Silver nanoparticles supported on P, Se-codoped g-C3N4 nanosheet as a novel heterogeneous catalyst for reduction of nitroaromatics to their corresponding amines

Article abstract of DOI:10.1016/j.molstruc.2021.130646

P, Se-codoped g-C₃N₄ (PSeCN) nanosheet was in situ prepared by facile thermal polymerization of melamine, phosphonitrilic chloride trimer, and selenium black powder as the precursors. It was found as a suitable support for the immobilization of silver nanoparticles (Ag NPs). The prepared nanocatalyst was fully characterized via standard analysis methods including EDX, ICP-OES, XRD, FT-IR, SEM, TEM, and BET. This PSeCN/Ag nanocatalyst with a higher specific surface area compared with CN, showed excellent catalytic activity towards the reduction of several nitroaromatic compounds using sodium borohydride (NaBH₄) in short reaction times with high efficiency and good selectivity in water as a green solvent. Significantly, the above-mentioned nanocomposite could be reused six times without appreciable loss of its catalytic activity.

Full text of DOI:10.1016/j.molstruc.2021.130646

Journal Pre-proof
Silver nanoparticles supported on P, Se-codoped g-C3N4 nanosheet
as a novel heterogeneous catalyst for reduction of nitroaromatics to
their corresponding amines
Mohadese Piri , Majid M. Heravi , Ali Elhampour ,
Firouzeh Nemati
PII:
S0022-2860(21)00779-1
DOI:
Reference:
https://doi.org/10.1016/j.molstruc.2021.130646
MOLSTR 130646
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Journal of Molecular Structure
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12 December 2020
4 May 2021
5 May 2021
Please cite this article as: Mohadese Piri , Majid M. Heravi , Ali Elhampour , Firouzeh Nemati , Silver
nanoparticles supported on P, Se-codoped g-C3N4 nanosheet as a novel heterogeneous catalyst for
reduction of nitroaromatics to their corresponding amines, Journal of Molecular Structure (2021), doi:
https://doi.org/10.1016/j.molstruc.2021.130646
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Silver nanoparticles supported on P, Se-codoped g-C3N4 nanosheet as a novel heterogeneous catalyst for
reduction of nitroaromatics to their corresponding amines
Mohadese Piria, Majid M. Heravia,*, m.heravi@alzahra.ac.ir, Ali Elhampourb,*,
Elhampour_ali@semnan.ac.ir, and Firouzeh Nematic
aDepartment of Chemistry, School of Science, Alzahra University, Vanak, Tehran, Iran
bLecturer, Semnan University, Semnan, Iran
cDepartment of Chemistry, Semnan University, Semnan, Iran
*Corresponding authors
Graphical Abstract
Highlights




Phosphorus and Selenium was doped into g-C3N4 by two-step calcination strategy.
PSeCN/Ag was synthesized by the stabilization of Ag NPs on the surface of PSeCN.
PSeCN/Ag exhibited excellent catalytic efficiency in the hydrogenation of nitroarenes.
PSeCN/Ag can be recovered and reused six runs without appreciable loss in its activity.

Abstract
P, Se-codoped g-C3N4 (PSeCN) nanosheet was in situ prepared by facile thermal polymerization
of melamine, phosphonitrilic chloride trimer, and selenium black powder as the precursors. It
was found as a suitable support for the immobilization of silver nanoparticles (Ag NPs). The
prepared nanocatalyst was fully characterized via standard analysis methods including EDX,
ICP-OES, XRD, FT-IR, SEM, TEM, and BET. This PSeCN/Ag nanocatalyst with a higher
specific surface area compared with CN, showed excellent catalytic activity towards the
reduction of several nitroaromatic compounds using sodium borohydride (NaBH4) in short
reaction times with high efficiency and good selectivity in water as a green solvent.
Significantly, the above-mentioned nanocomposite could be reused six times without appreciable
loss of its catalytic activity.
Keywords
P, Se-codoped graphitic carbon nitride, heterogeneous nanocatalyst, Ag NPs, reduction,
nitroarene
Introduction
In recent decades, due to industrial and economic growths, environmental pollution has always
been strongly considered a serious concern, worldwide [1]. Organic pollutants have strong
effects on the environment thus, should be intensively avoided [2-4]. Among the various
pollutants, nitroaromatic compounds are the most common ones which are found in industrial
and agricultural wastewaters [5].

