Cheng
Liu
,
Wei
Guo
,
Jinsong
Chen
,
Junhua
Zou
,
Zhiwen
Wang
and
Ling
Wu
*
State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou 350116, P.R. China. E-mail: wuling@fzu.edu.cn
First published on 30th October 2020
In this work, ZnTi-LDH nanosheets with several monolayer thickness were prepared as photocatalysts for the aerobic oxidation of aniline to yield nitrosobenzene under visible light irradiation. UV-vis DRS, in situ FTIR and XPS results jointly revealed that aniline molecules were efficiently chemisorbed and activated on ZnTi-LDH to form surface coordination active species, improving visible light absorption and inducing the photocatalytic reaction. The surface OH groups on ZnTi-LDH as the Brønsted base sites facilitated the reductive deprotonation of aniline to form the anilino anion, which was a key step in promoting aniline oxidation. ESR and XPS data suggested that oxygen vacancies (OVs) were formed due to the interaction between exposed OH groups and aniline molecules. The OVs, as the centers to capture photoelectrons, achieve the reduction of oxygen molecules to ˙O2− radicals, which further oxidize anilino species to produce nitrosobenzene. Finally, a possible mechanism was proposed to reveal the photocatalytic process based on the surface coordination activation theory.
Much work so far has focused on photocatalytic organic transformation, a green and sustainable process, in which an organic reaction could be initiated by light irradiation under mild conditions.10–12 Thus, photocatalysis is a promising candidate for the selective aerobic oxidation of aniline to nitrosobenzene. Recently, the photocatalytic aerobic oxidation of aniline under UV irradiation was demonstrated in three studies.13–15 However, nitrosobenzene was hardly detected in those processes as a result of the formation of azobenzene. Afterwards, selective photocatalytic oxidation of aniline to nitrosobenzene over Pt/TiO2 was reported.16 However, the Pt/TiO2 catalyst was prepared under harsh conditions and used an expensive noble metal (Pt). Therefore, it is desired to explore efficient catalysts for the selective photocatalytic oxidation of aniline to nitrosobenzene in an economical and convenient way.
Considerable research efforts have been devoted to ultrathin two-dimensional (2D) materials due to their high specific surface area, unique surface states, abundant active sites and other distinctive physicochemical properties.17–20 In our previous work, a series of ultrathin 2D nanosheets such as HNb3O8,21 Bi2MoO6,22,23 H1.4Ti1.65O4·H2O (ref. 24–27) and BiOCl (ref. 28) were developed for photocatalytic selective organic transformation. It was revealed in these studies that the ultrathin 2D material is an ideal model to explore the interaction between reactants and catalysts, which probably provides an in-depth understanding of the photocatalytic reaction mechanism at a molecular level.
Layered double hydroxides (LDHs) as an important class of 2D materials have been widely applied in photocatalysis, catalysis, biomedical science, electrochemistry and so on.29–32 LDHs have become promising photocatalysts owing to several outstanding advantages, including abundant surface hydroxyl (OH) groups, unique structure, uniform distribution of different metal cations, flexible tunability and high chemical stability. In the photocatalysis region, most of the literature about LDHs focus on water splitting,33–35 CO2 reduction36,37 and degradation of pollutant.38 However, little has been reported on the photocatalytic organic transformation by LDHs.39,40 Recently, our research group has reported the photocatalytic selective oxidation of benzyl alcohol over Au–Pd/MgAl-LDH and ZnTi-LDH under visible light irradiation.41,42 Both studies highlight the interaction between the basic sites on the surface of LDHs and reactant molecules. It was demonstrated by Hirai et al. that Pt/TiO2 exhibited excellent photocatalytic performance in aniline oxidation to nitrosobenzene because the Pt particles with high electron density behaved as Lewis base sites to achieve reductive deprotonation of aniline, which was the rate-determining step in the oxidation of aniline.16 Following this thought, the function of the basic sites was further emphasized in our two reported studies.43,44 Therefore, the question is whether the surface OH groups of LDHs as the Brønsted base sites could facilitate the reductive deprotonation of aniline to increase the catalytic activity.
