Bi2MoO6 Composite for Partial Oxidation of

Subscriber access provided by REGIS UNIV

Article

Synthesis of TiO2/Bi2MoO6 Composite for Partial Oxidation of Aromatic Alkanes under Visible-light Illumination Lu-Na Song, Feng Ding, Ya-Kun Yang, Du Ding, Lang Chen, Chak-Tong Au, and Shuang-Feng Yin ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04403 • Publication Date (Web): 21 Oct 2018 Downloaded from http://pubs.acs.org on October 22, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

Synthesis of TiO2/Bi2MoO6 Composite for Partial Oxidation of Aromatic Alkanes under Visible-light Illumination Lu-Na Songa, Feng Dinga, Ya-Kun Yanga, Du Dinga, Lang Chena,*, Chak-Tong Aub, and ShuangFeng Yina,* a

State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and

Chemical Engineering, Provincial Hunan Key Laboratory for Cost-effective Utilization of Fossil Fuel Aimed at Reducing Carbon-dioxide Emissions, Hunan University, Lushan South Road, Changsha 410082, Hunan, People’s Republic of China. b

College of Chemistry and Chemical Engineering, Hunan Institute of Engineering, Fuxing East

Road, Xiangtan 411104, Hunan, People’s Republic of China E-mail address: [email protected] (L Chen), [email protected] (SF Yin). KEYWORDS: Photocatalysis; Selective oxidation; Aromatic alkane; Benzaldehyde; TiO2/Bi2MoO6 composite

1 Environment ACS Paragon Plus

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Novel TiO2/Bi2MoO6 heterostructures with TiO2 particles loaded on Bi2MoO6 sheet were fabricated by a simple solvothermal process. A series of characterization techniques such as SEM, TEM, XRD, XPS, UV-vis DRS and electrochemical impedance approach were involved to study the physicochemical properties of the as-prepared materials. The TiO2/Bi2MoO6 composites were used for partial oxidation of aromatic alkanes into benzaldehyde and its derivatives using O2 as oxidant under the irradiation of visible light. At a TiO2-toBi2MoO6 molar ratio of 0.10, the composite exhibits excellent performance with benzaldehyde production rate being 1036.8 μmol g-1 h-1. The outstanding performance is ascribed to enhanced separation of charge carriers by loading TiO2 nanoparticles. The as-prepared composites are reusable photocatalysts for partial oxidation of aromatic alkanes under ambient conditions.

INTRODUCTION It is of great significance to selectively activate and transform sp3 C–H bonds in aromatic alkanes to value-added products.1 Conventional thermocatalytic approaches for this kind of reactions usually involve the use of metal catalysts. Most of them are conducted at high temperature and/or high pressure, and the shortcomings are high energy consumption and generation of hazardous wastes.2,3 Therefore, to develop facile and green routes to obtain the target products with high selectivity under mild conditions is highly desired.4 Heterogeneous photocatalysis is regarded as one of the most attractive and promising alternatives in selective activation of C-H bonds because it was commonly conducted at room temperature and using green solar energy as driven source.5–7 However, it is still a big challenge to develop efficient photocatalysts that are high in activity, selectivity as well as stability under visible light irradiation. So far, photocatalysts such as TiO2,8 In2S3,9 CdS,10 g-C3N4,11 Bi2WO6,12 Bi2MoO613 and materials based on them14–17 were


Page 2 of 26

Page 3 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


applied for selective oxidation of aromatic alkanes and/or alkanes in the presence of O2 as green oxidant. However, the application of these photocatalysts is severely restricted owing to the need of ultraviolet light, as well as the serious recombination of hole-electron pairs and low stability of catalysts. Being chemically stable, low cost, and nontoxic, TiO2 has been used as photocatalyst in partial oxidation of aromatic alkanes.18 It was reported that TiO2 is an essential component in heterogeneous hybrid system. It works as an electron reservoir as well as a mediator for electron transport for the maintenance of photocatalytic activity.19 However, with a wide bandgap of 3.2 eV, TiO2 can only be excited under ultraviolet illumination, which takes up only 4% of solar light energy.20,21 Meanwhile, the serious recombination of photogenerated charge carriers is another factor restricts the application of TiO2 photocatalysts.22 To widen the use of TiO2, it is necessary to expand its light absorption ability as well as prevent the recombination of charges. Being nontoxic and superior in spectral property, bismuth molybdates with suitable band location are promising photocatalysts.23,24 It has been widely used in pollutant degradation,25 water splitting26 and organic production.16,27 In our previous works, it was disclosed that both holes (directly) and electrons (via the generation of radicals such as •O2- and 1O2) play essential roles in the selective oxidation of toluene into benzaldehyde.12,16,17 It is considered that the construction of active sites for respective capture of hot electrons and holes is an efficient way to prevent the combination of charge carriers in photoredox surface reactions. Thus placing TiO2 nanoparticles onto Bi2MoO6 sheet to function as electron reservoir may result in synergistic properties that lead to enhanced photocatalytic performances of Bi2MoO6.28,29



