Synthesis and photocatalytic CO2 reduction performance of aminated coal-based carbon nanoparticles

Article abstract of DOI:10.1039/c8ra06062b

To obtain high-efficiency, low-cost, environmentally friendly carbon-based photocatalytic material, we synthesized coal-based carbon dots with sp² carbon structure and multilayer graphene lattice structure by the hydrogen peroxide (H₂O₂) oxidation method to strip nano-scale crystalline carbon in the coal structure and link with oxygen-containing groups such as the hydroxyl group. N, S co-doped aminated coal-based carbon nanoparticles (NH₂-CNPs) were then obtained by thionyl chloride chlorination and ethylenediamine passivation. The physical properties and chemical structure of the synthesized NH₂-CNPs were studied and the photocatalytic CO₂ reduction performance was tested. The results show that NH₂-CNPs are vesicle-type spherical particles with particle size of 42.16 ± 7.5 nm and have a mesoporous structure that is capable of adsorbing CO₂. A defect structure was formed on the surface of the NH₂-CNPs due to the doping of N and S elements, thereby significantly improving the ability to photogenerate electrons under visible light along with the ability to efficiently separate the photo-generated carriers. The photocatalytic reduction products of CO₂ over NH₂-CNPs were CH₃OH, CO, C₂H₅OH, H₂ and CH₄. After 10 hours of reaction, the total amount of products was 807.56 μmol g^-1 cat, the amount of CH₃OH was 618.7 μmol g^-1 cat, and the calculated selectivity for conversion of CO₂ to CH₃OH was up to 76.6%.

Full text of DOI:10.1039/c8ra06062b

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Synthesis and photocatalytic CO reduction
2
performance of aminated coal-based carbon
Cite this: RSC Adv., 2018, 8, 35989
nanoparticles
Halidan Maimaiti, * Abuduheiremu Awati,
and Bo Xu
Dedong Zhang, Gunisakezi Yisilamu
To obtain high-efficiency, low-cost, environmentally friendly carbon-based photocatalytic material, we
2
synthesized coal-based carbon dots with sp carbon structure and multilayer graphene lattice structure
by the hydrogen peroxide (H
structure and link with oxygen-containing groups such as the hydroxyl group. N, S co-doped aminated
coal-based carbon nanoparticles (NH -CNPs) were then obtained by thionyl chloride chlorination and
ethylenediamine passivation. The physical properties and chemical structure of the synthesized NH
CNPs were studied and the photocatalytic CO reduction performance was tested. The results show that
NH
-CNPs are vesicle-type spherical particles with particle size of 42.16 ꢀ 7.5 nm and have
a mesoporous structure that is capable of adsorbing CO . A defect structure was formed on the surface
of the NH -CNPs due to the doping of N and S elements, thereby significantly improving the ability to
photogenerate electrons under visible light along with the ability to efficiently separate the photo-
generated carriers. The photocatalytic reduction products of CO over NH -CNPs were CH OH, CO,
2 2
O ) oxidation method to strip nano-scale crystalline carbon in the coal
2
2
-
2
2
2
2
Received 17th July 2018
Accepted 10th October 2018
2
2
3
ꢁ
1
C
2
H
5
OH, H
the amount of CH
CH OH was up to 76.6%.
2
and CH
4
. After 10 hours of reaction, the total amount of products was 807.56 mmol g cat,
DOI: 10.1039/c8ra06062b
ꢁ1
3
OH was 618.7 mmol g cat, and the calculated selectivity for conversion of CO
2
to
rsc.li/rsc-advances
3
4–6
electrons and holes by their own surface defects. Therefore,
they can be used as carriers for photocatalytic materials such as
1
. Introduction
The photocatalytic reduction of CO into hydrocarbon fuels has noble metals or semiconductors or can be used as a photo-
2
7–11
motivated the research and development of various potentially catalyst aer being modied by hetero elements.
high-efficiency photocatalysts. The joint efforts of scholars
Coal is a natural and inexpensive carbon source, whose
around the world have resulted in new photocatalytic materials microstructure contains amorphous carbon regions consisting
1
for photocatalytic reduction of CO
2
.
However, most of the of aliphatic groups and nanoscale crystalline carbon formed by
12
developed catalysts are noble metal- and metal oxide polyaromatic groups. The crystalline carbon regions are
semiconductor-based photocatalysts, such as Ag, TiO
and ZnO, abundant and each crystalline carbon has the size of a quantum
etc., which have wide bandgaps, are expensive, show low effi- dot, suitable for obtaining CNPs by various methods.
2
13,14
In our
we prepared coal-based CNPs used HNO
15–17
ciency for visible light utilization, and pollute the environment. previous study,
3
As environmentally friendly substances, carbon-based nano- pretreated coal samples from the Wucaiwan coalmine in Xin-
materials such as graphene, carbon nanoparticles and activated jiang, China. We stripped crystalline carbon from the coal by
carbon are abundant and easy to obtain, with low cost and low acid oxidation and hydrogen peroxide oxidation methods and
2
toxicity. Carbon nanoparticles (CNPs) are a class of zero- linked oxygen-containing groups such as hydroxyl groups on
dimensional uorescent nanomaterials that were accidentally the CNPs. We found that the prepared coal-based CNPs had
2
discovered by Scrivens et al. during the separation and puri- a multi-layered graphene structure with sp carbon and also
3
cation of single-wall carbon nanotubes. CNPs have uores- contained some metal and non-metallic elements such as Fe,
cence properties like traditional quantum dots and are also Ca, Al, Si, K, S, etc., thereby having the unique photoelectric
excellent electron acceptors and donors. Aer being excited by properties of metal oxide semiconductor materials.
