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Selective Photocatalytic Synthesis of Haloanilines from Halonitrobenzenes over Multifunctional AuPt/Monolayer Titanate Nanosheet Yujie Song, Huan Wang, Zhitong Wang, Binbin Guo, Kaiqiang Jing, Yanjun Li, and Ling Wu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02662 • Publication Date (Web): 11 Sep 2018 Downloaded from http://pubs.acs.org on September 11, 2018
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ACS Catalysis
Selective Photocatalytic Synthesis of Haloanilines from Halonitrobenzenes over Multifunctional AuPt/Monolayer Titanate Nanosheet Yujie Song, a Huan Wang, a Zhitong Wang, a Binbin Guo, a, b Kaiqiang, Jing, a Yanjun Li, c Ling Wu a, b*. a. State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou, Fujian 350116, P. R. China. b. State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Science, Fuzhou, Fujian 350002, P. R. China. c. Shanghai Institute of Measurement and Testing, 1500 Zhangheng Road, Shanghai, 201203, P. R. China. ABSTRACT: Bimetallic alloy AuPt nanoclusters supported on monolayer H1.07Ti1.73O4∙H2O nanosheets (AuPt/TN) jointly complete a rapid catalytic reaction towards hydrogenation of halonitrobenzene to haloaniline in methanol under ambient conditions using HCOONH4 as a hydrogen source. Especially, AuPt/TN with the Au/Pt molar ratio of 1:2 exhibits the high catalytic conversion efficiency for halonitrobenzene (>99%) with a high selectivity of haloaniline (>99%). In-situ FTIR spectra suggest that the TN affords surface Brønsted acid sites to chemisorb and activate the halonitrobenzene molecules via the surface hydrogen bond coordination. In-situ ESR experiments indicate that HCOONH4 would be decomposed to H+ and ∙CO2- radical by photogenerated holes, serving as the hydrogen source and reducing species for the reduction of NO2 group, respectively. Experiment results reveal that atom Pt in alloy is responsible for the hydrogenation, while Au represses the dehalogenation of haloanilines. Finally, a possible synergetic mechanism is discussed. This work highlights the multifunctional AuPt/TN catalyst with multiple active sites exerts the respective functions to cooperatively catalyze organic transformations toward desired target products. KEYWORDS: AuPt Nanoclusters, Monolayer titanate nanosheet, Photocatalytic organic transformation, Halonitrobenzene, Haloanilines.
1. INTRODUCTION Selective hydrogenation of nitroaromatics to anilines has always been a research hotspot because anilines are indispensable precursors or intermediates in chemical industries, especially, haloanilines. 1-3 As the key intermediates for the production of pigments, pharmaceuticals, pesticides and dyes, haloanilines are commercially produced from the reduction of halonitrobenzenes using the reductive metal Fe or Sn in HCl solution. 4-6 However, many co-products, such as aniline intermediate and nearly equimolar amounts of metal salts, are produced in these processes. Recently, heterogeneous catalytic processes are developed to synthesize haloanilines via the direct hydrogenation of halonitrobenzenes with H2 as hydrogen source. 7-12 A challenging topic is desired to improve the selectivity of haloanilines because the weak carbonhalogen (C-X) bond is easy to be dehalogenated forming aniline. Therefore, an alternative photocatalytic process may be potential to achieve the task. A variety of photocatalysts are used for hydrogenation of nitroaromatics to corresponding amines under mild conditions, such as TiO2-based, 13-17 CdS, 18-21 HNb3O8, 22 and so on. For the photocatalytic synthesis of haloanilines, only noble metals/TiO2 as catalysts are reported. 23
Au, Pt and Pd are usually used as a co-catalyst to activate and dissociate H2 for hydrogenation of halonitrobenzenes. However, they suffer from the issues that indiscriminate hydrogenation of halogen (-X) and -NO2 group results in the dehalogenation of haloaniline to aniline under the pressure of H2. How to rationally fabricate and construct appropriate photocatalysts for selective hydrogenation of –NO2 would be great challenge to achieve the highly efficient conversion of halonitrobenzenes to haloanilines. Two dimensional (2D) ultrathin structure materials exhibit various distinctive advantages different from their corresponding bulk materials. 2D structure has a huge specific surface areas and unique surface states, such as highly exposed specific crystal facets, abundant surface unsaturated metal sites and dangling bonds. 24-27 Ultrathin 2D nanosheets have been widely reported as efficient heterogeneous catalysts, mostly based on their Lewis acid sites supplied by surface unsaturated metal sites. 28-30 These surface sites would efficiently chemisorb relevant molecules. Moreover, 2D nanosheets allow them to support the highly dispersed metal nanoclusters for assembling into a multifunctional photocatalyst. The surface characters on the nanosheets would enhance the interface interactions with these metal nanoparticles (NPs). Very re-
