Photochemically Engineering the Metal-Semiconductor Interface for Roomerature Transfer Hydrogenation of Nitroarenes with Formic Acid

Article abstract of DOI:10.1002/chem.201404325

A mild photochemical approach was applied to construct highly coupled metal-semiconductor dyads, which were found to efficiently facilitate the hydrogenation of nitrobenzene. Aniline was produced in excellent yield (>99 %, TOF: 1183) using formic acid as hydrogen source and water as solvent at room temperature. This general and green catalytic process is applicable to a wide range of nitroarenes without the involvement of high-pressure gases or sacrificial additives. A mild photochemical approach was applied to construct highly coupled metal-semiconductor dyads (example in picture: carbon nitride), which were found to catalyze the hydrogenation of nitrobenzene. Aniline was produced in excellent yield (>99 %, TOF 1183) using formic acid as the hydrogen source and water as the solvent at room temperature. This process is applicable to many nitroarenes without the involvement of high-pressure gases or sacrificial additives.

Full text of DOI:10.1002/chem.201404325

DOI: 10.1002/chem.201404325
Full Paper
&
Heterogeneous Catalysis |Hot Paper|
Photochemically Engineering the Metal–Semiconductor Interface
for Room-Temperature Transfer Hydrogenation of Nitroarenes
with Formic Acid
[
a]
[a]
[a]
[a]
[a]
[b]
Xin-Hao Li, Yi-Yu Cai, Ling-Hong Gong, Wei Fu, Kai-Xue Wang, Hong-Liang Bao,
[
a]
[a]
Xiao Wei,* and Jie-Sheng Chen*
Abstract: A mild photochemical approach was applied to
construct highly coupled metal–semiconductor dyads, which
were found to efficiently facilitate the hydrogenation of ni-
trobenzene. Aniline was produced in excellent yield (>99%,
TOF: 1183) using formic acid as hydrogen source and water
as solvent at room temperature. This general and green cat-
alytic process is applicable to a wide range of nitroarenes
without the involvement of high-pressure gases or sacrificial
additives.
[10]
Introduction
and metal nanoparticles for better activity and/or selectivity.
Approaches for controlling the interfacial structure of metal
and semiconductor dyad to tune their catalytic performance
are still rare (Scheme 1). Herein, we demonstrate that the ma-
nipulation of the interface quality produces strong interaction
between the metal and semiconductor components. In
a model reaction, the highly coupled interface obtained by
a mild photochemical method outperforms its traditional
counterparts significantly, revealing the noteworthy approach
for promoting the heterogeneous catalysis.
Nitrogen-doped carbon or graphitic carbon nitride (CN, g-
C N )-supported noble metal nanoparticles were recently intro-
3
4
duced as typical Mott–Schottky catalysts for artificial photosyn-
[
1,2]
thesis.
The Mott–Schottky effect results in an elevated
Schottky barrier and thus enhances the charge separation at
the contacting interface of noble metal and g-C N . For Mott–
3
4
Schottky heterojunctions, many parameters can be applied to
vary the Schottky barrier and thus the electron distribution
[
3]
there. The compatibility of the band structure of the carbon-
containing materials with the work function of noble metal
nanoparticles is believed to be the key factor determining the
[4,5]
performance of the hybrid catalysts for specific reactions.
A third dimension that should be considered is the interface
[
6,7]
effect. Indeed, surface strain, defects, exterior ions, and addi-
tional layers were usually manipulated to tune the barrier
height and thus the electron distribution of metal–semicon-
ductor in solid state physics. For catalysis and phototcatalysis,
there is a consensus that the interface effect between the cata-
lyst supports and embedded noble metal nanoparticles can
Scheme 1. Procedures or parameters for varying the catalytic performance
of supported noble metal nanocrystals (previously reported and this work).
A highly coupled interface favors better electron transfer at the interface
and stronger interaction of metal and support. The effect of the coupled in-
terface on catalytic activity of supported noble metal nanocrystals is de-
scribed herein.
[
8,9]
significantly change the final catalytic performance.