Herein, TiO2/Bi2MoO6 composite photocatalysts were prepared by a simple solvothermal method. They were evaluated for the partial oxidation of aromatic alkanes under visible light irradiation. The effects of catalyst composition, structure as well as morphology on photocatalytic performance were investigated. At a TiO2-to-Bi2MoO6 molar ratio of 0.10, the composite catalyst shows a benzaldehyde production rate of 1036.8 μmol g-1 h-1 without using any solvent, which is the highest among the prepared photocatalysts. EXPERIMENTAL SECTION Synthesis: TiO2 nanoparticles were prepared by hydrothermal method according to that reported in literature.30 The heterostructured TiO2/Bi2MoO6 composites were prepared by a solvothermal method. Typically, 0.970 g of Bi(NO3)3•5H2O and 0.05 g of CTAB (hexadecyl trimethyl ammonium bromide) were dissolved in 20 mL of nitric acid (1.5 M). Separately 0.242 g of Na2MoO4•2H2O was dissolved in distilled water to get a transparent solution. After mixing of the two solutions, 45 mL of distilled water with a designated amount of TiO2 was slowly added with stirring and ultrasonic treatment. Then, the resulted mixture was transferred into a Teflon-lined stainless steel autoclave (100 mL) and heated at 160 °C for 12 h. After cooling down to room temperature, the resulted TiO2/Bi2MoO6 composites were isolated by centrifugation, washed with distilled water and dried at 80 °C overnight. The samples synthesized using 4.0, 8.0, 12.0 and 16.0 mg of TiO2 (i.e. TiO2-to-Bi2MoO6 mole ratio of 0.05, 0.10, 0.15 and 0.20) are herein denoted as BMOT-5, BMOT-10, BMOT-15 and BMOT-20, respectively. For the purpose of comparison, a Bi2MoO6 sample was synthesized following the procedure but without the addition of TiO2.


Page 4 of 26

Page 5 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Characterization: Morphology and microscopic structure of the as-prepared samples were investigated over Hitachi S-4800 scanning electron microscope and JEM-2100F transmission electron microscope. Bruker D8 Advance X-ray diffractometer was applied to analyze the crystal phase of samples (monochromatized Cu-Kα radiation, λ = 0.154 06 nm). X-ray photoelectron spectroscopic data of Bi, Mo, Ti and O were acquired over a VG Multilab 2000 instrument. Light absorption ability was studied over a Cary-100 spectrophotometer using BaSO4 as reference. Shimadzu RF-5301PC fluorescence spectrophotometer was used to acquire the PL spectra of samples at room temperature. The photocurrent measurement was performed on a CHI660E electrochemistry workstation while electrochemical impedance spectroscopic (EIS) measurement on an Autolab electrochemistry workstation at room temperature. Both photocurrent and EIS measurements were conducted using a standard three-electrode cells containing 0.5 mol/L Na2SO4 aqueous solution with platinum foil and saturated calomel electrode as counter and reference electrode, respectively. A 500 W Xe lamp with a cut-off filter (λ ≥ 400 nm) was used as light source. Photocatalytic activity: Photocatalytic oxidation of toluene into benzaldehyde at room temperature and atmospheric pressure was used to evaluate the catalytic performance of catalysts. Briefly, the reaction was conducted in a round bottom flask which contained 10 mmol of toluene and 50 mg of catalyst. The evaluation was carried under visible-light irradiation (300 W Xe lamp, PLS-SXE 300C, Perfectlight, λ ≥400 nm) with a flow of O2 (rate = 3 mL·min-1). The flask was equipped with a condenser pipe to trap reactants and products. After a designated period, catalyst and reaction solution were separated by centrifugation and the products analyzed over a SHIMADZU Gas Chromatograph. More details of the procedure are available elsewhere.12 We completed the mass balance according to the gross mass before and after



Page 6 of 26

reaction; the gross mass should have increased slightly after the reaction because the major end product is aldehyde. Instead, we found that the mass remains unchanged which is nearly unavoidable because of saturated vapor pressure of the system. Conversion of aromatics and selectivity to the corresponding aldehydes is defined as follows:

Conversion (%) 

Selectivity (%) 

 amountof each product (mmol ) amountof substrate (mmol )

 100%

amount of corresponding aldehyde (mmol ) 100%  amount of each product (mmol )

(1)

(2)

RESULTS AND DISCUSSION SEM images of TiO2 and Bi2MoO6 are depicted in Figure 1a and b. TiO2 is in the form of nanoparticles, exhibiting size of 20–30 nm. The Bi2MoO6 sample exists as single elliptical sheet. Figure 1c shows the SEM images of BMOT-10, revealing that the TiO2 nanoparticles are distributed on the Bi2MoO6 sheet. Figure 1d shows the HRTEM image of BMOT-10, in which lattice fringes with interplanar distance of 0.308 nm assignable to (220) planes of Bi2MoO6 (JCPDS 21-0102) can be seen. The fringes in contact with Bi2MoO6 are with lattice of 0.238 nm in agreement with the (004) crystallographic planes of TiO2 (JCPDS 65-5714). Overall, the results confirm the formation of heterojunctions between TiO2 and Bi2MoO6. Figure 2 shows the XRD patterns of the as-prepared samples. The TiO2/Bi2MoO6 composites display 2θ peaks at 28.3◦, 32.9◦, 35.9◦, 47.1◦, 55.9◦ and 58.5◦ which can be indexed to the (131), (200), (151), (062), (331) and (262) planes of orthorhombic phased Bi2MoO6 (JCPDS 21-0102). Weak peak at 2θ = 25.4◦ appeared in all composite samples is attributed to the (101)


Page 7 of 26

planes of anatase TiO2 (JCPDS 65-5714), and there is no detection of any phases ascribable to impurities.

