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Photocatalytic selective oxidation of benzyl alcohol over ZnTi-LDH: The effect of surface OH groups
Applied Catalysis B: Environmental 260 (2020) 118185
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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb
Photocatalytic selective oxidation of benzyl alcohol over ZnTi-LDH: The effect of surface OH groups Junhua Zoua, Zhitong Wanga, Wei Guoa, Binbin Guoa, Yan Yub, Ling Wua, a b
T
⁎
State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou, 350116, China Key Laboratory of Eco-materials Advanced Technology (Fuzhou University), Fujian Province University, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Photocatalysis Benzyl alcohol Layered double hydroxide Surface OH groups Surface coordination species
ZnTi-LDH nanosheets have been synthesized as photocatalysts for selective oxidation of benzyl alcohol (BA) to benzaldehyde (BAD) under visible light irradiation. Based on UV–vis DRS, in-situ FTIR and XPS results, it was revealed that BA would be efficiently chemisorbed and activated on ZnTi-LDH forming the surface coordination species, which contributed to harvesting visible light and induced the photocatalytic reaction. The ESR spectra suggested that oxygen vacancies would be produced in ZnTi-LDHs after the adsorption of BA molecules. More exposed surface OH groups would induce more oxygen vacancies, which would serve as the centers for capturing photoelectrons from the photoexcited surface coordination species and act as active sites for enhanced O2 adsorption and activation, resulting outstanding photocatalytic activity. Finally, a possible mechanism was proposed to illustrate the photocatalytic process which mediated by surface coordination species formed by the interaction between ZnTi-LDH and BA.
1. Introduction Selective oxidation reactions, a kind of fundamental and significant transformation for the large-scale production of fine chemicals, are usually conducted under high temperature and high pressure in industrial production, which result in undesired energy consumption and environmental pollution [1–3]. Solar energy, being a clean, infinitely available and easy-to-handle energy resource, possesses great potential in driving chemical reactions environmentally [4–7]. Recently, photocatalytic selective organic transformations have attracted wide attentions because of their promising potential for triggering reactions under mild conditions. To date, diverse semiconductors, such as TiO2 [8,9], BiOCl [10], CdS [11], g-C3N4 [12,13] and MOFs [14] have been developed for photoredox organic transformations. However, most researches focus on the separation of photoelectron-hole pairs or regulating the band structures [15–17], while concentrate less on the active sites on the surface of photocatalysts and activation of reactants in the field of organic transformations. Recently, two dimensional (2D) materials have been receiving much attention because of the unique electronic structures and distinctive physicochemical properties resulting from the reduction of dimensions [18–20]. Construction of 2D materials can provide an extremely high percentage of exposed specific crystal facet, huge specific surface area, and large content of active sites [21–23]. These unique characteristics ⁎
endow the 2D materials the promising applications in photocatalytic selective organic transformations. Moreover, 2D materials are a class of ideal catalysts for studying the adsorption activity of reactants, which may provide an in-depth understanding of the catalytic reaction mechanism. In our previous studies, 2D nanosheets such as HNb3O8 [24], Bi2MoO6 [25,26], H1.4Ti1.65O4·H2O [27] and Pd/H1.07Ti1.73O4%H2O [28,29] were developed and applied to photocatalytic selective organic transformations. It was revealed that these 2D nanosheets showed higher photocatalytic activities than their layered compounds, because of their ultrathin structure with high external surface area and an extremely high percentage of active sites. Layered double hydroxides (LDHs) are a large class of two-dimensional (2D) inorganic layered host materials with the general formula of z+ [MⅡ1-xMⅢ (An−)z/n·mH2O (MⅡ and MⅢ are divalent and trivax (OH)2] lent metals, respectively; An− is the interlayer anion compensating for the positive charge of the brucite-like layers) [30]. The anions and metal ions in these materials are adjustable. LDHs with a wide variety of properties could be prepared due to the wide tunability in composition [31,32]. Therefore, these materials have been widely used as catalytic, magnetic and optical functional materials. Moreover, LDHs with lots of basic sites in the brucite-like layers could be served as heterogeneous solid base catalysts and have been widely applied to organic synthesis, such as Michael additions, aldol and Claisen–Schmidt condensation as well as Knoevenagel condensation [33]. In addition, it
Corresponding author. E-mail address: [email protected] (L. Wu).
https://doi.org/10.1016/j.apcatb.2019.118185 Received 9 June 2019; Received in revised form 6 September 2019; Accepted 12 September 2019 Available online 12 September 2019 0926-3373/ © 2019 Elsevier B.V. All rights reserved.
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overnight at 60 °C. ZnTi-LDHs with different molar ratio of Zn2+/Ti4+ were also prepared by varying dosage of TiCl4. ZnTi-LDHs with difference molar ratio of Zn2+/Ti4+ were denoted as ZT-n/m (“n/m” is the molar ration of Zn2+/Ti4+).
has been reported that MgAl-LDH with small size supported on carbon nanofiber showed a significantly improved activity than the bulk catalyst in the transesterification of glycerol with diethyl carbonate at a certain temperature due to a large number of accessible base sites in the catalyst [34]. By incorporating appropriate metal elements (such as Zn, Ti, Fe or Cr) into LDHs, semiconductor photocatalysts with band gap in the range from 2.0 to 3.4 eV could be synthesized [35,36]. For instance, some LDH nanosheets such as NiTi-LDH [37], ZnAl-LDH [38] and CuCrLDH [39] have been investigated as promising photocatalysts for water splitting, photoreduction of CO2 and N2 fixation, respectively. To this end, LDHs could be considered as promising photocatalysts for photoredox organic transformations. Recently, our research group reported Au-Pd/MgAl-LDH for photocatalytic selective oxidation benzyl alcohol under visible light [40]. However, the thickness of as-synthesized MgAlLDH was not thin enough to expose the active sites completely. In addition, the composite catalyst involved noble metals, which faced the drawback of high cost. Therefore, we would like to develop a new kind of LDH nanosheet for photocatalytic selective organic transformations. On the basis of the aforementioned considerations, we prepared the ZnTi-LDH nanosheets for photocatalytic selective oxidation of BA molecules to benzaldehyde (BAD) using oxygen as oxidant under visible light irradiation. ZnTi-LDH with Zn2+/Ti4+ molar ratio of 2/1 was found to exhibit excellent photocatalytic performance. From BET and FT-IR analyses, ZnTi-LDH with Zn2+/Ti4+ molar ratio of 2/1 exposes more OH groups than the other ZnTi-LDHs. Moreover, the results of UV–vis DRS, FT-IR, Raman spectra indicate that ZnTi-LDH can active the BA molecules to form the surface coordination species via the surface OH groups. The surface coordination species could respond to visible light and initiate the photocatalysis. The surface electron structures and defects in the BA-adsorbed ZnTi-LDH have been thoroughly studied by XPS and ESR spectra. The ZnTi-LDH with more exposed surface OH groups would contribute to the inducing more oxygen vacancies after the absorption of BA molecules, which would provide active sites for enhanced O2 adsorption and activation. The activated O2 molecule are more likely to generate O2− via the action of photogenerated electrons, resulting an outstanding photocatalytic activity. Finally, a surface coordination species promoted photocatalytic mechanism has been proposed to explain the performance of photocatalytic selective oxidation of BA over ZnTi-LDH under visible light irradiation.
2.2.2. Synthesis of M/ZnTi-LDH nanosheet (M = Au, Pd or Pt) The metal particles loaded on the surface of ZnTi-LDH was prepared by photodeposition. An aqueous solution of HAuCl4·4H2O (10 mg/mL) (the HAuCl4·4H2O was changed to H2PdCl4 or H2PtCl6·6H2O for loading Pd or Pt) was injected into a colloidal solution of ZnTi-LDH, and 5 mL of methanol was added as a sacrificial agent. After being constant stirred for 30 min with N2 as protective atmosphere, the suspensions were irradiated with a 300 W Xenon lamp (Beijing Perfectlight Co. Ltd., PLSSXE300D) for 1 h. The products were collected and washed with water several times, and then dried at 60 °C for further characterization and use. 2.3. Characterization X-ray diffraction (XRD) patterns were performed on a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (λ = 0.15406 nm) operated at 40 kV and 40 mA to give the phase. The morphologies of the samples were obtained by Hitachi SU8010 field-emission scanning electron microscopy (FESEM). Transmission electron microscopy (TEM) image and higher-resolution transmission electron microscopy (HRTEM) image were collected by using an FEI Talos field emission transmission electron microscope at an accelerating voltage of 200 kV. A tapping-mode atomic force microscope (AFM, Bruker Dimension Icon) with a Si-tip cantilever was used to evaluate the morphology of the prepared nanosheets on the Si substrate. The Brunauer–Emmett–Teller (BET) surface area determination and Barret–Joyner–Halender (BJH) pore volume and size analysis were measured on Micromeritics 3500 M apparatus. Ultraviolet–visible diffuse reflectance spectra (UV–vis DRS) were obtained via a Agilent Cary 500 UV–vis spectrophotometer. The Fourier transform infrared (FT-IR) spectra of LDH was carried out on a Nicolet IS50 Fourier transform infrared spectrometer at a resolution of 4 cm−1. X-ray photoelectron spectroscopy (XPS) analysis was performed on a PHI Quantum 2000 XPS system with a monochromatic Al Kα source and a charge neutralizer. The Raman spectra were collected on the Renishaw 2000 inVia Raman spectrometer. The electron spin resonance (ESR) signals were collected on a Bruker A300 spectrometer at room temperature using a 300 W Xe lamp (Beijing Perfectlight Co. Ltd., PLS-SXE300D) as light source.
