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Urchin-Inspired TiO2@MIL-101 Double-Shell Hollow Particles: Adsorption and Highly Efficient Photocatalytic Degradation of Hydrogen Sulfide Haibo Sheng, Dongyun Chen,* Najun Li, Qingfeng Xu, Hua Li, Jinghui He, and Jianmei Lu* Collaborative Innovation Center of Suzhou Nano Science and Technology, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, 199 Ren’ai Road, Suzhou, 215123, P. R. China S Supporting Information *
ABSTRACT: Titanium dioxide (TiO2) is a commonly used photocatalysis for the oxidation of hydrogen sulfide (H2S). However, the low surface area and adsorption ability of TiO2 limit the photocatalytic decomposition rate. Here, a tunable metal− organic framework (MOF) coating is applied to hollow TiO2 nanoparticles using a versatile step-by-step self-assembly strategy. The hollow structure provides a high surface area, and the selected MIL-101 (Cr) MOF has a high and regenerable adsorption ability for H2S. The TiO2@MIL-101 double-shell hollow particles enable a catalytic cycle involving simultaneous adsorption and degradation of H2S, with considerably enhanced photocatalytic reaction rate. This work provides a method for improving photocatalytic performance through the design of hollow MOF-based materials that rationally combine the power of MOF and TiO2.
I. INTRODUCTION Hydrogen sulfide (H2S) is a hazardous and toxic gas with a strong foul odor, even at very low concentrations. At volume fractions of greater than 10−6, H2S seriously damages the human body.1 It also presents threats to the environment as the precursor of acid rain and a cause of global warming.2 H2S naturally occurs in the atmosphere and is widely used in various industries, such as petroleum refining and paper manufacturing. It is even added to natural gas as an odorous compound. There are many techniques for the treatment of H2S;3 these include wet scrubbing, thermal incineration, biofiltration, adsorption, and catalytic decomposition.4−6 Among these methods, photocatalytic decomposition is one of the most efficient and environmentally friendly remediation techniques. Titanium dioxide (TiO2) is frequently used for the photocatalytic oxidation of toxic gases at low concentrations, and there have been many studies on the decomposition of H2S using TiO2 since Canela et al. reported the remarkable effects of TiO2 on this decomposition process.7,8 The presence of sulfate (SO42−) on the TiO2 indicates that H2S decomposition has occurred.9 However, the low surface area and adsorption ability of TiO2 limit the photocatalytic H2S decomposition rate. For these reasons, TiO 2 has been prepared in different morphologies, for example, as hollow nanoparticles, nanofibers, and hierarchical spheres, to increase the specific surface area.10−12 However, even with these modifications, the catalytic performance has not improved sufficiently. Metal−organic frameworks (MOFs), which are synthesized by assembling inorganic subunits with organic linkers such as carboxylates or phosphonates, have attracted great interest. © 2017 American Chemical Society
Their considerable applications in drug delivery, catalysis, gas storage, and selective adsorption can be attributed to their high surface area, porosity, and chemical tenability.13−16 A series of MOFs with excellent adsorption properties for noncorrosive gases, including hydrogen, methane, and carbon dioxide, have been reported. However, because of the acidity of H2S, only a few MOFs are stable toward H2S sorption and easily regenerated. One suitable MOF is MIL-101 (Cr), which is rigid and contains large pores, and this MOF strongly adsorbs H2S. With this MOF, the maximum adsorbed quantity of H2S at 2 MPa is reported to be 38.4 mmol g−1.17,18 To date, there have been many studies on the photocatalytic oxidation of H2S with TiO2 and on the adsorption of H2S by MOFs,19−24 but reports on enhancing the photocatalytic activity of TiO2 