The hydrogenation of toxic nitroarenes has been drawn much attention because the resultant
aromatic amines are non-toxic and widely used as key intermediates and raw materials (major
feedstocks) for the production of dyes, pigments, pharmaceuticals, herbicides, agrochemicals,
and functional polymers [6-8].
There are several synthetic approaches reported in the chemical literature for the reduction of
nitroarenes, such as electrolytic reduction [9], metal/acid reduction [10, 11], catalytic
hydrogenation [12-15], etc.
The electrolytic processes are usually performed in alkaline or acidic media, requiring high
energy consumption and the desired product yields are usually very low.
Metal/acid systems display poor selectivity and need strong acidic medium and stoichiometric
amounts of reducing agents that eventually produce a large number of wastes, causing serious
environmental problems (hence, are not economic and environmentally benign processes).
Among all known developed strategies, the catalytic hydrogenation of nitro compounds by noble
metal nanoparticles (NMNPs) is a promising method as it can be carried out under mild reaction
conditions, does not produce acid effluents, and demonstrates high selectivity and efficiency [16,
17].
Especially, the silver nanoparticles (Ag NPs), due to their biocompatibility, good conductivity,
stability, high catalytic activity, anti-viral, antibacterial, and antifungal properties, and lower
price as compared with other noble metals (Pt, Pd, Au) have attracted considerable attention in
organic synthetic chemists community [18, 19]. However, free Ag NPs can easily agglomerate
because of their high surface energy and small size, leading to poor durability and a remarkable
decrease in catalytic efficiency. Immobilization of Ag NPs on the appropriate solid supports is an
effective approach to circumvent these problems and thus enhance the catalytic activity and

stability. Toward this end, the design and synthesis of cost-effective and environmentally benign
supports that can provide a good platform for stabilizing NMNPs are highly desirable.
In recent years, 2D materials have garnered significant attention [20], among them, 2D graphitic
carbon nitride (g-C3N4), a metal-free polymeric semiconductor, with the layered structure and
ultrahigh nitrogen content (theoretically up to 60 wt.%) has emerged as appealing catalyst
support for the development of efficient and robust heterogeneous catalytic systems because of
its dazzling advantages including unique optical, electronic and morphological peculiarities, high
thermal and chemical stability, nontoxicity, abundant and cheap raw materials (ammonium
thiocyanate, thiourea, urea, cyanamide, dicyandiamide, and melamine), and facile synthesis [21-
24]. Unfortunately, despite these merits, applications of pure g-C3N4 in the catalysis are
restricted by some disadvantages such as low specific surface area, low electrical conductivity,
rapid carrier recombination rate, poor harvesting of the visible‐ light irradiation, weak π-π
conjugated stacked structure, and lack of active sites [25-28].
Many innovative strategies have been developed to overcome these deficiencies such as
protonating by strong acids [29], preparation of porous g-C3N4 [30], controlling morphology [31,
32], coupling with other semiconductors [33, 34], copolymerization [35, 36], and exfoliation to
two-dimensional nanosheets [37]. The 2-D g-C3N4 nanosheets because of having a larger
specific surface area than bulk g-C3N4 can improve catalytic activity. Also, heteroatom doping
(e.g., B, F, S, P, and I) is an efficient and simple technique to tailor the electronic structures,
chemical nature as well as texture properties, reduce the defect densities, improve catalytic
performance and enhance active sites of g-C3N4 [38-40]. In particular, doping with two
heteroatoms can significantly make more active sites to interact with immobilized nanoparticles
than singularly doped ones [41].

