Single-atom Fe-N4site for the hydrogenation of nitrobenzene: theoretical and experimental studies

Article abstract of DOI:10.1039/d1dt01227d

The hydrogenation of nitrobenzene to aniline is an important process in the industry of fine chemicals, but developing inexpensive catalysts with expected activity and selectivity still remains a challenge. By using density functional theory calculations, we demonstrated that the isolated Fe atom not only can weaken the adsorption of reactants and reaction intermediates as compared to Fe nanoparticles, but also remarkably decrease the reaction barrier for the hydrogenation of nitrobenzene to aniline. Thus, the Fe single-atom (Fe SA) catalyst is considered as an ideal catalyst for this reaction. This theoretical prediction has been subsequently confirmed by experimental results obtained for the Fe SAs loaded on N-doped hollow carbon spheres (Fe SAs/NHCSs) which achieved a conversion of 99% with a selectivity of 99% for the hydrogenation of nitrobenzene. The results significantly outperformed the Fe nanoparticles for this reaction. This work provides theoretical insight for the rational design of new catalytic systems with excellent catalytic properties.

Full text of DOI:10.1039/d1dt01227d

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Single-atom Fe–N4 site for the hydrogenation of
nitrobenzene: theoretical and experimental
studies†
Cite this: Dalton Trans., 2021, 50,
7995
Yan Liu,a Wenzhuang Zhang,a Yamin Zheng,a Konglin Wu, b Panpan Dong,a
a
Rong He, c Ning Lua and Junjie Mao
*
The hydrogenation of nitrobenzene to aniline is an important process in the industry of fine chemicals,
but developing inexpensive catalysts with expected activity and selectivity still remains a challenge. By
using density functional theory calculations, we demonstrated that the isolated Fe atom not only can
weaken the adsorption of reactants and reaction intermediates as compared to Fe nanoparticles, but also
remarkably decrease the reaction barrier for the hydrogenation of nitrobenzene to aniline. Thus, the Fe
single-atom (Fe SA) catalyst is considered as an ideal catalyst for this reaction. This theoretical prediction
has been subsequently confirmed by experimental results obtained for the Fe SAs loaded on N-doped
hollow carbon spheres (Fe SAs/NHCSs) which achieved a conversion of 99% with a selectivity of 99% for
the hydrogenation of nitrobenzene. The results significantly outperformed the Fe nanoparticles for this
reaction. This work provides theoretical insight for the rational design of new catalytic systems with excel-
lent catalytic properties.
Received 14th April 2021,
Accepted 6th May 2021
DOI: 10.1039/d1dt01227d
rsc.li/dalton
metal catalysts. The composite structure of catalysts can also
be used in the hydrogenation reaction. For example, Xiao et al.
Introduction
As an important raw material in the industry of fine chemicals, reported that the Au/Sn-TiO2 catalyst exhibited high catalytic
aniline is widely used for the production of pesticides, medi- activity in the hydrogenation of nitroarenes.11 However, the
cines, polymeric surfactants, and dyes.1–5 In the production of complex nanostructure/nanointerface is not conducive to the
aniline, the catalytic hydrogenation of nitrobenzene is the understanding of the catalytic active center and also brings
commonly used synthetic strategy. Noble metal-based cata- difficulties to the deeper mechanistic research. In this regard,
lysts, such as Pt, Pd, and Ru, are highly active for the hydrogen- it is highly desired to develop inexpensive catalysts with well-
ation reaction.6–10 For example, Li and co-workers demon- defined catalytic structure and adequate activity and selectivity
strated that corroded PtNi3 alloy showed high catalytic activity for the conversion of nitrobenzene to aniline. Single atom (SA)
and selectivity towards the hydrogenation of nitrobenzene.10 catalysts with the highest atomic utilization and a unique elec-
However, the scarcity and high cost of noble metal-based cata- tronic structure have received considerable attention
lysts hamper their practical use. Moreover, the hydrogenation recently.12–35 For example, Wang and co-workers developed a
of nitrobenzene to aniline is complex, involving several parallel series of synthetic strategies for preparing SAs with various
and cascade reactions that cause the formation of by-products, applications including electrocatalysis, photocatalysis and
such as hydroxylamine and azo-compounds. The intermediate organic catalysis,36,37 providing great opportunities for achiev-
products show poor selectivity over the commonly used noble ing high performance catalysts toward industrial applications.