Figure 1. SEM images of (a) TiO2, (b) Bi2MoO6, (c) BMOT-10 and HRTEM image of (d)

(331) (262)

(062)

(131)

(020)

(200) (151)

BMOT-10.

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


BMOT-20 BMOT-15 BMOT-10 BMOT-5 TiO2 Bi 2MoO6

10

20

30

40

50

60

70

80

2 Theta (degree) Figure 2. XRD patterns of the as-prepared samples.



The composition of the as-prepared composites was analyzed by ICP. The Ti-to-Bi molar ratio of BMOT-5, BMOT-10, BMOT-15 and BMOT-20 is 0.026, 0.134, 0.163 and 0.192, respectively, in agreement with those of theoretical values. The chemical states of surface elements were investigated using X-ray photoelectron spectroscopy (XPS). As displayed in Figure 3a, two peaks in the Ti 2p region were detected. One at 464.2 eV binding energy is ascribed to Ti 2p1/2 while that at 458.6 eV to Ti 2p3/2.29 Compared with those of TiO2 nanoparticles, the Ti 2p peaks of BMOT-10 show no shift in binding energy, suggesting that the structure of TiO2 remains intact in the BMOT-10 composite. Figure 3b reveals peaks at 157.8 and 163.1 eV that are characteristics of Bi4f7/2 and Bi4f5/2, respectively.31 As for Figure 3c, the peaks at around 232.2 and 235.3 eV can be assigned to Mo 3d peaks.32 The O 1s profile (Figure 3d) can be fitted with three components: 529.9 eV for Bi–O bond, 530.6 eV for Mo–O bond and 531.7 eV for Ti–O bond.32 Both the XRD and XPS results give evidences for the coexistence of Bi2MoO6 and TiO2 in the BMOT-10 composite.


Page 8 of 26

Page 9 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


BMOT-10

(c)

Figure 3 XPS spectra: (a) Ti 2p, (b) Bi 4f, (c) Mo 3d, (d) O 1s. The light absorption ability of TiO2, Bi2MoO6, BMOT-5, BMOT-10, BMOT-15 and BMOT-20 are shown in Figure 4a. Compared with the absorption edge of TiO2 (about 400 nm), those of TiO2/Bi2MoO6 are red shifted to visible light region, which is an indication of visible light absorption. In the cases of Bi2MoO6 and TiO2/Bi2MoO6, the absorption edges are the same. To calculate the band gap energies of TiO2, Bi2MoO6 and TiO2/Bi2MoO6, the transformed Kubelka-Munk function was plotted against light energy (Figure 4b). It is found that the TiO2/Bi2MoO6 heterostructures are similar to Bi2MoO6 in band gap energy. The results imply that there is no incorporation of TiO2 into the lattice of Bi2MoO6 for the modification of band gap energy.


(a)

1.2

1.0

(b)

TiO2 Bi 2MoO6

10

0.4

0.2 200

eV) TiO2 Bi 2MoO6 BMOT-5 BMOT-10 BMOT-15 BMOT-20 300

BMOT-5 BMOT-10 BMOT-15 BMOT-20

8 6

2

0.6

2

0.8

Page 10 of 26

14 12

(Ah 

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abosorbance (a.u.)


4 2 0

400

500

600

Wavelength (nm)

700

800

2.45eV

3.08eV

-2 2.4

2.6

2.8

3.0

3.2

3.4

h  eV)

Figure 4. (a) UV-vis absorption and (b) the corresponding (Ahν)2 versus hν plots of the asprepared samples. To illustrate separation enhancement of charge carriers over the TiO2/Bi2MoO6 composites, photoelectrochemical characterization was carried out. Figure 5a shows the photocurrent-time curves obtained over TiO2, Bi2MoO6, BMOT-5, BMOT-10 and BMOT-20 in light on-off cycles. It can be observed that BMOT-10 exhibits the highest photocurrent intensity among all samples, indicating that the Bi2MoO6 sheet promotes the generation of excited electrons, and there is fast injection of hot electrons into the TiO2 particles, resulting in enhancement of charge separation. The charge separation and collection efficiency at the interface can be further confirmed by the spectra collected in electrochemical impedance spectroscopic (EIS) investigation. As depicted in Figure 5b, the small arc radius of BMOT-10 demonstrates that introducing TiO2 nanoparticles onto Bi2MoO6 favors electron transfer and weakens charge recombination in Bi2MoO6, which are beneficial factors for the enhancement of photocatalytic activity.