light, CNPs can generate electrons and holes and, at the same
Modication of carbon nanomaterials and metal oxide
time, can suppress the combination of the photogenerated semiconductors by doping (especially doping with non-metallic
elements such as N, S) is a method for controlling the photo-
electric properties of the products. For carbon nanomaterials,
the sp hybrid carbon skeleton can be doped to obtain unpaired
Institute of Chemistry and Chemical Industry, Xinjiang University, Urumqi 830046,
2
Xinjiang, China. E-mail: m15899160730@163.com
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electrons, thereby generating a delocalized P negative charge and 30 mL of distilled water was added to the mixture with
system and promoting the electron transport properties of the constant stirring for 3–4 hours. Next, the reaction liquid was
18
materials. For metal oxide semiconductors, creating states in centrifuged at 10 000 rpm for 10 min. The supernatant was
the band gap that absorb visible light by doping can improve placed in a dialysate bag and dialyzed for 72 h (with constant
19
the visible light efficiency of the materials. Since coal-based water changing) and the aminated coal-based CNPs (NH
2
-CNPs)
CNPs have the characteristics of both general carbon nano- water dispersion was obtained. Finally, the NH -CNPs were
2
materials and metal oxide semiconductors, they are expected to separated by blowing dry the solution with N
2
.
simultaneously increase their electron transport properties and
visible light efficiency if they are doped with N and S elements.
Thus, in this paper, sulfonyl chloride chlorination and ethylene
diamine passivation were adopted to dope the synthesized
CNPS with nitrogen and sulfur, and aminated coal-based
2
.4 Structural characterization of the product
An H-600 transmission electron microscope (TEM) was employed
to measure the morphology of the product. The elemental anal-
ysis was performed by an EDX energy spectrometer (HITACHI-
SU8010). The thermogravimetric (TG-DTG) analysis was per-
formed by a thermal analyzer (PE-DTA/1700) with temperature
carbon nanoparticles (NH
2
-CNPs) were obtained. The struc-
reduction performance were
ture and photocatalytic CO
investigated.
2
ꢂ
scanning range of 25–800 C. The XRD patterns of the product
were acquired by an X-ray diffractometer (M18XHF22-SRA). The
2
. Experimental section
experimental conditions were as below: Cu Ka radiation (l ¼
ꢂ
0
.154056 nm), and scan range 2q ¼ 10–80 . The confocal Raman
2.1 Raw materials and reagents
spectra of the product were acquired by a Raman spectrometer
BRUKER VERTEX 70). The N adsorption–desorption perfor-
The coal samples were acquired from Wucaiwan coalmine in
Zhundong coaleld, Xinjiang, China. The samples were ground
and ltered by a 200 mesh, followed by drying at 105 C for 4 h.
(
2
mance was tested by a specic surface and aperture distribution
meter (Autosorb-IQ2), and the specic surface and aperture
distribution were calculated by the BET and BJH method. The
FTIR spectrum of the product was obtained by a Fourier trans-
form infrared spectrometer (FTIR, Bruker-EQUINOX 55). The
surface states of the product were analyzed by an X-ray photo-
ꢂ
The analytical grade sulfonyl chloride (SOCl
from Aladdin Reagent Co. Ltd. The analytical grade diamine
EDA) was purchased from Tianjin Zhiyuan Chemical Reagent
2
) was purchased
(
Co. Ltd. Analytical grade DMF, THF, H O , and HNO were
purchased from common chemical stores.
2
2
3
electron spectrometer (XPS, ESCALAB 250). The CO
programmed desorption performance (CO -TPD) was measured
by TP-5080 automatic adsorption–desorption dynamic
2
temperature
Ready-to-use dialysate bags (intercepted molecular weight of
2
500–1000, Spectrum Labs, U.S.) were purchased from Shanghai
a
Tong Shan Biological Science and Technology Co., Ltd.
analyzer. The ultraviolet-visible diffuse reection spectrum (UV-
vis DRS) of the product was measured by a UV-vis DRS spec-
trometer (Shimadzu UV-3100). The electrochemical impedance
2.2 Preparation of coal-based CNPs
Reux treatment was conducted on coal samples from Wucai- (EIS) and photoionization density were measured by an electro-
wan, Xinjiang with 2.5 M HNO
for 24 h. The pre-treated chemical workstation (CHI 660C).
3
samples were placed in a ball mill with distilled water, and
ground for 3 h at room temperature and 600 rpm. Aer drying,
2
.5 Photocatalytic CO reduction by visible light
2
0
.5 g of ultra-ne coal powder was dispersed in 10 mL of
NH -CNPs (0.2 g) were added to 100 mL of 0.1 M NaOH solution
2
deionized water by sonication, then 30 mL of 30% H was
added to the solution, followed by constant magnetic stirring
and reaction at 85 C for 3 h (5 mL of 30% H O was added to
the solution every 10 min for a total of four times). Aer the
reaction stopped, the reaction liquid was transferred to
a centrifuge tube for centrifuge separation (10 000 rpm for 10
min). The supernatant was blown dry by N
brown coal-based CNPs were obtained.
2 2
O
and dispersed by sonication for 15 min in the dark. The
suspension was transferred to a quartz bottle in a photocatalytic
reduction reactor, as shown in Fig. 1.