2. Experimental section 2.1. Materials Zinc nitrate hexahydrate (Zn(NO3)26H2O, 99%), urea (CH4N2O, 99%), titanium tetrachloride (TiCl4, 98%), benzaldehyde (BAD, 98.5%), Chloroauric acid (HAuCl4·4H2O, 99.7%), Chloroplatinic acid (H2PtCl6·6H2O, 99.7%) and PdCl2 (Palladium chloride, 99.9%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Benzyl alcohol (BA, 99%) was purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. Benzotrifluoride (BTF, 99%) was purchased from J&K Scientific Ltd. All these chemicals were used as received without further purification.
2.4. Solid state nuclear magnetic resonance (SSNMR) spectroscopy The as-synthesized ZnTi-LDHs was suspended in D2O for deuteration. After the mixture was stirred vigorously at room temperature for 2 days, the deuterated ZnTi-LDHs was centrifuged and finally vacuumdried at 60 °C. The 1H SSNMR spectra were obtained on a Bruker AVANCE Ⅲ 500 spectrometer using a 4 mm MAS probe. 2.5. Simulated in situ FTIR measurement
2.2. Catalysts preparation
The in-situ FTIR spectra of BA adsorbed on the sample were obtained on a Nicolet Is50 Fourier transform infrared spectrometer at a resolution of 4 cm−1. A total of 64 scans were performed to obtain each spectrum. Firstly, the powder samples (ZnTi-LDH mixed with KBr at a mass ratio of 50%) were first pressed into a self-supporting IR disk (18 mm diameter, 20 mg), and then the disk was placed into the sample holder which could be moved vertically along a quartz tube. Before initiating the FTIR measurements, the disk was treated under 6 Pa at 220 °C for 3 h to remove surface contaminants. After the disk cooling to room temperature, 10 μL of BA was spiked into the tube with a syringe via the septum. After adsorption equilibrium was reached, the FTIR
2.2.1. Synthesis of ZnTi-LDH nanosheet ZnTi-LDH was prepared by a modified co-precipitation method [41]. The Synthetic procedure is as follows: 2.38 g of Zn(NO3)26H2O and 3 g of urea were dissolved in 100 ml of deionized water under constant magnetic stirring. Then 0.44 ml of TiCl4 was dropped into the above mixed solution under vigorous stirring. After stirring around 20 min, the clear solution was aged in an autoclave at 130 °C for 48 h. Upon completion, the obtained white precipitate was centrifuged and washed by deionized water for several times, and then dried in an oven 2
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spectra of the samples were collected. The physisorbed BA was removed by a further evacuation at 150 °C for 3 min under 6 Pa, and then, another FTIR spectrum of the sample was taken. 2.6. Electrochemistry measurement The working electrode was prepared on fluorine-doped tin oxide (FTO) glass, which was cleaned by sonicating in ethanol and water for 30 min, respectively. The 5 mg sample was dispersed in 0.5 mL of N,Ndimethylformamide by sonication to get slurry. Then the slurry was spread onto the pretreated FTO glass. After drying at 393 K for 2 h, a copper wire was connected to the side part of the FTO glass using a conductive tape. The uncoated parts of the electrode were isolated with an epoxy resin and the exposed area of the electrode was 0.38 cm2. The electrochemical measurements were performed in a conventional three electrode cell, using a Pt plate and a saturated Ag/AgCl electrode as counter electrode and reference electrode, respectively. The working electrode was immersed in a 0.2 M Na2SO4 aqueous solution (pH = 6.8) without any additive for 30 s before measurement. The photocurrent measurements were conducted on a CHI660D workstation. A 300 W xenon lamp was used as the visible light source. 2.7. Photocatalytic activity test The photocatalytic selective oxidation of BA was performed in a 15 mL quartz tube. In the process, 20 mg of a catalyst, 0.1 mmol of BA, and 1.5 mL of BTF were mixed in the quartz tube. Then the tube was filled with oxygen at a pressure of 1 bar and stirred to make the catalyst blend evenly in the solution. Finally, the suspension was irradiated by a 300 W Xe lamp (Beijing Perfectlight Co. Ltd., PLS-SXE300D) with a 400 nm cut-off filter. After 4 h of irradiation, the mixture was centrifuged to completely remove the catalyst particles. The remaining solution was analyzed with an Agilent gas chromatograph (GC-7898B). The conversion of BA and the selectivity for BAD were defined as the follows: Conversion (%) = [(C0 – CBA)/C0] × 100%
Fig. 1. Powder XRD patterns for the ZnTi-LDHs with different molar ratios (A) and SEM image of ZT-2/1 (B).
Selectivity (%) = [CBAD/(C0 – CBA)] × 100% Where C0 is the initial and final concentration of BA and CBA and CBAD are the concentrations of BA and BAD, respectively.
the well dispersed nanosheets with thickness of 5.25˜5.95 nm are observed (Fig. 2D). Based on the interlayer distance of 0.66 nm (XRD results) and the thickness of 5.25˜5.95 nm (AFM results) for the asprepared ZnTi-LDH nanosheets, it is therefore estimated that the thickness of ZnTi-LDH nanosheets is about 8˜9 monolayers [42,43]. To demonstrate the specific surface area and porosity of the asprepared ZnTi-LDH, nitrogen adsorption-desorption was carried out. As shown in Fig. 3A, the N2 adsorption–desorption isotherms of the asprepared ZnTi-LDHs are type IV isotherm with a H3-type hysteresis loop, indicating the presence of mesopores. The size of mesoporous is approximately distributed in 5–6 nm, which has a large contribution to the specific surface area (Fig. 3B). According to the result of Nitrogen adsorption-desorption isotherm, all as-prepared samples possess high specific surface area (106.50 m2/g, 88.83 m2/g and 80.04 m2/g for ZT2/1, ZT-3/1 and ZT-4/1, respectively). Furthermore, the optical property of as-prepared ZnTi-LDHs were investigated via UV–vis DRS spectroscopy. As shown in Figure S2, the absorption edges (λabs) of ZnTi-LDHs were 382 nm, corresponding to a band gap of 3.25 eV estimated via the empirical equation of 1240/λabs. The results suggest that as-prepared ZnTi-LDHs are a kind of ultraviolet light responsive materials. The as-prepared ZnTi-LDHs were applied to photocatalysis for selective oxidations of BA at 298 K with oxygen atmosphere using BTF as the solvent. As shown in Table 1, ZT-2/1 displayed a high photocatalytic activity (61.0% conversion) under 4 h full spectrum of light irradiation. It is worth noting that under visible light irradiation ZT-2/1 could also show a high photocatalytic activity (38.8% conversion),
3. Results and discussions As shown by the powder XRD patterns for ZnTi-LDHs in Fig. 1A, a strong reflection (0 0 3) is observed at 2θ ≈ 13.4° and the patterns also show other reflections of (0 0 6), (0 0 9), (1 0 0), (1 0 1), (0 1 2), (1 1 0) and (1 1 3) which can be indexed to typical LDH materials [41]. These results indicate that the ZnTi-LDH with high crystallinity was successfully prepared. The basal interlayer distance (d003) of ZnTi-LDH was calculated to be 0.66 nm, which is lower than that of MgAl-LDH (d003 = 0.75 nm) reported previously [40]. The decrease of interlayer distance was considered as that the presence of Ti4+ results in a strong electrostatic interaction between inorganic layer and guest carbonate. From the FESEM images (Fig. 1B and Figure S1), it is obviously seen that the as-synthesized ZnTi-LDHs show hierarchical structure consisting of 2D layered nanosheet. The hierarchical structure with lots of macropores with different size, may enhance the contact between ZnTiLDH and reactants. Transmission electron microscopy (TEM) image (Fig. 2A) of the as-synthesized ZnTi-LDH indicates that the sample have a plate-like morphology with lateral dimension of 400˜600 nm. In HRTEM image (Fig. 2B), the lattice with a spacing of 0.25 nm is observed, corresponding to the (0 0 9) plane of the ZnTi-LDH phase. This numerical value is in accordance with the in-plane structural parameter of ZnTi-LDH crystal determined from the XRD characterization (d009 = 0.25 nm). Fig. 2C shows the AFM image of ZT-2/1, from which 3
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Fig. 2. TEM image (A), HRTEM image (B), AFM image(C) and the corresponding height profiles (D) of ZT-2/1.
