TiO2 composite

Applied Surface Science 500 (2020) 144214

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Plasmon-enhanced and controllable synthesis of azobenzene and hydrazobenzene using Au/TiO2 composite Qiuwen Liu, Jingwen Zhang, Fangshu Xing, ChuChu Cheng, Yawei Wu, Caijin Huang

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State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou 350116, People’s Republic of China

A R T I C LE I N FO

A B S T R A C T

Keywords: Au/TiO2 catalyst Plasmon Au–H species Azobenzene Hydrazobenzene

The selective reduction of nitroaromatics has long been the focus of fundamental and practical interest. The challenging issue lies in how to finely control the product selectivity. Herein, we report a light-assisted thermocatalytical process to convert nitrobenzene into azobenzene and hydrazobenzene with high selectivity and high yields by simply controlling the reaction time. The light-enhanced efficiency of Au/TiO2 catalyst is up to 14.8% at 90 °C due to the localized surface plasmon resonance (LSPR) effect of Au nanoparticles. Moreover, the hydrogen-delivery rate here is much faster than that of current reported systems. The reaction mechanism was also investigated by EPR technique, where Au–H intermediate species is found to account for the high efficiency in this reduction process. The favorable results demonstrated that plasmon-enhanced catalytic reaction might be more attractive and applicable in industrial organic synthesis.

1. Introduction Hydrazo and azo compounds, as critical building blocks, are highly desired intermediates in fine chemistry of organic synthesis, which have been widely used in material science, medicine, dyes, and agrochemicals [1,2]. Compared with traditional methods where biologically harmful byproducts are formed by diazotization [3], selective reduction of nitroaromatics has long been the focus of fundamental and practical interest [4]. Considerable efforts have been devoted to developing selective reduction of nitroaromatics systems in order to achieve the desirable synthesis of target products with high yield and good selectivity under greener conditions. During the transformation of nitroaromatics into corresponding azo derivatives, various reducing agents are involved. Reductive gases such as H2 and CO are once favored in the reduction systems, however, accompanying with high pressure and high temperature [5,6]. Commonly used reducing agents including NH2NH2 [7], NaBH4 [8], and NH3BH3 [9] are also employed. Interestingly, inexpensive alcohols, such as isopropanol, have also been adopted as the hydrogen source and have obtained good results, which exhibit remarkable advantages in regard to economy and safety [10,11]. On the other hand, the vast majority of reported systems for selective conversion of nitrobenzene focus exclusively on obtaining one reductive product. From a view of industrial application, it would be additionally attractive to realize the controlled reduction of nitrobenzene into more than one desirable product in a

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single reducing system, whose special value lies in its step-economical potential and cost-efficiency. Interestingly, an example of controllable synthesis of azoxybenzenes and anilines with alcohol as the reducing agent had been reported [12], this method, however, failed to obtain the high yield with good selectivity (72–75% yield of desired azoxybenzenes). In addition to the effect of reductant, catalysts play vital roles in the selective reduction of nitroaromatics, especially in conversion efficiency and selectivity. Among various candidate catalyst systems, supported gold nanoparticles (Au NPs) have been recognized as one of the most intensively investigated catalysts for reduction of nitroaromatics since Corma and García reported that Au/TiO2 can serve as an efficient catalyst for conversion of nitroaromatics into azo compounds with yields above 98% through a two-step, one-pot reaction [5]. However, it was reported that over-reduced lower-value products, such as aniline, are apt to be formed as the main products and an extra oxidation step is thus needed to obtain the target azobenzene. Encouragingly, Zhu’s group first established a photocatalysis route to synthesize azo compounds based on the localized surface plasmon resonance (LSPR) effect of plasmonic metal nanoparticles [13]. Following the continuous studies on Au catalysis, transient AueH species is claimed to be formed in the reductive process from supported Au NPs in the presence of isopropyl alcohol [13–15], which played an essential role in controlling the reduction of the nitro group at different intermediate stages [16]. Despite the fact that much progress has been made

Corresponding author. E-mail address: [email protected] (C. Huang).

https://doi.org/10.1016/j.apsusc.2019.144214 Received 19 July 2019; Received in revised form 26 September 2019; Accepted 27 September 2019 Available online 08 October 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

