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Facile synthesis of SiO2@CuxO@TiO2 heterostructures for catalytic reductions of 4-nitrophenol and 2-nitroaniline organic pollutants
Applied Surface Science 393 (2017) 110–118
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Facile synthesis of SiO2 @Cux O@TiO2 heterostructures for catalytic reductions of 4-nitrophenol and 2-nitroaniline organic pollutants Osman Ahmed Zelekew, Dong-Hau Kuo ∗ Department of Materials Science and Engineering, National Taiwan University of Science and Technology, No.43, Sec. 4, Keelung Road, Taipei 10607, Taiwan
a r t i c l e
i n f o
Article history: Received 8 July 2016 Received in revised form 17 September 2016 Accepted 4 October 2016 Available online 4 October 2016 Keywords: 4-Nitrophenol Reduction Nitro-aromatic p-n Junction
a b s t r a c t Herein, we designed the p-type Cux O (x = 1 or 2) nanoparticles deposited on SiO2 spherical particle inside and coated with thin layered n-type TiO2 semiconductors outside for reduction purpose. The composite material, abbreviated as SiO2 @Cux O@TiO2 , was characterized. The catalytic performance of the composite was tested for the reductions of 4-nitrophenol (4-NP) and 2-nitroaniline (2-NA). Complete reductions of 4-NP and 2-NA took, 210 and 150 s, respectively. The catalytic efficiency of the composite material may be associated with electron and hole separation resulted from the p-n junction formation between p-type Cux O and n-type TiO2 and the built-in electric field. Moreover, the hydride ion and electrons released from NaBH4 together with outward electrons from n-type TiO2 , synergistically, are also responsible for the reduction of nitro aromatic compounds. Our design of composite material from low-priced metal oxides was successful towards reduction of nitro-aromatic compounds. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Nitro-aromatic compounds are an organic materials used as precursors for manufacturing of organic products such as pharmaceuticals, pesticides, explosives, and other industrial chemicals [1]. However, the presence of nitro-aromatic compounds such as 2-NA and 4-NP in water would be dangerous to aquatic life and human health because of their toxicity, potential carcinogenic and mutagenic effects. Recently, reports suggested that the presence of even low concentration of 2-NA in water has resulted in environment contamination and public health problems [2–4]. Moreover, the 4NP is also an industrial pollutant with high solubility and stability in water and can invade the body through the skin, respiratory system, and digestive system to lead to major adverse effects on the blood, liver, and central nervous system [5]. Therefore, the Unites States Environmental Protection Agency has listed 4-NP and 2-NA as hazardous wastes and toxic pollutants [3,4]. Hence, the green chemical transformation that can be used green solvents, nontoxic chemicals and ambient reaction conditions are required [6]. Among the processes contributing to the remediation, reduction of the nitro group into the corresponding amine functional group is the most characteristic [7,8].
∗ Corresponding author. E-mail address: [email protected] (D.-H. Kuo). http://dx.doi.org/10.1016/j.apsusc.2016.10.016 0169-4332/© 2016 Elsevier B.V. All rights reserved.
Aromatic amines are important organic compounds used as a starting and an intermediates material for the manufacturing of a variety of chemicals such as biologically active natural products, pharmaceuticals, and dyes [9,10]. For this reason, catalytic reductions of nitro compounds in to aromatic amine with cost effective catalyst is important. Hence, metal oxide semiconductor catalysts are attractive materials because of their low cost and facile nature [11]. Particularly, titanium dioxide (TiO2 ) semiconductor has been extensively studied because it is one of the most affordable, stable, nontoxic, and active photocatalysts [12,13]. Although the catalytic activity of TiO2 towards reduction reaction is limited, it has been used together with precious metals such as Ag and Pt nanoparticles for reduction of 4-NP at different reaction condition, recently [14–16]. However, precious metal nanoparticles are not still cost effective and limited for practical applications. Hence, coupling of TiO2 with cost effective metal oxides such as p-type Cu2 O and CuO is significant towards reduction reaction. Moreover, Copper oxides based catalysts have been also given much attention because of low-cost and abundant, nontoxic, easily prepared materials. The design and fabrication of suitable composite catalysts from Cu2 O have been reported recently [17–19]. Huang et al. synthesized Cu2 O octahedrons on h-BN nanosheets for p-nitrophenol reduction [20]. Guo et al. used Cu2 O/Au catalyst for reduction of 4-nitrophenol [21]. Moreover, CuO composite catalyst also applied as a catalyst for reduction of nitroarenes [22,23]. However, there is no report on p-type Cux O oxides inside combined with n-type TiO2 outside aiming for the development of p-n het-
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erojunction and changing inactive TiO2 into active for the purpose of reduction reaction. Recently, our group used heterostructured Ag-deposited SiO2 @TiO2 composite spheres to enhance catalytic activity towards photo degradation of AB 1 dye [24]. In addition to this, the doublelayered p-type Ag2 O inside and n-type TiO2 outside had been also applied for rapid reduction of 4-NP [25]. Herein, we designed the composite catalyst from p-type Cu2 O and in situ generated CuO, and coated with n-type TiO2 semiconductor outside on SiO2 sphere as a support. The formation of p-n heterojunction in the composite is used for electrons and hole separation. The inactive n-type TiO2 semiconductor becomes active after combined with p-type Cux O towards reduction of nitro aromatic compounds, which is the aim of this work. The electron and hydride ion released from BH4 -are responsible for the reduction of aromatic compounds. Hence, our design with low cost metal oxide towards reduction was successful. The 2-NA and 4-NP compounds were selected as a model reaction to test the catalytic activity of the composite catalyst. 2. Experimental methods 2.1. Chemicals All chemicals used in this work were analytical grade and used without further purification. 2.2. Synthesis of SiO2 spherical particles SiO2 spherical particles were fabricated according to our previous work [25]. Briefly, 1.2 gm of CTAB was added into a mixture of 240 mL DI water and 160 mL absolute ethanol and ultrasonicated for 1 min. Then, 3.2 mL of ammonium hydroxide was dropped into the reaction mixture and stirred about 30 min. Subsequently, 4 mL of TEOS was added drop by dropand stirred for another 2 h. The resulting white precipitate was centrifuged, washed, and dried. Finally, the powder was calcinated at 550 ◦ C for 3 h and the SiO2 spherical particles were obtained. 2.3. Deposition of Cu2 O nanoparticles on the surface of SiO2 sphere To deposit Cu2 O nanoparticles on the surface SiO2 sphere, calcinated SiO2 (0.2 gm) were first dispersed into 20 mL of distilled water. Then, 2 mL of CuCl2 (0.1 M) solution was dropped into the dispersed solution and stirred for 10 min. Subsequently, 4 mL of NaOH (0.3 M) solution was added into the mixture drop by drop and the solution was changed to light blue color. After another 10 min stirring, 2 mL of N2 H4 ·H2 O (0.1 M) solution was added and the color of the mixture was changed to orange. The mixture was aged for 10 min under magnetic stirring and recovered by centrifugation, washed, and dried. Finally SiO2 @Cu2 O composite was obtained. 2.4. Coating of SiO2 @Cu2 O with TiO2 nanoparticles To coat TiO2 on SiO2 @Cu2 O composite sphere, 0.1 g of SiO2 @Cu2 O was added to 4 mL of isopropanol and 0.5 mL of dodecane mixture and treated with ultrasonication for 15 min. After ultrasonication, 0.1 mL of titanium isobutoxide (Ti(i-BO)) solution was dropped in to reaction mixture. Then, 0.1 mL of DI water was added and stirred for 4 h at room temperature. The resulting precipitate was collected by centrifugation and washed with ethanol. The composite of sphere was then annealed at 450 ◦ C for 3 h in air. Scheme 1 shows the schematic synthetic procedure and reduction of 2-NA and 4-NP.
