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Simple synthesis of WO3-Au composite and their improved photothermal synergistic catalytic performance for cyclohexane oxidation
Molecular Catalysis 473 (2019) 110389
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Simple synthesis of WO3-Au composite and their improved photothermal synergistic catalytic performance for cyclohexane oxidation ⁎
Jijin Mai, Yanxiong Fang, Jincheng Liu , Jinhong Zhang, Xiaolan Cai, Yuying Zheng
T
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School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Tungsten oxide Nanosheets Au nanoparticles Photothermal catalysis Cyclohexane oxidation
Partial oxidation of cyclohexane (CHA) to KA oil (cyclohexanone (-one) and cyclohexanol (-ol)) with dry air is one of the most important industrial process. Despite great efforts have made in developing new catalysts, industrial processes are still limited by low conversion and selectivity with harsh condition. Here we report a significantly improved cyclohexane conversion and KA oil selectivity under solar light irradiation by anew photothermal synergistic catalyst, which was prepared by loading Au nanoparticles (Au NPs) onto WO3 nanosheets (WO3 NSs) with simple ultrasonic method. A high conversion of 9.0% and a high selectivity of 99.0% are acquired by WO3 nanosheets-Au nanoparticles (WO3-Au) composite under dry air, which are much higher than the simple sum of photocatalysis and thermal catalysis. The significantly improved catalytic properties of cyclohexane oxidation by the WO3-Au composite are mainly attributed to the improved light absorption, the effective charge separation and the synergistic effect of photocatalysis and thermal catalysis. The photothermal synergy to improve catalytic activity may be widely used in green industrial catalytic processes.
1. Introduction Selective oxidation of single CeH bond to form CeOH and C]O is very difficult, thus limiting the use of CeH compounds [1–3]. The selective oxidation of cyclohexane shows extreme importance in the industrial production of KA oil (the mixture of cyclohexanol and cyclohexanone), which is main raw materials for the production of for the production of nylon-6 and nylon-66 [4–6]. The annual global market size of KA oil prepared by this method reaches 106 tons. However, the industrialized cyclohexane oxidation must be carried out at high temperature (˜425 K) under high pressure (˜2 MPa), and the cyclohexane conversion (< 5%) and KA oil selectivity (< 85%) is low [7,8]. Therefore, it is necessary to develop effective methods and catalysts to increase catalytic efficiency under mild conditions at low cost. The high conversion and high selectivity can be obtained by a photocatalytic cyclohexane oxidation process under mild reaction conditions, but the low photocatalytic activity and the slow reaction rate limit its development and further applications [9,10]. Meanwhile, the thermal catalytic oxidation of cyclohexane is an effective method widely used in the industry and has been extensively studied. Unfortunately, the thermal catalytic reaction is limited by the low selectivity (< 90%) of KA oil and the harsh reaction conditions (150 °C, 1 MPa) [11,12]. In order to combine the advantages of photocatalysis
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and thermal catalysis, the concept of photothermal synergy catalysis is proposed. Cong Wu and colleagues report that Ag @GQD (graphene quantum dot) hybridization promotes the catalytic efficiency of 4-nitrophenol reduction by SPR-mediated photothermal local heating [13]. Connor Kang NuoPeh and colleagues found that TiO2 nanotube-CuO composites have excellent photothermal catalytic performance under UV LED lamps at 90 °C [14]. The hydrogen yield was 5–7 times higher at room temperature than most other TiO2-basedphotocatalysts, and the apparent quantum yield was also calculated to be 66.9%. The application of photothermal catalytic process to cyclohexane oxidation may also improve its catalytic performance. WO3 is a type of widely applied photocatalyst, which can utilize the visible light with a wavelength less than 450 nm [1,15–18]. However, the high electron-hole recombination rate of pure WO3 determines the photocatalytic efficiency, which limits its use in cyclohexane oxidation. The photocatalytic activity can be improved by loading the precious metals such as Au (or Ag), which is caused by the extended visible light absorption range and the accelerated electron transfer kinetics for a improved charge separation effect [19].In addition, Au can be used as an electron trap to accelerate the motion of photogenerated electrons, delay carrier lifetime, suppress photoelectron-hole pair recombination, contribute to photoelectron hole separation, and improve the catalytic efficiency [20,21]. Moreover, gold nanoparticles are good thermal
Corresponding authors. E-mail addresses: [email protected] (J. Liu), [email protected] (Y. Zheng).
https://doi.org/10.1016/j.mcat.2019.04.018 Received 4 January 2019; Received in revised form 19 April 2019; Accepted 23 April 2019 Available online 14 May 2019 2468-8231/ © 2019 Elsevier B.V. All rights reserved.
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WO3 and WO3-Au composite was taken by a JEM-2100 electron microscopy. Surface characterization of the samples was performed by Xray photoelectron spectroscopy (XPS) using a Crates Axis Ultra Spectrometer. Electrochemical measurements were performed on a Multi Auto lab M204 with a standard three electrode. Fourier transform infrared spectroscopy (FTIR) was examined at room temperature using an infrared spectrometer (Broker). The UV–vis (UV–vis) spectra were performed by Shimadzu UV2450 spectrophotometer. The photoluminescence (PL) tests were excited at 325 and 425 nm at room temperature using a luminescence spectrometer of Horiba Join Yvon Fluoro Max 4.
catalysts for the cyclohexane oxidation. Wu Pingping and his colleagues reported a high TOF value of cyclohexane oxidation (69,091 h−1) catalyzed by gold nanoparticles with the support of mesoporous silica [22].Mario J.et al. used the combination of light and heat energy sources to degrade 2-propanol with high activity [23].The combination of Au nanoparticles and single layer WO3 NSs is expected to further enhance the photothermal catalytic activity for the cyclohexane oxidation. In our work, we prepared WO3-Au composite catalyst by loading Au NPs onto WO3 NSs using a simple ultrasonic method at room temperature. Owing to the SPR effect of Au NPs and photothermal synergistic catalysis, excellent cyclohexane oxidation catalytic performance under dry air is obtained. The photocatalytic activity and stability of WO3-Au composite catalyst and pure WO3 NSs for cyclohexane oxidation were studied and compared. The effects of Au NPs on the catalytic cyclohexane oxidation and the photothermal catalytic interaction mechanism were studied. The photocatalytic mechanism of WO3-Au composite system was discussed in detail. The strategy of this study provides a good path for highly-efficient cyclohexane catalytic oxidation.
