Insights into the Relationship of the Heterojunction Structure and

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Insights into the relationship of the hetero-junction structure and excellent activity: photo-oxidative coupling of benzyl-amine on CeO2-rod/g-C3N4 hybrid under mild reaction conditions Yuanyuan Chai, Lu Zhang, Qianqian Liu, Fengli Yang, and Wei-Lin Dai ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b01865 • Publication Date (Web): 05 Jul 2018 Downloaded from http://pubs.acs.org on July 6, 2018

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ACS Sustainable Chemistry & Engineering

Insights into the relationship of the hetero-junction structure and excellent activity: photo-oxidative coupling of benzyl-amine on CeO2-rod/g-C3N4 hybrid under mild reaction conditions Yuanyuan Chai †, Lu Zhang †, Qianqian Liu †, Fengli Yang †, Wei-Lin Dai *† †

Department of Chemistry & Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200438, P. R. China; Corresponding author: [email protected]

KEYWORDS: photo-oxidation, benzyl-amine, hetero-junction, CeO2, g-C3N4

ABSTRACT: Oxidation of amines to imines under light irradiation has been widely studied in the field of heterogeneous catalysis. For the first time, we demonstrate a facile mixing calcination approach for the preparation of CeO2/g-C3N4-x as a hetero-junction catalyst for the photo-oxidative coupling of benzyl-amine under the irradiation of a 300 W Xe arc lamp at 308 K with the air balloon. It was found that the rate constant of CeR/CN-66% was 3 times as high as that of pure CeO2 or g-C3N4. All kinds of structural characterizations suggested the formation of hetero-junction between the CeO2 and g-C3N4, serving as a tunnel for the transfer of photo-induced charge, which may contribute to the improvement of photo-activity. What’s more, the 1

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Ce3+ ions and oxygen vacancy significantly promoted the adsorption and activation of substrate or O2 molecular. Therefore, the efficient separation of the charges, the prolonged photo-induced electron lifetime, and the abundant defect structure (increased content of Ce3+ ions or the oxygen vacancy) were considered as the main factors for the higher photo-oxidation efficiency of the CeR/CN-66%. Another striking observation noticed that the CeR/CN-66% were recycled up to five cycles under the same condition and found to be highly efficient without any obvious decrease in the activity or selectivity due to the outstanding stability of the defect structure. Thus, highly efficient CeR/CN-x hetero-junction catalyst was synthesized in this work by a simple approach for the photo-oxidation of benzyl-amine under mild reaction conditions, and this finding may get wide application in other photocatalysis area.

2


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INTRODUCTION Imines are a class of nitrogen-based compounds with high reactivity due to the presence of unsaturated C=N double bonds and are important intermediates for the product of agricultural chemicals and pharmaceuticals.1,2 As a traditional approach for imine synthesis, the condensation reaction of amines and carbonyl compounds has been widely studied. However, the prolonged reaction time, activated aldehydes and dehydrating agents were required, which limited the use of this process practically and environmental-friendly.3,4 Recently, the direct oxidation of amines to imines has been considered as an important chemical transformation in the organic synthesis. The stoichiometric

oxidants such

as 2-iodoxybenzoic

acid,

N-tert-butyl-phenyl-

sulfinimidoyl chloride and permanganate have been used for the oxidation of the amines to imines.5 Unfortunately, the emission of a corresponding amount of undesirable toxic waste restricted the use of the stoichiometric oxidants. Besides, the difficulties in product separation were also present in the above system. Therefore, the development of heterogeneous catalysts for the aerobic amine oxidation reaction was captivating and demanding. Numbers of heterogeneous catalytic systems, including ligated metal based catalyst (i.e., Cu(I), Co(II), Ru(II)),6-8 graphene oxide,9 MnO210 and metal organic frameworks,11 have been reported for the oxidation of amines. Despite the higher activity or the good selectivity, most of them typically required relatively high reaction temperature or high pressure.12 Compared with the above thermochemical methods, the light-driven catalysis, where sunlight serves as the energy source for the reaction under the ambient temperature condition, is an attractive method. According to the previous literatures, TiO213 and Nb2O514 were 3



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efficient photo-catalysts for aerobic oxidation of various amines to imines, which responded only to UV light. Besides, Su et al. demonstrated the use of meso-porous g-C3N4 as the photo-catalyst to activate O2 for the selective oxidation of amines with visible light, but high oxygen pressure (0.5 MPa) was needed.15 Thus, the harsh reaction conditions may increase the operational complexities and the energy consumption. Therefore, it is of great interest to explore an efficient photo-catalyst for the highly selective oxidation of amines to imines under mild conditions. It is well known that, as one of the abundant rare earth oxides, CeO2 with the abundant surface oxygen vacancies and the Ce3+ ions have been extensively studied in the heterogeneous catalytic reaction.16-20 What’s more, CeO2, as a light-responsive photo-catalyst, has been demonstrated to be active in photo-catalytic degradation of the dye pollutants and the hydrogen evolution.21-22 Due to the broad band gap (Eg = 2.92 eV), CeO2 only absorbs light in the near UV region, which restricted its application in the visible light irradiation.23 Thus, doping with elements and forming hetero-junctions have been taken to enhance their activity under visible light irradiation.17,24,25 Recently, Huang et al. prepared CeO2/g-C3N4 composites by a simple mixing-calcination technique and found that the as-prepared sample exhibited higher activity than the pure CeO2 and g-C3N4 in the degradation of the methylene blue and the 4-chlorophenol under the visible light irradiation.26 Besides, the remarkably improved photo-redox activity was ascribed to the strong interfacial interaction, which induced more efficient separation and largely reduced recombination probability of photo-excited electron-hole pairs.27 Although the activity 4