Fig. 1. XRD patterns of (a) SiO2 sphere, (b) SiO2 @Cu2 O, (c) SiO2 @TiO2 , and (d) SiO2 @Cux O@TiO2 composite catalysts.
2.5. Characterizations The surface morphology and microstructural analysis of the composites were conducted by field-emission scanning electron microscopy (FE-SEM, JSM 6500F, JEOL, Tokyo, Japan) and highresolution transmission electron microscopy (HRTEM, Tecnai F20 G2, Philips, Netherlands). The element mapping was also examined by HRTEM. The X-ray diffraction (XRD) patterns of the catalysts were checked by a Bruker D2-phaser diffractometer using Cu K␣ radiation ( = 1.5418 Å). The X-ray photoelectron spectroscopy (XPS) measurements of the composite powders were done on a VG ESCA Scientific Theta Probe spectrometer with Al K␣ source. The UV–vis absorbance spectra of the composite and the reduction of 2NA and 4-NP were monitored by JASCO V-670 UV–vis-near-infrared spectrophotometer. 2.6. Catalytic ActivityTest The reductions of 2-NA and 4-NP were done according to the following procedure. Briefly, 8 mL of 2-NA aqueous solution (200 ppm) was added into a beaker containing 72 mL of DI water. Then, freshly prepared 8 mL of aqueous NaBH4 (0.1 M) was added. Then, 10 mg of catalyst was added into the reaction mixture under stirring at room temperature. The reduction progress was monitored by taking small portion of the supernatant concentration at different interval of time. Measurements were carried out using a JASCO V-670 UV–vis spectrophotometer. For reduction of 4-NP, the procedure was also similar to 2-NA. 3. Results and discussion 3.1. Characterization The XRD pattern of the as-prepared SiO2 , SiO2 @Cu2 O, SiO2 @TiO2 , and SiO2 @Cux O@TiO2 heterostructure samples were characterized. As it is observed from Fig. 1a and b, the SiO2 and SiO2 @Cu2 O spherical particles were amorphous. The amorphous phase of SiO2 @Cu2 O composite catalyst in Fig. 1b was because of the amorphous background of SiO2 that covers the intensities of small amount of Cu2 O nanoparticles deposited on it. However, the TiO2 peaks deposited on SiO2 for comparison purpose were appeared in an anatase phase (Fig. 1c). Moreover, the crystalline structures of SiO2 @Cux O@TiO2 heterostructure were also characterized after calcination of the composite at 450 ◦ C for 3 h. As shown from Fig. 1d, the anatase phase of TiO2 (1 0 1) planes was observed
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Scheme 1. Schematic illustrations of SiO2 @Cux O@TiO2 composite catalyst synthesis and catalytic applications for reducing 2-NA and 4-NP.
Fig. 2. High-resolution XPS spectra of (a) Ti 2p, (b) Cu 2p, (c) O 1s, and (d) Si 2p for SiO2 @Cux O@TiO2 composite catalyst.
(JCPDS No. 21-1272). However, the peaks represents for Cu2 O or CuO nanoparticles were not detected because Cu2 O or CuO peaks with low intensity were enclosed under amorphous SiO2 background and under TiO2 peaks. For this reason, XPS analysis to detect the presence of Cu(I) or Cu(II) in the composite is needed. The XPS analysis was performed to investigate the surface elemental composition and the chemical valence states of SiO2 @Cux O@TiO2 composite catalyst. The Ti, Cu, O, and Si elements were detected from XPS survey spectrum in the composite
catalyst. The binding energies of XPS spectra for Ti 2p3/2 and O 1s were observed at about 458.27 eV (Fig. 2a) and at 530.05 eV (Fig. 2c), respectively. The binding energies have a good agreement with the reported values of pure anatase [26,27]. Moreover, the high-resolution XPS spectrum of Cu 2p regions of the composite microspheres showed a representative curve with the binding energy of Cu 2p1/2 centered at 952.48 eV and the binding energy of Cu 2p3/2 centered at 932.50 eV (Fig. 2b). Our result revealed that Cu2 O nanoparticleswere formed which is also similar to the values
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Fig. 3. Scanning electron microscopy (SEM) images of: (a) pure SiO2 microspheres, (b) SiO2 @Cu2 O, and (c) SiO2 @Cux O@TiO2 composite catalyst; and (d) The EDS spectrum for SiO2 @Cux O@TiO2 composite catalyst.
reported in literature [28]. Furthermore, additional CuO nanoparticle was also detected during calcinations of TiO2 . The binding energies of CuO were observed at ∼934.38 eV for Cu 2p3/2 and at ∼953.56 eV for Cu 2p1/2 , in a good agreement with reported for values Cu++ in CuO [29]. Moreover, the binding energy of Si 2p XPS spectrum at 103.14 eV identified the formation of Si in SiO2 (Fig. 2d) [30]. Fig. 3a–c shows the typical SEM images of SiO2 , SiO2 @Cu2 O, and SiO2 @Cux O@TiO2 microsphere composites. As it is observed from Fig. 3a, the SiO2 spherical particles were uniform and had spherical smooth surface. The SEM image of SiO2 @Cu2 O showed the smaller Cu2 O nanoparticles deposited on the surface of SiO2 sphere and it was clearly observed from the inset of Fig. 3b. Furthermore, TiO2 nanoparticles were also uniformly coated on SiO2 @Cu2 O spherical particles (Fig. 3c). The presence of Ti, Cu, Si, and O elements in SiO2 @Cux O@TiO2 composite catalyst was shown by energy dispersive X-ray spectroscopy (EDS) analysis in Fig. 3d. Fig. 4a–c displays the TEM images at low, medium, and high resolutions of SiO2 @Cux O@TiO2 heterostructures, respectively. It is clearly shown that the TiO2 and Cu2 O or CuO nanoparticles were uniformly distributed and their morphologies can be viewed as the nanoparticles decorating on the surfaces of porous SiO2 spheres. The HRTEM image of SiO2 @Cux O@TiO2 composite further revealed that the d-spacing of 0.35 nm was attributed to anatase TiO2 (101) nanoparticle (Fig. 4d), indicating that TiO2 was well deposited and crystallized on the SiO2 sphere.
