TiO2-UiO-66-NH2 nanocomposites as efficient photocatalysts for the oxidation of VOCs

Journal Pre-proofs TiO2-UiO-66-NH2 nanocomposites as efficient photocatalysts for the oxidation of VOCs Jinhui Zhang, Yun Hu, Junxian Qin, Zhenxiang Yang, Mingli Fu PII: DOI: Reference:

S1385-8947(19)33229-2 https://doi.org/10.1016/j.cej.2019.123814 CEJ 123814

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Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

12 August 2019 7 December 2019 12 December 2019

Please cite this article as: J. Zhang, Y. Hu, J. Qin, Z. Yang, M. Fu, TiO2-UiO-66-NH2 nanocomposites as efficient photocatalysts for the oxidation of VOCs, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej. 2019.123814

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1

TiO2-UiO-66-NH2 nanocomposites as efficient photocatalysts

2

for the oxidation of VOCs

3

Jinhui Zhanga, Yun Hua,b,c*, Junxian Qina, Zhenxiang Yanga, Mingli Fua,b,c

4

a

5

Guangzhou 510006, PR China

6

b

7

Control, Guangzhou 510006, PR China

8

c

9

Ministry of Education, Guangzhou 510006, P. R. China

School of Environment and Energy, South China University of Technology,

Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution

The Key Lab of Pollution Control and Ecosystem Restoration in Industry Clusters,

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

*Corresponding author.

25

E-mail address: [email protected] (Y. Hu)

26

Tel: +86-20-39380573

27 28 29 30 1

1

Abstract

2

TiO2-UiO-66-NH2 nanocomposites with different mass ratios were successfully

3

prepared by a simple solvent evaporation method for photocatalytic oxidation of

4

toluene and acetaldehyde under UV light condition. The experimental results revealed

5

that the nanocomposites exhibited superior removal efficiency for toluene and

6

acetaldehyde than the pure TiO2, UiO-66-NH2 and other TiO2-based porous materials

7

during a 720 min of long-term evaluation under flowing conditions. The as-prepared

8

75TiO2-25UN nanocompoites exhibited the highest photocatalytic activity and CO2

9

selectivity. The conversion of 75TiO2-25UN for toluene and acetaldehyde were

10

72.7% and 70.74% respectively, which was about 9.7 and 10.5 times higher than that

11

of UiO-66-NH2, respectively. In addition, the CO2 production of 75TiO2-25UN for

12

toluene and acetaldehyde were 21.1 and 14.3 times higher than that of pure

13

UiO-66-NH2, respectively. N2 adsorption-desorption and SEM results revealed that

14

high surface area could enhance the dispersion of TiO2 and efficient contact to the

15

VOCs. Photoelectrochemical properties indicated that photoinduced carriers can be

16

efficiently separated and transferred via the interface of two components. The

17

outstanding deactivation resistance was achieved over 75TiO2-25UN sample due to

18

the efficient charge transfer and oxygen-rich condition. The possible mechanism of

19

VOCs oxidation was proposed based on radical scavenger experiments, which proved

20

that •O2- and h+ are the main active species. This work clearly demonstrated that

21

MOF-based composite can be used as efficient photocatalysts for the application of

22

VOCs oxidation.

23 24

Keywords

25

Metal-organic frameworks; TiO2; VOCs; Photocatalytic oxidation

26 27 28 29 30 2

1

1. Introduction

2

Volatile organic compounds (VOCs) are typical toxic organic pollutants in

3

atmosphere, which emitted from mobile sources and stationary sources (such as

4

industrial processes, indoor facilities, coal-fired power plants, etc.) [1,2]. VOCs

5

including various ketone, alcohols, aromatics (benzene, toluene), aldehydes

6

(acetaldehyde, formaldehyde) are most important precursors of ozone and PM2.5,

7

which harmful to human health [3-4]. Thus, many researcher have made great efforts

8

to develop efficient VOCs treatment technologies, such as adsorption, condensation,

9

membrane filtration, biological treatment and catalytic oxidation [5-6]. Among them,

10

photocatalytic oxidation technology has been proved to be an effective and

11

economical method for atmosphere purification due to the mild operating conditions,

12

cost-effective and the environmentally-friendly final products (CO2 and H2O) [7,8].

13

TiO2 is one of the most commonly used photocatalysts because of its high activity,

14

stability, non-toxicity and low cost [9,10]. However, TiO2 often deactivates during the

15

photocatalytic oxidation of VOCs due to the deposition of recalcitrant degradation

16

intermediates (carbonaceous residues) on the catalyst surfaces [11,12]. Einaga et al.

17

claimed that the activity of commercial TiO2 decreased 86% within 1 h for toluene

18

degradation [13]. Previous studies found that integration of TiO2 with porous

19

materials, such as activated carbon, zeolite and mesoporous silica, can significantly

20

improve the photocatalytic performance compared with pure TiO2 [14,15]. However,

21

it is difficult to achieve satisfactory efficiency because the porous materials only serve

22

as a carrier. Meanwhile, a study [16] also found that the porous materials with high

23

adsorption constants would prevent adsorbed pollutants transferring to the active sites

24

and decrease the photocatalytic activity. On the other hand, Choi's group [7,17] found

25

that TiO2 nanotube exhibited enhanced photocatalytic activity and deactivation

26

resistant for the photocatalytic degradation of aromatic compounds compared to TiO2

27

nanoparticle, since the nanotube structure is more beneficial to the diffusion of O2

28

molecules. Fu's group [18, 19] pointed out that oxygen-riched condition was helpful

29

for the oxidation of VOCs and could promote the interfacial electron transfer.

