Bio-inspired N,S-doped siligraphenes as novel metal-free catalysts for removal of dyes in the dark

Article abstract of DOI:10.1016/j.molliq.2019.111657

Bio-inspired porous Siligraphene (graphene-like SiC) catalyst and its N and N,S-doped SiC variants were synthesized via a low-temperature magnesiothermic method using natural precursors. The synthesized SiC catalysts were tested for the removal of anionic azo dyes (Congo red, Methyl orange, and Methyl red) in the dark. Si atoms in the siligraphene structure are more positive than C atoms due to the electronegativity of the latter. In doped structure, insertion of N and S atoms can further increase the positive charge of Si atoms and improve adsorption of oxygen and of the above anionic dyes on the surface of SiC. Thus, the catalytic activity of doped SiC for degradation of the dyes increased. A mechanism is proposed where O₂ molecules chemisorb onto the surface of SiC, O[sbnd]O bond length elongates, oxygen bond dissociates and radicals are produced. Quenching experiments show that ^?O₂ ^– and ^?OH radicals are the main reactive species in this process. Doping of nitrogen and sulfur in the presence of Si atoms in siligraphene structure has been shown to improve its capacity for dye removal compared to the corresponding carbon catalysts. Moreover, the SiC catalysts display good stability so that their performance did not significantly decrease even in continuous operation.

Full text of DOI:10.1016/j.molliq.2019.111657

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Bio-inspired N,S-doped siligraphenes as novel metal-free
catalysts for removal of dyes in the dark
Leila Seifikar Gomi, Maryam Afsharpour, Mitra Ghasemzadeh,
Panagiotis Lianos
PII:
S0167-7322(19)33935-2
DOI:
https://doi.org/10.1016/j.molliq.2019.111657
MOLLIQ 111657
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
15 July 2019
13 August 2019
28 August 2019
Please cite this article as: L.S. Gomi, M. Afsharpour, M. Ghasemzadeh, et al., Bio-inspired
N,S-doped siligraphenes as novel metal-free catalysts for removal of dyes in the dark,
Journal of Molecular Liquids(2019), https://doi.org/10.1016/j.molliq.2019.111657
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Bio-inspired N,S-Doped Siligraphenes as Novel Metal-
Free Catalysts for Removal of Dyes in the dark
Leila Seifikar Gomia, Maryam Afsharpoura,*, Mitra Ghasemzadeha, Panagiotis Lianosb
a Department of Inorganic Chemistry, Chemistry & Chemical Engineering Research Center
of Iran, 14335-186, Tehran, Iran
b Department of Chemical Engineering, University of Patras, 26500 Patras, Greece
*Corresponding author: Tel: +98 21 44787720; Fax: +98 21 44787812
E-mail address: afsharpour@ccerci.ac.ir

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Abstract
Bio-inspired porous Siligraphene (graphene-like SiC) catalyst and its N and N,S-doped SiC
variants were synthesized via a low-temperature magnesiothermic method using natural
precursors. The synthesized SiC catalysts were tested for the removal of anionic azo dyes
(Congo red, Methyl orange, and Methyl red) in the dark. Si atoms in the siligraphene structure
are more positive than C atoms due to the electronegativity of the latter. In doped structure,
insertion of N and S atoms can further increase the positive charge of Si atoms and improve
adsorption of oxygen and of the above anionic dyes on the surface of SiC. Thus, the catalytic
activity of doped SiC for degradation of the dyes increased. A mechanism is proposed where O2
molecules chemisorb onto the surface of SiC, O-O bond length elongates, oxygen bond

dissociates and radicals are produced. Quenching experiments show that O2 and OH radicals
are the main reactive species in this process. Doping of nitrogen and sulphur in the presence of Si
atoms in siligraphene structure has been shown to improve its capacity for dye removal
compared to the corresponding carbon catalysts. Moreover, the SiC catalysts display good
stability so that their performance did not significantly decrease even in continuous operation.
Keywords: Siligraphene; N and S doping; Metal-free catalyst; Dye Removal.