Fig. 5 demonstrates Cu2 O or CuO, and TiO2 nanoparticles distributions on the surface of amorphous SiO2 sphere. Fig. 5a shows the high-angle annular dark-field scanning TEM (HAADF-STEM) image of SiO2 @Cux O@TiO2 composite catalyst. Fig. 5b–f shows the elemental mapping of Si-K, O-K,Cu-K, Cu-L, and Ti-K, respectively, from the selected area of HAADF-STEM image. The elemental mapping of Si in Fig. 5b together with O in Fig. 5c shows the availability of SiO2 spherical particles in the composite. The element mapping of Cu in Fig. 5d and e also confirmed the uniform distribution of CuO or Cu2 O on SiO2 sphere. Fig. 5f shows the Ti elemental mapping. The elemental mapping of O in Fig. 5c contributes from SiO2 , Cu2 O, CuO, and TiO2 in the composite catalyst.
3.2. Catalytic activity test The catalytic reduction of 4-NP and 2-NA with NaBH4 has often been used as a model reaction to evaluate the catalytic performance of nanoparticles. Although reduction of 4-NP and 2-NA is thermodynamically feasible with NaBH4 , it is known that reduction does not spontaneously occur without catalyst due to the presence of a kinetic barrier [31,32]. The reduction reactions of both 4-NP and 2-NA compounds were easily achieved due to the formation of a single product in each case and were determined by measuring the change in UV–vis absorbance at different interval of reaction time [4].
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Fig. 4. (a) Low-magnification, (b) medium-magnification, and (c) high-magnification TEM images, and (d) HRTEM images of SiO2 @Cux O@TiO2 composite sphere.
Fig. 5. (a) HAADF STEM image and elemental mapping of (b) Si-K, (c) Cu-K, (d) Cu-L, (e) Ti-L, and (f) O-K.
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Fig. 6. (a) Absorption spectra of 4-NP before and after the addition of aqueous NaBH4 solution. (b) and (c) The time-dependent reductions of 4-NP after addition of SiO2 @Cux O@TiO2 , and SiO2 @Cu2 O composite catalyst, respectively. (d) Plots of Ct /C0 versus reaction time (s) for the reduction of 4-NP with SiO2 @TiO2 , SiO2 @Cu2 O, and SiO2 @Cux O@TiO2 . (e) Plots of ln(Ct /C0 ) versus reaction time (s) for SiO2 @Cu2 O and SiO2 @Cux O@TiO2 composite catalysts.
Fig. 6 illustrates the reduction reaction of 4-NP towards 4-AP. As it is shown from Fig. 6a, the initial aqueous solution of 4-NP provides a characteristic peak centered at about 317 nm. However, addition of NaBH4 causes the peak shifts to 400 nm which is attributed to the formation of 4-nitrophenolate ions under alkaline condition [33]. Moreover, the time taken for the conversion of 4-NP into the corresponding 4-AP was noted based on the disappearance of the peaks at 400 nm along with a simultaneous appearance of the peaks at about 300 nm after addition of catalyst [34,35]. Fig. 6b and c shows the time dependent reduction of 4-NP by SiO2 @Cu2 O and SiO2 @Cux O@TiO2 composite catalysts, respectively. The kinetics of the reduction of 4-NP in the presence of different catalysts was demonstrated with Ct /C0 ratio in terms of reaction time, where C0 referred to the initial concentration absorbance of 4-nitrophenolateat 400 nm. Fig. 6d shows Ct /C0 versus reaction time for reduction of 4-NP in the presence of SiO2 @TiO2 ,
SiO2 @Cu2 O, and SiO2 @Cux O@TiO2 composite catalysts. The presence SiO2 @TiO2 catalyst has no effect towards reduction of 4-NP which implies that TiO2 is not active without UV light source due to its higher band gap energy [36]. However, the presence of SiO2 @Cu2 O catalyst showed relatively better reduction ability due to lower band gap of Cu2 O and 72% of 4-NP was completed within 450 s. Hence, SiO2 @Cu2 O can accept and transfer electrons from NaBH4 towards nitroaromatic compounds at room temperature. Furthermore, SiO2 @Cux O@TiO2 composite exhibits much better catalytic performance toward reduction of 4-NP in to corresponding 4-AP and the reaction was completed within 210 s. The formation of p-n junction between p-type Cux O nanoparticles and thin layered n-type TiO2 semiconductors in SiO2 @Cux O@TiO2 composite probably facilitates the reduction reaction by decreasing the hole and electron recombination rate. In addition to this, the electrons released from BH4 − ion together with the hydride ion
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Fig. 7. (a) and (b) The time-dependent successive reductions of 2-NA after addition of SiO2 @Cux O@TiO2 and SiO2 @Cu2 O composite catalyst, respectively. (c) Plots of Ct /C0 versus reaction time (s) for the reduction of 2-NA with SiO2 @TiO2 , SiO2 @Cu2 O, and SiO2 @Cux O@TiO2 . (d) Plots of ln(Ct /C0 ) versus reaction time (s) for SiO2 @Cu2 O and SiO2 @Cux O@TiO2 catalysts.
attached on the surface of the catalyst makes the reduction reaction easier and effective. Fig. 6e shows the linear relationships between ln(Ct /C0 ) and the reaction time for SiO2 @Cu2 O and SiO2 @Cux O@TiO2 composite catalysts. The linear relationship of ln(Ct /C0 ) versus reaction time follows a pseudo-first-order kinetics reaction [37]. Considering the concentration of NaBH4 is much higher than 4-NP, the rate constantis calculated from the slope of the linear relationship between ln(Ct/ C0 ) and reaction time. The apparent rate constants (kapp ) of the as-prepared SiO2 @Cu2 O and SiO2 @Cux O@TiO2 catalyst were 0.0036 s−1 and 0.025 s−1 , respectively. The catalytic efficiencies of our catalyst also applied for reduction of 2-NA. Fig. 7a and b shows successive catalytic reduction reaction of 2-NA with SiO2 @Cux O@TiO2 and SiO2 @Cu2 O composite catalyst, respectively. The 2-NA showed a typical maximum absorbance peak at about 412 nm and gradually decreased as it was reduced into 1,2-benzenediamine. This gradual decrease in the peak intensity can be used to determine the progress of the reduction reaction [32]. As it is observed in Fig. 7c, the catalytic reduction of 2-NA into 1,2-benzenediamine with SiO2 @TiO2 is insignificant while SiO2 @Cu2 O has better catalytic capability, which is also similar with reduction of 4-NP. However, in the presence of SiO2 @Cux O@TiO2 catalyst, the reduction of 2-NA completed within 150 s. It could be illustrated that the reduction of 2-NA is also pseudo-first-order kinetics reaction at relatively higher concentration of NaBH4 . In a similar fashion with the rate constant of 4-NP, the catalytic rate constant could be evaluated from the curves plotted for the concentration of 2-NA ln(Ct /C0 ) against reaction time. Fig. 7d shows the linear relationship built between ln(Ct /C0 ) and time (s) for reduction of 2-NA. The corresponding kapp values calculated from the linear slope were 0.018 s−1 and 0.0026 s−1 for SiO2 @Cux O@TiO2 and SiO2 @Cu2 O catalysts, respectively.