Page 11 of 26

350

1.0

(a)

Bi2MoO6 Bi2MoO6 BMOT-5 BMOT-20

0.8

TiO2 TiO2 BMOT-10

(b)

Bi2MoO6 Bi2MoO6 TiO2 TiO2 BMOT-5 BMOT-10 BMOT-20

300 250

0.6

-Z''/ohm

Photocurrent (μA cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.4

200 150 100

0.2

50

0.0

0 65

85

105

125

145

165

185

0

Time (s)

200

400

Z'/ohm

600

800

Figure 5. (a) Photocurrent responses under visible-light irradiation and (b) electrochemical impedance spectroscopic (EIS) spectra over TiO2, Bi2MoO6, BMOT-5, BMOT-10 and BMOT20. Room temperature photoluminescence (PL) emission is commonly used to disclose the trapping, migration, and transfer efficiency of photogenerated electrons and holes, and can be adopted to understand the fate of charge carriers in semiconductors.29 Figure 6 shows the PL spectra of the as-prepared samples. Two distinct emission peaks at 450 and 525 nm belong to the emission of band gap transitions of TiO2.33 The Bi2MoO6 nanosheets also exhibit two strong emission peaks at about 429 and 539 nm, which might be assigned to the intrinsic luminescence properties of Bi2MoO6.34 In comparison with Bi2MoO6 and TiO2, the TiO2/Bi2MoO6 composites show obvious fluorescence decrease (or quenching). The phenomenon is attributed to interfacial transfer of photogenerated electrons from Bi2MoO6 to TiO2 which retards the charge recombination process. It is worth pointing out that among the catalysts, BMO-10 is the lowest in PL intensity, indicating highest ability in preventing the recombination of charge carriers. This phenomenon is in agreement with the reported results. At low TiO2 loading, the amount of sites



for charge transfer would not be enough whereas when TiO2 is in excessive, the probability of electron-hole collision would increase, and thus the high PL intensity.35

Bi2MoO6 Bi2MoO6 TiO2 TiO2 BMOT-5

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 26

300

BMOT-10 BMOT-15 BMOT-20

400

500

600

700

800

Wavelength (nm)

Figure 6. Photoluminescence (PL) spectra of the as-prepared samples (λEx = 325 nm). In the present study, partial oxidation of aromatic alkanes into aldehydes was used as model reaction to examine the photocatalytic performance of the catalysts. Table 1 displays the photocatalytic activity of TiO2, Bi2MoO6, BMOT-5, BMOT-10, BMOT-15 and BMOT-20. From the results of entries 1–3 one can get that the oxidation reaction is photocatalytic and O2 is indispensable. Apparently, all the TiO2/Bi2MoO6 composites exhibit better photocatalytic activity than single TiO2 and Bi2MoO6. TiO2 proportion in the heterostructures was found to be an important factor that affects the photocatalytic activity of TiO2/Bi2MoO6 composites. Under visible light irradiation, there is only trace amount of products (can be ignored) when TiO2 is used as photocatalyst (entry 4), whereas benzaldehyde is generated in a rate of 391.2 μmol·g-1·h-1 over Bi2MoO6. When TiO2 is introduced onto Bi2MoO6, the formation rate is 784.8 μmol·g-1·h-1 for BMOT-5, and reaches a maximum rate of 1036.8 μmol·g-1·h-1 over BMOT-10 with selectivity to benzaldehyde being around 97%. The enhanced photocatalytic performance could


Page 13 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


be ascribed to the reduction in the rate of electron-hole recombination, as indicated by EIS and PL measurements. With further increase of TiO2 content, the formation rate decreases to 851.1 μmol·g-1·h-1 and 517.3 μmol·g-1·h-1 over BMOT-15 and BMOT-20, respectively. It is plausible that an excessive presence of TiO2 in the heterostructures would increase the chance of electronhole collision and hence higher charge recombination rate in TiO2.35 Table 1. Partial oxidation of toluene into benzaldehyde over the as-prepared catalysts.a

aPhotocatalyst

Entry

Catalyst

1

none

Formation rate (μmol·g-1·h-1) -

2b

BMOT-10 (N2)

-

-

3c

BMOT-10 (dark)

-

-

4

TiO2

-

-

5

Bi2MoO6

391.2

97.8

6

BMOT-5

784.8

98.1

7

BMOT-10

1036.8

97.2

8

BMOT-15

851.1

98.2

9

BMOT-20

517.3

97.0

Sel.(%) -

(50 mg), substrate (10 mmol), no solvent, room temperature, O2 flow rate (3

mL·min-1), visible-light irradiation (λ ≥400 nm, 3 h); bReplacing O2 with N2; cWithout irradiation. At optimized condition, the benzaldehyde production rate could be as high as that achieved over Pd/Bi2WO6 (1140 μmol/g/h) reported by Yuan et al.36 As far as our knowledge goes, the benzaldehyde production rate in the present study is the highest ever reported over a heterogeneous photocatalyst using O2 as oxidant. It is worth pointing out that the selectivity to benzaldehyde over our TiO2/Bi2MoO6 composites (>97%) is higher than that over Pd/Bi2WO6 (90%).36 As shown in Figure 7, there is no significant loss of photoactivity in a test of five



recycling runs, and the benzaldehyde selectivity is constantly around 97%. Therefore, the TiO2/Bi2MoO6 composites are stable visible-light-driven photocatalysts apt for the partial oxidation of toluene to benzaldehyde under ambient conditions.