Howev-
er, current efforts were mainly focused on engineering the
morphology and composition of rationally selected supports
[a] Dr. X.-H. Li, Y.-Y. Cai, L.-H. Gong, Dr. W. Fu, Dr. K.-X. Wang, Dr. X. Wei,
Dr. J.-S. Chen
We engineered the interfacial structure of metal-CN catalyst
for selective hydrogenation of nitroarene compounds to aro-
matic amines here owing to the wide applications of the latter
School of Chemistry and Chemical Engineering
Shanghai Jiao Tong University, Shanghai 200240 (China)
E-mail: weixiao@sjtu.edu.cn
[11,12]
chemcj@sjtu.edu.cn
molecules in chemical and pharmaceutical industries.
As
[
b] Dr. H.-L. Bao
a typical aromatic amine, aniline with a worldwide production
exceeding four million tons per year was obtained mainly (ca.
Shanghai Synchrotron Radiation Facilities
Shanghai Institute of Applied Physics
Chinese Academy of Sciences, Shanghai 201204 (China)
97%) from hydrogenation of nitrobenzene. Excess amount of
H gas (>100 times in moles) was usually used for catalytic
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201404325.
2
transformation of nitrobenzene to aniline at a high pressure
Chem. Eur. J. 2014, 20, 16732 – 16737
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(
greater than 5 atm) and high temperature (>2008C), and the
[
13]
process is thus of “high-energy consumption”. Moreover, the
difficulties in transport and storage of H gas lead to additional
2
cost to the production of aniline. Alternatively, nitroarene com-
pounds can be reduced to aromatic amines by “extracting” hy-
drogen from hydrogen-donor molecules, that is, transfer hy-
[
14–20]
[21–23]
drogenation.
Among the available hydrogen donors,
FA, which may be derived from biomass with low toxicity and
high stability, is an abundant and cheap hydrogen source for
[
24–26]
safe hydrogen usage and storage.
The only byproduct of
dehydrogenation reaction of FA, CO , could be regenerated to
2
[24,25]
FA by chemical or biological routes in large quantities,
completing the sustainable cycles for the potential use of FA
as hydrogen source. Pd nanoparticles with the best activity for
both decomposition of FA and catalytic hydrogenation reac-
tions were thus selected as the model nanocatalyst in this
[
27,28]
work.
Results and Discussion
Figure 1. HRTEM images of Pd nanoparticles in A) Pd/CN-H and B) Pd/CN-PC
The catalyst was prepared via a modified wet impregnation
method, and the as-obtained sample was designated Pd/CN-
fresh. The Pd/CN-fresh catalyst was further reduced in 5% H2
at 3008C for 2 h to form a highly reduced Pd/CN-H catalyst.
The Pd/CN-fresh sample was also reduced under the irradiation
of commercial lamp (15 W) for one week at room temperature
(
Scale bar: 2 nm). C) XPS Pd3d spectra, D) Fourier-transformed EXAFS, and
E) the EXAFS Pd K-edge spectra of Pd/CN-PC (*) and control samples.
0
by the XPS analysis. Pd/CN-H was completely metallic Pd ac-
0
(for the detailed method, see the Supporting Information) and
cording to the sharp XPS Pd3d peaks of Pd and correspond-
the treated sample was denoted as Pd/CN-PC (Supporting In-
formation, Figure S1). The photoactive support CN can inject
photogenerated electrons to the Pd nanoparticles and trigger
ing temperature-programmed reduction (TPR) analysis (Sup-
porting Information, Figure S4). The extended X-ray absorption
fine structure spectra (EXAFS) of the Pd K-edge reveals the
presence of Pd coordination in the form of PdÀX (X=N or O;
Figure 1D; Supporting Information, Table S1). The calculated
coordination numbers (N) of the PdÀX shell for both Pd/CN-PC
and Pd/CN-H were at similar level and much lower than that of
Pd/CN-fresh. However, the N of Pd-Pd shell for Pd/CN-PC was
only half of that for Pd/CN-H, suggesting more surficial Pd
atoms for Pd/CN-PC. These observations reveal the success of
2
+
possible reduction of Pd species, especially for those at the
Pd-CN interface, making it possible to construct a highly cou-
pled interface. Other control samples, including carbon black
supported Pd NPs (Pd/C) nitrogen-doped layered carbon (Pd/
LC) and porous silica (Pd/SiO ) were also prepared for compari-
2
son.
