO2
260[b]
11411[c]
5150[b]
85[b]
1830[b]
128[b]
1490[b]
–
9
10
11
12
13
14
15
16
17
–
383[b]
–
–
9050[c]
9411[c]
10174[c]
6174[c]
[a] Reaction conditions: catalyst 10 mg; nitrobenzene 50 mmol; tempera-
ture 408C; pH2 6 MPa; reaction time 30 min. [b] Calculated on the basis
of nominal Pd contents. [c] Calculated on the basis of actual Pd contents
by ICP.
Figure 3. Raman spectra of the xPd/CeO2 catalysts calcined at 3508C.
ChemCatChem 2017, 9, 1 – 10
www.chemcatchem.org
3
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

Full Papers
the hydrogenation of nitrobenzene in the absence of solvent
at 30 min (entries 2–8). In the case of CeO2-supported noble
metal catalysts, except Pd, other metals such as Ag, Au, Pt, Rh,
and Ru were found to be almost inactive (<1.0% nitrobenzene
conversion) for the solvent-free hydrogenation of nitrobenzene
with H2 at 408C (entries 9–13). These results indicated that the
catalytic performance of the supported metal catalysts was
strongly dependent not only on the properties of metals and
supports but also on the preparation routes of supports and
catalysts for the hydrogenation of nitro aromatics with H2. For
the xPd/CeO2 catalysts with different Pd contents (x=1.0%,
2.0%, 2.5%, 3.0%, 5.0%; entries, 3, 14–17), the catalytic activi-
ties increased with the Pd content and became stable if the Pd
contents were more than 2.5%. Correspondingly, the turnover
frequencies (TOF, the number of converted molecules per Pd
atom per hour based on the total metal atoms) were increased
and exhibited a maximum value of 11411 hÀ1.
Table 3. Effect of reaction conditions (temperature, H2 pressure) on the
catalytic performance of the 2.5%Pd/CeO2 for the solvent-free hydroge-
nation of nitrobenzene.[a]
Entry
T
[8C]
pH2
[MPa]
Conv.[b]
[%]
TOF[b]
[hÀ1
]
1
2
3
4
5
6
7
9
40
40
40
40
40
40
30
50
60
80
100
0.1
1
2
4
6
8
6
6
6
2.6
6.0
1177[c]
2717[c]
12.3
20.4
25.2
27.9
14.2
36.0
49.9
89.5
100
5570[c]
9237[c]
11411[c]
12647[c]
6439[c]
17569[d]
24724[e]
44286[f]
69824[g]
10
11
12
6
6
Figure 4 illustrates the catalytic properties as a function of
reaction time over the 2.5%Pd/CeO2 catalyst for the selective
hydrogenation of nitrobenzene in the absence of solvent at
[a] Reaction conditions: catalyst 10 mg; nitrobenzene 50 mmol; tempera-
ture 408C; pH2 6 MPa; reaction time 30 min. [b] Calculated on the basis
of actual Pd contents by ICP. [c] Based on the conversions at 30 min.
[d] Based on 19.4% nitrobenzene conversion at 15 min. [e] Based on
18.2% nitrobenzene conversion at 10 min. [f] Based on 16.3% nitroben-
zene conversion at 5 min. [g] Based on 25.7% nitrobenzene conversion at
5 min.
pressure was raised from 0.1 MPa to 8.0 MPa, the conversion of
nitrobenzene increased from 2.6% to 27.9% at 30 min. As a
result, the TOF was enhanced from 1177 to 12647 hÀ1. It was
demonstrated in entries 7–12 of Table 2 that the nitrobenzene
conversion increased with the reaction temperature. When the
temperature was elevated from 30 to 1008C, the conversion of
nitrobenzene increased from 14.2% to 100%. Correspondingly,
the TOF values were enhanced from 6439 to 69824 hÀ1. This
TOF value was much higher than those over a variety of sup-
ported Pd or other metal catalysts for the selective hydrogena-
tion of nitrobenzene either in the presence of or absence of
solvents and additives, reported in the past years.[29,31,35,51,52] All
the reactions over the 2.5%Pd/CeO2 could achieve full conver-
sion and 100% aniline selectivity by prolonging the reaction
period, giving yields of approximately 100%, in the tempera-
ture range 30–1008C.
Figure 4. Catalytic properties as a function of reaction time over the
2.5%Pd/CeO2 catalyst. Reaction conditions: catalyst 10 mg; nitrobenzene
50 mmol; pH2 6.0 MPa; reaction temperature 408C.
408C. The conversion of nitrobenzene and the selectivity of
aniline increased almost linearly with the reaction time if the
conversion was lower than 50%. After the 3 h reaction, nitro-
benzene was completely converted and the selectivity to ani-
line was also enhanced to 100%, affording approximately
100% yield of aniline. During all the reactions, there was only
a certain amount of phenylhydroxylamine (PHA) observed in
the products in addition to aniline. The selectivity of PHA de-
creased with prolonging the reaction time, and finally tended
to zero in the products.