*

1200

*

*

-1

.h

-1

)

*

*

100

*

Benzaldehyde Selectivity

80

900

60

600

40

300

20

0

0

1

2

3

4

5

6

Selectivity (%)

1500

Formation rate (mol .g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 26

0 0

Recycle times

Figure 7. Recycle property of BMOT-10 in photocatalytic oxidation of toluene to benzaldehyde It was found that BMOT-10 is also efficient in the partial oxidation of toluene derivatives (Table 2). The yields of benzaldehyde derivatives with electron-donating groups (entries 2–4) on p-substituted toluene are higher than those with electron-withdrawing groups (entries 5–6). It is because the presence of an electron-donating group is beneficial for the formation of benzyl radicals.37 Comparing to the o- and m- substituted ones (entries 8–9), the p- substituted one shows higher activity, which could be due to steric effects. It is worth pointing out that owing to higher activity, the substrates with electron-donating groups show relatively lower selectivity to the corresponding aldehydes. In all our experiments using the TiO2/Bi2MoO6 catalysts, there is no detection of carbon dioxide and the absence of over oxidation can be ascertained.


Page 15 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Substrate scope for the photocatalytic oxidation of aromatic alkanes.a hυ (λ≥400 nm), 3 h R BMOT-10, O2

R

Entry

Substrate

Product

CHO

1

CHO

2

CHO

Formation rate

Conversion (%)

Selectivity (%)

1.6

97.2

1036.8

3.2

92.1

1964.8

3.5

96.3

2247.0

(μmol g-1 h-1)

3b

HO

4c

O

O

CHO

2.1

98.8

1383.2

5

Cl

Cl

CHO

1.4

>99

933.0

6

F

F

CHO

0.8

>99

533.3

7

NO2

NO2

0.6

>99

400.0

3.0

95.3

1906.0

2.6

95.8

1660.5

8 9

HO

CHO

CHO

CHO

CHO

a

substrate (10 mmol), BMOT-10 catalyst (50 mg), O2 flow rate (3 mL·min-1) without solvent,

b

benzotrifluoride (2 mL) as solvent, c acetonitrile (3 mL) as solvent. Capturing active species (commonly radicals) in photocatalytic reaction is a good strategy

to underlying the reaction mechanism. In this present work, quenching experiments with the addition of scavengers was performed to understand the role of different radicals.12 As shown in Figure 8, addition of TEMPO (scavenger for all radicals), the reaction was almost completely quenched. Similarly, the reaction was terminated after the addition of scavenger for holes (AO),



suggesting holes play an indispensable role in the partial oxidation of toluene. When BQ and K2S2O2 were added to capture •O2- and electron, respectively, the conversion of toluene are significantly inhibited. It is commonly known that, •O2- is produced through the activation of O2 by photogenerated electrons, indicating the participation of •O2- in the selective oxidation process. Furthermore, there is no apparent change in toluene conversion with the addition of •OH scavenger (TBA). The results of quenching experiments give clear evidence that photogenerated holes play an indispensible role in the partial oxidation of toluene over the TiO2/Bi2MoO6 composites.

Benzaldehyde (mmol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 26

160

TEMPO AO BQ K 2S2O8

120

Initial TBA

80

40

0

1

2

3

4

5

Reaction time (h)

Figure 8. Kinetic-experiments for the partial oxidation of toluene with or without the addition of scavengers: TEMPO (tetra-methylpiperidine N-oxide) for all radicals, BQ (benzoquinone) for superoxide radicals, TBA (tert-butyl alcohol) for hydroxyl radicals, AO (ammonium oxalate) for holes and K2S2O8 for electrons. Based on the discussions above, a possible reaction mechanism is tentatively proposed (Scheme 1). Under visible-light excitation, Bi2MoO6 with narrow band gap absorbs photons and there is excitation of electrons from valence band to conduction band. Then the excited electrons in the conduction band of Bi2MoO6 transfer to the conduction band edge of TiO2 in a stepwise


Page 17 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


manner, resulting in effective separation of photoinduced electron-hole pairs. The toluene adsorbed on the surface of BMOT-10 is oxidized to cationic radicals by the positive holes. Meanwhile, the hot electrons react with adsorbed O2 to give activated oxygen species (e.g., O2•– ).17 The activated oxygen species as well as O2 on the surface of catalyst selectively oxidize the cationic radicals, leading to the formation of benzaldehyde. It is noted that despite comparing to Bi2MoO6 and TiO2, the BMO composites show enhanced activity and high selectivity to benzaldehydes, the conversion of toluene and its derivatives are relatively low. On the basis of the proposed mechanism and the information provided in a recent work of Cao et al.,38 one can see that the separation of charge carriers, adsorption of reactant or O2 as well as the desorption of intermediates and/or products are important factors for the enhancement of activity. In our future research, we shall focus on two aspects. One is the design and fabrication of semiconductors or composites of novel structures and interfaces that can accelerate the separation and transfer of charge carriers. The other is to construct novel surfaces and interfaces which are beneficial to the adsorption of reactants and desorption of intermediates and/or products. CONCLUSIONS In summary, TiO2/Bi2MoO6 heterostructures were successfully synthesized and found to show good phtotocatalytic activity towards the partial oxidation of aromatic alkanes to benzaldehyde and its derivatives under visible light irradiation. Compared with the Bi2MoO6 and TiO2 single component, the TiO2/Bi2MoO6 composites have higher accessible surface area and better structural stability, and consequently better photocatalytic performance. Despite TiO2 by itself cannot be excited by visible light, it serves as a reservoir for hot electrons as well as a mediator for electron transport. In the TiO2/Bi2MoO6 composites, TiO2 greatly enhances the separation of photogenerated charges in Bi2MoO6, securing the location of the photoexcited electrons and