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of photo-degradation was improved by the combination of CeO2 and g-C3N4, the inherent relationship between the structure and the activity was not reported till now, such as the surface oxygen vacancy or the concentration of Ce3+. According to the reported data, Ahmad and co-workers demonstrated that the F-doped CeO2 with the higher Ce3+/Ce4+ redox couple showed the excellent performance in the oxidative coupling of amine to imine under higher temperature.28 Additionally, the presence of the surface-active Mn4+/Mn2+ couple and the enhanced defect structure of CeO2 nano-rods are found to be key factors for the high catalytic efficiency of the MnOx/CeO2 nano-rods.29 So far, there is no relative literature to study the application of CeO2 hetero-junctions for the photo-oxidation of amine to imine. The main aim of this work is to investigate the structure-activity relationships of the CeO2/g-C3N4 composite catalyst for the photo-oxidation of benzyl-amine. Theoretical and experimental results showed that the (110) terminated surface is more reactive than the (111) and (100) surfaces for better catalytic activity.30,31 And it has been demonstrated that ceria nano-rods preferentially expose both (100) and (110) facets, while ceria nanoparticles are dominated by (111) surfaces.32 Therefore, we developed a series of CeO2-rod/g-C3N4-x composites through a facile mechanical grinding and calcination method, and firstly investigated the application of the photo-catalysts for the oxidation coupling of benzyl-amines under simulated sunlight irradiation at 308 K with air as the oxidant. Interestingly, we found that the activity of the composites was greatly higher than pure CeO2 and pure g-C3N4 sample, especially for the CeR/CN-66% catalyst, which exhibited the best 5



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performance. For better understanding the relationship of the structure and activity for the obtained photo-catalysts, various characterization techniques, mainly including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), photoluminescence (PL), photocurrent-Time (PT), electrochemical impedance spectra (EIS), field-emission transmission electron microscope (FETEM), thermal gravity analysis (TG), H2-temperature programmed reduction (H2-TPR) and so on, were carried out. To the best of our knowledge, this work is the first report on CeO2-rod/g-C3N4 photo-catalyst for the oxidation coupling of benzyl-amines under mild reaction conditions.

EXPERIMENTAL SECTION

Materials. Cyanamide (5 g, 50% water solution), Ce(NO3)3·6H2O, NaOH and urea were purchased from Aladdin Industrial Inc. All of the reagents used were of analytical grade and were used without further purification. All aqueous solutions were prepared with the deionized water. Preparation of Photo-catalysts. g-C3N4 sample was synthesized by directly heating the mixture of cyanamide (5 g, 50% water solution) and urea (10 g) at 823 K in a muffle furnace for 2 h in a semi-closed system at a heating rate of 10 K min-1. CeO2 nano-rods were prepared via a hydrothermal process.32 Typically, 0.01 mol of Ce(NO3)3·6H2O were dissolved in 20 mL deionized water under vigorous stirring until the corresponding salts were completely dissolved. Meanwhile, 38.4 g of NaOH was dissolved in 60 mL deionized water. Subsequently, the two solutions were mixed 6


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together and kept stirring for 30 min, and the obtained mixed slurry was transferred to a stainless autoclave with 100 mL polytetrafluoroethylene liner and kept at 373 K for 24 h. Upon the stainless autoclave was cooled to room temperature, the precipitates were separated by centrifugation, washed with deionized water and ethanol until pH 7.0. After drying at 353 K overnight, the products were calcined at 823 K for 6 h in muffle oven with ramping rate of 5 K min-1. The CeO2-rod/g-C3N4 composites were prepared by a facile mechanical grinding and calcination method. Typically, a certain amount of CeO2 nano-rod and a suitable amount of g-C3N4 sample were added into a motor and then grounded for 30 min using a pestle. Then the obtained mixed powder was put into a crucible with a cover and then heated at 543 K for 2 h in a muffle furnace with a heating rate of 5 K min-1. The other CeO2-rod/g-C3N4-x composites with different proportions of CeO2 were prepared under the same conditions only by changing the amount of g-C3N4. And the various samples were marked as CeR/CN-x (x: wt% of the CeO2 nano-rod). Typical Procedure for Oxidation of Amines. The photo-catalytic selective oxidation was performed under the irradiation of a 300 W Xe arc lamp (CeauLight, CEL-HXF300, λ > 300 nm, light spectrum is similar with the solar light ) with continuous stirring in 25 mL flat-bottomed quartz container, which was sealed with a glass stopper, equipped with an air balloon and surrounded with water bath to kept at 308 K. Typically, amine (0.10 g) and catalyst (0.05 g) were introduced into the acetonitrile solvent (5.0 g) in the reactor. Prior to irradiation, the mixture was stirred for 0.5 h in dark to ensure the formation of homogeneous suspension. Then, the 7



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reaction was stirred at 308 K for 5 h. The product was quantitatively analyzed by GC-FID