2.5. Photocatalytic activity evaluation
2. Experimental
The photothermal catalytic reaction was carried out in a 100 mL Teflon photothermal reactor with the 1.5 MPa dry airs. After adding the dispersed catalyst of 20 mg in 5 ml of acetone and 10 mL of CHA, the reactor was reacted at 120 °C with continuous magnetic stirring under simulated sunlight of 220 W Xe lamps (Ceauligth 300).The radical trapping experiments were performed by adding appropriate amount of TBA or EDTA into the reactor. After the reaction, the product was analyzed by gas chromatography (GC, Agilent-7890 A).
2.1. Material
3. Results and discussion
Sodium hydroxide (NaOH, 98.0%), sodium borohydride (NaBH4, 95.0%), tungstic (VI) acid (H2WO4, 99.0%) and nitric acid (HNO3, 63.0%) are obtained from Guangzhou Chemical Reagent Factory. Tetrachloroauric (III) acid (HAuCl4, > 47.5%), 3-Mercaptopropionic acid (MPA, 98.0%), acetic acid glacial (C2H4O2, absolute), oleylamine (C18H37N, 99.0%), acetone (C3H6O), tert-butanol (TBA, 99.0%), ethylenediaminetetraacetic acid (EDTA, 98.0%) and 4-benzoquinone (BQ, 99.5%) were ordered from Aladdin Company and used without further pretreatment. Commercial WO3 nanoparticles (TUNGSTEN (VI) OXIDE, NANOPOWDER, < 100 NM&) were purchased from Sigma-aldrich Company used without further treatment.
3.1. Structural and morphological characteristics The WO3 nanosheets were synthesized by an oleyamine-assisted thermal exfoliation process and the organic ligand of oleylamine was removed by nitric acid. The WO3-Au composite were prepared by an ultrasonic mixing of WO3 NSs and Au NCs. Fig. 1 shows the TEM and HRTEM images of pure WO3 NSs and WO3-Au composite catalysts. From the TEM observation in Fig. 1a, the morphology of WO3 presents a 2D nanosheet structure with a size of about 200 nm. The almost transparent feature with several wrinkled structures indicates ultrathin thickness and flexibility of WO3 NSs. The TEM image in Fig. 1b is clearly showing that Au NPs are loaded onto the large surface of WO3 NSs, and the morphology of WO3 doesn’t have obvious change after compounding with Au nanoparticles. From Fig. 1c, the size of a single Au NCs is approximately 3–5 nm, which is bigger than the original Au clusters. As observed in Fig. 1d, the clear lattice stripes of WO3 NSs and gold nanoparticles in the HRTEM image indicates the good crystallinity of WO3 NSs and Au nanoparticles. The TEM and HRTEM observation confirms the formation of WO3-Au composite. The XRD patterns of pure WO3 NSs and WO3-Au composite catalysts are shown in Fig. 2a to investigate their structure. As shown, these diffraction peaks correspond well to a single layer of WO3 (JDPCS No. 18-1420) with a lattice parameter of 0.6960 nm. The strong and sharp peaks indicate that the WO3 NSs have perfect crystals with the predominantly oriented face. Because the highest loading amount of Au nanoparticles is only 2%, no diffraction peak of Au was found in the XRD patterns of WO3-Au composite [25]. Furthermore, the four similar XRD patterns in Fig. 2a indicates that the crystal size and structure of WO3 NSs are stable without clear change during the combination process with Au nanoparticles, which is consistent with our TEM observations. Fig. 2b shows the FT-IR spectra of pure WO3 NSs and WO3-Au composite catalysts with gold loadings of 0.5, 1.0, and 2.0 wt %.The peaks at 3420 and 1620 cm−1 are corresponded to the stretching vibration of OeH bonds. The peaks at 678 and 801 cm-1 are caused by the stretching vibration of OeWeO groups and the oscillation of OeW groups. The peak at 945 cm-1 can be assigned to the oscillation of O]W double bond in the WO3 NSs. Because the loading amount of Au is small, there are no any peaks from Au and the Au-SH coordination bonds. XPS was performed before and after the Au loading to obtain the
2.2. Preparation of Au nanoparticles Au NPs protected by 3-mercaptopropionic acid (MPA) were synthesized by previously reported methods requiring space [24]. All of these solutions were prepared with distilled water and treated under ice cooling. First, 0.25 mL of HAuCl4 solution (0.02 M) was required to add a space and 2 mL of MPA solution (0.005 M) to 2.35 mL with stirring to form an MPA-Au (I) complex. Next, 0.3 mL of NaOH (1 M) was introduced into the solution. After the quick injection of 0.1 mL NaBH4 solution ((0.1 mol L−1), the solution was kept stirring at 1000 rpm for 3 h. Then the MPA-Au NPs were obtained for the further application. 2.3. Preparation of WO3-Au composite catalyst The Monolayer WO3 NSs were synthesized by an oleyamine-intercalated exfoliation process, as reported by our earlier work [1]. To prepare the WO3 NSs-Au composite, 100 mg of WO3 NSs was dispersed into 20 mL of acetone by sonication for 30 min. An appropriate amount of the prepared Au nanoparticles was then added at room temperature under sonication for 10 min. The WO3-Au composite catalysts with the gold loading amount of 0.5 wt %, 1.0 wt %, and 2.0 wt % were obtained. The as-prepared catalysts were purified with acetone twice and further dispersed in acetone for the catalytic application. 2.4. Characterizations The phase of the sample was performed by X-ray diffraction (XRD) (Phillips X-pert) using Cook irradiation (20 kV, 20 a.m.) from 20 to 80° (2θ) at a scanning speed of 2°/min. The TEM and HRTEM observation of 2
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Fig. 1. (a) HRTEM image of pure WO3 NSs catalyst, (b) TEM and (c, d) HRTEM image of WO3-Au composite catalyst.