Herein, we fabricated ZnTi-LDH nanosheets 7–8 monolayers thick for the selective photocatalytic aerobic oxidation of aniline to nitrosobenzene under visible light irradiation. Three kinds of ZnTi-LDHs with different ZnII/TiIV molar ratios were prepared by varying the dosage of TiCl4. A unique ultrathin structure for ZnTi-LDH was exhibited through FESEM, TEM and AFM. The differences of the three kinds of ZnTi-LDHs in terms of specific surface area, light absorption, and the amount of surface OH groups were revealed by BET, UV-vis DRS and FTIR, respectively. The interaction between OH groups on the surface of ZnTi-LDH and aniline molecules was investigated by UV-vis DRS, in situ FTIR and XPS. The presence of oxygen vacancies produced by the interaction between ZnTi-LDH and aniline molecules was verified by ESR and XPS. The generation of superoxide radical (˙O2−) as the active oxidative species was further characterized by ESR combined with DMPO. Finally, a possible mechanism was proposed at the molecule level to illustrate the photocatalytic progress of the oxidation of aniline to nitrosobenzene based on the surface coordination activation theory.
Conversion (%) = [(C0 − CAL)/C0] × 100 |
Selectivity (%) = [CNSB/(C0 − CAL)] × 100 |
The details of the GC analysis are as follows: the remaining solution was determined using an Agilent GC-6898N instrument with a FID detector. An Agilent HP-5 column (15 m × 0.530 mm) was used as a separation column. The carrier gas was helium (99.999%) at the rate of 5.6 mL min−1. The temperatures of the injector and detector were respectively set as 300 °C and 270 °C. The injection volume was 10 μl. The column temperature was programmed from 40 °C to 160 °C at 20 °C min−1 and then held for 10.5 min at 160 °C.
Field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) were used to obtain the morphology and structure of the LDH samples. The FESEM image, shown in Fig. 2A, reveals that the prepared ZnTi-LDHs consist of two-dimensional thin nanoflakes. The rough surface of the hierarchical structure contributes to the contact between catalysts and reactants, which plays an important role in improvement of the catalytic performance. It is further confirmed by the TEM image in Fig. 2B that the hierarchical structure of the sample consists of ultrathin nanosheets with a lateral dimension of 500–700 nm. As can be seen from the HRTEM image in Fig. 2C, the interplanar spacing of the sample is 0.25 nm, which is consistent with the calculated result (d009 = 0.25 nm) based on the XRD results. In addition, the thickness of the ZnTi-LDH nanosheets measured by atomic force microscopy (AFM) in Fig. 2D–F is approximately 4.7–5.2 nm, which can be considered as a 7–8 monolayer thickness according to the interlayer spacing of 0.66 nm from XRD data.
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Fig. 2 FESEM image (A), TEM image (B), HRTEM image (C), AFM image (D) and the corresponding height profiles (E and F) for ZT-2/1. |
The specific surface area and porosity of the ZnTi-LDHs were investigated by low temperature nitrogen adsorption–desorption. The N2-sorption data in Fig. S1A† show a type IV isotherm with an H3-type hysteresis loop for the ZnTi-LDHs, which demonstrates that the prepared ZnTi-LDHs are mesoporous materials. The corresponding wide distribution of pore size in Fig. S1B† further confirms that the pore width of the samples is distributed in 6–7 nm, most likely resulting from the use of urea as the template during the hydrothermal synthesis. Meanwhile, the specific surface areas of ZT-2/1, ZT-4/1, and ZT-6/1 were measured to be 129.07 m2 g−1, 86.79 m2 g−1 and 48.08 m2 g−1, respectively. Furthermore, the optical property of the prepared catalysts was analyzed by ultraviolet-visible diffuse reflectance spectra (UV-vis DRS) spectroscopy. UV-vis DRS and the corresponding Tauc plot in Fig. S2† reveal that the bandgaps of ZT-2/1, ZT-4/1, and ZT-6/1 are about 3.25 eV, 3.34 eV, 3.37 eV, respectively. As a result, the three ZnTi-LDHs can only respond to ultraviolet light (λ ≤ 382 nm). In addition, Mott–Schottky plots (Fig. S3†) were used to measure the flat-band potentials of ZT-2/1, ZT-4/1 and ZT/6/1, which are respectively −1.1 V, −1.0 V and −0.8 V (vs. Ag/AgCl), corresponding to −0.9 V, −0.8 V and −0.6 V (vs. NHE). It is generally known that the position of the conduction band (CB) is close to that of the flat band, and compared with the O2/˙O2− potential (−0.28 V vs. NHE), the CBs of the ZnTi-LDH samples are more negative. Therefore, all of them are thermodynamically permissible for the reduction of O2 to ˙O2−, which is an important active oxygen species.
