Electrospun Nanofibers for Catalysts

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ELECTROSPUN NANOFIBERS FOR CATALYSTS

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Ping Lu1, Simone Murray1, Min Zhu2 Department of Chemistry and Biochemistry, Long Island University, Brooklyn, NY, United States1; Textile Development and Marketing Department, Fashion Institute of Technology, Manhattan, NY, United States2

23.1 INTRODUCTION Heterogeneous catalysis is of vital importance to the world’s economy and the sustainable development of our society (Ertl, 2008; Schauermann et al., 2013). It has been estimated that 90% of all chemical processes take place on the surface of heterogeneous catalysts. This contributes approximately 35% of global GDP (Fechete et al., 2012). Nanostructured materials are attractive candidates as heterogeneous catalysts (Kung and Kung, 2004), and include nanoparticles, nanowires, nanotubes, and nanofibers (Panthi et al., 2015b). Indeed, nanoparticles (e.g., platinum (Pt), palladium (Pd), and rhodium (Rh) supported on substrate surfaces (e.g., carbon and oxide) form the basis for the most widely used industrial catalysts (Norskov et al., 2008; Wegener et al., 2011). Recently, electrospinning has received great attention in catalysis research because of its versatility in the production of continuous one-dimensional nanofibers (Rezaee et al., 2017). Furthermore, electrospinning offers an easy way to construct three-dimensional hierarchically porous nanostructures made of nanofibers with diameters in the nanometer range (i.e., 1e1000 nm) (Lu and Xia, 2013a). Fibers on the nanoscale exhibit characteristically different physical and chemical properties that are not shown on a macro scale. For example, nanofibers have a much larger specific surface area than traditional textile fibers, which leads to increased activity in catalytic applications because more active surface is available and accessible by reactants in a chemical process (Lu et al., 2015). Moreover, electrospinning is capable of generating nanofibers from a wide range of organic and inorganic materials (e.g., polymers and oxides) to meet the needs of all kinds of applications. The simplicity of recovery and reuse of catalysts based on electrospun nanofibers makes them economically appealing and environmentally friendly (Astruc et al., 2005). As such, catalysts made of electrospun nanofibers are characterized as a type of “green” catalysts (Shao et al., 2012b; Ren et al., 2012). For instance, recyclable biocatalysts (enzymes immobilized on nanofibers) have been successfully employed in textile treatment, food processing, bioremediation, and protein digestion among other things (Li et al., 2012). When a reaction is carried out in harsh conditions, electrospun nanofibers can provide a suitable microenvironment to minimize enzyme deactivation (Ge et al., 2009). In addition, the immobilized enzymes have a longer lifetime as compared to free enzymes because their chemical structure and conformation are stabilized by the supporting nanofibers. Using traditional Electrospinning: Nanofabrication and Applications. https://doi.org/10.1016/B978-0-323-51270-1.00023-6 Copyright © 2019 Elsevier Inc. All rights reserved.

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metal-based catalysts, metals leaching into the environment are a huge concern in many applications, especially in the large-scale production of pharmaceuticals (Shao et al., 2012a). Attaching metal catalysts to nanofibers has proven to reduce the amount of leaching to an insignificant level, resulting in less product contamination and environmental pollution. Further, recovering the expensive metal catalysts on nanofibers can be as simple as filtering the reaction solution in most cases, or using a magnet for the separation of catalysts in other cases (Shao et al., 2012b; Ghasemi et al., 2015). Recovering and reusing these nanofibrous catalysts have been shown to have little to no effect on their catalytic performance. Nanofibers for catalysts are one of the most important topics in electrospinning applications, and many works have been published in this field in recent years (Wang et al., 2009; Panthi et al., 2015a,b; Hao et al., 2015; Ma et al., 2016; Siqueira et al., 2015; Pei and Leung, 2015; Oktay et al., 2015; Moreno et al., 2015). However, a dedicated review of the recent development, characterization, and design of heterogeneous catalysts based on nanofibers is still not available. This chapter does not attempt to give a comprehensive overview of the subject of nanofibers for catalysts, but concentrates on heterogeneous catalysts based on electrospun nanofibers with significant importance. Using this approach, the chapter begins with a brief introduction to methods for the preparation of nanofibrous catalysts and gives some examples of directly utilizing electrospun nanofibers as active catalysts. It further focuses on discussing catalysts supported on nanofibers and illustrating recent efforts toward the development and design of nanofibrous catalysts with greatly improved stability and activity. The chapter concludes with a personal perspective on the potential of nanofibrous catalysts in various important applications.

23.2 METHODS FOR PREPARING NANOFIBROUS CATALYSTS A plethora of methods have been used to prepare heterogeneous catalysts based on electrospun nanofibers. Generally, these methods can be classified into two categories: encapsulation through an electrospinning process, and postelectrospinning deposition or attachment.

23.2.1 ENCAPSULATION THROUGH ELECTROSPINNING PROCESS Encapsulation is the most straightforward method of producing nanofibrous catalysts, and involves mixing the catalysts or catalyst precursors with polymers to form a homogeneous and viscous solution. The mixed solution is then electrospun into nanofibers while the catalysts, usually tiny particles, are encapsulated inside polymer matrices during electrospinning. The obtained nanofibrous catalysts can be used with or without further treatment. Encapsulation is widely used for entrapping biocatalysts (i.e., enzymes or proteins) in polymer nanofibers to enhance their activity and stability. Nunes et al., (2016) encapsulated naringinase into poly(vinyl alcohol) (PVA) nanofibers by directly electrospinning aqueous buffer solutions containing 10% PVA and different amount of naringinase (Nunes et al., 2016). Naringinase, a hydrolytic enzymatic complex, is useful for biotransformation of steroids and deglycosylation of glycopeptide antibiotics, as well as for hydrolysis of glycolipids, flavonoids, and glycosides. The encapsulated naringinase in PVA nanofibers kept its 100% initial activity after 212 h operation for the hydrolysis of naringin at 25
C. Further chemical cross-linking with phenylboronic acid improved the thermal