30

Therefore, it is urgent to develop an effective photocatalyst that not only has excellent 3

1

photocatalytic activity but also can suppress deactivation.

2

Metal-organic frameworks (MOFs) are a fascinating class of porous crystalline

3

material because of their high surface areas, crystalline open structures and tunable

4

uniform cavities. Taking advantage of these distinct characteristics, MOFs have been

5

widely used in gas storage [20], gas separation [21], heterogeneous catalysis [22,23],

6

chemical sensing [24] and drug delivery [24]. Meanwhile, semiconductor MOFs, such

7

as MOF-5 [25], NH2-UiO-66 [26], MIL-100(Fe) [27], UiO-66 [28] and

8

NH2-MIL-125(Ti) [29], also draw great attention on their utilization as photocatalysts

9

in water treatment and air pollution control. Although MOF-based materials showed

10

some activity for gaseous VOCs under static condition [30-33], studies concerning

11

flowing conditions, which is close to real air treatment systems, is still scarce until

12

now and shows poor mineralization efficiency [34]. Thus, it is essential to develop

13

MOF-based catalysts with superior activity and CO2 selectivity under dynamic

14

condition.

15

In this work, a series of TiO2-UiO-66-NH2 nanocomposites with different contents

16

of UiO-66-NH2 were prepared by a facile solvent evaporation method for

17

photocatalytic oxidation of VOCs. UiO-66-NH2 was chosen to combine with TiO2 as

18

it has large surface area, interconnected 3D structure, excellent stability, light

19

responsiveness and semiconductor property. The nanocomposites exhibited an

20

enhanced photocatalytic activity, CO2 selectivity and stability than that of TiO2,

21

UiO-66-NH2 and other TiO2-based porous materials. Meanwhile, the possible

22

mechanism for the outstanding photocatalytic performance of TiO2-UiO-66-NH2

23

composites was also proposed. It is hoped that this work could provide important

24

development in MOFs-based photocatalysts with superior activity and deactivation

25

resistant for photocatalytic degradation of VOCs under flowing conditions.

26 27

2. Experiment

28

2.1. Materials

29

Zirconium chloride (ZrCl4) and 2-NH2-terephthalic acid (H2ATA) were purchased

30

from Aladdin Reagent Co., Ltd. (Shanghai, China). N,N-dimethylformamide (DMF) 4

1

and methanol (CH3OH) were obtained from Damao Chemical Reagent Factory

2

(Tianjin, China). Acetic acid was supplied by Sinopharm Chemical Reagent Co., Ltd.

3

(Shanghai, China). Titanium dioxide (TiO2) nanopowder is commercial P25 supplied

4

from Degussa Co., Ltd. (Germany). Activated carbon (AC) was obtained from

5

Nanjing XFNANO Materials Technique Cooperation. TS-1 was obtained from

6

Beijing HWRK Chem co., while SBA-15 was synthesized according to our previous

7

synthesis method [35,36]. All chemicals were analytical grade and used without

8

further modification.

9

Preparation of UiO-66-NH2: UiO-66-NH2 was synthesized using the same

10

procedure based on the previous work with some modifications [37]. In a typical

11

experiment, 0.2332 g of ZrCl4 and 0.1812 g of H2ATA were dissolved in DMF (50

12

mL). Subsequently, 6 mL of acetic acid was added, and then the mixture solution was

13

transferred into a 100 mL Teflon-lined stainless steel autoclave. The autoclave was

14

sealed and heated at 120 °C for 24 h under autogenous pressure. After cooling

15

naturally, the obtained sample was centrifuged and washed with anhydrous methanol

16

for several times to eliminate the residual DMF. The resulting pale yellow solid was

17

dried under vacuum at 100 °C for 12 h.

18

Preparation of TiO2-UiO-66-NH2 composites: TiO2-UiO-66-NH2 series composites

19

was prepared by a simple solvent evaporation method. In detail, a certain amount of

20

TiO2 was dispersed in 100 mL of anhydrous methanol to form a homogeneous

21

suspension by stirring for 30 min. Subsequently, an appropriate amount of

22

as-prepared UiO-66-NH2 sample was added to the above prepared solution with

23

continuous stirring. After stirring in the fume hood until dryness and then dried in an

24

oven at 100 °C for 12 h, then the final samples of TiO2-UiO-66-NH2 composites were

25

obtained. The as-synthesized TiO2-UiO-66-NH2 samples with 0, 10, 25, 50 and 75

26

wt% of UiO-66-NH2 were labeled as TiO2, 90TiO2-10UN, 75TiO2-25UN,

27

50TiO2-50UN and 25TiO2-75UN, respectively. 75TiO2-25SBA-15, 75TiO2-25TS-1

28

and 75TiO2-25AC were also prepared via the same method to compare with

29

75TiO2-25UN.

30 5

1

2.2. Characterizations

2

The X-ray diffraction (XRD) patterns were obtained using Cu Kα radiation

3

(λ=0.15418 nm) on a Bruker D8 ADVANCE diffractometer at 40 kV and 40 mA, the

4

range of 2θ is 5°-60°. The specific surface area and structural parameters tests of the

5

samples were carried out on an ASAP2020M by the low temperature N2

6

adsorption-desorption method after the samples were degassed in vacuum at 130 oC

7

for 12 h. UV-vis adsorption spectra were recorded on a UV-vis spectrophotometer

8

(Shimadzu, UV-2550) with BaSO4 as the reflectance standard. The morphologies of

9

the samples were characterized by scanning electron microscopy (SEM, Hitachi

10

S-4800, FE-SEM). Thermo-gravimetric (TG) analysis experiments were carried out

11

using a thermogravimetric analyzer (TA Q600-DSC) in N2 atmosphere with a heating

12

rate of 10 oC/min.