The selective photocatalytic oxidation of aniline (AL) to nitrosobenzene (NSB) was carried out over ZnTi-LDHs and P25 under visible light (λ ≥400 nm) irradiation. As shown in Table 1, the conversion of AL (33.5%) with high selectivity to NSB (76.7%) was achieved over ZT-2/1 after irradiating for 4 h, which exhibited an obviously superior catalytic performance than ZT-4/1 (15.4%), ZT-6/1 (8.1%) and P25 (13.6%). In addition, two by-products were produced in the photocatalytic reaction, azobenzene (AB) and nitrobenzene (NB). The formation of AB, another high-value chemical, results from the condensation between AL and a part of the produced NSB.11 NB is produced owing to the further oxidation of NSB. Table S1† summarizes the photocatalytic data of aniline oxidation between the current study and those reported in the literature. Compared to various photocatalysts, ZT-2/1 shows excellent photocatalytic performance in the conversion of AL and selectivity to NSB. It is worth mentioning that no product was detected under the reaction condition without ZnTi-LDH as a catalyst or in the dark condition. In addition, the photocatalytic oxidation reaction did not take place in an Ar atmosphere. On the basis of these results, it is concluded that ZnTi-LDH, light source and oxygen are all necessary for the photocatalytic oxidation of AL. Moreover, the reusability and stability of ZT-2/1 were also investigated. After recycling ZT-2/1 five times, its photocatalytic activity remains stable (Fig. S4†). The XRD patterns for fresh and used ZT-2/1 (Fig. S5†) show no obvious change, indicating the high stability of ZT-2/1. In order to explore the effect of solvents on the catalytic activity, reaction experiments using different solvents were carried out. As shown in Fig. S6,† the activity order is acetonitrile > BTF > toluene > hexane > ethyl acetate > EtOH, indicating that acetonitrile is the best option as the reaction solvent in this photocatalytic process. In the organic solvents without oxygen-containing functional groups, the photocatalytic conversion of aniline is improved with the increasing polarity of the solvents (acetonitrile > BTF > toluene > hexane). This is mainly because aniline can dissolve well in organic solvents with high polarity. In organic solvents with oxygen-containing functional groups (ethyl acetate and EtOH), the conversion considerably decreases but the selectivity of NSB obviously increases. This result may have contributed to the competitive adsorption of the solvent and aniline in this system.
Entry | Catalyst | Atm | Visible light (λ ≥400 nm) | Conv. (%) | Sel. NSB (%) | Sel. AB (%) | Sel. NB (%) |
---|---|---|---|---|---|---|---|
a Reaction conditions: aniline (0.026 mmol), catalyst (20 mg), acetonitrile (1.0 mL), temperature (25 °C), irradiation time (4 h). | |||||||
1 | ZT-2/1 | O2 | + | 33.5 | 76.7 | 16.1 | 7.2 |
2 | ZT-2/1 | O2 | — | — | — | — | — |
3 | None | O2 | + | — | — | — | — |
4 | ZT-2/1 | Ar | + | — | — | — | — |
5 | ZT-4/1 | O2 | + | 15.4 | 79.9 | 13.6 | 6.5 |
6 | ZT-6/1 | O2 | + | 8.1 | 86.4 | 6.2 | 7.4 |
7 | P25 | O2 | + | 13.6 | 62.9 | 33.3 | 3.8 |
Why do ZnTi-LDHs as ultraviolet light responsive materials have catalytic performance under visible light irradiation? The optical absorption property of aniline-adsorbed ZT-2/1 (AL/ZT-2/1) was analyzed by UV-vis DRS and the results are shown in Fig. 3. After the absorption of AL molecules, ZT-2/1 exhibits visible light absorption with a color change from white to yellow, similar to a phenomenon reported in our previous work.21,25,42 This result indicates that the interaction of AL with ZT-2/1 would induce the formation of surface coordination species, which exhibited optical absorption in the visible region due to the ligand-to-metal charge transfer (LMCT).47–49 In addition, photocurrent measurements (Fig. S7†) suggest that three types of aniline-adsorbed ZnTi-LDHs have photocurrent response under visible light irradiation. AL/ZT-2/1 shows enhanced photocurrent intensity than AL/ZT-4/1 and AL/ZT-6/1, indicating that AL/ZT-2/1 has higher separation efficiency of photoelectrons. Therefore, the surface coordination active species with the property of absorption in the visible region guarantees the photocatalytic oxidation of AL under visible light irradiation.