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stability of PVA nanofibers in aqueous media up to 121
C, leading to a threefold increase in the overall activity of encapsulated enzymes. Generally, catalyst precursors encapsulated inside electrospun nanofibers require postelectrospinning treatment to generate the desired active catalysts. The simplest method for the conversion of precursor to catalyst is calcination in air. Leung et al. synthesized novel TiO2eZnOeBi2O3 composite nanofibers with enhanced heterojunctions by a solegel-assisted electrospinning method and subsequent high-temperature calcination (Pei and Leung, 2015). Polyvinylpyrrolidone (PVP) was employed as the polymer matrix to encapsulate the precursors: titanium tetraisopropoxide, zinc acetate, and bismuth(II) nitrate. The collected precursor nanofibers were then converted to nanofibrous catalysts made of TiO2eZnOeBi2O3 by simple calcination at 650
C. The resultant TiO2e ZnOe Bi2O3 catalyst showed up to eightfold higher catalytic activity than commercially available TiO2 nanoparticles for the oxidation of nitrogen monoxide (NO). Moreover, the TiO2eZnOeBi2O3 nanofibrous catalyst was more stable in catalytic reaction than pure TiO2 nanoparticles. A reduction procedure can be coupled with the calcination to convert metal salts to metal nanoparticles. Bai et al. reduced the encapsulated PdCl2 in PdCl2epolystyreneepolyacrylonitrile (PdCl2ePSePAN) composite nanofibers to metallic Pd nanoparticles in the presence of hydrogen (H2) gas at 100
C before calcination (Guo et al., 2015). A hybrid catalyst composed of porous carbon nanofibers and encapsulated Pd nanoparticles was obtained after the thermal degradation of PS and the transformation of PAN to carbon at 500
C under a nitrogen (N2) atmosphere (Fig. 23.1).

FIGURE 23.1 Schematic illustrating the encapsulation of palladium (Pd) nanoparticles in porous carbon nanofibers through electrospinning, reduction, and calcination. Modified with permission from Guo, L., Bai, J., Wang, J., Liang, H., Li, C., Sun, W., Meng, Q., 2015. Fabricating series of controllable-porosity carbon nanofibers-based palladium nanoparticles catalyst with enhanced performances and reusability. Journal of Molecular Catalysis A: Chemical 400, 95e103, copyright 2015 Elsevier.

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23.2.2 POSTELECTROSPINNING DEPOSITION In postelectrospinning deposition, nanofibers without active catalysts are prepared by electrospinning, sometimes followed by calcination. Then the catalysts or catalyst precursors are deposited on the preformed nanofibers by various methods, including physical attachment, adsorption through ligand affinity, in situ synthesis/deposition, and chemical reaction to form covalent bonding. Lu and Hsieh fabricated polyacrylamide (PAAm) hydrogel nanofibers with high water absorbency and exceptional tensile strength for the entrapment of b-galactosidase, a large and bulky enzyme that catalyzes the breakdown of lactose sugar. The combination of physical steric hindrance, abundant hydrogen bonds, electric attractions, and other intermolecular interactions jointly entrapped the enzyme molecules in the PAAm nanofibers (Lu and Hsieh, 2009b). In addition, to attach enzymes physically to nanofibers, the same authors developed another method to immobilize enzymes on nanofibers through an affinity ligand. This dye ligand, called Cibacron Blue F3GA, was bound to the surface of cellulose nanofibers by a nucleophilic reaction between the triazinyl chloride in Cibacron Blue F3GA and the hydroxyl groups in cellulose. Because Cibacron Blue F3GA can interact with the active sites of enzymes by mimicking the structures of the substrates, cofactors, or binding agents, the celluloseedye affinity membrane was able to attract lipase, which hydrolyzes triglycerides into fatty acid and glycerol, through this strong specific ligandeenzyme interaction with a loading up to 54.4 mg/g nanofibers (Lu and Hsieh, 2009a). The amount of immobilized lipase was further increased by assembling Cibacron Blue F3GA and lipase into multiple alternating bilayers on cellulose nanofibers via electrostatic layer-by-layer deposition (Lu and Hsieh, 2010). Specifically, the first layer of Cibacron Blue F3GA was covalently bonded to the cellulose nanofibers. In a pH 4 acetate buffer solution, Cibacron Blue F3GA was negatively charged and attracted the positively charged lipase enzyme to construct the first bilayer. Each subsequent bilayer was constructed by repeating the alternating immersions in the anionic Cibacron Blue F3GA and the cationic lipase solutions (Fig. 23.2). Catalyst nanoparticles can also be deposited on electrospun nanofiber supports by in situ synthesis/ deposition. In this method, catalyst precursors, usually metal salts, are reduced to form tiny metal nanoparticles in the presence of electrospun nanofibers. Most of these synthesized catalyst nanoparticles are nucleated or deposited on the nanofiber surface, resulting in a stronger catalystesupport interaction than that of supported catalysts prepared by simple physical attachment. Moreno et al., (2015) synthesized gold (Au) nanoparticles on CeO2 electrospun nanofibers (Au/CeO2) by reducing gold(III) chloride (HAuCl4) with ethanol in a reaction solution containing CeO2 nanofibers (Moreno et al., 2015). The Au nanoparticles had an average size of around 3 nm and were distributed homogeneously on CeO2 nanofibers. Compared to catalysts prepared by deposition of preformed Au on to CeO2, the Au/CeO2 from in situ synthesis/deposition demonstrated better sintering resistance because of their stronger nanoparticleesupport interaction. In addition to solution-phase in situ synthesis/ deposition, catalyst nanoparticles can also be grown on nanofibers through gas-phase synthesis/ deposition such as atomic layer deposition (George, 2010). Zhang et al. deposited zinc oxide (ZnO) on electrospun silk nanofibers with atomic layer deposition by using diethyl zinc and water as precursors (Zhao et al., 2016). Because of the self-limiting nature of atomic layer deposition, the ZnO thickness was well controlled to obtain a series of ZnO/silk nanofibers by simply varying the growth cycles and operating temperatures. Sometimes these two in situ synthesis/deposition techniques can work together to produce catalysts with exotic nanostructures on nanofibers. Kayaci et al., (2014) first coated

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FIGURE 23.2 Schematic of the layer-by-layer (LbL) deposition of Cibacron Blue F3GA (CB) and lipase on cellulose (Cell) nanofibers. Modified with permission from Lu, P., Hsieh, Y.-L., 2010. Layer-by-layer self-assembly of Cibacron Blue F3GA and lipase on ultra-fine cellulose fibrous membrane. Journal of Membrane Science 348, 21e27, copyright 2010 Elsevier.