coordination species between ZnTi-LDH and BA molecules could be responsible for the active site, and thus promote the response to visible light. The surface coordination species formed by the interaction between ZnTi-LDH and BA molecules have been studied by UV–vis DRS. After BA was adsorbed, the color of ZnTi-LDH from white changed to yellow (Fig. 4A). BA-adsorbed ZnTi-LDH, therefore, exhibited intensive absorption in the visible region. Considering that BA could not absorb visible light, it could be assumed that this absorption in the visible light region was ascribed to the LMCT introduced by the surface coordination species. The BA-adsorbed ZnTi-LDH with absorption in the visible region could occur the photocatalytic selective oxidation under visible light irradiation. To research the effect of surface OH groups, ZnTi-LDHs with difference molar ratio of Zn2+/Ti4+ and the commercial TiO2 (P25) were also applied to photocatalytic selective oxidations of BA. The conversions for ZT-3/1, ZT-4/1 and P25 were 31.8%, 27.5% and 16.7%, respectively (Table 1, entry 10–12), which were all lower than that for ZT-2/1. In the FT-IR spectra (Fig. 4B), the peaks of OH group at 3100 cm−1 ˜ 3500 cm−1 for ZT-3/1 and ZT-4/1 are decreased in strength compared to that for ZT-2/1, suggesting that the amount of
while no over-oxidation product was detected under the reaction conditions (Table 1. entry 2). The oxidation reaction does not take place under photo-irradiation without ZnTi-LDH nor with the photocatalyst without irradiation (Table 1, entries 3–4). In the atmosphere of N2, conversion of ZT-2/1 decreased dramatically to 4.05% (Table 1, entries 5). These are demonstrated that O2, ZnTi-LDH and visible light are all necessary for photocatalytic oxidation of BA. Interestingly, it was found that para-substituted benzyl alcohols containing electron-donating groups (such as CH3 and OCH3; Table 1, entries 6–7) are more easily oxidized than those containing electron-withdrawing groups (such as F and Cl; Table 1, entries 8–9). The result indicates that the reaction may proceed through the intermediate of a carbocationic species [44]. From the UV–vis DRS spectrum, it suggests that the naked ZnTi-LDH with white color can only absorb the ultraviolet light but the photocatalysis could be triggered under visible light irradiation. In previous reports, the interaction of OH groups of TiO2 with 4-hydroxybenzyl alcohol would induce the surface coordination species, which exhibited strong absorption in the visible region due to the ligand-to-metal charge transfer (LMCT) [9,45]. Considering that the surface of LDHs exists abundant OH groups, we propose that the formation of surface 4
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Fig. 3. N2 sorption isotherms (A) and pore size distribution (B) of ZnTi-LDHs with different molar ratios.
surface OH groups in ZT-3/1 and ZT-4/1 are less than that in ZT-2/1. Therefore, the highest photocatalytic activity of ZT-2/1 in ZnTi-LDHs with difference molar ratio of Zn2+/Ti4+ may be due to the highest specific surface area (Fig. 3A) which suggests that the most surface OH groups are efficiently exposed in ZT-2/1. Moreover, the intensity of the peak of OH group for P25 is lower than that of ZnTi-LDHs. The result indicate that ZnTi-LDHs possess more OH group than P25, resulting in a higher photocatalytic performance than P25. In order to further explore the relationship between surface OH groups and photocatalytic activity, Au/ZT-2/1, Pd/ZT-2/1 and Pt/ZT-2/1 were also developed as photocatalysts for photocatalytic selective oxidation under visible light irradiation. As shown in Table 1 entry 13–15, it is obvious that the
Fig. 4. UV–vis DRS spectra of (a) ZT-2/1, (b) BA-adsorbed ZT-2/1 (A) and FT-IR spectra for the ZnTi-LDHs with different molar ratios (B).
conversions for Au/ZT-2/1, Pd/ZT-2/1, Pt/ZT-2/1 are 27.2%, 26.4% and 28.3%, respectively. They are all lower than that of ZT-2/1. To explore the reason of this phenomenon, FT-IR spectra were also carried out for the samples, and the results are shown in Figure S3. The peaks of OH group at 3100 cm−1 ˜ 3500 cm−1 for M/ZT-2/1 are decreased in strength compared to that for ZT-2/1. Due to the less amount of surface OH groups, these samples obtained lower photocatalysis activity than ZT-2/1. These results indicate that the surface OH groups of ZnTi-LDH may play an important role in photocatalytic selective oxidation benzyl
Table 1 Photocatalytic activity for the selective oxidation of benzylic alcohols over ZnTi-LDHs with visible light irradiationa. Entry
cat
R
Atm.
Visible light (λ ≥ 400 nm)
Conversion (%)
Selectivity (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
ZT-2/1 ZT-2/1 ZT-2/1 None ZT-2/1 ZT-2/1 ZT-2/1 ZT-2/1 ZT-2/1 ZT-3/1 ZT-4/1 P25 Au/ZT-2/1 Pd/ZT-2/1 Pt/ZT-2/1
H H H H H CH3 OCH3 F Cl H H
O2 O2 O2 O2 N2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2
UV-vis + – + + + + + + + + + + + +
61 38.8 – – 4.05 42.1 51.6 32.9 33.6 31.8 27.5 16.7 27.2 26.4 28.3
77 > 99 – – > 99 > 99 82 > 99 > 99 > 99 > 99 90 > 99 > 99 > 99
a
H H H
Recation condition: benzyl alcohol (0.1 mmol), photocatalyst (20 mg), benzotrifluoride (1.5 ml), temperature (25 ℃), irradiation time (4 h). 5
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with BA adsorbed, the electron spin resonance (ESR) spectroscopy was carried out. As displayed in Fig. 5B, the BA-adsorbed ZT-2/1 possesses the typical ESR signal centered around at g = 2.004, assigning to the oxygen vacancies [49]. However, no obvious ESR signal can be detected for the naked ZT-2/1. Moreover, BA molecules and BA-adsorbed ZnO also show no signals in the SER spectrum (Figure S5). The results indicate that the absorption of BA molecules on ZnTi-LDHs can result in the producing of oxygen vacancies in the photocatalysts. BA-adsorbed ZT-3/1 and ZT-4/1 also exhibit the typical ESR signals, but these signals all weaker than that of ZT-2/1. The more exposed surface OH groups in ZT-2/1 than those in ZT-3/1 and ZT-4/1 may result in higher concentration of oxygen vacancies on the surface of BA-adsorbed ZT-2/1. In previous work, surface oxygen vacancies can serve as the centers for capturing photoelectrons from the photoexcited surface coordination species. Moreover, they can also provide active sites for enhanced O2 adsorption and activation [10,50]. To explore the separation efficiency of photoelectrons and the photoexcited surface coordination species, electrochemical analysis was conducted. As shown in Figure S6, an enhanced photocurrent density of BA-adsorbed ZT-2/1 is clearly observed than that of ZT-3/1 and ZT-4/1 under visible light irradiation. However, the naked ZT-2/1, ZT-3/1 and ZT-4/1 show the similar photocurrent density (Figure S7). The results suggest that BA-adsorbed ZT-2/1 has higher efficiency of surface charge transfer than BA-adsorbed ZT-3/1 and ZT-4/1. Therefore, the higher photocatalytic activity of ZT-2/1 than that of ZT-3/1 and ZT-4/1 may due to the more oxygen vacances which was possible attributed by more exposed surface OH groups in the catalyst. To further understand the surface properties of BA-adsorbed ZnTiLDH, the XPS analyses were carried out. As shown in Fig. 6A, the Ti 2p XPS spectrum of BA-adsorbed ZT-2/1 exhibits an approximately 0.2 eV positive shift relative to the naked ZT-2/1, suggesting that the electron cloud density of surface Ti atoms is decreased. Thus, the result indicates that BA adsorbed onto the surface of ZnTi-LDH result in the redistribution of electrons from Ti atoms to BA molecules. However, BAadsorbed ZT-2/1 shows the identical Zn 2p XPS spectrum as naked ZT2/1 (Fig. 6B), indicating that chemical states of surface Zn atoms in ZnTi-LDH would not change after absorbing BA. In the O 1s XPS spectra for ZT-2/1 and BA-adsorbed ZT-2/1 (Fig. 6C), they all show the identical peaks at 529.6 eV, 532.1 eV and 532.9 eV which are attributed to