(Huaai,

GC9560)

and

determined

by

the

GC-MS

(Shimadzu,

GCMS-QP5050A) method. According to the decrease of the concentration for amine using anisole as the internal standard, the conversion was determined by GC. Each experiment was repeated twice to confirm the result, and the average values were used. After reaction, the catalyst was recovered and washed thoroughly with ethanol. Then recovered sample was dried at 353 K for 12 h and calcined at 543 K for 2 h. All recycle photo-catalytic reactions were carried out under the same experimental conditions. In view of unavoidable loss of photo-catalyst during the recycling test, several reactions were simultaneously performed for each cycle to collect enough spent catalyst samples. The trapping experiments of radicals and holes were performed according to the methods.33 For instance, 0.01 mL of tertiary butanol (t-BuOH), that used as hydroxyl radical (OH•) scavenger, were added to the above mentioned reaction system. In addition, 0.01 g of benzoquinone (BQ) was used as superoxide radical (•O2-) scavenger and 0.01 g of ammonium oxalate ((NH4)2C2O4) as hole (h+) scavenger. Characterization of Photo-catalyst. The wide-angle XRD patterns were collected on a Bruker D8 Advance X-ray diffractometer using nickel-filtered Cu Kα radiation (λ = 0.15406 nm) with a scanning angle (2θ) range of 20~90o, a scanning speed of 2o min-1, and a voltage and current of 40 kV and 40 mA, respectively. Specific surface areas of the samples were measured by nitrogen adsorption–desorption method at 77 K (Micromeritics Tristar ASAP 3000) using Brunauer–Emmett–Teller (BET) method. 8


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FETEM micrographs were obtained on a FEI Tecnai G2 F20 S-TWIN field-emission transmission electron microscope. Samples for electron microscopy observation were prepared by grinding and subsequent dispersing the powder in ethanol and applying a drop of very dilute suspension on carbon coated grids. XPS experiments were carried out with a RBD 147 upgraded Perkin–Elmer PHI 5000C ESCA systems equipped with a hemispherical electron energy analyzer. The Mg Kα (hν = 1253.6 eV) anode is operated at 14 kV and 20 mA. The spectra were recorded in the constant pass energy mode with a value of 46.95 eV, and all binding energies were calibrated using the carbonaceous C 1s line at 284.6 eV as reference. The experimental errors were within ±0.2 eV. PL spectra were obtained using a Hitachi F-4500 Fluorescence spectrophotometer with an excitation wavelength of 360 nm using a Xe lamp as the excitation source. Electrochemical measurements were performed on a CHI660E electrochemical workstation system with a standard three-electrode cell. The low-temperature EPR spectra were performed on a JES-FA200 spectrometer at 77 K. The FT-IR (Fourier transform infrared) spectra were used to identify the functional groups of the catalyst on a Nicolet Avatar-360 FT-IR spectrometer.

RESULTS AND DISCUSSION

Photo-oxidation

Activity

of

Benzyl-amine

over

Different

Catalysts.

Photo-oxidation of benzyl-amine to the N-benzylidene was carried out to evaluate the catalytic performance. In a typical experiment, 0.10 g benzyl-amine and 0.05 g anisole were dissolved in 5.0 g acetonitrile in the presence of 0.05 g CeR/CN-66% 9



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catalyst under air at 308 K for 5 h. As displayed in Table 1, blank reaction was performed in the absence of either photo-catalyst or light irradiation and almost no product was obtained, which confirmed that the transformation is truly driven by a photo-catalytic process. To our delight, in the presence of visible light under identical conditions, excellent conversion to corresponding imine was obtained (entry 3). In addition, we found that there was almost no increase in the conversion when the experiments were carried out under atmosphere of pure molecular oxygen, using O2 balloon (entry 4). However, there is no any products formed when the reaction was done under the N2 atmosphere (entry 5). Therefore, the interesting observations clearly suggest that the supply of oxygen from air is critical for the catalytic activity, which would be further explored in the mechanism part. Table 1. Oxidative coupling of benzyl-amine by CeR/CN-66% under different condition a

a

Entry

Catalyst

hʋ

Con. (%)b

Sel. (%)b

1

+

-

3.6

>99

2

-

+

1.3

>99

3

+

+

81.2

>99

4c

+

+

82.2

>99

5d

+

+

0.3

>99

Reaction condition: benzylamine, 100 mg; photocatalyst, 50 mg; anisole, 50 mg; acetonitrile, 5.0

g; Xe arc lamp; time, 5 h; air balloon; 308 K.

b

Determined by GC using anisole as internal

standard and confirmed by GC-MS. c Under an atmosphere of O2. d Under an atmosphere of N2.

Subsequently, time-dependent experiments were carried out for the photo-catalytic 10


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coupling of the benzyl-amine using the catalyst. As shown in Figure 1a, the conversion of benzyl-amines was remarkably increased along with the irradiation time. And it could be observed that the conversion of the benzyl-amine was 81.2% at 5 h reaction time over CeR/CN-66% composite. Obviously, the photo-catalytic activities of the CeO2 and g-C3N4 sample alone were extremely lower than that of the composite. The conversions were 45.8% and 43.2% for the CeO2 and g-C3N4, respectively. And the selectivity was all above 95%. Thus, it is expected that the present work could offer a useful direction on light-driven catalytic coupling of amines to imines under ambient conditions by using CeR/CN-66% composite as photo-catalyst. Compared with the CeO2NP/g-C3N4, the CeR/CN-66% composite

100

g-C3N4 CeO2

80

Conversion of benzylamine (%)

exhibited much higher activity under the same conditions (Figure S1).

