Al2O3 Nanofibrous Membranes

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Functional Inorganic Materials and Devices

Novel Flexible Self-Standing Pt/Al2O3 Nanofibrous Membranes: Synthesis and Multifunctionality for Environmental Remediation Yan Wang, Sihui Zhan, Song Di, and Xu Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07637 • Publication Date (Web): 12 Jul 2018 Downloaded from http://pubs.acs.org on July 19, 2018

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ACS Applied Materials & Interfaces

Novel

Flexible

Membranes:

Self-Standing

Synthesis

and

Pt/Al2O3

Nanofibrous

Multifunctionality

for

Environmental Remediation Yan Wang,a Sihui Zhan,b Song Di,a Xu Zhaoa* a

Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, P.R. China. b College of Environmental Science and Engineering, Nankai University, Tianjin 300350, PR China *

Corresponding author: E-mail: [email protected]

Abstract: In spite of intensive research investigating the prevalent Pt/Al2O3 catalysts, achieving macroscopic morphology beyond the powder form limitations remains highly challenging. Meanwhile, current impregnation-based preparation approaches show the drawbacks of tedious procedures and inefficient use of noble metals. Therefore, it is important to search for new methods for the fabrication of Pt/Al2O3 catalysts with novel morphology. In this study, a novel Pt/Al2O3 nanofibrous membrane catalyst is fabricated via a facile one–pot electrospinning process. The embedding of Pt nanoparticles is performed simultaneously with the formation of Al2O3 nanofibres. The Pt/Al2O3 membranes show remarkable mechanical properties with tensile stresses as high as 44.14 MPa. Notably, the Pt/Al2O3 membranes exhibit multifunctionality

with

excellent performance

characteristics.

The

catalytic

experiments indicate that 100% of bisphenol A is removed within 60 min, and 100% of CO is completely converted to CO2 at 242 °C when Pt/Al2O3 membranes are used as catalysts. The membranes also exhibit excellent filtration performance, clearly

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decreasing the turbidity of water, and meet the high efficiency particulate air (HEPA) filter standards. The excellent flexibility, satisfying mechanical property and multifunctionality extend the range of potential application of the Pt/Al2O3 membranes. Moreover, the facile synthesis suggests new possibilities for the fabrication of many membrane-form Al2O3-supported catalysts. Keywords: Pt/Al2O3, nanofibrous membrane, electrospun, environmental remediation 1. Introduction As one of the most widely used classic materials in practical applications, alumina (Al2O3) is widely studied as catalyst support owing to its many satisfactory properties, such as low cost, good stability, and nontoxicity.1-3 To date, various kinds of noble metal catalysts, such as Au,4,5 Pt,6 and Pd,7,8 supported on Al2O3 have been prepared. However, the preparation of noble metal-Al2O3 catalysts is mostly carried out by the two-step incipient wetness impregnation, which includes the preparation of the Al2O3 support followed by the loading of the noble metals (as summarized in Table S1).9-12 In spite of the advantages of the impregnation method, several drawbacks of the current procedures still need to be addressed.3 For example, the loading of noble metals is usually incomplete, causing an inefficient use of the noble metals. In addition, the preparation process involves two steps that are tedious and time-consuming. Hence, alternative approaches for the preparation of the noble metal-Al2O3 catalysts are urgently needed. Among the various noble metals, platinum (Pt) is of particular importance owing to its many merits such as outstanding electrical conductivity and good capability to induce catalytic reduction.13-15 Pt-based catalysts possess superior catalytic activity towards various reactions such as CO oxidation and activation of PMS for the