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1. Introduction
As a result of rapid growth of world population, water purification processes make
the most important challenges for researchers in wastewater management [1-4]. Organic
dyes are a main source of water pollution and they have a negative impact on human
health both directly and indirectly [5,6]. It is therefore essential to remove these pollutants
from wastewater. Current research has developed several advanced processes for dye
removal. Among them, removal by adsorption of dyes constitutes a powerful tool of great
practical importance. Numerous porous materials have been employed for this purpose
such as zeolites, biopolymers, silica, alumina, carbon and even Sahara desert sand [7-12].
As far as we know, there is only one reference on the use of silicon carbide (SiC) to
remove methylene blue dye by adsorption [13], although there are some literatures on
adsorption or photocatalytic degradation of dyes using SiC composites with metal or
metal oxides [14-16]. Silicon carbide is one of the most useful compounds due to its
specific properties such as thermal and chemical conductivity, great mechanical strength,
good chemical inertness, and oxidation resistance [17-19]. Therefore, SiC is broadly used
in sensors, drug delivery systems, composites, heterogeneous catalysts, hydrogen storage,
electronic, and optical devices [20-23]. In addition, previous reports show that SiC with
siligraphene structure has interesting properties due to its surface charge density [24,25].
SiC has been synthesized by several methods including sol-gel, pyrolysis of carbon-
containing siloxane polymers, and electrochemical etching of SiC wafers [26,27]. All
these methods are unable to control the morphology of silicon carbide nanostructures due
to their high process temperature (>1200°C). Also, these synthesis processes are costly
energy consumers using not environmentally friendly material precursors. In this respect,

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magnesiothermic method has been recently suggested as a low-temperature method (600-
800°C) for preparing SiC nanostructures which allowed to control the morphology and
production of porous materials. In this method, silica in the presence of Mg turns to silicon and
because of this highly exothermic reaction, silicon carbide can be synthesized at lower
temperature [28,29]. In addition in the present work, we have chosen cost effective porous
natural precursors to produce green porous SiC [24,30].
In this study, we report the synthesis of bio-inspired porous SiC and its doped
structures via magnesiothermic method. Diatomaceous earth and glucose were used as
natural silicon and carbon source, respectively. Surface area and chemical heterogeneities
of absorbents are important for efficient adsorption and also catalytic performance [31].
Thus, the effect of dopants on the adsorption capacity and the catalytic performance of
porous SiC has been investigated. In addition, a new mechanism of advanced oxidation
process is reported for dye degradation. In summary, the advantages of the presently proposed
SiC catalysts are the use of natural precursors for their synthesis, the relatively low temperature
synthesis procedure, and the mild conditions for their operation, i.e. dark, ambient temperature
and lack of additional oxidants.
2. Experimental
2.1. Materials
Diatomaceous earth (SiO2 content: 98%, porosity >93%, Density: 120g/L, pH: 7.8)
was used as Si source. Glucose, urea, and thiourea were purchased from Merck and used
as C, N, and N-S sources. Magnesium powder in the size of 0.06–0.3 mm (Merck) was
used in the synthesis of SiC (magnesiothermic method). Hydrofluoric acid and

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hydrochloric acid were used to wash magnesium and unreacted silica. Congo red (C.I.
Direct Red 28), Methyl Red (C.I. Acid Red 2), and Methyl orange (C.I. Acid Orange 52
were used as the anionic azo dyes.
2.2. Synthesis of SiC, N-doped SiC, and N, S-doped SiC samples
Diatomaceous earth was first washed with 1 M HCl solution to remove impurities,
and dried in an oven at 60 °C for 12 h. Then, 2 g of washed diatomaceous earth was
dispersed in a 250 ml of 8 g/L glucose solution (C source) and sonicated 5 times for 15
min for better penetration of glucose in the pores of diatomaceous earth. The mixture was
put in an oven and dried at 70 °C for 24 h. For the synthesis of SiC samples, the dried
mixture was carbonized at 700 °C for 1 h under an N2 atmosphere (5º C/min). Afterward,
SiO2/C composite was sufficiently mixed with magnesium powder in Si:Mg molar ratio
of 1:2 and put it in a furnace at 700 ºC for 5 h under N2 atmosphere to produce SiC
sample. Finally, to remove the magnesium oxide and its derivatives, the obtained SiC
samples were immersed in a 2 M HCl solution for 24 h at room temperature. After
washing it with water to remove magnesium chloride, the SiC product was immersed in a
2 M HF solution to remove unreacted silica. Then, the pure SiC sample was dried at 60°C
for 24 h in a vacuum oven.
To synthesize the doped SiC samples, 0.8 g of urea or thiourea was added to the
glucose solution and used to produce the N-doped and N,S-doped SiC according to the
reported procedure, respectively. In this article, SiC, N-doped SiC, and N, S-doped SiC
samples were briefly named
, respectively.