To find out the reason why ZT-2/1 has better photocatalytic performance than other kinds of ZnTi-LDHs, further material characterization was carried out. As shown in the FTIR spectra in Fig. 4, the intensity of the FTIR peak (3100–3500 cm−1) assigned to OH groups for ZT-2/1 significantly increases when compared with those for ZT-4/1, ZT-6/1 and P25, verifying that ZT-2/1 possesses a larger number of surface OH groups than the others. This is in good consistency with the results of specific surface area analyzed by N2-sorption. Consequently, it can be automatically concluded that more surface OH groups are efficiently exposed in ZT-2/1 owing to its higher specific surface area. It was demonstrated in three studies16,43,44 that basic sites could facilitate the reductive deprotonation of AL, which was the rate-determining step in the oxidation of AL. Taking into account the abundant OH groups on the surface of LDHs, we suppose that the OH groups on ZnTi-LDHs as the Brønsted base sites could also facilitate the reductive deprotonation of AL, resulting in the excellent photocatalytic performance in the oxidation of AL.
In situ FTIR analysis was further carried out to verify the interaction between ZnTi-LDH and AL molecules. As shown in Fig. 5c, after the adsorption of AL, several peaks assigned to AL molecules appeared in the spectrum compared with that of fresh ZT-2/1. The peaks also remained even after further evacuation (Fig. 5d), indicating that AL molecules efficiently chemisorbed on the surface of ZnTi-LDH. The peak at 1601 cm−1 could be ascribed to the stretching vibration of CC (σC
C) in the aromatic ring, while the two peaks at 1174 cm−1 and 1152 cm−1 could be assigned to the stretching vibration of C–H (σC–H) in the aromatic ring.50 Differently, the N–H bending vibration (δN–H) at 1621 cm−1 in the AL adsorbed on ZT-2/1 exhibited a shift toward a higher wavenumber compared with that in free AL molecules (1618 cm−1), indicating that AL molecules adsorbed on ZT-2/1 were transformed to the new species with a stronger N–H bond.51 It was proven in a previous study16 that the N–H vibrational band was strengthened via deprotonation by the base. In addition, the C–N stretching vibration (σC–N) at 1277 cm−1 in the AL/ZT-2/1 showed a similar move. These data clearly suggest that ZT-2/1 has some kind of basic sites to promote deprotonation of AL.