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Intensity (arb. unit)

in this area, it still suffers from some limitations. For example, the selectivity to the intermediate product is not controllable, especially noting hydrazo-compounds are often ignored due to their extremely low yield. Another limitation is that the inefficient hydrogen delivery rate, that is, the reaction rate of photocatalytic systems is insufficient to meet practical needs [17]. Harnessing the full potential of gold-based catalysts, recently emerged photo-thermal catalysis might shine a light on this reaction by combining light harvesting and unique catalytic function of Au plasmonic metal. Light-induced LSPR had been proved to enhance the catalytic performances in Fischer-Tropsch synthesis [18], CO2 conversion [19], Suzuki coupling reactions [20,21], etc. In this work, we demonstrated that Au/TiO2 composites enabled the controlled reduction of nitrobenzene into more than one product in one pot. We also surveyed several important catalytic characteristics associated with the effect of gold loading, the reaction temperature and light irradiation. A unique advantage of the present Au-based system is that the switchable products, including azobenzene and hydrazobenzene can be selectively obtained with high selectivity and high yields by simply varying the reaction time. Moreover, the reductive efficiency could be enhanced by the combination of thermal and light energy based on the LSPR effect of Au NPs. Most remarkably, TOF of nitrobenzene reduction is up to 404 h−1, much higher than those conventional catalytic hydrogenation processes. Stable Au–H intermediate species may account for the high efficiency in this reduction process. Our results herein give a new horizon for the application of gold-based catalysts in green and sustainable organic synthesis.

Au(200)

30% 20% 15% 10% 5% 1% TiO2 (101)

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Fig. 1. XRD patterns of x% Au/TiO2 samples.

frequency of 100 kHz. 3. Results and discussion The Au/TiO2 composites were prepared by wet reduction in deionized water. The X-ray diffraction (XRD) patterns were carried out to confirm the structure of as-prepared samples. As shown in Fig. 1, the main peaks locates at 25.3°, 37.8°, 48.0° corresponding to (1 0 1), (0 0 4), (2 0 0) planes of anatase (JCPDs 84-1285),[22] respectively. Moreover, these peaks of 38.2° and 44.4° are attributed to Au (1 1 1) and (2 0 0) facets (JCPDs 04-0784) [23] respectively. Note that the intensity of Au peaks is stronger with the increase in loading amount of Au. In addition, there is no obvious peak ascribed to other materials, indicating the purity of Au/TiO2 composites. For more insight into the samples component, X-ray photoelectron spectra (XPS) were employed. The survey XPS shows that 20% Au/TiO2 is comprised of Au, Ti and O element (see Fig. S1a in supporting information (SI)). In the Au 4f core-level spectrum (Fig. 2a), two peaks at 82.9 and 86.5 eV are attributed to Au 4f7/2 and 4f5/2 [24], respectively, suggesting that the Au element is metallic state in 20% Au/TiO2 composite. Similarly, the same results can be obtained in other Au/TiO2 composites with different amount of Au loading (see Fig. S1b–e in SI). The diffusion reflectance spectra (DRS) are presented in Fig. 2b, from which we can observe obvious optical absorption in the range 500–600 nm assigned to the LSPR effect of Au nanoparticles [25]. Difference in gold loading amounts can be responsible for different peak area and height. The transmission electron microscope (TEM) was performed to investigate the morphology of the resulting Au/TiO2 samples. As shown in Fig. 3, Au nanoparticles show heavy contrast to TiO2 support, and are highly dispersed for the 20% Au/TiO2 sample. According to the size distribution, the size of Au particles is estimated to be ca. 5 nm (Fig. 3b). The TEM images of 5% and 30% Au/TiO2 as well as size distribution of Au particles are given in Fig. S2 (SI). With the increasing amount of Au loading, a certain percentage of larger particles are also observed in some parts of the 30% Au/TiO2 sample, which can be attributed to the agglomeration of Au nanoparticles. Here, the selective reduction of nitrobenzene was selected as a model reaction to investigate the catalytic activity of Au/TiO2 catalysts with isopropyl alcohol serving as the reducing agent and solvent. KOH was also added to enhance the abstraction of hydrogen atom.[26] Initially, we estimated several important factors including Au loading, reaction temperature and light irradiation to optimize reaction conditions. As shown in Fig. 4a and b, nitrobenzene undergoes a gradual reduction process. There are three main products appearing sequentially, that is, azoxybenzene (AOB), azobenzene (AB) to hydrazobenzene (HAB) without other intermediates being observed. First, the dependence of the catalyst performance on Au loading

