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Unusual hollow AlO nanofibers with loofah-like skins: Intriguing catalyst supports for thermal stabilization of Pt nanocrystals Wanlin Fu, Yunqian Dai, Jerry Pui Ho Li, Zebang Liu, Yong Yang, Yibai Sun, Yiyang Huang, Rongwei Ma, Lan Zhang, and Yueming Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 02 Jun 2017 Downloaded from http://pubs.acs.org on June 4, 2017

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ACS Applied Materials & Interfaces

Unusual hollow Al2O3 nanofibers with loofah-like skins: Intriguing catalyst supports for thermal stabilization of Pt nanocrystals ‡

‡

‡

Wanlin Fu,† Yunqian Dai,*† Jerry Pui Ho Li, Zebang Liu, Yong Yang, Yibai Sun,§ Yiyang Huang,† Rongwei Ma,† Lan Zhang,† and Yueming Sun*†

†

State Key Laboratory of Bioelectronics, School of Chemistry and Chemical Engineering, Southeast

University, Nanjing, Jiangsu 211189, P. R. China  ‡

School of Physical Science and Technology, Shanghaitech University, Shanghai 200120, P. R. China

§Department

of Chemical and Pharmaceutical Engineering, Chengxian College, Southeast University,

Nanjing 210088, P. R. China

*Address corresponding to [email protected] and [email protected]

ACS Paragon Plus Environment


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Abstract Recently, hollow nanofibers could be fabricated by co-axis electrospinning method or template method. However, they are limited to applications because of the hardship in actual preparation. In this work, hollow γ-Al2O3 nanofibers with loofah-like skins were firstly fabricated by using a single spinneret during electrospinning. These intriguing nanofibers were explored as new Pt supports with excellently sinter-resistant performance up to 500 oC, attributed to the unique loofah-like surface of γ-Al2O3 nanofibers and the strong metal-support interactions between Pt and γ-Al2O3. When applied in the catalytic reduction of p-nitrophenol, the Pt/γ-Al2O3 calcined at 500 oC exhibited 4-times higher reaction rate constant (6.8 s-1·mg-1) over free Pt nanocrystals.

Keywords: platinum nanocrystal, electrospun nanofibers, γ-Al2O3, thermal-stable catalyst, supported catalyst


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Table of Content Intriguing electrospun hollow γ-Al2O3 nanofibers with loofah-like skins were firstly fabricated and explored as new Pt supports with excellently sinter-resistant performance up to 500 oC and superiorly catalytic activity.



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1. Introduction Alumina (Al2O3), a typical non-reducing ceramic material, is widely applied in the domains of catalysis,1 reinforcing components,2 and nanomaterials.3 Recently, electrospun Al2O3 nanofibers are used as catalyst substrates due to their large surface area, superiorly fine structures and the strong metal-support interactions (SMSI) with supported noble metals. Besides, the Lewis acid sites on the surface of Al2O3 have a major effect on the distribution and state of the active components and catalytic activity in different reactions.4 However, Al2O3 has a poor thermal-resistance, and usually loses their pristine fine structures due to the collapse of fibrous features during a heat-treatment, especially under a quick heating condition.5 Hollow nanofibers have attracted great attentions due to their unique electrochemical, electrical, and catalytic performances which are closely related to their high surface-to-volume ratio.6 Two elaborate electrospun approaches have been developed to fabricate hollow metal oxide nanofibers. One is template method, using electrospun nanofibers as template and coating desirable precursors. Additional removal with proper solvent or calcination at high temperatures was required for generating a typical hollow structure.7 The other is coaxial electrospinning two immiscible liquid with a coaxial spinneret, followed by removal of the core materials by extraction or combustion.8 These fussy and costly procedures pose great obstacles to practical preparation of hollow nanofibers. Therefore, a facile and cost-effective synthesis of hollow Al2O3 nanofibers with thermal-stable fibrous structure is highly demanded. To achieve supported catalyst with excellent activity, selectivity, thermal-stability and resistance to deactivation, choosing a proper substrate is of great importance.9 Recent advances in nanomaterials enabled noble metal nanocrystals, such as Platinum (Pt), to be readily prepared with tunable sizes, shapes and compositions.10,11 However, at a high temperature, typically above 300 oC, Pt nanocrystals usually aggregate irreversibly, and thus lose their outstanding catalytic performance dramatically. In practical reactions, including automobile exhaust gas treatment, CO oxidation,12 and combustion reactions,13 catalysts are usually operated at temperatures above 300 oC. In this regard, Pt supported catalysts with excellent stability at high reaction temperatures are highly demanded. A few cases have been designed to enhance the thermal-stability of metal nanocrystals through steric stabilization by an overlayer of inorganic oxide shell, such as silica10, 14-18 and ceria.19,20 However, the improved thermal-stability of such catalysts comes at a sacrifice of their catalytic activity since the protective shell would occupy the active sites of encapsulated nanocrystals more or less and block reactants to access them, which is a vital impediment to practical utilization.14,21 Hence, restoring the abundant highly active sites with superiorly thermal stability of metal nanocrystals catalyst at high temperatures is still of great challenge.


