Golden Carbon Nanotube Membrane for Continuous Flow Catalysis


Feb 22, 2017 - She joined Guangdong University of Technology as an Associate ... Tiankai Chen received his B.S. degree in materials chemistry from Pek...
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Golden Carbon Nanotube Membrane for Continuous Flow Catalysis Yanbiao Liu, Yuying Zheng, Bowen Du, Ricca Rahman Nasaruddin, Tiankai Chen, and Jianping Xie Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b00357 • Publication Date (Web): 22 Feb 2017 Downloaded from http://pubs.acs.org on February 26, 2017

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Golden Carbon Nanotube Membrane for Continuous Flow Catalysis Yanbiao Liu,1,3* Yuying Zheng,2,4 Bowen Du,2 Ricca Rahman Nasaruddin, 2 Tiankai Chen,2 Jianping Xie2*

1.

School of Environmental Science and Engineering, Donghua University, 2999 North Renmin Road, Shanghai, 201620 China. E-mail: [email protected]; Fax: +86 21 6779 2522; Tel: +86 21 6779 2545.

2.

Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117585 Singapore. E-mail: [email protected]; Fax: +65 6516 1936; Tel: +65 6516 1067.

3.

NUS Environmental Research Institute, National University of Singapore, 5A Engineering Drive 1, 117411 Singapore.

4.

Faculty of Chemical Engineering and Light Industry, GuangDong University of Technology, 100 Waihuan Xi Road, Guangzhou, 510006 China

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ABSTRACT: In this work, a high-performance catalytic membrane, composed of ultrasmall gold nanoclusters (AuNCs) and high aspect-ratio carbon nanotubes (CNTs), was designed for the continuous-flow catalytic reactions. In this hybrid catalytic membrane, the Au core of the NCs serves as high-performance catalyst, and the ligand of the NCs plays two key roles: 1) as well-defined surfactant assembly to effectively dissolve CNTs in aqueous solution, and 2) as efficient protecting ligand for Au core to avoid agglomeration. Due to the abovementioned features, a homogeneous 3D self-support catalytic membrane can be readily fabricated by vacuum filtration of the hybrid AuNCs/CNTs. The catalytic activity of the as-designed catalytic membrane was evaluated using 4-nitrophenol hydrogenation as a model catalytic reaction. The data suggest that the continuous flow catalytic reactor could achieve complete conversion of the substrate (i.e., 4-nitrophenol) within a single flow through the membrane with a hydraulic residence time (τ) of 3.0 sec. The catalytic membrane also showed enhanced catalytic kinetics as compared to the conventional batch reactor due to the convectively enhanced mass transfer. In addition, three important parameters, including the Au loading amount, substrate concentration, and flow rate, were identified as key factors that could affect the performance of the catalytic membrane.

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1. INTRODUCTION Supported gold (Au) particles on high surface area substrate are highly desirable for various heterogeneous catalytic reactions.[1, 2] Numerous studies have demonstrated that the catalytic reactivity of Au particles is highly size dependent, especially when the particles are in the nanoscale size range, where the physicochemical properties of Au nanoparticles are distinctively different from their bulk counterparts.[3-5] There are many successful attempts in using Au nanoparticles as efficient catalysts. However, the current synthesis protocols of supported Au nanocatalyts, such as co-precipitation, impregnation, and in situ reduction, often led to the formation of Au nanoparticles with a broad size distribution, mainly due to the relatively weak affinity of Au (or precursors) with the support and/or relatively high surface energy of these nanoparticles (therefore they tend to form agglomeration).[6-8] Furthermore, some fundamental aspects of Au nanocatalysts are also not well understood. For example, the origin of the high catalytic activity of Au nanocatalysts is still not very clear.[9-11] Different trends in the size-activity correlation of Au nanocatalysts were reported in different reaction systems.[12, 13] Further decreasing the size of Au particles to 2 nm or below, one can start to observe another distinctive change in their physicochemical properties. These sub-2 nm sized Au nanoparticles, often described as Au nanoclusters (AuNCs), have attracted recent interest in the research community due to their unique role in bridging the “missing link” between single Au atom and relatively large Au nanoparticles (e.g., particle size >2 nm).[14-21] In addition, ultrasmall AuNCs often feature with well-defined size/composition (at atomic precision), structure (molecular-like), and surface (well-organized), which could provide a good platform to address the abovementioned challenging issues in the Au nanoparticle system, such as the broad size distribution and complicated surface structure. [22-27]