lattice oxygen, adsorbed water and C–O functional group, respectively [51,52]. However, the BA-adsorbed ZT-2/1 shows a positive shift of the binding energy for OH groups by 0.2 eV compared to the naked ZT-2/1, suggesting that the adsorption of BA on the ZnTi-LDH may also result in the redistribution of electrons from the surface OH groups of ZnTi-LDH to BA. Moreover, the O 1s spectrum of ZT-2/1 with adsorbed BA molecules exhibits an additional peak at 530.2 eV which can be ascribed to the contribution from the adsorbed oxygen species at the vacancy sites [17,53]. In view of the experimental results, it is rational to infer that surface coordination species formed by BA molecules being adsorbed either on the Ti sites or OH sites of ZnTi-LDH [45]. The O 1s XPS spectra for ZT-3/1 and ZT-4/1 were also carried out to compare the amount of OH group in ZnTi-LDHs with difference molar ratio of Zn2+/Ti4+. From Figure S8, it can be observed that the O 1s XPS spectra for ZT-3/1 and ZT-4/1 also show peaks at 529.6 eV, 531.3 eV, 532.1 eV and 532.9 eV which are attributed to lattice oxygen, OH groups, adsorbed water and C–O functional group, respectively. The proportion of each oxygen signal for the three kinds of ZnTi-LDHs has been calculated and summarized in Table S1. It can be clearly observed that the proportion of OH group for ZT-2/1 is higher than that for ZT-3/1 and ZT-4/1. The result indicates that the amount of surface OH group in ZT-2/1 is more than that in ZT-3/1 and ZT-4/1, which is consisted with the FTIR result. To further explore the surface OH group of ZnTi-LDHs, the solid state nuclear magnetic resonance spectroscopy has been performed. Figure S9 shows 1H SSNMR spectra of deuterated ZnTi-LDHs. The peaks may be deconvoluted into six signals at 0.7, 1.1, 3.3, 4.7, 5.1 and 5.8 ppm, which are ascribed to the 1H chemical shifts of isolated
Fig. 5. (A) In suit FT-IR spectra of ZT-2/1 absorbed BA molecules. (a) Disk degassed at 220 °C for 3 h. (b) Adsorption for 30 min at RT (physisorption + chemisorption). (c) A further evacuation of excess BA at 6 Pa for 3 min after absorbed BA 30 min (chemisorption). (B) EPR spectrum of ZT-2/1, BA-adsorbed ZT-2/1, BA-adsorbed ZT-3/1 and BA-adsorbed ZT-4/1.
alcohol. In order to further explore the interactions between BA and ZnTiLDH, in situ FT-IR spectra were carried out for the BA absorption experiments to reveal the interactions between the catalyst and reagent molecules. As shown in Fig. 5A, the BA molecules can be chemisorbed and physisorbed on the surface of ZnTi-LDH. There three peaks are witnessed about 1454 cm−1, 1208 cm−1 and 1022 cm−1 assigned to CH2 scissor bending vibration, the CH in-plane bending vibration and the C–O bond stretching vibration peak, respectively. It is worth noting that the characteristic C–O band of BA adsorbed on the ZnTi-LDH surface (1022 cm−1) showed a red shift compared to free BA molecule (1080 cm−1). The results indicate that BA molecules can efficiently chemisorb at ZnTi-LDH and be activated. Furthermore, Raman spectra were measured to confirm the interaction between ZnTi-LDH and BA molecules. As shown in Figure S4, there four peaks are witnessed about 620 cm−1, 1001 cm−1, 1029 cm−1 and 1210 cm−1 are assigned to the Phenyl CCC in-plane bending vibration, phenyl ring breathing vibration, the C–O bond stretching vibration and the aromatic C–H vibration peak, respectively [46–48]. These results indicate that BA molecules could be absorbed at ZnTi-LDH to form surface coordination species. To understand the variation in the microstructure of the catalyst
6
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Fig. 6. The Ti 2p (A), Zn 2p (B) and O 1s (C) XPS spectra of ZT-2/1 before and after the adsorption of benzyl alcohol.
Zn−OH, isolated Ti−OH, Zn3−OH, interlayer water, Zn2Ti−OH and ZnTi2−OH, respectively [54,55].The result suggests that there are five kind of OH groups in ZnTi-LDHS, indicating that the metals, Zn and Ti are randomly distributed on the metal hydroxide sheets (Scheme S1) [56]. Notably, the intensity of the peak for Zn2Ti−OH and ZnTi2−OH in ZT-2/1 is higher than that in ZT-3/1 and ZT-4/1, indicating ZT-2/ 1with the more Zn2Ti−OH and ZnTi2−OH. The excellent photocatalytic performance for ZT-2/1 may be assigned to the more exposed Zn2Ti−OH and ZnTi2−OH which is in well agreement with the FTIR and XPS results. To elucidate the roles of O2 molecules in photocatalysis, a simulated in-situ ESR technique coupled with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was carried out. As shown in Fig. 7, the typical signal for ZT-2/
1 in the presence of BA verified the formation of DMPO−OOH, a spin derivative of DMPO−O2−, confirming the generation of O2− [49]. In contrast, no apparent signals are examined in the absence of BA molecules or under darkness. The result demonstrates that the generation of O2− under visible light was dependent on the surface coordination species, which could absorb visible light and initiate the photocatalysis. In addition, the BA-adsorbed ZT-2/1 possessed a superior ability for O2 activation to O2− than BA-adsorbed ZT3-1 and ZT-4/1. The result is due to the fact that more oxygen vacancies on the BA-adsorbed ZT-2/1 can act as the active sites to trap O2 molecular, contributing to enhancing photocatalytic oxidation organic reactions. Therefore, the O2− radical may be generated via the transfer of photoinduced electrons from the surface coordination species formed by ZnTi-LDH and BA molecules to oxygen molecules trapped by oxygen vacancies under visible light irradiation. According to the aforementioned experimental results and discussion, a possible photocatalytic mechanism is proposed to elucidate the photocatalytic oxidation of BA to BAD under visible light irradiation (Scheme 1). First, the BA can be chemisorbed either on the Ti sites or OH sites of ZnTi-LDH, and then form the surface coordination species at the interface through surface coordination [45]. Under visible light irradiation, the surface coordination species would be excited. And the adsorbed BA on ZnTi-LDH would be deprotonated, forming active intermediate species (C7H7O+). Then the photogenerated electrons transfer to the adsorbed oxygen molecules to produce O2− radicals. Finally, the C7H7O+ would further release a proton under the assistance of O2− to form BAD. 4. Conclusions In summary, we have explored ZnTi-LDH nanosheets for the photocatalytic selective oxidation of benzyl alcohol to benzaldehyde in the
Fig. 7. 5,5-dimethyl-1-pyrroline-N-oxide trapping ESR spectra of the samples. 7
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Scheme 1. Possible mechanism for selective oxidation of BA to BAD over ZnTi-LDH under visible light. (light blue spheres: Zn/Ti atoms; red spheres: oxygen atoms; red dotted circle: oxygen vacancy).
presence of O2 under visible light irradiation. BA molecules were adsorbed onto the surface of ZnTi-LDH to form the surface coordination species which could extend light absorption into the visible range and induced the photocatalytic reaction under visible light. Moreover, oxygen vacancies would be produced in the ZnTi-LDHs after BA molecules being adsorbed. With the more surface OH groups on ZnTi-LDH, the more oxygen vacancies would be induced. Oxygen vacancies would promote the separation of photoelectrons from the photoexcited surface coordination species. Moreover, the oxygen vacancies may also provide active sites for enhanced O2 adsorption and activation, generating O2− radicals via the trapped photoelectrons. Benefiting from more exposed OH groups, ZT-2/1 was found to exhibit higher photocatalytic performance under visible-light irradiation than ZT-3/1 and ZT-4/1. This work has not only developed an efficient photocatalyst for photocatalytic oxidation of BA, but also provided the deep understanding for the interactions between photocatalysts and reactant molecules.