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2.3. Synthesis of carbon catalysts
Carbon catalysts were synthesized by heating glucose at 700 °C for 1 h under an N2
atmosphere (5º C/min). Doped carbon catalysts were synthesized by adding 10 wt% of
urea or thiourea to glucose as N or N-S sources.
2.4. Characterization and Methods
The morphologies of porous SiC samples were obtained on a TESCAN, VEGA3
scanning electron microscope (SEM) equipped with an X-ray energy dispersive
spectroscope (EDX). Powder diffraction data were assembled by BrukerAxs, D8
Advance. The infrared spectroscopy was done by a Bruker, Vector instrument. N2
adsorption/desorption isotherms and the pore size distributions were obtained from the
Belsorp mini system. XPS analysis was performed on a Thermo Scientific, ESCALAB
250Xi system using a spectrometer with MgKα radiation (1253.6 eV). Raman
spectroscopy was done by a Bruker, Senterra micro-Raman (785 nm laser wavelength).
Dye removal tests were performed with a Perkin-Elmer, Lambda-35 UV-Vis
spectrometer.
All the calculations were done by the Gaussian 03 program. First, the structures were
optimized at the BP86/6-31+G** level and then NBO calculations were carried out using
NBO 3.1 in order to obtain the atomic charges.
2.5. Methods for Dye Removal
Batch experiments were carried out in the dark at ambient temperature in the presence
of SiC catalysts (SiC, N-SiC, and NS-SiC samples). Different concentrations of Congo

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red dye (30, 50, 70, and 100 ppm) were prepared to test the removal capacity of SiC
samples. 0.01 g of synthesized SiC samples were dispersed in 10 ml of certain dye
solution under stirring (120 rpm) in a dark chamber. Air as a supply of oxygen was
bubbled to the system to saturate the solution with oxygen during the reaction. The
concentration of dye in solution was measured by UV-Vis spectroscopy at 500 nm after
each 5 min.
All of the experiments were repeated three times to ensure data accuracy. The amount of dye
removal (Qe) was determined using the following equations:
Qe = (C0-Ce) V/M
(1)
(2)
Removal % = (C0-Ce)/C0 * 100
Where C0 and Ce are the initial and equilibrium dye concentrations (mg L-1), M is the mass
of SiC samples (g), and V is the volume of dye solutions (L).
In addition to Congo red, Methyl red and Methyl orange anionic azo dyes were also
tested to evaluate the activity of the SiC catalyst.
Quenching experiments were done by introducing 2 mM of IPA (isopropanol), BQ



(p-benzoquinone), and AO (ammonium oxalate) as OH, O2 , and h+ quenchers
respectively.
3. Result and Discussion
3.1. Characterization
FT-IR spectra for SiC, N-SiC, and NS-SiC samples are shown in Fig. 1. Peaks
appearing at 816, 822, and 819 cm-1 are related to the Si-C stretching vibration in SiC, N-

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SiC, and NS-SiC silicon carbide samples, respectively. The peaks that were observed at
3420 cm-1 in all samples can be mainly assigned to the O-H stretching vibration of
oxygen-functional groups and chemisorbed water on the surface of SiC.
XPS analysis of synthesized SiC samples is recorded in Fig. 2a-h. XPS survey
spectra of all samples showed Si2p (includes Si2p 3/2 and Si2p 1/2), C1s, and O1s peaks
which indicate the presence of SiC associated with oxygen-containing functional groups.
The Si2p 3/2 XPS spectra for SiC sample (Fig. 2a) can be deconvoluted into two bands at
99.31 and 100.82 eV, corresponding to Si-Si and Si-C species, respectively. This result
shows the presence of silicon in the SiC sample. While, the Si2p XPS peaks of N-SiC and
NS-SiC sample were fitted to two bonds that are related to Si-C and O-Si-C species at
about 101 and 102 eV, respectively (Fig. 2c,e). The C1s peaks of N-SiC and NS-SiC
show C-Si, C-C, and C-N bonds at about 283, 284.5, and 286 eV, respectively (Fig. 2d,f).
The N1s XPS peaks in N-SiC and NS-SiC (Fig. 2g,h) show doublet peaks at about 398
and 400 eV for pyridinic and pyrrolic nitrogen, respectively. The S2p XPS peak of NS-
SiC is attributed to the C-S-C bond (169.54 eV) (Fig. 2i).
The surface atomic percentage of Si, C, O, N, and S for SiC, N-SiC, and NS-SiC
samples that were obtained from EDX analysis and derived from the corresponding peak
areas of XPS spectra are summarized in Table 1. The results show that the presently
prepared SiC samples mainly consist of Si, C, and O elements. The co-doped NS-SiC
sample shows a higher N concentration (5.66 atom%) than the singly N-doped sample (N-
SiC) (4.05 atom%). The amount of sulfur in NS-SiC was measured to be about 0.54
atom%.

