A Strategy for One-Pot Conversion of Organic Pollutants into Useful Hydrocarbons through Coupling Photodegradation of MB with Photoreduction of CO2

Article abstract of DOI:10.1021/acscatal.6b01729

A strategy was developed to couple photocatalytic oxidation with photocatalytic reduction technology to realize one-pot conversion of MB into hydrocarbons for the first time. In this approach, organic pollutants were first decomposed into CO₂ by photodegradation and then the as-obtained CO₂ was converted into CH₃OH, C₂H₅OH, and CH₄ through photocatalytic reduction of CO₂ under solar spectrum irradiation by using GQDs/V-TiO₂ catalysts. The experimental results show that 5%GQDs/V-TiO₂ has the best photocatalytic activity and the product rates of CH₃OH, C₂H₅OH, and CH₄ are 13.24, 5.65, and 0.445 μmol g^-1 h^-1, respectively. The corresponding apparent quantum efficiency is 4.87% at 420 nm. The one-pot conversion of MB into hydrocarbons was demonstrated by a series of experiments. The photocatalytic mechanisms of one-pot conversion of MB into hydrocarbons were proposed to explain the detailed photocatalytic process.

Full text of DOI:10.1021/acscatal.6b01729

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Article
A Strategy for One-Pot Conversion of Organic
Pollutants into Useful Hydrocarbons through Coupling
Photodegradation of MB with Photoreduction of CO2
Jian-Ping Zou, Dan-Dan Wu, Jinming Luo, Qiu-Ju Xing, Xu-Biao Luo,
Wen-Hua Dong, Shenglian Luo, Hong-Mei Du, and Steven L. Suib
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01729 • Publication Date (Web): 06 Sep 2016
Downloaded from http://pubs.acs.org on September 7, 2016
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A Strategy for Oneꢀpot Conversion of Organic Pollutants
into
Useful
Hydrocarbons
through
Coupling
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Photodegradation of MB with Photoreduction of CO2
†
†
‡
†
†
JianꢀPing Zou, DanꢀDan Wu, Jinming Luo, QiuꢀJu Xing, XuꢀBiao Luo,
†
†
∗
†
§∗
WenꢀHua Dong, ShengꢀLian Luo, HongꢀMei Du, Steven L. Suib
†
Key Laboratory of Jiangxi Province for Persistent Pollutants Control and
Resources Recycle, Nanchang Hangkong University, Nanchang, Jiangxi, P. R.
China
‡
Key Laboratory of Drinking Water Science and Technology, Research Center for
EcoꢀEnvironmental Sciences, Chinese Academy of Sciences, Beijing, P. R. China
§
Department of Chemistry, University of Connecticut, Storrs, CT, USA
ABSTRACT
A strategy was developed to couple photocatalytic oxidation with
photocatalytic reduction technology to realize oneꢀpot conversion of MB into
hydrocarbons for the first time. In this approach, organic pollutants were firstly
decomposed into CO by photodegradation and then the asꢀobtained CO was
2
2
converted into CH
3
OH, C
2
H
5
OH, and CH
4
through photocatalytic reduction of CO
2
under solar spectrum irradiation by using GQDs/VꢀTiO
results show that the 5%GQDs/VꢀTiO has the best photocatalytic activity and the
OH, and CH are 13.24 and 5.65, and 0.445
2
catalysts. The experimental
2
product rates of CH
3
OH, C
2
H
5
4
ꢀ1 ꢀ1
ꢁ
mol·g ·h , respectively. The corresponding apparent quantum efficiency is 4.87%
at 420 nm. The oneꢀpot conversion of MB into hydrocarbons was demonstrated by a
series of experiments. The photocatalytic mechanisms of oneꢀpot conversion of MB
into hydrocarbons were proposed to explain the detailed photocatalytic process.
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KEYWORDS: CO
2
Reduction, GQDs, Organic Pollutants, Photocatalysis,
Photocatalytic Oxidation.
INTRODUCTION
With rapid industrialization and poor management of energy, there are issues
that threaten human health and global environment. Among them, the discharge of
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wastewater and the release of CO are major concerns. Since the first discovery of
Fujishima and Honda in 1972 on the photocatalytic splitting of water over TiO
2
1
electrodes under ultraviolet irradiation, clean and lowꢀcost photocatalysts are used
to tackle the energy crisis and environmental contamination simultaneously. The
photocatalysis technologies are based on oxidation and reduction reactions. For
example, in the degradation of organic pollutants, solar energy is used to oxidize the
2
compounds to CO
2
and H
2
O over catalysts. On the other hand, solar energy is
2 2
converted to chemical energy in reduction processes such as CO reduction and H
3
evolution from water splitting by using catalysts.