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degradation of BPA.8,11 Owing to these merits, Pt/Al2O3 catalysts have received continuous attention for use in environmental remediation area, including gas and water treatment (as summarized in Table S1).8,16 Nevertheless, the morphology of the catalysts has been limited to the powder form.7,8,17 Considering the elaborate separation process and the potential hazards caused by the possible catalysts loss, the membrane-form catalysts are more favourable in practical applications owing to their ease of operation and separation properties. In addition, the membrane-form catalysts may meet the increasing need for the comprehensive treatment of complex pollution. For example, the car exhaust pollutants contain not only toxic gases, such as CO, but also respirable particulate matter (PM).18-20 The conventional ceramics are quite brittle and have intrinsic cracks that grow under stress, while the membrane catalyst exhibit advantages and show promise in automotive emission treatment.21 In wastewater, the organic pollutants and PM coexist in many cases.22 While Pt/Al2O3 catalysts are effective for CO oxidation, the powder form catalysts cannot filtrate PM.17,23 Conversely, nanofibrous membranes show excellent filtration performance owing to their unique characteristics, such as small fibre diameter and highly interconnected pore structures.24-26 Thus, ideally, the Pt/Al2O3 catalysts should be fabricated in the form of nanofibrous membranes. Motivated by the considerations described above, a novel Pt/Al2O3 membrane-form catalyst was prepared in this work by a one-pot preparation process via the facile electrospinning method. Electrospinning is generally used to fabricate one-dimensional materials.27-29 In this work, we use the rapid evaporation of the solvent to drive the [PtCl6]2- movement to the surface of the fibres during the electrospinning process. The Pt nanoparticles on the surface of the fibres endow the Pt/Al2O3 membranes with good catalytic activity. Meanwhile, good flexibility as well



as high tensile stress of the Pt/Al2O3 membranes were obtained by fine-tuning of the electrospinning conditions. The Pt/Al2O3 membranes exhibit multifunctionality and can catalyse CO oxidization, oxidize organic pollutants with the addition of PMS, and filtrate the fine particulates both in gas and water. The easy preparation process, high tensile stress, good flexibility and excellent multifunctionality of the Pt/Al2O3 membranes mean that these membranes have considerable potential for use in the field of environmental remediation. 2. Experimental Section 2.1 Preparation procedure All materials, including HCOOH, CH3COOH, Al powder, polyethylene oxide ((PEO, Mw=500000), polyvinylpyrrolidone (PVP, Mw=1300000), and H2PtCl6·6H2O, were analytical grade. The electrospinning precursor was prepared by a modification of the procedure reported in our previous work.30 Typically, 30.16 mL HCOOH, 34.29 mL CH3COOH, 86.40 mL H2O and 5.40 g aluminium powder were mixed and stirred in a round-bottom flask for 24 h with refluxing at 80 °C. Next, 10 mL of the reaction solution was placed into a beaker followed by the addition of PEO (0.035 g) and PVP (0.035 g) with stirring. After that step, 216 µL H2PtCl6·6H2O (100 g/L) was dropped to the solution, followed by stirring with 5 minutes to obtain the 1 wt% Pt/Al2O3 electrospinning precursor, and the final membrane was denoted as 1% Pt/Al2O3 membrane. Similarly, 432, 864 and 1296 µL H2PtCl6·6H2O were added to obtain the 2, 4, and 6 wt% Pt/Al2O3 electrospinning precursor, and the final membranes were denoted as 2%, 4%, and 6% Pt/Al2O3 membranes, respectively. The above precursor was electrospun on the electrospinning mechine (SS-2535H, Ucalery Inc., China). A 10.0 mL plastic syringe with the metallic needle with the