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Declaration of Competing Interest The authors declare no competing financial interest. Acknowledgement This work was supported by the National Natural Science Foundation of China (21872032, 51672048, 51672046). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcatb.2019.118185. References [1] Z. Guo, B. Liu, Q. Zhang, W. Deng, Y. Wang, Y. Yang, Recent advances in heterogeneous selective oxidation catalysis for sustainable chemistry, Chem. Soc. Rev. 43 (2014) 3480–3524. [2] X. Lang, W. Ma, C. Chen, H. Ji, J. Zhao, Selective Aerobic Oxidation Mediated by
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Photocatalytic selective oxidation of benzyl alcohol over ZnTi-LDH: The effect of surface OH groups Junhua Zoua, Zhitong Wanga, Wei Guoa, Binbin Guoa, Yan Yub, Ling Wua, a b
T
⁎
State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou, 350116, China Key Laboratory of Eco-materials Advanced Technology (Fuzhou University), Fujian Province University, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Photocatalysis Benzyl alcohol Layered double hydroxide Surface OH groups Surface coordination species
ZnTi-LDH nanosheets have been synthesized as photocatalysts for selective oxidation of benzyl alcohol (BA) to benzaldehyde (BAD) under visible light irradiation. Based on UV–vis DRS, in-situ FTIR and XPS results, it was revealed that BA would be efficiently chemisorbed and activated on ZnTi-LDH forming the surface coordination species, which contributed to harvesting visible light and induced the photocatalytic reaction. The ESR spectra suggested that oxygen vacancies would be produced in ZnTi-LDHs after the adsorption of BA molecules. More exposed surface OH groups would induce more oxygen vacancies, which would serve as the centers for capturing photoelectrons from the photoexcited surface coordination species and act as active sites for enhanced O2 adsorption and activation, resulting outstanding photocatalytic activity. Finally, a possible mechanism was proposed to illustrate the photocatalytic process which mediated by surface coordination species formed by the interaction between ZnTi-LDH and BA.
1. Introduction Selective oxidation reactions, a kind of fundamental and significant transformation for the large-scale production of fine chemicals, are usually conducted under high temperature and high pressure in industrial production, which result in undesired energy consumption and environmental pollution [1–3]. Solar energy, being a clean, infinitely available and easy-to-handle energy resource, possesses great potential in driving chemical reactions environmentally [4–7]. Recently, photocatalytic selective organic transformations have attracted wide attentions because of their promising potential for triggering reactions under mild conditions. To date, diverse semiconductors, such as TiO2 [8,9], BiOCl [10], CdS [11], g-C3N4 [12,13] and MOFs [14] have been developed for photoredox organic transformations. However, most researches focus on the separation of photoelectron-hole pairs or regulating the band structures [15–17], while concentrate less on the active sites on the surface of photocatalysts and activation of reactants in the field of organic transformations. Recently, two dimensional (2D) materials have been receiving much attention because of the unique electronic structures and distinctive physicochemical properties resulting from the reduction of dimensions [18–20]. Construction of 2D materials can provide an extremely high percentage of exposed specific crystal facet, huge specific surface area, and large content of active sites [21–23]. These unique characteristics ⁎
endow the 2D materials the promising applications in photocatalytic selective organic transformations. Moreover, 2D materials are a class of ideal catalysts for studying the adsorption activity of reactants, which may provide an in-depth understanding of the catalytic reaction mechanism. In our previous studies, 2D nanosheets such as HNb3O8 [24], Bi2MoO6 [25,26], H1.4Ti1.65O4·H2O [27] and Pd/H1.07Ti1.73O4%H2O [28,29] were developed and applied to photocatalytic selective organic transformations. It was revealed that these 2D nanosheets showed higher photocatalytic activities than their layered compounds, because of their ultrathin structure with high external surface area and an extremely high percentage of active sites. Layered double hydroxides (LDHs) are a large class of two-dimensional (2D) inorganic layered host materials with the general formula of z+ [MⅡ1-xMⅢ (An−)z/n·mH2O (MⅡ and MⅢ are divalent and trivax (OH)2] lent metals, respectively; An− is the interlayer anion compensating for the positive charge of the brucite-like layers) [30]. The anions and metal ions in these materials are adjustable. LDHs with a wide variety of properties could be prepared due to the wide tunability in composition [31,32]. Therefore, these materials have been widely used as catalytic, magnetic and optical functional materials. Moreover, LDHs with lots of basic sites in the brucite-like layers could be served as heterogeneous solid base catalysts and have been widely applied to organic synthesis, such as Michael additions, aldol and Claisen–Schmidt condensation as well as Knoevenagel condensation [33]. In addition, it
Corresponding author. E-mail address: [email protected] (L. Wu).
https://doi.org/10.1016/j.apcatb.2019.118185 Received 9 June 2019; Received in revised form 6 September 2019; Accepted 12 September 2019 Available online 12 September 2019 0926-3373/ © 2019 Elsevier B.V. All rights reserved.
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overnight at 60 °C. ZnTi-LDHs with different molar ratio of Zn2+/Ti4+ were also prepared by varying dosage of TiCl4. ZnTi-LDHs with difference molar ratio of Zn2+/Ti4+ were denoted as ZT-n/m (“n/m” is the molar ration of Zn2+/Ti4+).
has been reported that MgAl-LDH with small size supported on carbon nanofiber showed a significantly improved activity than the bulk catalyst in the transesterification of glycerol with diethyl carbonate at a certain temperature due to a large number of accessible base sites in the catalyst [34]. By incorporating appropriate metal elements (such as Zn, Ti, Fe or Cr) into LDHs, semiconductor photocatalysts with band gap in the range from 2.0 to 3.4 eV could be synthesized [35,36]. For instance, some LDH nanosheets such as NiTi-LDH [37], ZnAl-LDH [38] and CuCrLDH [39] have been investigated as promising photocatalysts for water splitting, photoreduction of CO2 and N2 fixation, respectively. To this end, LDHs could be considered as promising photocatalysts for photoredox organic transformations. Recently, our research group reported Au-Pd/MgAl-LDH for photocatalytic selective oxidation benzyl alcohol under visible light [40]. However, the thickness of as-synthesized MgAlLDH was not thin enough to expose the active sites completely. In addition, the composite catalyst involved noble metals, which faced the drawback of high cost. Therefore, we would like to develop a new kind of LDH nanosheet for photocatalytic selective organic transformations. On the basis of the aforementioned considerations, we prepared the ZnTi-LDH nanosheets for photocatalytic selective oxidation of BA molecules to benzaldehyde (BAD) using oxygen as oxidant under visible light irradiation. ZnTi-LDH with Zn2+/Ti4+ molar ratio of 2/1 was found to exhibit excellent photocatalytic performance. From BET and FT-IR analyses, ZnTi-LDH with Zn2+/Ti4+ molar ratio of 2/1 exposes more OH groups than the other ZnTi-LDHs. Moreover, the results of UV–vis DRS, FT-IR, Raman spectra indicate that ZnTi-LDH can active the BA molecules to form the surface coordination species via the surface OH groups. The surface coordination species could respond to visible light and initiate the photocatalysis. The surface electron structures and defects in the BA-adsorbed ZnTi-LDH have been thoroughly studied by XPS and ESR spectra. The ZnTi-LDH with more exposed surface OH groups would contribute to the inducing more oxygen vacancies after the absorption of BA molecules, which would provide active sites for enhanced O2 adsorption and activation. The activated O2 molecule are more likely to generate O2− via the action of photogenerated electrons, resulting an outstanding photocatalytic activity. Finally, a surface coordination species promoted photocatalytic mechanism has been proposed to explain the performance of photocatalytic selective oxidation of BA over ZnTi-LDH under visible light irradiation.
2.2.2. Synthesis of M/ZnTi-LDH nanosheet (M = Au, Pd or Pt) The metal particles loaded on the surface of ZnTi-LDH was prepared by photodeposition. An aqueous solution of HAuCl4·4H2O (10 mg/mL) (the HAuCl4·4H2O was changed to H2PdCl4 or H2PtCl6·6H2O for loading Pd or Pt) was injected into a colloidal solution of ZnTi-LDH, and 5 mL of methanol was added as a sacrificial agent. After being constant stirred for 30 min with N2 as protective atmosphere, the suspensions were irradiated with a 300 W Xenon lamp (Beijing Perfectlight Co. Ltd., PLSSXE300D) for 1 h. The products were collected and washed with water several times, and then dried at 60 °C for further characterization and use. 2.3. Characterization X-ray diffraction (XRD) patterns were performed on a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (λ = 0.15406 nm) operated at 40 kV and 40 mA to give the phase. The morphologies of the samples were obtained by Hitachi SU8010 field-emission scanning electron microscopy (FESEM). Transmission electron microscopy (TEM) image and higher-resolution transmission electron microscopy (HRTEM) image were collected by using an FEI Talos field emission transmission electron microscope at an accelerating voltage of 200 kV. A tapping-mode atomic force microscope (AFM, Bruker Dimension Icon) with a Si-tip cantilever was used to evaluate the morphology of the prepared nanosheets on the Si substrate. The Brunauer–Emmett–Teller (BET) surface area determination and Barret–Joyner–Halender (BJH) pore volume and size analysis were measured on Micromeritics 3500 M apparatus. Ultraviolet–visible diffuse reflectance spectra (UV–vis DRS) were obtained via a Agilent Cary 500 UV–vis spectrophotometer. The Fourier transform infrared (FT-IR) spectra of LDH was carried out on a Nicolet IS50 Fourier transform infrared spectrometer at a resolution of 4 cm−1. X-ray photoelectron spectroscopy (XPS) analysis was performed on a PHI Quantum 2000 XPS system with a monochromatic Al Kα source and a charge neutralizer. The Raman spectra were collected on the Renishaw 2000 inVia Raman spectrometer. The electron spin resonance (ESR) signals were collected on a Bruker A300 spectrometer at room temperature using a 300 W Xe lamp (Beijing Perfectlight Co. Ltd., PLS-SXE300D) as light source.