Conversion of benzylamine (%)

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


a

CeR/CN-66% 60 40 20

80

b

60

40

20

0

0 0

1 2 3 Irradiation Time (h)

4

5

% 3% 5% N4 O2 6% 0% 85 -3 -7 -6 -5 C 3 Ce N N N N Ng C C C C / C R R/ R/ R/ R/ Ce Ce Ce Ce Ce

Figure 1 (a) Effect of reaction time on the photo-oxidation of benzyl-amine over different catalyst. (b) Performance of various catalysts for the photo-catalytic coupling of the benzyl-amine with 5 h irradiation. Reaction conditions: catalyst (0.05 g), benzyl-amine (0.1 g), anisole (0.05g), acetonitrile (5.0 g), in air, 308 K. For the sake of investigation on the interaction of CeO2 and g-C3N4, the activity of 11



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CeR/CN-x composites with different ratio of CeO2 was also evaluated under the same conditions. As shown in Figure 1b, the conversion of benzyl-amine for the photo-catalyst

followed

the

sequence:

CeR/CN-66%

>

CeR/CN-75%

>

CeR/CN-50% > CeR/CN-85% > CeR/CN-33% > g-C3N4 = CeO2. Evidently, the composites exhibited much higher activity than the pure CeO2 and g-C3N4 sample in the photo-catalytic coupling of the benzyl-amine. It can be found that the CeR/CN-66% catalyst demonstrated the highest conversion. Additionally, the kinetics rate constants (k) of the benzyl-amine oxidation rate for the samples was presented in Table 2, and it is obviously found that the reaction rate constant of CeR/CN-66% was 3 times as high as that of CeO2 or g-C3N4 for the photo-catalytic coupling of benzyl-amine. Interestingly, we found that the ratios of CeO2 and g-C3N4 have significant effect on the photo-activity. And compared with CeR/CN-66% sample, the CeR/CN-33% and CeR/CN-50% composites exhibited relatively lower specific activity. However, the activity significantly decreased when the ratio of the CeO2 was increased to 75% and 85%. The reasons would be discussed in the following sections. Table S1 exhibited the performance of the various catalysts reported elsewhere in this reaction. Compared with these materials, the CeR/CN-66% sample reveals the excellent activity under the mild reaction conditions. As a kind of heterogeneous catalyst, the BET surface area of the material is a key factor in the photo-catalytic coupling of the benzyl-amine. The specific BET surface area of the pure CeO2 is 87.9 m2•g-1, 1.4 times higher than that of the CeR/CN-66% sample, as shown in Table 2. Meanwhile, the reaction kinetic rate constant of the 12


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CeR/CN-66% was 2.8 times higher than that of the pure CeO2. What's more, the surface area of CeR/CN-33%, CeR/CN-50%, CeR/CN-66%, CeR/CN-75% and CeR/CN-85% were 33.4, 48.1, 61.2, 55.1 and 58.7 m2•g-1, respectively. Although the CeR/CN-66% with the highest activity has the largest surface area in the composites, the order does not match well with the conversion of benzyl-amine for the other composites. These results indicated that the high specific surface area is important but not the only reason for the enhancement of the photo-catalytic activities in the photo-catalytic coupling of benzyl-amine. Table 2. Surface area, kinetic rate constant, lattice parameter and atomic ratios of the various catalysts. CeR/CN -50%

CeR/CN -66%

CeR/CN -75%

33.4

48.1

61.2

55.1

58.7

0.115

0.185

0.247

0.320

0.279

0.213

0.160

0.171

0.179

0.189

0.185

0.174

Sample

g-C3N4

BET (m2•g-1)

29.2

87.9

k (h-1)

0.117

Ce3+ /(Ce +Ce4+ )a

---

3+

a

CeO2

CeR/CN -33%

CeR/CN -85%

Determined by XPS spectra.

Catalysts Characterizations. We speculate that the interaction between CeO2 nano-rod and g-C3N4 may be a vital reason for the enhanced photo-activity of the CeR/CN-x composites. And so far, there is no relative report about the effect of structural properties on the activity in the photo-catalytic coupling of benzyl-amine for the CeR/CN-x composites. Therefore, many kinds of characterization techniques were performed to explore the structure of the composites and to reveal the dominant reasons for the higher activity of the CeR/CN-x sample. 13