ꢂ
2
2
The system was placed under vacuum several times to
remove air, and high-purity CO
2
gas was owed into the
was 20
suspension at 1 atm with stirring (ow rate of CO
2
2
, and the black-
ꢁ1
mL min for 30 min). Next, the whole reaction system was
sealed, and illuminated with a 300 W xenon lamp (with a UV
cut-off lter to obtain visible light with wavelength over 420 nm)
2
2
.3 Fabrication of NH -CNPs
2 2
to start the photocatalytic CO /H O reduction reaction. Aer
CNPs (1 g) were placed in a reaction bottle with 20 mL DMF and reaction for 6 h, the liquid and gas products were acquired
1
dispersed by sonication for 2 h at room temperature. Aer the separately and subjected to H NMR and GC-MS to identify the
CNPs were fully dispersed to form a suspension, 15 mL of SOCl
2
species generated in the photocatalytic CO
2
reduction by NH
2
-
was added to the solution, followed by reux-stirring and CNPs. The above CO
2
/H
2
O photocatalytic reduction was then
ꢂ
reaction for 12 h at 65 C. DMF and unreacted SOCl2 were repeated, and the samples were acquired every 2 h and injected
removed by centrifugation. The reaction liquid was washed into a gas chromatograph; the liquid and gas products were
twice with THF to obtain the chloridized intermediate CNPs-Cl analyzed by the standard curve method (liquid phase: FID
ꢂ
ꢂ
and mixed with 30 mL EDA for reux reaction for 12 h at 130 C. detector, PEG-21 lling column, gas phase: TCD detector, TDX-
The temperature of the reaction liquid was decreased to 60 C, 01 lling column).
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2 2
Fig. 1 Schematic of the experimental setup for the photoreduction of CO with H O.
linked to N and O containing groups such as –NO
OH. Compared to CNPs, NH -CNPs have lower inorganic
particle content and apparently higher N, S, and Cl content,
According to Fig. 2a, coal powder has an irregular size distri- which is due to the aminated reaction.
bution and shape, and it cannot be dispersed in water. However,
Fig. 3A shows the TG-DTG results for coal powder, CNPs, and
the CNPs synthesized by H
have good water dispersion and NH
3
, –COOH,
3
. Results and discussions
–
2
2
3.1 Structure of NH -CNPs
2
1
2
O
2
2
-CNPs. We can see from the published literature that the
ꢂ
spherical shape with a diameter of 4.51 ꢀ 0.82 nm (Fig. 2b). The rst weight loss peak of coal powder under 100 C is caused by
ꢂ
high magnication TEM image (inset) shows that the surface of water volatilization, and the second peak at 445 C corresponds
the CNPs has clear lattice strips. The spacing between the strips to the weight loss peak of devolatilization during the pyrolysis
20
is 0.21 nm, close to the (100) plane of graphite, indicating that process. As the temperature increases, especially exceeding
ꢂ
the coal-based CNPs have a graphite-like crystalline carbon 600 C, the weight loss is relatively at, mainly caused by the
structure. It can be seen from Fig. 2c that the NH -CNPs are polycondensation stage.
2
spherical particles with vesicle structure, formed by EDA linking
The coal-based CNPs also showed a peak for the loss of water
ꢂ
and entangling with CNPs treated by SOCl
2
; the particle size of under 100 C but there was also a large weight loss peak at
ꢂ
4
2.16 ꢀ 7.5 nm is larger compared to the CNPs. The NH
2
-CNPs 288 C, which was obviously different from the weight loss
can also be uniformly dispersed in water.
behavior of raw coal, corresponding to the volatilization of
From the EDX results for the coal powder, product CNPs and small molecular compounds and the decomposition or dehy-
NH
-CNPs shown in Table 1, it can be seen that the O and N dration of oxygen-containing groups such as hydroxyl or
2
22
content in the coal-based CNPs are signicantly higher carboxyl groups. This indicates that a large amount of active
compared to the coal powder, while the C content is decreased. oxygen-containing functional groups appeared in the coal
This could be the result of the crystalline carbon being stripped structure as a result of oxidative stripping by H
out from the pulverized coal during the HNO
pretreatment, the coal-based CNPs showed a weight loss peak between 500–
ball milling, tank grinding, and H
oxidation processes, and 600
O . In addition,
2 2
3
ꢂ
2
O
2
C, corresponding to the condensation and
2
Fig. 2 TEM images of (a) coal powder, (b) as-grown CNPs and (c) NH -CNPs.
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Table 1 Elemental analysis data for the coal, CNPs and NH
2
-CNPs
a
Elemental analysis, w%
Sample
Processing method
C
N
O
Al
Si
S
Ca
Fe
Cl
0
Coal powder
Coal samples were
ground and ltered
by a 200 mesh
75.04
0.58
17.79
0.89
3.35
0.52
0.71
1.12
CNPs
As Section 2.2
As Section 2.3
47.34
40.64
6.65
14.43
43.40
36.45
0.24
0.18
1.36
0.93
0.14
6.31
0.28
0.17
0.59
0.21
0
0.68
NH
2
-CNPs
a
By EDX.
dehydrogenation of the aromatic structures in the coal; the peak condensation dehydrogenation of the aromatic structure into
1
6
ꢂ
position is lower than the pyrolysis temperature of the coal semicoke. Above 600 C, there was 57.30% undecomposed
ꢂ
powder (600–800 C). The possible reason is that the CNPs remnant, which was made up of the graphitized carbon and
contain more oxygen-containing groups, which can induce a small amount of inorganic particles in NH
26
2
-CNPs. The TG-
pyrolysis of the organic compounds, thereby lowering the DTG results further prove that the CNPs were successfully
23
pyrolysis temperature. The results indicate that the coal-based aminated. Fig. 3B shows the XRD patterns of coal powder, CNPs
CNPs obtained by the H
aromatic structures linked by oxygen-containing groups.