Hydrogenation of substituted nitroarenes
The selective hydrogenation protocol of the 2.5%Pd/CeO2 cat-
alyst was further extended to the conversion of various substi-
tuted nitroarenes to the corresponding amines to demonstrate
the general applicability in the absence of solvents at pH2 =
2.0 MPa. For all tested substituted nitroarenes, the hydrogena-
tion reactions were conducted at temperatures higher than
the melting points. Table 4 summarizes the hydrogenation of
various functional nitroarenes. It could be seen that the
2.5%Pd/CeO2 catalyst could smoothly transform each liquid
functional nitroarene fully, and exhibited excellent selectivities
in the hydrogenation to the corresponding aromatic amines in
the absence of solvent under the given conditions. All the ni-
The influences of the reaction conditions (temperature and
H2 pressure) were examined over the 2.5%Pd/CeO2 catalyst for
the hydrogenation of nitrobenzene in the absence of solvent,
and the results are listed in Table 3. The conversion of nitro-
benzene increased with the H2 pressure (entries 1–6). If the H2
&
ChemCatChem 2017, 9, 1 – 10
www.chemcatchem.org
4
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Papers
Table 4. Reaction results for the solvent-free selective hydrogenation of
various substituted nitroarenes to the corresponding aromatic amines
over the 2.5%Pd/CeO2 catalyst.[a]
Table 4. (Continued)
Entry
Substrate
T[b]
[8C]
t
[h]
Conv.
[%]
Sel.
[%]
Entry
Substrate
T[b]
[8C]
t
[h]
Conv.
[%]
Sel.
[%]
21
150 (145)
70 (57)
10
15
>99
>99
>99
>99
1
2
3
40 (6)
1
>99
>99
>99
>99
>99
>99
22
75 (54)
45 (2)
0.5
3.5
23
90 (76)
10
>99
88
4
5
55 (40)
60 (54)
16
>99
>99
>99
>99
24
25
26
210 (202)
120 (115)
95 (75)
7.5
>99
>99
>99
>99
>99
>99
1
1.5
3
6
7
8
110 (96)
55 (50)
8
>99
>99
>99
>99
>99
>99
6
100 (81)
1.5
27
28
95 (91)
50 (20)
7.5
3.5
>99
>99
>99[c]
>99[d]
9
95 (73)
1.5
1.5
>99
>99
>99
>99
10
120 (112)
29
30
31
32
90 (57)
105 (92)
110 (73)
115 (98)
20
59
>99
>99
>99
89
>99
>99
>99
11
12
120 (107)
45 (2)
23
>99
>99
>99
>99
6
5
2
13
55
11
>99
>99
17
14
15
55 (47)
9.5
>99
>99
87
85
[a] Reaction conditions: catalyst 16 mg; substrate 10 mmol, pH2 2.0 MPa.
[b] Melting point of substrate in parentheses. [c] 1,3-Diaminobenzene.
[d] 4-Ethylaniline.
110 (101)
15
13
16
17
125 (119)
120 (106)
>99
>99
77
troarenes with non-reducible groups such as -CH3, -CF3, -OCH3,
-CH3OH, -OH, and -NH2 (entries 1–11) could be quantitatively
hydrogenated into the corresponding amines without discerni-
ble byproducts. It was known that the dehalogenation of halo-
gen-substituted nitroarenes is a serious issue in selective hy-
drogenation. Herein, to our great delight, when halogen (F, Cl)-
substituted nitroarenes were completely converted, the che-
moselectivities of the corresponding aromatic amines could
reach ꢁ85% (entries 12–16). It was indicated for the examples
of entries 17–26 that reducible functional groups such as alde-
hyde, ketone, ester, amide, nitrile, sulfurylchloride could not be
hydrogenated in the selective hydrogenation of the nitro
groups in the benzene rings in the absence of solvent, there-
15
1
>99
18
19
145 (142)
90 (80)
>99
>99
>99
>99
1.5
20
50 (23)
6
>99
>99
ChemCatChem 2017, 9, 1 – 10
www.chemcatchem.org
5
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

Full Papers
fore giving >99% selectivities to the corresponding amines.
