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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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ACS Catalysis

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,† Sheng-Lian Luo,†∗ Hong-Mei 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

Eco-Environmental 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 one-pot conversion of MB into hydrocarbons for the first time. In this approach, organic pollutants were firstly 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 the 5%GQDs/V-TiO2 has the best photocatalytic activity and the product rates of CH3OH, C2H5OH, and CH4 are 13.24 and 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.

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KEYWORDS: CO2 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 wastewater and the release of CO2 are major concerns. Since the first discovery of Fujishima and Honda in 1972 on the photocatalytic splitting of water over TiO2 electrodes under ultraviolet irradiation,1 clean and low-cost 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 compounds to CO2 and H2O over catalysts.2 On the other hand, solar energy is converted to chemical energy in reduction processes such as CO2 reduction and H2 evolution from water splitting by using catalysts.3 Until now, considerable numbers of photocatalysts were explored in order to study photocatalytic mechanisms and to improve the efficiency of pollutants degradation or CO2 reduction. Zou et al. reported good stability and catalytic performance of a novel hetero-structured few-layered WS2-Bi2WO6/Bi3.84W0.16O6.24 composite in RhB degradation under visible light irradiation.4 Daia et al. observed excellent photocatalytic performance in the degradation of methylene blue over g-C3N4/F-TiO2 nanosheets under visible light irradiation.5 He et al. synthesized Ag3PO4/g-C3N4 composites that show good photocatalytic activity for CO2 reduction driven by visible light.6 Kuriki et al. reported the use of a Ru/C3N4 photocatalyst for the reduction of CO2 to formic acid.7 Wang et al. used mesoporous ZnCo2O4 nanorods as co-catalyst to promote CO2-to-CO conversion.8 Despite the great many reports on photocatalytic oxidation of organic pollutants and reduction of CO2, there is no report on the combination of these two issues for one-pot 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 “molecule-like” character. GQDs show many advantages, such as ease of preparation, good visible light absorption, and electro-optic properties of quantum dots.9-11 In view of the electron acceptor and transporter, photosensitizer, and upconversion properties of GQDs, we try to combine GQDs with Vanadium doped TiO2 (V-TiO2) in a composite system to use the full spectrum of sunlight and effectively separate photo-generated electrons and holes.12-13 We successfully combined GQDs with V-doped TiO2 mesoporous material to prepare GQDs/V-TiO2 composites with a facile solution method. Herein, we report the design of a photocatalytic system to realize one-pot conversion of organic pollutants to useful organic compounds by using the GQDs/V-TiO2 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 one-pot 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 V-TiO2 catalysts were synthesized by a previously reported procedure14 and GQDs/V-TiO2 were obtained by adding GQDs aqueous. As shown in Figure 1a, the as-prepared samples well match with the diffraction patterns of the anatase TiO2 (JCPDS 01-084-1286). And there are no graphene or impurity of vanadium oxide diffraction peaks observed in the V-doped TiO2 and GQDs/V-TiO2 composites. But the existence of GQDs can be confirmed by the below results of Raman, XPS, SEM, and TEM. Compare with TiO2, the diffraction peaks of the V-doped TiO2 has a slight shift to large angles (Fig. S2), suggesting that the Ti4+ in TiO2 was partially substituted by V5+ because the ionic size of V5+ (0.054 nm) is 3


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slightly smaller than that of Ti4+ (0.0605 nm).14 The Raman spectroscopy as a more sensitive method was carried out. As shown in Figure 1b, all Raman peaks of the as-prepared samples show the vibrational modes of the typical anatase TiO2, which are 143, 395, 515, and 636 cm−1.15 Meanwhile, there are no corresponding vanadium oxide peaks among the 5%V-TiO2 and GQD/V-TiO2 samples, which is consistent with the results of XRD. The XRD and Raman analyses demonstrate vanadium has been successfully doped into TiO2. As shown in Fig. S3, with the increase of the GQDs loading amount, the peak intensities of the disordered (D) band at 1350 cm-1 and the crystalline (G) band at 1590 cm-1 increase, assigned to sp3 defects and in-plane vibration of sp2 carbon in graphene,16 respectively.