Recently, reductions of nitro aromatic compounds have been done by using metal oxide. The p-type metal oxides such as Cu2 O, CuO, Fe2 O3 , and NiO were active but n-type metal oxides such as TiO2 and ZnO were inactive towards reduction of 4-NP [20,36]. Researches had been also conducted by coupling of TiO2 with Au, and Ag metal nanoparticles towards reduction of 4NP. Furthermore, using precious metal nanoparticles with n-type semiconductor active, is not cost effective [15,38]. The catalytic reduction towards 4-NP by SiO2 @Cux O@TiO2 catalysts is comparable with different catalysts in literatures shown in Table 1 below. The activity parameters used for the comparison of the catalytic activity of SiO2 @Cux O@TiO2 composite catalyst with the reported metal oxides-based and noble metal based catalysts used for the reduction of 4-NP are shown in Table 1. According to the literature, CuO and Co3 O4 catalysts have the activity parameters of about 0.19 and 0.13 s−1 g−1 , respectively [36]. The Ag-NP/C composite and TAC-Ag-1.0 catalysts have also the activity parameters of about 1.69 and 1.3 s−1 g−1 , respectively [39,40]. In the case of SiO2 @Cux O@TiO2 composite catalyst, the ratio activity parameter constant is 2.5 s−1 g−1 , which is comparable with reported literature values. In our design, we combined the cheapest p-type and n-type semiconductors together. The results showed that coupling of ntype TiO2 with p-type Cux O semiconductors was effective and changed the two oxide system catalyst more active than individual semiconductors. Scheme 2 shows the reduction of 4-NP (1) and 2-NA (2) at room temperature (RT). The possible catalytic reaction mechanism for reduction of 4-NP is illustrated in Fig. 8. The mechanism involves with chemisorption mechanism which implies that the nucleophile adsorbs on the surface of the composite catalyst. The electrophile also adsorbs simultaneously on the catalyst surface [41]. Furthermore, the nucleophile (i.e., BH4 − ions) donates electrons to the electrophile
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Table 1 Comparison of the catalytic capacities of various catalysts reported in literature for the reduction of 4-NP to 4-AP by NaBH4 . Catalyst/amount
Time Taken (s)
Kinetic rate constant, kapp (s−1 )
Ratio constant, K (s−1 g−1 )
Reference
Ag-NP/C composite/1.0 mg CuO/0.1 g Co3 O4 /0.1 g Cu2 O–Cu–CuO/1 mg TAC-Ag-1.0/4 mg SiO2 @Cu2 O@TiO2 /10 mg
1500 40 120 180 415 210
0.00169 0.019 0.013 0.0156 0.00519 0.025
1.69 0.19 0.13 15.6 1.3 2.5
[39] [36] [36] [9] [40] This work
Scheme 2. Reductions of 4-NP and 2-NA.
Fig. 8. The schematic reaction mechanism for the reduction of 4-NP with SiO2 @Cux O@TiO2 composite catalyst.
(4-NP) through the catalyst support. The aqueous medium also provides the required H+ ions to complete the reduction reaction. Moreover, the p-n junction and electric field formation between ptype Cux O and n-type TiO2 will also enhance the adsorption of the reactants on the surface of the catalyst. Even without the light illumination, the narrow band gap Cux O with Cu2 O (2.1 eV) and CuO (1.2 eV) can be thermal excited by local temperature in the near infrared light to produce electron-hole pairs. Those electrons will be electrically driven to the TiO2 side for enhancing the catalytic reduction. With the absorbed H+ ions on the catalyst surface, those thermally generated electrons on TiO2 layer with the electrically driven electrons can facilitate the electrons released from NaBH4 to survive, stay, and migrate for reduction reaction. Together with the survival electrons and protons, the amino functional group of 4-NP is discharged and 4-NP transforms to 4-AP. Fig. 8 shows the schematic reaction mechanism for the reduction of 4-NP. The stability of SiO2 @Cux O@TiO2 composite catalyst was tested by using 2-NA as a reactant. The catalyst was reused three-times towards reduction of 2-NA after centrifuged, washed, and dried for each catalytic run. As it is observed from Fig. 9, the reduction of 2NA into 1,2-benzenediamine was completed at about 5 and 7.5 min for the second and third cycles, respectively. Although the catalyst works properly for second and third run, the reaction rates were not as fast as the first run. It is suggested that the catalytic activities of the catalyst decreases as the recycling increases. This could be ascribed to the reduction the activity site of catalysts [20,23]. Therefore, further work is needed to make the catalyst to be as good as the firstcycle. Moreover, the high-resolution XPS spectrum for Cu 2p in the composite after three-time reuse is illustrated in Fig. 10. The peaks with the binding energy of Cu 2p1/2 at 952.0 eV and the binding energy of Cu 2p3/2 at 932.01 eV are observed. This shows the binding energies of Cu+ or Cu (0), which are very close to each other and
Fig. 9. Reusability of the catalyst for reduction of 2-NA into 1,2-benzenediamine.
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and by the National Taiwan University of Science and Technology through Grant 105H451714. References
Fig. 10. XPS spectrum of Cu 2p after three-time use for catalytic reduction of 2-NA.