Until now, considerable numbers of photocatalysts were explored in order to
study photocatalytic mechanisms and to improve the efficiency of pollutants
degradation or CO
2
reduction. Zou et al. reported good stability and catalytic
performance of a novel heteroꢀstructured fewꢀlayered WS ꢀBi WO /Bi3.84W0.16O6.24
2
2
6
4
composite in RhB degradation under visible light irradiation. Daia et al. observed
excellent photocatalytic performance in the degradation of methylene blue over
5
gꢀC
Ag
3
N
4
/FꢀTiO
2
nanosheets under visible light irradiation. He et al. synthesized
3
PO
4
/gꢀC
3
N
4
composites that show good photocatalytic activity for CO
2
6
reduction driven by visible light. Kuriki et al. reported the use of a Ru/C
3
N
4
7
photocatalyst for the reduction of CO
2
to formic acid. Wang et al. used mesoporous
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ZnCo O nanorods as coꢀcatalyst to promote CO ꢀtoꢀCO conversion.
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4
2
Despite the great many reports on photocatalytic oxidation of organic pollutants
and reduction of CO , there is no report on the combination of these two issues for
2
oneꢀpot conversion of organic pollutants to useful organic compounds. It is urgent to
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develop new catalysts that can fulfill simultaneous photocatalytic oxidation and
reduction.
Graphene quantum dots (GQDs) are graphene fragments that have more
“moleculeꢀlike” character. GQDs show many advantages, such as ease of
preparation, good visible light absorption, and electroꢀoptic properties of quantum
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dots.
In view of the electron acceptor and transporter, photosensitizer, and
upconversion properties of GQDs, we try to combine GQDs with Vanadium doped
TiO
2
(VꢀTiO
2
) in a composite system to use the full spectrum of sunlight and
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effectively separate photoꢀgenerated electrons and holes.
We successfully
combined GQDs with Vꢀdoped TiO
2
mesoporous material to prepare GQDs/VꢀTiO
2
composites with a facile solution method. Herein, we report the design of a
photocatalytic system to realize oneꢀpot conversion of organic pollutants to useful
2
organic compounds by using the GQDs/VꢀTiO catalyst and a mechanism is
proposed to explain the photocatalytic steps involved in the conversion of MB to
methanol, ethanol and methane and to confirm this oneꢀpot conversion. The present
work not only provides an ideal scenario of photodegradation and photoreduction
coupling, but also opens a new avenue to achieve the degradation and better use of
organic pollutants.
RESULTS AND DISCUSSION
CHARACTERIZATIONS
The nanocrystalline VꢀTiO
2
catalysts were synthesized by a previously reported
were obtained by adding GQDs aqueous. As shown
in Figure 1a, the asꢀprepared samples well match with the diffraction patterns of the
anatase TiO (JCPDS 01ꢀ084ꢀ1286). And there are no graphene or impurity of
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procedure and GQDs/VꢀTiO
2
2
vanadium oxide diffraction peaks observed in the Vꢀdoped TiO and GQDs/VꢀTiO2
2
composites. But the existence of GQDs can be confirmed by the below results of
Raman, XPS, SEM, and TEM. Compare with TiO
2
, the diffraction peaks of the
4+
Vꢀdoped TiO has a slight shift to large angles (Fig. S2), suggesting that the Ti in
2
5+
5+
TiO
2
was partially substituted by V because the ionic size of V (0.054 nm) is
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slightly smaller than that of Ti (0.0605 nm). The Raman spectroscopy as a more
sensitive method was carried out. As shown in Figure 1b, all Raman peaks of the
2
asꢀprepared samples show the vibrational modes of the typical anatase TiO , which
−1 15
are 143, 395, 515, and 636 cm . Meanwhile, there are no corresponding vanadium
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oxide peaks among the 5%VꢀTiO and GQD/VꢀTiO samples, which is consistent
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with the results of XRD. The XRD and Raman analyses demonstrate vanadium has
been successfully doped into TiO . As shown in Fig. S3, with the increase of the
GQDs loading amount, the peak intensities of the disordered (D) band at 1350 cm
2
ꢀ1
ꢀ1
3
and the crystalline (G) band at 1590 cm increase, assigned to sp defects and
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inꢀplane vibration of sp carbon in graphene, respectively.
Figure 1. XRD patterns (a) and Raman spectra (b) of the asꢀprepared TiO
2
and
GQDs/VꢀTiO composites with different loadings of GQDs.