and 88.11 eV are corresponded to the zero-valence metallic gold (Au°), and the two small peaks with binding energies of 85.08 and 88.68 eV are assigned to the of Au+ [26]. Fig. 4a shows the UV–vis absorption spectra of pure WO3 NSs and WO3-Au composite catalysts. As seen from Fig. 4a, all the samples show the absorption band onset of around 510 nm, which is related to the main band-gap absorption of WO3 NSs. For WO3-Au composite catalysts, a weak adsorption from 500 to 600 nm can be observed, which is caused by the SPR effect from the Au nanoparticles. The Tauc plot (Fig. 4b) of the absorption data reveal that the band gap energy of WO3 NSs is 2.30 eV, which is lower than that of 2.80 eV for WO3 as reported previously [6]. Furthermore, it can be observed that the estimated band gap energies of WO3-Au composite with the Au loading content of 0.5, 1.0 and 2.0 wt % has no clear changes compared to that of pure WO3
chemical valence of Au and WO3. The full-scale XPS spectra for all the samples are presented in Fig. 3a. The clear Au 4f and W 4f peaks confirm the presence of Au and W in the WO3-Au composite. The W 4f 7/2 and W 4f 5/2 peaks at 35.80 and 37.98 eV are belong to the W6+ oxidation state. Compared with pure WO3 NSs, it shows a 0.4 eV positive shift after Au loading, which is caused by the reduced electron cloud density. This indicates that there is strong chemical interaction between WO3 NSs and Au. In Fig. 3c, the O 1 s peaks from the O 1 s orbital of the WeO bonds for the pure WO3 and WO3-Au samples locate at the binding energy of 530.81 and 542.22 eV A transition of 0.3 eV to higher energy for the WO3-Au composite indicates that more electrons were removed from O in the WO3-Au composite. The high resolution XPS spectra of Au 4f orbital are presented at Fig. 3d. From the peak fittings of Au 4f7/2 and Au 4f 5/2 orbital, the two main peaks at 84.48
Fig. 2. (a) XRD patterns and (b) FT-IR spectra of pure WO3 and WO3-Au composite. 3
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Fig. 3. XPS patterns of pure WO3 and 2.0 wt % WO3-Au composite.
Fig. 4. (a) UV–vis absorption spectra, (b) Tauc plots and (c) PL intensities of pure WO3 and WO3-Au composite with 0.5, 1.0 and 2.0 wt % gold loading.
content increases, the PL intensity decreases gradually, indicating that the effective charge transfer in the WO3-Au composite and the prevented photoelectron-hole recombination. Besides, the PL results under the excitation wavelength of 425 nm show the same trend (Fig. 4d).
NSs. It can be concluded that the band gap energy can be narrowed by the single layer structure of WO3 NSs, and the introduction of Au nanoparticles will further extend the utilization of light spectra. It is well known that the quenching of semiconductor fluorescence is related to the photogenerated electron-hole recombination rate. To further understand the interaction between Au NPs and WO3 NSs, the PL spectra were examined with the excitation wavelength of 325 and 425 nm. Fig. 4c show the PL spectra of pure WO3 NSs and WO3-Au composite catalysts excited at the wavelength of 325 nm. As the gold
3.2. Photothermal catalytic oxidation of cyclohexane using WO3-Au composite catalysts The as-prepared WO3 NSs and WO3-Au composite are used for the 4
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Fig. 5. (a) Time series data for conversion (solid) and selectivity (dash) efficiency of cyclohexane oxidation under photocatalysis (PC), thermal catalysis (TC) or photo[HYPHEN] thermal catalysis (PTC) by 2.0 wt % WO3-Au composite ; (b) Time series data for conversion and selectivity of cyclohexane oxidation by pure WO3 (□) and WO3-Au composite with 0.5 (○), 1.0 (△) and 2.0 wt % (◇) gold loading; (c) Temperature series data for conversion (solid) and selectivity (dash and dot) efficiency of cyclohexane oxidation on by 2.0 wt % WO3-Au composite; (d) The conversion and selectivity of cyclohexane oxidation by pure WO3 (dot), 2.0 wt % WO3-Au composite (solid) and commercial WO3 (dash) over 5 cycling times (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
increased to 9.0%, which is higher than the simple sum of photocatalysis and thermal catalysis. This trend is illustrated in both Fig. 5a and Table 1 under the catalysis of pure WO3 NSs and WO3-Au composite catalysts with gold loadings of 0.5, 1.0 and 2.0 wt %.With the increase of Au loading amount, both the cyclohexane conversion and the KA-oil electivity increase quickly when the loading amount is less than 1.0 wt %. After the loading amount of Au is larger than 1.0 wt %, the photothermal catalytic reaction rate of WO3-Au composite is increased slightly. The apparent reaction rates of pure WO3 and WO3-Au composite with 0.5, 1.0 and 2.0 wt % gold loading are 0.082, 1.08, 1.16, and 1.14%•h−1. The reaction temperature also has important effects on the photothermal catalytic of cyclohexane oxidation. The catalytic results by 2.0 wt % WO3-Au composite at different temperatures are shown in Fig. 6c. When increasing the reaction temperature from 80 ℃to 140 ℃, the photothermal catalytic conversion of cyclohexane increases first and then decreases. The highest cyclohexane conversion of 9.0%and the highest cyclohexanone selectivity of 52.7% was gained at 120 ℃. When the reaction temperature was raised to 140 ℃, the cyclohexane conversion, the selectivity of cyclohexanone and total KA oil selectivity are decreased. However, the cyclohexanol selectivity slightly increases, which may be due to the decomposition of cyclohexanone at a high temperature. The long-term stability of the catalysts has an important indicator of practical application. It is noteworthy that after five
Table 1 Solar light-driven cyclohexane oxidation with died air by WO3-Au catalysts with different Au loading amount.a Entry
1 2 3 4 5 6 7 8 9 10 11 12
Catalystb
Pure 0.5 wt 1.0 wt 2.0 wt Pure 0.5 wt 1.0 wt 2.0 wt Pure 0.5 wt 1.0 wt 2.0 wt
% % % % % % % % %
Typec
PC PC PC PC TC TC TC TC PTC PTC PTC PTC
Conv. (%)
0.2 0.3 0.4 0.5 4.6 7.2 7.7 8.0 6.3 8.4 8.9 9.0
Sele. of KA (%)d
51.6 60.5 63.9 71.4 97.9 98.7 99.0 98.9 98.5 98.9 99.0 99.0
Error Conv. (%)
Sele. of KA (%)
0.02 0.05 0.03 0.05 0.15 0.09 0.14 0.06 0.14 0.12 0.07 0.09
3.10 1.60 2.00 1.20 0.50 0.70 0.70 0.40 0.70 0.80 0.50 0.70
a
10 mL of Cyclohexane, 20 mg of catalyst, 5 mL of aceton, 1.5 Mpa of dried air, temperature of 120 ℃, time of 8 hs, 220 W Xe lamps. b The Catalysts involve pure WO3 and WO3-Au composite with 0.5, 1.0 and 2.0 wt % gold loading. c Different type of catalysis, PC = photocatalysis, TC = thermal catalysis, PTC = photo-thermal catalysis. d Product selectivity = the content of KA oil /content (mmol) of all product) × 100%.