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cently, NPs/2D nanosheets, such as Au/TiS2, 31 Au/C3N4, 32 Pt/C3N4, 33 Pt/TiO2, 34 Pd/C3N4, 35, Pd/H1.07Ti1.73O4, 36 Pd/TiO2, 37 have been reported in photocatalysis. Compared with monometallic NPs, bimetallic alloy NPs with bifunctional catalytic sites usually exhibit high catalytic activity and selectivity due to the synergy of bimetallic alloy nanoparticles. 38-40 Additionally, their electronic configuration would be redistributed via tuning the component of an alloy, governing the catalytic performance. Thus, based on an idea of the cooperation between bimetallic alloy and ultrathin 2D nanosheet, a composite photocatalyst with multifunctional active sites may be designed for the selective hydrogenation of halonitrobenzenes to haloanilines. Herein, the bimetallic AuPt alloy nanoclusters supported on the monolayer H1.07Ti1.73O4∙H2O nanosheets (AuPt/TN) are prepared via an in-situ photoreduction method to photocatalyze the selective hydrogenation of halonitrobenzenes to haloanilines. On the basis of the surface hydroxyl (-OH, Brønsted acid sites) in H1.07Ti1.73O4∙H2O, Au and Pt (-OH-Au-Pt), the bimetallic AuPt/H1.07Ti1.73O4∙H2O bearing three active sites as a multifunctional photocatalyst has cooperatively catalyzed the selective hydrogenation of halonitrobenzenes to haloanilines in the presence of HCOONH4 at room temperature and O2 free condition. In-situ FTIR spectra are carried out to fully reveal the surface adsorption behaviors of relevant molecules on hydroxyl (-OH) in TN. Controlled experiments are conducted to elucidate the influence of Au, Pt and supports on activity and selectivity. Furthermore, the interface electrons transfer in bimetallic alloy nanoclusters are also deeply studied by XAFS analysis. Moreover, roles of HCOONH4 are also examined by the results of experiments and characterization in detail. Combining with the ESR technique, we propose a possible three active sites cooperatively catalytic mechanism for the highly efficient photocatalytic hydrogenation of halonitrobenzenes to haloanilines.
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added. The suspensions were irradiated under a Xenon lamp for 4h (PLS-SXE300D, 1000 mw/cm2, Beijing Perfect light Co. Ltd.) with stirring in N2 atmosphere. The final products were washed and collected, labelled as Pt/TN, Au2Pt1/TN, Au4Pt1/TN, Au6Pt1/TN, Au/TN, Au1Pt1/TN, Au1Pt2/TN, Au1Pt4/TN and Au1Pt6/TN respectively. For comparison, the Au1Pt2/layered H1.07Ti1.73O4∙H2O (Au1Pt2/LT) and Au1Pt2/TiO2 were prepared via the same method. 2.3 Photocatalytic reaction for hydrogenation of halonitrobenzenes to haloanilines. 20 mg of catalyst, 0.5 mmol halonitrobenzenes and 3.5 mmol HCOONH4 were placed in a Pyrex reactor with a jacket under vacuum condition to remove oxygen. Reaction temperatures were controlled via a circulating condensate water. Methanol of 4 mL with oxygen eliminated was transferred to the reactor. The mixed suspension was firstly stirred under darkness for 20 min to promote fully dispersion of catalysts and dissolution of reactants. The suspension was then irradiated under a Xe-arc lamp of 300 W (PLSSXE300D, 1000 mw/cm2, Beijing Perfect light Co. Ltd.). The IR-cut filter was also used to remove the wavelengths that are longer than 800 nm. After definite reaction time, the catalyst was removed from the mixed suspension via centrifugation. The supernatant with final products was determined by an Aglient GC 6890 (FID detector) and a mass spectrometer of HP-5973.
3. RESULTS AND DISCUSSION
2. EXPERIMENTAL SECTION 2.1 Synthesis for H1.07Ti1.73O4∙H2O nanosheets (TN). The TN was prepared via an exfoliation process by intercalating the layered H1.07Ti1.73O4∙H2O (LT) precursor using the TBAOH (40 %) as intercalation agent. 36, 45, 55, 56 The mole ratio for LT and TBAOH is about 1: 1.2. The solution was stirred continuously for more than 7 days at room temperature. The resulting white colloidal solution was flocculated by adding 1 M HCl solution to obtain the solid product of TN. Then, the obtained sample was washed and dried at 60℃. 2.2 Preparation for metal nanoclusters / H1.07Ti1.73O4∙H2O nanosheets (M/TN). 1% wt of AuPt alloy supported on the TN was prepared via a photodeposition method. 36 The aqueous solutions of HAuCl4∙4H2O (10 mg/mL) and H2PtCl6∙6H2O (10 mg/mL) were mixed with the Au:Pt mole ratio of 0:1, 2:1, 4:1, 6:1, 1:0, 1:1, 1:2, 1:4 and 1:6, respectively. The mixed solution was injected into the colloidal solution of TN. Then, 5 mL of methanol, as the hole sacrificial agent, was drop-wise
Fig.1. XRD patters of the prepared bimetallic AuPt/TN with several of mole ratios.