Selenium is a metabolizable element in organisms and very eco-friendly and safe with
exceptional and incomparable features within the periodic table [42]. Selenium possesses higher
polarizability, facilitating charge localization, and larger atomic size than sulfur and lone pairs of
selenium interact easily with the surrounding molecules. When Se is linked to metals, it indicates
more metalline, resulting in excellent chemical stability [43, 44]. Therefore, it can be considered
as an interesting candidate for doping into the framework of carbon nitrides. Also, P atom doping
can increase the active sites of the carbon compounds [45]. Nonetheless, to the best of the
authors' knowledge, there is no report in the literature about the co-doping of g-C3N4 with
selenium and phosphorus heteroatoms.
In continuation of our studies on the design and synthesis of heterogeneous nanocatalysts [46-51]
and armed with experience in the reduction of nitroaromatic compounds to their corresponding
amines catalyzed by nanoparticles [52] in the present paper, we wish to report on our successful
attempts of in situ preparation of P, Se-codoped g-C3N4 nanosheet (designated as PSeCN) by
two-step calcination strategy using melamine, phosphonitrilic chloride trimer, and selenium
black powder as the precursors and next used as the ideal support for the immobilization of Ag
nanoparticles to obtain PSeCN/Ag. Taking into account all these considerations, the present
study aimed to expand a highly efficient, robust, and easily separable nanocatalyst for the
catalytic hydrogenation of nitroaromatic compounds in the aqueous media.
2. Experimental
2.1. Materials

Melamine, selenium black 99+, phosphonitrilic chloride trimer, silver nitrate, sodium
borohydride, and various nitroarenes were purchased from Merck and used without any further
purification.
2.2. Instrumentations
The formation of the nanocatalyst was confirmed by different identification techniques, such as
Fourier transform infrared spectroscopy (FT-IR, Tensor IR 27 spectrometer) in the range of
4000-400 cm-1, powder X-ray diffraction (XRD, Rigaku Ultima IV diffractometer under Cu-Ka
radiation at 2θ range of 10-90°), field-emission scanning electron microscopy (SEM, SIGMA
VP), energy dispersive X-ray spectroscopy (EDX, Oxford instrument), transmission electron
microscopy (TEM, CM30300Kv), and N2 adsorption and desorption (BELSORP-mini ll at the
temperature of liquid nitrogen (77 K), and inductively coupled plasma optical emission
spectroscopy (ICP-OES, DV 5300). Also, the promotion of the reaction was monitored by thin
layer chromatography (TLC) on commercial aluminum-backed plates coated with layers of
Merck silica gel 60 F254.
2.3. Preparation of nanocatalyst
2.3.1 Fabrication of P, Se-codoped g-C3N4 (PSeCN) bulk
In a general synthesis route, melamine (2.8 g), phosphonitrilic chloride trimer (0.6 g), and
selenium black powder (0.6 g) were thoroughly ground. The resulting mixture was placed in a
ceramic crucible with a lid and pyrolyzed at 550 °C at a rate of 2 °C min-1 for 4 h. After cooling
to ambient temperature, the synthesized product was repeatedly washed with deionized water
and ethanol and put in an oven at 80 °C for 12 h to dry.
2.3.2 Fabrication of PSeCN nanosheets

The light yellow powder of the bulk P, Se-codoped g-C3N4 bulk was covered into P, Se-codoped
g-C3N4 nanosheet through in situ thermal condensation at 600 °C at a rate of 5 °C min-1 for 2 h.
The prepared deep yellow powder was washed with deionized water and ethanol three times
centrifuged repeatedly to eliminate impurities, then dried at 70 °C in the air for 12 h.
2.3.3 Fabrication of PSeCN/Ag nanocatalyst
To immobilize Ag NPs on PSeCN, PSeCN nanosheet (470 mg) was added to 100 mL of distilled
water and sonicated. Then, aqueous AgNO3 solution (4.72 mL, 10 mg.mL-1) was added to the
above suspension. After that, under ultrasonic, a solution of 210 mg NaBH4 in H2O (15 mL) was
injected drop by drop and stirred continuously for 12 h in ambient temperature. Finally, the
PSeCN/Ag was separated by centrifugation and dried in an oven for 24 h.
2.4. The catalytic activity of PSeCN/Ag in the reduction of nitroarenes
Typically, 4-nitroaniline (1 mmol), NaBH4 as reducer (5 mmol), PSeCN/Ag nanocatalyst (20
mg) were added into a 10 mL vial containing 3 mL H2O as the solvent. Then, the reaction
mixture was stirred for a certain time at 75 °C. The reaction was checked by TLC. Upon the
completion of the reaction, the PSeCN/Ag was isolated by centrifugation and washed with EtOH
and H2O to use for another consecutive run. Then, the product was extracted with ethyl acetate
and purified by plate or column chromatography.
3. Results and discussion
3.1. Characterization
3.1.1. X-ray diffraction (XRD) studies
To study the crystallinity and phase structure of PSeCN and PSeCN/Ag, the XRD technique was
conducted (Figure. 2). Both diffractograms reveal two prominent peaks at 2θ of 27.32° and

