TiO2 @ Carbon Microspheres Derived from Chitin for Highly

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Pd/TiO2 @ Carbon Microspheres Derived from Chitin for Highly Efficient Photocatalytic Degradation of Volatile Organic Compounds Ling Nie, Bo Duan, Ang Lu, and Lina Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05426 • Publication Date (Web): 07 Dec 2018 Downloaded from http://pubs.acs.org on December 10, 2018

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ACS Sustainable Chemistry & Engineering

Pd/TiO2 @ Carbon Microspheres Derived from Chitin for Highly Efficient Photocatalytic Degradation of Volatile Organic Compounds Ling Nie, Bo Duan, Ang Lu*, Lina Zhang* College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China E-mail: [email protected] (L. Zhang*); [email protected] (A. Lu*); [email protected] (L. Nie), [email protected] (B. Duan) Phone: +86-27-87219274.

Abstract: Facing the serious air pollution caused by the volatile organic compounds (VOCs), the highly efficient degradation and removal of VOCs have attracted great attentions. To develop efficient novel photocatalysts, Pd/TiO2 @ carbon nanofibrous microspheres were constructed by carbonizing the TiO2/chitin microspheres, followed by immobilizing Pd nanoparticles on the TiO2/carbon microspheres via a green pathway. The composite microspheres were composed of carbon nanofibers, on which TiO2 nanoparticles and ultra-small Pd nanoparticles with mean size of 1 nm were well immobilized and distributed, supported by transmission electron microscope and elemental mappings results. The Pd/TiO2 @ carbon microspheres were used for the gas phase photodegradation of VOCs. The results demonstrated that the conversion of toluene and benzene to CO2 and H2O reached 96.4% and 91.7%, respectively, which were higher than the reported values in the literatures. The combination of TiO2 and Pd nanoparticles played an important role on the enhancement of the catalytic activity. Moreover, the Pd/TiO2 @ carbon microspheres could be recycled and reused. This work

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provided a new strategy to eliminate VOCs, which was beneficial to a sustainable world. Keyword: Chitin microsphere, carbon nanofibers, catalyst TiO2 nanoparticles, Pd nanoparticles,

degradation

of

volatile

2


organic

compounds

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Introduction Annually millions of tons of volatile organic compounds (VOCs) in the air are emitted from the household products, industrial process and petrochemicals, endangering both the human health and environment even at very low concentration.14

Besides the most commonly sensed compounds such as benzene and toluene, many

other VOCs are more toxic and carcinogenic,5,

6

causing diseases such as asthma,

allergy and cancer.7-9 Thus, developing effective techniques to eliminate VOCs by the catalyst are of great significance and urgency. VOCs removing methods such as condensation and membrane separation, thermal oxidation, biofiltration and adsorption have been developed,10-14 among which the most recent techniques are the adsorption by the porous materials and the catalytic oxidation by catalysts supported on the porous or composite materials.15 The activated carbon and derivatives are the most frequently utilized adsorbents due to their easy operation and low cost.16,

17

However, the

disadvantages of adsorption such as pore clog, hygroscopicity, and other problems associated with regeneration have been constantly encountered.18-20 The catalytic oxidation is considered as a promising and innovative option to remove VOCs, especially in the small industrial plants with low pollutant concentration in the exhaust gas. Among the catalytic oxidation, TiO2 is an active and popular photocatalyst for the applications in the elimination of diverse organic pollutants due to its photosensitivity, chemical stability, and low cost.21-24 However, TiO2 are often utilized as nanoparticles and easy to aggregate, leading to the decreased photocatalytic efficiency and difficulties in the recovering and recycling.25,

26

Therefore, an effective supporter, which can 3



immobilize the TiO2 nanoparticles evenly, is essential for the successful development of TiO2 photocatalysts with high performance. In order to improve the catalytic site of TiO2, adding the metal nanoparticles is an effective method. Meanwhile, the support material is supposed to effectively fix and disperse the nanoparticles. Renewable chitin from exoskeletons of crabs and shrimps contain plentiful hydroxyl groups and acetylamino groups, which has strong ability to capture the metal nanoparticles.27 In our laboratory, chitin has been successfully dissolved in NaOH/urea aqueous solution at low temperature, from which novel functional materials have been constructed.28, 29 Moreover, the chitin microspheres and their carbon microspheres have been demonstrated to be good supporting materials of the metal catalysts, and chitin is an eco-friendly matrix.30-32 Thus, we attempted to immobilize TiO2 metal catalysts on the nanofibers in the chitin based microspheres for photocatalysis. Our strategy is to immobilize TiO2 nanoparticles on the carbon nanofibers and Pd nanoparticles as cocatalyst to improve catalytic activity. In the present work, the TiO2 @ chitin nanofibrous microspheres (TNCM) were constructed from TiO2/chitin blend solution in NaOH/urea aqueous system with cooling. Subsequently, the microspheres were carbonized to obtain TiO2 @ carbon nanofibrous microspheres (CTNCM), followed by introducing Pd nanoparticles to construct Pd/TiO2 @ carbon nanofibrous microspheres (PCTNCM). In our findings, the nanofibrous structure of the chitin microspheres and the carbonaceous microspheres favored the immobilization and homogeneous distribution of TiO2 and Pd nanoparticles, and supplied plenty of active 4