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inner diameter of 0.9 mm was used to feed the precursor. The specific conditions were as follows: [push speed] = 0.06 mm min-1, [distance from needle to collector] = 30.0 cm, [applied voltage] = 12 kV. After electrospinning, the precursor membranes were obtained, and subsequently, the membranes were calcined in a muffle furnace. The calcination procedure was performed by heating at the rate of 1 °C min-1 to 500 °C for 2 h, and the corresponding membranes were denoted as Pt/Al2O3-500. Similarly, the Pt/Al2O3-600, -700 and -800 membranes were obtained when the calcination temperatures were 600, 700 and 800 °C, respectively. 2.2 Characterization The morphologies of the fibres were observed by field emission scanning electron microscopy (FESEM, SU-8020, Hitachi, Japan), and scanning transmission electron microscopy (STEM, JEM-2100F, at 200 kV). Thermo-gravimetric and differential scanning calorimetry (TG-DSC) measurements were analysed using a Mettler TGA-1 thermogravimetric analyser in air. X-ray diffraction (XRD) measurements were performed with an X`pert PRO MPD PC system using Cu Kα irradiation (λ = 0.15 418 nm, applied tube voltage = 40 kV, electric current = 40 mA). X-ray photoelectron spectroscopy (XPS) was analysed by a Phi Quantern apparatus, and binding energy is calibrated according to C1s=284.8eV. The tensile stresses of the membranes were measured on an XG-1A tensile tester (Shanghai New Fibre Instrument, 5 mm clamp distance, 1 mm min-1 drawing speed). N2 adsorption-desorption isotherms were investigated using a ASAP 2460 equipment (Version 2.01, Micromeritics Instrument Corporation, USA) , and the samples were degassed in vacuum (40 oC, 15 h) before analysis. CO pulse chemisorption was tested on a AutoChem II 2920 (Micromeritics Instrument Corp., USA) (see the details in Text S1).



The catalytic performance of the Pt/Al2O3 membranes in water was evaluated by the degradation of bisphenol A (BPA) with the addition of peroxymonosulphate (PMS). The catalytic oxidaton of CO was performed in a stainless steel tubular reactor at atmosphereic pressure. The gas filtration performance was evaluated by the DOP method using a TSI model 8130 automated filter tester. Oily dioctyl phthalate (DOP) aerosols (charge neutralized, and with 300 nm mass median diameter) were obtained. The water filtration performance of the membranes was characterized using the Co3O4 nanoparticles with diameters of ~427 nm as the simulated particular matter. The error bars were obtained by carrying out three parallel experiments for each group, and the error range was ≤5%. The details of the above multifunctionality tests are described in the supporting information (Text S1). 3. Results and Discussion 3.1 Fabrication and structure of Pt/Al2O3 fibrous membranes The formation processes of flexible Pt/Al2O3 nanofibrous membranes are presented in Figure 1. The precursor for the electrospun contains evenly distributed Al3+, HCOO-, CH3COO-, H2O, PEO and [PtCl6]2-. During electrospinning, the solvent (H2O) evaporates immediately. Meantime, the hydroxyl and carboxyl groups bonded to aluminium ions further polymerized with the formation of–Al–O–C–polymer,30 while [PtCl6]2- ions migrate to the surface of the fibres. The ambient humidity is vital and is controlled to below 20% during electrospinning. When the humidity is greater than 20%, the solvent (H2O) cannot evaporate rapidly, leading to insufficient migration of [PtCl6]2- to the surface of the fibres. Moreover, only fragile pieces of the xerogel fibres, rather than the flexible membranes, were obtained when the evaporation of H2O was not sufficiently rapid (Figure S1). In contrast, when the humidity below 20%, the membranes show large sizes (50×50 cm2), good flexibility


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and have no defects (Figure S2a). The fibres that compose the xerogel fibrous membranes have smooth surfaces and diameters of ~620 nm (Figure S2b). Examination of the TG-DSC curves (Figure S2c) presents that the weight loss (70%) occurs primarily before 500 °C, which is caused by the removal of H2O and organics from the fibers. Correspondingly, several exothermic peaks attributed to the burning of the organics appeared in the DSC curve. Therefore, the membranes are calcined at temperatures higher than 500 °C to ensure the complete removal of water and organics. During the calcinations, [PtCl6]2- is reduced to Pt nanoparticles owing to the pyrolysis. After the calcinations, a flexible self-standing Pt/Al2O3 nanofibrous membrane with uniformly distributed Pt nanoparticles on the surface of the Al2O3 fibres is obtained.

Figure 1 Fabrication processes of Pt/Al2O3 fibrous membranes.