by coating it with a MOF to improve H2S sorption remain limited. Herein, we present a general stepwise strategy for the synthesis of urchin-inspired double-shell hollow TiO2@MIL-101 particles that both adsorb and catalytically degrade H2S and thus enhance the photocatalytic activity of TiO2 (see Scheme 1). The H2S conversion ratio reached 90.1%, which improved the catalytic efficiency by 31% and 114% compared to the hollow and bulk TiO2 after 60 min, respectively, and the H2S concentration was reduced rapidly and equilibrium was achieved in approximately 60 min, which is almost half of the time required for the hollow TiO2. In certain aspects, namely, their high surface area, low density, and shell Received: March 27, 2017 Revised: June 12, 2017 Published: June 12, 2017 5612
DOI: 10.1021/acs.chemmater.7b01243 Chem. Mater. 2017, 29, 5612−5616
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the resulting powder was dried in an oven at 60 °C. The PS@TiO2 core−shell particles were then annealed in air at 500 °C for 2 h at a heating rate of 3 °C/min to get the hollow TiO2 particles. Synthesis of TiO2@MIL-101 Double-Shell Hollow Particles. A 0.45 g portion of succinic anhydride was dissolved in 30 mL of DMF, and 1 mL of (3-aminopropyl)triethoxysilane was added gradually. After the mixture was stirred for 3 h at room temperature, 2 mL of deionized water and 10 mL of the suspension of the hollow TiO2 particles in DMF (0.05 g/mL) were added. The mixture was then stirred for a further 8 h. The product was then washed several times with EtOH and collected by centrifugation. The freshly prepared carboxylate-terminated hollow TiO2 was dispersed in 10 mL of EtOH, and 10 mL of an EtOH solution of CrNO3·9H2O (2 mM) was added. After the solution was stirred for 15 min at room temperature, 10 mL of an EtOH solution of terephthalic acid (2 mM) was added, and the mixture was stirred for a further 30 min at 25 °C. The product was washed twice with EtOH and collected by centrifugation. This step was repeated several times, and then the product was dried in an oven at 75 °C. Photocatalytic Oxidation. A 1.6 L batch reactor containing a quartz glass was used for the photocatalytic oxidation of H2S. First, 0.5 g of the prepared catalyst was deposited onto the quartz glass and the reactor was evacuated. Next, 0.8 L of wet ultra zero air and 0.8 L of a mixed gas containing H2S (400 ppm) and nitrogen were introduced into the batch reactor. The UV light was switched on to start the reaction, and the concentration of H2S was analyzed by a gas chromatograph equipped with a flame ionization detector−methanizer. The conversion of H2S was calculated according to eq 1
Scheme 1. Schematic Illustration of the Fabrication of Double-Shell Hollow TiO2@MIL-101 Particles
permeability, hollow structures are superior to solid particles. Therefore, a hard-templating method was used to prepare hollow TiO2.25 Then, a versatile step-by-step strategy was used to synthesize the MOF shell. The MOF shell could be varied by using different framework building blocks.26−28
⎛ C⎞ η = ⎜1 − ⎟ × 100% C0 ⎠ ⎝
II. EXPERIMENTAL SECTION Instrumentation. Transmission electron microscopy (TEM) images were obtained on a FEI Tecnai G-20 transmission electron microscope, and scanning electron microscopy (SEM) was conducted using a Hitachi S-4700 scanning electron microscope. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and scanning transmission electron microscopy (STEM) elemental mapping images were measured on a FEI Tecnai F-20 transmission electron microscope. X-ray diffraction (XRD) was measured on an X’Pert-Pro MPD to determine the crystal phase. Compositional analysis of the TiO2@MIL-101 double-shell hollow particles was performed using X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). Energy-dispersive X-ray (EDX) spectra were obtained using a FEI Tecnai G-20 transmission electron microscope equipped with a Horiba EMAX 6853-H EDX system. Fourier transform infrared (FTIR) spectra were recorded on a Nicolet 4700 spectrometer. The UV−visible (UV−vis) diffuse reflection spectra were measured on a Cary 50 UV−vis spectrophotometer. The percentage of MOF in the catalyst was analyzed by thermogravimetric analysis (TGA, TG209 F1 Libra). Synthesis of Polystyrene Nanospheres. Polystyrene (PS) nanospheres were used as the template and were prepared as follows. In brief, 0.05 g of sodium dodecyl sulfate and 15 g of styrene monomer were added to 80 mL of water with stirring, and the temperature was raised to 80 °C under nitrogen. Then, 20 mL of an aqueous solution containing 0.15 g of potassium persulfate was added dropwise. The mixture was stirred for another 10 h, and the PS spheres were precipitated by addition of sodium chloride. The final product was washed several times with deionized water and dried in an oven at 60 °C. Synthesis of Hollow TiO2 Particles. A mixed ethanol (EtOH)/ acetonitrile (ACN) (3/1, v/v) solvent was dried over anhydrous sodium sulfate for at least 24 h. Then, 0.5 mL of tetrabutyl titanate was added to 20 mL of the mixed solvent and stirred for 10 min. In a separate flask, 0.017 g of PS nanospheres we synthesized was added to 90 mL of the mixed EtOH/ACN solvent, and the suspension was sonicated to disperse the particles. Next, 0.3 mL of ammonia, 0.06 mL of deionized water, and 20 mL of the mixed solvent containing tetrabutyl titanate were added dropwise to the prepared PS suspension in succession, and the mixture was stirred for 1 h at room temperature. The product was washed with EtOH and centrifuged three times, and
(1)
where c0 and c were the initial and measured concentrations of H2S, respectively. The concentration of H2S was measured every 15 min during the experiment.
III. RESULTS AND DISCUSSION Hollow TiO2 was prepared using a hard-templating method, and polystyrene (PS) spheres were used as the template. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images of these highly monodispersed PS spheres are shown in Figure 1a,b. The PS@TiO2
Figure 1. TEM images of (a) polystyrene (PS) spheres and (c) hollow TiO2. SEM images of (b) polystyrene (PS) spheres and (d) hollow TiO2. 5613
DOI: 10.1021/acs.chemmater.7b01243 Chem. Mater. 2017, 29, 5612−5616
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Chemistry of Materials core−shell particles were prepared as previously reported.29 EtOH/ACN (3/1, v/v) was used as the reaction solvent, and the ratio of EtOH/ACN was found to play an important role. A volume ratio of 3:1 resulted in an appropriate diffusion rate for the accumulation of a proper amount of tetrabutyl titanate and its hydrolysate on the surface of the PS spheres. It was also important to ensure that the water in the mixed solvent was removed to prevent tetrabutyl titanate from hydrolysis. The PS@TiO2 core−shell particles were annealed in air at 500 °C to remove the template, and hollow TiO2 particles were obtained. As shown in Figure 1c, a smooth and uniform TiO2 shell with a thickness of about 20 nm was obtained. The SEM image in Figure 1d also demonstrates the synthesis of complete, smooth hollow TiO2 spheres. The (3-aminopropyl)triethoxysilane (APTES) silane coupling agent and succinic anhydride were then used to modify the hollow TiO2 particles to enable the step-by-step selfassembly process of MOFs on the surface. Next, metal ions were coordinated to the resulting carboxylate-terminated hollow TiO2 to initiate MOF growth, and then terephthalic acid (PTA) was added as the organic ligand. This is a similar method to that used to prepare functionalized self-assembled monolayers. With repeated cyclic growth, an MIL-101 shell was obtained. The TEM and SEM images of the TiO2@MIL-101 double-shell hollow particles are shown in Figure 2a,b, in which
Figure 3. XPS patterns of hollow TiO2@MIL-101: (a) survey pattern, (b) Ti 2p narrow spectra, and (c) Cr 2p. (d) EDX of hollow TiO2@ MIL-101.