2. Experimental section 2.1. Materials Zinc nitrate hexahydrate (Zn(NO3)26H2O, 99%), urea (CH4N2O, 99%), titanium tetrachloride (TiCl4, 98%), benzaldehyde (BAD, 98.5%), Chloroauric acid (HAuCl4·4H2O, 99.7%), Chloroplatinic acid (H2PtCl6·6H2O, 99.7%) and PdCl2 (Palladium chloride, 99.9%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Benzyl alcohol (BA, 99%) was purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. Benzotrifluoride (BTF, 99%) was purchased from J&K Scientific Ltd. All these chemicals were used as received without further purification.
2.4. Solid state nuclear magnetic resonance (SSNMR) spectroscopy The as-synthesized ZnTi-LDHs was suspended in D2O for deuteration. After the mixture was stirred vigorously at room temperature for 2 days, the deuterated ZnTi-LDHs was centrifuged and finally vacuumdried at 60 °C. The 1H SSNMR spectra were obtained on a Bruker AVANCE Ⅲ 500 spectrometer using a 4 mm MAS probe. 2.5. Simulated in situ FTIR measurement
2.2. Catalysts preparation
The in-situ FTIR spectra of BA adsorbed on the sample were obtained on a Nicolet Is50 Fourier transform infrared spectrometer at a resolution of 4 cm−1. A total of 64 scans were performed to obtain each spectrum. Firstly, the powder samples (ZnTi-LDH mixed with KBr at a mass ratio of 50%) were first pressed into a self-supporting IR disk (18 mm diameter, 20 mg), and then the disk was placed into the sample holder which could be moved vertically along a quartz tube. Before initiating the FTIR measurements, the disk was treated under 6 Pa at 220 °C for 3 h to remove surface contaminants. After the disk cooling to room temperature, 10 μL of BA was spiked into the tube with a syringe via the septum. After adsorption equilibrium was reached, the FTIR
2.2.1. Synthesis of ZnTi-LDH nanosheet ZnTi-LDH was prepared by a modified co-precipitation method [41]. The Synthetic procedure is as follows: 2.38 g of Zn(NO3)26H2O and 3 g of urea were dissolved in 100 ml of deionized water under constant magnetic stirring. Then 0.44 ml of TiCl4 was dropped into the above mixed solution under vigorous stirring. After stirring around 20 min, the clear solution was aged in an autoclave at 130 °C for 48 h. Upon completion, the obtained white precipitate was centrifuged and washed by deionized water for several times, and then dried in an oven 2
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spectra of the samples were collected. The physisorbed BA was removed by a further evacuation at 150 °C for 3 min under 6 Pa, and then, another FTIR spectrum of the sample was taken. 2.6. Electrochemistry measurement The working electrode was prepared on fluorine-doped tin oxide (FTO) glass, which was cleaned by sonicating in ethanol and water for 30 min, respectively. The 5 mg sample was dispersed in 0.5 mL of N,Ndimethylformamide by sonication to get slurry. Then the slurry was spread onto the pretreated FTO glass. After drying at 393 K for 2 h, a copper wire was connected to the side part of the FTO glass using a conductive tape. The uncoated parts of the electrode were isolated with an epoxy resin and the exposed area of the electrode was 0.38 cm2. The electrochemical measurements were performed in a conventional three electrode cell, using a Pt plate and a saturated Ag/AgCl electrode as counter electrode and reference electrode, respectively. The working electrode was immersed in a 0.2 M Na2SO4 aqueous solution (pH = 6.8) without any additive for 30 s before measurement. The photocurrent measurements were conducted on a CHI660D workstation. A 300 W xenon lamp was used as the visible light source. 2.7. Photocatalytic activity test The photocatalytic selective oxidation of BA was performed in a 15 mL quartz tube. In the process, 20 mg of a catalyst, 0.1 mmol of BA, and 1.5 mL of BTF were mixed in the quartz tube. Then the tube was filled with oxygen at a pressure of 1 bar and stirred to make the catalyst blend evenly in the solution. Finally, the suspension was irradiated by a 300 W Xe lamp (Beijing Perfectlight Co. Ltd., PLS-SXE300D) with a 400 nm cut-off filter. After 4 h of irradiation, the mixture was centrifuged to completely remove the catalyst particles. The remaining solution was analyzed with an Agilent gas chromatograph (GC-7898B). The conversion of BA and the selectivity for BAD were defined as the follows: Conversion (%) = [(C0 – CBA)/C0] × 100%
Fig. 1. Powder XRD patterns for the ZnTi-LDHs with different molar ratios (A) and SEM image of ZT-2/1 (B).
Selectivity (%) = [CBAD/(C0 – CBA)] × 100% Where C0 is the initial and final concentration of BA and CBA and CBAD are the concentrations of BA and BAD, respectively.
the well dispersed nanosheets with thickness of 5.25˜5.95 nm are observed (Fig. 2D). Based on the interlayer distance of 0.66 nm (XRD results) and the thickness of 5.25˜5.95 nm (AFM results) for the asprepared ZnTi-LDH nanosheets, it is therefore estimated that the thickness of ZnTi-LDH nanosheets is about 8˜9 monolayers [42,43]. To demonstrate the specific surface area and porosity of the asprepared ZnTi-LDH, nitrogen adsorption-desorption was carried out. As shown in Fig. 3A, the N2 adsorption–desorption isotherms of the asprepared ZnTi-LDHs are type IV isotherm with a H3-type hysteresis loop, indicating the presence of mesopores. The size of mesoporous is approximately distributed in 5–6 nm, which has a large contribution to the specific surface area (Fig. 3B). According to the result of Nitrogen adsorption-desorption isotherm, all as-prepared samples possess high specific surface area (106.50 m2/g, 88.83 m2/g and 80.04 m2/g for ZT2/1, ZT-3/1 and ZT-4/1, respectively). Furthermore, the optical property of as-prepared ZnTi-LDHs were investigated via UV–vis DRS spectroscopy. As shown in Figure S2, the absorption edges (λabs) of ZnTi-LDHs were 382 nm, corresponding to a band gap of 3.25 eV estimated via the empirical equation of 1240/λabs. The results suggest that as-prepared ZnTi-LDHs are a kind of ultraviolet light responsive materials. The as-prepared ZnTi-LDHs were applied to photocatalysis for selective oxidations of BA at 298 K with oxygen atmosphere using BTF as the solvent. As shown in Table 1, ZT-2/1 displayed a high photocatalytic activity (61.0% conversion) under 4 h full spectrum of light irradiation. It is worth noting that under visible light irradiation ZT-2/1 could also show a high photocatalytic activity (38.8% conversion),
3. Results and discussions As shown by the powder XRD patterns for ZnTi-LDHs in Fig. 1A, a strong reflection (0 0 3) is observed at 2θ ≈ 13.4° and the patterns also show other reflections of (0 0 6), (0 0 9), (1 0 0), (1 0 1), (0 1 2), (1 1 0) and (1 1 3) which can be indexed to typical LDH materials [41]. These results indicate that the ZnTi-LDH with high crystallinity was successfully prepared. The basal interlayer distance (d003) of ZnTi-LDH was calculated to be 0.66 nm, which is lower than that of MgAl-LDH (d003 = 0.75 nm) reported previously [40]. The decrease of interlayer distance was considered as that the presence of Ti4+ results in a strong electrostatic interaction between inorganic layer and guest carbonate. From the FESEM images (Fig. 1B and Figure S1), it is obviously seen that the as-synthesized ZnTi-LDHs show hierarchical structure consisting of 2D layered nanosheet. The hierarchical structure with lots of macropores with different size, may enhance the contact between ZnTiLDH and reactants. Transmission electron microscopy (TEM) image (Fig. 2A) of the as-synthesized ZnTi-LDH indicates that the sample have a plate-like morphology with lateral dimension of 400˜600 nm. In HRTEM image (Fig. 2B), the lattice with a spacing of 0.25 nm is observed, corresponding to the (0 0 9) plane of the ZnTi-LDH phase. This numerical value is in accordance with the in-plane structural parameter of ZnTi-LDH crystal determined from the XRD characterization (d009 = 0.25 nm). Fig. 2C shows the AFM image of ZT-2/1, from which 3
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Fig. 2. TEM image (A), HRTEM image (B), AFM image(C) and the corresponding height profiles (D) of ZT-2/1.