difficult to identify by XPS. However, the binding energy of Cu++ , like the catalyst before use in Fig. 2b, was not observed. Hence, reduction of Cu++ into Cu+ or Cu (0) probably occurred because of the effect of NaBH4 as a reducing agent during reaction. These phenomena were also reported in previous reports [9,20]. Furthermore, the disappearance of the characteristic shakeup satellite peak, peculiar to the Cu(II) species [41], indicates the reduction of the Cu++ species. 4. Conclusions The SiO2 @CuxO@TiO2 heterostructured composite catalyst has been designed and fabricated by a facile method with the concept of p-n heterojunction formation for effective electrons and hole separation. The composite catalyst was characterized by different techniques. The catalytic efficiency of the composite was tested for reduction of 2-NA and 4-NP at room temperature. The SiO2 @Cux O@TiO2 composite catalyst took 210 s and 150 s for complete reduction of 4-NP and 2-NA, respectively. Results shown in this study clearly demonstrated that the formation of p-n junction between p-type Cux O and n-type TiO2 and the outward electrons from n-type TiO2 together with hydride ion, synergistically, make the catalyst highly active. The plausible mechanism for the catalytic reduction of nitroarenes also shown. This work provides a cheap and new two-oxide catalyst system towards reduction of nitroarenes in aqueous solutions, and may also have a possible application in industrial production. Acknowledgments This work was supported by Ministry of Science and Technology of the Republic of China under Grant MOST104-2218-E-011-007
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Full Length Article
Facile synthesis of SiO2 @Cux O@TiO2 heterostructures for catalytic reductions of 4-nitrophenol and 2-nitroaniline organic pollutants Osman Ahmed Zelekew, Dong-Hau Kuo ∗ Department of Materials Science and Engineering, National Taiwan University of Science and Technology, No.43, Sec. 4, Keelung Road, Taipei 10607, Taiwan
a r t i c l e
i n f o
Article history: Received 8 July 2016 Received in revised form 17 September 2016 Accepted 4 October 2016 Available online 4 October 2016 Keywords: 4-Nitrophenol Reduction Nitro-aromatic p-n Junction
a b s t r a c t Herein, we designed the p-type Cux O (x = 1 or 2) nanoparticles deposited on SiO2 spherical particle inside and coated with thin layered n-type TiO2 semiconductors outside for reduction purpose. The composite material, abbreviated as SiO2 @Cux O@TiO2 , was characterized. The catalytic performance of the composite was tested for the reductions of 4-nitrophenol (4-NP) and 2-nitroaniline (2-NA). Complete reductions of 4-NP and 2-NA took, 210 and 150 s, respectively. The catalytic efficiency of the composite material may be associated with electron and hole separation resulted from the p-n junction formation between p-type Cux O and n-type TiO2 and the built-in electric field. Moreover, the hydride ion and electrons released from NaBH4 together with outward electrons from n-type TiO2 , synergistically, are also responsible for the reduction of nitro aromatic compounds. Our design of composite material from low-priced metal oxides was successful towards reduction of nitro-aromatic compounds. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Nitro-aromatic compounds are an organic materials used as precursors for manufacturing of organic products such as pharmaceuticals, pesticides, explosives, and other industrial chemicals [1]. However, the presence of nitro-aromatic compounds such as 2-NA and 4-NP in water would be dangerous to aquatic life and human health because of their toxicity, potential carcinogenic and mutagenic effects. Recently, reports suggested that the presence of even low concentration of 2-NA in water has resulted in environment contamination and public health problems [2–4]. Moreover, the 4NP is also an industrial pollutant with high solubility and stability in water and can invade the body through the skin, respiratory system, and digestive system to lead to major adverse effects on the blood, liver, and central nervous system [5]. Therefore, the Unites States Environmental Protection Agency has listed 4-NP and 2-NA as hazardous wastes and toxic pollutants [3,4]. Hence, the green chemical transformation that can be used green solvents, nontoxic chemicals and ambient reaction conditions are required [6]. Among the processes contributing to the remediation, reduction of the nitro group into the corresponding amine functional group is the most characteristic [7,8].
∗ Corresponding author. E-mail address: [email protected] (D.-H. Kuo). http://dx.doi.org/10.1016/j.apsusc.2016.10.016 0169-4332/© 2016 Elsevier B.V. All rights reserved.
Aromatic amines are important organic compounds used as a starting and an intermediates material for the manufacturing of a variety of chemicals such as biologically active natural products, pharmaceuticals, and dyes [9,10]. For this reason, catalytic reductions of nitro compounds in to aromatic amine with cost effective catalyst is important. Hence, metal oxide semiconductor catalysts are attractive materials because of their low cost and facile nature [11]. Particularly, titanium dioxide (TiO2 ) semiconductor has been extensively studied because it is one of the most affordable, stable, nontoxic, and active photocatalysts [12,13]. Although the catalytic activity of TiO2 towards reduction reaction is limited, it has been used together with precious metals such as Ag and Pt nanoparticles for reduction of 4-NP at different reaction condition, recently [14–16]. However, precious metal nanoparticles are not still cost effective and limited for practical applications. Hence, coupling of TiO2 with cost effective metal oxides such as p-type Cu2 O and CuO is significant towards reduction reaction. Moreover, Copper oxides based catalysts have been also given much attention because of low-cost and abundant, nontoxic, easily prepared materials. The design and fabrication of suitable composite catalysts from Cu2 O have been reported recently [17–19]. Huang et al. synthesized Cu2 O octahedrons on h-BN nanosheets for p-nitrophenol reduction [20]. Guo et al. used Cu2 O/Au catalyst for reduction of 4-nitrophenol [21]. Moreover, CuO composite catalyst also applied as a catalyst for reduction of nitroarenes [22,23]. However, there is no report on p-type Cux O oxides inside combined with n-type TiO2 outside aiming for the development of p-n het-
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erojunction and changing inactive TiO2 into active for the purpose of reduction reaction. Recently, our group used heterostructured Ag-deposited SiO2 @TiO2 composite spheres to enhance catalytic activity towards photo degradation of AB 1 dye [24]. In addition to this, the doublelayered p-type Ag2 O inside and n-type TiO2 outside had been also applied for rapid reduction of 4-NP [25]. Herein, we designed the composite catalyst from p-type Cu2 O and in situ generated CuO, and coated with n-type TiO2 semiconductor outside on SiO2 sphere as a support. The formation of p-n heterojunction in the composite is used for electrons and hole separation. The inactive n-type TiO2 semiconductor becomes active after combined with p-type Cux O towards reduction of nitro aromatic compounds, which is the aim of this work. The electron and hydride ion released from BH4 -are responsible for the reduction of aromatic compounds. Hence, our design with low cost metal oxide towards reduction was successful. The 2-NA and 4-NP compounds were selected as a model reaction to test the catalytic activity of the composite catalyst. 2. Experimental methods 2.1. Chemicals All chemicals used in this work were analytical grade and used without further purification. 2.2. Synthesis of SiO2 spherical particles SiO2 spherical particles were fabricated according to our previous work [25]. Briefly, 1.2 gm of CTAB was added into a mixture of 240 mL DI water and 160 mL absolute ethanol and ultrasonicated for 1 min. Then, 3.2 mL of ammonium hydroxide was dropped into the reaction mixture and stirred about 30 min. Subsequently, 4 mL of TEOS was added drop by dropand stirred for another 2 h. The resulting white precipitate was centrifuged, washed, and dried. Finally, the powder was calcinated at 550 ◦ C for 3 h and the SiO2 spherical particles were obtained. 2.3. Deposition of Cu2 O nanoparticles on the surface of SiO2 sphere To deposit Cu2 O nanoparticles on the surface SiO2 sphere, calcinated SiO2 (0.2 gm) were first dispersed into 20 mL of distilled water. Then, 2 mL of CuCl2 (0.1 M) solution was dropped into the dispersed solution and stirred for 10 min. Subsequently, 4 mL of NaOH (0.3 M) solution was added into the mixture drop by drop and the solution was changed to light blue color. After another 10 min stirring, 2 mL of N2 H4 ·H2 O (0.1 M) solution was added and the color of the mixture was changed to orange. The mixture was aged for 10 min under magnetic stirring and recovered by centrifugation, washed, and dried. Finally SiO2 @Cu2 O composite was obtained. 2.4. Coating of SiO2 @Cu2 O with TiO2 nanoparticles To coat TiO2 on SiO2 @Cu2 O composite sphere, 0.1 g of SiO2 @Cu2 O was added to 4 mL of isopropanol and 0.5 mL of dodecane mixture and treated with ultrasonication for 15 min. After ultrasonication, 0.1 mL of titanium isobutoxide (Ti(i-BO)) solution was dropped in to reaction mixture. Then, 0.1 mL of DI water was added and stirred for 4 h at room temperature. The resulting precipitate was collected by centrifugation and washed with ethanol. The composite of sphere was then annealed at 450 ◦ C for 3 h in air. Scheme 1 shows the schematic synthetic procedure and reduction of 2-NA and 4-NP.