However, 1,3-dinitrobenzene and 4-nitrostyrene were found to
generate 1,3-diaminobenzene (entry 27) and 4-ethylaniline
(entry 28), respectively. Interestingly, the 2.5%Pd/CeO2 could
also transform heterocyclic nitroarenes containing N element
to the corresponding amines in the absence of solvent, afford-
ing yields of >99% (entries 30–32). The higher selectivity for
the nitro group in the benzene ring was likely owing to the
higher reactivity of the nitro group than other reducible
groups in the hydrogenation reactions over the 2.5%Pd/CeO2
catalyst. This point could be supported by several separate in-
termolecular competitive reductions. If the mixtures of nitro-
benzene and other reducible group-substituted benzene (e.g.,
benzoic acid, aldehyde, ketone, ester, amide, nitrile) were hy-
drogenated with H2 in the liquid phase without solvent, nitro-
benzene was completely converted into aniline, whereas other
reactants showed no reaction.
onstrated that the 2.5%Pd/CeO2 catalyst was reusable for the
hydrogenation of nitroarenes in the absence of solvent with
H2. After the seventh run, the spent 2.5%Pd/CeO2 catalyst was
analyzed by ICP-AES. The results showed that the content of
Pd was 2.34 wt%, in good accordance with that (2.35 wt%) in
the fresh catalyst (Table 1). In addition, the Pd and Ce contents
in the filtrate were negligible (<0.1 ppm) by ICP-AES, indicat-
ing that there was no leaching of the components in the hy-
drogenation procedure in the absence of solvent. Notbaly, the
spent 2.5%Pd/CeO2 catalyst, which was not treated in air at
3008C, exhibited clear deactivation for the hydrogenation of
nitrobenzene (approximately 48% nitrobenzene conversion).
X-ray photoelectron spectroscopy (XPS) analysis demonstrated
that some Pd2+ ions were transformed into metallic Pd in the
hydrogenation process (Figure S2c in the Supporting Informa-
tion), although after calcination at 3508C in air, the metallic Pd
was converted into PdOx and PdxCe1ÀxO2Às solid solution again
(Figure S2b in the Supporting Information).
Recycling of catalyst
Active sites of Pd/CeO2 catalyst
The recycling of catalysts is of great importance for industrial
applications of liquid-phase catalytic reactions. The selective
hydrogenation of nitrobenzene with approximately 80% con-
version and 99% aniline selectivity in the first cycle was used
as an example to examine the reusability of the 2.5%Pd/CeO2
catalyst for the hydrogenation of nitroarenes with H2 in the ab-
sence of solvent. The catalyst after every reaction run was re-
covered by simple filtration of the reaction mixture, washing
with ethanol, followed by calcination at 3508C. Only seven re-
action cycles were performed because of the loss of the cata-
lyst in each recovery process. The cycling results are illustrated
in Figure 5. It can be seen that the conversion of nitrobenzene
in each cycle was almost constant at approximately 80%, and
the aniline selectivity remained at about 99%. This result dem-
The Pd3d spectra in Figure 1 demonstrated that Pd species
could react with CeO2 to form a PdxCe1ÀxO2Às solid solution
and exist in the form of isolated Pd2+ ions with ÀPd2+ÀO2ÀÀ
Ce4+À linkages on the catalyst surface. As the Pd content was
raised, PdOx species appeared on the surface of CeO2, and the
relative amount of PdOx increased. In Table 2, pure CeO2 sup-
port showed no activity (entry 1), whereas the xPd/CeO2 cata-
lysts exhibited excellent catalytic activities and showed a maxi-
mum TOF value at the Pd content of 2.5% for the hydrogena-
tion of nitrobenzene with H2 in the absence of solvents (en-
tries 3, 14–17). On increasing the Pd content further to 3.0%
and 5.0%, respectively, the catalytic activities showed no signif-
icant change, which was likely owing to the saturation of Pd2+
on the surface of the PdxCe1ÀxO2Às solid solution. These results
clearly demonstrated that the Pd2+ ions on the surface of the
PdxCe1ÀxO2Às were the dominant active sites of the xPd/CeO2
catalysts and the PdOx species on the surface were less active
for the hydrogenation of nitrobenzene with H2. The increases
in the TOF with raising the Pd content in the range 1.0–2.5%
could be attributed to the fact that the relatively larger
amount of Pd2+ ions was diffused into the interior of the CeO2
crystallites, which were not accessible to the reactants, owing
to stronger interaction between Pd2+ ions and CeO2 at lower
Pd loadings.
To further prove the activity of Pd2+ ions for the hydrogena-
tion of nitrobenzene in the absence of solvent, the 2.5%Pd/
CeO2 catalyst was reduced with H2 at 3508C by using different
times (2 h, 4 h, 8 h, 24 h, 48 h). Figure 6 illustrates the Pd3d
XPS spectra of the reduced 2.5%Pd/CeO2 catalysts. It could be
found that, except for the peaks at 337.6 eV and 342.9 eV for
the Pd2+ ions, after the reduction, two new peaks positioned
at 335.6 eV and 340.9 eV, which were assigned to Pd3d5/2 and
Pd3d3/2 of metal Pd0,[51] appeared and the peak intensities in-
creased with the reduction time. This result demonstrated that
the number of the Pd2+ ions reduced on the surface of the
PdxCe1ÀxO2Às solid solution increased with the reduction time.