2
The morphologies of the asꢀobtained 5%VꢀTiO
2
, and 5%GQDs/VꢀTiO
2
materials
were investigated by SEM and TEM. In Figures 2aꢀb, the asꢀprepared 5%VꢀTiO
2
and 5%GQDs/VꢀTiO show compact and smooth microspheres with an average
2
diameter of 1~2 ꢁm. TEM images (Fig. 2cꢀd) reveal that the microspheres are
composed of a great number of nanoparticles. In Figure 2e, the HRTEM images of
the 5%GQDs/VꢀTiO
surface of VꢀTiO microspheres with diameters of ~10 nm (Fig. 2e inset). And the
lattice spacing distances of 0.35 and 0.229 nm correspond to the (101) planes of the
2
show that the asꢀprepared GQDs are well dispersed on the
2
1
6,17
anatase phase of TiO
2
and the (1120) facet of graphene, respectively.
EDS
analysis confirms the existence of Ti, O, V, and C (Fig. S4 in SI). The above results
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indicate that GQDs are successfully attached to the surfaces of the VꢀTiO
2
. The BET
2
2
of the VꢀTiO microspheres is 255.6 m /g, much larger than that of P25 (70 m /g)
2
2
(Table S1 in SI). And after the loading of GQDs, the BET of the VꢀTiO has a little
increase. And the asꢀprepared samples show Type IV isotherm curves with a
typeꢀH2 hysteresis loop, which is a typical characteristic of mesoporous materials
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(
Fig. S5). The pore size distribution plots (Fig. S5, inset) show that the asꢀprepared
VꢀTiO sample have a narrow pore size distribution, indicating that the samples
2
have very uniform mesopores.
Figure 2. SEM images of 5%VꢀTiO
2
(a) and 5%GQDs/VꢀTiO
2
(b); TEM images of
(e).
5
%VꢀTiO
2
(c) and 5%GQDs/VꢀTiO
2
(d); HRTEM image of 5%GQDs/VꢀTiO
2
In order to investigate the composition and the chemical bonding environment
of the asꢀprepared samples, Xꢀray photoemission spectroscopy (XPS) were carried
out. In the Ti 2p spectrum of TiO , two peaks were observed at 458.5 and 464.4 eV
(Fig. 3a) that is ascribed to Ti 2p3/2 and Ti 2p1/2, respectively. Compared with the
2
1
5
XPS spectra of TiO
2 2 2
, Ti 2p peaks in VꢀTiO and 5%GQDs/VꢀTiO slightly shift
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toward higher binding energy after V doped into TiO lattice. The curve fitting of
O1s (Fig. 3b) was achieved using component peaks. Three peaks can be identified as
−
follows: (1) the peak in the range of 529.5-530.5 eV can be assigned to “O
2
” ions in
the lattice (blue curve); (2) the peak in the range of 531−532 eV can be assigned to
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the adsorbed OH species, O species or oxygen vacancies (red curve); and (3) the
peak in the range of 532.5-533.5 eV can be assigned to the adsorbed molecular
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water (green curve). As shown in Figure 3c, the peak of the C 1s of
%GQDs/VꢀTiO can be fitted with five components. The main peak at binding
5
2
energy of 284.6 eV is assigned to CꢀC bonding in graphene, while the peaks at 285.3
and 286.2eV are related to the CꢀN and CꢀO bonds, and the other two peaks, located
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at 288.0 and 289.1eV, could be assigned to the C=O and OꢀC=O components,
respectively. The peaks at 516.2ꢀ517.4 eV for 5%VꢀTiO and 5%GQDs/VꢀTiO are
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5
+
assigned to V 2p3/2 (Fig. 3e), which are similar to those of V
2
O
5
reported in the
1
4
literature. The results indicate the V atom was successfully doped into TiO
the valence of V did not change after photocatalytic reaction. The XPS survey
spectra of 5%VꢀTiO and 5%GQDs/VꢀTiO show the existence of the Ti 2p, O 1s, V
p and C 1s (Fig. 3d), and the intensity of C 1s of 5%GQDs/VꢀTiO is much higher
than that of 5%VꢀTiO . The above results indicate that the GQDs have successfully
loaded on the surface of 5%VꢀTiO
2
and
2
2
2
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Figure 3. XPS spectra for Ti 2p and V 2P of TiO , 5%VꢀTiO , and
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5%GQDs/VꢀTiO (a); O 1s of TiO , 5%VꢀTiO and 5%GQDs/VꢀTiO (b); C 1s of
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%GQDs/VꢀTiO
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p of 5%VꢀTiO
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and 5%GQDs/VꢀTiO before and after photocatalytic reaction
2
(
e).
PHOTOCATALYTIC ACTIVITY
The simultaneous degradation of MB and reduction of CO
asꢀprepared samples was performed in a closed circulation system. In the dark, there
is no detection of any products. Under solar spectrum irradiation, CH OH, C OH,
and CH can be generated by the asꢀprepared VꢀTiO , and GQDs/VꢀTiO (Fig. 4a).