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sites for TiO2. This would be a simple and green approach to fabricate the photocatalysts to eliminate VOCs with superior catalytic performance. This work is important to solve the hazardous gas wastes, which are harmful to the public health and the environment. Experimental section Materials. Chitin powder was bought from Golden-Shell Biochemical Co., Ltd. (Zhejiang, China). Chitin powder was purified with 5 wt% NaOH, 7 wt% HCl and 4 wt% H2O2 aqueous solutions to remove the impurities such as protein, mineral substance, and pigments. Palladium acetate (Pd(OAc)2, 99%) was purchased from Beijing Bailing Wei Technology Co., Ltd. Sodium borohydride (NaBH4, 98%) was obtained from Shanghai Guoyao Chemical Co., Ltd. Titanium dioxide was bought from Shenshi Chemical Reagents. Other chemical reagents were commercially analytical grade and used without further purification. The water used was deionized water. Fabrication of nanofibrous chitin microspheres. Chitin was dissolved in 11 wt% NaOH/4 wt% urea aqueous solution at 30 oC via the freezing/thawing according to our previous report.33 11 g Span 85 and 250 g isooctane were added to 1 L vessel and stirred at 1000 rpm for 40 min. Then chitin solution was poured to the suspension at 0 °C and stirred for 40 min. Subsequently, the mixture containing 7.5 g isooctane and 6 g Tween 85 was then dropped into the emulsion for 50 min. 80 °C water was added to the vessel to induce the formation of the nanofibrous chitin microspheres, and then the pH value of the suspension was adjusted to 7.0 by addition of diluted hydrochloric acid. After isolating by filtration, the regenerated nanofibrous chitin microspheres were 5



obtained, which were then washed with ethanol and deionized water, respectively. Finally, the nanofibrous chitin microspheres were exchanged with tert-butyl alcohol (tBuOH), followed by freeze-dried, and coded as NCM. Fabrication of TiO2 @ carbon nanofibrous microspheres. TiO2 nanoparticles were dispersed in the transparent chitin solution to obtain the homogeneous TiO2/chitin solution, and then the TiO2 @ chitin nanofibrous microspheres were prepared by the same process of NCM mentioned above. The TiO2 @ carbon nanofibrous microspheres were fabricated by the carbonization of the as-prepared TiO2 @ chitin nanofibrous microspheres under following N2 atmosphere at 500 °C for 2 h in a tubular furnace. After cooled down to 25 °C, the black products were washed with dilute HCl and deionized water, respectively, and then dried at 80 °C for a night to give the TiO2 @ carbon nanofibrous microspheres, coded as CTNCM. Fabrication of Pd/TiO2 @ carbon nanofibrous microspheres. 200 mg TiO2 @ carbon nanofibrous microspheres were dispersed in 20 mL Pd(OAc)2 aqueous solution, and stirred at room temperature for about 12 h, then washed with deionized water for five times, isolated by filtration, followed by the addition of 5 mL 30 mM sodium borohydride aqueous solution. After reacting for 5 minutes, the mixture was filtered to obtain Pd/TiO2 @ carbon nanofibrous microspheres, coded as PCTNCM. Characterization. Scanning electron microscopy (SEM) images were obtained by a field emission scanning electron microscope (FESEM, zeiss, SIG-MA). Transmission electron microscope (TEM) (JEM-2100, JEOL Ltd., Japan) was used for TEM images with an accelerating voltage of 200 kV and the samples were deposited 6


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on a copper grip. Fourier-transform infrared (FT-IR) spectra of the samples were analyzed by a FT-IR spectrometer (FTIR5700, Thermo Scientific, America). The Xray diffraction (XRD) patterns were recorded on a XRD diffraction micrometer (Rigaku Miniflex600, shimadzu, Japan) with Cu-Kα radiation (λ=0.15406 nm). The thermal properties of the samples were carried out on a pyrisdiamond TA Lab system (Q500, TA, America) at a heating rate of 10 °C min-1 from 25 °C to 700 °C under the nitrogen atmosphere. X-ray photoelectron spectra (XPS) (ESCALAB250Xi, Thermo Fisher Scientific, America) analyses were recorded using a Kratos XSAM800 X-ray photoelectron spectrometer. The Brunner-Emmet-Teller (BET) surface areas (Autosorb-2, Quantachrome, USA) were measured and calculated by the nitrogen sorption-desorption isotherms using an adsorption analyzer. The reactants and products were analyzed by a gas chromatography (GC9560, Shanghai, Huaai). Photocatalytic activity measurements. The photocatalytic performance was evaluated according to reference.34 A 125 W high pressure Hg lamp (China, Shanghai Yaming, GYZ, λnm > 340 nm, the light spectrum with peaks around 365, 400, 440, 550 and 580 nm) was used as light source. The intensity of UV light in the region of 320400 nm on the surface of sample was 0.96 mW/cm2. The distance between the sample surface and the lamp was 150 mm. Firstly, the sample was dispersed on a 15 cm glass dish, and put on the continuous-flow closed cylindrical stainless-steel gas-phase reactor with volume of 5.56 L at ambient temperature (See Figure S1). And then, 2 μL of benzene or toluene liquid was injected in the reactor, and the sample was irradiated by the light to up to that the liquid benzene or toluene completely evaporated. 7