Figure 1. XRD patterns (a) and Raman spectra (b) of the as-prepared TiO2 and GQDs/V-TiO2 composites with different loadings of GQDs. The morphologies of the as-obtained 5%V-TiO2, and 5%GQDs/V-TiO2 materials were investigated by SEM and TEM. In Figures 2a-b, the as-prepared 5%V-TiO2 and 5%GQDs/V-TiO2 show compact and smooth microspheres with an average diameter of 1~2 µm. TEM images (Fig. 2c-d) reveal that the microspheres are composed of a great number of nanoparticles. In Figure 2e, the HRTEM images of the 5%GQDs/V-TiO2 show that the as-prepared GQDs are well dispersed on the surface of V-TiO2 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 anatase phase of TiO2 and the (1120) facet of graphene, respectively.16,17 EDS analysis confirms the existence of Ti, O, V, and C (Fig. S4 in SI). The above results 4


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indicate that GQDs are successfully attached to the surfaces of the V-TiO2. The BET of the V-TiO2 microspheres is 255.6 m2/g, much larger than that of P25 (70 m2/g) (Table S1 in SI). And after the loading of GQDs, the BET of the V-TiO2 has a little increase. And the as-prepared samples show Type IV isotherm curves with a type-H2 hysteresis loop, which is a typical characteristic of mesoporous materials (Fig. S5). The pore size distribution plots (Fig. S5, inset) show that the as-prepared V-TiO2 sample have a narrow pore size distribution, indicating that the samples have very uniform mesopores.

Figure 2. SEM images of 5%V-TiO2 (a) and 5%GQDs/V-TiO2 (b); TEM images of 5%V-TiO2 (c) and 5%GQDs/V-TiO2 (d); HRTEM image of 5%GQDs/V-TiO2 (e). In order to investigate the composition and the chemical bonding environment of the as-prepared samples, X-ray photoemission spectroscopy (XPS) were carried out. In the Ti 2p spectrum of TiO2, 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.15 Compared with the XPS spectra of TiO2, Ti 2p peaks in V-TiO2 and 5%GQDs/V-TiO2 slightly shift 5


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toward higher binding energy after V doped into TiO2 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 “O2−” ions in the lattice (blue curve); (2) the peak in the range of 531−532 eV can be assigned to 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 water (green curve).15 As shown in Figure 3c, the peak of the C 1s of 5%GQDs/V-TiO2 can be fitted with five components. The main peak at binding energy of 284.6 eV is assigned to C-C bonding in graphene, while the peaks at 285.3 and 286.2eV are related to the C-N and C-O bonds, and the other two peaks, located at 288.0 and 289.1eV, could be assigned to the C=O and O-C=O components,18,19 respectively. The peaks at 516.2-517.4 eV for 5%V-TiO2 and 5%GQDs/V-TiO2 are assigned to V5+ 2p3/2 (Fig. 3e), which are similar to those of V2O5 reported in the literature.14 The results indicate the V atom was successfully doped into TiO2 and the valence of V did not change after photocatalytic reaction. The XPS survey spectra of 5%V-TiO2 and 5%GQDs/V-TiO2 show the existence of the Ti 2p, O 1s, V 2p and C 1s (Fig. 3d), and the intensity of C 1s of 5%GQDs/V-TiO2 is much higher than that of 5%V-TiO2. The above results indicate that the GQDs have successfully loaded on the surface of 5%V-TiO2.

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Figure 3. XPS spectra for Ti 2p and V 2P of TiO2, 5%V-TiO2, and 5%GQDs/V-TiO2 (a); O 1s of TiO2, 5%V-TiO2 and 5%GQDs/V-TiO2 (b); C 1s of 5%GQDs/V-TiO2 (c); survey scans of 5%V-TiO2 and 5%GQDs/V-TiO2 (d), and V 2p of 5%V-TiO2 and 5%GQDs/V-TiO2 before and after photocatalytic reaction (e). PHOTOCATALYTIC ACTIVITY The simultaneous degradation of MB and reduction of CO2 over the as-prepared samples was performed in a closed circulation system. In the dark, there is no detection of any products. Under solar spectrum irradiation, CH3OH, C2H5OH, and CH4 can be generated by the as-prepared V-TiO2, and GQDs/V-TiO2 (Fig. 4a). Compared with V-TiO2, there is a marked improvement of photocatalytic activity for GQDs/V-TiO2. And the loading amount of GQDs has a significant influence on the photocatalytic activity of GQDs/V-TiO2. With the increase of the loading amount of GQDs, the photocatalytic activity of GQDs/V-TiO2 is significantly 7