13 14

2.3. Photoelectrochemical measurements

15

Photoelectrochemical measurements were performed on a conventional three

16

electrode configuration with a Pt plate as the counter electrode and a saturated

17

calomel electrode as the reference electrode. 0.5 M Na2SO4 aqueous solution was

18

used as the electrolyte. The working electrode was prepared by the following process:

19

20 mm × 10 mm indium-tin oxide (ITO) glass was used in the present study. One

20

edge of the ITO glass was carefully covered with insulating tape, leaving a 10 mm ×

21

10 mm exposed conductive side of ITO glass. 10 mg of photo-catalyst was dispersed

22

in a mixed solution of 1 mL of ethanol and 10 µL of naphthol to make a homogeneous

23

suspension, in which the exposed area of the ITO glass dip-coated. After drying in an

24

oven at 80 oC for 10 h, the insulating tape on ITO glass was removed. The

25

photocurrent was detected by the UV (125 W high pressure mercury lamp) on-off

26

process with a pulse of 50 s on-off cycle by chronoamperometry. The electrochemical

27

impedance spectroscopy (EIS) was performed in a frequency range from 10-2 to 105

28

Hz with a bias potential of 2.0 V.

29 30

2.4. Activity test 6

1

The photocatalytic degradation of gaseous toluene was performed at ambient

2

temperature in a continuous quartz photoreactor [38]. Briefly, the gaseous toluene was

3

obtained by flowing N2 into a saturator filled with toluene solution at ice bath, the

4

flow rate of N2 was regulated by mass flow controllers to 0.25 mL/min. The 20:80

5

vol% O2:N2 compressed air was divided into two paths, one served for the

6

humidification of air and the other one transported dry air (Scheme 1). The

7

humidification was obtained by bubbling air through a saturator containing deionized

8

water at room temperature. Both air flows were regulated by flow meters and the

9

relative humidity in the reactor remain constant at 60%. Finally, the toluene gas was

10

mixed with air to obtain a flow rate of 100 mL/min with 25 ppm toluene in the gas

11

mixture. Two fans were fixed around the lamp to keep room temperature in the flow

12

system. 100 mg of photocatalyst power was evenly placed on a 250-mesh screen in

13

the middle of the quartz reactor for each experiment (Scheme 1c). A 125 W high

14

pressure mercury lamp (Philips) with an intensity of 50 mW/cm2 (UV at 365 nm as

15

shown in Scheme 1b) was used as a UV light source. The concentration of the outlet

16

toluene was determined via 100 uL effluent gas was automatically injected into the

17

gas chromatograph (GC-2014, Shimadzu, Japan) system equipped with a flame

18

ionization detector (FID) at each 20 min interval. A methanizer, using a Ni-based

19

catalyst, was integrated into the GC system to analyze outlet CO2 concentration. The

20

GC was calibrated via external standard method and the conditions were set as

21

follows: N2 constant flow of 36 mL/min, injector temperature 200 °C, detector

22

temperature 250 °C, methanizer temperature 360 °C and automatic injection valve

23

temperature 60 °C.

24

The photocatalytic degradation of gaseous acetaldehyde under UV light was same

25

to that of toluene but using a quartz reactor (L × i.d.= 10 cm × 6 mm) (Scheme 1d)

26

instead of 120 mL quartz photo-reactor.

27

The active species trapping experiments were conducted with different radical

28

scavengers of AgNO3 for e-, triethanolamine (TEOA) for h+, p-benzoquinone (BQ)

29

for •O2-, isopropanol (IPA) for •OH [39-41]. Typically, 1 mmol of agents were

30

dissolved in 10 mL of ethanol by ultrasonic at room temperature. Then 500 mg of the 7

1

photocatalysts were added and ultra-sonicated for 30 min to form a uniform

2

suspension. After that, the suspension was dried at 60 °C overnight. The testing

3

methods for photocatalytic oxidation of toluene with the catalyst treated by

4

scavengers is the same as mentioned above.

5

The VOCs conversion is evaluated using Eqs. (1), (2) and (3) : Toluene or acetaldehy conversion (%) 

CO  Ct 100% CO

(1)

(2) (3) 6

where C0 is the concentration of toluene (C0 = 25 ppm) or acetaldehyde (C0 = 30 ppm)

7

at the start time of the reaction (t0 = 0), and C tCO 2 and Ct are the concentrations of

8

CO2 and toluene or acetaldehyde at the reaction time (ti= t), respectively.

9 10

3. Results and discussion

11

3.1. XRD analysis

12

The XRD patterns were measured to investigate the crystal phase, crystallinity and

13

purity of pure TiO2, UiO-66-NH2 and TiO2-UiO-66-NH2 series composites. As shown

14

in Fig. 1, the sharp intensity of diffraction peak demonstrate good crystallinity of

15

as-synthesized UiO-66-NH2. For TiO2, the XRD peaks at 2= 25.4°, 37.8°, 48.0°, 54.0°

16

and 55.1° can be indexed to the (101), (004), (200), (105), (211) and (204) crystal

17

planes of anatase TiO2, respectively. In addition, characteristic diffraction peaks at

18

2= 27.4°, 36.1° and 41.2° can be attributed to the (110), (101) and (111) faces of

19

rutile TiO2, respectively. As for the TiO2-UiO-66-NH2 samples, the XRD

20

characteristic peaks of UiO-66-NH2 and TiO2 are clearly displayed in the composites,

21

indicating a two-phase composition of UiO-66-NH2 and TiO2. Meanwhile, the peak

22

intensities ascribed to TiO2 gradually decreased and those assigned to UiO-66-NH2

23

increased with the increase the content of UiO-66-NH2 in the nanocomposites.