In this study, we demonstrated, by means of systematic
theoretical analysis, that Fe single atoms (Fe SAs) are ideal cat-
aKey Laboratory of Functional Molecular Solids, Ministry of Education, Anhui
alysts for the hydrogenation of nitrobenzene. Specifically, we
Province Key Laboratory of Optoelectric Materials Science and Technology and Anhui
simulated all the possible reaction pathways of the hydrogen-
ation of nitrobenzene over Fe SAs and Fe nanoparticles (Fe
NPs). Density functional theory (DFT) calculations showed that
adsorption of nitrobenzene and reaction intermediates on Fe
SAs is not only weaker than that on Fe NPs, but these SAs
largely decreased the energy barrier of the hydrogenation reac-
tion. To confirm the theoretical prediction, we synthesized the
Laboratory of Molecule-Based Materials, Anhui Normal University, Wuhu, 241002,
China. E-mail: maochem@ahnu.edu.cn
bSchool of Chemistry and Chemical Engineering, Anhui University of Technology,
Maanshan 243002, China
cState Key Laboratory of Environment-friendly Energy Materials, Southwest
University of Science and Technology, Mianyang, 621010, China
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1dt01227d
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Fe single atoms on the N-doped hollow carbon spheres (Fe action between the reactant and FeN4 catalyst, which is favor-
SAs/NHCSs) and used them for the catalytic hydrogenation of able for the reaction.
nitrobenzene. Notably, Fe SAs/NHCSs exhibited a conversion
To further evaluate the catalytic activity of the Fe SA catalyst,
of nitrobenzene of 99% with a selectivity to aniline of 99%, we simulated all possible reaction pathways of the nitro-
which considerably outperformed Fe NPs/NHCSs. The success- benzene hydrogenation reaction. As shown in ESI,
ful development of Fe SAs/NHCSs as hydrogenation catalysts is Scheme S1,† four hydrogenation pathways of nitrobenzene to
a prototype for the rational design of catalytic systems at the aniline are possible.38,39 The common features of them must
atomic level based on the theoretical calculation.
undergo the dissociation of N–O bonds and the formation of
N–H (O–H) bonds, whereas the difference between them is
whether the N–O bond dissociation occurs first during the
second and fourth steps of the hydrogenation reaction. To
identify the optimal reaction path, we calculated the reaction
energies and the activation energies of all the corresponding
elementary steps (ESI, Table S1†). Fig. 2 displays the energy
profiles of all the reaction pathways, and the optimal reaction
pathway is shown with a red line. The first step of all these
routes is the transfer of the H atom to the O atom of the nitro
group to form an O–H bond and generate C6H5NOOH, a
process needing to overcome an energy barrier of 0.60 eV. In
the second step, two competitive pathways exist. One was the
hydrogenation of C6H5NOOH to form C6H5N(OH)2, whereas
the other corresponds to the dissociation of N–O bonds in
C6H5NOOH to form C6H5NO. The formation of C6H5N(OH)2 is
an exothermic process and releases an energy of 1.61 eV.
However, the activation barrier of the subsequent dehydroxyla-
tion to C6H5NOH (0.89 eV) is 0.22 eV higher than that of the
formation of C6H5NO (0.67 eV). Thus, it is more likely that the
dissociation of the N–O bond occurs for the production of
C6H5NO. Next, the formation of C6H5NOH requires a barrier of
0.02 eV, while it releases an energy of 1.27 eV. Unlike the
second step of the hydrogenation reaction, the H atom in the
hydrogenation process of C6H5NOH prefers to bind with N to
form an N–H bond, which is easier than the dissociation of
the N–O bond in C6H5NOH. The activation energy of 0.87 eV
for the fifth step is the highest, which makes it the rate-deter-
mining step of the whole reaction process. In this step, an
alternative pathway of the five-membered ring transition state
for N–O bond cleavage was also investigated (ESI, Fig. S3†).
This alternative pathway exhibited similar adsorption energy
and activation energy to the original pathway in Fig. 2. Finally,
H easily transfers to N to form C6H5NH2 via an exothermic
process, releasing an energy of 2.57 eV. Thus, the reaction
mechanism on the Fe SA catalyst involves several consecutive
reactions, which follow the order of C6H5NO2 → C6H5NOOH →
C6H5NO → C6H5NOH → C6H5NHOH → C6H5NH → C6H5NH2.
For comparison, we also investigated the hydrogenation
reaction pathway of nitrobenzene on the surface of the Fe NP
catalyst. The energy profiles for all the hydrogenation reaction
pathways on the Fe NP catalyst are shown in Fig. 3, while the
corresponding activation and reaction energies are summar-
Results and discussion
To investigate the stable structure of the Fe SA catalyst, geome-
try optimizations of all possible adsorption configurations of a
single Fe atom on the graphene were performed, and the
corresponding binding energies were calculated (ESI, Fig. S1†).