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a conformal layer of ZnO on electrospun PAN nanofibers with oxygen vacancies and grain boundaries by atomic layer deposition; then single crystalline ZnO nanoneedles were grown on the ZnO layer by a solution-phase hydrothermal process, creating a hierarchical nanostructure with enhanced photocatalytic activity (Kayaci et al., 2014). The last method for attaching catalysts to electrospun nanofibers involves a single step or a series of chemical reactions to link the catalysts and the nanofibers covalently with or without spacer molecules. Li et al. directly immobilized Pseudomonas cepacia lipase on electrospun PAN nanofibers via covalent bonding to catalyze transesterification reactions without a spacer molecule (Li et al., 2011). In detail, the nitrile groups (-CN) on PAN nanofibers were activated by an amidination reaction with dry HCl and absolute ethanol. Upon the completion of activation, the amine groups of P. cepacia lipase reacted with the intermediate imidoester derivative to form a covalent bond between the carbon of PAN nitrile group and the nitrogen of lipase amine group. To minimize the undesirable conformational effect and the steric hindrance due to the extremely short distance between the nanofiber and the catalyst, spacer molecules can be used to distance the tethered enzymes from the underlying surface. Wang and Hsieh, (2004) used poly(ethylene glycol) (PEG) diacylchloride as a spacer molecule to connect cellulose nanofibers and Candida rugose lipase (Wang and Hsieh, 2004). The distance between the enzyme and the nanofiber can be easily controlled by using PEG molecules of different chain lengths.

23.3 ELECTROSPUN NANOFIBERS AS CATALYSTS 23.3.1 POLYMER NANOFIBERS AS CATALYSTS Generally, electrospun nanofibers are used as supporting substrates for the immobilization and incorporation of solid active catalysts such as enzymes, metal nanoparticles, and oxide nanostructures. However, sometimes they can be used directly as catalysts with simple treatment or chemical modification. For example, Shao et al. used electrospun PAN nanofibers functionalized with sulfonic groups as a solid acid catalyst to replace the sulfuric acid in chemical reactions (Shao et al., 2012b). Acids have played significant role in the catalysis of various important chemical processes. However, as an unrecyclable catalyst, homogeneous sulfuric acid can cause serious environmental pollution. By reacting preoxidized PAN nanofibers with chlorosulfuric acid, SO3H-bearing PAN nanofibers were synthesized and used to catalyze acetalization and esterification reactions. In these reactions, the sulfonated PAN nanofibers showed high catalytic activity with up to 99.99% conversion rates and up to 99.80% yields, which are even better figures than for homogeneous H2SO4. The sulfonated PAN nanofibers were recovered from the reaction solution by simple filtration and reused six times without losing any considerable activity.

23.3.2 METAL NANOFIBERS AS CATALYSTS Kim et al. fabricated Pt nanofibers by electrospinning a PVP solution containing hexachloroplatinic acid (H2PtCl6$6H2O), which is a Pt precursor (Kim et al., 2013). The composite nanofibers were then converted to pure Pt nanofibers via thermal annealing at 450
C to remove the polymer matrix and decompose the salt to form metallic Pt. These Pt nanofibers with diameters ranging from 40 to 70 nm were fused together to form an interconnected web, which provided a large surface area for improved catalytic activity. The transparent Pt nanofibrous web served as a catalyst layer in dye-sensitized solar

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cells and improved the power efficiency by 6.0%, and with its small fiber size and thickness it displayed sufficient durability and stability even under harsh conditions.

23.3.3 OXIDE NANOFIBERS AS CATALYSTS Electrospun oxide nanofibers have been extensively studied for catalytic applications. Without incorporating any other active components, certain oxide nanofibers hold some level of catalytic ability in themselves in various chemical processes (Ruiz-Rosas et al., 2012). For example, titanium dioxide (TiO2) is one of the most widely investigated metal oxides in a range of catalytic applications (e.g., hydrogen evolution, photo oxidation, and hydrogenation). Chuangchote et al., (2009) fabricated TiO2 nanofibers from PVP and titanium(IV) butoxide by a combination of electrospinning and solegel techniques (Chuangchote et al., 2009). After calcination, each TiO2 nanofiber consisted of a bundle of nanofibrils of 20e25 nm aligned along the fiber direction (Fig. 23.3). These small nanofibrils significantly improved the catalytic activity of TiO2 in a hydrogen evolution reaction, surpassing commercial TiO2 nanoparticles and TiO2 nanofibers prepared by hydrothermal synthesis. By introducing a second oxide component into TiO2 nanofibers, the catalytic activity of the composite nanofibers can be greatly enhanced in some chemical reactions (Kanjwal et al., 2015; Ma et al., 2016). For example, in catalyzing the degradation of toluene, pure TiO2 nanofibers had a removal efficiency of 69.6%; but after doping with 10% SiO2 the removal efficiency reached 90.6%. In fact, composite TiO2eSiO2 nanofibers with different doping ratios all showed higher removal efficiency than pure TiO2 nanofibers, due to the enlarged surface area and chemical characteristics of composite nanofibers with the TieOeSi bond (Zhan et al., 2014). Pascariu et al. doped zinc oxide (ZnO) nanofibers with tin oxide (SnO2) to enhance the degradation of rhodamine B under visible light irradiation (Pascariu et al., 2016). ZnO has been broadly studied as a photocatalyst due to its large band gap, large binding energy, and low cost, but its catalytic activity is

FIGURE 23.3 (A) Scanning electron microscope (SEM) and (B) Transmission electron microscope (TEM) images of TiO2 nanofibers showing the nanofibrils aligned in the fiber direction. Modified with permission from Chuangchote, S., Jitputti, J., Sagawa, T., Yoshikawa, S., 2009. Photocatalytic activity for hydrogen evolution of electrospun TiO2 nanofibers. ACS Applied Materials & Interfaces 1, 1140e1143, copyright 2009 American Chemical Society.