coordination species between ZnTi-LDH and BA molecules could be responsible for the active site, and thus promote the response to visible light. The surface coordination species formed by the interaction between ZnTi-LDH and BA molecules have been studied by UV–vis DRS. After BA was adsorbed, the color of ZnTi-LDH from white changed to yellow (Fig. 4A). BA-adsorbed ZnTi-LDH, therefore, exhibited intensive absorption in the visible region. Considering that BA could not absorb visible light, it could be assumed that this absorption in the visible light region was ascribed to the LMCT introduced by the surface coordination species. The BA-adsorbed ZnTi-LDH with absorption in the visible region could occur the photocatalytic selective oxidation under visible light irradiation. To research the effect of surface OH groups, ZnTi-LDHs with difference molar ratio of Zn2+/Ti4+ and the commercial TiO2 (P25) were also applied to photocatalytic selective oxidations of BA. The conversions for ZT-3/1, ZT-4/1 and P25 were 31.8%, 27.5% and 16.7%, respectively (Table 1, entry 10–12), which were all lower than that for ZT-2/1. In the FT-IR spectra (Fig. 4B), the peaks of OH group at 3100 cm−1 ˜ 3500 cm−1 for ZT-3/1 and ZT-4/1 are decreased in strength compared to that for ZT-2/1, suggesting that the amount of
while no over-oxidation product was detected under the reaction conditions (Table 1. entry 2). The oxidation reaction does not take place under photo-irradiation without ZnTi-LDH nor with the photocatalyst without irradiation (Table 1, entries 3–4). In the atmosphere of N2, conversion of ZT-2/1 decreased dramatically to 4.05% (Table 1, entries 5). These are demonstrated that O2, ZnTi-LDH and visible light are all necessary for photocatalytic oxidation of BA. Interestingly, it was found that para-substituted benzyl alcohols containing electron-donating groups (such as CH3 and OCH3; Table 1, entries 6–7) are more easily oxidized than those containing electron-withdrawing groups (such as F and Cl; Table 1, entries 8–9). The result indicates that the reaction may proceed through the intermediate of a carbocationic species [44]. From the UV–vis DRS spectrum, it suggests that the naked ZnTi-LDH with white color can only absorb the ultraviolet light but the photocatalysis could be triggered under visible light irradiation. In previous reports, the interaction of OH groups of TiO2 with 4-hydroxybenzyl alcohol would induce the surface coordination species, which exhibited strong absorption in the visible region due to the ligand-to-metal charge transfer (LMCT) [9,45]. Considering that the surface of LDHs exists abundant OH groups, we propose that the formation of surface 4
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Fig. 3. N2 sorption isotherms (A) and pore size distribution (B) of ZnTi-LDHs with different molar ratios.
surface OH groups in ZT-3/1 and ZT-4/1 are less than that in ZT-2/1. Therefore, the highest photocatalytic activity of ZT-2/1 in ZnTi-LDHs with difference molar ratio of Zn2+/Ti4+ may be due to the highest specific surface area (Fig. 3A) which suggests that the most surface OH groups are efficiently exposed in ZT-2/1. Moreover, the intensity of the peak of OH group for P25 is lower than that of ZnTi-LDHs. The result indicate that ZnTi-LDHs possess more OH group than P25, resulting in a higher photocatalytic performance than P25. In order to further explore the relationship between surface OH groups and photocatalytic activity, Au/ZT-2/1, Pd/ZT-2/1 and Pt/ZT-2/1 were also developed as photocatalysts for photocatalytic selective oxidation under visible light irradiation. As shown in Table 1 entry 13–15, it is obvious that the
Fig. 4. UV–vis DRS spectra of (a) ZT-2/1, (b) BA-adsorbed ZT-2/1 (A) and FT-IR spectra for the ZnTi-LDHs with different molar ratios (B).
conversions for Au/ZT-2/1, Pd/ZT-2/1, Pt/ZT-2/1 are 27.2%, 26.4% and 28.3%, respectively. They are all lower than that of ZT-2/1. To explore the reason of this phenomenon, FT-IR spectra were also carried out for the samples, and the results are shown in Figure S3. The peaks of OH group at 3100 cm−1 ˜ 3500 cm−1 for M/ZT-2/1 are decreased in strength compared to that for ZT-2/1. Due to the less amount of surface OH groups, these samples obtained lower photocatalysis activity than ZT-2/1. These results indicate that the surface OH groups of ZnTi-LDH may play an important role in photocatalytic selective oxidation benzyl
Table 1 Photocatalytic activity for the selective oxidation of benzylic alcohols over ZnTi-LDHs with visible light irradiationa. Entry
cat
R
Atm.
Visible light (λ ≥ 400 nm)
Conversion (%)
Selectivity (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
ZT-2/1 ZT-2/1 ZT-2/1 None ZT-2/1 ZT-2/1 ZT-2/1 ZT-2/1 ZT-2/1 ZT-3/1 ZT-4/1 P25 Au/ZT-2/1 Pd/ZT-2/1 Pt/ZT-2/1
H H H H H CH3 OCH3 F Cl H H
O2 O2 O2 O2 N2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2
UV-vis + – + + + + + + + + + + + +
61 38.8 – – 4.05 42.1 51.6 32.9 33.6 31.8 27.5 16.7 27.2 26.4 28.3
77 > 99 – – > 99 > 99 82 > 99 > 99 > 99 > 99 90 > 99 > 99 > 99
a
H H H
Recation condition: benzyl alcohol (0.1 mmol), photocatalyst (20 mg), benzotrifluoride (1.5 ml), temperature (25 ℃), irradiation time (4 h). 5
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with BA adsorbed, the electron spin resonance (ESR) spectroscopy was carried out. As displayed in Fig. 5B, the BA-adsorbed ZT-2/1 possesses the typical ESR signal centered around at g = 2.004, assigning to the oxygen vacancies [49]. However, no obvious ESR signal can be detected for the naked ZT-2/1. Moreover, BA molecules and BA-adsorbed ZnO also show no signals in the SER spectrum (Figure S5). The results indicate that the absorption of BA molecules on ZnTi-LDHs can result in the producing of oxygen vacancies in the photocatalysts. BA-adsorbed ZT-3/1 and ZT-4/1 also exhibit the typical ESR signals, but these signals all weaker than that of ZT-2/1. The more exposed surface OH groups in ZT-2/1 than those in ZT-3/1 and ZT-4/1 may result in higher concentration of oxygen vacancies on the surface of BA-adsorbed ZT-2/1. In previous work, surface oxygen vacancies can serve as the centers for capturing photoelectrons from the photoexcited surface coordination species. Moreover, they can also provide active sites for enhanced O2 adsorption and activation [10,50]. To explore the separation efficiency of photoelectrons and the photoexcited surface coordination species, electrochemical analysis was conducted. As shown in Figure S6, an enhanced photocurrent density of BA-adsorbed ZT-2/1 is clearly observed than that of ZT-3/1 and ZT-4/1 under visible light irradiation. However, the naked ZT-2/1, ZT-3/1 and ZT-4/1 show the similar photocurrent density (Figure S7). The results suggest that BA-adsorbed ZT-2/1 has higher efficiency of surface charge transfer than BA-adsorbed ZT-3/1 and ZT-4/1. Therefore, the higher photocatalytic activity of ZT-2/1 than that of ZT-3/1 and ZT-4/1 may due to the more oxygen vacances which was possible attributed by more exposed surface OH groups in the catalyst. To further understand the surface properties of BA-adsorbed ZnTiLDH, the XPS analyses were carried out. As shown in Fig. 6A, the Ti 2p XPS spectrum of BA-adsorbed ZT-2/1 exhibits an approximately 0.2 eV positive shift relative to the naked ZT-2/1, suggesting that the electron cloud density of surface Ti atoms is decreased. Thus, the result indicates that BA adsorbed onto the surface of ZnTi-LDH result in the redistribution of electrons from Ti atoms to BA molecules. However, BAadsorbed ZT-2/1 shows the identical Zn 2p XPS spectrum as naked ZT2/1 (Fig. 6B), indicating that chemical states of surface Zn atoms in ZnTi-LDH would not change after absorbing BA. In the O 1s XPS spectra for ZT-2/1 and BA-adsorbed ZT-2/1 (Fig. 6C), they all show the identical peaks at 529.6 eV, 532.1 eV and 532.9 eV which are attributed to lattice oxygen, adsorbed water and C–O functional group, respectively [51,52]. However, the BA-adsorbed ZT-2/1 shows a positive shift of the binding energy for OH groups by 0.2 eV compared to the naked ZT-2/1, suggesting that the adsorption of BA on the ZnTi-LDH may also result in the redistribution of electrons from the surface