Page 14 of 38

Firstly, the XRD patterns of CeO2, g-C3N4 and CeR/CN-x samples were presented in Figure 2. The diffraction peaks of CeR/CN-x assigned to the (111), (200), (220), (311) and (222) planes are indexed to the cubic fluorite structure of CeO2 crystals, and the strongest peak of the CeO2 is attributed to (111) plane, which was consistent well with the following HRTEM images. Besides, there is no peak attributed to the g-C3N4, and the phenomenon was also reported by Zhang’s group.27 What’s more, the characteristic peaks of g-C3N4 do not occur in the WO3/g-C3N4 XRD pattern.34 This finding may be resulted from the covering of g-C3N4 by the CeO2 or the lower crystallinity of g-C3N4. In addition, we surprisingly found that the strongest diffraction peaks at 28.60o for pure CeO2 sample was slightly shift to 28.44o when the introduction content of g-C3N4 was 34%. However, the (200) peak was shifted to higher degree with increasing the CeO2 content in the composites, indicating that the coupling has important influence on the lattice parameters of CeO2. According to the XRD pattern, we could calculate the lattice parameters, and the values for pure CeO2, CeR/CN-33%, CeR/CN-50%, CeR/CN-66%, CeR/CN-75% and CeR/CN-85% were 5.447 Å, 5.454 Å, 5.464 Å, 5.476 Å, 5.465 Å, 5.458 Å, respectively. The enlargement trend of lattice parameter was caused by the lattice dilation induced by the replacement of Ce4+ (0.97 Å) by slightly larger radius of Ce3+ (1.13 Å). The existence of Ce3+ would be further discussed on the following XPS section. And the real content of CeO2 in the composites performed by TG analysis was similar with that of theoretical one, as shown in Table S2.

14


Page 15 of 38

g-C3N4

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


CeR/CN-85% CeR/CN-75% CeR/CN-66% CeR/CN-50% CeR/CN-33% CeO2 10

20

30

40 2Θ

50

60

70

80

Figure 2 XRD patterns of CeO2, g-C3N4, and CeR/CN-x samples. While, the HRTEM images and the elements mapping (Figure 3) would further confirm the phase of g-C3N4. As shown in Figure S2, the CeO2 nano-rods with 30 ∼ 80 nm in length and 10 ± 1 nm in diameter were attached on the surface and edge of the g-C3N4, suggesting that a hetero-junction structure was formed. As can be observed in Figure 3, the lattice fringes with a spacing of 0.34 nm was corresponding to the interlayer stacking reflection, indexed as the (002) peak of the g-C3N4. And it can be concluded that the CeR/CN-x hetero-junction structure was formed based on the combination of the (002) plane of g-C3N4 and the (111) and (100) facets of CeO2, which was favor to the electron transfer. As shown in Figure 3, the element mapping results obviously present the uniform dispersity of C, N, Ce and O, which indicating the combination of the g-C3N4 and CeO2.

15



Page 16 of 38

Figure 3 FETEM images and SEM mapping of the CeR/CN-66% sample. Herein, based on the XRD and TEM results, it can be inferred that the hetero-junction structure brings in the generation of intense interaction between the interface of both materials, which would further result in the change of the surface composition and the chemical states of the CeR/CN-x. 16


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XPS analysis was carried out to characterize the valence state of Ce ions. The Ce 3d spectra were deconvoluted into eight well-resolved peaks: ν (~881.5 eV), νˊ (~884.6 eV), νˊˊ (~887.7 eV), νˊˊˊ (~897.6 eV), µ (~900.2 eV), µˊ (~902.8 eV), µˊˊ (~906.5 eV), µˊˊˊ (~915.8 eV). As shown in Figure 4a, the v and u represent the spin-orbit coupling of Ce 3d5/2 and Ce 3d3/2, respectively.35 The peaks labeled as ν, νˊˊ, νˊˊˊ, µ, µˊˊ and µˊˊˊ are characteristic of Ce4+ species, whereas the peaks denoted as νˊ and µˊ are assigned to Ce3+ ions.36 After fitting the Ce 3d core-level spectra, the Ce3+ and Ce4+ species both existed in the CeO2 and the CeR/CN-x composites, and the concentration of Ce3+ was calculated using formula (1): 37 Ce3+ concentration = I(Ce3+)/[ I(Ce3+) + I(Ce4+)]

(1)

The estimated results are presented in Table 2, and the percentage of Ce3+ to the total Ce for pure CeO2 is 16%. Interestingly, it was found that the concentration of Ce3+ increased from 17.1% to 18.9% for the CeR/CN-x composites. Obviously, the CeR/CN-66% catalyst exhibited the highest proportion of Ce3+ species, indicating the presence of stronger interactions between the CeO2 nano-rods and g-C3N4, which is in accordance with the XRD and TEM analysis.38 To our delight, the valence states of the Ce (+3, +4) are found to play a crucial role in the catalytic efficiency of the cerium oxide catalyst. For instance, Sudarsanam et al. revealed that the excellent performance of MnOx/CeO2 nano-rods can be attributed to the presence of surface-active Mn4+/Mn2+ couple and the enhanced defect structure of nano-rods (i.e., higher numbers of Ce3+ ions and abundant O vacancies) which could enhance the mobility of the oxygen.29 And Zhao et al. found that the rich surface defects including surface 17



oxygen vacancies and Ce3+ ions are the origin of the enhanced water oxidation performance of the CeO2 NRs treated under reduced atmosphere.39 What's more, Ahmad group found that the F-doped CeO2 exhibited higher activity for oxidation coupling of benzyl-amine to imine, which may be due to the higher Ce3+ concentration.28 And in the Cu/CeO2 system, the authors claimed that the best catalytic performance in CO oxidation was attributed to its abundance on defects and O vacancies as well as the high population of Cu+/Ce3+ redox pairs.40 Based on the above results, it is obvious that the distribution of the Ce3+ and Ce4+ species have a crucial influence on the catalytic activity of the title reaction. ν′ ν′′

ν′′′ µ µ′ µ′′

µ′′′

b Intensity (a.u.)