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affected by the fast recombination of photogenerated holes and electrons; hence pure ZnO nanofibers only degraded 23% of the dye in 6 h. The degradation rate was more than doubled by using SnO2doped ZnO nanofibers with a Sn/Zn molar ratio of 0.03. The increased catalytic activity is ascribed to the formation of local heterojunctions between ZnO and SnO2 which facilitate the separation of electron/hole pairs. Another study by Chae et al., (2017) demonstrated how adding a compatible metal oxide to ceria (CeO2) nanofibers could improve their catalytic activity (Chae et al., 2017). CeO2-based materials are widely used for oxygen storage, gas sensors, luminescent materials, and catalysts due to cerium’s excellent redox ability between Ce3þ and Ce4þ. In photocatalytic oxidation of methylene blue, pristine CeO2 nanofibers showed a degradation efficiency of 68%, but with the addition of copper(I) oxide (Cu2O) the photocatalytic ability rose to 92% under the same reaction conditions (Fig. 23.4). The enhanced photocatalytic activity of CeO2eCu2O composite nanofibers is due to the improved separation of electron/hole pairs by Cu2O as well as the increased surface area and enhanced absorption effect. To achieve even better catalytic performance, more than two metal oxides can be combined to form a homogeneous solid solution in electrospun nanofibers (Pei and Leung, 2015). For example, Barakat et al. incorporated ZnO and Fe2O3 into TiO2 nanofibers by electrospinning PVP solutions containing these metal precursors (Barakat et al., 2014). After calcination in air TiO2eZnOeFe2O3 nanofibers with different metal ratios were generated and used as a catalyst for water splitting under the irradiation of visible light. For a catalyst to split water efficiently under visible light (l ¼ 400e800 nm), the band gap should be small enough for the absorption of visible light and the following water oxidation/reduction; however, TiO2 has a relatively large band gap of 3.2 eV. It was found that the band gap of TiO2 nanofibers decreased to 2.85 eV after the addition of 5% ZnO, but further increasing the ZnO content failed to lower the band gap of the composite nanofibers any more. By adding a third metal oxide, 10% Fe2O3, to the TiO2eZnO nanofibers, the band gap decreased to 1.89 eV. However, TiO2eZnOeFe2O3 nanofibers containing 6% Fe2O3 with a band gap of 2.25 eV showed the optimum catalytic activity in water splitting because of the synergistic effect of band gap, chemical defects, light absorption, surface area, photocorrosion resistance, and recombination centers. The resultant TiO2eZnOeFe2O3 nanofibers achieved an unprecedented hydrogen production rate of 0.12 mL/min mgcat (Fig. 23.5).

23.4 CATALYSTS SUPPORTED ON ELECTROSPUN NANOFIBERS 23.4.1 BIOCATALYSTS ON NANOFIBERS Enzymes are well-known biocatalysts with a high degree of substrate specificity for applications such as pharmaceutical synthesis, food processing, and production of fine chemicals (Ge et al., 2009). However, the use of enzymes in industry has been greatly limited by their poor stability and impractical reusability. Electrospun nanofibers are shown to be effective supporting materials to maintain enzymes’ activity, enhance their thermal stability, and improve their recyclability (Wang et al., 2009). Many enzymes have been successfully immobilized on to electrospun nanofibers, including lipases from fungal species such as Candida rugosa, Thermomyces lanuginosus, Pseudomonas cepacia, Pseudomonas fluorescens, and Candidas antarctica, as well as a-amylase, b-galactosidase, laccase, urease, and catalase peroxidase among others (Weiser et al., 2016; Maryskova´ et al., 2016; Gupta et al., 2013; El-Aassar, 2013; Huang et al., 2011; Chen et al., 2009; Daneshfar et al., 2015; Li et al., 2012; Xu et al., 2013; Siqueira et al., 2015; Wang et al., 2006; Oktay et al., 2015).

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FIGURE 23.4 (A) Full spectra showing photocatalytic degradation (or oxidation) of methylene blue by CeO2eCu2O composite nanofibers with time. (B) Comparative photocatalytic activity of pristine CeO2 nanofibers and CeO2eCu2O composite nanofibers. Modified with permission from Chae, B.W., Amna, T., Hassan, M.S., Al-Deyab, S.S., Khil, M-S., 2017. CeO2-Cu2O composite nanofibers: synthesis, characterization photocatalytic and electrochemical application. Advanced Powder Technology, 28, 230e235, copyright 2017 Elsevier.

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FIGURE 23.5 Influence of Fe2O3 content in composite nanofibers (ZnO ¼ 5%) on hydrogen production. Modified with permission from Barakat, N.A.M., Taha, A., Motlak, M., Nassar, M.M., Mahmoud, M.S., Al-Deyab, S.S., El-Newehy, M., Kim, H.Y., 2014. ZnO&Fe2O3-incorportaed TiO2 nanofibers as super effective photocatalyst for water splitting under visible light radiation. Applied Catalysis A: General, 481, 19e26, copyright 2014 Elsevier.