Compared with VꢀTiO , there is a marked improvement of photocatalytic activity
for GQDs/VꢀTiO . And the loading amount of GQDs has a significant influence on
the photocatalytic activity of GQDs/VꢀTiO . With the increase of the loading
2
over the
3
2 5
H
4
2
2
2
2
2
2
amount of GQDs, the photocatalytic activity of GQDs/VꢀTiO is significantly
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enhanced, and the optimum GQDs content is 5 wt%. The product rate of CH OH,
ꢀ1
ꢀ1
C H OH, and CH is 13.24, 5.65 and 0.445 ꢁmol·g ·h over the 5%GQDs/VꢀTiO ,
2
5
4
2
respectively. The apparent quantum efficiency (AQE) of 5%GQDs/VꢀTiO
2
was
measured under similar conditions with light irradiation at 420 nm by using a
bandꢀpass filter and the corresponding AQE reaches 4.87% at 420 nm (Details can
be seen in SI). This phenomenon can be explained since higher loadings of GQDs
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result in more light absorption of the VꢀTiO (Fig. S6a in SI), and that leads to better
photocatalytic performance, whereas the photocatalytic performance of the catalyst
becomes worse when the loading amount of GQDs is above 5 wt% because much
more GQDs decreases the light absorption of the VꢀTiO
the photocatalytic activities of GQDs and the physical mixture of GQDs and VꢀTiO
were investigated. The results show that there are no photocatalytic products when
using GQDs, whereas the physically mixed GQDs and VꢀTiO has a little better
photocatalytic activity than VꢀTiO but much lower than 5%GQDs/VꢀTiO (Fig. S7
in SI). Besides liquid products of CH OH and C H OH, gaseous CH was found in
2
. In addition, as a reference,
2
2
2
2
3
2
5
4
the photocatalytic process of MB degradation over the asꢀprepared catalysts. The
removal rate of MB can reach above 99% after 8 h solar spectrum irradiation over
5
%GQDs/VꢀTiO
system (Fig. S8 in SI). The total organic carbon (TOC) result (Figs. S9 in SI) also
show that MB can be nearly completely degraded to CO and water over the
2
during the process of oneꢀpot MB conversion in the closed gas
2
5%GQDs/VꢀTiO
2
after 8 h solar spectrum irradiation.
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3 2 5 4
Figure 4. The generation rate of CH OH, C H OH and CH via oneꢀpot conversion
of MB to useful hydrocarbons under solar spectrum irradiation (a) and under visible
light irradiation (λ≥420 nm) (b) over the asꢀprepared catalysts.
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In addition, the photocatalytic activity of the asꢀprepared catalysts under visible
light (λ≥420 nm) was investigated. As shown in Figure 4b, similar to the case under
solar spectrum irradiation, GQDs play a vital role in photocatalytic activity, and the
optimum GQDs content is 5%. The generation rates of CH
3
OH, C
2 5 4
H OH, and CH
ꢀ1 ꢀ1
are 6.7, 4.1 and 0.275 ꢁmol·g ·h over the 5%GQDs/VꢀTiO , respectively. The
2
stability of the 5%GQDs/VꢀTiO
oneꢀpot conversion of MB to useful hydrocarbons. The activity of the
5%GQDs/VꢀTiO is stable across four cycling runs (Fig. 5). There is just a little
decrease for the generation rate of CH OH and C H OH via oneꢀpot conversion of
2
was investigated by a fourꢀrun cycling test of
2
3
2
5
MB to useful hydrocarbons under solar spectrum irradiation after 4 runs. And XPS
of the V 2p transition of 5%GQDs/VꢀTiO before and after irradiation showed that
2
the chemical valence of vanadium did not change (Fig. 3e), indicating the
photocatalytic reduction and oxidation in this system are not due to the conversion
5+
4+
of V /V .
Figure 5. The cycling runs of oneꢀpot conversion of MB to useful hydrocarbons
over the 5%GQDs/VꢀTiO
2
under solar spectrum for 32 h.
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In order to further confirm the role of catalyst and the origin of carbon of
CH OH, C H OH, and CH , a series of reference experiments were done. When MB
3
2
5
4
solution was replaced by deionized water but with the other reaction conditions
unchanged, there is no formation of any products. When highꢀpurity CO was
bubbled into the system containing deionized water, CH OH, C H OH, and CH can
2
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
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9
0
1
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0
1
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0
1
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4
5
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7
8
9
0
3
2
5
4
be generated and the GQDs/VꢀTiO
2
also exhibits the highest photocatalytic activity
(Fig. S10 in SI). In this system, NaOH could act a sacrificial agent of
20,21
photoꢀgenerated holes during the photoreduction of CO
highꢀpurity Ar gas rather than CO was bubbled, no products can be detected.