ν

a

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 18 of 38

CeR/CN-85% CeR/CN-75% CeR/CN-66%

CeO2 CeR/CN-66%

CeR/CN-50% CeR/CN-33% CeO2 880

890 900 910 Binding Energy (eV)

920

1.92

1.95

g

1.98

2.01

Figure 4 (a) High resolution XPS spectra of Ce 3d of the pure CeO2 and CeR/CN-x samples and (b) EPR spectrum of CeO2 and CeR/CN-66% samples at 77 K. On account of the charge conservation, the presence of abundant Ce3+ ions on the ceria surface in the composites provided by Ce 3d XPS probably induces surface oxygen vacancies, another important kind of defect sites in metal oxides for the catalytic oxidation reactions.41 The formation and stabilization of Ce3+ and oxygen vacancies in CeO2 lattice of CeR/CN-x composites is demonstrated by the following 18


Page 19 of 38

equation (2).42 2 Ce4+ + OO → 2 Ce3+ + V•• •O + 1/2 O2

(2)

Where OO indicates an oxygen atom on an oxygen lattice sites of ceria, and V•• •O signifies an oxygen vacancy with double positive charge in CeO2 lattice. In order to confirm the relative content of the oxygen vacancies defect sites, the low-temperature EPR was also carried out for CeO2 and CeR/CN-66% samples. As we all know, the ESR peak intensity was consistent with the concentration of paramagnetic species. As shown in Figure 4b, it is clear that the height of the EPR peak in CeR/CN-66% sample was higher than that of the pure CeO2, which indicated that there were more oxygen vacancy defects in the CeR/CN-66%. Therefore, we could infer that the intense interaction between the CeO2 and the g-C3N4 have a significant effect on the structure of CeO2, the increasing of defect sites was in accordance with the XPS results (Figure 4a). 2500

g-C3N4 CeR/CN-33% CeR/CN-50% CeR/CN-66% CeR/CN-75% CeR/CN-85%

2000

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


1500 1000 500 0 400

450

500

550

600

650

Wavelength (nm)

Figure 5 Photoluminescence spectra of the g-C3N4 and CeR/CN-x composites. In addition, the H2-TPR experiments was also carried out in Figure S3. As shown, compared with the pure CeO2, the peak that attributed to the surface reduction of 19



Page 20 of 38

cerium oxide, shifted to higher temperature for CeR/CN-66% sample, indicating the reduction of the cerium oxide was inhibited due to the interaction between CeO2 and g-C3N4.43 The results were well consistent with the XPS and EPR analysis. What’s more, the peak centered at above 900 K was corresponding to the decomposition of the g-C3N4. Therefore, the addition of g-C3N4 was favorable to the redox properties for the CeO2, making CeR/CN-x catalysts expose enough Ce3+ species and oxygen vacancies, bridging a charge transfer between CeO2 and g-C3N4, which may directly contribute to the enhanced conversion for the CeR/CN-x composites. The strong interaction between the two components could be further confirmed by PL spectra. Generally, the PL emission results mainly from the recombination of free carriers.44 As demonstrated in Figure 5, compared with the g-C3N4 sample, the CeR/CN-x composites displayed the much lower emission intensity, suggesting that the CeO2 nano-rod dramatically hindered the charge recombination, especially for the CeR/CN-66% sample. On the other hand, it is found that a minor blue-shift of the recombination peak for the CeR/CN-x composites. This finding could reveal the presence of the more oxygen vacancies.28 Therefore, lower PL emission intensity means the lower electron-hole recombination rates, which is beneficial to the improvement of photo-catalytic activity.

20


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0.5

5000

a

CeR/CN-66%

b

0.4

CeO2 CeR/CN-66% g-C3N4

4000

0.3 0.2

-Z'' (ohm)

Photocurrent (uA)

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


g-C3N4

0.1 CeO2

3000

2000

1000

0.0

0

-0.1 0

50

100

150 200 Time (s)

250

300

0

100

200

300

400

500

Z' (ohm)

Figure 6 (a) Transient photo-current profiles and (b) EIS spectra of CeO2, g-C3N4 and CeR/CN-x samples under the 300 W Xe lamp irradiation. Furthermore, the photo-current measurements were performed to explore the efficient charge separation resulted from the interaction between CeO2 and g-C3N4. It can be clearly observed that the current occurred steadily and quickly when switching the light on (Figure 6a). As compared with pure CeO2 and g-C3N4, the CeR/CN-66% composites exhibited much higher photo-current of 8 or 3 times than that of the counterparts, respectively. Interestingly, the photocurrent of CeR/CN-66% sample exhibited a little decrease in the presence of light, when the light was turned off, the photocurrent gradually decreased to zero, which may be induced by the released electrons from the re-oxidation process of Ce3+/Ce4+ in CeR/CN-66%. The similar phenomenon was also found in the CoSx/g-C3N4 system.45 While, the photocurrent has a little increase with the light irradiation for the pure g-C3N4 sample, which may be resulted from the slower electron transfer. Therefore, the significant enhancement in photocurrent was arisen from the efficient separation of the photo-induced charges that attributed to the strong interaction between each other. Besides, EIS analysis was carried out to clarify the efficient charge separation of CeR/CN-x samples from 21