The Pd/CN-H and Pd/CN-PC exhibit similar mean sizes but
totally different shapes as revealed by the high-resolution
transmission electron microscopy (HRTEM) analysis (Figure 1;
Supporting Information, Figures S2,S3). As expected, the Pd/
CN-H has a regularly round shape, because the thermal treat-
ment can help to decrease surface energy of tiny particles by
reducing the percentage of surface atoms and thus result in
the spherical particles. The shape of Pd/CN-PC is much more ir-
regular, rather speaking for a strong interaction between the
our photochemical method in removing the PdO species with-
x
out obviously changing the morphology of the Pd nanoparti-
cles. More importantly, the enriched electron intensity of Pd
nanoparticles in Pd/CN-PC was confirmed by EXAFS analysis
with the highest intensity of white line in EXAFS K-edge spec-
tra of all samples (Figure 1E), suggesting that the photochemi-
cal method is sufficient to construct a highly coupled Pd-CN in-
terface. Such an electron transfer from CN to PdNPs was also
ascertained by both the significantly decreased photolumines-
cence intensity (Supporting Information, Figure S5A) and the
lowered LUMO position (Figure S5B) of Pd/CN-PC without ob-
vious change in the band gap (Figure S5C).
[
28]
support CN and Pd nanoparticles.
X-ray photoelectron spectroscopic (XPS) analysis results (Fig-
ure 1C) reveal that the as-formed nanoparticles of Pd/CN-fresh
0
2+
are not completely metallic Pd with around 40 at% of Pd
species presumably in the form of PdO . It is reasonable to pre-
We initially tested the possibility of transfer hydrogenation
of nitrobenzene at room temperature by using water as the
solvent and FA as the only hydrogen source. For a blank test,
no conversion was observed in the absence of catalyst, noble
metals, or FA (Table 1, entries 1, 2, 5). The Pd/CN-fresh could
trigger the transfer hydrogenation of nitrobenzene (NB) at
room temperature in water and gave a selectivity >99% to
aniline and a conversion of 10% within five minutes. These ob-
x
dict that the more oxidative part of the Pd nanoparticles of
Pd/CN-fresh were mainly distributed at the Pd-CN interface as
the outer surface of the Pd nanoparticles were much easier to
be reduced owing to the involvement of excess amount of
NaBH in the synthetic process. Pd/CN-fresh remained stable in
4
2
+
vacuum without photoirradiation. The Pd content of Pd/CN-
PC was obviously lower than that of Pd/CN-fresh, as revealed
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Table 1. The reaction conditions for transfer hydrogenation of nitroben-
[
a]
zene to aniline.
Entry
Catalyst
NB:FA
t [min]
Con. [%]
Sel. [%]
1
2
3
4
5
6
7
8
9
1
–
1:5
1:0
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:1
1:2
1:3
60
60
5
–
–
100
100
–
trace
13
9
–
–
>99
>99
–
–
>99
98
–
>99
–
Pd/CN-PC
Pd/CN-PC
Pd/CN-PC
CN
Pd/C
Pd/LC
Pd/SiO
PdO /CN
Pd/CN-H
Pd/C-H
Pd/CN-PC
Pd/CN-PC
Pd/CN-PC
[
b]
5
60
60
60
60
60
5
60
5
5
2
x
–
0
23
trace
26
51
75
1
1
1
1
1
2
3
4
>99
>99
>99
5
[a] Standard conditions: H
2
O (10 mL), nitrobenzene (NB; 1 mmol), FA
(5 mmol), catalyst (20 mg), 258C. Conversions (Con.) and selectivity (Sel.)
were determined by gas chromatography (GC) using an FID detector.
b] The reaction was conducted in N
Figure 2. Conversions of nitrobenzene over various catalysts. Pd/CN-H was
obtained by reducing Pd/CN-fresh in 5% H at 3008C for 2 h, whilst Pd/CN-
2
[
2
.