3b) is also composed of two major peaks; the Ti 2p1/2 and Ti 2p3/2 orbits are located at about 464.92 and 459.03 eV, respectively. The Cr 2p XPS spectrum (Figure 3c) is composed of two major peaks; the Cr 2p1/2 and Cr 2p3/2 orbits are located at about 587.25 and 578.34 eV, respectively. A TEM equipped with an EDX system was used to obtain the EDX spectra of the TiO2@MIL-101 double-shell hollow particles, and Ti, Cr, O, and C peaks were observed at about 0.19, 0.36, 4.31, and 5.39 keV, respectively (Figure 3d). The intensities of the C and O peaks are clearly higher than those of the Ti and Cr peaks; this is because the organic linker is composed of carbon and oxygen. These results thus indicate that the TiO2@MIL-101 doubleshell hollow particles were synthesized successfully. Thermogravimetric analysis (TGA) (Figure S4) was used to quantitatively determine the composition of hollow TiO2 and the TiO2@MIL-101 double-shell hollow particles. Rutile TiO2, synthesized by calcination in air at 500 °C, has high thermal stability, and no weight loss was observed in the TG curves of the hollow TiO2. After the particles were coated with the MOF shell, the content of rutile TiO2 in the core−shell particles decreased to 80 wt %. The TG curves of the TiO2@MIL-101 double-shell hollow particles show that the MOF shell was unstable at temperatures higher than 200 °C because the organic components decompose at high temperatures. However, weight loss was observed for these particles only at high temperatures, which reveals the stable existence of the MOF shell. The hollow TiO2 and TiO2@MIL-101 double-shell hollow particles were used to remove H2S in the presence of oxygen, and demonstrated photocatalytic air purification abilities. The optical properties of the samples were characterized with UV− visible (UV−vis) diffuse reflectance spectroscopy (Figure 4a). The absorption band around 600 nm is attributed to the d−d spin-allowed transition of the Cr3+ centers.30−32 The intercepts of tangents of the plots of (αhν)2 versus the photon energy for hollow TiO2 and hollow TiO2@MIL-101 are shown in the inset of Figure 4a. The estimated band gaps were 3.11 and 2.89 eV for hollow TiO2 and hollow TiO2@MIL-101, respectively, and the photocurrent transient response (Figure S5) also indicates that the MOF shell did not weaken the light-absorption properties of TiO2.
Figure 2. (a) TEM image of hollow TiO2@MIL-101. (b) SEM image of hollow TiO2@MIL-101. (c) HAADF-STEM image of local hollow TiO2@MIL-101 and (d−f) STEM EDX mappings of (d) Ti, (e) Cr, and (f) O.
shells with a number of spherical protrusions were clearly observed. The Brunauer−Emmett−Teller (BET) surface area was 286 m2 g−1. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and STEM EDX mappings are shown in Figure 2c−f and Figure S6, confirming the presence of the MOF layer. In addition, the powder exhibited a color change (Figure S1), which also demonstrates the successful growth of the MOF on the surface of the carboxylate-terminated hollow TiO2. The composition of the TiO2@MIL-101 double-shell hollow particles was determined by X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDX). Figure 3a shows the XPS survey spectrum of the TiO2@MIL-101 double-shell hollow particles. It contains clear peaks corresponding to Ti, Cr, O, and C, which demonstrates the presence of both TiO2 and MIL-101. The Ti 2p XPS spectrum (Figure 5614
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enhances the photocatalytic efficiency. The photocatalytic process under UV irradiation can be explained by two possible pathways. The OH radical that forms on the TiO2 surface can oxidize H2S to H2SO4 [eqs 3−5]. Alternatively, eqs 6 and 7 show another route to the destruction of H2S at TiO2 that considers the highly oxidizing atmosphere inside the reactor.33 Finally, a cycle involving the simultaneous adsorption and catalytic decomposition of H2S, which considerably enhances the photocatalytic degradation of H2S, is generated. Figure 4. (a) UV−vis diffuse reflectance spectra and plots of (αhν)1/2 versus photon energy for hollow TiO2 and hollow TiO2@MIL-101 (inset). (b) H2S conversion of hollow TiO2 and TiO2@MIL-101 double-shell hollow particles.