holes at separate active sites. The enhanced photocatalytic performance of TiO2/Bi2MoO6 is ascribed to the restricted recombination of electron-hole pairs owing to the stepwise energy levels between Bi2MoO6 and TiO2. A possible reaction mechanism has been proposed based on the results of quenching experiments, and h+, e-, and O2•– are major active species participating in the reaction system.

Scheme 1. Plausible reaction mechanism involved in the partial oxidation of toluene into benzaldehyde over BMOT-10 AUTHOR INFORMATION Corresponding Author E-mail address: [email protected] (L Chen), [email protected] (SF Yin). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDEGEMENTS


Page 18 of 26

Page 19 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


This project was financially supported by the National Natural Science Found of China (Grants 21476065, 21671062, 21776064 and 21725602), the Natural Science Foundation of Hunan Province (Grant 2015JJ3033), and the Science and Technology Project of Hunan Province (2015JC3051). C. T. Au thanks HNU for an adjunct professorship.



REFERENCES [1] Copéret, C. C–H bond activation and organometallic intermediates on isolated metal centers on oxide surfaces. Chem. Rev. 2010, 110, 656–680, DOI 10.1021/cr900122p. [2] Kesavan, L.; Tiruvalam, R.; Rahim, M. H. A.; Saiman, M. I.; Enache, D. I.; Jenkins, R. L.; Dimitratos, N.; Lopez–Sanchez, J. A.; Taylor, S. H.; Knight, D. W.; Kiely, C. J.; Hutchings, G. J. Solvent–free oxidation of primary carbon–hydrogen bonds in toluene using Au–Pd alloy nanoparticles. Science 2011, 331, 195–199, DOI 10.1126/science.1198458. [3] Li, X. H.; Chen, J. S.; Wang, X.; Sun, J.; Antonietti, M. Metal–free activation of dioxygen by graphene/g–C3N4 nanocomposites: functional dyads for selective oxidation of saturated hydrocarbons. J. Am. Chem. Soc. 2011, 133, 8074–8077, DOI 10.1021/ja200997a. [4] Marzo, L.; Pagire, S. K.; Reiser, O.; Konig, B. Visible-light photocatalysis: does it make a difference in organic synthesis? Angew. Chem. Int. Ed. 2018, 57, 10034–10072, DOI 10.1002/anie.201709766. [5] Chen, X.; Mao, S.S. Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications. Chem. Rev. 2007, 107, 2891–2959, DOI 10.1021/cr0500535. [6] Sarina, S.; Zhu, H.; Zheng, Z.; Bottle, S. E.; Chang, J.; Ke, X.; Zhao, J. C.; Huang, Y. Sutrisno, A.; Willans, M.; Li, G. Driving selective aerobic oxidation of alkyl aromatics by sunlight on alcohol grafted metal hydroxides. Chem. Sci. 2012, 3, 2138–2146, DOI 10.1039/C2SC20114C. [7] Fukuzumi, S.; Doi, K.; Itoh, A.; Suenobu, T.; Ohkubo, K.; Yamada, Y.; Karlin, K.;D. Formation of a long-lived electron-transfer state in mesoporous silica-alumina composites enhances photocatalytic oxygenation reactivity. P. Natl. Acad. Sci. USA. 2012, 109, 15572– 15577, DOI 10.1073/pnas.1119994109.


Page 20 of 26

Page 21 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[8] Ide, Y.; Kawamoto, N.; Bando, Y.; Hattori, H.; Sadakane, M.; Sano, T. Ternary modified TiO2 as a simple and efficient photocatalyst for green organic synthesis. Chem. Commun. 2013, 49, 3652–3654, DOI 10.1039/C3CC41174E. [9] Ye, X.; Chen, Y.; Ling, C.; Zhang, J.; Meng, S.; Fu X.;, Wang, X.; Chen, S. Chalcogenide photocatalysts for selective oxidation of aromatic alcohols to aldehydes using O2 and visible light: A case study of CdIn2S4, CdS and In2S3. Chem. Eng. J. 2018, 348, 966–977, DOI 10.1016/j.cej.2018.05.035. [10]Zhang, Y.; Zhang, N.; Tang, Z. R.; Xu, Y. J. Transforming CdS into an efficient visible light photocatalyst for selective oxidation of saturated primary C–H bonds under ambient conditions. Chem. Sci. 2012, 3, 2812–2822, DOI 10.1039/C2SC20603J. [11]Wang, Y.; Wang, X.; Antonietti, M. Polymeric graphitic carbon nitride as a heterogeneous organocatalyst: from photochemistry to multipurpose catalysis to sustainable chemistry. Angew. Chem. Int. Ed. 2012, 51, 68–89, DOI 10.1002/anie.201101182. [12]Liu, Y.; Chen, L.; Yuan, Q.; He, J.; Au, C. T.; Yin, S. F. A green and efficient photocatalytic route for high-selective oxidation of saturated alpha-carbon C–H bond in aromatic alkanes over flower-like Bi2WO6. Chem. Commun. 2016, 52, 1274–1277, DOI 10.1039/c5cc07586f. [13]Cai, K.; Lv, S. Y.; Song, L. N.; Chen, L.; He, J.; Chen, P.; Au, C. T.; Yin, S. F. Facile preparation of ultrathin Bi2MoO6 nanosheets for photocatalytic oxidation of toluene to benzaldehyde under visible light irradiation. J. Solid State Chem. 2019, 269, 145–150, DOI 10.1016/j.jssc.2018.09.027. [14]Yuan, R.; Fan, S.; Zhou, H.; Ding, Z.; Lin, S.; Li, Z.; Zhang, Z.; Xu, C.; Wu, L.; Wang, X.; Fu, X. Chlorine-radical-mediated photocatalytic activation of C-H Bonds with visible light. Angew. Chem. Int. Ed. 2013, 52, 1035–1039, DOI 10.1002/anie.201207904.