another perspective. In general, the smaller arc radius of EIS plot implied a higher charge transfer efficiency. As displayed in Figure 6b, the arc radius of the CeR/CN-x composites were smaller than that of the pure CeO2 or g-C3N4, particularly for CeR/CN-66% hetero-junction composite, revealing that the surface phase junctions between CeO2 and g-C3N4 could largely improve the separation of the photo-induced electrons and holes. Mechanism Discussions. Based on the above discussion, we found that the interaction between the CeO2 nano-rod and the g-C3N4, on one hand, resulted in the more surface defects of CeO2 nano-rod, mainly including Ce3+ and oxygen vacancies; on the other hand, accelerated the separation and transfer of the free chargers, which may be key factors for the increased catalytic performance of the CeR/CN-x composites. To understand the reaction mechanism responsible for the oxidative coupling of benzyl-amine photo-catalyzed by CeR/CN-x composites, the main oxidative species in the photo-oxidation process were tested through the radicals and holes trapping experiments. a Blank

CeR/CN-66%

50 40 30 20

60

t-BuOH (NH4)2C2O4 BQ

b

Pure CeO2

50

Conversion %

60

Conversion %

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 22 of 38

40

Blank

BQ

30 20

10

10

0

0

t-BuOH

(NH4)2C2O4

Figure 7 Effect of the addition of various radical scavengers on the photo-oxidation of benzyl-amine. Reaction condition: benzylamine (100 mg); photocatalyst, (50 mg); 22


Page 23 of 38

anisole, (50 mg); acetonitrile, (5.0 g); Xe arc lamp; 3 h; air; 308 K. As shown in Figure 7b, we found that the holes and OH•are the main active species in the photo-catalytic benzyl-amine oxidation of the pure CeO2. However, tert-butanol (OH• scavenger) does not show any apparent effect on benzyl-amine oxidation for CeR/CN-66% sample (Figure 7a). Meanwhile, the presence of the ammonium oxalate (hole scavenger) or the benzoquinone (BQ, •O2- scavenger) led to the decrease of conversion to 44% and 30%, respectively, implying the involvement of both holes and •O2-, where •O2- is the predominant active species in the photo-catalytic process for CeR/CN-66% sample. Thus, it is interesting that the combination of CeO2 nano-rod and g-C3N4 have obvious influence on the active species, which implies that a different photo-catalytic mechanism involved.

a 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


CeR/CN-66%

CeO2

3184 3186 3188 Magnetic Field (G)

Figure 8 (a) ESR spectra of the samples, and (b) Potential energy diagram for samples and O2. In addition, ESR spectroscopy with a spin trapping method was carried out to further explore the involvement of •O2- species (Figure 8a). The ESR signal produced on the CeR/CN-66% sample was significant higher than that of CeO2, indicating more 23



Page 24 of 38

superoxide active species were generated during the photo-oxidation process. This finding was in accordance with the results of radical trapping experiments (Figure 7). The striking observation indicates a new transfer pathway formed for the photo-generated charge carriers of CeR/CN-x. As shown in Figure 8b, the potential of the conduction band (CB) of C3N4 was higher than that of CeO2, allowing the photo-induced electrons transfer from g-C3N4 to CeO2. The electron could lead to the partial reduction of Ce4+ to Ce3+, meanwhile, the oxygen vacancy appeared, as proved by XPS and EPR. And then, the oxygen molecule adsorbed on the defect sites of the CeO2, such as Ce3+ and the oxygen vacancy, were activated to create the superoxide radicals by the photo-induced electrons. Thus, the abundant oxygen vacancy and Ce3+ could enhance the mobility of oxygen in the CeR/CN-x, which facilitates O2 activation, a crucial step in this reaction. Therefore, there is almost no activity with the N2 atmosphere. While the holes could transfer to the valence band (VB) of the C3N4. Besides the promotion in the effective separation of the electron-hole pairs, the g-C3N4 could enhance the adsorption of the aromatic reagents attributed to the π-π bonding between g-C3N4 and benzene skeleton of the benzyl-amine, which could be further confirmed by the FTIR experiments (Figure 9).

24


Page 25 of 38

a

b

Fresh g-C3N4

Fresh CeR/CN-66% Adsorbed CeR/CN-66%

Adsorbed g-C3N4

1300

Intensity (a.u.)

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


1400

1500

1600

1700

1300

Wavenumber (cm-1)

1400

1500 1600 Wavenumber (cm-1)

1700

Figure 9 FTIR spectra of samples after expose to benzylamine. The vibrations δNH2 from benzyl-amine appeared, indicating that non-dissociative species adsorbed on the surface of the g-C3N4. This result has also been observed by Mahyari’s group, and it was found that the S, N: GQDs which could promote the oxygen and substrate adsorption on the bridge sites of GQDs, was beneficial to the activity.46 Moreover, the reaction was not completely suppressed as the addition of the BQ, therefore, we speculate that Ce3+ may have important impact on the enhancement of photo-oxidation reaction for CeR/CN-66%.