PC was prepared by photochemical reduction of Pd/CN-fresh. Reaction con-
ditions: solvent (10 mL), nitrobenzene (NB; 1 mmol), FA (5 mmol), catalyst
(20 mg), 258C, 5 min. Selectivities were all determined to be 99.9% for reac-
tions over Pd/CN-fresh, Pd/CN-H, and Pd/CN-PC.
servations reveal that the supported Pd nanoparticles func-
tioned as the active sites for transferring H atoms from FA to
nitrobenzene in water. The control experiment showed that
PdO nanoparticle-based catalyst (PdO /CN) could not allow
catalysts described herein (Figure 2). Simply by optimizing the
molecular ratios of substrate nitrobenzene (n ) over Pd (n ;
x
x
access to the reduction of nitrobenzene, excluding the possi-
NB
Pd
2
+
ble scenario of Pd function as the active center. Moreover,
Supporting Information, Table S3), we increased the turnover
frequencies (TOF) of transfer hydrogenation of nitrobenzene to
the PdO part might also act as resistant layer to destroy the
x
Mott–Schottky contact and thus uncoupled interface between
Pd and CN, and were unwanted in the current catalytic system.
Until now, only a few examples of heterogeneous catalysts, in-
a very high level. The TOF value could reach up to
À1
À1
1183 mol(NB)mol Pdh at 298 K under optimized conditions.
Such a TOF value is one or two orders of magnitude greater
than those (<33) of previous heterogeneous catalysts for this
reaction even under more critical conditions (Figure 3A). The
TOF value of 1183 achieved here is also comparable or better
than the homogeneous catalysts for transfer hydrogenation of
[
22]
cluding TiO -supported Au nanoparticles
and carbon-sup-
2
[
23]
ported Pd nanoparticles, have been reported for transfer hy-
drogenation of nitroarenes from FA or its derivatives under
mild conditions with low turnover frequencies (TOF<33). For
these catalytic systems, inert protection gas, bases, or expen-
sive organic ligands were also required. Pd/CN-fresh with
[14]
nitroarene compounds (Supporting Information, Table S4).
More importantly, the conversion rate of FA in this catalytic
system is rather high. It is worth noting that the TOF values of
FA for transfer hydrogenation of nitroarenes were usually
much lower than that for direct dehydrogenation of FA to re-
À1
À1
a TOF of 24 mol(NB)mol Pdh at 298 K, which is comparable
to the bench-mark heterogeneous catalyst, promises a huge
space for further enhancing its catalytic activity as a recyclable
catalyst in water under ambient conditions. Pd/CN-H gave
a much higher conversion (23%) and TOF (184 mol(NB)
lease H , which is presumably due to the slightly higher activa-
2
tion barrier of nitro compounds as compared with that of FA.
Surprisingly, the TOF value of FA over the optimized catalyst
À1
À1
mol Pdh at 298 K) for the same reaction, surpassing the
catalytic performance of all previously reported heterogeneous
catalysts. For comparison, carbon or nitrogen-doped carbon
supported Pd nanoparticles with similar loadings and size were
also prepared (Supporting Information, Figure S6, Table S2)
and these catalysts could only offer rather limited conversion
of nitrobenzene (<13% within one hour as listed in Table 1,
entries 6–8) under standard conditions. Thus it is reasonable to
attribute the high performance of Pd-CN mainly to the inter-
face coordination or metal–support interaction, excluding the
promoting effect of structure feature or size of Pd nanoparti-
cles.
À1
À1
was up to 3549 mol FA mol Pd h at 298 K, surpassing the
bench-mark heterogeneous catalysts for all catalytic reactions
using FA or formates as hydrogen source (Figure 3B) for both
hydrogen evolution reactions and transfer hydrogenation reac-
[23,27,29–31]
tions.