2. Experimental section 2.1. Catalyst preparation A certain amount (1.045, 5.225, 10.450, 15.675, 20.900, or 31.135 mL) of HAuCl4 (10 g/L) and 0.5 g of TiO2 powder were added to the 31 mL deionized water. 4 mL of lysine (310 mg/mL) was then added, and the mixture was stirred vigorously for 20 min. Following that, 20 mL of aqueous solution of NaBH4 (3.8 mg/mL) was dropped into the mixture in 20 min. After keeping stirring for 1 h, the sample was left in the dark for 16 h without stirring. The precipitate was filtrated and washed with deionized water and ethanol, and then dried at 60 °C. The resulting sample was defined as x% Au/TiO2 (x = 1, 5, 10, 15, 20, or 30), where x% represents the mass ratio of Au in the composites. 2.2. Activity test Nitrobenzene (1.5 mmol) and KOH (0.3 mmol) were dissolved in 15 mL isopropyl alcohol, then 50 mg catalyst (Au/TiO2 composites) was added. The reaction was carried out in nitrogen at 90 °C under stirring unless noted otherwise. The products were analyzed using an Agilent 7820A GC equipped with HP-FFAP column and a Varian Cary 500 Scan UV–Vis spectrometer. 2.3. Characterization Powder X-ray diffraction (XRD) patterns were obtained at room temperature on a Bruker D8 Advance diffractometer with Cu kα1 radiation. Fourier transform infrared spectra (FTIR) were collected on a nicolet 670 FTIR spectrometer using KBr as a transmission standard and a diluting reagent. UV–Vis spectra were recorded on a Varian Cary 500 Scan UV–Vis system with BaSO4 as reflectance standard. The morphologies and microstructures of the samples were investigated by a field emission scanning electron microscopy (FESEM) on a Hitachi New Generation SU8100 apparatus and transition electron microscopy (TEM) on a TECNAIF30 instrument under an acceleration voltage of 200 kV. ESR measurements were made on a Bruker-A 300 at room temperature, X-band spectrometer using a magnetic field modulation 2

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Fig. 4. Conversion and selectivity for x% Au/TiO2 at 90 °C. Reaction conditions: nitrobenzene 1.5 mmol, KOH 0.3 mmol, isopropyl alcohol 15 mL, catalyst 50 mg, N2 atmosphere, visible light irradiation (λ > 400 nm), reaction time 30 min.

irradiation (λ > 400 nm). We could observe that AOB is transiently generated and gradually transferred into AB and NB is almost completely transformed into AB after a reaction time of 10 min. As the reaction proceeds, the accumulated AB begin to be reduced to HAB with prolonging time and HAB became the final product with the solution turning from orange-reddish to colorless. Most remarkably, even when prolonging the reaction time to 90 min, no further reduced products such as aniline were detected. This result is extremely welcome, not only because the reaction can harvest AB and HAB without any other over-reduced products, but also because the yield of desired products with high selectivity can be simply controlled by monitoring the reaction time. It is worth noting that the reduction of NB in the absence of light irradiation gave the similar results with inferior efficiency. For the insight into the role of light irradiation, the catalytic performance under different reaction conditions was investigated. The conversion and selectivity over 20% Au/TiO2 with or without visible light at 40 °C are studied (see Fig. S3 in SI). Obviously, the reaction rate is slower without visible light irradiation. We further investigated NB reduction reaction over 20% Au/TiO2 at 25 °C. As depicted in Fig. S4a (SI), under visible light irradiation, it shows a 96% conversion of NB in prolonged reaction time as long as 10 h, however, with a moderate selectivity for the generation of azoxy- and azo-compounds and negligible yield of HAB is obtained. Moreover, the NB conversion reached