The morphologies of the Pt/Al2O3 membranes were investigated (Figure 2). As shown in Figure 2a, the membrane was composed of randomly arranged nanofibres, and many inter-connected open pores were observed in the membranes. Moreover, the



membrane was continuous and intact without any cracks (Figure S3a). The influence of calcination temperatures and Pt content on the fiber diameter is small, which were all between 300~400 nm for the 1%-6% Pt/Al2O3 fibers after calcined at 500-800 oC (the detailed analysis are in Text S2, Figure S3 and Table S2). The surface of the Pt/Al2O3 fibres was observed by scanning transmission electron microscopy (STEM). As shown in Figure 2b, highly distributed Pt nanoparticles with small particle size (less than 3 nm) were clearly observed on the surface of the Pt/Al2O3 fibres, which is in contrary to the smooth surface of individual Al2O3 fiber (Figure S4). The agglomeration of Pt particles was not observed on the 2% Pt/Al2O3-500 fibres, while becoming evident with the increase of Pt content (See the details in Text S3, Figure S4). The average Pt particle diameter of 6% Pt/Al2O3 membrane was estimated to be 13.47 nm by the CO pulse chemisorption, which was larger than that observed in STEM images due to the possibly agglomerated Pt particles in the fibers. TEM images further indicate the Pt nanoparticles distribute uniformly in the fibers (Figure 2c,d). The element distributions of the Pt/Al2O3 fibre was analysed by Energy dispersive spectroscopy (EDS). As shown in Figure 2e~h, Three elements including Pt, Al and O, were observed in the fibre. Al and O elements were distributed more uniformly than the Pt element in the fibres because the [PtCl6]2- migrated during the electrospinning process. The weight ratios of Al, O, and Pt in the fibre were 51%, 47.1% and 1.9%, respectively (Figure S5), which were similar to the calculated results (Al: 52%, O: 46.1%, Pt: 2%).


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Figure 2 (a-d) morphologies and (e-h) EDS analysis of 2% Pt/Al2O3-500 fibers. (a) SEM image, (b) STEM image, (c) Low-magnification TEM image, (d) high-magnification TEM image, and (e) EDS layered image, (f) Al K 1, (g) O Kα1, (h) Pt Lα1.

X-ray diffraction (XRD) results were analysed to obtain an insight into the crystal structure of the membranes. To clearly determine the crystal phase of Pt, 6% Pt/Al2O3 membrane was used owing to the easier detection of Pt at higher concentrations. As shown in Figure 3a, no diffraction patterns appeared for the xerogel fibrous membranes, indicating the presence of amorphous structure when the membranes were not calcined. By contrast, four peaks at 39.8°, 46.2°, 67.5° and 81.3° corresponding to (111), (200), (220), (311) crystal face of Pt appeared when the membrane was calcined at 500 °C, indicating that formation of Pt nanoparticles by the pyrolysis of [PtCl6]2- at high temperatures. Meanwhile, the intensity of the peaks gradually increased with the increase of the calcination temperatures (except at 800 °C), suggesting that the Pt nanoparticles grow gradually at higher temperatures. In particular, the Pt peak intensity increased sharply when the calcination temperature



was 800 °C. In addition, the alumina phases also changes as a function of thermal treatment.31-33 As shown in Figure 3a, there is no obvious peaks for Al2O3 when the calcination temperature≤700 oC, while the peaks for γ-Al2O3 (PDF card: 50-0741) appeared when calcined at 800 oC. The results indicate that the alumina phases changed from amorphous (≤700 oC) to γ-Al2O3 (800 oC) as a function of thermal treatment. Table S3 summarized the crystal sizes of Pt calculated according to the Debye-Scherrer formula. The Pt crystal sizes were 2.7, 6.5, 8.2 and 26.2 nm when the membranes were calcined at 500, 600, 700 and 800 °C, respectively. It was reported that the nanosized Pt particles show excellent catalytic performance34,35 and that the catalytic performance is inversely correlated with the particle size.36 Therefore, the membranes calcined at 500 °C should show excellent catalytic performance characteristics. N2 adsorption-desorption isotherms were tested to investigate the BET surface areas of the membranes. As shown in Figure 3b, the isotherms show type-Ⅲ like adsorption when the membrane calcined at 500-700 oC, while show type-Ⅳ like adsorption when calcined at 800 oC. The results indicate that the membranes are nonporous materials when calcined at 500-700 oC, while some mesoporous pores generates when calcined at 800 oC. Correspondingly, There is no noticeable changes on BET surface areas when the calcination temperature increased from 500 to 700 oC, which were 4.32, 8.36, and 6.69 m2/g when calcined at 500, 600, and 700 oC, respectively; while the BET surface area increased to 57.59 m2/g when calcined at 800 oC. The small surface area may be beneficial for the mechanical property of the membranes.