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XRD patterns of SiC samples are depicted in Fig. 3. XRD patterns of SiC, N-SiC, and
NS-SiC show peaks at about 2θ = 33.86, 35.57, 37.95, 41.32, 60.04, and 71.78° that are
assigned to (101), (102), (103), (104), (110), and (202) planes of 6H-SiC structure
(JCPDS 29-1128). Also, in SiC sample, three diffraction peaks were observed at 2θ =
28.39, 47.26, and 56.07° can be contributed to the (111), (220), and (311) planes of cubic
phase of silicon (JCPDS 27-1402).
Raman spectra of synthesized SiC catalysts are shown in Fig. 4. In all samples, two
specified bands of graphene, D bond at about 1350 cm-1 and G band at about 1570 cm-1
were observed. The presence of these two peaks on the structure of SiC samples
demonstrate the formation of siligraphene structure [24]. The siligraphene is a graphene-
like structure with high amount of Si elements that are doped in carbon with graphene
structure [32-34].
As we know, the D band is activated by lattice defects such as substitution of
heteroatoms, vacancies, or functional groups on the surface of graphene structures. So,
the intensity ratio of peaks D to G can indicate the degree of disorder and amount of sp2
in a sp3 network. The ID/IG ratio in SiC, N-SiC, and NS-SiC were found to be 0.92, 0.93,
and 0.95, respectively. The larger ID/IG ratio can result from more defects in doped
structures due to the substitution of doped atoms in the carbon skeletons. So, NS-SiC
attained the highest ID/IG value due to the largest amount of dopants.
SEM images of synthesized SiC samples (SiC, N-SiC, and NS-SiC) are shown in Fig.
5b-d. Fig. 5a presents the morphology of silicon precursor (diatomaceous earth). The
SEM of SiC sample (SiC without doping) clearly shows a porous structure like to its
initial precursor (diatomaceous earth) (Fig. 5b). It was concluded that the diatomaceous

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earth performed as hard template for the synthesis of the porous SiC sample. By adding
nitrogen and sulfur in the SiC matrix, the morphologies of N-SiC and NS-SiC were
changed and a different porosity was observed (Fig. 5c,d).
The porosity of prepared SiC samples was studied by N2 adsorption/desorption
measurements which demonstrated a type IV isotherm that illustrates the presence of
micro and mesopores (Fig. 6). The specific surface areas of SiC, N-SiC, and NS-SiC were
found to be 101, 135, and 133 m2/g, respectively. The results show that the surface area
increased after doping the elements in the SiC structure. It can be attributed to the
interaction between the N and S sources and glucose which forms a new matrix with
higher porosity. As shown in Table 2, NS-SiC show a significant increase in the
micropore content (14.73%). NS-SiC sample shows the lowest total pore volume (0.291
cm3/g) and pore diameter (8.759 nm) due to the higher nitrogen and sulfur dopant
contents.
3.2. Dye Removal Results
Fig. 7 shows the removal of Congo red using SiC catalysts with different initial
concentrations of dye (30-100 ppm). All the experiments were done at an optimized
amount of catalyst in the dark under air bubbling to saturate the solution with oxygen
(Table 3). The amount of dye adsorbed (Qe), equilibrium concentrations (Ce), time of
removal, and the percentage of dye removal are given in Table 3. Both doped-SiC
catalysts can completely remove lower concentration of dye after a short time (5 min).
Based on catalytic data, it was revealed that the removal percentage was decreased by
increasing the initial concentration of dye. Also, it is obvious that by increasing the