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In this work, we demonstrate a thermal-stable catalyst composed of Pt nanocrystals functionalized γ-Al2O3 nanofibers (referred as Pt/γ-Al2O3). The novel hollow γ-Al2O3 nanofibers with loofah-like skins were firstly fabricated by using a facile single spinneret during electrospinning process followed by a simple calcination. Benefiting from the physical blockage set by the unique wrinkles on the loofah-like skins, the Pt/γ-Al2O3 catalyst achieved excellently thermal stability up to 500 oC in an oxidative atmosphere. Simultaneously, the new catalyst exhibited greatly catalytic activity after being calcined at 500 oC in both of the catalytic reduction of p-nitrophenol and the CO oxidation reaction, possibly due to the synergistic effects between Pt and γ-Al2O3 nanofibers. Moreover, their extraordinarily thermal stability allowed the Pt/γ-Al2O3 catalyst to be highly active during long-term reaction and even can be regenerated easily through a portable heat-treatment.

2. Experimental Section 2.1 Chemicals. Polyvinylpyrrolidone (PVP, Mw ≈ 1.3×106 or 55,000), chloroplatinic acid hydrate (H2PtCl6·xH2O, 99.995%), sodium borohydride (NaBH4, 99.99%) and aluminum acetylacetonate (Al(acac)3) were obtained from Alfa Aesar. Mercaptopropionic acid (MPA) was purchased from Sigma Aldrich. All chemicals were used as received. The water used in all experiments was filtered through a Millipore filtration system with a resistivity of 18 MΩ·cm. 2.2 Electrospinning of Al2O3 nanofibers. The Al(acac)3/PVP composite nanofibers were prepared by electrospinning a precursor containing 0.3 g of Al(acac)3, 0.3 g of PVP (Mw ≈ 1.3×106), 2 mL of ethanol and 3 mL of acetone with a flow rate of 0.3 mL/h, at 15 kV. A piece of folded aluminium foil was used to collect the nanofibers. The as-spun Al(acac)3/PVP nanofibers were kept in air overnight and then converted to Al2O3 nanofibers after a calcination at a given temperature for 2 h in air with ramping rate of 10 oC/min. 2.3 Synthesis of 3 nm Pt nanocrystals. 3 nm Pt nanocrystals were prepared by a polyol method. 4 mL of ethylene glycol was added to a glass vial and heated in oil bath at 110 oC for 30 min. Then, 5.6 mg of PVP (Mw ≈ 55,000) and 4.1 mg of H2PtCl6 were dissolved separately in 0.5 mL of ethylene glycol at room temperature, and then added simultaneously into the system at a rate of 0.67 mL/min. The reaction was continued with heating at 110 oC for 1.5 h and finally cooled down to room temperature. 2.4 Loading of Pt nanocrystals on Al2O3 nanofibers. The as-prepared Pt nanocrystals were loaded on the surface of Al2O3 nanofibers by a simple impregnation method. 5 mg of Al2O3 nanofibers were immersed in a certain amount of diluted Pt suspension, followed by a gentle stirring at room temperature for 2 h. The as-prepared Pt/Al2O3 were washed with ethanol for six times, and then dispersed in ethanol. The centrifugation speed for washing was generally 3000 rpm/min.