24 8

1

3.2. Thermogravimetric analysis

2

The TG analysis in N2 atmosphere were conducted to investigate the thermal

3

stability of TiO2, UiO-66-NH2 and TiO2-UiO-66-NH2 series composites. As shown in

4

Fig. 2, only one weight loss peak at 50~200 ºC can be observed for the pure TiO2,

5

attributing to the desorption of surface water by evaporation. The TG analysis results

6

of UiO-66-NH2 and TiO2-UiO-66-NH2 samples are similar and both presented three

7

weight loss steps. The first weight loss peak at 50~150 C can be attributed to surface

8

water evaporation, the second one at 200~400 C can be assigned to DMF

9

decomposition, and the third one at 400~600 C can be attributed to the framework

10

collapse of UiO-66-NH2 [42]. Thus, the pure UiO-66-NH2 exhibits a high thermal

11

stability and the results agreed with previous reports [43]. Moreover, it is worthy note

12

that the nanocomposites possess relatively higher thermal stability compared with the

13

pristine UiO-66-NH2. Notably, with the increase of TiO2 content in the composite

14

samples, the thermal stability of the composite become higher. This is because the

15

decomposition amount of DMF and H2ATA ligands decreases with the increase of

16

TiO2 content in nanocomposites [20,44]. Thus, it can be concluded that the

17

nanocomposites maintain a stronger thermal stability.

18

Furthermore, the real contents of TiO2 and UiO-66-NH2 in MOF-based

19

nanocomposites also quantified by TG. The residual quantity of UiO-66-NH2, TiO2

20

and TiO2-UiO-66-NH2 series composites can be obtained from Fig. 2. Then the TiO2

21

to UiO-66-NH2 mass ratio (

22

equation [27]:

23

mT ) in composites can be calculated by the following m NU

r r mT  TUN UN m UN rT  rTUN

(4)

24

where rTUN , rT and rUN are the residual mass fraction (%) of TiO2-UiO-66-NH2,

25

pure TiO2 and pristine UiO-66-NH2, respectively. As shown in Table 1, the real TiO2

26

and UiO-66-NH2 contents are consistent with the theoretically values.

27

9

1

3.3. Isothermal N2 adsorption-desorption

2

The specific surface area and porosity of the synthesized samples were analyzed by

3

N2 adsorption-desorption measurements at 77 K. As shown in Fig. 3a, the isotherm of

4

UiO-66-NH2 exhibits a typical type I curve with a sharp increase at low pressure

5

regions, indicating its microporous characteristics [45]. The commercial TiO2 shows a

6

type IV isotherm with a H3 typical hysteresis loop due to the aggregations of TiO2

7

crystallites. The isotherm of 75TiO2-25UN exhibits hybrid type I/IV, indicating the

8

presence of a combination of micropores and mesopores. 75TiO2-25SBA-15,

9

75TiO2-25TS-1 and 75TiO2-25AC were also measured to compare with

10

75TiO2-25UN. As shown in Fig. 3b, the hysteresis loops of all the composites exhibit

11

type H3 at high-pressure zone (P/Po > 0.85), suggesting the presence of mesopores.

12

75TiO2-25SBA-15 exhibits a bimodal hysteresis loop distributing at both low relative

13

pressure zone (0.6 < P/Po < 0.8) and high-pressure zone ( P/Po > 0.85 ), which is

14

ascribed to H1 and H3 hysteresis loop, respectively. 75TiO2-25UN nanocomposites

15

shows higher specific surface area than other TiO2-based composites (Table 2).

16

Studies have pointed out that gas-solid photocatalytic reactions take place on the

17

surface of catalysts [46-47]. Thus, the higher specific surface area will increase the

18

dispersion of TiO2 and enhance the light absorption capability, which are expected to

19

enhance the photocatalytic reactivity.

20

The pores distribution of TiO2-UiO-66-NH2 series composites can be confirmed by

21

Fig. 3f. The pore size distribution curves depicted for UiO-66-NH2 and 75TiO2-25UN

22

are similar (Fig. 3c) due to pure TiO2 is non-porous material. Micropore size

23

distribution analyses indicated that the diameter of tetrahedral and octahedral pore in

24

UiO-66-NH2 has a diameter of 0.68 and 1.49 nm, respectively (Fig. 3c and Table 2),

25

in accordance with the crystal model (Fig. 3e) [48]. While the micropore pore size of

26

75TiO2-25UN (0.64 and 1.26 nm) is a little smaller than that of the pure UiO-66-NH2

27

due to the blockage by TiO2 nanoparticles. 75TiO2-25AC and 75TiO2-25TS-1 are

28

both micropores distribution because AC as an industrial scale porous materials has

29

enrich micropore structure distribution [49] and TS-1 is a typical micropore materials

30

[50] (Fig. 3d and Table 2). As for 75TiO2-25SBA-15, there is no micropore size value 10

1

because SBA-15 is a typical mesoporous molecular sieve (Fig. 3d and Table 2).

2

Ordered and regular channel structure would facilitate oxygen diffusion to the active

3

site to boost the oxidation capacity of photocatalytic [7,17,38]. Thus, appropriate

4

pores structure is beneficial to the photocatalytic oxidation of toluene or acetaldehyde

5

and deactivation resistance of catalysts.