Bimetallic AuPt/H1.07Ti1.73O4∙H2O nanosheets (AuPt/TN) with different mole ratio were prepared via an in-situ photo-reduction method. For the determination of bulk phase features, as-obtained samples were characterized by XRD. Fig. 1 shows the XRD patterns for TN loaded by AuPt alloy (Au1Pt6/TN, Au1Pt4/TN Au1Pt2/TN, Au/TN, Pt/TN, Au1Pt1/TN, Au2Pt1/TN, Au4Pt1/TN and Au6Pt1/TN). The characteristic peak around 38.4° is assigned to the AuPt (111) crystal face. 41 Compared to that of Pt (111, 2θ: 39.1°), the peak shifts to a lower 2θ angle indexed to the higher d space (d (111) = 0.229 nm). This d space is larger than that of Pt (d (111) = 0.222 nm) and smaller than that
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of Au (d (111) = 0.234 nm), AuPt alloy.
ACS Catalysis 42
indicating the formation of
TEM images of the prepared Au1Pt2/TN are shown in Fig. 2. It is observed that the metal nanoclusters are highly dispersed on the ultrathin H1.07Ti1.73O4∙H2O nanosheets (Fig. 2a). The nanoclusters show an average size of about 3nm (inserted in Fig. 2a). The high-resolution image concentrated on a single metal nanocluster shows the clear lattice fringes with about 0.229 nm spacing (Fig. 2b), which would be assigned to the fcc-type (face-centered cubic) AuPt (111) planes with that of 0.226 nm for fcc-type Pt (111) and 0.235 nm for fcc-type Au (111). 41 Moreover, the lattice fringes with the distance of 0.218 nm would be attributed to the new planes in bimetallic AuPt, because lattice fringe spaces of 0.203 and 0.226 nm are corresponding to Au (200) and Pt (111), respectively. 43 A cub model of single AuPt nanocrystal is schematically reproduced in Fig. 2b (inserted). These results suggest the successful formation of intermetallic nanoclusters on the TN. Furthermore, EDS further confirms the existence of Au, Pt and Ti (Supporting Information, Fig. S1).
Element-specific X-ray absorption near edge structure spectroscopy (XANES) is a power technique to reveal the electronic structures of AuPt alloy. With regard to the L3edge XANES of Au and Pt, electronic transition takes place from 2p3/2 to 5d5/2, and the peak intensity of white line (peak of absorption edge) increases with the unoccupied degree of 5d5/2. 44 The bulk average electronic states of Au and Pt in AuPt nanoclusters were further investigated by XANES via the comparisons in intensity of L3 absorption edges (white line) from Pt and Au. As shown in Fig. 3a, single Au metal shows no intense white line for L3 edge due to a filled d10 orbital. However, the L3-edge spectra of Au in AuPt alloy shows a higher white line peak intensity than the Au foil, suggesting the electrons loss in d character of Au. Conversely, the Pt L3 edge of white line peak in the prepared sample is weaker than that in the single Pt foil (Figure 3b), arising from the gain of dcharges at Pt atom. According to the literature, 44 electrons transfer would take place from a metal of more occupied valence band to another metal in an alloy for the stability of bimetallic bonds. Therefore, the d charges depletion in Au atom and the d-charge gain in Pt atom support the fact that Au-Pt bond exists in AuPt alloy, which coincided with the results of TEM, XRD and XPS.
Fig. 2 (a) TEM images of the prepared Au1Pt2/TN (Magnified image has been inserted). (b) HRTEM image of AuPt alloy nanoclusters (inserted is the AuPt cub model).
Fig. 3 XANES spectra of as prepared Au1Pt2/TN. (a) L3-edge of Au and foil reference, (b) L3-edge of Pt and Pt foil reference.