As discussed earlier, enzymes can be immobilized on to electrospun nanofibers by encapsulation, physical adsorption, or covalent bonding. A variety of polymers have been investigated as supporting substrates for enzymes, chosen based on whether they are able to provide a suitable microenvironment for the attached enzymes (Wang et al., 2009). Some polymers, including cellulose and PVA, are hydrophilic and thus biocompatible with enzymes under physiological conditions (Huang et al., 2011; Lu and Hsieh, 2009a, 2010; Weiser et al., 2016). Other polymers, such as PAN and polyamide, offer high mechanical strength and thermal resistance (Gupta et al., 2013; Daneshfar et al., 2015). Hydrophilic polymers such as chitosan are unable to form uniform nanofibers by themselves, but polymers such as poly(lactic acid) which have appropriate solubility and compatibility with chitosan can be blended with chitosan to facilitate the fabrication of nanofibers by electrospinning (Maryskova´ et al., 2016; Siqueira et al., 2015). By combining polymers with different properties, the obtained composite nanofibers are not only nontoxic, biocompatible, and biodegradable, but also mechanically strong and thermally stable. Siqueira et al., (2015) used PVA as the polymer matrix for the encapsulation of lipase by electrospinning (Siqueira et al., 2015). Due to its excellent hydrophilicity and biocompatibility, the active conformation of the encapsulated lipase was successfully stabilized against deactivation in the acylation of racemic secondary alcohols in organic media. The catalytic activity and selectivity of the encapsulated lipase were conserved after eight repetitive reaction cycles. Chen et al. fabricated poly(methyl methacrylate) nanofibers with different diameters for lipase immobilization through physical adsorption (Chen et al., 2009). The authors investigated the relationship between the fiber diameter and the lipase loading and activity. Lipase was physically adsorbed on to poly(methyl methacrylate)

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nanofibers with diameters of 36, 100, 300, and 500 nm to form a monolayer cover. Lipase loading was reduced 7.7 fold from 332 to 43 mg/g by increasing the fiber diameter from 36 to 500 nm, resulting from the decreased specific surface area of nanofibers with the increase in fiber size. The specific activity of adsorbed lipase stayed in a range of 43%e50%, independent of the fiber diameter. Although the maximal velocity (Vmax, a kinetic parameter) decreased after attaching lipase to nanofibers, the Michaelis constant (Km) of adsorbed lipase was essentially identical to the free lipase, due to the favorable mass transfer effect on the surface of small nanofibers. Huang et al., (2011) immobilized lipase on to electrospun cellulose nanofibers by covalent bonding to enhance the enzyme’s thermal stability and durability (Huang et al., 2011). The surface of cellulose nanofibers was first oxidized by NaIO4 to generate aldehyde groups. Then amino groups on lipase were coupled with these aldehyde groups on cellulose to form covalent linkages. At 60
C the half-life of immobilized lipase (203 min) was twofold longer than that of free lipase (100 min), which is a clear indication of improved thermal stability and high-temperature resistance. The immobilized lipase retained more than 60% of its original activity after 30 days while the free lipase had less than 10% left after the same period, demonstrating much better storage stability for the immobilized lipase. Gupta et al., (2013) compared the activity and reusability of lipase immobilized on PAN nanofibers by physical adsorption and covalent bonding (Gupta et al., 2013). The physical means was simple adsorption of lipase on to the support, and the chemical means involved the activation of surface nitrile (-CN) groups in the presence of dry HCl/C2H5OH. The maximum lipase loading through covalent bonding was twice that through physical adsorption; furthermore, the maximum lipase immobilization was completed in 90 min by the chemical method while it took 150 min in the physical method. The possible explanation could be that equilibrium between adsorbed lipase and free lipase in the physical method took longer to establish than in the chemical reaction: the adsorbed lipase tended to detach easily, while the covalently bound lipase rarely dissociated. However, adsorbed lipase showed the highest hydrolytic and transesterification activities among the native, adsorbed, and covalently bound lipases (Table 23.1). Specifically, hydrolytic activity increased 3.6-fold and 1.8-fold for adsorbed and covalently bound lipases, respectively, as compared to native lipase. An even higher increase was observed for transesterification activity, at 32-fold and 9-fold for adsorbed and bound lipases, respectively. This showed that immobilization of lipase by physical and chemical protocols led to hyperactivation of the enzyme, probably due to the hydrophobic nature of PAN. Since the covalent linkage might have distorted the active site of lipase while physical adsorption did not, the increased

Table 23.1 Hydrolytic and Transesterification Activity of Native Lipase and Immobilized Lipase

Biocatalyst

Protein Content (mg/g of Total Biocatalyst)

Hydrolytic Activity (mmol/min mg-Protein)

Transesterification Activity (mmol/h mg-Protein)

Native Adsorption Covalent bonding

80 13 22

138 461 227

1.25 39.74 11.36

Modified with permission from Gupta, A., Dhakate, S.R., Pahwa, M., Sinha, S., Chand, S., Mathur, R.B., 2013. Geranyl acetate synthesis catalyzed by Thermomyces lanuginosus lipase immobilized on electrospun polyacrylonitrile nanofiber membrane. Process Biochemistry, 48, 124e132, copyright 2013 Elsevier.

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activity of adsorbed lipase was higher than that of covalently bound lipase. The covalently bound lipase did, however, exhibit higher activity retention after multiple reuse cycles. Typically, covalently bound lipase had 75% residual activity after eight cycles while the adsorbed lipase kept only 45%. In comparison, free lipase lost all activity after eight cycles.