2
O
2
oxidation method mainly contain and NH
2
-CNPs. According to the gure, CNPs demonstrate
ꢂ
a broad diffraction peak at 15–40 , corresponding to the (002)
27
The primary weight loss of NH -CNPs has four stages. The crystal plane of graphite; this characteristic peak does not
2
ꢂ

rst stage occurs below 100 C with a loss amount of 2.84%, appear in the XRD pattern of the coal sample. Aer amination,
which is attributed to the water in the sample. The second stage the corresponding graphite (002) peak in the NH
2
-CNPs shied
ꢂ
happens at around 192 C with a loss of 9.63%, corresponding toward lower angle. This is because the surface of CNPs is
24
to the heat cracking of EDA groups linked to the CNPs surface.
2
linked to NH – and other groups containing N and S, leading to
In the third stage, the TG loss is 8.22%, which occurs at around the larger particle size of NH
2
-CNPs compared to CNPs.
ꢂ
267 C and can be attributed to the decomposition of oxygen-
Raman spectrometry is an effective method for evaluating
containing groups such as carboxyl and hydroxyl into CO or the structure and performance of carbon nanoparticles. As
2
2
5
CO at high temperatures. The oxygen-containing groups were shown in the Raman spectrum in Fig. 3C, both CNPs and NH -
2
ꢁ
1
introduced in the synthesis process (such as coal pre-treatment CNPs demonstrate two obvious peaks at 1350 and 1580 cm
,
by HNO
3
or H
2
O
2
oxidation). The fourth stage happened at near corresponding to the D band and G band, respectively; however,
ꢂ
551 C with a loss amount of 28.28%, corresponding to the the coal powder does not display the characteristic peaks. The D
2
Fig. 3 Physical characterizations of coal powder, CNPs, and NH -CNPs. (A) TG-DTG; (B) XRD; (C) Raman; (D) BET.
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ꢁ
1
ꢁ1
band is related to the defects in carbon materials, and the G 3400 cm (curve a). At 1635 cm , the peak corresponds to the
2
band reects the sp hybrid carbon atom content in carbon stretching–vibration absorption peak of –C–O–. The peak at
28
ꢁ1
material. The ratio of the peak intensity (I /I ) between the D 1350 cm
corresponds to the stretching–vibration peak of
D
G
ꢁ1
band and G band can be used to evaluate the graphitization and COO–. The peak at 1093 cm is the stretching–vibration peak
defects of the carbon material. Using the raw data from Fig. 3C, of C–O. The peak at 797 cm is the stretching–vibration peak of
ꢁ
1
ꢁ
1
the calculated I
respectively. The I
were aminated, suggesting their high microstructural integrity. peaks, there are apparent amido (CONH–) peaks at 3245 and
D
/I
G
for CNPs and NH
2
-CNPs is 0.76 and 0.98, C–NO
2
. The absorption peak at 605 cm corresponds to inor-
D
/I was apparently increased aer the CNPs ganic particles. For NH
G
2
-CNPs, besides the above characteristic
ꢁ
1
ꢁ1
The aminated material also has defect edges, which favor the 2922 cm . At 1520, 1443 and 1219 cm , the absorption peaks
29
effective separation of photogenerated electrons and holes,
correspond to the amide I band, amide II band and amide III
band, respectively. The peak at 583 cm corresponds to the O–S
ꢁ
1
leading to enhanced photocatalytic CO reduction activity.
2
N2 adsorption–desorption tests were performed on coal absorption peak. The results suggest that CNPs have success-
powder, CNPs and NH -CNPs. The specic surface area of the fully reacted with EDA to give the aminated product NH -CNPs.
2
2
sample was calculated using the BET equation, and the pore There is no apparent absorption peak for inorganic particles or
size and distribution were calculated by the BJH equivalent free ethylenediamine, suggesting the inorganic content was
cylinder model; the results are shown in Fig. 3D. The hysteresis lowered and the EDA was removed by dialysis.
loop of coal powder is not obvious, indicating that coal has
XPS can be used to detect the valence states of elements in
a complex pore structure that cannot be described by the BET nanomaterials. To further determine the product chemical
theory. The adsorption isotherms of the coal-based CNPs structure, we used XPS to analyze the surface states of CNPs and
belong to I-type isotherms, indicating that CNPs are micropo- NH -CNPs, as shown in Fig. 4(B–F). According to the full XPS
2
rous structural materials. The
isotherms for NH -CNPs are type-IV isotherms, indicating the similar XPS patterns, with four characteristic peaks at 160.0,
mesoporous structure. From the pore distribution graph (inset), 286.0, 401.0 and 530.9 eV, corresponding to S2p, C1s, N1s and
the CNPs have a pore size of 3–12 nm, and NH -CNPs have small 1s. The results show that CNPs and NH -CNPs are mainly
pores of 2–9 nm and large pores of 10–60 nm. The corre- composed of C, N, O, and S, and the N and S contents in NH
2 2
N adsorption–desorption spectra of CNPs and NH -CNPs (Fig. 4B), the two products show
2
2
O
2
2
-
2
ꢁ1
sponding BET specic surface areas are 29.36 m g for CNPs, CNPs are apparently higher as compared to those in CNPs. The
and 172.65 m g for NH -CNPs. The total pore volume for O content of NH -CNPs is lower than CNPs, possibly due to the
2
ꢁ1
2
2
3
ꢁ1
3
ꢁ1
CNPs and NH -CNPs is 0.0114 cm
g
and 0.320 cm
g
,
replacement of oxygen-containing groups by NH – groups.
2
2
respectively. Generally, a larger specic surface area and pore Fig. 4C shows the high-resolution XPS spectrum of C1s for CNPs
volume are favorable for reactant adsorption, which can and NH -CNPs. The NH -CNPs have ve splitting peaks at
2
2
3
0
signicantly enhance the photocatalytic activity.