Figure 5. Reuse profiles of the 2.5%Pd/CeO2 catalyst for the solvent-free hy-
drogenation of nitrobenzene. Reaction conditions: catalyst 20 mg; nitroben-
zene 50 mmol; pH2 2.0 MPa; 408C; 2 h in the first cycle; the catalyst/nitro-
benzene ratio was kept same as that of the first cycle in the subsequent
cycles.
&
ChemCatChem 2017, 9, 1 – 10
www.chemcatchem.org
6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Papers
ent change with the [Pd2+]/[Pd2+ +Pd0] ratio. These results
demonstrated that metallic Pd generated on the CeO2 might
also be inactive just as the PdOx and other noble metals on
CeO2 had shown, or its activity was significantly lower than
that of Pd2+ ions on the surface of the PdxCe1ÀxO2Às solid solu-
tion for the hydrogenation of nitroarenes with H2 in the ab-
sence of solvents.
Proposed hydrogenation pathway of nitrobenzene over Pd/
CeO2 catalyst
It is generally accepted that under reducing conditions, the hy-
drogenation of nitroarenes can occur through two reaction
pathways: direct and indirect. The direct pathway is that the
nitro group in the aromatic ring is first hydrogenated to the ni-
troso, and then further to the hydroxylamine in two consecu-
tive steps, and finally, the hydroxylamine is reduced to the
amine; whereas in the indirect pathway, the nitroso and hy-
droxylamine intermediates condense to generate azo inter-
mediates, which subsequently undergo hydrogenation and
cleavage of the azo bond to release aniline.[51,53,54] In the selec-
tive hydrogenation of nitrobenzene as a model reaction
(Figure 4), only phenylhydroxylamine was detected in the
products in addition to aniline, and its selectivity declined with
the reaction time, and eventually becoming zero. There was
no azoxybenzene or azobenzene detected in the products.
These results demonstrated that the solvent-free hydrogena-
tion of nitrobenzene to aniline was implemented over the Pd/
CeO2 catalyst through the direct pathway instead of the indi-
rect pathway. This assumption was also supported by other
two separate experiments over the 2.5%Pd/CeO2. It was found
that the hydrogenation of nitrosobenzene in the absence of
solvents generated the phenylhydroxylamine immediate, and
further reaction to aniline without azo intermediates occurred
at 408C and at pH2 =6.0 MPa, whereas the hydrogenation of
azoxybenzene to aniline was very slow under the same condi-
tions.
Figure 6. Pd3d XPS spectra of the 2.5%Pd/CeO2 catalysts reduced with H2 at
3508C for different times.
Notably, there were always small amounts of Pd2+ ions unre-
duced on the surface of the 2.5%Pd/CeO2 catalysts, even at
3508C for 48 h or at an elevated temperature of 5008C. Table 5
summarizes the [Pd2+]/[Pd2+ +Pd0] molar ratios in the reduced
2.5%Pd/CeO2 catalysts estimated by the peak areas of the
Pd3d spectra in Figure 6. When the reduction time was pro-
longed to 48 h, the [Pd2+]/[Pd2+ +Pd0] ratio declined to 0.23.
The XRD patterns (Figure S3 in the Supporting Information) of
the reduced 2.5%Pd/CeO2 showed no diffraction peaks associ-
ated with metallic Pd, indicating that metallic Pd was highly
dispersed on the surface of the catalysts owing to strong inter-
actions between the metal and support. The reduced 2.5%Pd/
CeO2 catalysts were used for the solvent-free hydrogenation of
nitrobenzene, and the catalytic activities are listed in Table 5. It
could be seen that the conversion of nitrobenzene decreased
clearly with the decrease in the [Pd2+]/[Pd2+ +Pd0] ratio. From
Table 5, it was found that the TOFs based on the Pd2+ ions un-
reduced on the surface of the 2.5%Pd/CeO2 showed no appar-
It has been demonstrated in Table 2 that Pd catalysts on
other various supports such as Al2O3, La2O3, SiO2, TiO2, and
ZrO2 were less active, and Pd2+ ions on the CeO2 support were
the dominant active sites for the hydrogenation of nitroarenes.
Therefore, the superior catalytic performance could be attribut-
ed to the cooperative effect between Pd2+ ions and unique
surface sites of CeO2. It was proposed that the synergistic
action of Pd2+ ions and oxygen vacancies on the surface of
PdxCe1ÀxO2Às activated molecular hydrogen to active surface H
atoms, which reacted with nitro groups adsorbed on the cata-
lyst surface to produce nitroso and hydroxylamine in two con-
secutive steps, and finally to the amine (Scheme 1). In the hy-
drogenation of nitrobenzene, H atoms on the catalyst surface
might also react with Pd2+ ions to form metallic Pd, attenuat-
ing the catalytic activity of the Pd/CeO2.