photocatalytic, thermal catalytic and photothermal catalytic oxidation process of cyclohexane. The detailed catalytic properties of cyclohexane oxidation under certain conditions are shown in Fig. 5. As shown in Fig. 5a, the conversions of cyclohexane oxidation catalyzed by the 2.0 wt % WO3-Au composite are only 0.49 and 7.61% when just reacted under simulated sunlight (220 W Xe lamps) without external heating and at 120 °C in the dark. However, the highest conversion of 9.0%and the highest selectivity of 99.0% can be acquired after 8 hs reaction of a photothermal synergetic catalysis process (Table 1). The results show that both photocatalysis and thermal catalysis can effectively catalyze the oxidation of cyclohexane, but the conversion rate of thermal catalytic conversion is much higher than that of photocatalysis. The conversion under photothermal synergistic catalysis can be further
Fig. 6. Photothermal catalytic activities of 2.0 wt % WO3-Au composite in reactive species trapping experiments with three types of reactive species scavengers. 5
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Fig. 7. (a) Photocurrent response patterns and (b) electrochemical impedance spectroscopy of pure WO3 and WO3-Au composite with 0.5 1.0 and 2.0 wt % gold loading.
mainly due to the loss of SPR effect and high electron-hole recombination rate. The electrochemical research results further confirm that Au plays a key role in promoting charge transfer and thus producing photocurrent. Based on the above analysis and in combination with the thermal catalytic process, we proposed the reaction mechanism of cyclohexane oxidation. As shown in Scheme 1, in the presence of heating, a partially positively charged gold atom (Au+) acquires an electron from cyclohexane to form Au° to generate the radical of Cy%. At the same time, photogenerated electrons on WO3 NSs can be excited to the CB under visible light irradiation, and then inject into the gold core (Au°) to make the gold core a reduction center (Au (e−)). Multi-electron reduction between electrons and O2 takes place to produce H2O and a small amount of hydrogen peroxide (H2O2), and Au (e−) offers from multielectron oxidation into Au+, which facilitates the formation of the radical of Cy%. On the other hand, holes having strong oxidizing ability can directly oxidize cyclohexane (CyH) to obtain Cy%, which can easily react with O2 to promote the photocatalytic oxidation reaction.
catalytic cycles, the catalyst remained effective without evident decreases in the conversion and selectivity, indicating that our synthesized WO3 and WO3-Au composite have the same excellent long-term stability with that of the commercial WO3 nanoparticles (Fig. 5d).Compared to the commercial WO3 (Conv. 5.9%), catalysts in our system show more outstanding performance obviously. 3.3. Possible catalytic mechanism Hydroxyl radicals and superoxide radicals are the main active centers in the photocatalytic process. In order to investigate the effective catalytic active centers, several control photothermal catalytic experiments were performed with the addition of t-butanol (TBA), benzquinamide (BQ) and ethylenediaminetetraacetic acid (EDTA), which are scavengers for hydroxyl radicals (%OH), O2%− radical and hole [18]. As presented in Fig. 7, with the addition of TBA and BQ, the conversion and selectivity only have no obvious change, indicating O2%−and %OH radical has nearly no effect in this reaction. However, after the hole scavenger of EDTA was added into the reaction solution, the conversion decreased greatly from 9.0–6.18 %, indicating that the hole may play a vital role in the photocatalytic cyclohexane oxidation as the main active species. The conduction band position of WO3 ( + 0.5 eV vs. NHE) is too low to reduce O2 to superoxide radical (O2▪)(-0.13 eV), which is in good agreement with our scavenger experimental result [6]. Instead, the conduction band electrons of WO3 can produce hydrated hydrogen peroxide by a multi-electron reduction procedure, which may be helpful for the cyclohexane oxidation. The valence band potential of WO3 ( + 3.20 eV vs. NHE) is very positive with strong oxidation capability to oxidize H2O to generate %OH radical. However, there is no water in our cyclohexane oxidation process, which interpret the reason why the %OH radical is not main active center in this cyclohexane oxidation process [27]. The hole generated in the VB of WO3 has significant influence in the catalytic activity of cyclohexane oxidation, which may be caused by the strong oxidizing capability to facilitate the cyclohexane oxidation reaction. The current densities in the photocurrent response curves of the catalysts in Fig. 6a confirms the improved current density with the increased loading amount of Au nanoparticles, which shows the reduced order for the 2.0 wt % WO3-Au composite, 1.0 wt % WO3-Au composite, 0.5 wt % WO3-Au composite and WO3 NSs. This suggests that the electron transfer to Au NPs is more quickly in 2.0 wt % WO3-Au composite catalysts. It indicates that the gold loading promotes the separation of photogenerated electron- holes in WO3 NSs and WO3-Au composite and contributes to an improved catalytic activity in the cyclohexane oxidation. Impedance measurements are given in Fig. 6b. The charge transfer resistance of WO3-Au composite is higher than that of pure WO3 NSs, which may be caused by the ligands in the surface of small Au nanoparticles. Although pure WO3 NSs have the lowest electrical resistance, the photothermal catalytic activity is still low, which is
4. Conclusions The high quality WO3-Au composite was prepared by the combination of WO3 NSs and Au nanoparticles for the improved photothermal catalytic cyclohexane oxidation with dry air under solar lightirradiation. The TEM and HRTEM image confirmed that Au NPs with the size of approximately 3–5 nm are attached on the surface of large monolayer MoO3 nanosheets. The morphology, size and crystalline of WO3 NSs shows no clear changes with the combination of Au NPs. The XPS test suggests the existence of W +6 and Au+. The catalysts exhibit significantly improved photothermal catalytic performance with a high conversion (9.0%) and high KA oil selectivity (99%) achieved on WO3Au composite catalysts with 2.0 wt % gold loaded. The performance of
Scheme 1. The proposed photothermal catalytic process of cyclohexane oxidation by the WO3-Au composite under solar-light irradiation. 6
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this catalyst is 1.5–2 times higher than the commercial one. The high selectivity for cyclohexane oxidation on WO3-Au composite is due to the Surface Plasmon Resonance (SPR) of Au NPs and the suppression of superoxide radical (O2%−) producing of WO3 NSs. The findings in this work may provide a new method to increase the catalytic oxidation activity of cyclohexane.
[12]
[13]
[14]
Acknowledgements [15]
Authors would like to acknowledge the National Natural Science Foundation of China (Grant No.21776049), the Natural Science Foundation of Guangdong Province (Grant No. 2018A030313174) and the Guangdong Science and Technology Plan Grant (No.2016A010103028) support for this work.