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2.5 Characterizations. Transmission electron microscopy (TEM) images were collected using a transmission electron microscope (Tecnai G2 T20, FEI) operated at 200 kV. The crystal structure information was obtained with X-ray diffraction (Bruker, D8 advance using Cu-Ka radiation, λ=1.5406 Å). The Pt mass was determined by Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) (Optima 7300DV, Perkin Elmer Corporation). The UV-vis spectra were recorded on a UV-Vis spectrometer (Cary 60). Zeta-potential was tested by a Nano ZS90 in alcohol at room temperature. Brunauer-Emmett-Teller (BET) surface area was tested by Nova 1200e (Quantachrome, USA). 2.6 Evaluation of catalytic activity 2.6.1 Catalytic reduction of p-nitrophenol. As a model reaction, the reduction of p-nitrophenol to p-aminophenol by NaBH4 was chosen to estimate the catalytic activities. In a typical procedure, 50 µL of p-nitrophenol (7.4 mmol/L) and 50 µL of NaBH4 (2.4 mol/L) were added into 2.5 mL of Millipore water. Then, a given amount of the composite catalyst was quickly added into the system to start the catalytic reaction. The kinetic process of the reduction was monitored by measuring the UV absorption of the solution at 400 nm. 2.6.2 Isothermal CO titration. The surface area of Pt in the catalyst was measured by selective chemisorption of oxygen atoms in the CO titration procedure.22 Briefly, 12 mg of the calcined catalyst was loaded in the middle of a 6 mm quartz reactor tube and heated to 150 oC under Ar flow (20 sccm) at 10 oC/min. Once 150 oC is reached, the catalyst was reduced in a flow of 75 % H2 in Ar (20 sccm) and held for 30 mins. This was then cooled to 100 oC, followed by oxidation in a flow of 3 % O2 (20 sccm) and held for 30 mins. This was followed by flushing in an Ar stream, followed by cooling to the 50 oC and held. An injection of 0.05 % CO was then put into the system. The CO oxidation is according to the reaction: Pt-O + CO (g) Pt + CO2 (g) The total CO2 products equate to the surface oxygen adsorbates population, thus the Pt bound sites. The CO2 products and all fed gases are monitored by online MS during the titration process. This includes CO (m/z – 28), O2 (m/z – 32), and CO2 (m/z – 44). It should also be noted that it has been found that Pt obtains a maximum of O2 33% coverage.22 The Pt dispersion has then been determined by using the following equation: CO2 titration products ሺmolሻ /0.33 Dispersion (%) = ( ) ×100 Total amount of Pt in sample (mol)


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Assuming an atom density of 105 atoms/cm2 and a spherical shape of the Pt particles. The average Pt particle size was estimated to be ~10 nm. 2.6.3 CO oxidation. The Pt/γ-Al2O3 samples were tested for its activity in CO oxidation reaction. First, a gas mixture of 0.05% CO and 0.025% O2 in Ar (total flow 20 sccm) was fed into the reactor. The reactor temperature was set to 40 oC and then heated up with ramping rate of 10 oC/min, up to the calcining temperature for the catalyst sample. The CO2 products and all fed gases are monitored by online MS during the titration process. 2.7 Regeneration of Pt/γ-Al2O3. When the reduction of p-nitrophenol had proceeded for about 3 min, 20 µL of MPA (10% vol) was added into the system. The obtained product was washed by the ultrapure water after the reaction is terminated. To recycle the catalyst, the reclaimed product was heated to 350 oC in air with ramping rate of 10 oC/min and held for 2 h.

3. Results and Discussion 3.1 Fine-structure of hollow Al2O3 nanofibers with loofah-like skins. The hollow Al2O3 nanofibers with loofah-like skins were fabricated by a facile electrospinning method using a single spinneret. Figure 1 and S1 show the TEM and SEM images of the as-spun composite nanofibers before and after being calcined at 350, 500, 700, 900 and 1100 oC. By measuring ~100 nanofibers, the diameter distribution of each sample could be counted as Fig S2. As shown in Fig 1A and G, the as-spun composite nanofibers exhibited a smooth surface with high length-diameter ratio. Distinct from the conventional reported electrospun nanofibers,8,21 a unique graded distribution along the axis was observed in the composite nanofibers. After being calcined at 350 oC for 2 h in air, as shown in Fig 1B and H, the interior of composite nanofibers became porous, and the average diameter decreased to ~290 nm. When the calcination temperature was raised up to 500 oC (Fig 1C and I), wrinkles aroused on the surface of porous nanofibers. In Fig 1D and J, the skins of nanofibers became coarser after being calcined at 700 oC. When further increasing the calcination temperature to 900 oC, these nanofibers (Fig 1E and K) exhibited loofah-like surfaces with obvious wrinkles, belt-like shape and hollow structure surprisingly. As shown in the inset of Fig 1E, a closer examination of the nanofibers revealed the tubular cross section with wall thickness of ~30 nm. Elevating the calcination temperature to 1100 oC (Fig 1F and L), the nanofibers lost their loofah-like surface and showed a smooth belt-like appearance with increased diameters of ~440 nm instead. Moreover, large crystal grains appeared on the margin of the nanofibers. Previous works reported that the Al2O3 nanofibers fractured easily and thus lost the capability of keeping fibrous structure during heat treatment at high temperatures.5 However, our well-designed Al2O3 nanofibers still kept fibrous