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Fig. 5 In situ FTIR spectra of aniline absorption on ZT-2/1. (a) FTIR spectra of free aniline. (b) Degassing stage. (c) Physical and chemical adsorption stage. (d) Chemical adsorption stage. |
To further explore the active sites for the interaction between ZnTi-LDH and AL molecules, X-ray photoelectron spectroscopy (XPS) characterization was used to compare the surface properties of naked ZT-2/1 with those of AL-adsorbed ZT-2/1. As shown in Fig. 6A, the Zn 2p XPS spectrum of AL/ZT-2/1 was identical to that of fresh ZT-2/1, providing the evidence of no interaction between Zn atoms and AL molecules. Nevertheless, the Ti 2p XPS spectrum of AL/ZT-2/1 in Fig. 6B exhibits a higher binding energy shift (0.1 eV) relative to that of the naked ZT-2/1, inferring the decrease in electron density in the Ti nucleus. This result reveals the transport of electrons from Ti atoms to AL molecules after the adsorption of AL. The O 1s XPS spectra for ZT-2/1 and AL/ZT-2/1 in Fig. 6C show three similar 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.52,53 However, AL/ZT-2/1 shows a higher binding energy shift (0.3 eV) for OH groups compared to the naked ZT-2/1 (531.3 eV), suggesting the migration of electrons from surface OH groups to AL molecules due to the adsorption of AL. Moreover, the O 1s spectrum of AL/ZT-2/1 exhibits an extra peak at 530.2 eV, which is ascribed to the adsorbed oxygen species at the vacancy sites.54,55 From the results we have already obtained, it can be concluded that the interaction between ZnTi-LDH and AL molecules was triggered via Ti sites and OH sites as the active sites. As a result of the interaction, surface coordination active species as well as oxygen vacancies (OVs) were formed at the same time. In addition, combined with the results of in situ FTIR analysis and abundant OH sites on LDH materials, it was further inferred that the basic site to promote deprotonation of AL was OH sites on the surface of ZT-2/1.
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Fig. 6 The Zn 2p (A), Ti 2p (B) and O 1s (C) XPS spectra of ZT-2/1 before and after the adsorption of aniline. |
Subsequently, the existence of OVs in AL-adsorbed ZnTi-LDH was further confirmed by the analysis of electron spin resonance (ESR) spectra. As shown in Fig. 7, no apparent ESR signal can be examined for the fresh ZT-2/1. On the contrary, AL/ZT-2/1 exhibited the typical ESR signal centered at g = 2.004, which was ascribed to the electrons trapped in the OVs.56 It was evidenced again that OVs were produced when AL molecules were adsorbed on ZT-2/1. As for AL/ZT-4/1 and AL/ZT-6/1, they obviously exhibited the weaker ESR signals, indicating the formation of a smaller number of OVs compared to AL/ZT-2/1. Therefore, the more surface OH groups exposed on ZnTi-LDH, the more OVs it could produce via the adsorption of AL molecules.
The roles of O2 molecules in the photocatalysis reaction was evidenced by a simulated in situ ESR measurement coupled with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin trapping reagent. As shown in Fig. 8, no obvious ESR signal was detected in the absence of AL molecules or in the dark condition. In contrast, ZT-2/1 with the addition of AL under visible light irradiation exhibited the typical signal assigned to the DMPO–˙OOH adduct, a spin derivative of DMPO–˙O2−, confirming the presence of ˙O2−.55,57 The results reveal that the generation of ˙O2− under visible light irradiation results from the surface coordination active species formed between ZnTi-LDH and AL molecules, which could absorb visible light and initiate the photocatalysis reaction. Moreover, the ESR signal intensity for ZT-2/1 is obviously stronger than those for ZT-4/1 and ZT-6/1, indicating that ZT-2/1 possessed a superior performance to produce ˙O2− than the others after the adsorption of AL molecules. It was demonstrated in a number of studies that OVs not only serve as the centers to capture photoelectrons from the photoexcited surface coordination species but also provide active sites for adsorption and activation of O2.58,59 Therefore, more OVs produced by the interaction between the ZnTi-LDH and AL molecules are definitely helpful to increase the performance to form ˙O2−, promoting photocatalytic oxidation organic reactions.
On the basis of the findings described above, a possible photocatalytic mechanism for the aerobic oxidation of aniline under visible light irradiation is explained in Scheme 1. Firstly, AL molecules are efficiently chemisorbed and activated on the surface of ZnTi-LDH via the Ti sites or OH sites as the active sites, resulting in not only formation of the surface coordination active species and OVs but also facilitation of the reductive deprotonation of AL to produce anilino species (I). Under visible light irradiation, the surface coordination species are excited to produce photogenerated electrons, which would be captured by OVs to reduce the adsorbed and activated oxygen molecules to ˙O2− radicals (II). Finally, ˙O2− radicals further oxidize anilino species to form nitrosobenzene (III).
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Scheme 1 Possible mechanism for selective oxidation of aniline to nitrosobenzene over ZnTi-LDH under visible light. |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0cy01721c |
This journal is © The Royal Society of Chemistry 2021 |