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Molecular Catalysis journal homepage: www.elsevier.com/locate/mcat
Simple synthesis of WO3-Au composite and their improved photothermal synergistic catalytic performance for cyclohexane oxidation ⁎
Jijin Mai, Yanxiong Fang, Jincheng Liu , Jinhong Zhang, Xiaolan Cai, Yuying Zheng
T
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School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Tungsten oxide Nanosheets Au nanoparticles Photothermal catalysis Cyclohexane oxidation
Partial oxidation of cyclohexane (CHA) to KA oil (cyclohexanone (-one) and cyclohexanol (-ol)) with dry air is one of the most important industrial process. Despite great efforts have made in developing new catalysts, industrial processes are still limited by low conversion and selectivity with harsh condition. Here we report a significantly improved cyclohexane conversion and KA oil selectivity under solar light irradiation by anew photothermal synergistic catalyst, which was prepared by loading Au nanoparticles (Au NPs) onto WO3 nanosheets (WO3 NSs) with simple ultrasonic method. A high conversion of 9.0% and a high selectivity of 99.0% are acquired by WO3 nanosheets-Au nanoparticles (WO3-Au) composite under dry air, which are much higher than the simple sum of photocatalysis and thermal catalysis. The significantly improved catalytic properties of cyclohexane oxidation by the WO3-Au composite are mainly attributed to the improved light absorption, the effective charge separation and the synergistic effect of photocatalysis and thermal catalysis. The photothermal synergy to improve catalytic activity may be widely used in green industrial catalytic processes.
1. Introduction Selective oxidation of single CeH bond to form CeOH and C]O is very difficult, thus limiting the use of CeH compounds [1–3]. The selective oxidation of cyclohexane shows extreme importance in the industrial production of KA oil (the mixture of cyclohexanol and cyclohexanone), which is main raw materials for the production of for the production of nylon-6 and nylon-66 [4–6]. The annual global market size of KA oil prepared by this method reaches 106 tons. However, the industrialized cyclohexane oxidation must be carried out at high temperature (˜425 K) under high pressure (˜2 MPa), and the cyclohexane conversion (< 5%) and KA oil selectivity (< 85%) is low [7,8]. Therefore, it is necessary to develop effective methods and catalysts to increase catalytic efficiency under mild conditions at low cost. The high conversion and high selectivity can be obtained by a photocatalytic cyclohexane oxidation process under mild reaction conditions, but the low photocatalytic activity and the slow reaction rate limit its development and further applications [9,10]. Meanwhile, the thermal catalytic oxidation of cyclohexane is an effective method widely used in the industry and has been extensively studied. Unfortunately, the thermal catalytic reaction is limited by the low selectivity (< 90%) of KA oil and the harsh reaction conditions (150 °C, 1 MPa) [11,12]. In order to combine the advantages of photocatalysis
⁎
and thermal catalysis, the concept of photothermal synergy catalysis is proposed. Cong Wu and colleagues report that Ag @GQD (graphene quantum dot) hybridization promotes the catalytic efficiency of 4-nitrophenol reduction by SPR-mediated photothermal local heating [13]. Connor Kang NuoPeh and colleagues found that TiO2 nanotube-CuO composites have excellent photothermal catalytic performance under UV LED lamps at 90 °C [14]. The hydrogen yield was 5–7 times higher at room temperature than most other TiO2-basedphotocatalysts, and the apparent quantum yield was also calculated to be 66.9%. The application of photothermal catalytic process to cyclohexane oxidation may also improve its catalytic performance. WO3 is a type of widely applied photocatalyst, which can utilize the visible light with a wavelength less than 450 nm [1,15–18]. However, the high electron-hole recombination rate of pure WO3 determines the photocatalytic efficiency, which limits its use in cyclohexane oxidation. The photocatalytic activity can be improved by loading the precious metals such as Au (or Ag), which is caused by the extended visible light absorption range and the accelerated electron transfer kinetics for a improved charge separation effect [19].In addition, Au can be used as an electron trap to accelerate the motion of photogenerated electrons, delay carrier lifetime, suppress photoelectron-hole pair recombination, contribute to photoelectron hole separation, and improve the catalytic efficiency [20,21]. Moreover, gold nanoparticles are good thermal
Corresponding authors. E-mail addresses: [email protected] (J. Liu), [email protected] (Y. Zheng).
https://doi.org/10.1016/j.mcat.2019.04.018 Received 4 January 2019; Received in revised form 19 April 2019; Accepted 23 April 2019 Available online 14 May 2019 2468-8231/ © 2019 Elsevier B.V. All rights reserved.
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WO3 and WO3-Au composite was taken by a JEM-2100 electron microscopy. Surface characterization of the samples was performed by Xray photoelectron spectroscopy (XPS) using a Crates Axis Ultra Spectrometer. Electrochemical measurements were performed on a Multi Auto lab M204 with a standard three electrode. Fourier transform infrared spectroscopy (FTIR) was examined at room temperature using an infrared spectrometer (Broker). The UV–vis (UV–vis) spectra were performed by Shimadzu UV2450 spectrophotometer. The photoluminescence (PL) tests were excited at 325 and 425 nm at room temperature using a luminescence spectrometer of Horiba Join Yvon Fluoro Max 4.
catalysts for the cyclohexane oxidation. Wu Pingping and his colleagues reported a high TOF value of cyclohexane oxidation (69,091 h−1) catalyzed by gold nanoparticles with the support of mesoporous silica [22].Mario J.et al. used the combination of light and heat energy sources to degrade 2-propanol with high activity [23].The combination of Au nanoparticles and single layer WO3 NSs is expected to further enhance the photothermal catalytic activity for the cyclohexane oxidation. In our work, we prepared WO3-Au composite catalyst by loading Au NPs onto WO3 NSs using a simple ultrasonic method at room temperature. Owing to the SPR effect of Au NPs and photothermal synergistic catalysis, excellent cyclohexane oxidation catalytic performance under dry air is obtained. The photocatalytic activity and stability of WO3-Au composite catalyst and pure WO3 NSs for cyclohexane oxidation were studied and compared. The effects of Au NPs on the catalytic cyclohexane oxidation and the photothermal catalytic interaction mechanism were studied. The photocatalytic mechanism of WO3-Au composite system was discussed in detail. The strategy of this study provides a good path for highly-efficient cyclohexane catalytic oxidation.