To reveal surface chemical states of the prepared AuPt alloy, XPS (X-ray photoelectron spectroscopy) analysis was conducted. Full XPS spectra for Pt/TN, Au/TN and Au1Pt2/TN are shown in Fig. S2a. Elements of C, Ti, Au, Pt and O can be observed clearly, with their characteristic peaks at the binding energy values of 73.7 (Pt 4f), 83 (Au 4f), 285 (C 1s), 459 (Ti 2p) and 530 eV (O 1s) respectively. Fig. S1b shows the high-resolution for Au of Au/TN and Au1Pt2/TN. For Au1Pt2/TN, the characteristic peaks with binding energy values of 83.9 and 87.6 eV are attributed to Au 4f7/2 and Au 4f5/2 respectively, indicating the metallic Au0. 41-43 Furthermore, these peaks shift to the higher binding energy values, compared with those for Au/TN (83.6 and 87.3eV). These results would be attributed to the redistribution of electrons between Au and Pt in AuPt alloy. Moreover, the binding energy values of Pt 4f exhibit shift from 70.9 (4f7/2) 74.2eV (4f3/2) for Pt/TN to 70.5 and 73.8eV for Au1Pt2/TN, indicating that the electron cloud distribution moves from Au atom to Pt atom in the AuPt alloy nanoclusters. These results support the interaction of Au and Pt, in accordance with the results of XRD and TEM.
Fig. 4 Photocatalytic performance of prepared samples. Reaction conditions: catalyst, 20 mg, 4-chloronitrobenzene, 0.5 mmol, HCOONH4, 3.5 mmol, solvent, 4 mL methanol, atmosphere, 1 atm N2, 298 K. Reaction temperature was controlled via a jacket connected to a circulating cold water source. Xenon lamp with an IR-cut filter to remove all wavelengths longer than 800 nm.
The photocatalytic performances of prepared samples were examined for the hydrogenation of halonitroben-
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zene in methanol solvent at N2 atmosphere. Fig.4 shows the photocatalytic activities of the prepared samples. It is obvious that Pt/TN, Au1Pt2/TN, Au1Pt4/TN and Au1Pt2/TN give the high conversions of 4-chloronitrobenze (＞99%) after 4h, whereas, Au/TN, Au1Pt2/TN, Au2Pt1/TN, Au4Pt1/TN and Au6Pt1/TN exhibit the high selectivity for the 4-chloroaniline. In particular, Au1Pt2/TN shows both the excellent conversion of 4-chloronitrobenzene and selectivity to 4-chloroaniline (>99%). These results suggest that Pt would contribute to the conversion of 4chlorinenitrobenzene, and Au is responsible for the selectivity of the desire product. The turnover frequency (TOF) and turnover number (TON) are also calculated by the moles of converted 4-chloronitrobenzene per mole of AuPt in catalyst per hour and produced 4-chloroaniline per mole of AuPt, respectively. Thus, the TOF for Au1Pt2/TN is 122.3 h-1, and TON is calculated as 489.2 in 4 h. As shown in Fig. 5, the yield variations of 4chloroaniline with time were also recorded over Pt/TN and Au1Pt2/TN. For Pt/TN (Fig. 5a), it has been observed that the yield of 4-chloroaniline is firstly increased from 0 to 42% within 2 h and then decreased to 20% as the reaction time prolonged. However, the yield of byproduct aniline is continuously increased from 0 to 80% within reaction time. A small amount of nitrobenzene is also detected due to the dehalogenation of 4chloronitrobenzene. These results indicate that Pt/TN has no selectivity for the hydrogenation of -Cl and -NO2. Fig. 5b shows that the yield of 4-chloroaniline is constantly increased with the reaction time prolonged. The byproducts aniline and nitrobenzene are not detected in the whole process. All of these results demonstrate that the introduction of Au in Pt/TN would inhibit the dechlorination of 4-chloroaniline and 4-chloronitrobenzene. Combined with the XANES spectra, it can be deduced that the electrons transfer from Au to Pt atom in AuPt alloy would dominate the selectivity of 4-chloroaniline via the recombination of electronic structures.
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that photocatalysis is mainly responsible for the performances. Furthermore, the layered H1.07Ti1.73O4∙H2O (LT) and TiO2 (Anatase) were also used as the supports to reveal the roles of TN. It is obvious that Au1Pt2/TiO2 and Au1Pt2/LT show the disappointing catalytic performances under the identical conditions. These results indicate that the selective hydrogenation of 4-chloronitrobenzene is driven by AuPt alloy with the assistance of TN. As a photoactive semiconductor material, the photoabsorption property would influence its catalytic performance. The TN only adsorb UV-light with the wavelength less than 360 nm (λ≤360 nm). The deposition of AuPt alloy would promotes TN to adsorb more visible light based on the surface plasma resonance of metal nanoclusters (Fig. S3). Thus, we further examine the light effects of Au1Pt2/TN. As Fig. S4 shows that, under the visible light irradiation (420 nm≤λ≤800nm), 27% conversion of 4chloronitrobenzene with 98% selectivity of 4chloroaniline is achieved in the same conditions. Considering that TN would not be excited by visible light, AuPt nanoclusters would be responsible for photocatalysis, due to their hot electron actions. This is why the Au1Pt2/TN also show a thermal catalytic activity. Remarkably, a considerable accelerating effect is observed under a light (350nm≤λ≤800nm) irradiation. The enhanced photocatalytic activity would be attributed to the excitation of TN. These results revealed that TN would act as a photocatalyst to supply photogenerated electrons and holes.