6 7

3.4. Surface morphology

8

SEM is used to investigate the morphology, crystallization and distribution of the

9

catalysts. As shown in Fig. 4a, UiO-66-NH2 displays a smooth and regular octahedral

10

shape with sharp edges, and the average size of the edge are approximately 200 nm.

11

TiO2 exhibits an agglomerated pellets structure (Fig. 4b). The SEM images of the

12

composites (Fig. 4c~f) exhibit that TiO2 is well dispersed on the surface of

13

UiO-66-NH2. The dispersed TiO2 particles provides more active sites to be irradiated,

14

which is beneficial to the contact between TiO2 and VOCs in the photocatalytic

15

oxidation process.

16 17

3.5. Optical properties

18

To investigate the interaction between the two components in the composites, the

19

catalysts were further studied by UV-vis diffuse reflectance spectra. As shown in Fig.

20

5a, the pure UiO-66-NH2 shows a strong absorption in the region of approximately

21

300~450 nm. Pure TiO2 exhibits a steep absorption edge only in the UV region. The

22

optical property of UiO-66-NH2 are well inherited to TiO2-UiO-66-NH2 series

23

composites, and the adsorption bands of composites exist between that of TiO2 and

24

UiO-66-NH2. The absorption band of the composite exhibits a red shift with the

25

increase of the UiO-66-NH2 content. For comparison, SBA-15, TS-1, AC and their

26

composites were also measured by the same way (Fig. 5b). Since the color of the AC

27

is black, AC and 75TiO2-25AC both have good absorption in the region of 230~800

28

nm. As presented in Fig. 5b, it can found that light absorption of TS-1 and SBA-15

29

are very poor in the region of 230~800 nm, especially SBA-15 almost no light

30

absorption. Thus, the light absorption of 75TiO2-25TS-1 and 75TiO2-25SBA-15 are 11

1

similar with pure TiO2.

2

The band gap energy (Eg) of TiO2 and HOMO-LOMO gap of UiO-66-NH2 are

3

displayed in Fig. 5c and d, and the band gap of UiO-66-NH2 and TiO2 were calculated

4

to be 2.85 and 3.28 eV, respectively. Some researches indicated that LUMO potential

5

of UiO-66-NH2 is at -0.6 eV [26,44], thus its HOMO potential is at 2.25 eV.

6

Meanwhile, studies point out that ECB potential of TiO2 is at -0.28 eV [29,51], thus its

7

EVB potential is at 3.00 eV.

8 9 10

3.6 Photocatalytic activity and stability 3.6.1 Photocatalytic activity

11

The photocatalytic activity of TiO2-UiO-66-NH2 series composites was evaluated

12

by photocatalytic oxidation of gaseous toluene in a continuous flow reactor under

13

room temperature and atmosphere pressure, and the results are presented in Fig. 6.

14

Under the dark condition, the sag curves were caused by the toluene

15

adsorption/desorption process and the adsorption capacity decreased in the order of:

16

UiO-66-NH2 > 25TiO2-75UN > 50TiO2-50UN > 75TiO2-25UN ≈ 90TiO2-10UN ≈

17

TiO2 (Fig. 6a), but the channel structure of these catalyst are similar except pure TiO2

18

(Table 2). Thus, it can be conclude that regular channel can not improve adsorption.

19

When the UV light was turned on, the toluene concentration rapidly decreased and

20

gradually reached a steady state concentration for all samples. For TiO2, the

21

conversion rate decreased from 80% to 44% after 240 min of UV light irradiation

22

because of the blockage of active sites by carbonaceous residues, which was

23

consistent with previous reports [7,17]. The photocatalytic activity of pure

24

UiO-66-NH2 was very poor, indicating that UiO-66-NH2 itself could not act as an

25

effective photocatalyst. In contrast, TiO2-UiO-66-NH2 series composites exhibited

26

excellent photocatalytic activity, and the conversion of toluene over the

27

TiO2-UiO-66-NH2 catalysts increased with the UiO-66-NH2 content added from 10

28

wt% to 25 wt%, and then decreased with further increase of UiO-66-NH2 content.

29

Thus, the 75TiO2-25UN catalyst exhibited the highest photocatalytic activity with

30

72.70% of toluene conversion after reaction for 240 min, which was higher than the 12

1

sum of conversion on TiO2 (44.22%) and UiO-66-NH2 (7.48%). Therefore, the

2

conversion of 75TiO2-25UN is 1.64 and 9.72 times higher than that of pure TiO2 and

3

pristine UiO-66-NH2, respectively. In contrast, 25TiO2-75UN, 50TiO2-50UN and

4

90TiO2-10UN removed 47.16%, 60.00% and 62.76% of toluene in the air-flow,

5

respectively.

6

The production amount of CO2 from toluene removal on these samples are shown

7

in Fig. 6b. Under dark condition, CO2 was not detected, meaning that UV light was a

8

requirement for the reaction. When the light was turned on, the initial CO2

9

concentration in the outlet gas increased rapidly and then gradually decreased to be

10

stable, which was in accordance with the photocatalytic toluene activity. The

11

TiO2-UiO-66-NH2 nanocomposites achieved higher CO2 production than TiO2 (38.4

12

ppm) and UiO-66-NH2 (4.0 ppm), especially 75TiO2-25UN showed the highest CO2

13

production (84.5 ppm) and selectivity (66.5%), that its CO2 production is about 2.2

14

and 21.1 times higher than commercial TiO2 and UiO-66-NH2, respectively, within

15

240 min of light irradiation.