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concentration of the dye, the differences in catalytic performances of synthesized SiCs
were better observed. Results clearly show that doping the heteroatoms increases the
catalytic property in the removal of dye in the order NS-SiC > N-SiC > SiC.
As dye removal reactions were performed in the dark, no photocatalytic reactions
could occur. Congo red is adsorbed on positive sites of SiC samples through the oxygen
atoms of the sulfonate group of dye molecules. More adsorptions can occur in N-SiC and
NS-SiC samples due to their higher surface areas than in the SiC sample. Also, doping the
heteroatoms on the SiC matrix increases the defects and can enhance the adsorption.
However, the surface area of SiC catalysts was not very high (lower than 150 m2/g),
while results show that the dye can be completely removed in a short time. This indicates
the existence of another mechanism besides adsorption. We then suggest a mechanism of
dye degradation via oxidation processes by chemisorption of oxygen on the surface of
SiC catalysts.
Fig. 7 also shows that NS-SiC can completely remove dye up to an initial
concentration of 70 ppm. By increasing the concentration above 70 ppm, a decrease in

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removal percentage was observed. Obviously, a high dye concentration can lead to a
saturation of the surface of the NS-SiC catalyst and limit oxygen adsorption thus
decreasing catalytic activity.
3.3. Proposed Mechanism
In our proposed mechanism, the O2 molecule chemisorbs to the surface of SiC
samples by bonding with two adjacent silicon atoms and forms an epoxide like structure
(Fig. 8). The Si-O bond length is a strong chemical bond and oxygen molecule can easily
adsorb on the SiC surface. After chemisorption of O2, the O-O bond length elongates and



a charge transfer from SiC to O2 occurs. Then the oxygen bond dissociates and O, O2 ,
OH, or OOH radicals are produced. In another adsorption configuration, H2O molecules
can also adsorb on the SiC surface. As shown in Fig. 8, both O2 and H2O prefer to adsorb
on the Si atoms that have a more positive charge. The H2O molecule has a lower affinity
for SiC surface and it may be physisorbed on the Si sites. When the water molecule is
adsorbed, a proton in the H2O molecule can be dissociated and forms OH groups by
connecting to the dissociated O2 molecules.
Theoretical study (Fig. 9) shows that the charge of Si atoms increases by doping the
heteroatoms and the co-doped SiC then becomes more positive than. The charge of Si
atoms was calculated to be about 1.96, 1.94, and 1.83 for NS-SiC, N-SiC, and SiC,
respectively. Since the charge of Si atoms in the co-doped SiC is more positive, the NS-
SiC shows the higher affinity for oxygen adsorption.

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To prove the proposed mechanism, the catalytic test was repeated by purging N2 and
O2 to evaluate the effect of oxygen in the removal of dye. The results show that purging
oxygen into the system increases the rate of reaction and NS-SiC removed the Congo red
(70 ppm) completely after 10 min (dye was removed in the air after 20 min). Purging N2
shows a 42% dye removal, which is obviously related solely with dye adsorption.
In addition, quenching experiments were done to measure the effects of active species


during the catalytic reaction. Fig. 10 clearly shows that in the presence of BQ as O2

quencher, the degradation of the dye was significantly limited, which suggest that the O2
radical is the main active species in the catalytic degradation of the dye. The result also
reveals that the degradation of the dye was reduced by addition of IPA, which indicates

the involvement of OH radical as a reactive species. However, h+ quenching by AO
showed negligible effect on the removal of the dye (Fig. 10), thus suggesting the absence
of any photocatalytic effect.
It is well known that the surface property of the catalyst is an important parameter in
adsorption and catalytic activity. The doped SiC samples have more surface areas and
show higher dye removal. Also, NS-SiC shows the highest removal due to the more N
and S doping that should be attributed to the more positive charge on the Si atoms in SiC
sample. Also, doping with heteroatoms can increase the affinity of SiC surface to oxygen
groups and improve the catalytic properties.