2.5. Photocatalytic activity evaluation
2. Experimental
The photothermal catalytic reaction was carried out in a 100 mL Teflon photothermal reactor with the 1.5 MPa dry airs. After adding the dispersed catalyst of 20 mg in 5 ml of acetone and 10 mL of CHA, the reactor was reacted at 120 °C with continuous magnetic stirring under simulated sunlight of 220 W Xe lamps (Ceauligth 300).The radical trapping experiments were performed by adding appropriate amount of TBA or EDTA into the reactor. After the reaction, the product was analyzed by gas chromatography (GC, Agilent-7890 A).
2.1. Material
3. Results and discussion
Sodium hydroxide (NaOH, 98.0%), sodium borohydride (NaBH4, 95.0%), tungstic (VI) acid (H2WO4, 99.0%) and nitric acid (HNO3, 63.0%) are obtained from Guangzhou Chemical Reagent Factory. Tetrachloroauric (III) acid (HAuCl4, > 47.5%), 3-Mercaptopropionic acid (MPA, 98.0%), acetic acid glacial (C2H4O2, absolute), oleylamine (C18H37N, 99.0%), acetone (C3H6O), tert-butanol (TBA, 99.0%), ethylenediaminetetraacetic acid (EDTA, 98.0%) and 4-benzoquinone (BQ, 99.5%) were ordered from Aladdin Company and used without further pretreatment. Commercial WO3 nanoparticles (TUNGSTEN (VI) OXIDE, NANOPOWDER, < 100 NM&) were purchased from Sigma-aldrich Company used without further treatment.
3.1. Structural and morphological characteristics The WO3 nanosheets were synthesized by an oleyamine-assisted thermal exfoliation process and the organic ligand of oleylamine was removed by nitric acid. The WO3-Au composite were prepared by an ultrasonic mixing of WO3 NSs and Au NCs. Fig. 1 shows the TEM and HRTEM images of pure WO3 NSs and WO3-Au composite catalysts. From the TEM observation in Fig. 1a, the morphology of WO3 presents a 2D nanosheet structure with a size of about 200 nm. The almost transparent feature with several wrinkled structures indicates ultrathin thickness and flexibility of WO3 NSs. The TEM image in Fig. 1b is clearly showing that Au NPs are loaded onto the large surface of WO3 NSs, and the morphology of WO3 doesn’t have obvious change after compounding with Au nanoparticles. From Fig. 1c, the size of a single Au NCs is approximately 3–5 nm, which is bigger than the original Au clusters. As observed in Fig. 1d, the clear lattice stripes of WO3 NSs and gold nanoparticles in the HRTEM image indicates the good crystallinity of WO3 NSs and Au nanoparticles. The TEM and HRTEM observation confirms the formation of WO3-Au composite. The XRD patterns of pure WO3 NSs and WO3-Au composite catalysts are shown in Fig. 2a to investigate their structure. As shown, these diffraction peaks correspond well to a single layer of WO3 (JDPCS No. 18-1420) with a lattice parameter of 0.6960 nm. The strong and sharp peaks indicate that the WO3 NSs have perfect crystals with the predominantly oriented face. Because the highest loading amount of Au nanoparticles is only 2%, no diffraction peak of Au was found in the XRD patterns of WO3-Au composite [25]. Furthermore, the four similar XRD patterns in Fig. 2a indicates that the crystal size and structure of WO3 NSs are stable without clear change during the combination process with Au nanoparticles, which is consistent with our TEM observations. Fig. 2b shows the FT-IR spectra of pure WO3 NSs and WO3-Au composite catalysts with gold loadings of 0.5, 1.0, and 2.0 wt %.The peaks at 3420 and 1620 cm−1 are corresponded to the stretching vibration of OeH bonds. The peaks at 678 and 801 cm-1 are caused by the stretching vibration of OeWeO groups and the oscillation of OeW groups. The peak at 945 cm-1 can be assigned to the oscillation of O]W double bond in the WO3 NSs. Because the loading amount of Au is small, there are no any peaks from Au and the Au-SH coordination bonds. XPS was performed before and after the Au loading to obtain the
2.2. Preparation of Au nanoparticles Au NPs protected by 3-mercaptopropionic acid (MPA) were synthesized by previously reported methods requiring space [24]. All of these solutions were prepared with distilled water and treated under ice cooling. First, 0.25 mL of HAuCl4 solution (0.02 M) was required to add a space and 2 mL of MPA solution (0.005 M) to 2.35 mL with stirring to form an MPA-Au (I) complex. Next, 0.3 mL of NaOH (1 M) was introduced into the solution. After the quick injection of 0.1 mL NaBH4 solution ((0.1 mol L−1), the solution was kept stirring at 1000 rpm for 3 h. Then the MPA-Au NPs were obtained for the further application. 2.3. Preparation of WO3-Au composite catalyst The Monolayer WO3 NSs were synthesized by an oleyamine-intercalated exfoliation process, as reported by our earlier work [1]. To prepare the WO3 NSs-Au composite, 100 mg of WO3 NSs was dispersed into 20 mL of acetone by sonication for 30 min. An appropriate amount of the prepared Au nanoparticles was then added at room temperature under sonication for 10 min. The WO3-Au composite catalysts with the gold loading amount of 0.5 wt %, 1.0 wt %, and 2.0 wt % were obtained. The as-prepared catalysts were purified with acetone twice and further dispersed in acetone for the catalytic application. 2.4. Characterizations The phase of the sample was performed by X-ray diffraction (XRD) (Phillips X-pert) using Cook irradiation (20 kV, 20 a.m.) from 20 to 80° (2θ) at a scanning speed of 2°/min. The TEM and HRTEM observation of 2
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Fig. 1. (a) HRTEM image of pure WO3 NSs catalyst, (b) TEM and (c, d) HRTEM image of WO3-Au composite catalyst.