1% 5% 10% 15% 20% 30% TiO2

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Fig. 2. (a) Au 4f core-level spectrum of 20% Au/TiO2. (b) DRS of x % Au/TiO2 samples.

concentration was investigated. Fig. 4a shows conversion and selectivity of the evolution in nitrobenzene reduction reaction over various Au loading catalysts at 90 °C. It should be noted that the selectivity of final product HAB exhibits volcano shape, which reaches nearly 100% with complete conversion over 20% Au/TiO2. Thus, it could be concluded that 20% Au/TiO2 is the optimum one. In order to understand the distribution of products versus reaction time, the products at different reaction time were analyzed. Fig. 5a shows the NB conversion and products (AB, HAB, AOB) selectivity at different reaction time over 20% Au/TiO2 with xenon lamp light

a b 20%Au/TiO2

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Fig. 3. TEM image (a) and Au particles size distribution (b) of 20% Au/TiO2. 3

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noteworthy increase with the TOF up to 344 h−1. Additionally, the difference between light and dark reactions varies with temperature in the range of 25 to 90 °C. With the introduction of visible light, the TOF values of increases from 344 h−1 to 404 h−1 at 90 °C, the light-enhanced efficiency is determined to be 14.8%. More obvious light-enhancement in reaction activity at higher temperature is attributed to reduced energy threshold for the activation of the NeO bonds. Thus absorbed light energy is able to yield more excited energetic electrons with energy above the threshold and is sufficient to overcome the activation barrier of reaction [30]. It should be underlined that this value is nearly one or two order of magnitude greater than those in previously reported heterogeneous protocols, including Ru, Pt, Ni, Cu and other Au supported catalyst systems, as summarized in Table S1(SI). This photoassisted contribution displayed obviously photo and thermal synergistic effect of Au/TiO2 composites on NB reduction [31,32]. Moreover, the long-term stability of Au/TiO2 composites was investigated by cycle experiment. The TOF holds stable after 5 runs (Fig. S5), showing the superior stability of the catalyst Au/TiO2 during the catalytic reaction. The similar XRD patterns of the fresh and used 20% Au/TiO2 indicate that there is no obvious structural change after the catalytic reaction (Fig. S6). It is worth noting that the effective separation of Au/TiO2 and products can be easily achieved by standing the reaction system, which is helpful for the reuse of catalyst Au/TiO2. The direct and condensation routes are generally accepted for the catalytic reduction of aromatic nitro compounds [33]. It’s usually envisioned that only in the condensation route, a chemical coupling step (nitrosobenzene and hydroxylamine) occurs readily in basic or neutral media, resulting in the yield of azoxy compounds followed by further reduction to azo and/or hydrazo compounds. Electron spin resonance spectroscopy is often regarded as a powerful tool to identify paramagnetic intermediates in reduction process. In this work, ESR with DMPO spin-trap technique was carried out. As depicted in Fig. 7, the reaction system in absence of NB produced nine characteristic peaks identified as a DMPO–H adduct by comparison with the literature (Fig. 7a, aN = 14.98, aH(1) = aH(2) = 19.83 G) [34]. The 3-lines signal (aN = 14 G) is suggested to originate from a product of DMPO cleavage (Fig. 7c) [35]. The failure of ∙H radical formation suggests the critical role of KOH in facilitating the hydrogen transfer in this reaction. Hydrogen atoms should be abstracted from isopropyl alcohol in the first stage and bound to the surface of Au nanoparticle to form stable AueH species. With the addition of NB, these AueH species capture oxygen atoms from NeO (or N]O) bonds to yield AOB, AB, AueOH species which are further consumed by isopropanol. Under visible light irradiation, plasmonically generated hot electrons with sufficient energy can be transferred from the Au NPs to weaken the NeO bonds, thus facilitating this process [36]. The six characteristic signals can be

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Fig. 5. Conversion and selectivity to time plot for 20% Au/TiO2 with (a) or without (b) visible light irradiation (λ > 400 nm). Reaction conditions: nitrobenzene 1.5 mmol, KOH 0.3 mmol, isopropyl alcohol 15 mL, catalyst 50 mg, N2 atmosphere, temperature 90 °C. Table 1 The catalytic results of the reduction of nitrobenzene at different conditions over Au/TiO2 NPs. Temp (°C)

90 90 90 90 40 40 40 40 25 25

30

Fig. 6. The TOF of 20% Au/TiO2 catalyst at different reaction conditions. Reaction conditions: nitrobenzene 1.5 mmol and KOH 0.3 mmol were dissolved in isopropyl alcohol 15 mL, catalyst 50 mg, N2 atmosphere.