Fig. 4A shows the infrared spectrum of CNPs and NH
The CNPs synthesized by H
the stretching–vibration peak of the hydroxyl groups at and are consistent with the high-resolution XPS spectrum of C1s
284.8 eV, 286.1 eV, 287.3 eV, 288.0 eV and 292.4 eV, corre-
-CNPs. sponding to C–C, C–O, C–O–C, C]O and O]C–O bonds,
2
2 2
O oxidation and separation show respectively. The C–C, C–O and C]O bond have high content
2
Fig. 4 Structural characterizations of CNPs and NH -CNPs: (A) FT-IR spectra; (B–F) XPS spectra.
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for CNPs (inset). This shows that the skeleton of the NH
2
-CNPs
Fig. 5C shows the time-dependent photocatalytic conversion of
comes from CNPs. Fig. 4D shows the high-resolution XPS CO into hydrocarbons over NH -CNPs. It can be seen that the
2
2
spectrum of O1s for CNPs and NH -CNPs. The CNPs have peaks yield of CH OH is signicantly higher than that of CH , H ,
2
3
4
2
at 530.0 eV and 532.8 eV, which correspond to the C–O and C H OH and CO. In addition, the amount of the photocatalytic
2
5
3
1
C]O groups of the graphene structure (inset). NH
a single peak between 530.0 eV and 532.8 eV, which is attributed indicating that NH
to C–O–H. This suggests that the oxygen content is lowered rate. Aer the 10 hours of reaction, the total amount of products
2
-CNPs have product was substantially unchanged aer 10 hours of reaction,
2
-CNPs have a faster photocatalytic reaction
ꢁ
1
during amination. Fig. 4E shows the high-resolution XPS was 807.56 mmol g cat, and the amount of CH
3
OH was 618.7
-CNPs. There are three spitting peaks at mmol g cat. Therefore, the calculated selectivity for the conver-
98.28 eV, 399.98 eV and 401.68 eV. The peaks at 398.28 eV and sion of CO to CH OH was 76.6%. The inset of Fig. 5C shows the
ꢁ
1
spectrum of N1s for NH
2
3
3
2
3
99.98 eV have low binding energies, corresponding to the change in CH OH yield produced by photocatalytic CO reduction
3
2
amide bond (CONH) and an amine with a positive charge using NH -CNPs aer three cycles. In each cycle, the reaction
2
3
+
32
(
–NH ). The peak at 401.68 eV corresponds to graphite liquid was changed, and the illumination time was 6 h. The results
nitrogen, which is formed by replacing the carbon atoms with show that aer reusing 3 times, NH -CNPs gave a stable perfor-
OH production. The CH OH yield in
-CNPs. It can be seen that the products have four the third cycle was 89.4% of the yield in the rst cycle. This
types of S structure. The peaks at 163.96 eV, 164.9 eV, 168.4 eV, suggests that the NH -CNPs are suitable for photocatalytic CO
and 169.7 eV correspond to C–S–C, C]S, C–SO –C, and C–SO – reduction. We also examined the non-aminated CNPs photo-
2
33
nitrogen atoms. Fig. 4F is the high-resolution XPS spectrum of mance with repeatable CH
2p in NH
3
3
S
2
2
2
2
3
34–37
C groups,
respectively.
catalytic CO reduction products. It was found that the CH OH
2 3
ꢁ
1
yield in the liquid products was only 91.70 mmol g cat, about 1/
th of the CH OH yield when using the NH -CNPs photocatalyst.
To prove that the catalyst, illumination, and CO are the
To conrm the photocatalytic products and selectivity of the necessary requirements for photocatalytic CO reduction to
OH and CO, we conducted three control
experiments under non-catalyst, non-illumination, and non-
6
3
2
3
2 2
.2 Photocatalytic CO reduction performance by NH -CNPs
2
2
1
NH
2
3 4 2 2 5
-CNPs, we used H NMR and GC-MS to analyze the liquid CH OH, CH , H , C H
and gas products obtained aer reaction for 6 hours.
1
Fig. 5A shows the H NMR results for the liquid products CO
conditions. The experiments were performed according to
2
from the photocatalytic CO reduction by NH -CNPs. The single Section 2.5, with illumination time (if any) of 6 h. No products
2
2
peak at d ¼ 3.34 ppm is the chemical shi of hydrogen protons were detected.
at –CH of CH OH. The triple peak at d ¼ 1.17 ppm and the
3
3
quadruple peak at d ¼ 3.67 ppm represent the hydrogen protons
3
.3 CO
O solvent. This result shows that the liquid products of To reveal that the NH
photocatalytic CO reduction are CH OH and C OH. reduction of CO activity than CNPs, we rst analyzed the type of
We also identied the gas products by GC-MS. The results basic sites and their distribution on the catalyst surface by CO
2
photocatalytic reduction mechanism for NH
2
-CNPs
at –CH
2
of CH
3
CH
2
OH. The single peak at d ¼ 4.79 ppm is the
D
2
2
-CNPs have greater photocatalytic
2
3
2
H
5
2
2
-
show that the gas products of photocatalytic CO reduction by TPD. The results are shown in Fig. 6A. Generally, the area of the
2
NH -CNPs contain H , CO, and CH Fig. 5B shows the GC-MS desorption peak reects the number of active sites on the
2
2
4.
results of the gas products when we used the isotopic marker catalyst surface, while the temperature of the desorption peak
1
3
CO
2
as the only carbon source in the system for photocatalytic reects the adsorption intensity of the corresponding species at
38
CO
sent CH
2
reduction. According to the results, m/z ¼ 17 and 29 repre- active sites. NH
2
-CNPs exhibited a weak a peak in the low-
1
3
13
ꢂ
4
and CO, respectively, which indicate that the CO temperature zone (162 C), which is generally considered as
and CH come from photocatalytic CO
4
2
2
reduction by NH -CNPs. the CO
2
physical adsorption peak of the catalyst. At around
1
Fig. 5 (A) H NMR spectra of the aqueous samples after 6 h of irradiation. (B) GC-MS spectral (m/z ¼ 16, 17, 28, 29) analyses of the carbon source
1
3
for the evolved CH
hydrocarbons over NH
CH OH, with replacement of the solution and photocatalyst (recovered) every 6 h).