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structure well after being heat-treated at an extremely high temperature, indicating an excellently thermal stability. N2 adsorption hysteresis of composite nanofibers calcined at different temperatures were tested and the results were listed in Table 1. In the range of room temperature to 500 oC, the specific surface area of nanofibers increased from 1.8 m2/g to 180.2 m2/g, which is 20-times higher than that of reported metal-oxide nanofibers, such as Fe2O3-Al2O3 composite nanofibers23 and TiO2 nanofibers.24 Further increasing the calcination temperature, the surface area decreased gradually, possibly due to the enlargement of crystal grains. 3.2 Formation mechanism of unusual hollow γ-Al2O3 nanofibers with loofah-like skins. To reveal the growth mechanism of the intriguing loofah-like hollow Al2O3 nanofibers, we conducted several control experiments. We chose ethanol only as the solvent in the electrospinning solution, without adding acetone, to uncover the effect of solvents. This solution was gently heated to improve the poor solubility of Al(acac)3 in ethanol, so as to obtain a homogenous precursor and thus a stable electrospinning process. However, as shown in Fig S4A, the as-spun Al(acac)3/PVP composite nanofibers exhibited distinct core-shell structure. The interior of nanofibers was speculated to be the Al(acac)3 crystal grains, while the shell was supposed to be the composition of PVP and Al(acac)3. When calcined at 500 oC or 900 oC, the nanofibers exhibited smooth surface, and no hollow nor loofah-like features were observable (Fig S4B and C). In addition, when acetone was used as the only solvent in electrospinning precursor, amount of PVP could not be solved well despite extra heat was supplied, so that a typical electrospinning process could not be conducted successfully. Thus it can be seen that both of the acetone and ethanol played a vital role in generating homogeneous Al(acac)3/PVP composite nanofibers thanks to their good solubility of Al(acac)3 and PVP respectively, which may evolve to hollow morphology with fine structures during calcination at high temperatures. Furthermore, the TGA test of PVP, Al(acac)3 and Al(acac)3/PVP composite nanofibers (Fig 2A) showed that the first weight-loss stage of the composite nanofibers began at 40 oC, which was associated with the evaporation of adsorbed water of PVP. The second degraded stage of Al(acac)3/PVP composite nanofibers occurred between 180 oC to 250 oC, which was slightly earlier than the fastest thermal-decomposition of Al(acac)3. Later, the composite nanofibers experienced a dramatically weight-loss ranging from 250 oC to 550 oC, possibly due to the oxidation of PVP.25 PVP and Al(acac)3 in the nanofibers decomposed completely at around 600 oC, and then the weight of nanofibers had no obvious change. Interestingly, the thermal stability of Al(acac)3 and PVP in the composite nanofibers have been improved compared to pure Al(acac)3 and PVP. Moreover, as the XRD pattern (Fig 2B) shows, the Al2O3 nanofibers were still amorphous after being calcined at 700 oC, then transformed to γ-Al2O3 at 900 oC and finally to α-Al2O3 at 1100 oC.26,27 As Fig S3 illustrated, the γ-Al2O3 (111)


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interplanar spacing is 4.56 Å, which perfectly matches the data from the powder diffraction file (PDF 10-0425). Besides, there is no epitaxial crystalline structure was observed, confirming the polycrystalline γ-Al2O3 nanofibers as a result. Taken together, the growth mechanism of these new Al2O3 nanofibers can be summarized as Scheme 1: (i) During electrospinning process, acetone evaporated faster than ethanol, and thus the undissolved PVP gradually distributed along the axis. In early range of room temperature to 350 oC, Al(acac)3 decomposed a lot and PVP started to decompose, producing a porous inner structure, with the diameters of nanofibers decreased rapidly. (ii) In the range of 350 oC to 500 °C, both of PVP and Al(acac)3 decomposed quickly and thus generated a large amount of gas, which caused a high inner pressure and expanded the whole nanofibers outward, leaving a porous fine structure with increased diameter. (iii) In the range of 500 oC to 700 oC, Al2O3 tended to migrate to the surface of nanofibers under a rapid heating condition, causing the wrinkles to be more vigorous. (iv) In the range of 700 oC to 900 oC, amorphous-Al2O3 was transformed into polycrystalline γ-Al2O3, within the increasing size of crystalline grains. The further migration of the enlarged γ-Al2O3 crystalline grains led them to gather on the margin of the nanofibers, producing an intriguing deformation to belt-like hollow fibers. Simultaneously, the growth of γ-Al2O3 crystalline grains decreased the specific surface area slightly as a result. (v) At a high temperature of 1100 oC, α-Al2O3 was formed and the crystalline grains turning to bigger and bigger, even blocking the inner channel of the fibers, and decreasing specific surface area as well. The belt-like structure become more vigorous, and the diameter of nanofibers increased a lot. However, the fibrous structure was kept well due to the remarkably thermal stability of the well-designed Al2O3 nanofibers. 3.3 γ-Al2O3 nanofibers applied as intriguing catalytic supports. Electrospun nanofibers have proven to be highly efficient catalytic supports owing to the high porosity and large surface area. There are a number of methods for decorating Pt nanocrystals on electrospun nanofibers, including the gas-phase decomposition, reduction process followed by a simple impregnation and direct growth of metal nanoparticles on substrates.28 In our previous research, we have demonstrated a method for depositing uniform Pt nanocrystals of ~3 nm in diameter made by a polyol method (as shown in Fig S5) on the electrospun nanofibers.16 Using this method, the loading density of the Pt nanocrystals can be readily controlled by adjusting the dosage ratio of Pt to Al2O3 nanofibers in the impregnation process, as shown in TEM images (Fig S6) and the corresponding EDX results (Table S1), offering an excellent system for the study of catalytic performance of Pt nanocrystals supported on ceramic nanofibers. To investigate the feasibility of loofah-like hollow γ-Al2O3 nanofibers (as-spun nanofibers calcined at 900 oC) to be a potential appropriate substrate for thermal-stable catalyst, amorphous-Al2O3 nanofibers (as-spun nanofibers calcined at 500 oC) were also used as Pt substrates for better comparison. It can be