16

To verify the superiority of UiO-66-NH2 compared with traditional porous

17

materials for the photocatalytic oxidation of toluene, the photocatalytic activity of

18

different TiO2-based porous material were also investigated. As shown in Fig. 7a and

19

b, 75TiO2-25UN exhibited much higher and more stable toluene conversion and CO2

20

selectivity than the traditional TiO2-based porous composites, suggesting the

21

superiority of UiO-66-NH2 as a component of composite to enhance the

22

photocatalytic activity of TiO2. Meanwhile, we also compared our work with some

23

relevant studies (Table 3), and the difficulty of each reaction conditions was divided

24

into ‘Very difficult’, ‘Difficult’ and ‘Moderate’ based on the flow rate (static or

25

dynamic), residence time, photocatalytic activity and CO2 selectivity. The materials

26

and photocatalytic system in this work exhibit higher photocatalytic activity and CO2

27

selectivity under shorter residence time compare to the other studies in Table 3. Thus,

28

our reaction conditions are considered more difficult than the other work, and labeled

29

‘Very difficult’ in Table 3. The 75TiO2-25UN nanocomposites has higher surface area

30

than traditional TiO2-based porous material (Table 2), and studies also pointed out 13

1

that large surface area could enhance the activity due to the dispersal of active species

2

and light harvest. Thus, surface area plays an important role in attaining the high

3

photocatalytic oxidation of toluene. Moreover, it was found that all materials existed

4

porous structure (Fig. 3), suggesting that oxygen could easily and quickly diffused

5

into pores and produced oxygen-rich conditions around the active sites. Thus,

6

MOF-based samples have a unique impact on enhancing the photocatalytic

7

performance for VOCs oxidation due to the advantages of large surface area, porous

8

structure and semiconductor property.

9

Also, the photocatalytic degradation of acetaldehyde on the different samples were

10

carried out under UV light irradiation. As can be seen in Fig. 8, under the dark

11

condition, pure TiO2 has higher acetaldehyde adsorption capacity than the other

12

TiO2-based porous materials, and all TiO2-based porous materials exhibited ordered

13

and regular channel except pure TiO2 (Fig. 3d and Table 2). Therefore, it can be

14

concluded that channel structure cannot play an important role for acetaldehyde

15

adsorption. When the light turned on, the 75TiO2-25UN composites showed much

16

higher acetaldehyde conversion (70.74%) and CO2 production (41.54 ppm) than other

17

TiO2-based porous materials, and 98% of the removed acetaldehyde was mineralized

18

into harmless CO2. So the conversion and CO2 production of 75TiO2-25UN is 10.5

19

and 14.3 higher than that of pure UiO-66-NH2, respectively. Thus, it indicated that the

20

combination of TiO2 and UiO-66-NH2 can significantly enhance the photocatalytic

21

performance of the composite for VOCs oxidation.

22 23

3.6.2 Catalyst stability

24

The stability of the photocatalyst is very important for practical application. The

25

75TiO2-25UN composite was used as the representative sample to examine the

26

long-term stability under UV light irradiation. As shown in Fig. 8, the photocatalytic

27

activity of 75TiO2-25UN did not exhibit a significant decrease after 720 min of

28

irradiation. Furthermore, XRD and IR results of the used samples had no apparent

29

change compared with the fresh one (Fig. 9a and b). Additionally, the morphology

30

structure of the fresh and used samples were almost the same (Fig. 9c and d). These 14

1

results indicated that the prepared TiO2-UiO-66-NH2 composite had a high stability

2

during the photocatalytic oxidation of toluene and acetaldehyde.

3 4

3.7 Reaction mechanism

5

To unveil the separation efficiencies of the photogenerated electron-hole pairs,

6

photocurrent analyses were conducted for TiO2, UiO-66-NH2, 75TiO2-25UN and

7

other TiO2-based porous composite through on-off cycles under UV light irradiation.

8

As shown in Fig. 10a, the pure UiO-66-NH2 exhibits a very low photocurrent density

9

due to the fast recombination of photogenerated carriers. The photocurrent response

10

of 75TiO2-25UN was obviously enhanced compared with pure TiO2 and pristine

11

UiO-66-NH2, indicating the efficient photogenerated carriers transfer between the

12

interface of TiO2 and UiO-66-NH2. Compared with other TiO2-based porous

13

composite, 75TiO2-25UN also exhibited the highest photocurrent density (Fig. 10b),

14

suggesting the much higher efficiency of charge transfer between TiO2 and

15

UiO-66-NH2. This evidence was also supported by the electrochemical impedance

16

spectroscopy (EIS) results (Fig. 10c and d). It can be found that 75TiO2-25UN

17

exhibited the smallest arc semicircle, indicating that a “nanoscale mixing” would

18

promote electron transfer. These observations indicated the efficient charge transfer of

19

75TiO2-25UN which was responsible for the excellent photocatalytic activity.

20

To elucidate the possible photocatalytic reaction mechanism for the oxidation of

21

toluene over TiO2-UiO-66-NH2, a series active species trapping experiments were

22

performed with different radical scavengers. In this case, BQ, TEOA, AgNO3 and IPA

23

are used as scavengers for •O2-, h+, e- and •OH, respectively. As shown in Fig. 11, the

24

catalyst almost deactivated when it was treated by BQ, meaning that •O2- should be

25

the main active species for oxidation of toluene. When the catalyst was treated by

26

TEOA and AgNO3, the toluene removal efficiency was depressed, suggesting that the

27

h+ and e- play important roles in the photocatalytic oxidation reaction. When the

28

sample was treated by IPA, the toluene conversion just slightly decreased, implying

29

that •OH radicals were not the main active species. Thus, the photocatalytic reaction