4
and CO in the photocatalytic conversion of CO
2
on NH
2
-CNPs. (C) Time-dependent photocatalytic conversion of CO
2
into
2
-CNPs (the inset shows the long-term time curve for the recovered NH
2
-CNPs from the photocatalytic CO synthesis of
2
3
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Fig. 6 (A) The CO
2
-TPD profiles of the CNPs (a) and NH
2
-CNPs (b), (B) DR UV-vis spectra of the CNPs (a) and NH
2
-CNPs (b). (C) The calculation
2
2
of the band gap of the CNPs (a) and NH -CNPs (b) based on the (Ahv) versus hv curve.
ꢂ
3
50 C, the CO -TPD spectra of both CNPs and NH -CNPs
Aer the catalyst is excited by light, the photogenerated
2
2
showed a strong b peak, corresponding to a medium-strong electrons and holes have two pathways: (i) migrating to the
adsorption of catalyst, indicating that both CNPs and NH surface of the catalyst and triggering the subsequent photo-
-CNPs demonstrated a broad g catalytic reduction reaction; (ii) recombining with each other.
adsorption sites in NH Apparently, to enhance the photogenerated electron–hole pair
-CNPs is separation efficiency, the catalyst surface must have clear
2
-
CNPs have CO
peak at 460 C, suggesting strong CO
2
adsorption. NH
2
ꢂ
2
2
-
CNPs. In addition, the adsorption peak area of NH
2
apparently larger than CNPs. Based on the above results, it can pathways for electrons. It is generally considered that the semi-
be concluded that NH -CNPs have improved basic site types and circle in the EIS curve represents the transfer resistance for
2
distribution compared to CNPs. This is an important reason electrons in the pores of the composite material. Semi-circles
why NH
2
-CNPs have higher activity than CNPs in CO
2
photo- with larger diameters correspond to poorer electron transfer
catalytic reduction.
capability. Thus, to investigate the separation efficiency of
The photocatalytic process oen involves the light excitation photogenerated electron–hole pairs, we measured the EIS
of the catalyst, photocarrier separation, migration, and recom- curves for CNPs and NH -CNPs under the open circuit potential;
bination, as well as the generation of active species and the the results are shown in Fig. 7A. According to the Nyquist curve,
subsequent oxidation and reduction reactions. Among them, aer the CNPs are aminated, the impedance of NH -CNPs is
2
2
the absorption range and intensity are two important factors signicantly lower compared to the CNPs. This is likely due to
affecting the CO photocatalytic reduction performance. Fig. 6B the defects in NH -CNPs, which can trap more photogenerated
2
2
shows the measured UV-vis spectra of CNPs and NH
2
-CNPs. electrons via the quantum connement effect, enhancing the
According to the gure, both materials can absorb visible light, separation efficiency of electron–hole pairs in the composite
while NH -CNPs have a broader absorption range and intensity catalyst, leading to lower sample impedance.
than CNPs. Thus, aer being aminated, the CNPs' response to The transient photocurrent–time curve is an effective way to
2
visible light is enhanced. In this work, we employed excitation study the transfer of photogenerated carriers on the surface of
light with a wavelength greater than 420 nm for the photo- the catalyst. Fig. 7B shows the photocurrent–time curve of CNPs
catalytic reaction. The highest photon energy was 2.95 eV. The and NH -CNPs electrodes in a 0.5 M Na SO solution with
2
2
4
incident photon energy must be equal to or greater than the illumination by a xenon lamp (l > 420 nm) under open circuit
material band gap to excite electrons from the VB to CB and potential and at room temperature. There are three differences
39
generate holes in the VB for the subsequent redox reaction.
Thus, to prove that the CNPs and NH -CNPs can be excited by NH
visible light, we measured their DR UV-vis curves, and used eqn density than CNPs. This suggests that NH
between the transient photocurrent–time curves of CNPs and
-CNPs: (i) NH -CNPs have larger steady-state photocurrent
-CNPs can excite
1) to calculate and plot the relationship between (Ahv) and more photogenerated electrons for photocurrent compared to
2
2
2
2
2
(
40
hv. By extending the straight-line portion of the curve to the x- CNPs. (ii) Upon turning on the light, the photocurrent of CNPs
axis, the energy band gaps (E ) of CNPs and NH -CNPs were increases quickly, while the photocurrent of NH -CNPs
g
2
2
obtained, which were 2.42 and 2.02 eV, respectively (Fig. 6C). increases slowly. This is because upon illumination, CNPs can
This further suggests that NH -CNPs have stronger visible light be quickly excited and generate electrons, while part of the
absorption than CNPs to generate electrons and holes. This is carriers in NH -CNPs are trapped and cannot transfer in time,
another important reason why NH -CNPs have higher activity leading to a slow increase to the maximum. (iii) Upon cutting-
than CNPs in CO photocatalytic reduction. off the light, the photocurrent of CNPs quickly drops to zero,
while the photocurrent of NH -CNPs decreases exponentially to
2
2
41
2
2
2
À
Á
1
2
K hv ꢁ E
g
a “tail” rather than zero. This is because the previously trapped
charges are gradually released, generating low current and
slowing down the decrease of the photocurrent to a non-zero
value. Based on the transient photocurrent–time curve, it
can be concluded that NH -CNPs can generate more
A ¼
(1)
is
hv
where A is the absorption coefficient, K is a constant, and E
g
42
the energy band gap.