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AB

HAB

0.26 0.95 – – 13.8 20.1 – – 32.1 49.9

98.76 98.04 – 2.04 85.5 78.8 84.3 87.4 66.75 47.9

0.98 1 99.95 97.96 0.7 1.1 15.7 12.6 1.1 2.2

only 73% at 10 h without visible light irradiation (Fig. S4b). The catalytic results of the reduction of nitrobenzene were summarized in Table 1. Such repressed reductive process in in dark condition at different temperatures indicates that visible light irradiation indeed contributes to the reduction of NB, which could be attributed to the LSPR effect of Au nanoparticles [13,27,28]. Because LSPR effect promotes the formation of excited energetic electrons with higher energy levels [29], which assists the activation of the nitrobenzene molecules on the Au NPs, thus leading to enhanced catalyst performance. Fig. 6 displays the turnover frequency (TOF) of 20% Au/TiO2 at different reaction temperatures with or without visible light. In the lower temperature range, the TOF value increases quite slowly, which are 4.86 h−1 for TOF at 25 °C, and 13.11 h−1 at 40 °C, respectively. While temperature reaches 90 °C, the NB conversion rate exhibits a 4

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important intermediate that could account for the high efficiency in this reduction process. Our findings in this study reveal an effective pathway to optimize catalytic activity of supported Au NPs. This work could harvest multi-product with high selectivity, making the reaction attractive for industrial applications and providing a reference to coming studies on the photo-thermal catalytic process.

Intensity (arb. unit)

c: Au/TiO2+isopropanol+NB+DMPO

b: Au/TiO2+isopropanol+KOH+NB+DMPO

Declaration of Competing Interest a: Au/TiO2+isopropanol+KOH+DMPO

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements

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The authors are grateful for the financial support of National Natural Science Foundation of China (Nos. U1662112, 21273038, 21543002 and 11305091) and the Open Project Program of the State Key Laboratory of Photocatalysis on Energy and Environment (Grant Nos. SKLPEE-KF201703, SKLPEE-KF201807), Fuzhou University.

Magnetic field (G) Fig. 7. ESR spectra under different conditions.

Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.144214. References [1] Z. Yue, T. Ikeda, Smart Light-Responsive Materials: Azobenzene-Containing Polymers and Liquid Crystals, Wiley, 2009. [2] T. Ikeda, O. Tsutsumi, Optical switching and image storage by means of azobenzene liquid-crystal films, Science 268 (1995) 1873–1875. [3] L. Hu, X. Cao, L. Shi, F. Qi, Z. Guo, J. Lu, H. Gu, A highly active nano-palladium catalyst for the preparation of aromatic azos under mild conditions, Org. Lett. 13 (2011) 5640–5643. [4] E. Merino, Synthesis of azobenzenes: the coloured pieces of molecular materials, Chem. Soc. Rev. 40 (2011) 3835–3853. [5] A. Grirrane, A. Corma, H. Garcia, Gold-catalyzed synthesis of aromatic azo compounds from anilines and nitroaromatics, Science 322 (2008) 1661–1664. [6] L. He, L.C. Wang, H. Sun, J. Ni, Y. Cao, H.Y. He, K.N. Fan, Efficient and selective room-temperature gold-catalyzed reduction of nitro compounds with CO and H2O as the hydrogen source, Angew. Chem. Int. Ed. 48 (2009) 9538–9541. [7] Y. Gao, D. Ma, C. Wang, J. Guan, X. Bao, Reduced graphene oxide as a catalyst for hydrogenation of nitrobenzene at room temperature, Chem. Commun. 47 (2011) 2432–2434. [8] F. Hamon, F. Djedaini-Pilard, F. Barbot, C. Len, Azobenzenes—synthesis and carbohydrate applications, Tetrahedron 65 (2009) 10105–10123. [9] E. Vasilikogiannaki, C. Gryparis, V. Kotzabasaki, I.N. Lykakis, M. Stratakis, Facile reduction of nitroarenes into anilines and nitroalkanes into hydroxylamines via the rapid activation of ammonia borane complex by supported gold nanoparticles, Adv. Synth. Catal. 355 (2013) 907–911. [10] H. Yang, X. Cui, X. Dai, Y. Deng, F. Shi, Carbon-catalysed reductive hydrogen atom transfer reactions, Nat. Commun. 6 (2015) 6478. [11] J.H. Kim, J.H. Park, Y.K. Chung, K.H. Park, Ruthenium nanoparticle-catalyzed, controlled and chemoselective hydrogenation of nitroarenes using ethanol as a hydrogen source, Adv. Synth. Catal. 354 (2012) 2412–2418. [12] R.P. Wei, F. Shi, Controllable synthesis of azoxybenzenes and anilines with alcohol as the reducing agent promoted by KOH, Synth. Commun. 49 (2019) 1–9. [13] H. Zhu, X. Ke, X. Yang, S. Sarina, H. Liu, Reduction of nitroaromatic compounds on supported gold nanoparticles by visible and ultraviolet light, Angew. Chem. Int. Ed. 49 (2010) 9657–9661. [14] X. Ke, X. Zhang, J. Zhao, S. Sarina, J. Barry, H. Zhu, Selective reductions using visible light photocatalysts of supported gold nanoparticles, Green Chem. 15 (2013) 236–244. [15] J. Li, S. Song, Y. Long, L. Wu, X. Wang, Y. Xing, R. Jin, X. Liu, H. Zhang, Investigating the hybrid-structure-effect of CeO2-encapsulated Au nanostructures on the transfer coupling of nitrobenzene, Adv. Mater. 30 (2018) 1704416–1704422. [16] X. Liu, S. Ye, H.-Q. Li, Y.-M. Liu, Y. Cao, K.-N. Fan, Mild, selective and switchable transfer reduction of nitroarenes catalyzed by supported gold nanoparticles, Catal. Sci. Technol. 3 (2013) 3200–3206. [17] S. Li, F. Wang, Y. Liu, Y. Cao, Highly chemoselective reduction of nitroarenes using a titania-supported platinum-nanoparticle catalyst under a CO atmosphere, Chin. J. Chem. 35 (2017) 591–595. [18] S. Yu, T. Zhang, Y. Xie, Q. Wang, X. Gao, R. Zhang, Y. Zhang, H. Su, Synthesis and characterization of iron-based catalyst on mesoporous titania for photo-thermal FT synthesis, Int. J. Hydrogen Energy 40 (2015) 870–877. [19] A.A. Upadhye, I. Ro, X. Zeng, H.J. Kim, I. Tejedor, M.A. Anderson, J.A. Dumesic,

Scheme 1. A plausible reaction pathway for the reduction of nitrobenzene with Au/TiO2 NPs.

assigned to the DMPO-OOC(OH)(CH3)2 adducts (Fig. 7b) [34], implying that AueOH species could be consumed by isopropanol to give birth to the carboperoxyl species in the reaction system. Following that, AB is subsequently reduced to be HAB. Since no formation of AN (aniline) was detected even prolonging the reaction time, we have ruled out the pathway involving further reduction of intermediates to aniline, thus resulting in a high selectively of the nitroaromatic conversion into azo- and hydrazo- compunds. The efficient reduction of NB to AB could be the alkaline environment due to suitable amount of KOH introduced into the reaction system [16,37]. While HAB has a saturated NeON bond and might not interact with the Au–H species, thus the further reduction does not occur [38]. It seems condensation route must be involved in the hydrogenation of nitrobenzene. The possible reaction pathway here follows the condensation route depicted in Scheme 1.

4. Conclusions In summary, Au/TiO2 composite can efficiently catalyze the chemoselective hydrogenation of nitroaromatic under mild conditions. More importantly, we achieved the selective reduction of nitrobenzene to both hydrazobenzene and azobenzene with high selectivity by simply controlling the reaction time. Visible light irradiation can contribute to the reduction of NB. Additionally, Au–H species was found to be an 5

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