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Figure 3 (a) XRD results and (b) N2 adsorption-desorption isotherms for 6% Pt/Al2O3 calcined at various temperatures. (●: γ-Al2O3 (PDF card: 50-0741),

: Pt (PDF card:

04-0802)) The specific chemical environments of the elements on the surface of the Pt/Al2O3 membranes were identified by X-ray photoelectron spectroscopy (XPS). In the XPS survey spectrum (Figure 4a), four elements namely, Al, O, Pt, C were observed. The C element peak was due to the unavoidable adsorption of CO2 during the experiment, and Al, O, Pt were the main elements of the membranes. In addition, there was no obvious peak of Cl element that may derive form the H2PtCl6 precursor , indicating the extremely low content of Cl in the membranes (see the details in Text S4). The O1s spectrum (Figure 4b) was fitted by two peaks centred at 531.1 and 532.8 eV, corresponding to the oxygen in Al-O-Al and in the surface hydroxyl groups (–OH), respectively.6 In the Al 2p spectra (Figure 4c), the binding energy at 74.2 eV corresponds to Al3+ in Al-O-Al, while the peak centred at 70.8 eV is assigned to the Pt 4f7/2 of Pt0. In the Pt 4d spectra (Figure 4d), the peaks centred at 314.7 and 331.5 eV were assigned to Pt 4d5/2 and Pt 4d3/2 of Pt0.37 Notably, the concentration of Pt on the surface of the fibres was calculated to be 6.5 wt% by XPS analysis, which is considerably higher than the theoretically calculated value (2 wt%). These results further confirmed the migration of [PtCl6]2- to the surface of the fibres during the



electrospinning process.

Figure 4 XPS analysis of 2% Pt/Al2O3-500 (a) XPS survey (b) O 1s, (c) Al 2p, (d) Pt 4d spectra.

Figure 5 shows the optical images of the membranes. It can be observed that the membrane is intact without any defects. The good flexibility of the membranes could be directly observed by bending and twisting the membranes, and the membranes could restore their original shape without any damage after bending and twisting. Moreover, the membrane can support a hanging weight (50 g), clearly showing the good mechanical property of the membranes. Figure 6 presents the membrane’s typical tensile stress-strain curves. As presented in Figure 6a, the tensile stresses of 2% Pt/Al2O3 membranes calcined at 500, 600, 700 and 800 °C were 40.04, 35.96, 33.91, and 22.34 MPa, respectively. A slight decrease in the tensile stress of the membranes was observed when the calcination temperature increased, which may be caused by the gradual increase of the Pt particle size when the samples are calcined at higher temperatures. Figure 6b presents the effect of Pt contents on the mechanical


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property of the Pt/Al2O3 membranes. The tensile stresses were 37.66, 40.04, 43.12, and 44.14 MPa for the membranes with 1%, 2%, 4% and 6% Pt contents, respectively. The differences between the tensile stresses obtained with various Pt contents were very small, and only a slight increase was observed with the increase of the Pt content, which was possibly due to the generation of more grain boundaries at higher Pt content. Remarkably, the tensile stresses of the Pt/Al2O3 membranes are considerably higher than those of the previously reported electrospun inorganic membranes, such as the SiO2 membranes (7.2 MPa),38 Al2O3 membranes (2.98 MPa),30 and TiO2-CeO2 membranes (1.38 MPa),39 and even certain polymer membranes, such as polyamide-56 fibrous membranes (12.33 MPa)40 and PEO@PAN/PSU membranes (9.26 MPa).41 The excellent mechanical properties of the Pt/Al2O3 membranes are highly beneficial for their practical applications.