CH OH, C OH, and CH cannot be detected in the MB solution in the absence of
the GQDs/VꢀTiO under visible light irradiation. The results indicate that the
photoreduction of CO to CH OH, C H OH, and CH is definitely facilitated by the
2
.
In addition, when
2
3
2
H
5
4
2
2
3
2
5
4
catalysts.
PHOTOCATALYTIC MECHANISM
The optical absorption edge and the MottꢀSchottky (MS) plots of VꢀTiO show
2
22
that the conduction band potentials of VꢀTiO
2
is ꢀ0.33 V, (Fig. 6a). Combine with
the optical absorption edge of VꢀTiO
2
2
(Fig. S6b in SI), the EVB of VꢀTiO is
calculated as 2.47 V (ECB = EVB ꢀ E ). Figure 6b shows the upconversion properties
g
of GQDs, the PL spectrum of GQDs excited by long wavelength from 600 to 800
nm with the upconverted emissions located in the range of 454 to 478 nm. This
upconverted PL property of GQDs is attributed to multiphoton active processes
23
similar to those previous reports. In addition, the transient photocurrent response
measurements show that the 5%GQDs/VꢀTiO exhibits the largest photocurrent
2
response among the asꢀprepared catalysts (Fig. 6c).
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2 2
Figure 6. MottꢀSchottky (MS) plots of the TiO and 5%VꢀTiO (a); upconverted PL
properties of GQDs excited by long wavelength from 600 to 800 nm (b);
photocurrent transient responses of TiO , 5%VꢀTiO and GQDs/VꢀTiO samples
2
2
2
2 4 2 4
electrodes in Na SO (Na SO 0.5 M) solution under solar spectrum irradiation (c);
2
removal rate of MB over the 5%GQDs/VꢀTiO after the addition of different
scavengers (d).
To clarify the mechanism of oneꢀpot conversion of MB to methanol, ethanol,
and methane, we performed trapping experiments to investigate the main active
2
species in the photocatalytic oxidation of MB over 5%GQDs/VꢀTiO . Since the
oneꢀpot conversion experiment was carried out in a closed glass gas system in the
+
absence of air, we only considered hydroxyl radicals (·OH) and holes (h ) by adding
24
+
1
.0 mM isopropanol and triethanolamine. From Figure 6d, h is the main active
species and the second is ·OH for the degradation of MB to CO and H O in the
2
2
2
absence of air or O .
The generated intermediates and final products were detected by using LC−MS
and the MB degradation pathway was proposed. As shown in Fig. S11, the MS
fragmentation pattern at m/z = 284 is ascribed to the Cl atom among MB ionized to
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ꢀ
Cl . With the increase of photoreaction time, the characteristic pattern at m/z = 284
significantly decreases, and MS patterns (except for the weak peak at m/z 362)
disappeared after 8 h photoreaction under solar spectrum irradiation, which indicates
the nearly complete mineralization of MB, in consistent with the result of TOC. And
it can be found some mass peaks at m/z 256, 270, 284, 290, 318, 334 and 362,
suggesting the occurrence of the hydroxylations and demethylation reactions of MB
molecules. The peaks at m/z = 256 and 270 are produced by demethylation of MB
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2
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4
5
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8
9
0
2
5
(
–CH
3
elimination), which results in the concomitant N–H formation. And the
signals at m/z = 300, 318, 334 and 362 suggest successive hydroxylations of MB by
·
OH radicals. The probable main routes of mineralization are shown in Fig. S12.
+
During the process of photocatalytic reaction, h and ·OH radicals act as oxidizing
+
agents. For the first rout, ·OH first attacks CꢀS =C functional group of MB to form
+
CꢀS(=O)ꢀC, which was identified by the pattern at m/z 300. And h or ·OH
successively attacked the aromatic ring to generate some intermediates (MS patterns
2
4
ꢀ
ꢀ
at m/z 318, 334, and 362), and ultimately to demineralize to CO , SO , NO and
2
3
25ꢀ27
H
2
O.
NꢀCH
aromatic ring (m/z = 290) to form CO
hydroxylation. Combine with Fig. S11, we know that the first rout could be the
major one of MB mineralization. The results confirm that CH OH, C OH and
CH should be formed from the photoreduction of CO but not from the
As for the second rout, the ·OH radicals abstract H from the –CH
3
to make
3
scission (m/z = 270 and 256), and then ·OH radicals further attack the
ꢀ
2ꢀ
4
2
, H
2
O, NO
3
, and SO
after successive
3
2 5
H
4
2
intermediates during MB degradation according to the proposed pathway.