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enhanced, and the optimum GQDs content is 5 wt%. The product rate of CH3OH, C2H5OH, and CH4 is 13.24, 5.65 and 0.445 µmol·g-1·h-1 over the 5%GQDs/V-TiO2, respectively. The apparent quantum efficiency (AQE) of 5%GQDs/V-TiO2 was measured under similar conditions with light irradiation at 420 nm by using a band-pass 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 result in more light absorption of the V-TiO2 (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 V-TiO2. In addition, as a reference, the photocatalytic activities of GQDs and the physical mixture of GQDs and V-TiO2 were investigated. The results show that there are no photocatalytic products when using GQDs, whereas the physically mixed GQDs and V-TiO2 has a little better photocatalytic activity than V-TiO2 but much lower than 5%GQDs/V-TiO2 (Fig. S7 in SI). Besides liquid products of CH3OH and C2H5OH, gaseous CH4 was found in the photocatalytic process of MB degradation over the as-prepared catalysts. The removal rate of MB can reach above 99% after 8 h solar spectrum irradiation over 5%GQDs/V-TiO2 during the process of one-pot MB conversion in the closed gas 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 CO2 and water over the 5%GQDs/V-TiO2 after 8 h solar spectrum irradiation.

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Figure 4. The generation rate of CH3OH, C2H5OH and CH4 via one-pot conversion of MB to useful hydrocarbons under solar spectrum irradiation (a) and under visible light irradiation (λ≥420 nm) (b) over the as-prepared catalysts. In addition, the photocatalytic activity of the as-prepared 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 CH3OH, C2H5OH, and CH4 are 6.7, 4.1 and 0.275 µmol·g-1·h-1 over the 5%GQDs/V-TiO2, respectively. The stability of the 5%GQDs/V-TiO2 was investigated by a four-run cycling test of one-pot conversion of MB to useful hydrocarbons. The activity of the 5%GQDs/V-TiO2 is stable across four cycling runs (Fig. 5). There is just a little decrease for the generation rate of CH3OH and C2H5OH via one-pot conversion of MB to useful hydrocarbons under solar spectrum irradiation after 4 runs. And XPS of the V 2p transition of 5%GQDs/V-TiO2 before and after irradiation showed that 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 of V5+/V4+.

Figure 5. The cycling runs of one-pot conversion of MB to useful hydrocarbons over the 5%GQDs/V-TiO2 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 CH3OH, C2H5OH, and CH4, a series of reference experiments were done. When MB solution was replaced by deionized water but with the other reaction conditions unchanged, there is no formation of any products. When high-purity CO2 was bubbled into the system containing deionized water, CH3OH, C2H5OH, and CH4 can be generated and the GQDs/V-TiO2 also exhibits the highest photocatalytic activity (Fig. S10 in SI). In this system, NaOH could act a sacrificial agent of photo-generated holes during the photoreduction of CO2.20,21 In addition, when high-purity Ar gas rather than CO2 was bubbled, no products can be detected. CH3OH, C2H5OH, and CH4 cannot be detected in the MB solution in the absence of the GQDs/V-TiO2 under visible light irradiation. The results indicate that the photoreduction of CO2 to CH3OH, C2H5OH, and CH4 is definitely facilitated by the catalysts. PHOTOCATALYTIC MECHANISM The optical absorption edge and the Mott-Schottky (MS) plots of V-TiO2 show that the conduction band potentials of V-TiO2 is -0.33 V,22 (Fig. 6a). Combine with the optical absorption edge of V-TiO2 (Fig. S6b in SI), the EVB of V-TiO2 is calculated as 2.47 V (ECB = EVB - Eg). Figure 6b shows the upconversion properties 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 similar to those previous reports.23 In addition, the transient photocurrent response measurements show that the 5%GQDs/V-TiO2 exhibits the largest photocurrent response among the as-prepared catalysts (Fig. 6c).

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Figure 6. Mott-Schottky (MS) plots of the TiO2 and 5%V-TiO2 (a); upconverted PL properties of GQDs excited by long wavelength from 600 to 800 nm (b); photocurrent transient responses of TiO2, 5%V-TiO2 and GQDs/V-TiO2 samples electrodes in Na2SO4 (Na2SO4 0.5 M) solution under solar spectrum irradiation (c); removal rate of MB over the 5%GQDs/V-TiO2 after the addition of different scavengers (d). To clarify the mechanism of one-pot conversion of MB to methanol, ethanol, and methane, we performed trapping experiments to investigate the main active species in the photocatalytic oxidation of MB over 5%GQDs/V-TiO2. Since the one-pot 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 1.0 mM isopropanol and triethanolamine.24 From Figure 6d, h+ is the main active species and the second is ·OH for the degradation of MB to CO2 and H2O in the absence of air or O2. 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 11