Fig. 1. XRD patterns of (a) SiO2 sphere, (b) SiO2 @Cu2 O, (c) SiO2 @TiO2 , and (d) SiO2 @Cux O@TiO2 composite catalysts.
2.5. Characterizations The surface morphology and microstructural analysis of the composites were conducted by field-emission scanning electron microscopy (FE-SEM, JSM 6500F, JEOL, Tokyo, Japan) and highresolution transmission electron microscopy (HRTEM, Tecnai F20 G2, Philips, Netherlands). The element mapping was also examined by HRTEM. The X-ray diffraction (XRD) patterns of the catalysts were checked by a Bruker D2-phaser diffractometer using Cu K␣ radiation ( = 1.5418 Å). The X-ray photoelectron spectroscopy (XPS) measurements of the composite powders were done on a VG ESCA Scientific Theta Probe spectrometer with Al K␣ source. The UV–vis absorbance spectra of the composite and the reduction of 2NA and 4-NP were monitored by JASCO V-670 UV–vis-near-infrared spectrophotometer. 2.6. Catalytic ActivityTest The reductions of 2-NA and 4-NP were done according to the following procedure. Briefly, 8 mL of 2-NA aqueous solution (200 ppm) was added into a beaker containing 72 mL of DI water. Then, freshly prepared 8 mL of aqueous NaBH4 (0.1 M) was added. Then, 10 mg of catalyst was added into the reaction mixture under stirring at room temperature. The reduction progress was monitored by taking small portion of the supernatant concentration at different interval of time. Measurements were carried out using a JASCO V-670 UV–vis spectrophotometer. For reduction of 4-NP, the procedure was also similar to 2-NA. 3. Results and discussion 3.1. Characterization The XRD pattern of the as-prepared SiO2 , SiO2 @Cu2 O, SiO2 @TiO2 , and SiO2 @Cux O@TiO2 heterostructure samples were characterized. As it is observed from Fig. 1a and b, the SiO2 and SiO2 @Cu2 O spherical particles were amorphous. The amorphous phase of SiO2 @Cu2 O composite catalyst in Fig. 1b was because of the amorphous background of SiO2 that covers the intensities of small amount of Cu2 O nanoparticles deposited on it. However, the TiO2 peaks deposited on SiO2 for comparison purpose were appeared in an anatase phase (Fig. 1c). Moreover, the crystalline structures of SiO2 @Cux O@TiO2 heterostructure were also characterized after calcination of the composite at 450 ◦ C for 3 h. As shown from Fig. 1d, the anatase phase of TiO2 (1 0 1) planes was observed
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Scheme 1. Schematic illustrations of SiO2 @Cux O@TiO2 composite catalyst synthesis and catalytic applications for reducing 2-NA and 4-NP.
Fig. 2. High-resolution XPS spectra of (a) Ti 2p, (b) Cu 2p, (c) O 1s, and (d) Si 2p for SiO2 @Cux O@TiO2 composite catalyst.
(JCPDS No. 21-1272). However, the peaks represents for Cu2 O or CuO nanoparticles were not detected because Cu2 O or CuO peaks with low intensity were enclosed under amorphous SiO2 background and under TiO2 peaks. For this reason, XPS analysis to detect the presence of Cu(I) or Cu(II) in the composite is needed. The XPS analysis was performed to investigate the surface elemental composition and the chemical valence states of SiO2 @Cux O@TiO2 composite catalyst. The Ti, Cu, O, and Si elements were detected from XPS survey spectrum in the composite
catalyst. The binding energies of XPS spectra for Ti 2p3/2 and O 1s were observed at about 458.27 eV (Fig. 2a) and at 530.05 eV (Fig. 2c), respectively. The binding energies have a good agreement with the reported values of pure anatase [26,27]. Moreover, the high-resolution XPS spectrum of Cu 2p regions of the composite microspheres showed a representative curve with the binding energy of Cu 2p1/2 centered at 952.48 eV and the binding energy of Cu 2p3/2 centered at 932.50 eV (Fig. 2b). Our result revealed that Cu2 O nanoparticleswere formed which is also similar to the values
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Fig. 3. Scanning electron microscopy (SEM) images of: (a) pure SiO2 microspheres, (b) SiO2 @Cu2 O, and (c) SiO2 @Cux O@TiO2 composite catalyst; and (d) The EDS spectrum for SiO2 @Cux O@TiO2 composite catalyst.
reported in literature [28]. Furthermore, additional CuO nanoparticle was also detected during calcinations of TiO2 . The binding energies of CuO were observed at ∼934.38 eV for Cu 2p3/2 and at ∼953.56 eV for Cu 2p1/2 , in a good agreement with reported for values Cu++ in CuO [29]. Moreover, the binding energy of Si 2p XPS spectrum at 103.14 eV identified the formation of Si in SiO2 (Fig. 2d) [30]. Fig. 3a–c shows the typical SEM images of SiO2 , SiO2 @Cu2 O, and SiO2 @Cux O@TiO2 microsphere composites. As it is observed from Fig. 3a, the SiO2 spherical particles were uniform and had spherical smooth surface. The SEM image of SiO2 @Cu2 O showed the smaller Cu2 O nanoparticles deposited on the surface of SiO2 sphere and it was clearly observed from the inset of Fig. 3b. Furthermore, TiO2 nanoparticles were also uniformly coated on SiO2 @Cu2 O spherical particles (Fig. 3c). The presence of Ti, Cu, Si, and O elements in SiO2 @Cux O@TiO2 composite catalyst was shown by energy dispersive X-ray spectroscopy (EDS) analysis in Fig. 3d. Fig. 4a–c displays the TEM images at low, medium, and high resolutions of SiO2 @Cux O@TiO2 heterostructures, respectively. It is clearly shown that the TiO2 and Cu2 O or CuO nanoparticles were uniformly distributed and their morphologies can be viewed as the nanoparticles decorating on the surfaces of porous SiO2 spheres. The HRTEM image of SiO2 @Cux O@TiO2 composite further revealed that the d-spacing of 0.35 nm was attributed to anatase TiO2 (101) nanoparticle (Fig. 4d), indicating that TiO2 was well deposited and crystallized on the SiO2 sphere.