Figure 5 Optical images of 2% Pt/Al2O3-500 membranes.



Figure 6 Tensile stress-strain curves of (a) 2% Pt/Al2O3 membranes at various calcination temperatures and (b) Pt/Al2O3 -500 membranes with various Pt contents.

3.2 Potential applications of the Pt/Al2O3 membranes In the field of environment remediation, Pt/Al2O3 are the prevalent catalysts for water treatment and exhaust emission control.8,17 Meanwhile, the electrospun fibrous membranes exhibit great potential as fine particulate filtration media.42,43 Hence, the electrospun Pt/Al2O3 fibrous membranes should possess multifunctionality in environment remediation. In this study, four types of potential applications of the prepared Pt/Al2O3 membranes were investigated, namely, catalysis of organic pollutants degradation, catalysis of CO oxidation, and water and gas filtration of fine particulates. The catalytic performance of Pt/Al2O3 membranes for organic pollutants degradation was investigated by degrading Bisphenol A with the addition of peroxymonosulphate. Bisphenol A (BPA) is typical organic pollutant that is frequently detected in water (see the details in Text S5).44 And peroxymonosulphate (PMS) is usually used as the catalytic promoter in the degradation of organic pollutants.7,8,45 Ultrapure water (Liyuan-30S, Beijing Liyuan company, China) was used to prepare the simulated wastewater. The properties including the salt


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concentrations of the ultrapure water were listed in Table S4. It can be seen that the salt concentrations were very low, which scarcely affected the degradation process. Prior to the main experiments, the adsorption capacity of the Pt/Al2O3 membranes toward BPA were investigated. As shown in Figure S6, there was negligible adsorption of BPA, and the adsorption equilibrium was reached within 30 min. The poor adsorption capacity of the membranes was due to the small surface area of the membranes. The catalytic performance of the Pt/Al2O3 membranes with different calcination temperatures were investigated. As presented in Figure 7a, the removal efficiencies of BPA by the 2% Pt/Al2O3 membranes calcined at 500, 600, 700 and 800 °C were 100%, 96%, 69%, and 12% at 90 min, respectively. The removal efficiency of BPA gradually decreased when the calcination temperature increased from 500 °C to 700 °C, while it decreased significantly when calcined at 800 °C. It can be observed from the XRD analysis described above (Figure 3 and Table S3) that the crystal size of Pt increased with increasing calcination temperature. The opposite tendency of the catalytic performance and the Pt sizes with the calcination temperatures is in accordance with previous reports which indicated that catalytic performance is negatively correlated with the Pt particle size.36 Hence, these results confirmed that the Pt/Al2O3 membranes calcined at 500 °C possess the best catalytic performance. Figure 7b shows the effect of the Pt content on the catalytic performance of the Pt/Al2O3 membranes. The removal percentages of BPA by the membranes with 0%, 1%, 2%, 4%, 6% Pt content at 60 min were 10%, 28%, 78%, 89% and 100%, respectively. BPA was completely removed at 90 min when the Pt content is 2%-6% in the membranes. The degradation rate of BPA increased with the increase of the Pt content owing to the greater number of active sites present in the membranes with



higher Pt content. Notably, the catalytic activities for the activation of PMS to degrade BPA by 2%-6% Pt/Al2O3-500 membranes are superior to those of the powder form catalysts reported recently (summarized in Table S5). For example, BPA was 95.2% removed within 60 min by CuFe2O4,44 and was totally removed in 60 min by Ag/mpg-C3N4 under visible light irradiation.46

Figure 7 Removal efficiencies of BPA using (a) 2% Pt/Al2O3 membranes with different calcination temperatures and (b) Pt/Al2O3-500 membranes with various Pt contents. Reaction conditions: bisphenol A 20 mg/L, PMS 2 mM.