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seen from Fig S5, Fig 3A and E that Pt nanocrystals remain their size and dispersity on the surface of nanofibers, which could be attributed to the electrostatic repulsion between positively charged Pt surfaces (ζ=1.4±0.6 mV in ethanol, Table S2). As shown in the HRTEM images (the inset of Fig 3E) and the corresponding Fast-Fourier transform (FFT) pattern, the supported Pt nanocrystals exhibited truncated octahedral morphology, and the lattice distance of 2.3 Å and 1.9 Å were assigned to the {111} and {200} facets.29 Meanwhile, Al2O3 nanofibers had a negatively charged surface in ethanol (Table S2), and thus absorbed the positively charged Pt nanocrystals as a result of electrostatic attraction. Certainly, the porous structure, large surface area and loofah-like skins helped the supported Pt nanocrystals to be anchored tightly as well. It is worth noting that amorphous-Al2O3 nanofibers have a ζ-potential of -7.4±1.0 mV in ethanol, while that of γ-Al2O3 nanofibers is -4.2±0.6 mV (Table S2). Due to the similar surface charges, the loading density of Pt nanocrystals on amorphous-Al2O3 nanofibers was almost the same as that on γ-Al2O3 nanofibers (as shown in Fig 3A and 3E). And the ICP-OES results confirmed that the Pt content in Pt/amorphous-Al2O3 is 0.51 wt%, similar to that in Pt/γ-Al2O3 (0.46 wt%), facilitating our further evaluation of the thermal stability and catalytic activity. The thermal stability of Pt/Al2O3 nanofibers was investigated by calcining them at a series of given temperatures. As Fig 3 and Fig S7 shown, the Pt nanocrystals on Pt/γ-Al2O3 remain the size of 2.9±0.4 nm after being calcined at 500 oC, approximately the same as before heat-treatment. When the temperature was raised up to 600 oC, the supported Pt nanocrystals started to sinter in some extent with the size slightly increased to 4.8±1.6 nm. However, the supported Pt nanocrystals aggregated together dramatically at 700 oC and their size increased to 10.7±3.6 nm. It can be seen from Fig 3 that, Pt/amorphous-Al2O3 exhibited a similarly outstandingly thermal stability compared to Pt/γ-Al2O3, indicating that the sinter-resistance depends much on the fine structure rather than the crystal phase. The ultra-high porosity of the electrospun Al2O3 nanofibers provide numerous loading sites for Pt nanocrystals, and the remarkable wrinkles act as physical barriers to prevent loaded Pt nanocrystals from aggregating, attributing to such an excellent sinter-resistance (up to 500 oC) under a high loading density. 3.4 Catalytic reduction of p-nitrophenol. The reduction of p-nitrophenol to p-aminophenol was chosen to quantitatively estimate the catalytic activity of Pt/Al2O3, which was widely applied as a model reaction in previous researches.14,30,31 A certain amount of Pt/Al2O3 (2.1×10-6 M of Pt) was added into 1.42×10-4 M of p-nitrophenol and 4.62×10-2 M of NaBH4 aqueous solution to start a typical reaction and the whole reaction was monitored by a UV-Vis spectrophotometer immediately. As several researches reported, the time prior to UV measurement effect the catalytic result a lot.14,31 It is found that an induction period (the initial flat region in Figure 4B, S8 and S9) was required before the reaction initiated. This induction time is probably associated with the first adsorption of p-nitrophenol onto the metallic surface.14 To simplify the reaction