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Subsequently, the reactants and products were analyzed by a gas chromatograph every 15 min. The photocatalytic cycle test was carried out for 5 times in the same previous reaction condition. The removal rate (W) of benzene and toluene was calculated as following formulas: W (%) =

c ― c0 c0

× 100%

(1)

Where c was the CO2 concentration at a given time, ppm; c0 referred to the CO2 concentration at the initial time, ppm. Results and discussion Morphology and structure of metal nanoparticles and microspheres. Chitin, as one of the most abundant marine biopolymers derived from seafood waste, is a sustainable polymer material. As shown in Scheme 1, chitin was dissolved in a NaOH/urea aqueous solution with cooling to form a transparent solution. The microspheres, fabricated via emulsion method, could tightly immobilize and uniformly disperse the TiO2 nanoparticles. The Pd nanoparticles were also effectively fixed on the carbon microspheres containing TiO2 nanoparticles. Thus, Pd/TiO2 @ carbon nanofibrous microspheres as the composite catalyst (PCTNCM) were obtained. Figure 1 shows the SEM images of the NCM, CTNCM and PCTNCM as well as their size distribution. A spherical shape of NCM was observed and the size distribution was in the range from 10 to 73 μm with an average diameter of 40 μm. After blended with TiO2 and carbonization, CTNCM maintained the spherical shape, and the average diameter decreased slightly from 40 to 35μm. After introducing Pd nanoparticles, PCTNCM maintained the spherical shape, and had a similar diameter with CTNCM, 8


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suggesting that the introduction of Pd changed hardly the morphology and size of the carbon microspheres. This suggested that the microspheres were good matrix to support the Pd and TiO2 nanoparticles. To clarify the detailed structure of the microspheres, high magnification SEM images were observed, as shown in Figure 2d, e, f. The chitin microspheres were weaved with chitin nanofibers with average diameter of 28 nm without pore wall. It was not hard to imagine that the unique structure of the microspheres could not only favor the evenly distribution of the metal oxide catalyst, but also improve the gas diffusion of VOC and the light transmittance channels, leading to the improvement of the reacting sites and utilization rate of the catalyst. As shown in Figure 2e, the carbon nanofiber from chitin was a good support matrix for the TiO2 catalysts. In view of these results, CTNCM maintained the highly hierarchical porous and interconnected network structures of NCM, and TiO2 nanoparticles with a mean diameter of about 16 nm were tightly immobilized on the carbon nanofibers. Such structure could increase the loading areas of TiO2 nanoparticles, and provide more active sites for the photocatalytic degradation. After Pd loading (Figure 2f), the size and morphology of the TiO2 nanoparticles on the carbon nanofibers were same as those before loading on the whole. The distribution of Pd on the composite microsphere was characterized by the energy dispersive spectrum (EDS) elemental mapping. As shown in Figure 2g, the TiO2 and Pd nanoparticles were uniformly distributed in the microspheres. The results further confirmed the excellent dispersity of TiO2 and Pd nanoparticles as well as the homogeneous structure of the composite microspheres. The size and size distribution of the TiO2 and Pd species in PCTNCM was further 9



investigated, as displayed in Figure 3. Obviously, TiO2 nanoparticles (Figure 3h), with a narrow size distribution (from 11 to 21 nm) and an average size of 16 nm, were tightly immobilized on the carbon nanofibrous networks (Figures 3a and b), suggesting the strong physical interactions between the nanofibers and the metal nanoparticles. The dispersion of ultra-small Pd nanoparticles (Figure 3f) on PCTNCM indicated that PCTNCM provided enough anchoring sites for the ultra-small Pd nanoparticles. This could explain that the strong interactions between Pd and TiO2 with carbon nanofibers occurred.35 Interestingly, the ultra-small Pd nanoparticles (Figure 3i) exhibited a narrow size distribution ranged from 0.5 to 1.7 nm, with an average size of 1.0 nm, and the smaller size would endow the microspheres with efficient photocatalytic degradation of VOC. The porous structure of the catalyst supporter was very important to improve the catalytic efficiency. The pore structure of the NCM, CTNCM, and PCTNCM were analyzed by nitrogen adsorption/desorption measurements. As shown in Figure 4, the good agreement of the absorption and desorption isotherms of both NCM and CTNCM or PCTNCM indicated that the composite microspheres maintained the similar porous structure during the carbonization process. Usually, the specific surface area of the microspheres would increase after carbonization, but that of CTNCM decreased, possibly as a result of occupation of TiO2. Compared with CTNCM, the specific surface area of PCTNCM further decreased. However, PCTNCM still had a specific surface area of 167 m2g-1, which was sufficient for the gas diffusion of VOC and the light transmittance channels. It was worth mentioning that PCTNCM exhibited hierarchical 10


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pore structure. The results of the pore size distribution revealed the existence of the micropores (

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