Table 5. Catalytic activities of the 2.5%Pd/CeO2 catalysts with [Pd0]/
[Pd0 +Pd2+] molar ratios reduced with H2 at 3508C for different times (t).
Entry
t
[h]
[Pd2+]/[Pd2+ +Pd0] molar ratio
[%]
Conv.[a]
[%]
TOF[b]
[hÀ1
]
1
2
3
4
5
6
0
2
4
8
24
48
100
85
73
52
32
23
25.2
21.3
17.1
13.7
8.8
11411
11973
10608
11930
12404
12797
6.5
Conclusions
[a] Reaction conditions: catalyst 10 mg; nitrobenzene 50 mmol; pH2
6.0 MPa; time 30 min, temperature 408C. [b] Based on the Pd2+ ions in
the reduced 2.5%Pd/CeO2 estimated by XPS.
CeO2-supported palladium catalysts were prepared by a simple
impregnation method with Na2CO3, and exhibited superior cat-
ChemCatChem 2017, 9, 1 – 10
www.chemcatchem.org
7
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

Full Papers
Commercial (Al2O3, CeO2, La2O3, SiO2, TiO2, ZrO2,)-supported cata-
lysts containing 2.5 wt% Pd were prepared by the same method
as the 2.5%Pd/CeO2 except different supports were used. For dis-
tinguishing, commercial CeO2 supported Pd was labelled as
2.5%Pd/CeO2-C.
2.5%Ag/CeO2, 2.5%Au/CeO2, 2.5%Pt/CeO2, 2.5%Rh/CeO2, and
2.5% Ru/CeO2 catalysts were prepared by the same method as the
2.5%Pd/CeO2 except different metal salts were used (AgNO3,
HAuCl4·4H2O, H2PtCl6, RhCl3·3H2O, RuCl3·3H2O).
Scheme 1. Possible hydrogenation pathway of nitroarenes on Pd/CeO2.
Characterization of catalysts
alytic activity and high chemoselectivity for the hydrogenation
of a variety of substituted nitroarenes including those with re-
ducible functional groups to the corresponding aromatic
amines in the absence of solvent and additives under mild re-
action conditions. Pd2+ ion species existed as isolated single
atoms with ÀPd2+ÀO2ÀÀCe4+À linkages on the surface of the
PdxCe1ÀxO2Às solid solution and served as active sites for the
selective hydrogenation of nitroarenes in the absence of sol-
vent. The superior catalytic performance could be attributed to
the synergistic effect between Pd2+ ions and unique surface
sites of CeO2. The Pd/CeO2 catalyst could be recovered easily
for at least seven recycling reactions without the loss of cata-
lytic properties. The present study will also contribute to the
development of efficient and environmentally friendly metal
catalysts and provide a new insight into the catalytic behavior
of supported Pd and other noble metals for various organic re-
actions.
The actual weight percentages of Pd in the catalysts were analyzed
by ICP-AES conducted with a PerkinElmer emission spectrometer.
XRD analysis was performed with a Rigaku D/MAX-2500 apparatus
using CuKa radiation (l=0.15418 nm) at a voltage of 40 kV and a
current of 40 mA. TEM images were recorded with a JEOL JEM-
2010F field emission microscope operating at 200 kV. The sample
was prepared by placing a drop of ethanol solution of well-ground
catalyst powder on a carbon-coated copper grid, followed by etha-
nol evaporation. XPS were measured by using an ESCALAB 250Xi
spectrometer equipped with monochromatized AlKa radiation
(hv=1486.6 eV) operated at a pressure of ca. 1ꢁ10À9 torr. The
spectra were calibrated by using the binding energy of the C1s
peak at 284.6 eV. Specific surface areas of the samples were mea-
sured by using a Micromeritics ASAP 2020 Sorptometer at liquid ni-
trogen temperature (À1968C). Before the measurement, the
sample was degassed at 3008C for 10 h. The specific surface area
was evaluated by using the BET method. The Raman spectra were
obtained with a Renishaw inVia Raman spectrometer under ambi-
ent conditions at an excitation wavelength of 514 nm.