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model and calculation, we focused on the catalytic rate after the induction time. Fig 4A shows the quick decline

of the absorption at 400 nm in a decelerated fashion, indicating the reduction of p-nitrophenol with the increasing reaction time. And the increase of absorption at 300 nm suggests the generation of p-aminophenol at the same time. For further investigating the effects of substrate and calcination temperature on the catalytic activity of Pt/Al2O3, a series of control experiments were carried out as Fig 4B and Fig S8 shows. The UV absorption at 400 nm of the p-nitrophenol reduction system treated with pure γ-Al2O3 or amorphous-Al2O3 was detected to exclude the adsorption produced by those porous nanofibers. However, the UV adsorption of p-nitrophenol declined very slightly, possibly since the dosage of Al2O3 nanofibers is too little which is equal to the amount in Pt/Al2O3 composite. Thus, the change of the p-nitrophenol concentration could be totally ascribed to the decomposition induced by Pt/Al2O3 catalyst instead of the adsorption. As the concentration of NaBH4 is much higher than that of p-nitrophenol in the reaction system, the pseudo-first-order kinetics could be applied to calculate the apparent rate constant (K) of Pt/Al2O3.14 Moreover, in order to make the calculated K values comparable between catalysts in control experiments, the best-fit K values could be correlated to the Pt content (Km). We found that the supported Pt nanocrystals before calcination exhibited a similarly catalytic activity as the free Pt nanocrystals.29,32 It can be seen from Fig 4C that in the range of room temperature to 500 oC, the activity of Pt/Al2O3 was promoted dramatically, probably due to the thermal-decomposition of PVP that capped on the surface of Pt nanocrystals when preparing Pt nanocrystals. It is worth noting that Pt/γ-Al2O3 exhibited a 4-times higher Km value (6.8 s-1·mg-1) after being calcined at 500 oC than that of free Pt nanocrystals, which shows an obvious superiority compared with Pt/amorphous-Al2O3. This surprising ascendency could be benefited from the SMSI between γ-Al2O3 and Pt nanocrystals.33 It is well known that the substrate material can significantly alter the catalytic activity and selectivity of a catalytic nanoparticle by affecting its structure and the electron density.34 Actually, metal nanocrystals usually bond with Al atoms to make γ-Al2O3 reach a saturated coordination state, and thus the stability and activity could be improved as a result.1 However, when the temperature of calcination was raised up to 600 oC or even 700 oC, the supported Pt nanocrystals suffered from aggregating and lost their catalytic activity, confirming the strong size-dependent catalytic activity of supported noble metals.10 In previous works, SiO2 and CeO2 shell are widely utilized to enhance the thermal stability of supported nanocrystals, but they sacrifices the catalytic activity inevitably.13-20 However, the greatly thermal stability with excellently catalytic activity are obtained simultaneously in this novel Pt/γ-Al2O3 catalyst without additional protective shell. 3.5 CO titration and oxidation. The surface area of the Pt nanocrystals loaded on the loofah-like skins of γ-Al2O3 nanofibers were measured by CO titration of chemisorbed oxygen (Table S3) at 50



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o

C.22,35 The highest accessible Pt surface area (11.58 m2/g) and dispersion (3.56 %) were obtained after