Fig. 5 demonstrates Cu2 O or CuO, and TiO2 nanoparticles distributions on the surface of amorphous SiO2 sphere. Fig. 5a shows the high-angle annular dark-field scanning TEM (HAADF-STEM) image of SiO2 @Cux O@TiO2 composite catalyst. Fig. 5b–f shows the elemental mapping of Si-K, O-K,Cu-K, Cu-L, and Ti-K, respectively, from the selected area of HAADF-STEM image. The elemental mapping of Si in Fig. 5b together with O in Fig. 5c shows the availability of SiO2 spherical particles in the composite. The element mapping of Cu in Fig. 5d and e also confirmed the uniform distribution of CuO or Cu2 O on SiO2 sphere. Fig. 5f shows the Ti elemental mapping. The elemental mapping of O in Fig. 5c contributes from SiO2 , Cu2 O, CuO, and TiO2 in the composite catalyst.
3.2. Catalytic activity test The catalytic reduction of 4-NP and 2-NA with NaBH4 has often been used as a model reaction to evaluate the catalytic performance of nanoparticles. Although reduction of 4-NP and 2-NA is thermodynamically feasible with NaBH4 , it is known that reduction does not spontaneously occur without catalyst due to the presence of a kinetic barrier [31,32]. The reduction reactions of both 4-NP and 2-NA compounds were easily achieved due to the formation of a single product in each case and were determined by measuring the change in UV–vis absorbance at different interval of reaction time [4].
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Fig. 4. (a) Low-magnification, (b) medium-magnification, and (c) high-magnification TEM images, and (d) HRTEM images of SiO2 @Cux O@TiO2 composite sphere.
Fig. 5. (a) HAADF STEM image and elemental mapping of (b) Si-K, (c) Cu-K, (d) Cu-L, (e) Ti-L, and (f) O-K.
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Fig. 6. (a) Absorption spectra of 4-NP before and after the addition of aqueous NaBH4 solution. (b) and (c) The time-dependent reductions of 4-NP after addition of SiO2 @Cux O@TiO2 , and SiO2 @Cu2 O composite catalyst, respectively. (d) Plots of Ct /C0 versus reaction time (s) for the reduction of 4-NP with SiO2 @TiO2 , SiO2 @Cu2 O, and SiO2 @Cux O@TiO2 . (e) Plots of ln(Ct /C0 ) versus reaction time (s) for SiO2 @Cu2 O and SiO2 @Cux O@TiO2 composite catalysts.
Fig. 6 illustrates the reduction reaction of 4-NP towards 4-AP. As it is shown from Fig. 6a, the initial aqueous solution of 4-NP provides a characteristic peak centered at about 317 nm. However, addition of NaBH4 causes the peak shifts to 400 nm which is attributed to the formation of 4-nitrophenolate ions under alkaline condition [33]. Moreover, the time taken for the conversion of 4-NP into the corresponding 4-AP was noted based on the disappearance of the peaks at 400 nm along with a simultaneous appearance of the peaks at about 300 nm after addition of catalyst [34,35]. Fig. 6b and c shows the time dependent reduction of 4-NP by SiO2 @Cu2 O and SiO2 @Cux O@TiO2 composite catalysts, respectively. The kinetics of the reduction of 4-NP in the presence of different catalysts was demonstrated with Ct /C0 ratio in terms of reaction time, where C0 referred to the initial concentration absorbance of 4-nitrophenolateat 400 nm. Fig. 6d shows Ct /C0 versus reaction time for reduction of 4-NP in the presence of SiO2 @TiO2 ,
SiO2 @Cu2 O, and SiO2 @Cux O@TiO2 composite catalysts. The presence SiO2 @TiO2 catalyst has no effect towards reduction of 4-NP which implies that TiO2 is not active without UV light source due to its higher band gap energy [36]. However, the presence of SiO2 @Cu2 O catalyst showed relatively better reduction ability due to lower band gap of Cu2 O and 72% of 4-NP was completed within 450 s. Hence, SiO2 @Cu2 O can accept and transfer electrons from NaBH4 towards nitroaromatic compounds at room temperature. Furthermore, SiO2 @Cux O@TiO2 composite exhibits much better catalytic performance toward reduction of 4-NP in to corresponding 4-AP and the reaction was completed within 210 s. The formation of p-n junction between p-type Cux O nanoparticles and thin layered n-type TiO2 semiconductors in SiO2 @Cux O@TiO2 composite probably facilitates the reduction reaction by decreasing the hole and electron recombination rate. In addition to this, the electrons released from BH4 − ion together with the hydride ion
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Fig. 7. (a) and (b) The time-dependent successive reductions of 2-NA after addition of SiO2 @Cux O@TiO2 and SiO2 @Cu2 O composite catalyst, respectively. (c) Plots of Ct /C0 versus reaction time (s) for the reduction of 2-NA with SiO2 @TiO2 , SiO2 @Cu2 O, and SiO2 @Cux O@TiO2 . (d) Plots of ln(Ct /C0 ) versus reaction time (s) for SiO2 @Cu2 O and SiO2 @Cux O@TiO2 catalysts.
attached on the surface of the catalyst makes the reduction reaction easier and effective. Fig. 6e shows the linear relationships between ln(Ct /C0 ) and the reaction time for SiO2 @Cu2 O and SiO2 @Cux O@TiO2 composite catalysts. The linear relationship of ln(Ct /C0 ) versus reaction time follows a pseudo-first-order kinetics reaction [37]. Considering the concentration of NaBH4 is much higher than 4-NP, the rate constantis calculated from the slope of the linear relationship between ln(Ct/ C0 ) and reaction time. The apparent rate constants (kapp ) of the as-prepared SiO2 @Cu2 O and SiO2 @Cux O@TiO2 catalyst were 0.0036 s−1 and 0.025 s−1 , respectively. The catalytic efficiencies of our catalyst also applied for reduction of 2-NA. Fig. 7a and b shows successive catalytic reduction reaction of 2-NA with SiO2 @Cux O@TiO2 and SiO2 @Cu2 O composite catalyst, respectively. The 2-NA showed a typical maximum absorbance peak at about 412 nm and gradually decreased as it was reduced into 1,2-benzenediamine. This gradual decrease in the peak intensity can be used to determine the progress of the reduction reaction [32]. As it is observed in Fig. 7c, the catalytic reduction of 2-NA into 1,2-benzenediamine with SiO2 @TiO2 is insignificant while SiO2 @Cu2 O has better catalytic capability, which is also similar with reduction of 4-NP. However, in the presence of SiO2 @Cux O@TiO2 catalyst, the reduction of 2-NA completed within 150 s. It could be illustrated that the reduction of 2-NA is also pseudo-first-order kinetics reaction at relatively higher concentration of NaBH4 . In a similar fashion with the rate constant of 4-NP, the catalytic rate constant could be evaluated from the curves plotted for the concentration of 2-NA ln(Ct /C0 ) against reaction time. Fig. 7d shows the linear relationship built between ln(Ct /C0 ) and time (s) for reduction of 2-NA. The corresponding kapp values calculated from the linear slope were 0.018 s−1 and 0.0026 s−1 for SiO2 @Cux O@TiO2 and SiO2 @Cu2 O catalysts, respectively.