Fig. 6 Yield of 4-chloroaniline with the various content of HCOONH4. Au1Pt2/TN 20 mg, solvent, methanol 4 mL, 4chloronitrobenzene, 0.5 mmol, atmosphere, N2 1atm, λ≤ 800 nm, 298 K, reaction time 4 h. Fig. 5 The yield variations of 4-chloroaniline with time over Pt/TN (a) and Au1Pt2/TN (b). Identical reaction conditions as described in Fig. 4.
Moreover, series of control experiments were conducted to better understand this reaction. The results are listed in Table S1. No desired product is detected with absence of a catalyst or alone TN (entries 1-2), suggesting that AuPt alloy is indispensable for this reaction. It is obvious that 14% of 4-chloronitrobenzene is catalyzed to 4chloroaniline under the darkness at 298K. Even elevating reaction temperature to 353K, the conversion of 4chloronitrobenzene only increases to 31 % with 91 % of selectivity for 4-chloroaniline (entries 3-4), suggesting
To better elucidate the roles of HCOONH4, Control experiments are carried out (Fig.6). Negligible 4chloroaniline is detected over Au1Pt2/TN in the absence of HCOONH4. The yield of the 4-chloroaniline increases with the increase of HCOONH4 content. When 3.5 mmol of HCOONH4 is used, the yield of 4-chloroaniline reaches the highest value with the yield of 99%. With the content of HCOONH4 further increase, the yield of desired product then decreases. Meanwhile, a large quantity of aniline is detected, which would be attributed to over hydrogenation of 4-chloroaniline. It should be noted that the generation rate of 4-chloroaniline firstly increases and then decreases as the increased content of HCOONH4, indicat-
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ACS Catalysis
ing that the induced process of HCOONH4 may suffer from other competitive reactions. It can be inferred that the HCOONH4 may act as the hydrogen source. We also studied the influence of reaction atmosphere on the induced process of HCOONH4. It is obvious that a sharply decreased conversion and selectivity are observed at air atmosphere, compared with that at Ar or N2 atmosphere (Fig. 7). These results demonstrate that the induced HCOONH4 may produce some active intermediate species, participating in the reaction under the production of oxygen free atmosphere. Such catalytic process would be explicitly explored in the later content.
The stability of a catalyst is extremely important in application. We also tested the cyclical stability of Au1Pt2/TN in this reaction. To avoid the interference of excessive catalyst, the halved Au1Pt2/TN (10 mg) was used to examine the catalytic stability. As shown in Fig. S5, Au1Pt2/TN maintains the stable catalytic conversion for the 4-chloronitrobenzene with the high selectivity of 4chloroaniline even after the five recycles. After the reaction, the filtrate was analyzed by inductively coupled plasma atomic emission (ICP-AES). No Au or Pt was detected, confirming the stability of the Au1Pt2/TN catalyst. Table 1. Selectively Photocatalytic Hydrogenation of Halonitrobenzenes to Haloanilines over the bimetallic Au1Pt2/TN.
Entry
Fig.7 The photocatalytic activities of Au1Pt2/TN for selective hydrogenation of 4-chloronitrobenzene at different reaction conditions. Au1Pt2/TN, 20 mg, methanol 4mL, 4chloronitrobenzene, 0.5 mmol, HCOONH4, 3.5 mmol, N2/Ar/Air/O2 1atm, λ≤800 nm, 298 K, reaction time 4 h.
To better understand the effect of solvents on catalytic efficiency, water, toluene, acetonitrile and benzene were used as the solvent for photocatalytic hydrogenation of 4chloronitrobenzene over Au1Pt2/TN at 298 K under N2 atmosphere. Results are listed in Table S2. In water solvent, only 6.9% yield of 4-chloroaniline is obtained after the reaction time of 4 h due to the small solubility of 4chloronitrobenzene. Moreover, in organic solvents, the conversion of 4-chloronitrobenzene increases with the increasing polar of the solvent (Acetonitrile > Toluene >Benzene). However, all of them suffer from the low conversion of 4-chloronitrobenzene and selectivity of 4chloroaniline. As shown in Table S2, only 21% conversion of 4-chloronitrobenzene and 7% yield of 4-chloroaniline are detected in acetonitrile solvent under the identical conditions. Moreover, 11% and 6% conversion of 4chloronitrobenzene are obtained in toluene and benzene solvent, respectively. However, the desired 4chloroaniline is undetected (Table S2, entries 3-4). The insolubility of HCOONH4 in acetonitrile, toluene and benzene solvents may also result in the deficiency of hydrogen source to restrain the hydrogenation of 4chloronitrobenzene. Compared with toluene, acetonitrile and benzene, methanol is a more suitable solvent for this reaction due to the better dissolution for 4chloronitrobenzene and HCOONH4.