Further electrochemical analysis (Supporting Information,
Figure S7) was conducted to demonstrate the electrochemical
2
À1
surface area (ESA) of Pd/CN-PC (0.13 m g ), Pd/CN-H
2
À1
2
À1
(0.83 m g ), and Pd/C-H (3.9 m g ). Such a result again ex-
0
cludes the possible effect of numbers of Pd active cites on the
final catalytic activity in this work. The maximum TOF value
À1
À1
Owing to the highly coupled Pd-CN interface and thus high
electron intensity of Pd nanoparticles, the Pd-CN-PC offered
the highest conversion of NB (100% within 5 min) among the
was estimated to be 1430 mol(NB)mol active sitess , sur-
passing all catalysts available for hydrogenation of nitroben-
zene.
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[
a]
Table 2. Pd/CN-catalyzed transfer hydrogenation of nitroarenes.
[
b]
Entry
1
Substrates
Products
t [min]
Y. [%]
99
40
2
3
30
50
99
99
4
5
120
93
15
5
99
99
99
99
[
c]
6
7
8
20
20
9
1
20
20
99
99
0
Figure 3. A) TOF values of nitrobenzene with varied ratios of catalysts (nPd)/
substrates (nsub) and B) TOF values of FA for transfer hydrogenation of nitro-
11
5
99
99
benzene over Pd/CN (*), Pd/LC (
&
), and bench-mark heterogeneous cata-
from Ref. [23]. Black
[
d]
12
60
lysts. Purple line: the TOF of nitrobenzene over Au/TiO
2
and gray bars: the TOF of FA over heterogeneous catalysts from
Refs. [29,30,31], respectively).
1
3
4
160
5
93
12
1
The scope of the reaction was further demonstrated in the
transfer hydrogenation of various substituted nitroarenes
under mild conditions. Water was chosen as a green and
cheap dispersing medium for all of the catalytic reactions, al-
though other organic solvents including ethanol and acetoni-
trile can also act as an effective solvent for such a transfer hy-
drogenation reaction (Supporting Information, Tables S5 and
S8). The Pd/CN-PC was generally effective in transfer hydroge-
nation of nitroarenes with different substituents (Table 2), in-
cluding both electron-deficient substituents (for example,
fluoro groups; Table 2, entry 5) and electron-rich species (for
example, methoxy and hydroxy; Table 2, entry 1–4, 6). When 6-
nitroquinoline was converted into 6-aminoquinoline with
a high yield at an elevated temperature (Table 2, entry 12), the
heterocyclic ring was kept intact as well. Under optimized con-
ditions, all of the substrates were transformed to correspond-
ing aromatic amines with high conversion and selectivity
[
2
a] Standard conditions: H O (10 mL), nitrobenzene (NB; 1 mmol), FA
(5 mmol), catalyst (20 mg), 258C. [b] Yields (Y.) were determined by GC
and GC-MS. [c] The yield was determined by a UV/Vis spectrum of the 4-
nitrophenol. [d] 508C.
(Supporting Information, Figure S10A) and the size of support-
ed Pd nanoparticles (Figure S10B) remained unchanged after
multiple cycles of reuse, rather speaking for the outstanding
reusability of the catalyst. Identical conversions and selectivity
were obtained when the reactions were scaled up for ten
times from 10 mL to 50 mL to 100 mL (Supporting Information,
Figure S11). Given that the present catalytic system shows no
sensitivity of the hydrogen transfer reactions to air, water, or
acid, it is feasible to scale up the synthesis of aniline in large
quantities.
(
>99%), suggesting the good tolerance of various functional
For the hydrogenation reaction of nitroarenes to corre-
sponding amines, two routes (routes 2 and 3 in Figure 4) have
groups in this Pd/CN-PC-based catalytic system.
[32,33]
Stability and recyclability of catalysts are important for prac-
tical applications. After the catalytic reaction, we recycled Pd/
CN-PC by base-washing for further use without obvious loss in
catalytic activity in the following nine runs (Supporting Infor-
mation, Figure S9). The fact that the concentration of leached
Pd in the filtrate after reaction was below the detection level
been proposed.