23.4.2 METAL NANOPARTICLES ON NANOFIBERS Transition metal nanoparticles, including Ni, Cu, Au, Ag, Pd, Pt, and Rh, are the most widely used industrial heterogeneous catalysts (Shao and Qi, 2013; Shao et al., 2012a; Moreno et al., 2015; Hao et al., 2015; Liu et al., 2015b; Lu et al., 2013; Obuya et al., 2011; Yousef et al., 2012). The size, shape, and composition of the active metal nanoparticles, as well as the interaction between the nanoparticles and their support, are crucial for the activity and selectivity of metal catalysts (Chng et al., 2013). Electrospun nanofibers made of various polymers and oxides have been utilized as supporting materials for metal catalysts, and are able to increase the activity and selectivity, improve the reusability, and enhance the stability of supported catalysts. Certain functional groups on polymer nanofibers can chelate with metals to offer extra stability to metal catalysts (Shao and Qi, 2013; Shao et al., 2012a). For example, PVA nanofibers can chelate with a variety of transition metals due to the abundant surface hydroxyl groups. Shao et al. prepared porous PVA nanofibers containing a Pd precursor salt (Na2PdCl4) by electrospinning to catalyze coupling reactions of aromatic halides (Shao et al., 2012a). The divalent Pd2þ was reduced to zerovalent Pd0 by the hydroxyl groups of PVA at temperatures above 150
C. Meanwhile, the PVA was partially crosslinked under the thermal treatment, which effectively prevented the polymer matrix from deformation and dissolution in organic solvents. The obtained Pd/PVA was used to catalyze Ullmann reductive homocoupling of iodobenzene in dimethyl sulfoxide, which achieved 100% conversion at 110
C within 7 h. The Pd/PVA was recycled five times without much activity loss. The same catalyst was tested in the HeckeMizoroki cross-coupling of several representative aromatic halides with acrylates, demonstrating more than 95% yields and more than 97% selectivity within 4 h. The catalyst retained its catalytic activity after eight repeated uses. In Sonogashira cross-coupling of phenylacetylene with several aromatic halides, a Pd/PVA nanofiber catalyst achieved high yields in the range of 88%e99%. The remarkable catalytic activity and stability of the Pd/PVA nanofiber in these three coupling reactions are attributed to the chelation of Pd with PVA nanofibers. Inorganic nanofibers are excellent supporting materials for catalyst nanoparticles because of their high specific surface area, high thermal and chemical stability, and high mechanical strength. Yoon et al., (2012) developed a highly reactive and sinter-resistant catalytic system based on Pt nanoparticles embedded in the inner surfaces of CeO2 hollow fibers (Ptencap/CeO2) (Yoon et al., 2012). The Ptencap/ CeO2 catalytic system was prepared in three steps via a simple, template-based route: deposition of Pt nanoparticles (w3 nm) on to the surface of plasma-treated PS fibers; growth of a uniform sheath of CeO2 on Pt/PS fibers by hydrolysis; and removal of PS and other organics by calcination (Fig. 23.6A). It was observed that the Pt nanoparticles played an important role in the nucleation and growth of a porous sheath of crystalline CeO2 on the PS fibers (Fig. 23.6B and C). In addition to porous walls, the hollow fibers had open ends to ensure high permeation and mass transfer rates for species involved in a catalytic reaction (Fig. 23.6C and D). The turnover frequency of this Ptencap/CeO2 catalytic system for CO oxidation was found to be two to three orders of magnitude higher than those based on conventional Pt/SiO2 and Pt/TiO2 systems (Fig. 23.6E). Pt/CeO2 catalysts are known to exhibit strong

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FIGURE 23.6 (A) The three-step procedure developed for the preparation of Ptencap/CeO2 hollow fibers with open ends: (1) deposition of Pt nanoparticles; (2) coating of CeO2 sheath; (3) removal of PS fiber at 400
C. SEM images of PS fibers with surfaces coated with Pt nanoparticles and CeO2 sheaths (B) before and (C) after calcination (inset: TEM image confirming the hollow structure). (D) High-magnification TEM images revealing the porous wall structures of the Ptencap/CeO2 hollow fibers after calcination in air. (E) Turnover frequencies (TOFs) at an O2/CO ratio of 1:2 versus temperature for Pt/CeO2 hollow fibers calcined at 600 and 700
C for 2 h. For comparison, TOFs are shown for conventional Pt/SiO2 and Pt/TiO2 catalysts (5 wt% Pt) under the same reaction conditions. Modified with permission from Yoon, K., Yang, Y., Lu, P., Wan, D., Peng, H-C., Stamm Masias, K., Fanson, P.T., Campbell, C.T., Xia, Y., 2012. A highly reactive and sinter-resistant catalytic system based on platinum nanoparticles embedded in the inner surfaces of CeO2 hollow fibers. Angewandte Chemie International Edition, 51, 9543e9546, copyright 2012 Wiley-VCH.

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metalesupport interaction effects, with a potential to enhance catalytic activities for reactions involving rapid oxygen and/or electron transfer between the metal and the support (Vayssilov et al., 2011). But such a high increase in activity for CO oxidation was unprecedented. This could be due to a unique structure and composition for maximizing the Pt and CeO2 interface. In the hollow Ptencap/ CeO2 fibers, the CeO2 nanocrystalline sheath tightly surrounding the Pt nanoparticles with a maximized PteCeO2 interface greatly enhanced the catalytic CO oxidation rate. The CeO2 layer on Pt was suggested to accelerate the overall reaction rate by participating in accepting and donating oxygen atoms. This strong metalesupport interaction, it effectively stabilized the Pt nanoparticles against sintering up to 700
C. Dai et al., (2010) demonstrated a sinter-resistant catalytic system based on TiO2 nanofibersupported Pt nanoparticles covered by a porous sheath of SiO2 (Dai et al., 2010). The porous SiO2/ Pt/TiO2 system was prepared in three steps: deposition of Pt nanoparticles (w3 nm) on the surface of TiO2 nanofibers (w100 nm); coating of SiO2 on Pt/TiO2 with cetyltrimethylammonium bromide (CTAB) as a pore-generating agent; and generating a porous sheath of SiO2 by calcination to remove the CTAB and other organics. The TEM image and particle size analyses of the calcined porous SiO2/ Pt/TiO2 nanofibers revealed that the Pt nanoparticles retained their sizes without forming big aggregates. The authors also proved that the Pt surface in these materials was accessible to reactants by measuring the Pt surface area using selective chemisorption and titration (CO titration of O adatoms on Pt). For samples before and after silica coating, the average Pt dispersions were measured to be 42% and 29%, respectively. The porous silica shell served as an effective physical barrier to prevent sintering of Pt nanoparticles while providing channels for chemical species to reach the active sites on the surface of nanoparticles, thus allowing the catalytic reaction to occur. Indeed, the protected Pt nanoparticles did not exhibit a morphological change upon calcination at temperatures up to 750
C. In sharp contrast, the Pt nanoparticles were observed to aggregate in the absence of silica sheaths at temperatures as low as 350
C. The porous silica coating calcined at 750
C was still permeable for chemical species in the aqueous hydrogenation reaction of methyl red, with catalytic conversion of 61%. Furthermore, the Pt nanoparticles trapped inside the silica matrix did not show any leakage in any reaction. Enclosed catalytic systems based on electrospun nanofibers may enjoy improved activity by generating pores on the protective layers to increase the catalyst accessibility. However, it is highly likely that these tiny pores can be blocked by impurities (e.g., carbon), or collapse and sinter into a nonporous solid at elevated temperatures and pressures in real applications. Indeed, a number of oxides, such as CeO2 and SiO2, have been observed to aggregate and sinter to form bulk nonporous crystals (Adijanto et al., 2013; Yoon et al., 2012; Dai et al., 2010). To overcome these potential problems, Lu et al. developed a simple approach to fabricate a sinter-resistant catalytic system with an exposed Pt surface by building an energy barrier around catalyst nanoparticles (Lu et al., 2013). In the new system, porous rutile TiO2 fibers (Fig. 23.7) were used as a strongly interacting oxide support with higher surface area, porosity, and thermal stability than the solid anatase TiO2 fibers. The porous fibers were prepared by hydrolyzing titanium tetraisopropoxide in the void spaces of PS yarns, followed by calcination at 800
C. By simply reducing the coverage density of PVP on the Pt nanoparticles, SiO2 selectively deposited only on the porous TiO2 support while leaving the Pt mainly uncovered (Fig. 23.8), thus the Pt surface was exposed while the nanoparticles were supported on TiO2 and isolated from each other by SiO2. The competitive interactions of Pt with the strongly interacting TiO2 support and the weakly interacting SiO2 layer provide an effective energy barrier, in addition to a