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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 (–CH3 elimination), which results in the concomitant N–H formation.25 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 C-S+=C functional group of MB to form C-S(=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 at m/z 318, 334, and 362), and ultimately to demineralize to CO2, SO42-, NO3- and H2O.25-27 As for the second rout, the ·OH radicals abstract H from the –CH3 to make N-CH3 scission (m/z = 270 and 256), and then ·OH radicals further attack the aromatic ring (m/z = 290) to form CO2, H2O, NO3-, and SO42- after successive hydroxylation. Combine with Fig. S11, we know that the first rout could be the major one of MB mineralization. The results confirm that CH3OH, C2H5OH and CH4 should be formed from the photoreduction of CO2 but not from the intermediates during MB degradation according to the proposed pathway.

Figure 7. (a) Inorganic ions detected by ion chromatogram over the 5%GQDs/V-TiO2 of the standard MB solution, the MB solution after 8h reaction 12


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under solar spectrum irradiation and that after 1 h dark reaction, as well as those in deionized water; (b) The amount of CO32- in the MB solution variation with the irradiation

time

in

the

process

of

one-pot

MB

conversion

over

the

5%GQDs/V-TiO2 . 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 the MB degradation process, while NO3−, NO2-, SO42− and CO32- were detected after 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 concentrations of Cl-, CO32-, SO42- and NO3- are markedly improved in the final solution after 8 h one-pot conversion of MB over the 5%GQDs/V-TiO2 (Fig. 7a), which demonstrates the 5%GQDs/V-TiO2 really possesses the ability of the mineralization of MB. And the amount of the CO32- increases with increase of illumination time and then decrease after 5 h irradiation (Fig. 7b), while the amounts of CH3OH, C2H5OH, and CH4 increase during the one-pot conversion of MB. The above results confirm that carbon of CH3OH, C2H5OH, and CH4 originated from the CO2 generated from MB degradation but not from the intermediates of MB degradation or GQDs. Furthermore, the gaseous product of CH4 further confirms that the 5%GQDs/V-TiO2 catalyst can make CO2 reduce to CH4. In other words, one-pot conversion of MB to CH3OH, C2H5OH, and CH4 can be achieved in the photocatalytic system through coupling photodegradation of MB and photoreduction of CO2 over the GQDs/V-TiO2 catalyst. As for CO2, it is hard to detect it because of the low sensitivity of packed column (GC-7900, TCD, HayeSep A) for the trace of CO2. In our one-pot study, CH3OH, C2H5OH, and CH4 are the products, and there is no detection of CO or HCHO. Based on the results and those reported in the literature,28-30 we deduced the possible reaction steps for photocatalytic reduction of CO2 over 5%GQDs/V-TiO2 (Eqs. (1)-(8)). In Equations (1)-(3), CO2 or CO32- first 13


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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 ·OCH3 (Eqs. (4-5)). The ·OCH3 species can undergo protonation to form CH3OH or dimerization/hydrogenation to form C2H5OH,31 or even react with H+ to form CH4 (Eqs. (6-8)). CO2 + e− + H+ → HCOO·

(1)

CO32- + e− + 3H+ → HCOO· + H2O

(2)

HCOO· + e− + H+ → ·CO + H2O

(3)

·CO + e− + H+ → ·CHO

(4)

·CHO + 2e− + 2H+ → ·CH3O

(5)

·CH3O + e− + H+ → CH3OH

(6)

·CH3O + ·CH3O + 2e− +2H+ → CH3CH2OH + H2O

(7)

·CH3O + 3e− + 3H+ → CH4 + H2O

(8)