The reuse property of the 6% Pt/Al2O3-500 membrane for BPA degradation was investigated, the Pt/Al2O3 membranes was regenerated by calcinations at 500 oC to remove the adsorbed intermediates after each use. As shown in Figure S7, the removal percentages of BPA at 45 min were 99% in the first cycle, 87% in the second cycle, and 53% in the third cycle, respectively. The removal rate gradually decreased with the increase of recycle number. The Pt nanoparticles may grow up during recalcinations, thus leading to to the decrease of catalytic ability of the Pt/Al2O3 membranes during resue. The decline of the catalytic ability of the catalysts on the degradation of organics in water was also observed in previous reports.8,45


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To investigate the catalytic mechanism of the Pt/Al2O3 membranes for BPA degradation, the electron spin-resonance (ESR) spectra as well as radical scavenging experiments were carried out. As shown in Figure S8a, there is no signals such as DMPO-SO4•− or DMPO-•OH observed in the ESR spectra. In contrast, it was reported that signals of DMPO-SO4•− and DMPO-•OH would appear if the catalytic mechanism was based on a free radical reaction, such as that in CuFe2O4/PMS and cobalt phthalocyanine systems.44,47 In the radical scavenging experiments, excess methyl alcohol (MeOH) was added to quench both •OH and SO4•−. Because MeOH could react rapidly with SO4•− (k = 2.5 × 107 M−1 s−1) and •OH (k = 9.7 × 108 M−1 s−1).48,49 It was reported that if the degradation based on free radicals mechanisms, the addition of methanol would inhibit the degradation rate.8,50 In contrast，there was no obvious retardation in BPA degradation when Pt/Al2O3 was used for PMS activation (Figure S8b). These results indicate that free radicals including •OH and SO4•− were hardly generated and participated in the degradation of BPA in this Pt/Al2O3/PMS reaction. The degradation mechanism was more like the nonradical activation mechanism that studied recently.8,51,52 The Pt nanoparticles on the surface of Pt/Al2O3 fibers possess good electrical conductivity. Thus the electrons could transfer from BPA to HSO5- by the media of Pt nanoparticles. Consequently, HSO5- was reduced but without the formation of SO4·-, and BPA was oxidative degraded. Figure 8 shows the catalytic performance of the Pt/Al2O3 membranes for CO oxidation. The membrane with 1 wt% Pt content was chosen as the representative membrane because it has the usual Pt loading amount for CO oxidation.1,35 Notably, the membrane was used directly without trituration or mixing with quartz sand. The trituration and mixture with quartz sand process is usually used for the powder form catalysts in order to enhance the dispersion.35,53,54 Figure 8a shows the results of CO



conversion as a function of the reaction temperature. It can be observed that approximately 50% of CO was converted to CO2 at 220 °C, and 100% of CO was converted to CO2 at 242 °C. The results are consistent with the results obtained for the powder-form Pt/Al2O3 catalysts reported in the literature (Table S6),1,23,36 indicating that the membrane form of the Pt/Al2O3 catalysts does not affect their catalytic activity. Moreover, the catalytic process could run continuously for 26 h with 100% CO conversion (Figure 8b), demonstrating the good stability of the catalytic activity of the membranes. The reused property of the Pt/Al2O3 membrane was also investigated. As shown in Figure S9, there is no noticeable changes for CO oxidation during the cycling test. The 100% CO conversion temperature was maintained at ~242 °C in the first, second and third cycle test, indicating the good recyclability of Pt/Al2O3 membranes for CO oxidation. The good catalytic activity and stability may arise from the high dispersion and strong fixation of the Pt nanoparticles in the fibres.

Figure 8 CO oxidation (a) activity and (b) stability of 1% Pt/Al2O3-500 membrane.