Figure 7. (a) Inorganic ions detected by ion chromatogram over the
5%GQDs/VꢀTiO
2
of the standard MB solution, the MB solution after 8h reaction
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under solar spectrum irradiation and that after 1 h dark reaction, as well as those in
2
ꢀ
deionized water; (b) The amount of CO3 in the MB solution variation with the
irradiation time in the process of oneꢀpot MB conversion over the
2
5%GQDs/VꢀTiO .
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To further confirm the mineralization of MB in the system, inorganic ions in
ꢀ
the liquid products are analyzed by ion chromatography. The Cl could be detected
at the beginning of the reaction, indicating that the S−Cl bond was the first broken in
−
ꢀ
2−
2ꢀ
the MB degradation process, while NO , NO , SO and CO3 were detected after
3
2
4
the MB molecule was totally oxidized. In addition, compared to those in deionized
water, the standard MB solution and the MB solution under dark reaction for 1 h, the
ꢀ
2ꢀ
3
2ꢀ
4
ꢀ
concentrations of Cl , CO
, SO
3
and NO are markedly improved in the final
solution after 8 h oneꢀpot conversion of MB over the 5%GQDs/VꢀTiO (Fig. 7a),
2
which demonstrates the 5%GQDs/VꢀTiO
mineralization of MB. And the amount of the CO
illumination time and then decrease after 5 h irradiation (Fig. 7b), while the amounts
of CH OH, C OH, and CH increase during the oneꢀpot conversion of MB. The
above results confirm that carbon of CH OH, C OH, and CH originated from the
CO generated from MB degradation but not from the intermediates of MB
2
really possesses the ability of the
2
ꢀ
3
increases with increase of
3
2
H
5
4
3
2
H
5
4
2
degradation or GQDs. Furthermore, the gaseous product of CH
that the 5%GQDs/VꢀTiO catalyst can make CO reduce to CH
oneꢀpot conversion of MB to CH OH, C OH, and CH
photocatalytic system through coupling photodegradation of MB and photoreduction
of CO over the GQDs/VꢀTiO catalyst. As for CO , it is hard to detect it because of
4
further confirms
2
2
4
. In other words,
3
2
H
5
4
can be achieved in the
2
2
2
the low sensitivity of packed column (GCꢀ7900, TCD, HayeSep A) for the trace of
CO2.
In our oneꢀpot study, CH
3 2 5 4
OH, C H OH, and CH are the products, and there is
no detection of CO or HCHO. Based on the results and those reported in the
2
8ꢀ30
literature,
we deduced the possible reaction steps for photocatalytic reduction of
2
ꢀ
CO over 5%GQDs/VꢀTiO
2
2
(Eqs. (1)ꢀ(8)). In Equations (1)ꢀ(3), CO
2
or CO
3
first
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+
combine with an electron and H to form HCOO· that undergoes hydrogenation to
give ·CO and water. The ·CO hydrogenates to form ·CHO that undergoes
hydrogenation to generate ·OCH
3
(Eqs. (4ꢀ5)). The ·OCH
3
species can undergo
3
1
protonation to form CH
3
OH or dimerization/hydrogenation to form C H OH, or
2 5
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8
9
0
+
even react with H to form CH (Eqs. (6ꢀ8)).
4
−
+
CO
2
3
+ e + H → HCOO·
(1)
2
ꢀ
−
+
CO
+ e + 3H → HCOO· + H
2
O
(2)
(3)
(4)
(5)
−
+
2
HCOO· + e + H → ·CO + H O
−
+
·
CO + e + H → ·CHO
−
+
·
3
CHO + 2e + 2H → ·CH O
−
+
·
CH
3
O + e + H → CH
3
OH
(6)
(7)
(8)
−
+
·
CH O + ·CH O + 2e +2H → CH CH OH + H O
3
3
3
2
2
−
+
3 4 2
·CH O + 3e + 3H → CH + H O
Based on the above results and discussion, we put forth a plausible mechanism
for oneꢀpot conversion of MB to methanol, ethanol, and methane over
GQDs/VꢀTiO
2
under solar spectrum and visible light irradiation (Scheme 1). As
shown in Scheme 1, when the GQDs/VꢀTiO
2
catalyst is irradiated under solar
spectrum, GQDs can absorb visible light (600ꢀ800 nm) and then emit short
ꢀ
wavelength light (< 470 nm) that is absorbed by the VꢀTiO
2
to generate electrons (e )
+
and holes (h ). Meanwhile, the VꢀTiO
2
can simultaneously absorb UV light to
ꢀ
+
generate electrons (e ) and holes (h ). The electrons are excited from the VB to CB
of VꢀTiO , and then transferred to the surface of GQDs, whereas the holes in the VB
2
react with MB on the catalyst surface to form CO
photogenerated electrons on the surface of GQDs, they can reduce CO
from MB degradation, to ·COOH, and the ·COOH species that are further reduced
to CH OH, C OH, and CH . However, no products of CO and HCHO in the
2 2
and H O. As for the
2
, originated
3
2
H
5
4
process of photocatalytic reaction were detected because the potential of CB of
32,33
VꢀTiO
2
is more positive than the redox potentials of CO and HCHO.