and 88.11 eV are corresponded to the zero-valence metallic gold (Au°), and the two small peaks with binding energies of 85.08 and 88.68 eV are assigned to the of Au+ [26]. Fig. 4a shows the UV–vis absorption spectra of pure WO3 NSs and WO3-Au composite catalysts. As seen from Fig. 4a, all the samples show the absorption band onset of around 510 nm, which is related to the main band-gap absorption of WO3 NSs. For WO3-Au composite catalysts, a weak adsorption from 500 to 600 nm can be observed, which is caused by the SPR effect from the Au nanoparticles. The Tauc plot (Fig. 4b) of the absorption data reveal that the band gap energy of WO3 NSs is 2.30 eV, which is lower than that of 2.80 eV for WO3 as reported previously [6]. Furthermore, it can be observed that the estimated band gap energies of WO3-Au composite with the Au loading content of 0.5, 1.0 and 2.0 wt % has no clear changes compared to that of pure WO3
chemical valence of Au and WO3. The full-scale XPS spectra for all the samples are presented in Fig. 3a. The clear Au 4f and W 4f peaks confirm the presence of Au and W in the WO3-Au composite. The W 4f 7/2 and W 4f 5/2 peaks at 35.80 and 37.98 eV are belong to the W6+ oxidation state. Compared with pure WO3 NSs, it shows a 0.4 eV positive shift after Au loading, which is caused by the reduced electron cloud density. This indicates that there is strong chemical interaction between WO3 NSs and Au. In Fig. 3c, the O 1 s peaks from the O 1 s orbital of the WeO bonds for the pure WO3 and WO3-Au samples locate at the binding energy of 530.81 and 542.22 eV A transition of 0.3 eV to higher energy for the WO3-Au composite indicates that more electrons were removed from O in the WO3-Au composite. The high resolution XPS spectra of Au 4f orbital are presented at Fig. 3d. From the peak fittings of Au 4f7/2 and Au 4f 5/2 orbital, the two main peaks at 84.48
Fig. 2. (a) XRD patterns and (b) FT-IR spectra of pure WO3 and WO3-Au composite. 3
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Fig. 3. XPS patterns of pure WO3 and 2.0 wt % WO3-Au composite.
Fig. 4. (a) UV–vis absorption spectra, (b) Tauc plots and (c) PL intensities of pure WO3 and WO3-Au composite with 0.5, 1.0 and 2.0 wt % gold loading.
content increases, the PL intensity decreases gradually, indicating that the effective charge transfer in the WO3-Au composite and the prevented photoelectron-hole recombination. Besides, the PL results under the excitation wavelength of 425 nm show the same trend (Fig. 4d).
NSs. It can be concluded that the band gap energy can be narrowed by the single layer structure of WO3 NSs, and the introduction of Au nanoparticles will further extend the utilization of light spectra. It is well known that the quenching of semiconductor fluorescence is related to the photogenerated electron-hole recombination rate. To further understand the interaction between Au NPs and WO3 NSs, the PL spectra were examined with the excitation wavelength of 325 and 425 nm. Fig. 4c show the PL spectra of pure WO3 NSs and WO3-Au composite catalysts excited at the wavelength of 325 nm. As the gold
3.2. Photothermal catalytic oxidation of cyclohexane using WO3-Au composite catalysts The as-prepared WO3 NSs and WO3-Au composite are used for the 4
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Fig. 5. (a) Time series data for conversion (solid) and selectivity (dash) efficiency of cyclohexane oxidation under photocatalysis (PC), thermal catalysis (TC) or photo[HYPHEN] thermal catalysis (PTC) by 2.0 wt % WO3-Au composite ; (b) Time series data for conversion and selectivity of cyclohexane oxidation by pure WO3 (□) and WO3-Au composite with 0.5 (○), 1.0 (△) and 2.0 wt % (◇) gold loading; (c) Temperature series data for conversion (solid) and selectivity (dash and dot) efficiency of cyclohexane oxidation on by 2.0 wt % WO3-Au composite; (d) The conversion and selectivity of cyclohexane oxidation by pure WO3 (dot), 2.0 wt % WO3-Au composite (solid) and commercial WO3 (dash) over 5 cycling times (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
increased to 9.0%, which is higher than the simple sum of photocatalysis and thermal catalysis. This trend is illustrated in both Fig. 5a and Table 1 under the catalysis of pure WO3 NSs and WO3-Au composite catalysts with gold loadings of 0.5, 1.0 and 2.0 wt %.With the increase of Au loading amount, both the cyclohexane conversion and the KA-oil electivity increase quickly when the loading amount is less than 1.0 wt %. After the loading amount of Au is larger than 1.0 wt %, the photothermal catalytic reaction rate of WO3-Au composite is increased slightly. The apparent reaction rates of pure WO3 and WO3-Au composite with 0.5, 1.0 and 2.0 wt % gold loading are 0.082, 1.08, 1.16, and 1.14%•h−1. The reaction temperature also has important effects on the photothermal catalytic of cyclohexane oxidation. The catalytic results by 2.0 wt % WO3-Au composite at different temperatures are shown in Fig. 6c. When increasing the reaction temperature from 80 ℃to 140 ℃, the photothermal catalytic conversion of cyclohexane increases first and then decreases. The highest cyclohexane conversion of 9.0%and the highest cyclohexanone selectivity of 52.7% was gained at 120 ℃. When the reaction temperature was raised to 140 ℃, the cyclohexane conversion, the selectivity of cyclohexanone and total KA oil selectivity are decreased. However, the cyclohexanol selectivity slightly increases, which may be due to the decomposition of cyclohexanone at a high temperature. The long-term stability of the catalysts has an important indicator of practical application. It is noteworthy that after five
Table 1 Solar light-driven cyclohexane oxidation with died air by WO3-Au catalysts with different Au loading amount.a Entry
1 2 3 4 5 6 7 8 9 10 11 12
Catalystb
Pure 0.5 wt 1.0 wt 2.0 wt Pure 0.5 wt 1.0 wt 2.0 wt Pure 0.5 wt 1.0 wt 2.0 wt
% % % % % % % % %
Typec
PC PC PC PC TC TC TC TC PTC PTC PTC PTC
Conv. (%)
0.2 0.3 0.4 0.5 4.6 7.2 7.7 8.0 6.3 8.4 8.9 9.0
Sele. of KA (%)d
51.6 60.5 63.9 71.4 97.9 98.7 99.0 98.9 98.5 98.9 99.0 99.0
Error Conv. (%)
Sele. of KA (%)
0.02 0.05 0.03 0.05 0.15 0.09 0.14 0.06 0.14 0.12 0.07 0.09
3.10 1.60 2.00 1.20 0.50 0.70 0.70 0.40 0.70 0.80 0.50 0.70
a
10 mL of Cyclohexane, 20 mg of catalyst, 5 mL of aceton, 1.5 Mpa of dried air, temperature of 120 ℃, time of 8 hs, 220 W Xe lamps. b The Catalysts involve pure WO3 and WO3-Au composite with 0.5, 1.0 and 2.0 wt % gold loading. c Different type of catalysis, PC = photocatalysis, TC = thermal catalysis, PTC = photo-thermal catalysis. d Product selectivity = the content of KA oil /content (mmol) of all product) × 100%.