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As the charge surface is an important role in catalytic performance of doped-SiC, the
effect of pH on dye removal was tested for NS-SiC catalyst (Fig. 11).
It is well-known
that in the pH higher than pHzpc of catalyst, the surface of catalyst became positive
which is proper for adsorption of negative dyes and oxygen species. So, the best dye
removal results was observed at pH higher than 7. The decrease of removal in lower pH
can be attributed to decomposition of dye.
Fig. 12 shows the UV-Vis spectra that were plotted during the removal process of 100
ppm dye solution. Results show the intensities of all peaks in Congo red dye were which
confirms the destruction of azo-linkage (500 nm) and also, the benzoic (235 nm), and
naphthalene rings (347 nm) in Congo red structure. Furthermore, no new bands appeared
in the UV-Vis region. Also, degradation products formed at the end of catalytic process
were analyzed by LC–MS. Main species detected in the solution are presented in Fig. S1
including LC–MS spectra for the aqueous solution of 30 ppm Congo red dye which was
reacted with NS-SiC for 10 min. Only two peaks at 135 and 143 m/z ratios was observed
with a very low intensity which prove the significant decolorization and mineralization to
give CO2 and H2O during this catalytic process.
3.4. Recovery of Catalysts
The reusability of the catalysts was tested by using treated and also washed catalysts.
First, the catalysts were filtered from dye solution and used in another run without any
further recovery (Fig. 13a). After 3 runs, only a little decrease was observed in the

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activities of SiC catalysts. It confirms that the catalysts, especially NS-SiC, can remove
dye by degradation in addition to adsorption. These claims can be confirmed by FT-IR
data (Fig. 13b). No obvious peaks were observed on the FT-IR spectra of used SiC
catalysts after the first run, especially in the case of NS-SiC. This result shows that
catalysts can completely degrade the dye, therefore, no dye residues were detected on the
surface of the catalysts. However, after five runs, catalytic activities decrease due to the
saturation of the surface of catalysts with adsorbed dye and this result to a decrease of dye
degradation.
The recovery of NS-SiC catalyst was tested after washing with water (Fig. 14a). It
shows no significant decrease in the activity of this catalyst after 5 runs. FTIR and diffuse
reflectance spectra of washed NS-SiC catalyst were compared with fresh and used NS-
SiC catalysts (Fig. 14b,c). Results confirm that no traces of dye remained on the surface
of the catalyst and the chemical structure of the catalyst has not changed after the
degradation experiment and washing process.
3.5. Comparison of Catalytic Activities
To better appreciate their catalytic activity, corresponding carbon catalysts were also
synthesized. Fig. 15 shows the comparison of Congo red removal by using carbon and
SiC catalysts. It is seen that all SiC catalysts show better catalytic properties than their
corresponding carbon catalysts, although, dye removal was increased by doping the
heteroatoms onto the carbon matrix. This effect of heteroatoms doping is also observed in
the results of SiC catalysts.

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Based on the proposed mechanism, the activities of SiC catalysts were compared with
corresponding carbon catalysts (Fig. 15). Results clearly show the influence of doping of
heteroatoms in catalytic properties. Carbon catalyst without doping can remove dye only
by adsorption process. But, doped carbon catalysts can remove dye according to
adsorption and oxidation reaction simultaneously. Doping N and S atoms in the graphene
structure induces a positive charge on the carbon atoms which chemisorb oxygen
molecules and produce radicals which assist oxidation process. Comparison of carbon
with SiC catalysts showed higher catalytic performance in SiC catalysts due to the
presence of Si atoms on the graphene structure. According to the proposed mechanism,
the Si atoms in siligraphene structure are the active sites to adsorb oxygen molecules due
to the higher positive charge. By doping heteroatoms into the siligraphene structure, their
properties their properties are further enhanced due to the increase of the Si
electropositivity. The positive charge on the Si atoms can chemisorb O2 molecules from
the solution and generate oxygen radicals which enhance the oxidation process and
destroy the organic dye.
Two other anionic azo dyes (Methyl red and Methyl orange) were also tested for
further evaluation of catalytic properties of doped SiC. For this reason, NS-SiC as the best
catalyst was selected and the catalytic removal of Congo red was compared with Methyl
red and Methyl orange (Fig. 16). As shown in Fig. 16, NS-SiC removed Congo red and
Methyl orange completely after 10 min and removed 98% of methyl red after 15 min.
These results confirm that this catalyst can remove different organic dyes via an oxidation
process.