Experimental Section
Catalyst reaction and analysis
Chemicals
The selective hydrogenation of nitroarenes was performed in a
100 mL high-pressure reactor with magnetic stirring and pressure
control system. For a typical catalytic reaction, catalyst (10 mg) and
substrate (50 mmol) were first added into the reactor. The reaction
mixture was heated to the desired temperature (Æ0.28C) under
vigorous stirring for 1 h under N2 atmosphere. After this, the reac-
tor was flushed five times with 1.0 MPa H2, and then the reaction
was performed at the set H2 pressure and at a stirring rate of
900 rpm, at which the mass transport or diffusion limitation was
confirmed to be negligible in the reaction. At the end of the reac-
tion, the remaining hydrogen was discharged slowly to atmospher-
ic pressure, and was immediately switched to N2 at room tempera-
ture, and then ethanol (~20 mL) and n-decane (100 mL) as standard
were added into the reactor. The reaction mixture was separated
by filtration and further diluted with ethanol and then was dried
with anhydrous Na2SO4. The products were analyzed by a gas chro-
matography-mass spectrometry (GC-MS) and a GC with a capillary
column and a flame ionization detector. Turnover frequencies
(TOFs) were calculated on the basis of the total metal amount in
the catalysts. The carbon balance was more than 98%.
All chemicals were analytical grade, purchased from Sinopharm
Chemical Reagent Co. Ltd., and used in this work as received with-
out purification. Commercial Al2O3 (159 m2 gÀ1), CeO2 (15 m2 gÀ1),
La2O3 (<2 m2 gÀ1), SiO2 (438 m2 gÀ1), TiO2 (38 m2 gÀ1), and ZrO2
(9 m2 gÀ1) were used for comparison. Deionized water was used
throughout all the experiments.
General procedure for catalyst preparation
Preparation of CeO2 support: In a typical synthesis, Ce(NO3)3·6H2O
(8.7 g) was dissolved in water (50 mL) at room temperature. Then,
an aqueous solution of Na2CO3 (0.5 molLÀ1) was added dropwise
to the above aqueous solution with vigorous stirring until the pH
value increased to approximately 7.5. The precipitate was filtered
and washed at least three times with deionized water, and finally
was dried at 1008C overnight and calcined in air at 5008C for 5 h
with a ramp rate of 58CminÀ1
.
Preparation of supported Pd catalysts: a series of xPd/CeO2 cata-
lysts with different Pd mass percentage (x=1.0%, 2%, 2.5%, 3.0%,
5.0%) were prepared by a simple impregnation method. Typically,
the as-prepared CeO2 powder (1 g) was dispersed into an aqueous
solution (15 mL) containing the desired amount of palladium ace-
tate (Pd(OAc)2) under stirring at room temperature for 2 h. The
mixture was stirred at 708C to evaporate water, and then was
dried in an oven at 1008C overnight. Finally, the obtained solid
was calcined at 3508C for 5 h.
Acknowledgments
This research was supported by the Open Project of State Key
Laboratory of Advanced Special Steel of Shanghai University
(SKLASS2015-Z052), National Basic Research Program of China
(973 Program, No. 2014CB643403), the National Natural Science
&
ChemCatChem 2017, 9, 1 – 10
www.chemcatchem.org
8
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Papers
[28] O. Verho, K. P. J. Gustafson, A. Nagendiran, C. W. Tai, J. E. Backvall, Chem-
CatChem 2014, 6, 3153–3159.
[29] J. Li, X. Y. Shi, Y. Y. Bi, J. F. Wei, Z. G. Chen, ACS Catal. 2011, 1, 657–664.
[30] J. Lyu, J. G. Wang, C. S. Lu, L. Ma, Q. F. Zhang, X. B. He, X. N. Li, J. Phys.
Chem. C 2014, 118, 2594–2601.
Foundation of China (No. 51574164), and Basic Major Research
Program of Science and Technology Commission Foundation of
Shanghai (No. 14JC1491400).
[31] Y. Hu, K. Tao, C. Z. Wu, C. Zhou, H. F. Yin, S. H. Zhou, J. Phys. Chem. C
2013, 117, 8974–8982.
[32] Z. P. Dong, K. Liang, C. X. Dong, X. L. Li, X. D. Le, J. T. Ma, RSC adv. 2015,
5, 20987–20991.
Keywords: ceria · hydrogenation · nitroarenes · palladium ·
solvent-free conditions
[1] R. V. Jagadeesh, A. E. Surkus, H. Junge, M. M. Pohl, J. Radnik, J. Rabeah,
H. Huan, V. Schunemann, A. Brꢂckner, M. Beller, Science 2013, 342,
1073–1076.
[33] K. Shen, L. Chen, J. L. Long, W. Zhong, Y. W. Li, ACS Catal. 2015, 5,
5264–5271.
[34] C. Wang, F. Yang, W. Yang, L. Ren, Y. Zhang, X. Jia, L. Zhang, Y. Li, RSC
Adv. 2015, 5, 27526–27532.
[2] J. F. Hartwig, Nature 2008, 455, 314–322.
[3] B. Gnanaprakasam, J. Zhang, D. Milstein, Angew. Chem. Int. Ed. 2010, 49,
1468–1471; Angew. Chem. 2010, 122, 1510–1513.