The fact that the conversion of azoben-
zene (Figure 4) was much slower than that of nitrobenzene
under fixed experimental conditions excludes the possibility of
route 3 in this Pd/CN-FA catalytic system. When the molecular
ratio of FA and nitrobenzene was varied from 1 to 5, aniline
was the only reaction product observed (on the basis of GC
analysis) with a selectivity >99% and various conversions de-
pending on the amount of FA (Supporting Information, Fig-
ure S12). Byproducts including nitrosobenzene and N-phenyl-
(0.1 ppm) of the inductively coupled plasma mass spectrome-
try (ICP-MS) suggests that the catalytic reactions proceeded
heterogeneously. Both the structure of carbon nitride support
Chem. Eur. J. 2014, 20, 16732 – 16737
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solution, promising the great potential of Pd/CN-FA catalytic
system for green industrial applications.
Experimental Section
Synthesis
mpg-C N (CN): Cyanamide (5 g) was dissolved in a Ludox HS40
3
4
solution (7.5 g; dispersion of 14 nm SiO particles with 40 wt% in
2
water) and heated at 658C overnight to remove water. The as-
formed white powder was heated at a temperature of 6008C for
À1
4
h (ramp: 2.38C min ) under the protection of N . The resulting
2
brown–yellow powder was treated with a 4m HF acid for 24 h to
remove the silica template. The powders were then centrifuged
and washed three times with distilled water and twice with etha-
nol. Finally the powders were dried at 608C under vacuum over-
night.
Figure 4. The proposed reaction routes of hydrogenation of nitroarenes. Ni-
trobenzene was reduced (route 1) by consuming six hydrogen atoms
bonded on the surface of the Pd catalyst in one step, or (route 2) through
multi-step reactions via partly reduced intermediates (nitrosobenzene and
N-phenylhydroxylamine) to aniline, or (route 3) via a coupling intermediate
Pd/CN-fresh: Mesoporous carbon nitride (100 mg) was dispersed
into water (20 mL) by sonication and vigorous stir. A mixed solu-
tion of PdCl2 with 2m HCl solution containing Pd component
(9 mg) was added into the dispersion and stirred overnight. A cer-
(azobenzene) to aniline.
tain amount of 4m NaOH solution was added to tune the pH value
to around 13. Then, 0.1m NaBH (10 mL) was added dropwise to
4
hydroxylamine could only be detected when deactivated cata-
lysts were used (Supporting Information, Figure S9B). As
a result, the reduction of nitrobenzene proceeded by consum-
ing six hydrogen atoms bonded on the surface of the Pd cata-
lyst in one step (route 1) even when hydrogen source was far
less than the amount of nitrobenzene (Supporting Information,
Figure S12). The pre-adsorption of the nitrobenzene and corre-
sponding oxygen-containing intermediates on the surface of
this suspension at a temperature of 10–158C. After 6 h, the ob-
tained mixture was separated by centrifugation, washed thorough-
ly with distilled water and ethanol, and finally dried at 608C for
1
2 h.
Pd/CN-PC: Pd/CN-fresh was reduced by a photochemical method
under the irradiation of a commercial lamp (15W) for one week at
room temperature, resulting in the Pd/CN-PC sample. The reactor
is shown in the Supporting Information, Figure S1.
Pd/CN-H: Pd/CN-fresh (100 mg) was heated to 3008C for one hour
with N protection, and then reduced with 5% H at 3008C for 2 h.
[
4,34]
carbon nitrides should be a key step.