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FIGURE 23.7 (A) SEM and (B) TEM images of highly porous TiO2 fibers obtained by calcination of PS yarns whose interior pores had been deposited with amorphous TiO2 derived from a Ti(OiPr)4 precursor. The inset shows an individual TiO2 (rutile phase) nanocrystal removed from the fiber. Modified with permission from Lu, P., Xia, Y., 2013b. Novel nanostructures of rutile fabricated by templating against yarns of polystyrene nanofibrils and their catalytic applications. ACS Applied Materials & Interfaces, 5, 6391e6399, copyright 2013 American Chemical Society.

physical barrier, to inhibit the diffusion, migration, and coalescence of the Pt nanoparticles. As a result, the new triphasic catalytic system with an “islands in the sea” configuration on electrospun nanofibers demonstrated significantly enhanced resistance to sintering up to 700
C while the Pt surface was exposed. Most importantly, the exposed Pt catalysts on the surface were highly active in catalyzing the reduction of p-nitrophenol even after calcination at high temperatures. Lu et al. deposited highly dispersed Pt nanoparticles on to porous CeO2 nanofibers for a wateregas shift reaction (Lu et al., 2015). The porous CeO2 nanofibers were fabricated using cerium(III) acetylacetonate as the precursor and PAN as the polymer matrix. The PAN matrix played a critical role in stabilizing the porous structure to prevent collapse during calcination in air up to 800
C. In the presence of PVP and 4-benzyolbenzoic acid, highly uniform Pt nanoparticles with an average diameter of 1.7 nm were synthesized and deposited in situ on the surface of CeO2 nanofibers under the irradiation of ultraviolet (UV) light from a quartz mercury vapor arc lamp. In the photo-induced reduction, the solvent ethanol served as the sacrificial reducing agent to provide electrons, PVP as the dispersing agent, and 4-benzyolbenzoic acid as the photo initiator. The photoreduction of Pd4þ to Pd0 was extremely fast, resulting in the formation of distinctly smaller Pt nanoparticles compared to those from thermal reduction. Furthermore, the Pt nanoparticles distributed uniformly across the entire surface of porous CeO2 nanofibers. In the wateregas shift reaction, the Pt/CeO2 nanofiber catalyst demonstrated a catalytic activity of 95% CO conversion at 450
C, while the Pt aggregates on CeO2 nanofibers showed only 5% conversion under the same condition. Moreover, the Pt/CeO2 nanofiber catalyst was very stable, and maintained its original activity for 10 h in the reaction condition.

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FIGURE 23.8 TEM images showing the selectivity of SiO2 deposition on the surface of TiO2 versus the surface of Pt nanoparticles with increasing reaction time: (A) 0, (B) 1, (C) 2, (D) 3, (E) 4, and (F) 5 h. The average thickness of the SiO2 layer is marked on each image. Modified with permission from Lu, P., Campbell, C.T., Xia, Y., 2013. A sinter-resistant catalytic system fabricated by maneuvering the selectivity of SiO2 deposition onto the TiO2 surface versus the Pt nanoparticle surface. Nano Letters, 13, 4957e4962, copyright 2013 American Chemical Society.

23.4.3 OXIDE NANOPARTICLES ON NANOFIBERS Oxide nanoparticles such as TiO2 and ZnO are often grown on or incorporated in to electrospun nanofibers to enhance their catalytic activity and reusability (Ren et al., 2012; Kim et al., 2014; Pant et al., 2013; Wang et al., 2011). Many of these oxide catalysts belong to photocatalysts which use UV/ visible light as the energy source for the degradation of small molecules, such as various dye molecules, hydrogen production, and redox reactions (Rezaee et al., 2017; Ren et al., 2012; Pant et al., 2013; Wang et al., 2011; Lavanya et al., 2014; Barakat et al., 2014; Kim et al., 2014; Al-Enizi et al., 2014; Walmsley et al., 2013). A wide variety of nanofibers, including polymer, carbon, and oxide, have been used to support oxide nanoparticles. Rezaee et al., (2017) incorporated tungsten oxide (WO3) nanoparticles on to PVA nanofibers for photocatalytic degradation of dye molecules under visible light (Rezaee et al., 2017). Highly reactive