[15]Tripathy, J.; Lee, K.; Schmuki, P. Tuning the selectivity of photocatalytic synthetic reactions using modified TiO2 nanotubes. Angew. Chem. Int. Ed. 2014, 53, 12605–12608, DOI 10.1002/anie.201406324. [16]Song, L. N.; Chen, L.; He, J.; Chen, P.; Zeng, H. K.; Au, C. T.; Yin, S. F. First synthesis of Bi self-doped Bi2MoO6-Bi2Mo3O12 composite and its excellent photocatalytic performance for selective oxidation of aromatic alkanes under visible light irradiation. Chem. Commun. 2017, 53, 6480–6483, DOI 10.1039/c7cc02890c. [17]He, J.; Chen, L.; Ding, D.; Yang, Y. K.; Au, C. T.; Yin, S. F. Facile fabrication of novel Cd3(C3N3S3)2/CdS porous composites and their photocatalytic performance for toluene selective oxidation under visible light irradiation. Appl. Catal. B: Environ. 2018, 233, 243– 249, DOI 10.1016/j.apcatb.2018.04.008. [18] Fujihira, M.; Satoh, Y.; Osa, T. Heterogeneous photocatalytic oxidation of aromatic compounds on TiO2. Nature 1981, 293, 206–208, DOI 0028-0836/81/380206-0301.00. [19]Daghrir, R.; Drogui, P.; Robert, D. Modified TiO2 for environmental photocatalytic applications: a review. Ind. Eng. Chem. Res. 2013, 52, 3581–3599, DOI 10.1021/ie303468t. [20]Won, D. I.; Lee, J. S.; Ji, J. M.; Jung, W. J.; Son, H. J.; Pac, C.; Kang, S. O. Highly robust hybrid photocatalyst for carbon dioxide reduction: tuning and optimization of catalytic activities of dye/TiO2/Re(I) organic-inorganic ternary systems. J. Am. Chem. Soc. 2015, 137, 13679–13690, DOI 10.1021/jacs.5b08890. [21]Li, J.; Liu, X.; Piao, X.; Sun, Z.; Pan, L. Novel carbon sphere@Bi2MoO6 core–shell structure for efficient visible light photocatalysis. RSC Adv. 2015, 5, 16592–16597, DOI 10.1039/C4RA16777E. [22]Zhou, Z. J.; Fan, J. Q.; Wang, X; Sun, W. Z.; Zhou, W. H.; Du, Z. L.; Wu, S. X. Solution


Page 22 of 26

Page 23 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


fabrication and photoelectrical properties of CuInS2 nanocrystals on TiO2 nanorod array. ACS Appl. Mater. Interfaces 2011, 3, 2189–2194, DOI 10.1021/am200500k. [23]Long, J. L.; Wang, S.; Chang, H.; Zhao, B.; Liu, B.; Zhou, Y.; Wei, W.; Wang, X.; Huang, L.; Huang, W. Bi2MoO6 nanobelts for crystal facet-enhanced photocatalysis, Small 2014, 10, 2791–2795, DOI 10.1002/smll.201302950. [24]Schuh, K.; Kleist, W.; Høj, M.; Trouillet, V.; Jensen, A. D.; Grunwaldt, J. D. One-step synthesis of bismuth molybdate catalysts via flame spray pyrolysis for the selective oxidation of propylene to acrolein. Chem. Commun. 2014, 50, 15404–15406, DOI 10.1039/C4CC07527G. [25]Li, Z.; Wu, Z.; Zhang, S.; Shen, J.; Feng, W.; Du, Y.; Wan, L.; Zhang, S. Defect state of indium-doped bismuth molybdate nanosheets for enhanced photoreduction of chromium(vi) under

visible

light

illumination.

Dalton

Trans.