As shown in Fig. 1a, a single Fe atom coordinated with four N
atoms (FeN4) can be strongly adsorbed on graphene, with a
binding energy of 6.51 eV. The difference charge density
shown in Fig. 1b reveals the electronic structure of a single Fe
atom coordinated with N. Therefore, we selected the FeN4
structure as a model to study the mechanism of the hydrogen-
ation of nitrobenzene to aniline on the surface of the Fe SA
catalyst. Because the adsorption of reactants plays an impor-
tant role in the heterogeneous catalytic reactions, we first
simulated the possible adsorption configurations of nitro-
benzene on the FeN4 structure. The adsorption configurations
of nitrobenzene were classified into two types, i.e., vertical and
parallel adsorption. All the optimized structures and adsorp-
tion energies of these configurations are displayed in ESI,
Fig. S2.† The adsorption energy of the parallel configuration
(0.65 eV) was higher than that of the vertical adsorption con-
figuration (0.30 eV). Accordingly, it can be stated that nitro-
benzene prefers to adsorb on the surface of the Fe SA catalyst
in the parallel mode (Fig. 1c). Afterwards, we simulated the
difference charge of the parallel adsorption configuration
(Fig. 1d). The significant charge transfer between the nitro
group of nitrobenzene and Fe atom suggests a strong inter-
Fig. 1 (a) The optimized geometry and the binding energy of the FeN4 ized in ESI, Table S2.† First, a stable Fe6 configuration was
used to simulate Fe NPs, as previously reported.40 The calcu-
lated configuration of nitrobenzene and adsorption energies
structure. (b) Difference charge density of the FeN4 structure. (c) The
optimized geometry and the adsorption energy of C6H5NO2 on the
FeN4 surface. (d) Difference charge density of the FeN4 structure with
the adsorption of nitrobenzene. C, N, O, H and Fe atoms are represented
on the Fe NP catalyst are depicted in ESI, Fig. S4.† As noticed,
the most stable adsorption mode of nitrobenzene was the par-
allel adsorption configuration, which was similar to that found
by grey, blue, red, white, and orange balls, respectively. The positive and
negative changes are shown in yellow and cyan, respectively.
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Fig. 2 Energy profiles of four possible reaction pathways for the hydrogenation of nitrobenzene to aniline on the FeN4 structure. The red route rep-
resents the optimal reaction pathway. The green numbers represent the energy barriers of the rate-determining steps for the whole reaction
process. Optimized structures of the intermediates and transition states for the optimal reaction pathway are exemplified.
Fig. 3 Energy profiles of four possible reaction pathways for the hydrogenation of nitrobenzene to aniline on the Fe nanoparticles. The red route
represents the optimal reaction pathway. The green numbers represent the energy barriers of the rate-determining steps for the whole reaction
process. Optimized structures of the intermediates and transition states for the optimal reaction pathway are exemplified.
for Fe SAs. The adsorption energy between the reactant and C6H5NO2 → C6H5NOOH → C6H5NO → C6H5NOH → C6H5N →
catalyst is as high as 2.25 eV, implying a difficult occurrence of C6H5NH → C6H5NH2. The hydrogenation of C6H5NH to
the subsequent hydrogenation reaction (ESI, Table S3†). In the C6H5NH2 is the rate-determining step of the whole process,
light of calculation results, the optimal reaction pathway on with a barrier of 2.03 eV, and a reaction energy endothermic by
the Fe NPs is different from that on the Fe SA catalyst, and it is 0.68 eV. Obviously, the hydrogenation reaction of nitrobenzene
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on the Fe NP catalyst has to overcome a higher energy barrier
The precursor (PS@Fe(III)-APFR) and final sample (Fe SAs/
than that on the Fe SA catalyst. Therefore, the above results NHCSs) were characterized in relation to their composition,
prove that the Fe SA catalyst is a better catalyst for this hydro- morphology, and structure. The chemical compositions of Fe
genation reaction.