2
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Fig. 7 (A) Nyquist plots of the CNPs (a) and NH
2
-CNPs (b). (B) The transient photocurrent responses of the CNPs (a) and NH
2
-CNPs (b) in 0.5 M
2 4
Na SO aqueous solution under visible-light (l > 420 nm) irradiation.
+
ꢁ
0
redox
photocarriers, and due to surface defects, the photo-generated
carriers can be effectively separated. Even aer the light is
CO + 2H + 2e / CO + H O, E
¼ ꢁ0.53 V
(5)
2
2
+
ꢁ
0
turned off, NH
charges for photocatalytic CO
2
-CNPs can still release a small amount of
reduction. This is another reason
CO
CO
CO + 12H + 12 e / C
2
+ 6H + 6e / CH
OH + H
3
2
O, Eredox ¼ ꢁ0.38 V (6)
2
+
ꢁ
0
2
+ 8H + 8e / CH
4
+ 2H
2
O, Eredox ¼ ꢁ0.24 V
(7)
that NH -CNPs have better photocatalytic performance.
2
Based on the above analysis, we concluded that the mecha-
+
ꢁ
0
2
2
2
H
5
OH + 3H
2
O, Eredox ¼ ꢁ0.74 V(8)
nism of photocatalytic CO
shown in Fig. 8. The analysis of UV-vis DRS and band gap curves
revealed that the band gap of NH -CNPs decreased from 2.42 eV
for CNPs to 2.02 eV, indicating that the NH -CNPs have
2 2 2
/H O reduction by NH -CNPs is as
2
2
4
. Conclusions
enhanced response to visible light and can generate more
electrons and holes (eqn (2)). The PL, EIS, and I–T results show
3
Using HNO pre-treated coal samples from Wucaiwan, Xinjiang,
that the photogenerated electron–hole pairs in NH -CNPs can
2
China, nano-scale crystalline carbon was fabricated from coal
by the hydrogen peroxide oxidation method. Aer being linked
with oxygen-containing groups such as hydroxyl, the coal-based
be easily separated, and the electrons can be transferred to the
surface of the catalyst for the CO
8)). The reduction potential of NH
electron quantities are likely to be the same as the reduction
potential for CH OH production and the required electron
amount, respectively. Therefore, the primary product of pho-
tocatalytic CO reduction by NH -CNPs is CH OH.
2
reduction reaction (eqn(3)–
(
2
-CNPs and the generated
2
CNPs with sp carbon architecture and multilayer graphene
fragmentation structure were obtained. Next, sulfonyl chloride
chlorination and ethylenediamine passivation were adopted to
dope the CNPs with nitrogen and sulfur and produce aminated
coal-based carbon nanoparticles (NH
CO reduction. The results show that NH
enhanced visible light response, leading to more photo-
generated electrons that contribute to the photocurrent. More-
over, since the surface of NH -CNPs has a defect structure, the
3
2
2
3
2
-CNPs) for photocatalytic
-CNPs possess
ꢁ
+
2
2
NH
2
-CNPs + hv / e + h
(2)
(3)
ꢁ
+
ꢁ
H
2
O + e / H + OH
2
+
ꢁ
0
2
H + e / H , E
¼ ꢁ0.41 V
(4) photogenerated carriers can be effectively separated. Even upon
2
redox
turning off the illumination, the NH -CNPs can still release
2
2
a small amount of charge for photocatalytic CO reduction,
improving the overall catalytic performance. This work
contributes to better controlling the optoelectronic properties
of carbon nanomaterials, and provides a new approach to
further utilize coal resources for high-performance materials.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
The authors are grateful for the nancial support of the
National Science Foundation of China (21663028) and the Key
Laboratory of Coal Clean Conversion & Chemical Engineering
Process of Xinjiang Uygur Autonomous Region.
Fig. 8 Schematic of the mechanism of the photocatalytic CO
reduction with water on NH -CNPs.
2
2
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2
2 A. J. Fletcher, Y. Uygur and K. M. Thomas, J. Phys. Chem. C,
References
2007, 111, 8349–8359.
1
2
3
W. Tu, Y. Zhou and Z. Zou, Adv. Mater., 2014, 26, 4607–4626. 23 B. Ruiz, J. B. Parra, J. A. Pajares and J. J. Pis, J. Anal. Appl.
H. Sun and S. Wang, Energy Fuels, 2014, 28, 22–36.
Pyrolysis, 2006, 75, 27–32.
X. Xu, R. Ray, Y. Gu, H. J. Ploehn, L. Gearheart, K. Raker and 24 Z. Yun-fei, H. Maimaiti and Z. Bo, RSC Adv., 2017, 7, 2842–
W. A. Scrivens, J. Am. Chem. Soc., 2004, 126, 12736–12737.
2850.
X. Wang, L. Cao, F. Lu, M. J. Meziani, H. Li, G. Qi, B. Zhou, 25 Z. Li, L. Zhu, W. Wu, S. Wang and L. Qiang, Appl. Catal., B,
4
5
6
B. A. Harruff, F. Kermarrec and Y.-P. Sun, Chem. Commun.,
009, 3774–3776, DOI: 10.1039/b906252a.
2016, 192, 277–285.
26 A. Sachdev and P. Gopinath, Analyst, 2015, 140, 4260–4269.
2
L. Cao, X. Wang, M. J. Meziani, F. Lu, H. Wang, P. G. Luo, 27 Y. Badhe, K. Balasubramanian and R. Gupta, RSC Adv., 2015,
Y. Lin, B. A. Harruff, L. M. Veca, D. Murray, S.-Y. Xie and
Y.-P. Sun, J. Am. Chem. Soc., 2007, 129, 11318–11319.