When the
GQDs/VꢀTiO
2
catalyst is irradiated under visible light, only GQDs can absorb
visible light (600ꢀ800 nm) and then emit short wavelength light (< 470 nm). Then
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ꢀ
2
the VꢀTiO absorbs the emitted UV light (< 450 nm) to generate electrons (e ) and
+
holes (h ). Similar to the case under solar spectrum irradiation, the photogenerated
holes and electrons can effectively separate due to the presence of GQDs, and
further degrade MB to CO
proposed mechanism well illustrates why the photocatalytic activity of the
GQDs/VꢀTiO under visible light irradiation is worse than that under solar spectrum
irradiation. Furthermore, the mechanism also explains the multiple roles of GQDs in
the GQDs/VꢀTiO composite and supports the oneꢀpot conversion process of MB to
2 3 2 5 4
and finally form CH OH, C H OH, and CH . Thus the
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2
2
methanol, ethanol, and methane.
Scheme 1. Proposed mechanism of the oneꢀpot conversion of MB to useful organic
compounds over the 5%GQDs/VꢀTiO2.
CONCLUSION
In summary, for the first time we put forward a novel strategy to couple
photocatalytic oxidation with photocatalytic reduction for oneꢀpot conversion of
organic pollutants to useful organic compounds over the GQDs/VꢀTiO
Experimental results show that 5%GQDs/VꢀTiO performs the best for the
conversion of MB to CH OH, C OH and CH . A photocatalytic mechanism is
2
catalysts.
2
3
2
H
5
4
proposed to illustrate the detailed conversion processes of MB to useful organic
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compounds, and the mechanism well explains the multiple roles of GQDs. The
present work demonstrates that it is possible to have oneꢀpot conversion of organic
pollutants to useful organic compounds. It is a new approach to harness solar energy
while having the problem of environmental pollution and energy crisis tickled
simultaneously.
0
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0
METHOD SECTION
Synthesis of 5%VꢀTiO
2
. All chemicals were analytical grade and used without
materials were
further purification. The vanadium doped mesoporous TiO
2
synthesized via a modified nonaqueous solvothermal process based on the previous
14,15
reported method.
In a typical synthesis, 0.05 mL of diethylenetriamine (DETA,
9
9%, NewTop Shang hai) was added to 40 mL of isopropyl alcohol under vigorous
stirring, and then 0.06 mL of triisopropoxyvanadium (97%, Redꢀchemical) and 1.5
mL of titanium isopropoxide (TIP, 99.999%, aladdin) was added under vigorous
stirring. The mixture was stirred for 10 min and then transferred to a 100 mL
Teflonꢀlined stainlessꢀsteel autoclave. After treatment at 200℃ for 24 h,
yellowꢀbrown precipitate was collected by centrifugation, washed with ethanol and
deionized several times. After dried at 110℃ , yellowishꢀbrown power of 5%VꢀTiO
was obtained.
2
34
Synthesis of GQDs/VꢀTiO . GQDs was synthesized by the reported method.
2
Typically, 0.525 g urea and 0.6006 g citric acid were dissolved into 12 mL
deionized water, and stirred to form a clear solution. Then the solution was
transferred into a 25 mL Teflonꢀlined stainlessꢀsteel autoclave, treatment at 160℃
for 8 h to obtain lightꢀyellow GQDs solution. And then 0.1025 g of the asꢀprepared
5%VꢀTiO
deionized water, and then the aqueous solution was vigorously stirred for 24h.
Finally, 5%GQDs/VꢀTiO composites were collected by centrifugation and dried at
0 ℃ for 4 h. The 1%GQDs/VꢀTiO and 10%GQDs/VꢀTiO were prepared by the
2
sample and 2.5 mL GQDs (0.205 mg/mL) was dispersed into 10 mL
2
8
2
2
same procedure with the addition of 0.5 mL and 5 mL GQDs, respectively.