2018,

47,

8110–8120,

DOI

10.1039/C8DT01807C. [26]Ke, J.; Duan, X.; Luo, S.; Zhang, H.; Sun, H.; Liu, J.; Tade, M.; Wang, S. UV-assisted construction of 3D hierarchical rGO/Bi2MoO6 composites for enhanced photocatalytic water oxidation. Chem. Eng. J. 2017, 313, 1447–1453, DOI 10.1016/j.cej.2016.11.048. [27]Guo, J.; Shi, L.; Zhao, J.; Wang, Y.; Tang, K.; Zhang, W.; Xie, C.; Yuan, X. Enhanced visible-light photocatalytic activity of Bi2MoO6 nanoplates with heterogeneous Bi2MoO6x@Bi2MoO6

core-shell structure. Appl. Catal. B: Environ. 2018, 224, 692–704, DOI

10.1016/j.apcatb.2017.11.030. [28]Tian, J.; Hao, P.; Wei, N.; Cui, H. Z.; Liu, H. 3D Bi2MoO6 nanosheet/TiO2 nanobelt heterostructure: enhanced photocatalytic activities and photoelectochemistry performance. ACS Catal. 2015, 5, 4530–4536, DOI 10.1021/acscatal.5b00560.



[29]Li, H.; Zhang, T. X.; Pan, C.; Pu, C. C.; Hu, Y.; Hu, X. Y.; Liu, E. Z.; Fan, J. Self-assembled Bi2MoO6/TiO2 nanofiber heterojunction film with enhanced photocatalytic activities. Appl. Surf. Sci. 2017, 391, 303–310, DOI 10.1016/j.apsusc.2016.06.167. [30]Li, J.; Liu, X.; Sun, Z.; Pan, L. Novel Bi2MoO6/TiO2 heterstructure microspheres for degradation of benzene series compounds under visible light irradiation. J. Colloid Interface Sci. 2016, 463, 145–153, DOI 10.1016/j.jcis.2015.10.055. [31]Meng, Q. Q.; Zhou, Y. S.; Chen, G.; Hu, Y. D.; Lv, C. D.; Qiang, L. S.; Xing, W. N. Integrating both homojunction and heterojunction in QDs self-decorated Bi2MoO6/BCN composites to achieve an efficient photocatalyst for Cr(VI) reduction. Chem. Eng. J. 2018, 334, 334–343, DOI 10.1016/j.cej.2017.07.134. [32]Cai, J.; Huang, J.; Lai, Y. 3D Au–decorated Bi2MoO6 nanosheet/TiO2 nanotube arrays heterostructure with enhanced UV and visible–light photocatalytic activity for organic pollutants. J. Mater. Chem. A. 2017, 5, 16412–16421, DOI 10.1039/C7TA02077E. [33]Zhang, M.; Shao, C.; Mu, J.; Zhang, Z.; Guo, Z.; Zhang, P.; Liu, Y. C. One–dimensional Bi2MoO6/TiO2 hierarchical heterostructures with enhanced photocatalytic activity. CrystEngComm 2012, 14, 605–612, DOI 10.1039/C1CE05974B. [34]Tian, G.; Chen, Y.; Zhou, J.; Tian, C.; Li, R.; Wang, C.; Fu, H. G. In suit growth of Bi2MoO6 on reduced graphene oxide nanosheets for improved visible-light photocatalytic activity. CrystEngComm 2014, 16, 842–849, DOI 10.1039/C3CE41999A. [35]Li, N.; Zhu, L.; Zhang, W. D.; Yu, Y. X.; Zhang, W. H.; Hou, M. F. Modification of TiO2 nanorods by Bi2MoO6 nanoparticles for high performance visible-light photocatalysis. J.Alloy Compd. 2011, 509, 9770–9775, DOI 10.1016/j.jallcom.2011.07.102. [36]Yuan, B.; Zhang, B.; Wang, Z.; Lu, S.; Li, J.; Liu, Y.; Li, C. Photocatlaytic aerobic


Page 24 of 26

Page 25 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


oxidation of toluene and its derivatives to aldehydes on Pd/Bi2WO6. Chin. J. Catal. 2017, 38, 440–446, DOI 10.1016/S1872-2067(17)62757-8. [37]Su, F.; Mathew, S. C.; Lipner, G.; Fu, X.; Antonietti, M.; Blechert, S.; Wang, X. mpg-C3N4catalyzed selective oxidation of alcohols using O2 and visible light. J. Am. Chem. Soc. 2010, 132, 16299–16301, DOI 10.1021/ja102866p. [38]Cao, X.; Chen, Z.; Lin, R.; Cheong, W. C.; Liu, S.; Zhang, J.; Peng, Q.; Chen, C.; Han, T.; Tong, X.; Wang, Y.; Shen, R.; Zhu, W.; Wang, D.; Li, Y. A photochromic composite with enhanced carrier separation for the photocatalytic activation of benzylic C-H bonds in toluene. Nat. Catal. 2018, 1, 704–710, DOI 10.1038/s41929-018-0128-z.



For Table of Contents Use only

O2

O2•–

TiO2/Bi2MoO6 composites prepared by a simple solvothermal method, with TiO2 nanoparticles being electron reservoir, are stable and efficient photocatalysts for the partial oxidation of aromatic alkanes under ambient conditions.


Page 26 of 26

Bi2MoO6 Composite for Partial Oxidation of

Recommend Documents