The results for the water filtration performance towards fine particulates of the Pt/Al2O3 membranes are presented in Figure S10. The simulated particles were nanoparticles with uniformly distributed diameters (Figure S10A). The particles were


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highly dispersed in the aqueous solution, as was identified by the laser particle analyser. Only two peaks centred at 170 nm and 600 nm were observed, and the average diameter was 427 nm (Figure S10B). The diameter of the simulated particles is lower than the size of the common microorganisms such as cryptosporidium (4~7 µm) and giardia (1~5 µm) in water. As shown in Figure S10C, the solution containing the simulated particles was cloudy, while it became dramatically clarified after the filtration. The turbidity of the solutions was 232 NTU before filtration, and decreased to 32.1 and 30.4 NTU when after filtration by Al2O3 membrane and Pt/Al2O3 membrane, respectively. It can be seen that both Al2O3 and Pt/Al2O3 membranes exhibit good water filtration performance. The Pt nanoparticles play a small role in the Al2O3 membrane when it used as filtration media. But notably, the membrane morphology of Pt/Al2O3 is superior to the conventional powder form Pt/Al2O3 catalysts. If the Pt/Al2O3 membrane used in water treatment, it can degrade organic pollutants and filtrate fine particulates simultaneously. The filtration performance of the Pt/Al2O3 membranes towards fine particulates in air was tested using the dioctyl phthalate (DOP) aerosols. The DOP method is the internationally accepted method for the evaluation of the filtration performance of a filter. In this work, Pt/Al2O3 membranes were used as self-standing filtration media owing to their high tensile stress, and DOP formed the 300 nm liquid smoke by atomization to represent the fine particulate in air. The 1% Pt/Al2O3-500 membrane, which is the same as the membrane used for CO oxidation was used. As shown in Figure 9, the filtration efficiencies with increased basis weight were 99.838, 99.922, 99.971, 99.986 and 99.991%, and the corresponding pressure drops were 241, 293, 339, 415, and 487 Pa, respectively. The filtration efficiency first rose sharply and later remained steadily above the high value of 99.97%, which is the requirement of the



high efficiency particulate air (HEPA) filter standards.41,55 The pressure drop increased linearly, which was consistent with the results reported in the literature.34 The smaller pressure drop is preferred in practical applications.41 In terms of the HEPA filter standards and the small pressure drop, the membrane with the basis weight of 11.40 g/m2 exhibited satisfactory filtration performance with 99.971% filtration efficiency and 339 Pa pressure drop. The filtration performance of the Pt/Al2O3 membranes is quite high and comparable to the state-of-the-art results reported in the literature (Table S7). Moreover, such membranes as polyvinylidene fluoride,22 polyamide-6,56 polyimide,57 and polyacrylonitrile58 membranes are polymer membranes. It is reported that most polymer membranes could only operate at temperatures below 100 °C, and polyimide nanofibres exhibit higher thermal stability from 25 to 370 °C.57 By contrast, Pt/Al2O3 membranes possess superior thermal stability at temperatures above 500 °C, as could be inferred from the calcination process during their preparation. Hence, the excellent filtration performance suggests that the Pt/Al2O3 membranes are promising candidate materials for fine particulate filtration applications.

Figure 9 Filtration performance of 1% Pt/Al2O3-500 membrane with various basis weights. (face velocity: 30 L/min)


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4. Conclusions In summary, novel Pt/Al2O3 membranes with good flexibility, high tensile stress and multifunctionality were successfully prepared by electrospinning technique. The Pt/Al2O3 membranes are composed of randomly arranged nanofibres, and Pt nanoparticles with diameters less than 3 nm exists evenly on the fibre surface. The membranes exhibit multifunctionality in the field of environment remediation, such as the excellent catalytic performance for organic pollutant degradation and CO oxidation, and good filtration performance for water and gas filtrations. We believe that this facile electrospinning preparation process will extend the opportunities for the synthesis of Al2O3-supported catalysts. Meanwhile, the novel membrane form and the excellent multifunctional performance of the Pt/Al2O3 catalysts will indicate new possibilities for their applications.

Acknowledgements This work was financially supported by projects of the National Key Research and Development Programme of China (No. 2016YFA0203101), National Natural Science Foundation of China (No. 21722702, No. 51708544), and Tianjin Commission of Science and Technology as a key technologies R&D project (No. 16YFXTSF00440).

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35x15mm (600 x 600 DPI)


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Al2O3 Nanofibrous Membranes

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