-X
Time (h)
Conv. (%)
Sel. (%)
1
4-Cl
4
＞99
99
2
3-Cl
3.7
＞99
99
3
2-Cl
4
＞99
96
4
4-Br
4.6
99
98
5
3-Br
4.2
99
90
6
2-Br
4.5
99
87
7
4-I
6
99
96
8
3-I
6
99
94
9
2-I
8
98
68
Reaction conditions: Au1Pt2/TN, 20 mg, HCOONH4, 3.5 mmol, halonitrobenzene, 0.5 mmol, methanol 4 mL, N2, 1 atm, 298 K, Xe-lamp with an IR-cut filter (λ≤800 nm).
Moreover, several halonitrobenzenes are expanded to detect the general applicability of the catalyst. The prepared Au1Pt2/TN was used in hydrogenation of a series of halonitrobenzenes in the presence of HCOONH4. The results are shown in Table 1. It is obvious that 2- and 3chloronitrobenzenes can be selectively catalyzed into corresponding chloroaniline (entries 2-3). Similar with -Cl, nitrobenzene with the -Br and -I substituted are also hydrogenated to haloanilines with the high selectivity (entries 4-9). It should be noted that the 2- and 3- substituted halonitrobenzenes are easier to be dehalogenated producing anilines, due to the spatial effect of -X and -NO2. 11 It would be observed that the stronger the electronwithdrawing ability for substituent -Cl, -Br, -I, the higher the catalytic activities are. 45 These would be attributed that the functional groups with electron-withdrawing would weaken bond energy of N-O to promote its activation. Thus, the reaction rate follows the order of -I< -Br < -Cl, which is similar to the previous studies. 11, 45 To further elucidate the active species, a simulated insitu electron paramagnetic resonance technic (in-situ EPR) coupled with DMPO spin-trapping was carried out at N2 atmosphere under light irradiation. Results are shown in Fig. 8. It can be observed that a characteristic
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peak with sextet of DMPO-∙CO2- is observed in presence of HCOONH4 for Au1Pt2/TN, Pt/TN and Au/TN. No apparent signals are examined in absence of HCOONH4 or under darkness, suggesting that the formation of ∙CO2derives from HCOONH4 with assistance of TN via a photo-induced process (Fig. 9). ∙CO2- radicals possess the strong reducibility (E0 (CO2/∙CO2-) = -1.8 V), 22, 46-47 which would be fleetly consumed at air or O2 atmosphere. Thus, the intensity of the ∙CO2- signal is decreased enormously in air atmosphere (Fig. 9), resulting in a lower catalytic efficiency (Fig. 7). The dehydrogenation of HCOONH4 and the reduction of nitro compounds process are shown from Eqs. (1) to (6). HCOONH4 would dissolve in methanol solution to produce HCOO- and H+ (Eqs. (1)). Under the light irradiation, electrons and holes are generated in TN (Eqs. (2)). HCOO- is induced to produce H+ and ∙CO2by photo-generated holes, because of the more positive valence band position (2.42 V vs. NHE) (Eqs. (3)). 22, 46-48 Meanwhile, the negative charges accumulated in AuPt nanoclusters would also promote the dehydrogenation of HCOO- to form ∙CO2- (Eqs. (4)). 2, 22 As a electrons sink, surface AuPt would reserve the photo-generated electrons to improve separation of electron-hole pairs and promote the reduction of H+ (Eqs. (5)). 49, 50 The unstable AuPt-H would decompose to produce H for the hydrogenation of –NO2. 51, 52 The reductive ∙CO2- and H released from AuPt corporately complete the reduction of -NO2 group. Finally, the -NH2 group is formed (Eqs. (6)). 17
Fig. 8 EPR spectra of Au/TN, Pt/TN and Au1Pt2/TN in a methanol solvent with presence of DMPO at N2 atmosphere.
Fig. 9 EPR spectra of Au1Pt2/TN in methanol solvent with presence of DMPO at different atmosphere.