[4] Y. Liu, Y. Lu, M. Prashad, O. Pepic, T. J. Blacklock, Adv. Synth. Catal. 2005,
347, 217–219.
[5] S. M. Kelly, B. H. Lipshutz, Org. Lett. 2014, 16, 98–101.
[6] A. Corma, P. Serna, P. Concepcion, J. J. Calvino, J. Am. Chem. Soc. 2008,
130, 8748–8753.
[7] H. S. Wei, X. Y. Liu, A. Q. Wang, L. L. Zhang, B. T. Qiao, X. F. Yang, Y. Q.
Huang, S. Miao, J. Y. Liu, T. Zhang, Nat. Commun. 2014, 5, 5634.
[8] Z. Z. Wei, J. Wang, S. J. Mao, D. F. Su, H. Y. Jin, Y. H. Wang, F. Xu, H. R. Li,
Y. Wang, ACS Catal. 2015, 5, 4783–4789.
[35] S. Zhang, C. R. Chang, Z. Q. Huang, J. Li, Z. M. Wu, Y. Y. Ma, Z. Y. Zhang,
Y. Wang, Y. Q. Qu, J. Am. Chem. Soc. 2016, 138, 2629–2637.
[36] L. Vivier, D. Duprez, ChemSusChem 2010, 3, 654–678.
[37] M. Hatanaka, N. Takahashi, N. Takahashi, T. Tanabe, Y. Nagai, A. Suda, H.
Shinjoh, J. Catal. 2009, 266, 182–190.
[38] T. Sahu, S. S. Bisht, K. R. Das, S. Kerkar, Curr. Nanosci. 2013, 9, 588–593.
[39] V. R. Stamenkovic, B. Fowler, B. S. Mun, G. Wang, P. N. Ross, C. A. Lucas,
N. M. Markovic, Science 2007, 315, 493–497.
[40] R. Subbaraman, D. Tripkovic, D. Strmcnik, K. C. Chang, M. Uchimura,
A. P. Paulikas, V. Stamenkovic, N. M. Markovic, Science 2011, 334, 1256–
1260.
[9] D. Formenti, C. Topf, K. Junge, F. Ragaini, M. Beller, Catal. Sci. Technol.
2016, 6, 4473–4477.
[41] Y. Wang, F. Wang, Q. Song, Q. Xin, S. Xu, J. Xu, J. Am. Chem. Soc. 2013,
135, 1506–1515.
[10] W. Shi, B. S. Zhang, Y. M. Lin, Q. Wang, Q. Zhang, D. S. Su, ACS Catal.
2016, 6, 7844–7854.
[42] M. Tamura, H. Wakasugi, K. I. Shimizu, A. Satsuma, Chem. Eur. J. 2011,
17, 11428–11431.
[11] M. Takasaki, Y. Motoyama, K. Kigashi, S. H. Yoon, I. Mochida, H. Nagashi-
ma, Org. Lett. 2008, 10, 1601–1604.
[43] Y. Namai, K. I. Fukui, Y. Iwasawa, J. Phys. Chem. B 2003, 107, 11666–
11673.
[12] L. Liu, P. Concepcion, A. Corma, J. Catal. 2016, 340, 1–9.
[13] S. F. Cai, H. H. Duan, H. P. Rong, D. S. Wang, L. Li, W. He, Y. D. Li, ACS
Catal. 2013, 3, 608–612.
[14] P. R. Chen, F. K. Yang, A. Kostka, W. Xia, ACS Catal. 2014, 4, 1478–1486.
[15] T. Mitsudome, Y. Mikami, M. Matoba, T. Mizugaki, K. Jitsukawa, K.
Kaneda, Angew. Chem. Int. Ed. 2012, 51, 136–139; Angew. Chem. 2012,
124, 140–143.
[44] K. Anupriya, E. B. Vivek, B. Subramanian, J. Alloys Compd. 2014, 590,
406–410.
[45] S. Gnanam, V. Rajendran, J. Sol-Gel Sci. Technol. 2011, 58, 62–69.
[46] M. Tamura, K. Tomishige, Angew. Chem. Int. Ed. 2015, 54, 864–867;
Angew. Chem. 2015, 127, 878–881.
[47] L. Meng, J. J. Lin, Z. Y. Pu, L. F. Luo, A. P. Jia, W. X. Huang, M. F. Luo, J. Q.
Lu, Appl. Catal. B 2012, 119–120, 117–122.
[16] Z. L. Li, J. L. Li, J. H. Liu, Z. L. Zhao, C. G. Xia, F. W. Li, ChemCatChem
2014, 6, 1333–1339.