13.52° that are indexed as the (0 0 2) and (1 0 0) crystal planes of graphitic materials,
corresponding to the interlayer structural packing and in-plane repeated tri-s-triazine units,
respectively [53]. These results show that the original structure of g-C3N4 was preserved after P
and Se co-doping. Also, the absence of peaks related to Se and P indicates that these elements
have entered into the g-C3N4 network. After loading Ag, other obvious diffraction peaks appear
in the XRD pattern of PSeCN/Ag, at 2 = 38.6 , 44.32 , 64.34 , and 77 corresponding to (1 1
1), (2 0 0), (2 2 0), (3 1 1) diffractions of the face-centered-cubic structure of Ag nanoparticles
[54]. As a result, Ag nanoparticles have been successfully assembled on the surface of the
PSeCN.
<Figure 1>
3.1.2. Fourier transform spectroscopy (FT-IR) studies
To obtain information about the chemical structure of the synthesized specimens, FT-IR analysis
was performed. The FT-IR spectra of g-C3N4, PSeCN (nano), PSeCN/Ag present similar
absorption bands which confirm the formation of the graphitic structure of carbon nitride in all
samples, as shown in Figure 1. For all samples, the absorption bands at 807 and 885 cm−1
belong to the out-of-plane bending modes of) heptazine units and the cross-linked deformation
mode of N-H bonds, respectively [55]. The peaks at 1238 and 1319 cm-1 correspond to the
stretching vibration of connected units of C-N(-C)-C or C-NH-C [56]. The observed absorption
peaks in 1407, 1560, and 1640 cm-1 are assigned to the stretching vibrations of C-N and C=N in
heterocycles (in tri-s-triazine rings) [57]. The broad band located at 3000-3500 cm-1 is attributed
to the O-H and N-H stretching vibration modes possibly due to absorbed water molecules and

uncondensed amino groups [58]. No characteristic peaks related to covalent bonding of P and Se
with other elements were detected in the spectrum of PSeCN, possibly owning to overlap with
the vibration mode of C-N and low doping levels of P and Se. Also, the FT-IR spectrum of
PSeCN/Ag is similar to PSeCN, suggesting that the structure of the prepared nanocatalyst
remains unchanged after the deposition of Ag nanoparticles.
<Figure 2>
3.1.3. Scanning electron microscopy (SEM) studies
SEM images of the PSeCN and PSeCN/Ag elucidated their surface morphology and
nanostructure (Figure. 3). The SEM images of PSeCN exhibit plate-like structures with
smoothed surfaces (Figure. 3a and b). After the deposition of Ag NPs, the morphology did not
affect but a rough surface was observed (Figure. 3c and d) [59, 60].
<Figure 3>
3.1.4. Energy dispersive X-ray spectroscopy (EDX) studies
EDX analysis was applied to determine the chemical composition of PSeCN/Ag nanocatalyst
(Figure. 4). The presence of various elements such as C, N, P, Se, and Ag in the EDX spectrum
of PSeCN/Ag confirm the successful doping of P and Se in PSeCN as well as the deposition of
Ag NPs on the surface of the PSeCN/Ag.
<Figure 4>
3.1.5 Elemental mapping analysis