K. A. S. Fernando, S. Sahu, Y. Liu, W. K. Lewis, E. A. Guliants,
5, 23622–23634.
28 S. Ms and S. Sankararaman, J. Mater. Sci. Nanotechnol., 2017,
5, DOI: 10.15744/2348-9812.5.103.
A. Jafariyan, P. Wang, C. E. Bunker and Y.-P. Sun, ACS Appl. 29 X. Wang, X. Zhao, D. Zhang, G. Li and H. Li, Appl. Catal., B,
Mater. Interfaces, 2015, 7, 8363–8376.
2018, 228, 47–53.
S. Hu, R. Tian, L. Wu, Q. Zhao, J. Yang, J. Liu and S. Cao, 30 J. Jin, J. Yu, D. Guo, C. Cui and W. Ho, Small, 2015, 11, 5262–
Chem.–Asian J., 2013, 8, 1035–1041.
5271.
R. Wang, K.-Q. Lu, Z.-R. Tang and Y.-J. Xu, J. Mater. Chem. A, 31 T. Ishizaki, Y. Wada, S. Chiba, S. Kumagai, H. Lee,
7
8
9
2
017, 5, 3717–3734.
S. Hu, Q. Chang, K. Lin and J. Yang, Carbon, 2016, 105, 484–
89.
0 J. Di, J. Xia, X. Chen, M. Ji, S. Yin, Q. Zhang and H. Li, Carbon,
017, 114, 601–607.
A. Serizawa, O. L. Li and G. Panomsuwan, Phys. Chem.
Chem. Phys., 2016, 18, 21843–21851.
32 M.-X. Wang, Z. Guo, Z.-H. Huang and F. Kang, Catal.
Commun., 2015, 62, 83–88.
33 G. P. Mane, S. N. Talapaneni, K. S. Lakhi, H. Ilbeygi,
U. Ravon, K. Al-Bahily, T. Mori, D. H. Park and A. Vinu,
Angew. Chem., Int. Ed. Engl., 2017, 56, 8481–8485.
4
1
1
1
2
1 H. Yu, R. Shi, Y. Zhao, G. I. N. Waterhouse, L.-Z. Wu,
C.-H. Tung and T. Zhang, Adv. Mater., 2016, 28, 9454–9477.
2 R. Ye, C. Xiang, J. Lin, Z. Peng, K. Huang, Z. Yan, N. P. Cook, 34 T. Zhang, C. Li, Y. Gu, X. Yan, B. Zheng, Y. Li, H. Liu, N. Lu,
E. L. Samuel, C. C. Hwang, G. Ruan, G. Ceriotti, A. R. Raji,
A. A. Marti and J. M. Tour, Nat. Commun., 2013, 4, 2943.
3 C. Hu, C. Yu, M. Li, X. Wang, J. Yang, Z. Zhao, A. Eychmuller,
Y. P. Sun and J. Qiu, Small, 2014, 10, 4926–4933.
4 S. Hu, Z. Wei, Q. Chang, A. Trinchi and J. Yang, Appl. Surf.
Sci., 2016, 378, 402–407.
5 B. Zhang, H. Maimaiti, Y. Zhang and M. Wei, Int. J. Coal Sci.
Technol., 2017, 4, 342–353.
6 Z. Dedong, H. Maimaiti, A. Awati, G. Yisilamu, S. Fengchang
and W. Ming, Chem. Phys. Lett., 2018, 700, 27–35.
Z. Zhang and G. Feng, Talanta, 2017, 165, 143–151.
35 S. Bian, C. Shen, Y. Qian, J. Liu, F. Xi and X. Dong, Sens.
Actuators, B, 2017, 242, 231–237.
36 S. C. Wuang, K. G. Neoh, E. T. Kang, D. W. Pack and
D. E. Leckband, Adv. Funct. Mater., 2006, 16, 1723–1730.
37 Y.-T. Yang, X.-X. Yang, Y.-T. Wang, J. Luo, F. Zhang,
W.-J. Yang and J.-H. Chen, Fuel, 2018, 219, 166–175.
38 T. Witoon, N. Kachaban, W. Donphai, P. Kidkhunthod,
K. Faungnawakij, M. Chareonpanich and J. Limtrakul,
Energy Convers. Manage., 2016, 118, 21–31.
1
1
1
1
1
1
1
2
2
7 F. Sun, H. Maimaiti, Y.-e. Liu and A. Awati, Int. J. Energy Res., 39 H. Wang, L. Zhang, Z. Chen, J. Hu, S. Li, Z. Wang, J. Liu and
018, 1–12.
X. Wang, Chem. Soc. Rev., 2014, 43, 5234–5244.
8 K. A. Kurak and A. B. Anderson, J. Phys. Chem. C, 2009, 113, 40 F. Wang, P. Chen, Y. Feng, Z. Xie, Y. Liu, Y. Su, Q. Zhang,
2
6730–6734.
Y. Wang, K. Yao, W. Lv and G. Liu, Appl. Catal., B, 2017,
207, 103–113.
41 H. Zhang, L. Zhao, F. Geng, L.-H. Guo, B. Wan and Y. Yang,
Appl. Catal., B, 2016, 180, 656–662.
42 Z. Li, J. Zhang, Y. Li, S. Zhao, P. Zhang, Y. Zhang, J. Bi, G. Liu
and Z. Yue, Biosens. Bioelectron., 2018, 99, 251–258.
9 R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga,
Science, 2001, 293, 269.
0 H. Krishna Sadhanala, A. Maddegalla and K. K. Nanda, New
J. Chem., 2017, 41, 13742–13746.
1 L. Shi, Q. Liu, X. Guo, W. Wu and Z. Liu, Fuel Process.
Technol., 2013, 108, 125–132.
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