Based on the above results and discussion, we put forth a plausible mechanism for one-pot conversion of MB to methanol, ethanol, and methane over GQDs/V-TiO2 under solar spectrum and visible light irradiation (Scheme 1). As shown in Scheme 1, when the GQDs/V-TiO2 catalyst is irradiated under solar spectrum, GQDs can absorb visible light (600-800 nm) and then emit short wavelength light (< 470 nm) that is absorbed by the V-TiO2 to generate electrons (e-) and holes (h+). Meanwhile, the V-TiO2 can simultaneously absorb UV light to generate electrons (e-) and holes (h+). The electrons are excited from the VB to CB of V-TiO2, and then transferred to the surface of GQDs, whereas the holes in the VB react with MB on the catalyst surface to form CO2 and H2O. As for the photogenerated electrons on the surface of GQDs, they can reduce CO2, originated from MB degradation, to ·COOH, and the ·COOH species that are further reduced to CH3OH, C2H5OH, and CH4. However, no products of CO and HCHO in the process of photocatalytic reaction were detected because the potential of CB of V-TiO2 is more positive than the redox potentials of CO and HCHO.32,33 When the GQDs/V-TiO2 catalyst is irradiated under visible light, only GQDs can absorb visible light (600-800 nm) and then emit short wavelength light (< 470 nm). Then 14


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the V-TiO2 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 CO2 and finally form CH3OH, C2H5OH, and CH4. Thus the proposed mechanism well illustrates why the photocatalytic activity of the GQDs/V-TiO2 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/V-TiO2 composite and supports the one-pot conversion process of MB to methanol, ethanol, and methane.

Scheme 1. Proposed mechanism of the one-pot conversion of MB to useful organic compounds over the 5%GQDs/V-TiO2. CONCLUSION In summary, for the first time we put forward a novel strategy to couple photocatalytic oxidation with photocatalytic reduction for one-pot conversion of organic pollutants to useful organic compounds over the GQDs/V-TiO2 catalysts. Experimental results show that 5%GQDs/V-TiO2 performs the best for the conversion of MB to CH3OH, C2H5OH and CH4. A photocatalytic mechanism is proposed to illustrate the detailed conversion processes of MB to useful organic 15


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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 one-pot 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. METHOD SECTION Synthesis of 5%V-TiO2. All chemicals were analytical grade and used without further purification. The vanadium doped mesoporous TiO2 materials were synthesized via a modified nonaqueous solvothermal process based on the previous reported method.14,15 In a typical synthesis, 0.05 mL of diethylenetriamine (DETA, 99%, NewTop Shang hai) was added to 40 mL of isopropyl alcohol under vigorous stirring, and then 0.06 mL of triisopropoxyvanadium (97%, Red-chemical) 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 Teflon-lined stainless-steel autoclave. After treatment at 200℃ for 24 h, yellow-brown precipitate was collected by centrifugation, washed with ethanol and deionized several times. After dried at 110℃, yellowish-brown power of 5%V-TiO2 was obtained. Synthesis of GQDs/V-TiO2. GQDs was synthesized by the reported method.34 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 Teflon-lined stainless-steel autoclave, treatment at 160℃ for 8 h to obtain light-yellow GQDs solution. And then 0.1025 g of the as-prepared 5%V-TiO2 sample and 2.5 mL GQDs (0.205 mg/mL) was dispersed into 10 mL deionized water, and then the aqueous solution was vigorously stirred for 24h. Finally, 5%GQDs/V-TiO2 composites were collected by centrifugation and dried at 80 ℃ for 4 h. The 1%GQDs/V-TiO2 and 10%GQDs/V-TiO2 were prepared by the same procedure with the addition of 0.5 mL and 5 mL GQDs, respectively.

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Test of photocatalytic activity. The one-pot conversion of MB to the alcohols was carried out in a Pyrex top-irradiation reaction vessel connected to a glass closed gas system (Prefect Light, Beijing, Labsolar-III (AG), as shown in Fig. S1). A 300 W Xe lamp (Wavelength range: 320 nm ≤ λ ≤ 780 nm, light intensity: 160 mW/cm2) 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 high-purity Ar was flowed into the system until reaching ambient pressure. Before irradiation, the solution was stirred for 1 h in the dark to reach adsorption-desorption 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 XODC-0506, Nanjing Shunliu, China) during the reaction to maintain the good light-absorbing ability of the catalysts. The product of CH4 is online sampled (the volume is 1 mL) and analyzed by an online gas chromatograph (GC-7900, TCD, HayeSep A, Ar carrier, Shanghai Tianmei). For the yield of liquid, a certain amount of solution was taken from the reaction cell at given time intervals for subsequent analysis of CH3OH and C2H5OH concentration using a gas chromatograph (GC-7890, FID, DB-WAX, Agilent., America), as well as analysis of MB concentration with a UV-vis spectrophotometer (Hitachi U-3900H). The photocatalytic reduction of CO2 (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 CO2 (99.999%) was 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 U-3900H UV-visible spectrophotometer and the total organic carbon (TOC) was measured by a Shimadzu TOC (TOC-L CSH CN200). The intermediates were detected by LC-MS (Thermo, Finnigan, LCQ-Deca 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% 17