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4. Conclusion
To illustrate the catalytic properties of silicon carbide nanostructures in the removal
of anionic azo dyes, SiC, N- doped SiC, and N,S-doped SiC have been prepared from
natural sources (diatomeous earth as Si and glucose as C sources). This study has been
focused on the effect of doping onto the SiC with siligraphene structure. The obtained
catalysts showed different textural properties, surface area, and catalytic properties. The
catalytic results can be attributed to the fact that doping the N and S onto the SiC structure
leads to an increase in surface area and the increase of defects which enhance the
adsorption of dye. Furthermore, doping can increase the positive charge of Si atoms
which can adsorb oxygen and then, degradation of dye may take place due oxidation
processes. Also, the results of these SiC catalysts were compared with their corresponding
carbon analogues. Results show that the N and S doping in the presence of Si atoms in the
siligraphene structures enhance dye removal activities compared to the corresponding
carbon catalysts. Moreover, the recyclability of the catalysts was evaluated after multiple
uses. The SiC catalysts display good stability and especially in the N, S-doped SiC, the
performance of the catalyst did not decrease significantly even in continues reaction. So,
silicon carbides can be introduced as the high-performance metal-free catalysts in
degradation of organic compounds.
Acknowledgment
Support of this investigation by Chemistry and Chemical Engineering Research Center of
Iran is gratefully acknowledged.

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Figure Caption:
Figure 1. FT-IR spectra of SiC, N-SiC, and NS-SiC samples.
Figure 2.XPS analysis of Si2p(a) and C1s(b) of SiC; Si2p(c), C1s(d), and N1s(g) of N-SiC; and
Si2p(e), C1s(f), N1s(h), and S2p(i)of NS-SiC samples.
Figure 3. XRD patterns of SiC, N-SiC, and NS-SiC samples.
Figure 4. Raman spectra of SiC, N-SiC, and NS-SiC samples.
Figure 5. SEM images of Diatomaceous earth (a), SiC (b), N-SiC (c), and NS-SiC (d) samples.
Figure 6. N2 adsorption/desorption isotherms of SiC (a), N-SiC (b), and NS-SiC (c), and pore
size distribution of SiC (d), N-SiC (e), and NS-SiC (f) samples.
Figure 7. Removal of different concentrations of Congo red dye using SiC, N-SiC, and NS-SiC
catalysts (0.01 g catalyst, 10 ml dye, dark chamber, room temperature).
Figure 8. Proposed mechanism for catalytic performance of SiC catalysts.
Figure 9. Schematic structure of the different atoms on SiC samples and surface charges of
atoms.
Figure 10. Control experiments of catalytic removal of Congo red (50 ppm) in dark condition
with the addition of different radical quenchers.
Figure 11: Removal of Congo red (50 ppm) at different pH using NS-SiC (0.01 g catalyst, 10 ml
dye, dark chamber, room temperature).
Figure 12. Removal of Congo red dye (100 ppm) using SiC, N-SiC, and NS-SiC catalysts (0.01
g catalyst, 10 ml dye, dark chamber, room temperature).
Figure 13. Recovery of SiC, N-SiC, and NS-SiC catalysts after 5 runs without washing the
catalysts (a), and FT-IR spectra of used SiC catalysts (b) (0.01 g catalyst, 10 ml of 50
ppm dye, dark chamber, room temperature).

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Figure 14. Recovery of NS-SiC catalyst after 5 runs after washing the catalyst (a), FT-IR spectra
(b), and diffuse reflectance spectraof used and washed SiC catalyst (c) (0.01 g
catalyst, 10 ml of 50 ppm dye, dark chamber, room temperature).
Figure 15. Comparison of the catalytic properties of SiC samples with their related carbon
samples (0.01 g catalyst, 10 ml of 100 ppm dye, dark chamber, room temperature).
Figure 16. Comparison of the catalytic properties of NS-SiC sample in removal of different
anionic azo dyes (0.01 g catalyst, 10 ml of 50 ppm dye, dark chamber, room
temperature).
Table 1. Elemental analysis (atom%) of SiC samples.
Table 2. Textural characterization for the samples.
Table 3. Results of dye removal using SiC, N-SiC, and NS-SiC catalysts.

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Fig. 1

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(102
)
(110
)
(202
)
(101
)
(103
)
(104
)

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G
D
ID/IG = 0.95
ID/IG = 0.93
ID/IG = 0.92

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