The photocatalytic results are shown in Figure 4b, and all samples clearly removed H2S. However, hollow TiO2 had a greater photocatalytic effect than the bulk TiO2. The H2S conversion ratio for hollow TiO2 reached 68.8% after 60 min, and it improved the catalytic efficiency by 63% compared to the bulk TiO2 which exhibited a lower H2S conversion ratio of 42.2%. Bulk TiO2 exhibited the lowest photocatalytic rate, and it was expected owing to its low surface area. The TiO2@MIL101 double-shell hollow particles removed H2S at a fastest rate. The H2S conversion ratio was 90.1%, which improved the catalytic efficiency by 31% and 114% compared to the hollow and bulk TiO2 after 60 min, respectively. Furthermore, in the presence of TiO2@MIL-101 double-shell hollow particles, the H2S concentration was reduced rapidly and equilibrium was achieved in approximately 60 min, which is almost half of the time required for the hollow TiO2. This greater purification ability is caused by the MOF shell, which has a high surface area and H2S-adsorption ability. The photocatalytic performance was still excellent after reusing for four times without any regeneration process (Figure S7). These results demonstrate that TiO 2 @MIL-101 double-shell hollow particles can effectively remove ppm levels of H2S from air, indicating their excellent photocatalytic performance. Scheme 2 shows the reaction mechanism of the photocatalytic H2S removal. Adsorption of H2S by the MIL-101 (Cr) shell gives rise to a high local concentration of H2S. This increases the probability of contact with the catalyst and thus
TiO2 + hv → e− + h+
(2)
h+ + H 2Oads → ·OH + H+
(3)
h+ + OH−superf → ·OH
(4)
H 2S + 8·OH → SO4 2 − + 2H+ + 4H 2O
(5)
O2 + 2e− + 2H 2O → H 2O2
(6)
H 2S + 4H 2O2 → SO4 2 − + 2H+ + 4H 2O
(7)
IV. CONCLUSION Urchin-inspired TiO2@MIL-101 double-shell hollow particles were prepared as a photocatalyst for the removal of H2S and showed excellent purification ability. The H2S conversion ratio reached 90.1%, which improved the catalytic efficiency by 31% and 114% compared to the hollow and bulk TiO2 after 60 min, respectively, and the H2S concentration was reduced rapidly and equilibrium was achieved in approximately 60 min, which is almost half of the time required for the hollow TiO2. The MIL101 coating the surface of the hollow TiO2 enhanced the photocatalytic activity owing to its excellent H2S-adsorption ability. The MOF-based nanostructure was synthesized using a versatile step-by-step self-assembly strategy, which suggests that the MOF shell could be altered by using different framework building blocks. This would allow the adsorption and catalytic conversion of different gases at the same time.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01243. Photographs of hollow TiO2 and hollow TiO2@MIL101; FTIR spectra of hollow TiO2 and hollow TiO2@ MIL-101; XRD patterns of hollow TiO2 and hollow TiO2@MIL-101; TGA curves of hollow TiO2 and hollow TiO2@MIL-101 (PDF); photocurrent transient response and recycling test of hollow TiO2@MIL-101; HAADFSTEM image and EDX mappings of hollow TiO2@MIL101 (PDF)
Scheme 2. Schematic of the Reaction Mechanism for H2S Removal
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (J.L.). *E-mail:
[email protected] (D.C.). ORCID
Jianmei Lu: 0000-0003-2451-7154 Notes
The authors declare no competing financial interest. 5615
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ACKNOWLEDGMENTS This work was supported by the Collaborative Innovation Center of Suzhou Nano Science and Technology. We gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (51573122 and 21301125), the National Key Technology R&D Program (2015BAG20B03-06) and the Science and Technology Program for Social Development of Jiangsu (BE2015637), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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