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HCOONH4
NH4++HCO2-
(1)
NS + hⱱ
e- + h+
(2)
HCOO- + h+
∙CO2- (-1.8V vs NHE) + H+
(3)
AuPt + HCOO
-
AuPt + H+ + e-
∙CO2-
+ AuPt-H
AuPt-H
(4) (5)
6∙CO2- + 6AuPt-H + -NO2 6 CO2 + 6AuPt + -NH2 +2 H2O
(6)
It has been reported that the properties of supports, including surface acidity, basicity and unsaturated metal atoms, would affect the catalytic conversion and selectivity by dominating the adsorption of reactant or product molecules. In our previous studies, we also found that the nanosheets with abundant surface Lewis acid sites and OH groups would efficiently adsorb relevant molecules via forming surface coordination species, which significantly promotes the catalytic efficiency. 28, 36, 44, 53-54 To deeply reveal the role of TN, we used the in-situ FTIR technic to study the adsorption behaviors of 4chloronitrobenzene on the TN surface. As shown in Fig. 10 A, characteristic peaks assigned to 4chloronitrobenzene are evidently observed after the adsorption of 4-chloronitrobenzene on TN (Fig. 10 A (b)). Even after the further evacuation, these peaks also remain, suggesting the strong interactions between relevant molecules and TN (Fig. 10 A (c)). Moreover, the peaks at the wavenumber of 1311 and 1528 cm-1 attribute to ν-NO2 vibration for 4-chloronitrobenzene adsorbed on TN. Compared with those for free 4-chloronitrobenzene, these peaks show a shift towards higher wavenumber, while other characteristic peaks have no change (Fig. 10 A (d)). These results further demonstrate that it is the -NO2 instead of -Cl that coordinate with TN, and the interface charge redistribution takes place via the coordination species. Considering the abundant surface -OH groups (Brønsted acid sites) in TN, it is rational to propose that 4-chloronitrobenzene would be adsorbed on TN via a TiO-H∙∙∙O=N or Ti-O-H∙∙∙O-N coordination. 45 It is to say, the available surface -OH groups should be responsible to improving the adsorption efficiency. To confirm this deduction, the counterpart layered titanate (LT) with few content of surface -OH groups was used for comparison. Fig. 10 (B) shows that no obvious characteristic peaks attributed to 4-chloronitrobenzene are observed after an adsorption process (b), suggesting that a small amount of relevant molecules would be chemisorbed on the surface of LT. To further explore the influence of adsorbed nitrobenzene on light absorption, UV-DRS spectra of single TN and TN adsorbed by 4-chloronitrobenzene were examined. As shown in Fig. S6, the light adsorption of TN shows a slight red shift to visible light region after the adsorption of 4-chloronitrobenzene, suggesting that interface charge transfer takes place between TN and 4chloronitrobenzene. The formed surface coordination TiO-H∙∙∙O=N or Ti-O-H∙∙∙O-N species would greatly facilitate the activation of -N-O or -N=O bonds via interface charges redistribution. Thus, the coordinated 4chloronitrobenzene is easier to be converted. Under visible light irradiation (420 nm≤λ≤800 nm), 27% conver-
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sion of 4-chloronitrobenzene with 98% selectivity of 4chloroaniline is also obtained (Fig. S4). Even though TN is not excited by visible light, the activated –NO2 could be hydrogenated into 4-chloroaniline by HCOONH4 with assistance of Au1Pt2 nanoclusters, because the hot electrons of Au1Pt2 would induce the generation of ∙CO2- from HCOONH4 (Eqs. (4)). Furthermore, the deposition of Au1Pt2 would promotes the light adsorption of TN (Fig. S3), facilitating the catalytic process. Therefore, surface coordination and activation of –NO2 on TN are one of the crucial factors for high catalytic efficiency. The chemisorption of reactant molecules on catalysts would lead to significant reduction of kinetic barriers in the interface reaction due to the interface charge redistribution. Therefore, Au1Pt2/TN performed the superior catalytic activity than Au1Pt2/LT and Au1Pt2/TiO2 (Table S1, entries 5-6). All of these results support that TN not only serves as a support for AuPt alloy, but also provides abundant active sites for the coordination and activation of 4chloronitrobenzene. TN with more open structure and exposed surface acid sites could absorb more 4chloronitrobenzene molecules to greatly promote the conversion efficiency.
NO2 or surface Au1Pt2 nanoclusters. The direct electrons transfer take place from CB of TN to adsorbed –NO2 resulting in the direct reduction of -NO2. Then, surface Au1Pt2 nanoclusters induce the formation of active hydrogen via hot electrons for selective hydrogenation of reduced –NO2 to –NH2. Moreover, the photogenerated electrons would also migrate to the surface Au1Pt2 nanoclusters for the reduction of H+ into activated H to directly hydrogenate the activated -NO2 groups. Because of the electrons transfer from Au to Pt atom in AuPt alloy, the generation rate of H would be efficiently regulated, depressing the over hydrogenation of produced 4chloroaniline to aniline. Therefore, the cooperation of TN and bimetallic synergy jointly governs the conversion of 4-chloronitrobenzene to 4-chloroanline with a highly selectivity.