[17] F. A. Westerhaus, R. V. Jagadeesh, G. Wienhofer, M. M. Pohl, J. Radnik,
A. E. Surkus, J. Rabeah, K. Junge, H. Junge, M. Nielsen, A. Bruchner, M.
Beller, Nat. Chem. 2013, 5, 537–543.
[18] H. B. Fu, L. Zhang, Y. Wang, S. J. Chen, Y. Wan, J. Catal. 2016, 344, 313–
324.
[19] G. Vilꢃ, D. Albani, M. Nachtegaal, Z. Chen, D. Dontsova, M. Antonietti, N.
Lꢄpez, J. Pꢃrez-Ramꢅrez, Angew. Chem. Int. Ed. 2015, 54, 11265–11269;
Angew. Chem. 2015, 127, 11417–11422.
[48] R. V. Gulyaev, A. I. Stadnichenko, E. M. Slavinskaya, A. S. Ivanova, S. V. Ko-
scheev, A. I. Boronin, Appl. Catal. A 2012, 439–440, 41–50.
[49] R. V. Gulyaev, T. Y. Kardash, S. E. Malykhin, O. A. Stonkus, A. S. Ivanova,
A. I. Boronin, Phys. Chem. Chem. Phys. 2014, 16, 13523–13539.
[50] K. Priolkar, P. Bera, P. Sarode, M. Hegde, S. Emura, R. Kumashiro, N. Lalla,
Chem. Mater. 2002, 14, 2120–2128.
[51] Z. Hu, X. Liu, D. Meng, Y. Guo, Y. Guo, G. Lu, ACS Catal. 2016, 6, 2265–
2279.
[52] M. Tamura, K. Kon, A. Satsuma, K. I. Shimizu, ACS Catal. 2012, 2, 1904–
1909.
[20] J. Gu, Z. Zhang, P. Hu, L. Ding, N. Xue, L. Peng, X. Guo, M. Lin, W. Ding,
ACS Catal. 2015, 5, 6893–6901.
[21] S. Kataoka, Y. Takeuchi, A. Harada, T. Takagi, Y. Takenaka, N. Fukaya, H.
Yasuda, T. Ohmori, A. Endo, Appl. Catal. A 2012, 427, 119–124.
[22] R. Nie, J. Wang, L. Wang, Y. Qin, P. Chen, Z. Hou, Carbon 2012, 50, 586–
596.
[53] P. Serna, A. Corma, ACS Catal. 2015, 5, 7114–7121.
[54] A. Mahata, R. K. Rai, I. Choudhuri, S. K. Singh, B. Pathak, Phys. Chem.
Chem. Phys. 2014, 16, 26365–26374.
[55] T. Y. Guo, J. P. Du, J. P. Li, J. Mater. Sci. 2016, 51, 10917–10925.
[56] Q. G. Dai, H. Huang, Y. Zhu, W. Deng, S. X. Bai, X. Y. Wang, G. Z. Lu, Appl.
Catal. B 2012, 117 – 118, 360–368.
[23] D. He, H. Shi, Y. Wu, B. Q. Xu, Green Chem. 2007, 9, 849–851.
[24] Y. Chen, C. Wang, H. Liu, J. Qiu, X. Bao, Chem. Commun. 2005, 5298–
5300.
[25] F. Wang, J. Liu, X. Xu, Chem. Commun. 2008, 2040–2042.
[26] Z. Y. Sun, Y. F. Zhao, Y. Xie, R. T. Tao, H. Y. Zhang, C. L. Huang, Z. M. Liu,
Green Chem. 2010, 12, 1007–1011.
Manuscript received: April 13, 2017
Revised manuscript received: May 10, 2017
[27] H. G. Huang, X. G. Wang, M. W. Tan, C. J. Chen, X. J. Zou, W. Z. Ding,
X. G. Lu, ChemCatChem 2016, 8, 1485–1489.
Accepted manuscript online: June 27, 2017
Version of record online: && &&, 0000
ChemCatChem 2017, 9, 1 – 10
www.chemcatchem.org
9
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

FULL PAPERS
X. Shi, X. Wang,* X. Shang, X. Zou,
W. Ding, X. Lu*
Solvent-free hydrogenation: Cerium
oxide-supported palladium catalysts ex-
hibit exciting catalytic activity and high
chemoselectivity for solvent-free hydro-
genation of a variety of substituted ni-
troarenes under mild reaction condi-
tions. Pd2+ ions on the surface of
PdxCe1ÀxO2Às solid solutions are found
to be the active sites. The superior cata-
lytic performance can be attributed to
the cooperative effect between the
Pd2+ ions and unique surface sites of
CeO2.
&& – &&
High Performance and Active Sites of
a Ceria-Supported Palladium Catalyst
for Solvent-Free Chemoselective
Hydrogenation of Nitroarenes
&
ChemCatChem 2017, 9, 1 – 10
www.chemcatchem.org
10
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!