Recently, reductions of nitro aromatic compounds have been done by using metal oxide. The p-type metal oxides such as Cu2 O, CuO, Fe2 O3 , and NiO were active but n-type metal oxides such as TiO2 and ZnO were inactive towards reduction of 4-NP [20,36]. Researches had been also conducted by coupling of TiO2 with Au, and Ag metal nanoparticles towards reduction of 4NP. Furthermore, using precious metal nanoparticles with n-type semiconductor active, is not cost effective [15,38]. The catalytic reduction towards 4-NP by SiO2 @Cux O@TiO2 catalysts is comparable with different catalysts in literatures shown in Table 1 below. The activity parameters used for the comparison of the catalytic activity of SiO2 @Cux O@TiO2 composite catalyst with the reported metal oxides-based and noble metal based catalysts used for the reduction of 4-NP are shown in Table 1. According to the literature, CuO and Co3 O4 catalysts have the activity parameters of about 0.19 and 0.13 s−1 g−1 , respectively [36]. The Ag-NP/C composite and TAC-Ag-1.0 catalysts have also the activity parameters of about 1.69 and 1.3 s−1 g−1 , respectively [39,40]. In the case of SiO2 @Cux O@TiO2 composite catalyst, the ratio activity parameter constant is 2.5 s−1 g−1 , which is comparable with reported literature values. In our design, we combined the cheapest p-type and n-type semiconductors together. The results showed that coupling of ntype TiO2 with p-type Cux O semiconductors was effective and changed the two oxide system catalyst more active than individual semiconductors. Scheme 2 shows the reduction of 4-NP (1) and 2-NA (2) at room temperature (RT). The possible catalytic reaction mechanism for reduction of 4-NP is illustrated in Fig. 8. The mechanism involves with chemisorption mechanism which implies that the nucleophile adsorbs on the surface of the composite catalyst. The electrophile also adsorbs simultaneously on the catalyst surface [41]. Furthermore, the nucleophile (i.e., BH4 − ions) donates electrons to the electrophile
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Table 1 Comparison of the catalytic capacities of various catalysts reported in literature for the reduction of 4-NP to 4-AP by NaBH4 . Catalyst/amount
Time Taken (s)
Kinetic rate constant, kapp (s−1 )
Ratio constant, K (s−1 g−1 )
Reference
Ag-NP/C composite/1.0 mg CuO/0.1 g Co3 O4 /0.1 g Cu2 O–Cu–CuO/1 mg TAC-Ag-1.0/4 mg SiO2 @Cu2 O@TiO2 /10 mg
1500 40 120 180 415 210
0.00169 0.019 0.013 0.0156 0.00519 0.025
1.69 0.19 0.13 15.6 1.3 2.5
[39] [36] [36] [9] [40] This work
Scheme 2. Reductions of 4-NP and 2-NA.
Fig. 8. The schematic reaction mechanism for the reduction of 4-NP with SiO2 @Cux O@TiO2 composite catalyst.
(4-NP) through the catalyst support. The aqueous medium also provides the required H+ ions to complete the reduction reaction. Moreover, the p-n junction and electric field formation between ptype Cux O and n-type TiO2 will also enhance the adsorption of the reactants on the surface of the catalyst. Even without the light illumination, the narrow band gap Cux O with Cu2 O (2.1 eV) and CuO (1.2 eV) can be thermal excited by local temperature in the near infrared light to produce electron-hole pairs. Those electrons will be electrically driven to the TiO2 side for enhancing the catalytic reduction. With the absorbed H+ ions on the catalyst surface, those thermally generated electrons on TiO2 layer with the electrically driven electrons can facilitate the electrons released from NaBH4 to survive, stay, and migrate for reduction reaction. Together with the survival electrons and protons, the amino functional group of 4-NP is discharged and 4-NP transforms to 4-AP. Fig. 8 shows the schematic reaction mechanism for the reduction of 4-NP. The stability of SiO2 @Cux O@TiO2 composite catalyst was tested by using 2-NA as a reactant. The catalyst was reused three-times towards reduction of 2-NA after centrifuged, washed, and dried for each catalytic run. As it is observed from Fig. 9, the reduction of 2NA into 1,2-benzenediamine was completed at about 5 and 7.5 min for the second and third cycles, respectively. Although the catalyst works properly for second and third run, the reaction rates were not as fast as the first run. It is suggested that the catalytic activities of the catalyst decreases as the recycling increases. This could be ascribed to the reduction the activity site of catalysts [20,23]. Therefore, further work is needed to make the catalyst to be as good as the firstcycle. Moreover, the high-resolution XPS spectrum for Cu 2p in the composite after three-time reuse is illustrated in Fig. 10. The peaks with the binding energy of Cu 2p1/2 at 952.0 eV and the binding energy of Cu 2p3/2 at 932.01 eV are observed. This shows the binding energies of Cu+ or Cu (0), which are very close to each other and
Fig. 9. Reusability of the catalyst for reduction of 2-NA into 1,2-benzenediamine.
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and by the National Taiwan University of Science and Technology through Grant 105H451714. References
Fig. 10. XPS spectrum of Cu 2p after three-time use for catalytic reduction of 2-NA.
difficult to identify by XPS. However, the binding energy of Cu++ , like the catalyst before use in Fig. 2b, was not observed. Hence, reduction of Cu++ into Cu+ or Cu (0) probably occurred because of the effect of NaBH4 as a reducing agent during reaction. These phenomena were also reported in previous reports [9,20]. Furthermore, the disappearance of the characteristic shakeup satellite peak, peculiar to the Cu(II) species [41], indicates the reduction of the Cu++ species. 4. Conclusions The SiO2 @CuxO@TiO2 heterostructured composite catalyst has been designed and fabricated by a facile method with the concept of p-n heterojunction formation for effective electrons and hole separation. The composite catalyst was characterized by different techniques. The catalytic efficiency of the composite was tested for reduction of 2-NA and 4-NP at room temperature. The SiO2 @Cux O@TiO2 composite catalyst took 210 s and 150 s for complete reduction of 4-NP and 2-NA, respectively. Results shown in this study clearly demonstrated that the formation of p-n junction between p-type Cux O and n-type TiO2 and the outward electrons from n-type TiO2 together with hydride ion, synergistically, make the catalyst highly active. The plausible mechanism for the catalytic reduction of nitroarenes also shown. This work provides a cheap and new two-oxide catalyst system towards reduction of nitroarenes in aqueous solutions, and may also have a possible application in industrial production. Acknowledgments This work was supported by Ministry of Science and Technology of the Republic of China under Grant MOST104-2218-E-011-007
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