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OH radicals generated from the cleavage of H2O2 under light irradiation greatly increased the reaction rate. The WO3/PVA nanofiber catalyst degraded around 95% of methylene blue in the presence of H2O2 within 300 min. Pant et al., (2013) incorporated TiO2eZnO binary nanoparticles on to the surface of carbon nanofibers by hydrothermal treatment (Pant et al., 2013). The TiO2eZnO/carbon nanofibers degraded 100% of methylene blue in 150 min, while the TiO2eZnO without carbon nanofibers only degraded around 70% of the dye. The improved photocatalytic activity of TiO2eZnO/ carbon nanofibers is associated with the coupling effect of TiO2 and ZnO as well as the remarkable absorptivity of the carbon nanofibers to dye molecules. The TiO2eZnO/carbon nanofibers were reused twice without activity loss; but the third cycle took 180 min to degrade 100% of the dye, which was related to the blocking of active sites on the oxide by dye molecules. Wang et al. successfully grew rutile TiO2 nanowires on electrospun anatase TiO2 nanofibers through a wateredichloromethane interface-assisted hydrothermal method (Wang et al., 2011). The formation of this branched heterojunction involved two steps (Fig. 23.9): the rutile TiO2 nanoparticles were entrapped at the

FIGURE 23.9 (A) Schematic illustrating the formation process of branched TiO2 heterojunction, and SEM images showing the (B) low magnification and (C) high magnification rutile/anatase heterojunction. Modified with permission from Wang, C., Zhang, X., Shao, C., Zhang, Y., Yang, J., Sun, P., Liu, X., Liu, H., Liu, Y., Xie, T., Wang, D., 2011. Rutile TiO2 nanowires on anatase TiO2 nanofibers: a branched heterostructured photocatalysts via interface-assisted fabrication approach. Journal of Colloid and Interface Science, 363, 157e164, copyright 2011 Elsevier.

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wateredichloromethane interface; and then the trapped rutile TiO2 coalesced to form rutile TiO2 nanowires on electrospun anatase TiO2 nanofibers through Ostwald ripening. The density and length of rutile nanowires can be readily controlled by varying the reaction conditions. A heterojunction with a rutile/anatase ratio of 2.43 demonstrated the highest activity among pristine anatase nanofibers, rutile nanowires, TiO2/SnO2 branched heterojunctions, and rutile/anatase heterojunctions with other ratios, and degraded nearly 100% of rhodamine B in 40 min. Furthermore, the rutile/anatase heterojunction did not lose activity after three repeat uses. Al-Enizi et al., (2014) loaded nickel oxide (NiO) on to nitrogen-doped carbon nanofibers by chemical precipitation of Ni(OH)2 and thermal annealing for methanol oxidation (Al-Enizi et al., 2014). The NiO nanoparticles, with an average diameter of 7.5 nm, spread uniformly over the entire surface of the carbon nanofibers. The NiO/carbon nanofibers showed much better specific and mass activities for methanol oxidation than other nickel-based catalysts, with a specific activity of 0.300 A/cm2, which is 60 folds of nickel wires (0.005 A/cm2) and 12 folds of NiO particles (0.025 A/cm2), and a mass activity of 1.8 A/mg, which is 900 folds of nickel wires (0.002 A/mg) and 21 folds of NiO particles (0.084 A/mg).

23.4.4 OTHER CATALYST NANOPARTICLES ON NANOFIBERS There are some catalysts with combinations of different elements that do not fit into the above categories (Yu et al., 2016; Guo et al., 2017). For example, metal sulfides, including Cds, PdS, and ZnS supported on electrospun nanofibers, have been applied in the photocatalytic oxidation/reduction of molecules in water (Liu et al., 2015a; Panthi et al., 2015a). Liu et al. coated electrospun cellulose nanofibers with CdS nanoparticles by a wet chemical method for photodegradation of dye molecules (Liu et al., 2015a). These CdS nanoparticles had a cubic zinc-blende structure and dispersed uniformly on cellulose nanofibers. CdS/cellulose nanofibers with 20% CdS loading degraded 99% of rhodamine B within 40 min under visible light, whereas CdS nanoparticles only decomposed 51% of dye molecules under the same condition. The higher activity of CdS/cellulose nanofibers could be associated with the separation of CdS nanoparticles by cellulose nanofibers. CdS nanoparticles without support tended to form aggregates, which increased the recombination rate of photo-induced electron/hole pairs due to the reduced distance between trapped carriers. The CdS/cellulose nanofibers showed little loss of activity after three cycles. Panthi et al. synthesized PdSeZnS binary nanoparticles for dye photodegradation (Panthi et al., 2015a). The incorporation of PdS enhanced the visible light activity of ZnS. The PdSeZnS/PVA nanofibers degraded 100% of methylene blue in 100 min, while the ZnS/ PVA nanofibers only degraded 75% of the dye. The increased photocatalytic activity of PdSeZnS/PVA nanofibers is due to the coupling effect of PdS and ZnS, which reduced the recombination rate of photogenerated electron/hole pairs.

23.5 CONCLUSIONS This chapter discusses electrospun nanofibers for various catalytic applications. Nanofibrous catalysts can be prepared through encapsulation during electrospinning and postelectrospinning deposition. Some electrospun nanofibers made of polymers, metals, and oxides can be directly used as active catalysts with simple postelectrospinning treatments such as chemical activation, reduction, and

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calcination. Many electrospun nanofibers have been proven to be excellent supporting materials for various catalysts, including biocatalysts, metal nanoparticles, and oxide nanostructures, among others. Supporting catalysts on electrospun nanofibers significantly improves their catalytic activity, selectivity, stability, and reusability. Immobilizing enzymes on suitable polymer nanofibers enhances their activity and stability because of the benign microenvironment endowed by the supporting surface. Depositing metal nanoparticles on nanofibers prevents the catalysts from leaching into reaction media and sintering at high temperatures, due to the strong interaction between catalyst and support. Growing metal oxide nanostructures on nanofibers broadens their capability in photocatalytic degradation of pollutant molecules because of the coupling effect of catalysts. Virtually any type of catalyst supported on electrospun nanofibers achieves improved reusability. As such, electrospun nanofibers have become an indispensable component in the development of highly active and stable catalysts, and electrospun nanofiber-based catalysts can be confidently expected to have potential in various applications in energy production, environmental protection, and health sciences.

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