30

formulas can be expressed as following: 15

1

TiO2-UiO-66-NH2 + UV light → h+ + e-

2

e- + O2 → •O2-

3

•O2- + toluene → CO2 + H2O

4

h+ + toluene → CO2 + H2O

5

h+ + H2O → •OH + H+

6

•OH + toluene → CO2 + H2O

7

Based on the above experimental results, a possible photocatalytic reaction

8

mechanism for VOCs oxidation over the TiO2-UiO-66-NH2 composite was proposed

9

in Scheme 2. Single UiO-66-NH2 contributes little to the oxidation of toluene or

10

acetaldehyde. However, the high surface area of UiO-66-NH2 can increase the TiO2

11

dispersion and enhance the light absorption capability. Simultaneously, the

12

interconnected 3D porous network of UiO-66-NH2 is beneficial to the O2 diffusion

13

and light penetration. TiO2 is the active center for toluene and acetaldehyde

14

photocatalytic oxidation over the nanocomposite catalyst. Under light irradiation, O2

15

molecules can not only fast and easily diffuse into the pores of UiO-66-NH2 and be

16

adsorbed on the Zr3+ sites to form •O2-, but also produce an oxygen-rich condition

17

around TiO2 [17,18]. UiO-66-NH2 and TiO2 both can be excited by UV light due to

18

semiconductor properties. Thus, compared with pure TiO2 and UiO-66-NH2, the

19

TiO2-UiO-66-NH2 composite cooperate well to achieve outstanding catalytic activity

20

and CO2 selectivity. Moreover, a possible interface electron transfer behavior is

21

demonstrated. Under UV light irradiation, UiO-66-NH2 and TiO2 both can be excited

22

and generate electron-hole pairs. Since the CB potential of UiO-66-NH2 (-0.60 eV vs.

23

NHE) is more negative than that of TiO2 (-0.28 eV vs. NHE), the photoinduced

24

electrons on the CB of UiO-66-NH2 can directly and easily transfer to the CB of TiO2.

25

Moreover, the oxidation potential of O2/•O2- (-0.046 eV vs. NHE) is more negative

26

than the CB of UiO-66-NH2 and TiO2. Thus, the photoinduced electrons can be

27

captured by the dissolved O2 to produce •O2- radicals for toluene oxidation.

28

Simultaneously, the excited holes produced by TiO2 were injected into the VB of

29

UiO-66-NH2, and it can oxidize the toluene and acetaldehyde to CO2 directly. Thus,

30

the charge transfer efficiently inhibits the recombination of photogenerated 16

1

electron-hole pairs. In addition, the oxidation potential of H2O/•OH (1.99 eV vs NHE)

2

is lower than the VB of UiO-66-NH2 and TiO2. Thus, parts of photoinduced holes can

3

oxidize the adsorbed water molecules to form •OH radicals for toluene oxidation.

4 5

4. Conclusions

6

In summary, TiO2-UiO-66-NH2 series photocatalysts with high activity and

7

stability were successfully fabricated via a simple solvent evaporation method. The N2

8

adsorption-desorption isotherm and SEM results indicated the large surface area could

9

promote

the

TiO2

dispersion,

light

harvesting

and

mass

transfer.

The

10

photoelectrochemical property results confirmed the photo-excited charge carriers

11

could effective transfer and separate via interface due to semiconductor properties.

12

Photocatalytic oxidation of VOCs demonstrated that the photocatalytic activity and

13

the CO2 selectivity of TiO2-UiO-66-NH2 composites were significantly enhanced

14

compared with TiO2, UiO-66-NH2 and other traditional TiO2-based porous materials.

15

The composite with 25 wt% of UiO-66-NH2 presented the highest toluene and

16

acetaldehyde conversion and CO2 selectivity because the effective contact between

17

TiO2 and UiO-66-NH2 are able to promote the irradiation of light and make full use of

18

active sites. Moreover, the nanocomposite exhibited superior structure and reactivity

19

stability after 720 min of continuous evaluation. Active species trapping experiments

20

confirmed the photoinduced •O2- and h+ are the major active species for VOCs

21

oxidation. The above results indicated that the TiO2-UiO-66-NH2 nanocomposite

22

photocatalyst is an alternative candidate for possible practical application in VOCs

23

purification.

24 25

Acknowledgments

26

This work was supported by the National Natural Science Foundation of China

27

(21777047, 5157824) and the scientific research project of Guangzhou city

28

(201804020026).

29 30 17

1

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22 23 24 25 26 27 28

23

(a)

1

(b)

(c)

2

(d)

3 4

Scheme 1. The schematic diagram of the VOCs removal system (a), emission

5

spectrum of the UV lamp (b), schematic illustration of the used set-up (c and d).

6 7 8 9 10 11 12 13 14 24

1 2

Fig. 1. XRD patterns of TiO2, UiO-66-NH2 and TiO2-UiO-66-NH2 series composites

3 4 5 6 7 8 9 10 11 12 13 14 15 16

25

1 2

Fig. 2. TG cures of TiO2, UiO-66-NH2 and TiO2-UiO-66-NH2 series composites

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

26

(a)

(b)

(c)

(d)

1

2

(e)

3

(f)

4 5

Fig. 3. N2 adsorption-desorption isotherms (a,b), Horvath-Kawazoe (HK)

6

micropore size distribution(c,d), from left to right: structure, tetrahedral and

7

octahedral cages for UiO-66-NH2, respectively (e) and pore size distribution (f) of

8

samples. 27

(a)

(b)

(c)

(d)

(e)

(f)

1

2

3 4 5

Fig. 4. FE-SEM images of UiO-66-NH2 (a), TiO2 (b), 25TiO2-75UN (c), 50TiO2-50UN (d), 75TiO2-25UN (e) and 90TiO2-10UN (f).