calcination at 500 oC, which is much higher than that of Pt supported on commercial γ-Al2O3 (W.R. Grace, 0.49%), due to the fine-structure of loofah-like hollow γ-Al2O3 nanofibers with abundant active sites.36 Figure 5A compares the TOFs of Pt/γ-Al2O3 calcined at 350, 500, and 600 oC respectively, whereby each of the CO oxidation reactions were performed at a temperature range of 40-400 oC at a ramp rate of 10 oC/min. These TOFs were calculated based on the Pt surface area measured by the CO titration at 50 oC. The Pt/γ-Al2O3 calcined at 500 oC, referred as Pt/γ-Al2O3 (500 oC), shows the highest TOF in the comparable temperature range of 444-497 K, indicating its highest catalytic activity. Compared to Pt/γ-Al2O3 (350 oC), the activities of Pt/γ-Al2O3 (500 oC) and Pt/γ-Al2O3 (600 oC) both increased, which is likely because of the removal of the polymer during calcination. The CO oxidation rates observed from these catalysts are in line with Pt catalysts in the field (with Ea=107.81 ± 0.93 kJ/mol, Table S4) and appear to be more active than other Pt/Al2O3 in the field. By comparison, studies by Gorte and coworkers40 have demonstrated that smaller particles (1.7 nm Pt/Al2O3) are observed to exhibit lower CO oxidation rates (with Ea=178 kJ/mol) than larger particles (14 nm Pt/Al2O3) (with Ea=125 kJ/mol). This observation is correlated to the higher binding strength of CO on step sites present on the smaller particles (< 2 nm), which contain a higher fraction of CO bound to step sites, and thus affect CO oxidation kinetics in the desorption limited regime.41-43 These results indicate that the loofah-like hollow γ-Al2O3 nanofibers indeed work well at protecting the Pt nanocrystals under high-temperature heat treatment, due to its delicate morphology and the crystalline structure. 3.6 Long-term reaction and facile regeneration of Pt/γ-Al2O3 catalyst. The need for catalytic materials that remain stable and active over long periods, often in the presence of deactivating or even poisoning compounds, presents a challenge. A cycling test was done to study the stability of Pt/γ-Al2O3 by repeated chemical reduction of p-nitrophenol (Fig 6A, S9). After ten successful cycles, 1 mg of Pt/γ-Al2O3 (500 oC) still maintained a conversion of p-nitrophenol to p-aminophenol over 95% in 5 min, and showed no significant downward trend. As the TEM image in Fig S10 shows, the Pt/γ-Al2O3 catalyst kept the fine structures and no obvious aggregation of Pt nanocrystals was observed. The result clearly shows that the as-obtained catalyst is stable and active under long-term catalytic conditions. Moreover, in practical industrial applications, noble metal catalysts often suffer from sulfur-containing molecules which could poison catalyst and thus deactivate their activities easily and irreversibly. Based on the excellent thermal-stability of Pt/γ-Al2O3 catalyst, a simple circulating utilization of is demonstrated through a feasible heat treatment. Mercaptopropionic acid (MPA) was applied to provide sulphur and poison the Pt nanocrystals. As Fig 6B illustrated, once MPA was added, the ultraviolet absorption of p-nitrophenol at 400 nm kept unchanged, which indicated the deactivation of Pt nanocrystals. After a


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simple heat-treatment at 350 oC in air for 2 h, the regenerated Pt/γ-Al2O3 catalyst held the nano-structure (Fig S11) and thus exhibited extraordinary catalytic activity equivalent to the freshly prepared Pt/γ-Al2O3 catalyst, confirming the decomposing of thiols and PVP around Pt nanocrystals during calcination.

4. Conclusion In summary, porous and hollow γ-Al2O3 nanofibers with loofah-like skins were successfully fabricated by a simple electrospinning without using complex co-axial electrospinning device or high-cost templates. It is found that different solvents with distinct solubility and volatility could induce separated thermal-decomposing process. In addition, the migration and crystalline transformation of Al2O3 during heat-treatment also contributed to the formation of this intriguing fine-structure. Thanks to the unique morphology of γ-Al2O3 nanofibers and SMSI between Pt and γ-Al2O3, Pt nanocrystals loaded on the γ-Al2O3 nanofibers maintained their size and structure after being calcined at 500 oC. Moreover, both of the catalytic reduction of p-nitrophenol and the CO oxidation reaction confirmed the remarkably catalytic activity. In addition, the Pt/γ-Al2O3 catalyst exhibited superior stability during long-term reaction and even in the presence of poisoning agents. The formation of hollow γ-Al2O3 nanofibers with loofah-like skins could provide a new strategy to fabricate 1D nanomaterials with advanced constructions. Distinguished from commonly designed core-shell configuration, the Pt/γ-Al2O3 catalyst overcame the shortcoming that protective shell would block the active sites of metal catalyst. Herein, we believe that the new synthetic strategy demonstrated in this paper could be applied on other catalytic system with different compositions.

Associated Content Supporting Information SEM image of as-spun Al(acac)3/PVP composite nanofibers; TEM images of Al(acac)3/PVP composite nanofibers without adding acetone, as-prepared Pt nanocrystals, Pt/γ-Al2O3 with different dosage ratio between Pt nanocrystals and γ-Al2O3 nanofibers, Pt/γ-Al2O3 calcined at 700 oC and their corresponding size distribution, Pt/γ-Al2O3 after 10 cycles of catalytic reduction reaction and their corresponding size distribution, poisoned Pt/γ-Al2O3 after regeneration by facile thermal treatment and their corresponding size distribution; diameter distributions of as-spun Al(acac)3/PVP composite nanofibers before and after being calcined at given temperatures; HRTEM image of γ-Al2O3; UV-Vis spectra of p-nitrophenol catalyzed by Pt/amorphous-Al2O3 and the catalytic cycles of p-nitrophenol catalyzed by Pt/γ-Al2O3; Pt content of Pt/γ-Al2O3 with different dosage ratio between Pt nanocrystals and γ-Al2O3 nanofibers measured by EDX; ζ-potential of Pt nanocrystals, amorphous-Al2O3 nanofibers and