Figure 10 Reaction mechanism of photo-oxidative coupling of benzyl-amines by the CeR/CN-x. Based on our present observation and previously published reports, a plausible mechanism is presented in Figure 10. Under light irradiation, the excited electrons in 25



Page 26 of 38

g-C3N4 quickly transfer to the CB of CeO2 due to the interfacial interaction, and then the electrons are easily captured by the oxygen molecules adsorbed on the surface of the photo-catalyst with enough defect sites to create the superoxide radicals. Additionally, the holes accumulated on the VB of g-C3N4, could accept the electron of substrate adhered on the catalyst, resulting in the oxidation of benzyl-amine. Therefore, the photo-catalytic activity of the CeR/CN-x was significantly enhanced, and the mechanism was well matched with the results of the active radical species test. The remarkably improved photo-activity should be mainly attributed to the fabrication of the CeR/CN-x hetero-junction structure. On the one hand, the strong interfacial interaction was in favor of the separation of photo-induced charges on g-C3N4, leading to the longer photo-induced electrons lifetime. On the other hand, this interaction was beneficial to the adsorption and activation of the substrate or the oxygen due to the persistent existence of the Ce3+ and oxygen vacancy. In addition, the larger BET of the CeR/CN-x shows large contribution to the photo-oxidation of benzyl-amine. Obviously, the CeR/CN-66% with the highest proportion of Ce3+ and oxygen vacancy exhibited the most efficient separation of photo-induced carriers, which directly promoted the reaction process. We also carried out the recycling experiments of the CeR/CN-66% with 5 times for the photo-oxidation of benzyl-amine. As depicted in Figure 11a, no obvious decrease in conversion of substrate was observed, and there was no significant variation in the selectivity of imine product with the recycling test. In addition, the spent catalyst was performed by TEM and XPS to reveal the advantages of the hetero-junction 26


Page 27 of 38

composite material. 100

a

b

80

ν

ν ′ ′′ µ

ν′

Intensity (a.u)

ν′ ′

Conv (%)

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


60

40

µ′ µ′ ′

µ ′ ′′

Spent Ce3+: 21%

20

Fresh Ce3+: 22% 0 1

2

3

4

5

880

Recycle Test

890 900 910 Binding Energy (eV)

920

Figure 11 (a) Recyclability plot showing isolated yield of imine product, and (b) High resolution XPS spectra of Ce 3d of the fresh CeR/CN-66% and the spent catalyst with five cycle testing. The obtained TEM images presented that the morphology and the rod size have no significantly change after the repeated photo-oxidation reaction (Figure S4). According to the above mechanism assumption, it is no objection to conclude that the surface composition and the element chemical states own direct effect on the photo-oxidation activity for the benzyl-amine. The XPS study was conducted for the Ce element in spent CeR/CN-66% sample, and quite interesting observation was observed. The comparison of the Ce 3d spectrum for the spent and the fresh CeR/CN-66% sample clearly reveal that the concentration of the Ce3+ ions have no obvious decrease during the benzyl-amine photo-oxidation, as shown in Figure 11b. Compared with the previous reports by Ahmad and Zhao’s group,28,41 we found there was a small content of Ce4+ ions generated from the oxidation of Ce3+ ions in the CeR/CN-66% hetero-junction material through the reaction process, which could 27



Page 28 of 38

result in the excellent stability. In brief, the observations could indicate that the good stability of the CeR/CN-66% hetero-junction material was mainly attributed to the superior structure, which has great advantage in the reduction of the cost. Thus, our study provides an excellent highly efficient and stable CeR/CN-x catalyst for the photo-oxidation coupling reactions of benzyl-amine.

CONCLUSIONS In conclusion, we have synthesized a series of CeR/CN-x composite catalysts with a facile mixing-calcination methodology, which exhibited excellent photo-catalytic activity in the oxidation reaction of benzyl-amine under Xe light irradiation at 308 K. Interestingly, it was firstly reported that the rate constant of CeR/CN-66% was 3 times as high as that of pure CeO2 or g-C3N4. The XRD, XPS and TEM results revealed that the as-obtained CeR/CN-x sample with hybridization structure showed strong interfacial interaction between the CeO2 and g-C3N4, which could induce more efficient separation of photo-induced carriers and result in the generation of defect sites, such as Ce3+ and oxygen vacancy, as confirmed by the PL, EPR, H2-TPR and photocurrent experiments. The mechanism investigations declared that the remarkable activity improvement of CeR/CN-66% was ascribed to the easy transfer of photo-generated charges and the increase of adsorption of substrate and oxygen molecular. It was also found that the as-prepared CeR/CN-66% photo-catalysts could be repeatedly used for five times without obvious loss of activity and selectivity, suggesting its super stability. Thus, this study not only provided a deep insight into the 28


Page 29 of 38 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


relationship of the structure and the performance of the CeR/g-C3N4 composite, but also came up with a promising approach for the improvement of photo-oxidation coupling of amine under mild reaction conditions.

ASSOCIATED CONTENT

Supporting Information. TEM images of CeR/CN-66% sample, TEM images of the fresh CeR/CN-66% and the spent catalyst with five cycle testing, TG results of the CeR/CN-x samples, the oxidation of benzyl-amine over various catalysts and the H2-TPR profiles of CeO2 and CeR/CN-66% samples.

AUTHOR INFORMATION Corresponding Author * Fax: +86 3124 2978

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We would like to thank financial support by NNSFC (Project 21373054), and the 29



Page 30 of 38

Natural Science Foundation of Shanghai Science and Technology Committee (08DZ2270500). And the authors also thank the reviewers for their kind comments and constructive suggestions.

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TOC

The outstanding performance of simple CeO2-rod/g-C3N4 hybrid for photo-oxidative coupling of benzyl-amine under mild reaction conditions.

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Insights into the Relationship of the Heterojunction Structure and

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