6 7 8

28

(a)

(b)

(c)

(d)

1

2 3

Fig. 5. UV-vis diffuse reflectance spectra and Tauc plot of the samples.

4 5 6 7 8 9 10 11 12 13

29

(a)

1 2

Fig. 6. Toluene removal (a) and generated CO2 concentration (b) over

3

TiO2-UiO-66-NH2 composites under dark and then UV light conditions.

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

30

(b)

(a)

(b)

1 2

Fig. 7. Photocatalytic degradation of toluene (a) and generated CO2 concentration (b)

3

over different catalysts.

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

31

(a)

1 2 3

Fig. 8. Photocatalytic oxidation of acetaldehyde on different catalysts under UV light.

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

32

(b)

(a)

(b)

(c)

(d)

1

2 3

Fig. 9. XRD patterns (a), FT-IR spectra (b) and SEM [Fresh (c), Used (d)] images of

4

75TiO2-25UN before and after the photocatalytic reaction under UV light irradiation

5

for 720 min.

6 7 8 9 10 11 12 13

33

(a)

(b)

(c)

(d)

1

2 3

Fig. 10. Photocurrent responses (a, b) and EIS Nyquist plots (c, d) of the samples.

4 5 6 7 8 9 10 11 12

34

1 2

Fig. 11. Photocatalytic activities of 75TiO-25UN by different treatment to trap active

3

species under UV irradiation.

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

35

1 2 3

Scheme 2. Possible reaction mechanism for the photocatalytic oxidation of VOCs on the TiO2-UiO-66-NH2 composite.

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 36

1

Table 1 The UiO-66-NH2 and TiO2 contents in the composites Sample

Residual mass fraction (%)

TiO2 content (%)

UiO-66-NH2 content (%)

TiO2

98.12

100

0

90TiO2-10UN

93.65

9.39

90.38

9.62

75TiO2-25UN

87.42

3.34

76.96

23.04

50TiO2-50UN

75.41

1.04

50.98

49.02

25TiO2-75UN

62.38

0.30

23.08

76.92

UiO-66-NH2

51.68

0

0

100

mT / mUN

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 37

1

Table 2 Textural property of the as-prepared photocatalysts SBET

Total pore

Average pore

Micropore size

(m2/g)

volume (cm3/g)

size (nm)

(nm)

UiO-66-NH2

1121.80

0.73

2.59

0.68, 1.49

TiO2

55.99

0.22

15.89

==

75TiO2-25UN

280.56

0.43

6.13

0.64, 1.26

75TiO2-25AC

256.29

0.58

9.038

0.53, 0.92

75TiO2-25SBA-15

204.69

0.65

12.69

==

75TiO2-25TS-1

152.89

0.50

13.07

0.54, 1.70

Sample

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

38

1

Table 3 List of relevant studies in VOCs photocatalytic oxidation Reference

This work

Journal of Colloid and Interface Science, 2018, 522, 174-182 Applied Catalysis B: Environmental 2018, 224, 705-714 Applied Catalysis B: Environmental, 2014, 146: 123-130 Applied Catalysis B: Environmental, 2018, 221: 215-222.

Chemical Engineering Journal, 2018, 349, 708-718

Chemical Engineering Journal, 2018, 346, 77-84 Chemical Engineering Journal, 2018, 334, 206-215

Material

TiO2-UiO-66NH2

TiO2@UiO-66NH2

NiO-TiO2

commercial TiO2

CuInS2 QDs/Bi2WO6

0.5 wt% rGO-TiO2

1 wt% Pd/TiO2

PDA-PbO2/TiO 2 nanotube arrays

Reaction system Toluene (25 ppm); Acetaldehyde (30 ppm); Dynamic (100 mL/min); Residence time: 72 s (Toluene); 1.7 s (Acetaldehyde) UV light (50 mW/cm2); Time: 720 min; Styrene (30 ppm); Dynamic (35 mL/min); Visible light; Time: 600 min; Toluene (3000 ppm); Static; Solar light (500 mW/cm2); Time: 6 h; Toluene (400 ppmv); Static; UV light (3 mW/cm2); Time: 6 h; Toluene (1 μL toluene injected into a reaction cell of 120 ml); Static; Visible light; Time: 5 h o-xylene (25 ppm); Acetaldehyde (25 ppm); Dynamic (80 mL/min); Residence time: 225 s; UV light; Time: 160 min; Propylene (600 ppm); Static; Visible light (30.4 mW/cm2); Time: 100 min; Toluene (100-300 ppm); Static (liquid phase); UV light; Time: 120 min;

2 39

Conversion & Selectivity

Reaction conditions

Toluene: 72.7% & 66.5% Acetaldehyde: 70.74% & 98.4%

Very difficult

99% & 34.6%

Difficult

80% & 56%

Difficult

46-52% & NA

Difficult

63% & CO2 production of 0.72 mmol

Difficult

o-xylene: 54% & NA Acetaldehyde: 42% & NA

Difficult

63% & NA

Moderate

66 % & 62.3 mol%

Moderate

1

Graphical Abstract

2 3

4 5 6 7 8

Research Highlights

9



evaporation method.

10 11





16

The composites showed high stability during long-time reaction under flowing conditions.

14 15

The composites exhibited outstanding photocatalytic performance for VOCs degradation.

12 13

TiO2-UiO-66-NH2 nanocomposites were synthesized by a simple solvent



A mechanism for enhanced photocatalytic performance over TiO2-UiO-66-NH2 was proposed.

17 18 19 20 21 22

Notes The authors declare no competing financial interest. 40

1

41