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γ-Al2O3 nanofibers in ethanol; Surface areas and dispersions for Pt nanocrystals loaded on the surface of γ-Al2O3 nanofibers after being calcined at 200, 350, and 500 oC for 2 h; Pre-exponential factor and Ea of Pt nanocrystals loaded on the surface of γ-Al2O3 nanofibers after calcinations at 350, 500, and 600 oC for 2 h. This material is available free of charge via the Internet at http://pubs.acs.org.

Author Information Corresponding Author *E-mail: [email protected] and [email protected] Notes The authors declare no competing financial interest.

Acknowledgments This work was financially supported by the National Basic Research Program (973 program, 2013CB932902), the National Natural Science Foundation of China (21201034, 21310102005 and 21573148), the Science and Technology Support Program (Industry) Project of Jiangsu Province (BE 2013118), the Fundamental Research Funds for the Central Universities, the Priority Academic Program Development of Jiangsu Higher Education Institutions, Nanjing science and technology committee (2014-030002), Shanghai Pujiang Program (15PJ1405800) and the ‘Frontier Science” between Shell and Chinese Academy of Science. The authors acknowledge the assistance from Prof. Kuibo Yin and Hongtao Zhang in the School of Electronic Science and Engineering at Southeast University for helping us with the taking of HRTEM images.


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Figure 1. SEM (top row) and TEM (bottom row) images of as-spun Al(acac)3/PVP composite nanofibers before (A, G) and after being calcined at 350 (B, H), 500 (C, I), 700 (D, J), 900 (E, K) and 1100 oC (F, L) for 2 h in air. The insets of (A-F) are the corresponding cross section SEM images, and the scale bars represent 200 nm. The white dash lines and arrows highlight the tubular wall of nanofibers in (E).



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Table 1. BET surface area calculated from N2 adsorption hysteresis of composite nanofibers calcined at different temperatures. BET Surface Area (m2/g)

Sample AP/PVP

1.8 o

10.6

o

180.2

o

164.3

o

120.5

Al2O3 (350 C) Al2O3 (500 C) Al2O3 (700 C) Al2O3 (900 C) o

Al2O3 (1100 C)

46.8


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Figure 2. (A) TGA curves of Al(acac)3, Al(acac)3/PVP as-spun nanofibers and PVP calcined in the air respectively. (B) XRD spectra of Al(acac)3/PVP nanofibers calcined at 500, 700, 900 and 1100 oC respectively.



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Scheme 1. Schematic illustration of formation mechanism of novel hollow Al(acac)3/PVP nanofibers with loofah-like skin by using single spinneret. The yellow represents Al(acac)3 and Al2O3, while the blue represents PVP.


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Figure 3. TEM images of Pt nanocrystals loaded on amorphous-Al2O3 (left column) and γ-Al2O3 (right column) and their corresponding size distribution. The composite catalyst was performed before (A, E) and after being calcined at 350 (B, F), 500 (C, G) and 600 oC (D, H) respectively. The insets are corresponding HRTEM image and the Fast-Fourier transform (FFT) pattern, and the scale bar represents 1 nm.



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Figure 4. (A) The reduction of p-nitrophenol catalyzed by Pt/γ-Al2O3. (B) Time dependent reduction rate of p-aminophenol catalyzed by γ-Al2O3 nanofibers and Pt/γ-Al2O3 before and after being calcined at 350, 500, 600 and 700 oC respectively. (C) The kinetic constant of catalytic reaction against the calcination temperature.


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Fig 5. (A) Log (TOFPt surface) at an O2/CO ratio of 1:2 against temperature: 1000/T (K-1) displayed at the bottom axis, and T (K) displayed at the top axis, for Pt/γ-Al2O3 calcined at 350, 500 and 600 oC for 2 h. (B) Comparison of Pt/γ-Al2O3 catalyst data to literature Pt/SiO2 model catalyst [37], Pt (100) single crystal [38], and Pt/SiO2 technical catalyst data [39].



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Figure 6. (A) Catalytic cycles of Pt/γ-Al2O3. (B) Time dependent reduction rate of p-nitrophenol catalysed by regenerated Pt/γ-Al2O3, and the arrow indicate the time of adding mercaptopropionic acid (MPA).


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