Scheme 1. Proposed mechanism of photocatalytic hydrogenated reduction of 4-chloronitronenzene over the Au1Pt2/TN with presence of HCOONH4 under darkness. Fig.10. In-situ FTIR spectra of the TN (A) and LT (B) for 4chloronitrobenzene adsorption. (a) After degassing at 180 ℃ for 4 h. (b) Adsorption of 4-chloronitrobenzene for 30 min at room temperature (physisorption+chemisorption). (c) Further evacuation of excess probe molecules at 180 ℃ for 5 min (chemisorption). (d) The FTIR spectrum of the 4chloronitrobenzene in CCl4 solution.
Based on the aforementioned experimental results and discussion, we propose a possible mechanism for photocatalytic 4-chloronitrobenzene reduction to 4chloroanline. As shown in Scheme 1, 4chloronitrobenzene molecules would be adsorbed on the surface -OH groups of TN via the hydrogen bonds forming the Ti-O-H∙∙∙O=N or Ti-O-H∙∙∙O-N coordination, resulting in the activation of -N-O and -N=O due to the interface charges redistribution and transfer. HCOONH4 would exist in the form of HCOO- and NH4+ in polar methanol solution. Under the darkness or visible light irradiation, Au1Pt2 nanoclusters would induce HCOONH4 to produce hydrogen and strongly reductive ∙CO2- for reduction and hydrogenation of the activated –NO2. 22 Under simulated sunlight irradiation, electron and hole pairs are generated in excited TN (Scheme 2). The holes would be consumed by HCOO- promoting the generation of ∙CO2- radicals (HCOO-+h+→H++∙CO2-) 46-47 to further reduce the activated -N-O and -N=O. Synchronously, the generated electrons may directly transfer to adsorbed –
Scheme 2. Proposed mechanism of photocatalytic hydrogenated reduction of 4-chloronitronenzene over the Au1Pt2/TN with presence of HCOONH4 under simulated sunlight irradiation.
4. CONCLUSION In conclusion, a multifunctional Au1Pt2/TN photocatalyst with multiple active sites is designed to exert the respective functions for efficiently catalytic selective conversion of halonitrobenzenes to haloanilines. It is revealed that the TN afford abundant surface Brønsted acid sites to selectively chemisorb -NO2 group via hydrogen bonds forming the Ti-O-H∙∙∙O=N or Ti-O-H∙∙∙O-N coordination. The formed surface coordination species would activate the –N-O and –N=O bonds due to the interface
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charge redistribution. Moreover, TN is photoactived to generate electrons and holes. HCOONH4 is induced by holes to produce strong reductive ∙CO2- and H+. Meanwhile, generated electrons would directly transfer to the adsorbed –NO2 and surface Au1Pt2 alloy nanoclusters for the reduction of –NO2 and production of active H. It has been clarified that electronic structure of Au1Pt2 alloy would tune the generate rate of H due to the electrons transfer from Au to Pt atom, inhibiting the dechlorination of produced 4-chloroaniline and 4-chloronitrobenzene. The cooperation of TN with abundant surface acid sites and AuPt synergistic effect with tunable electron structure jointly improve the selectivity of 4-haloanilines with the high conversion of 4-halonitrobenzene. This work would provide a strategy for the rational design of multifunctional catalysts and help to understand the photocatalytic organic transformation at a molecular level.
ASSOCIATED CONTENT Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXXX. The detailed experiments and supplementary experimental results.
AUTHOR INFORMATION Corresponding Author * Email:
[email protected] (L. Wu).
Author Contributions In this manuscript, Y. Song, H. Wang, Z. Wang and K. Jing carried out the synthesis and characterizations of materials and the photocatalysis experiments. B. Guo and Y. Li did the XPS and TEM characterizations. L. Wu conceived the project, supervised the research work and discussed the results. All authors contributed to the paper writing. We thanks Hao Zhang (Shanghai Institute of Applied Physics, Chinese Academy of Sciences) very much for his great assistance in experiments of XANES.
Funding Sources This work was supported by the National Natural Science Foundation of China (21872032 and 51672048), the major science and technology projects of Fujian Province (2015YZ0001-1).
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT Thanks to the Shanghai Synchrotron Radiation Facility of China for the XAFS spectra measurements at the BL14W1 beam line.
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