SAs/NHCSs determined by inductively coupled plasma mass
To verify the theoretical results, we performed the experi- spectrometry (ICP-MS) show that the weight percentage of Fe
mental studies on Fe SAs and Fe NPs. The typical synthetic element in Fe SAs/NHCSs was 0.95%. The morphology was
strategy of Fe SAs/NHCSs is illustrated in Fig. 4a. First, poly- assessed by transmission electron microscopy (TEM) in com-
styrene (PS) nanospheres were prepared via a previously parison with that of pure PS. As shown in ESI, Fig. S5,† the PS
reported procedure and used as the template. The PS template template displays a sphere-like morphology with a homo-
was then uniformly dispersed in an aqueous solution contain- geneous size distribution. As shown in Fig. 4b, the PS@Fe(III)-
ing m-aminophenol and Fe(NO3)3·9H2O. After that, formal- APFR precursor displays a similar morphology to the PS tem-
dehyde was injected into the mixture to obtain the PS@Fe(III)- plate, indicating the morphology preservation during the
aminophenol formaldehyde (APFR) product through conden- polymerization process. For Fe SAs/NHCSs, the decomposition
sation polymerization. In this case, the PS@Fe(III)-APFR of the PS template led to a hollow sphere-like morphology with
retained the amino groups (–NH2) of the m-aminophenol pre- spheres of an average diameter of ca. 250 nm (Fig. 4c). The
cursor, which served as the coordination sites for Fe3+ ions. TEM image of an individual hollow sphere of Fe SAs/NHCSs
The as-prepared PS@Fe(III)-APFR was finally carbonized under shows that the shell thickness of the carbon layer is 10 nm
an Ar atmosphere to remove the PS template and form Fe SAs/ (Fig. 4d). As revealed by the results of N2 adsorption and de-
NHCSs.
sorption experiments, Fe SAs/NHCSs display a high specific
Fig. 4 (a) Schematic illustration of the synthesis of Fe SAs/NHCSs. (b) TEM images of PS@Fe(III)-APFR. (c) TEM images. (d) Magnified TEM images of
the Fe SAs/NHCSs. (e) HAADF-STEM image and the corresponding EDX maps of Fe SAs/NHCSs. (f) Representative AC HAADF-STEM image of the Fe
SAs/NHCSs. The red circles were drawn around the isolated Fe atom. (g) XRD pattern of Fe SAs/NHCSs and NHCSs.
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area of 112 m2 g−1 with a micropore size of 1.95 nm (ESI, +3. The EXAFS spectrum of Fe SAs/NHCSs displays a single
Fig. S6†). Fig. 4e shows the high-angle annular dark field scan- peak at 1.5 Å, which corresponds to the Fe–N bond (Fig. 5b).
ning transmission electron microscopy (HAADF-STEM) image Furthermore, no typical peak of Fe–Fe bonds with an intera-
and the corresponding energy dispersive X-ray (EDX) elemental tomic distance of above 2.2 Å could be found, demonstrating
mapping of the selected area for Fe SAs/NHCSs. The C, N, and the atomically dispersed Fe species in the N-doped hollow
Fe elements are homogeneously distributed on the whole carbon sphere. In addition, we performed a wavelet transform
hollow sphere. The existence of these elements on the surface (WT) to further illustrate the Fe K-edge EXAFS oscillation
was further confirmed by XPS (ESI, Fig. S7†). The high resolu- (Fig. 5c). Only one intensity maximum at around 3.6 Å−1
tion N 1s XPS spectrum displays signals at 398.4 eV, 400.2 eV corresponding to the Fe–N bond was observed in the WT
and 401.2 eV, proving the co-existence of the pyridinic N, pyr- contour plot of Fe SAs/NHCSs, which further confirms the iso-
rolic N, and graphitic N in Fe SAs/NHCSs (ESI, Fig. S8(a)†). In lation of Fe atoms in Fe SAs/NHCSs. The chemical environ-
addition, the Fe 2p XPS spectrum of Fe SAs/NHCSs indicates a ment and the corresponding structural parameters of Fe atoms
mixed valence of +2 and +3 (ESI, Fig. S8(b)†). The existing were determined by quantitative EXAFS fitting in the R space,
form of Fe was validated by the atomic-resolved HAADF-STEM k space and q space (Fig. 5d, e, and ESI, Fig. S9†). As shown in
image. Based on the heavier Z-contrast of Fe relative to N and ESI, Table S4,† Fe SAs/NHCSs exhibited a coordination
C, the isolated Fe atoms were recorded in the carbon layer in number of 4.35 with an average bond length of 2.02 Å, which
the single-atom state (Fig. 4f). As shown in Fig. 4g, the XRD shows that the Fe atom in Fe SAs/NHCSs is coordinated with
pattern of Fe SAs/NHCSs does not display the characteristic four N atoms, forming a typical Fe1N4 structure.
peaks of iron, which exclude the existence of Fe NPs.