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formic acid) (60:40, v/v). 2 uL extract was injected using the auto sampler. Different ions (including Cl-, CO32-, SO42- and NO3-) generated in the one-pot conversion of MB were analyzed by using an ion chromatograph (IC, DIONEX) equipped with Dionex IonPacTM AS18 columns (eluent: 23mM KOH, Dionex EGC-KOH II 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 as-prepared catalysts, UV-vis absorption spectra and TOC of MB in the process of one-pot conversion over the 5%GQDs/V-TiO2, Generation rate of CH3OH, C2H5OH and CH4 from the photoreduction of pure CO2 by using the as-prepared catalysts under solar spectrum irradiation, as well as the results of MS and proposed degradation pathway of MB by the 5%GQDs/V-TiO2 are available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (S.-L. Luo); Tel: +86 791 83863688; Fax: +86 791 83953373 and E-mail: [email protected] (S. L. Suib); Fax: +1 860 4862981 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, 20133ACB21001, 20122BCB23013, and 20114BAB203005), Opening Foundation of the Chinese National Engineering Research Center for Control and Treatment of Heavy metal Pollution (No. 2015CNERC-CTHMP-02), the Foundation of State Key Laboratory of Structural Chemistry (20100015), and the graduate student innovation 18


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fund of Jiangxi province (YC2014-S381). SLS acknowledges the support of the US Department of Energy, Office of Basic Energy sciences, Division of Chemical, Geological, and Biological Sciences under grant DE-FG02-86ER13622.A000. REFERENCES (1) Fujishima, A.; Honda, K. Nature 1972, 238, 37-8. (2) Wang, Q.; jia, Y.; Wang, M.; Qi, W.; Pang, Y.; Cui, X.; Ji, W. J. Phys. Chem. C. 2015, 119, 22066-22071. (3) Zhou, W.; Li, W.; Wang, J.; Qu, Y; Yang, Y.; Xie, Y.; Zhang, K.; Wang, L.; Fu, H.; Zhao, D. J. Am. Chem. Soc. 2014, 136, 9280-9283. (4) Zou, J.; Ma, J.; Yu, J.; He, J.; Meng, Y.; Luo, Z.; Luo, S.; Luo, X.; Suib, S. L. Appl. Catal. B-Environ. 2015, 179, 220-228. (5) Daia, K.; Lu, L.; Liang, C.; Liu, Q.; Zhu, G. Appl. Catal. B-Environ. 2014, 156-157, 331-340. (6) He, Y.; Zhang, L.; Teng, B.; Fan, M. Environ. Sci. Technol. 2015, 49, 649-656. (7) Kuriki, R.; Sekizawa, K.; Ishitani, O.; Maeda, K. Angew. Chem. Int. Ed. 2015, 54, 1-5. (8) Wang, S.; Ding, Z.; Wang, X. Chem. Comm. 2014, 00, 1-3. (9) M.; Bacon, Bradley, S. J.; Nann, T. Part. Part. Syst. Charact. 2013. (10) Yeh, T. F.; Teng, C. Y.; Chen, S. J.; Teng, H. Adv. Mater. 2014, 26, 3297-3303. (11) Peng, J.; Gao, W.; Gupta, B. K.; Liu, Z.; Song, L.; Marti, A. A.; Hayashi, T.; Zhu, J.; Ajayan, P. M. Nano. Lett. 2012, 12, 844-849. (12) Liu, H.; Wu, Y.; Zhang, J. ACS Appl. Mater. Inter. 2011, 3, 1757-1764. (13) Agegnehub, A. K.; Pan, C.; Tsaib, M.; Rick, J.; Sua, W.; Lee, J.; Hwang, B. Int. J. Hydrogen Energ. 2016, 41, 6752-6762. (14) Chen, J. S.; Tan, Y. L.; Li C. M.; Cheah, Y. L.; Luan, D.; Madhavi, S.; Lou, X. W. J. Am. Chem. Soc. 2010, 132, 6124-6130. (15) Luo, Z.; Poyraz, A. S.; Kuo, C. H.; Miao, R.; Meng, Y.; Chen, S. Y.; Jiang, T.; Wenos, C.; Suib, S. L. Chem. Mater. 2015, 27, 6-17.

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A Strategy for One-Pot Conversion of Organic ... - ACS Publications

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