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Test of photocatalytic activity. The oneꢀpot conversion of MB to the alcohols
was carried out in a Pyrex topꢀirradiation reaction vessel connected to a glass closed
gas system (Prefect Light, Beijing, LabsolarꢀIII (AG), as shown in Fig. S1). A 300
2
W Xe lamp (Wavelength range: 320 nm ≤ λ ≤ 780 nm, light intensity: 160 mW/cm )
10
11
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43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
was used as the source of simulated sunlight. The catalyst (50 mg) was dispersed (by
magnetic stirring) in a 50 mL solution containing MB (8mg/L) and NaOH (0.01 M).
The whole reaction setup was evacuated by using vacuum pump several times for
complete removal air, and highꢀpurity Ar was flowed into the system until reaching
ambient pressure. Before irradiation, the solution was stirred for 1 h in the dark to
reach adsorptionꢀdesorption equilibrium of photocatalyst and MB solution. The
temperature of the reaction solution was maintained at 6 ℃ by an external flow of
cold water (constant temperature device XODCꢀ0506, Nanjing Shunliu, China)
during the reaction to maintain the good lightꢀabsorbing ability of the catalysts. The
4
product of CH is online sampled (the volume is 1 mL) and analyzed by an online
gas chromatograph (GCꢀ7900, TCD, HayeSep A, Ar carrier, Shanghai Tianmei). For
the yield of liquid, a certain amount of solution was taken from the reaction cell at
3 2 5
given time intervals for subsequent analysis of CH OH and C H OH concentration
using a gas chromatograph (GCꢀ7890, FID, DBꢀWAX, Agilent., America), as well
as analysis of MB concentration with a UVꢀvis spectrophotometer (Hitachi
2
Uꢀ3900H). The photocatalytic reduction of CO (99.999%) was also examined
following the same process but having the MB solution replaced by deionized water.
After the reaction setup was evacuated several times, pure CO (99.999%) was
2
flowed into the system until reaching ambient pressure.
Analytic methods. The solution was detected by recording the absorbance at
the characteristic band of 664 nm using a Hitachi Uꢀ3900H UVꢀvisible
spectrophotometer and the total organic carbon (TOC) was measured by a Shimadzu
TOC (TOCꢀL CSH CN200). The intermediates were detected by LCꢀMS (Thermo,
Finnigan, LCQꢀDeca xp) equipped with an electrospray ionization (ESI) source. The
samples were injected at a flow rate of 0.2 mL/min under isocratic conditions. The
ion mode was set on positive mode and the mobile phase was methanol–water (0.1%
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formic acid) (60:40, v/v). 2 uL extract was injected using the auto sampler. Different
ꢀ
2ꢀ
3
2ꢀ
ꢀ
ions (including Cl , CO , SO and NO ) generated in the oneꢀpot conversion of
4
3
MB were analyzed by using an ion chromatograph (IC, DIONEX) equipped with
TM
Dionex IonPac
AS18 columns (eluent: 23mM KOH, Dionex EGCꢀKOH II
0
1
2
3
4
5
6
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0
1
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8
9
0
Cartridge). The flow rate is 1mL/min and the applied current is 57 mA.
ASSOCIATED CONTENT
Supporting Information. The details of Characterizations,apparent quantum
efficiency calculation and results of XRD, Raman spectra, EDS spectra, DSR
spectra of the asꢀprepared catalysts, UVꢀvis absorption spectra and TOC of MB in
the process of oneꢀpot conversion over the 5%GQDs/VꢀTiO
CH OH, C OH and CH from the photoreduction of pure CO
asꢀprepared catalysts under solar spectrum irradiation, as well as the results of MS
2
, Generation rate of
3
2
H
5
4
2
by using the
and proposed degradation pathway of MB by the 5%GQDs/VꢀTiO
free of charge via the Internet at http://pubs.acs.org.
2
are available
AUTHOR INFORMATION
Corresponding Author
* Eꢀmail: sllou@hnu.edu.cn (S.ꢀL. Luo); Tel: +86 791 83863688; Fax: +86 791
3953373 and Eꢀmail: steven.suib@uconn.edu (S. L. Suib); Fax: +1 860 4862981
8
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
We gratefully acknowledge the financial support of the NSF of China (20801026,
51238002, 51272099, and 51622806), the NSF of Jiangxi Province (KJLD12002,
2
0133ACB21001, 20122BCB23013, and 20114BAB203005), Opening Foundation
of the Chinese National Engineering Research Center for Control and Treatment of
Heavy metal Pollution (No. 2015CNERCꢀCTHMPꢀ02), the Foundation of State Key
Laboratory of Structural Chemistry (20100015), and the graduate student innovation
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fund of Jiangxi province (YC2014ꢀS381). SLS acknowledges the support of the US
Department of Energy, Office of Basic Energy sciences, Division of Chemical,
Geological, and Biological Sciences under grant DEꢀFG02ꢀ86ER13622.A000.
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