To evaluate the catalytic performance of Fe SAs/NHCSs, we
To explore the valence state and coordination environment performed the hydrogenation of nitrobenzene at 40 °C for 6 h.
of Fe species in Fe SAs/NHCSs, we carried out X-ray absorption The Fe nanoparticles loaded on N doped hollow carbon (Fe
near edge structure (XANES) and extended X-ray absorption NPs/NHCSs) and N doped hollow carbon (NHCSs) were used
fine structure (EXAFS) experiments of Fe SAs/NHCSs, Fe foil, as reference catalysts. The dispersed Fe nanoparticles on Fe
and Fe2O3. As shown in Fig. 5a, displaying the XANES spectra, NPs/NHCSs were verified by TEM, XRD and XPS measurements
the energy absorption edge of Fe SAs/NHCSs is located (ESI, Fig. S10–13†). As shown in Fig. 6a, nitrobenzene rarely
between that of Fe foil and Fe2O3, which indicates that the converted into aminobenzene without a catalyst. Additionally,
valence state of the Fe atom in Fe SAs/NHCSs is between 0 and Fe NPs/NHCSs and NHCSs exhibited poor catalytic activity
with 26% and 10% conversions, respectively. In contrast, Fe
Fig. 5 (a) Fe K-edge XANES spectra of Fe SAs/NHCSs, Fe foil, and
Fe2O3. (b) Fourier transform (FT) of Fe K-edge XANES spectra of Fe SAs/
NHCSs, Fe foil, and Fe2O3. (c) Wavelet transform (WT) plots of Fe SAs/
NHCSs, Fe foil, and Fe2O3. (d) The corresponding EXAFS fitting curve of
Fe SAs/NHCSs in the R space. (e) The EXAFS fitting curves of Fe SAs/
NHCSs in the k space.
Fig. 6 (a) The catalytic performance of Fe SAs/NHCS, Fe NPs/NHCS,
and NHCS catalysts. (b) Recycling test of the Fe SAs/NHCS catalyst. (c)
Hydrogenation of the functionalized nitro-compounds by Fe SAs/
NHCSs.
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SAs/NHCSs achieved a remarkable conversion of 99% with a Foundation of Anhui province (1908085QB45, 2008085QB81,
selectivity to aminobenzene of 99%, which was higher than and 2008085QA33) and the Education Department of Anhui
those of Fe NPs/NHCSs (ESI, Table S5†) and comparable to Province Foundation (KJ2019A0503). The calculations in this
those of the reported catalysts (ESI, Table S6†). The kinetic paper have been done using the supercomputing system of the
analysis indicated that the hydrogenation of nitrobenzene over National Supercomputing Center in Changsha. The authors
Fe SAs/NHCSs obeyed the first order kinetics (ESI, Fig. S14†). thank BL1W1B in the Beijing Synchrotron Radiation Facility
Fig. 6b shows the stability of the Fe SAs/NHCSs for the catalytic (BSRF) for help with characterization.
hydrogenation of nitrobenzene. Both the conversion and the
selectivity of Fe SAs/NHCSs exhibited negligible decay after 5
cycles, demonstrating high stability. The Fe SAs/NHCSs after
the stability test were characterized by HRTEM, elemental
Notes and references
mapping, XRD and XPS (ESI, Fig. S15–17†). All results demon-
strated the absence of Fe nanoparticles or clusters, which indi-
cated that the Fe single atoms in Fe SAs/NHCSs hardly aggre-
gated during the reusability tests. Consequently, Fe SAs/
NHCSs served as robust catalysts for the highly efficient hydro-
genation of nitrobenzene to aniline. Moreover, we performed
the hydrogenation of nitroaromatic compounds with different
substituents to study the hydrogenation scope of Fe SAs/
NHCSs (Fig. 6c), and found that the Fe SAs/NHCS catalyst exhi-
bits high activity and selectivity in the hydrogenation of all of
the investigated nitroarenes. The results further confirm the
high catalytic performance of the as-obtained Fe SAs/NHCS
catalyst.
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In this study, DFT calculations were used to predict the cata-
lytic activity of the Fe SA catalyst in the hydrogenation of nitro-
benzene to aniline. The results revealed that the isolated Fe
atom on the support can weaken the adsorption of nitro-
benzene on the catalyst surface, thus ensuring the appropriate
adsorption energy of the reactants on the surface of the cata-
lysts needed for an efficient catalytic process. Moreover, com-
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hydrogenation reaction on the Fe SA catalyst were much lower,
which significantly increased the reaction rate. To validate the
theory, Fe SAs/NHCSs were synthesized and the catalytic per-
formance for the hydrogenation of nitrobenzene was per-
formed. The experimental results were in good agreement with
the theoretical calculations, since Fe SAs/NHCSs achieved a
conversion of 99% with a selectivity of 99%, performance
exceeding that of the Fe NPs/NHCSs. Our finding provides a
new insight for the rational design of catalytic systems at the
atomic level.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21971002), the Natural Science
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