To further investigate the elemental composition and distribution in PSeCN/Ag, the EDX-
mapping analysis was carried out (Figure. 5). The map images also affirm the presence of C, N,
P, Se, and Ag elements, which are homogeneously distributed in the structure PSeCN/Ag.
<Figure 5>
3.1.6 Transmission electron microscopy (TEM) studies
The surface structures and morphology of the PSeCN/Ag nanocatalyst were further examined
using TEM. As shown in Figure 6, the lighter parts can be assigned to the sheet-like morphology
of PSeCN with lower density and the darker spots correspond to Ag NPs with higher density.
The Ag NPs have a spherical morphology and are uniformly dispersed over the PSeCN sheets
without aggregation.
<Figure 6>
3.1.7 Inductively coupled plasma-optical emission spectrometer (ICP‐ OES) analysis
In the following, ICP-OES analysis was exploited to determine the exact content of Ag in the
nanocatalyst. The loading amount of Ag in the PSeCN/Ag is reported to be 2.69% that is in good
agreement with the EDX analysis result.
3.1.8. Nitrogen adsorption-desorption analysis
To evaluate the textural and structural features in the as-fabricated samples, nitrogen sorption
analysis was performed. The N2 adsorption and desorption diagrams of PSeCN bulk, PSeCN
nano, and PSeCN/Ag are presented in Figure.7. Also, the specific surface area, pore size, and
pore volume are listed in Table 1. N2 adsorption-desorption diagram of all samples displays a
type IV curve based on the IUPAC standard which confirms the mesoporous structure. As

expected, the specific surface area of PSeCN bulk (9.64 m2 g-1) is nearly 4 times larger than that
of PSeCN nano (33.61 m2g-1). The higher specific surface area can provide more active sites and
increase catalytic performance. Besides, the PSeCN/Ag exhibits a lower specific surface area
compared to PSeCN. Decreasing the surface area of PSeCN/Ag indicates that the Ag NPs were
successfully anchored on the surface of PSeCN and filled the most often pores.
<Figure 7>
Table 1. Summary of BET measurements
Sample
Pore volume (cm3.g-1)
Specific surface area (m2.g-1) Pore size (nm)
PSeCN(bulk)
PSeCN(nano)
PSeCN/Ag
0.074
0.174
0.118
9.64
33.61
20.39
16.64
14.56
16.49
3.2 Evaluation of the catalytic activity of PSeCN/Ag
To extend the utilization of PSeCN/Ag nanocatalyst in different aromatic nitro compounds
catalytic reduction, its catalytic potential was examined in the reduction reaction of 4-nitroaniline
using NaBH4 as the reductant. For the optimization of the reaction conditions, the influence of
different parameters (dosage of the catalyst and reductant, solvent, and temperature) was
surveyed in detail. Firstly, various quantities of the catalyst were tested (Table 2, entries 1-4).
Entry 1 displays that in the absence of a catalyst, the reduction reaction cannot occur. Thus,
PSeCN/Ag is an essential factor for this reaction. As can be seen, 20 mg of catalyst is the most

appropriate amount (Table 2, entry 3). When the optimal amount of catalyst was obtained, the
NaBH4 loading was examined. With the decrease of the NaBH4 amount from 5 to 4 mmol, a
decrease in the product yield was observed (Table 2, entry 5). Next, temperature screening was
undertaken and 75 °C proved to be the optimum reaction temperature. When the reaction
temperature was reduced to 55 °C the product yield decreased to 60% and increase the reaction
temperature to 95 °C did not have a positive effect (appreciable influence) on the product
conversion percentage and rate of reaction (Table 2, entries 6 and 7). Finally, to assess the
solvent effect, the test reaction was performed in various solvents (Table 2, entries 8 and 9).
Among them, water, as a sustainable, cheap, and green solvent, exhibited the best performance
with 98% yield.
Therefore, the optimal reaction conditions were determined to be PSeCN/Ag nanocatalyst (20
mg) and NaBH4 (5 mmol) at 75 °C in H2O as solvent (Table 2, entry 3).
Table 2. Optimization of reaction conditionsa.
Temp.(oC)
Entry Catalyst (mg)
NaBH4
(mmol)
Solvent
Time(min) Yield(%)b
1
2
3
4
5
6
7
-
5
5
5
5
4
5
5
H2O
H2O
H2O
H2O
H2O
H2O
EtOH
75
75
75
75
75
55
95
25
25
25
25
25
25
25
-
10
20
30
20
20
20
75
99
98
99
60
98