Indeed, H can form
2
2
2
in our catalytic system via decomposition of FA with an FA
conversion of 1%, which is however much lower than that
Characterization
(60%) of the hydrogenation reaction of nitrobenzene under
fixed conditions (Supporting Information, Figure S13). A series
of control experiments were also conducted in acetonitrile as
the solvent and a similar trend was observed. It is concluded
The PXRD measurements were performed on a Rigaku D/Max 2550
X-ray diffractometer operating with Cu Ka1 radiation. The HRTEM
and TEM measurements were performed on JEOL JEM-2010HT. Ni-
trogen adsorption/desorption analyses were performed on a Micro-
meritics ASAP 2010 M1C nitrogen adsorption instrument (Microme-
tritices Inc., USA) at 77 K. The GC analysis was performed on Shi-
madzu GC-2014 gas chromatograph with a flame ionization detec-
tor (FID) and a Rtx-1 column (30 m in length and 0.25 mm in diam-
eter) using nitrogen as the carrier gas. The GC-MS analysis was
performed on Shimadzu QP2010E with a Rxi-5 ms column (30 m in
length and 0.25 mm in diameter) using He as the carrier gas. The
ICP-MS analysis was performed on iCAP6300. For cyclic voltamme-
try (CV) measurements, thin films of the mpg-C N and Pd/CN were
that the formation and activation of H are not essential steps
2
for hydrogenation of nitrobenzene in our reaction system. Fur-
ther investigation in the reaction mechanism is also in prog-
ress.
Conclusion
In summary, the highly coupled metal–CN interface has been
demonstrated to be the crucial factor underlying the high se-
lectivity and high TOF value. The Pd/CN-PC catalyst obtained
facilely by a mild photochemical method bears many advan-
tages, including low cost, high efficiency, safety, and sustaina-
bility, in contrast with conventional synthetic pathways for aro-
3
4
drop-casted on a glassy carbon electrode from a suspension of the
catalyst (5 mg), Nafion (95 mL), and ethanol (350 mL). The electro-
chemical study of the networks was investigated on CHI 700E in
an electrolyte solution of 0.1m TBAPF6 in acetonitrile. Potential
was calculated versus ferrocene (0.64 eV vs NHE). Scan rate:
À1
matic amines via activation of H . The excellent reusability and
100 mVs ; T=208C. To measure the electrochemical surface area
2
(
ESA) of Pd catalysts, CV measurements were carried out in 0.5m
reproducibility (Supporting Information, Figure S14) of the cat-
alytic system and its exceptional catalytic activity reported
herein show that it is now possible to combine efficacy and re-
usability in the same heterogeneous catalyst to facilitate trans-
fer hydrogenation of nitroarenes at room temperature. Huge
À1
H SO solutions at a sweep rate of 50 mVs . The electrochemically
2
4
active surface area of the Pd catalysts was calculated from the hy-
drogen adsorption/desorption charge in the CV curve. The XPS
measurements were performed on a Kratos AXIS ULTRA DLD X-ray
photoelectron spectrometer with a monochromatized X-ray source
(Al KR hu S2=1486.6 eV). The Pd X-ray absorption spectra (XAS)
were obtained on the BL14W1 beamline at the Shanghai Synchro-
H tanks and high-temperature fluidized bed reactors can be
2
generally replaced by a room-temperature formic acid aqueous
Chem. Eur. J. 2014, 20, 16732 – 16737
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tron Radiation Facility (SSRF), operated at 3.5 GeV with injection
currents of 140–210 mA. A Si(111) double-crystal monochromator
was used to reduce harmonic component of the monochrome
beam. All of the samples were measured in fluorescence mode.
The powder of a sample was pressed into a self-supporting disk,
which was then sealed with Kapton membrane and subjected to
XAS measurement. The IFEFFIT software was used to calibrate the
energy scale, to correct the background signal and to normalize
the intensity. Reliable parameters for the Z (Pd, Pd) and Z (Pd, N or
O) contributions were determined by multiple-shell fitting in r
[6] Y. Yamada, C. K. Tsung, W. Huang, Z. Huo, S. E. Habas, T. Soejima, C. E.
Aliaga, G. A. Somorjai, P. Yang, Nat. Chem. 2011, 3, 372.
[
[
[
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Acknowledgements
This work was supported by the National Basic Research Pro-
gram of China (2013CB934102, 2011CB808703) and the Nation-
al Natural Science Foundation of China (21331004, 21301116).
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Keywords: formic acid
·
heterogeneous catalysis
·
2
012, 134, 8926.
hydrogenation · interfaces · sustainable chemistry
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Received: July 9, 2014
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Published online on October 21, 2014
Chem. Eur. J. 2014, 20, 16732 – 16737
www.chemeurj.org
16737
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