photocatalytic, thermal catalytic and photothermal catalytic oxidation process of cyclohexane. The detailed catalytic properties of cyclohexane oxidation under certain conditions are shown in Fig. 5. As shown in Fig. 5a, the conversions of cyclohexane oxidation catalyzed by the 2.0 wt % WO3-Au composite are only 0.49 and 7.61% when just reacted under simulated sunlight (220 W Xe lamps) without external heating and at 120 °C in the dark. However, the highest conversion of 9.0%and the highest selectivity of 99.0% can be acquired after 8 hs reaction of a photothermal synergetic catalysis process (Table 1). The results show that both photocatalysis and thermal catalysis can effectively catalyze the oxidation of cyclohexane, but the conversion rate of thermal catalytic conversion is much higher than that of photocatalysis. The conversion under photothermal synergistic catalysis can be further
Fig. 6. Photothermal catalytic activities of 2.0 wt % WO3-Au composite in reactive species trapping experiments with three types of reactive species scavengers. 5
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Fig. 7. (a) Photocurrent response patterns and (b) electrochemical impedance spectroscopy of pure WO3 and WO3-Au composite with 0.5 1.0 and 2.0 wt % gold loading.
mainly due to the loss of SPR effect and high electron-hole recombination rate. The electrochemical research results further confirm that Au plays a key role in promoting charge transfer and thus producing photocurrent. Based on the above analysis and in combination with the thermal catalytic process, we proposed the reaction mechanism of cyclohexane oxidation. As shown in Scheme 1, in the presence of heating, a partially positively charged gold atom (Au+) acquires an electron from cyclohexane to form Au° to generate the radical of Cy%. At the same time, photogenerated electrons on WO3 NSs can be excited to the CB under visible light irradiation, and then inject into the gold core (Au°) to make the gold core a reduction center (Au (e−)). Multi-electron reduction between electrons and O2 takes place to produce H2O and a small amount of hydrogen peroxide (H2O2), and Au (e−) offers from multielectron oxidation into Au+, which facilitates the formation of the radical of Cy%. On the other hand, holes having strong oxidizing ability can directly oxidize cyclohexane (CyH) to obtain Cy%, which can easily react with O2 to promote the photocatalytic oxidation reaction.
catalytic cycles, the catalyst remained effective without evident decreases in the conversion and selectivity, indicating that our synthesized WO3 and WO3-Au composite have the same excellent long-term stability with that of the commercial WO3 nanoparticles (Fig. 5d).Compared to the commercial WO3 (Conv. 5.9%), catalysts in our system show more outstanding performance obviously. 3.3. Possible catalytic mechanism Hydroxyl radicals and superoxide radicals are the main active centers in the photocatalytic process. In order to investigate the effective catalytic active centers, several control photothermal catalytic experiments were performed with the addition of t-butanol (TBA), benzquinamide (BQ) and ethylenediaminetetraacetic acid (EDTA), which are scavengers for hydroxyl radicals (%OH), O2%− radical and hole [18]. As presented in Fig. 7, with the addition of TBA and BQ, the conversion and selectivity only have no obvious change, indicating O2%−and %OH radical has nearly no effect in this reaction. However, after the hole scavenger of EDTA was added into the reaction solution, the conversion decreased greatly from 9.0–6.18 %, indicating that the hole may play a vital role in the photocatalytic cyclohexane oxidation as the main active species. The conduction band position of WO3 ( + 0.5 eV vs. NHE) is too low to reduce O2 to superoxide radical (O2▪)(-0.13 eV), which is in good agreement with our scavenger experimental result [6]. Instead, the conduction band electrons of WO3 can produce hydrated hydrogen peroxide by a multi-electron reduction procedure, which may be helpful for the cyclohexane oxidation. The valence band potential of WO3 ( + 3.20 eV vs. NHE) is very positive with strong oxidation capability to oxidize H2O to generate %OH radical. However, there is no water in our cyclohexane oxidation process, which interpret the reason why the %OH radical is not main active center in this cyclohexane oxidation process [27]. The hole generated in the VB of WO3 has significant influence in the catalytic activity of cyclohexane oxidation, which may be caused by the strong oxidizing capability to facilitate the cyclohexane oxidation reaction. The current densities in the photocurrent response curves of the catalysts in Fig. 6a confirms the improved current density with the increased loading amount of Au nanoparticles, which shows the reduced order for the 2.0 wt % WO3-Au composite, 1.0 wt % WO3-Au composite, 0.5 wt % WO3-Au composite and WO3 NSs. This suggests that the electron transfer to Au NPs is more quickly in 2.0 wt % WO3-Au composite catalysts. It indicates that the gold loading promotes the separation of photogenerated electron- holes in WO3 NSs and WO3-Au composite and contributes to an improved catalytic activity in the cyclohexane oxidation. Impedance measurements are given in Fig. 6b. The charge transfer resistance of WO3-Au composite is higher than that of pure WO3 NSs, which may be caused by the ligands in the surface of small Au nanoparticles. Although pure WO3 NSs have the lowest electrical resistance, the photothermal catalytic activity is still low, which is
4. Conclusions The high quality WO3-Au composite was prepared by the combination of WO3 NSs and Au nanoparticles for the improved photothermal catalytic cyclohexane oxidation with dry air under solar lightirradiation. The TEM and HRTEM image confirmed that Au NPs with the size of approximately 3–5 nm are attached on the surface of large monolayer MoO3 nanosheets. The morphology, size and crystalline of WO3 NSs shows no clear changes with the combination of Au NPs. The XPS test suggests the existence of W +6 and Au+. The catalysts exhibit significantly improved photothermal catalytic performance with a high conversion (9.0%) and high KA oil selectivity (99%) achieved on WO3Au composite catalysts with 2.0 wt % gold loaded. The performance of
Scheme 1. The proposed photothermal catalytic process of cyclohexane oxidation by the WO3-Au composite under solar-light irradiation. 6
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this catalyst is 1.5–2 times higher than the commercial one. The high selectivity for cyclohexane oxidation on WO3-Au composite is due to the Surface Plasmon Resonance (SPR) of Au NPs and the suppression of superoxide radical (O2%−) producing of WO3 NSs. The findings in this work may provide a new method to increase the catalytic oxidation activity of cyclohexane.
[12]
[13]
[14]
Acknowledgements [15]
Authors would like to acknowledge the National Natural Science Foundation of China (Grant No.21776049), the Natural Science Foundation of Guangdong Province (Grant No. 2018A030313174) and the Guangdong Science and Technology Plan Grant (No.2016A010103028) support for this work.
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