8
9
20
20
5
5
EtOH: H2O
(1:1)
75
75
25
25
83
90
EtOH
aReaction condition: 4-nitro aniline (1.0 mmol), NaBH4, solvent (3 mL).
bIsolated yield.
To confirm the general applicability of PSeCN/Ag, the reduction of a variety of nitroarenes was
investigated under the optimum conditions, and outcomes are summarized in Table 3. Starting
materials having electron-rich groups such as NH2, OH, CH2OH, and NHNH2, and electron-poor
groups like I, Br, Cl, and CF3 were transformed into the relevant amines products with good to
excellent yields and selectively and no dehalogenated product was seen in halogenated
nitroarenes. More importantly, the aromatic nitro compounds bearing the challenging reducible
functional groups, including ketone and aldehyde were effectively converted into their
corresponding aniline derivatives with high selectivity and carbonyl groups remained unchanged
during the reduction process.
Table 3. Reduction of nitroarenes using PSeCN/Ag nanocatalysta
Entry
1
Substrate
Product
Time (min) Yield (%)
99
25

2
99
50
20
3
4
99
98
25
5
6
120
110
95
98
7
120
92
8
9
100
110
90
76
10
85
120

11
180
83
12
13
110
165
84
74
Reaction conditions: 4-nitroaniline (1 mmol), catalyst (20 mg), NaBH4 (5 mmol), solvent (3
mL), temperature (75°C).

3.3 Reusability of the catalyst
Finally, a reusability test was accomplished to explore the industrial applicability and
sustainability of the fabricated nanocatalyst. For this purpose, the catalyst was isolated at the end
of the reaction by centrifugation and reused in the next runs under identical conditions after
washing frequently with warm methanol and drying in an oven. The yield of the product slightly
reduced from 99 to 91% after six runs (Figure. 8). This result demonstrates that there is no
significant degradation in the catalytic activity of PSeCN/Ag. However, in run 7, the conversion
percentage of the product decreased to 85%.
<Figure 8>
4. Conclusion

In conclusion, the present study focused on the g-C3N4 modification using three efficient
techniques such as phosphorus and selenium co-doping, creating mesoporous construction
(sheet-like structure), and immobilization of Ag NPs on its surface. In situ phosphorus and
selenium co-doped graphitic carbon nitride nanosheet (PSeCN) was successfully prepared for the
first time by applying melamine, phosphonitrilic chloride trimer, and selenium black powder as
precursors through facile thermal polymerization strategy (method) and used as an excellent
platform for stabilization of Ag NPs. Then, synthesized PSeCN/Ag was exploited as a
heterogeneous nanocatalyst for promoting the reduction of aromatic nitro compounds in aqueous
media and indicated remarkable catalytic efficacy and selectivity. The outstanding performance
of PSeCN/Ag could be ascribed to two factors. First, attendance of support with nanometer size.
Secondly, 2-D confinement offers more active sites. Additionally, nanocatalyst could be easily
recovered and reused for five runs with negligible loss in its activity because of the strong
interaction of PSeCN with Ag NPs. Hopefully, this study would provide enormous opportunities
for the design and synthesis of other metal-based heteroatom-doped g-C3N4 materials for the
advanced hydrogenation process with challenging compounds.
Credit Author Statement
Mohadese Piri (Investigation, Data Curation, Visualization)
Majid M. Heravi (Conceptualization; Validation, Resources, Writing - Review & Editing,
Supervision, Project administration, Funding acquisition)
Ali Elhampour (Conceptualization, Methodology, Data Curation, Writing - Original Draft)
Firouzeh Nemati (Conceptualization, Supervision)

Declaration of interests
There are no conflicts to declare.
Acknowledgments
The authors are thankful from Alzahra University Research Council for financial support. MMH
also appreciates the granted individual research chair given by Iran National Scientific
Foundation (INSF).
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Figure 1. XRD patterns of (a) PSeCN and (b) PSeCN/Ag
Figure 2. FT-IR spectra of (a) g-C3N4, (b) PSeCN (nano), and (c) PSeCN/Ag
Figure 3. SEM images of (a, b) PSeCN (nano), and (c, d) PSeCN/Ag
Figure 4. EDX spectrum of PSeCN/Ag
Figure 5. Elemental mapping data of PSeCN/Ag
Figure 6. TEM images of PSeCN/Ag
Figure 7. N2 adsorption and desorption diagrams of (a) PSeCN bulk, (b) PSeCN nano, (c)
PSeCN/Ag
Figure 8. Catalyst recycling diagram