OH groups of ZnTi-LDH to BA. Moreover, the O 1s spectrum of ZT-2/1 with adsorbed BA molecules exhibits an additional peak at 530.2 eV which can be ascribed to the contribution from the adsorbed oxygen species at the vacancy sites [17,53]. In view of the experimental results, it is rational to infer that surface coordination species formed by BA molecules being adsorbed either on the Ti sites or OH sites of ZnTi-LDH [45]. The O 1s XPS spectra for ZT-3/1 and ZT-4/1 were also carried out to compare the amount of OH group in ZnTi-LDHs with difference molar ratio of Zn2+/Ti4+. From Figure S8, it can be observed that the O 1s XPS spectra for ZT-3/1 and ZT-4/1 also show peaks at 529.6 eV, 531.3 eV, 532.1 eV and 532.9 eV which are attributed to lattice oxygen, OH groups, adsorbed water and C–O functional group, respectively. The proportion of each oxygen signal for the three kinds of ZnTi-LDHs has been calculated and summarized in Table S1. It can be clearly observed that the proportion of OH group for ZT-2/1 is higher than that for ZT-3/1 and ZT-4/1. The result indicates that the amount of surface OH group in ZT-2/1 is more than that in ZT-3/1 and ZT-4/1, which is consisted with the FTIR result. To further explore the surface OH group of ZnTi-LDHs, the solid state nuclear magnetic resonance spectroscopy has been performed. Figure S9 shows 1H SSNMR spectra of deuterated ZnTi-LDHs. The peaks may be deconvoluted into six signals at 0.7, 1.1, 3.3, 4.7, 5.1 and 5.8 ppm, which are ascribed to the 1H chemical shifts of isolated
Fig. 5. (A) In suit FT-IR spectra of ZT-2/1 absorbed BA molecules. (a) Disk degassed at 220 °C for 3 h. (b) Adsorption for 30 min at RT (physisorption + chemisorption). (c) A further evacuation of excess BA at 6 Pa for 3 min after absorbed BA 30 min (chemisorption). (B) EPR spectrum of ZT-2/1, BA-adsorbed ZT-2/1, BA-adsorbed ZT-3/1 and BA-adsorbed ZT-4/1.
alcohol. In order to further explore the interactions between BA and ZnTiLDH, in situ FT-IR spectra were carried out for the BA absorption experiments to reveal the interactions between the catalyst and reagent molecules. As shown in Fig. 5A, the BA molecules can be chemisorbed and physisorbed on the surface of ZnTi-LDH. There three peaks are witnessed about 1454 cm−1, 1208 cm−1 and 1022 cm−1 assigned to CH2 scissor bending vibration, the CH in-plane bending vibration and the C–O bond stretching vibration peak, respectively. It is worth noting that the characteristic C–O band of BA adsorbed on the ZnTi-LDH surface (1022 cm−1) showed a red shift compared to free BA molecule (1080 cm−1). The results indicate that BA molecules can efficiently chemisorb at ZnTi-LDH and be activated. Furthermore, Raman spectra were measured to confirm the interaction between ZnTi-LDH and BA molecules. As shown in Figure S4, there four peaks are witnessed about 620 cm−1, 1001 cm−1, 1029 cm−1 and 1210 cm−1 are assigned to the Phenyl CCC in-plane bending vibration, phenyl ring breathing vibration, the C–O bond stretching vibration and the aromatic C–H vibration peak, respectively [46–48]. These results indicate that BA molecules could be absorbed at ZnTi-LDH to form surface coordination species. To understand the variation in the microstructure of the catalyst
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Fig. 6. The Ti 2p (A), Zn 2p (B) and O 1s (C) XPS spectra of ZT-2/1 before and after the adsorption of benzyl alcohol.
Zn−OH, isolated Ti−OH, Zn3−OH, interlayer water, Zn2Ti−OH and ZnTi2−OH, respectively [54,55].The result suggests that there are five kind of OH groups in ZnTi-LDHS, indicating that the metals, Zn and Ti are randomly distributed on the metal hydroxide sheets (Scheme S1) [56]. Notably, the intensity of the peak for Zn2Ti−OH and ZnTi2−OH in ZT-2/1 is higher than that in ZT-3/1 and ZT-4/1, indicating ZT-2/ 1with the more Zn2Ti−OH and ZnTi2−OH. The excellent photocatalytic performance for ZT-2/1 may be assigned to the more exposed Zn2Ti−OH and ZnTi2−OH which is in well agreement with the FTIR and XPS results. To elucidate the roles of O2 molecules in photocatalysis, a simulated in-situ ESR technique coupled with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was carried out. As shown in Fig. 7, the typical signal for ZT-2/
1 in the presence of BA verified the formation of DMPO−OOH, a spin derivative of DMPO−O2−, confirming the generation of O2− [49]. In contrast, no apparent signals are examined in the absence of BA molecules or under darkness. The result demonstrates that the generation of O2− under visible light was dependent on the surface coordination species, which could absorb visible light and initiate the photocatalysis. In addition, the BA-adsorbed ZT-2/1 possessed a superior ability for O2 activation to O2− than BA-adsorbed ZT3-1 and ZT-4/1. The result is due to the fact that more oxygen vacancies on the BA-adsorbed ZT-2/1 can act as the active sites to trap O2 molecular, contributing to enhancing photocatalytic oxidation organic reactions. Therefore, the O2− radical may be generated via the transfer of photoinduced electrons from the surface coordination species formed by ZnTi-LDH and BA molecules to oxygen molecules trapped by oxygen vacancies under visible light irradiation. According to the aforementioned experimental results and discussion, a possible photocatalytic mechanism is proposed to elucidate the photocatalytic oxidation of BA to BAD under visible light irradiation (Scheme 1). First, the BA can be chemisorbed either on the Ti sites or OH sites of ZnTi-LDH, and then form the surface coordination species at the interface through surface coordination [45]. Under visible light irradiation, the surface coordination species would be excited. And the adsorbed BA on ZnTi-LDH would be deprotonated, forming active intermediate species (C7H7O+). Then the photogenerated electrons transfer to the adsorbed oxygen molecules to produce O2− radicals. Finally, the C7H7O+ would further release a proton under the assistance of O2− to form BAD. 4. Conclusions In summary, we have explored ZnTi-LDH nanosheets for the photocatalytic selective oxidation of benzyl alcohol to benzaldehyde in the
Fig. 7. 5,5-dimethyl-1-pyrroline-N-oxide trapping ESR spectra of the samples. 7
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Scheme 1. Possible mechanism for selective oxidation of BA to BAD over ZnTi-LDH under visible light. (light blue spheres: Zn/Ti atoms; red spheres: oxygen atoms; red dotted circle: oxygen vacancy).
presence of O2 under visible light irradiation. BA molecules were adsorbed onto the surface of ZnTi-LDH to form the surface coordination species which could extend light absorption into the visible range and induced the photocatalytic reaction under visible light. Moreover, oxygen vacancies would be produced in the ZnTi-LDHs after BA molecules being adsorbed. With the more surface OH groups on ZnTi-LDH, the more oxygen vacancies would be induced. Oxygen vacancies would promote the separation of photoelectrons from the photoexcited surface coordination species. Moreover, the oxygen vacancies may also provide active sites for enhanced O2 adsorption and activation, generating O2− radicals via the trapped photoelectrons. Benefiting from more exposed OH groups, ZT-2/1 was found to exhibit higher photocatalytic performance under visible-light irradiation than ZT-3/1 and ZT-4/1. This work has not only developed an efficient photocatalyst for photocatalytic oxidation of BA, but also provided the deep understanding for the interactions between photocatalysts and reactant molecules.
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Declaration of Competing Interest The authors declare no competing financial interest. Acknowledgement This work was supported by the National Natural Science Foundation of China (21872032, 51672048, 51672046). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcatb.2019.118185. References [1] Z. Guo, B. Liu, Q. Zhang, W. Deng, Y. Wang, Y. Yang, Recent advances in heterogeneous selective oxidation catalysis for sustainable chemistry, Chem. Soc. Rev. 43 (2014) 3480–3524. [2] X. Lang, W. Ma, C. Chen, H. Ji, J. Zhao, Selective Aerobic Oxidation Mediated by
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