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One effective way to utilize these nanocomposite catalysts is to fabricate catalytic membranes embedded with Au nanocatalysts,[28-31] which allow the separation and catalytic reaction to occur simultaneously in the same reactor. The catalytic membrane could also avoid the post-separation/recovery of the catalysts from the conventional batch reactor. In this study, we chose a water-soluble thiolate-protected Au25(SR)18 cluster (where SR is the thiolate ligand) as the model catalyst, and carbon nanotubes (CNTs) with well-defined morphology and surface as catalyst support and membrane materials.[32, 33] The structure of Au25(SR)18 is well understood both experimentally and theoretically in the NC community (e.g., it consists of a Au13 icosahedral core, capped by six –SR-Au-SR-Au-SR- motifs). In addition, Au25(SR)18 is highly stable in solution, even under some harsh reaction conditions.[34-37] Both the Au core and thiolate ligands (e.g., 6-mercaptohexanoic acid, MHA) of Au25(SR)18 play indispensable role in the design of our catalytic membranes. In particular, the Au core in the NCs could serve as high-performance catalyst, and the ligand shell of the NCs could work as a good surfactant to facilitate the dispersion of CNTs in water, as well as to provide good protection for Au core to avoid agglomeration. The high aspect-ratio CNTs can then be readily fabricated into a homogeneous 3D self-support membrane by vacuum filtration, with a uniform distribution of Au25(SR)18 NCs throughout the membrane network, both horizontally and vertically. To the best of our knowledge, this is the first successful attempt in the integration of the atomically precise AuNCs and high surface area CNTs into a catalytic membrane for continuous flow catalysis. The catalytic performance of the asdesigned membrane was evaluated using 4-nitrophenol hydrogenation as a model catalytic reaction, due to its simplicity and well-established characterization protocols. The effects of several key experimental parameters on the catalytic performance were also systematically

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studied, shedding some light on the good design of Au nanocatalysts in the catalytic membrane in a continuous flow reactor.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. All chemicals and materials were commercially available and

used

without

further

purification.

Hydrogen

tetrachloroaurate

(III)

hydrate

(HAuCl4·3H2O) was purchased from Alfar Aesar. 6-mercaptohexanoic acid (MHA, 90%), 4mercaptobenzoic acid (p-MBA, 99%), sodium borohydride (NaBH4, ≥98%), nitric acid (HNO3, 70%), and 4-nitrophenol (4-NP, ≥99%) were purchased from Sigma-Aldrich (Singapore). Sodium hydroxide (NaOH, ≥99%) was purchased from Merck (Singapore). Aqueous solutions were prepared using deionized water (DI-H2O) with resistivity of 18.2 MΩ•cm. Carbon nanotubes (CNTs) were purchased from Suzhou Jiedi Nanotech. Pte Ltd. (China). Regenerated cellulose dialysis tubing with a molecular-weight cut-off (MWCO) of 3.5 kDa was purchased from Fisher Scientific (Singapore). 2.2. Fabrication and characterization of the catalytic membranes. The Au25(MHA)18 and Au25(p-MBA)18 NCs were synthesized according to our reported NaOH-mediated NaBH4-reduction protocol[34] and carbon monoxide (CO)-reduction protocol,[38] respectively. The corresponding UV-vis absorption spectra of the as-synthesized AuNCs are in good agreement with the previously reported results,[34] which indicates the successful synthesis of the thiolate-protected Au25 NCs (Figure S1 in Supplementary Information). The assynthesized AuNCs were purified using dialysis for 3 h at 0 °C. CNTs were further treated with HNO3 at 100 °C for 12 h to introduce some oxy-functional groups on the tube walls (hereafter referred to as HNO3-CNTs). Various hybrid membranes with different amounts of AuNC loading were fabricated and comparatively studied. In a typical process, 15 mg of HNO3-CNTs were dispersed into a 5

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different concentration of AuNC solution (with an atomic concentration of Au from 5 to 50 µmol). The mixture was then probe-sonicated for 30 min. After that, the homogeneous dispersion of HNO3-CNTs and AuNCs was vacuum-filtered onto a 5 µm Millipore JMWP PTFE membrane (Billerica, MA). The membrane was further washed with 150 mL of DIH2O to remove any residue reactants and impurities. A pristine HNO3-CNTs membrane was prepared by a similar method except using DI-H2O to replace the AuNC solution. The morphologies of the as-fabricated membranes were examined on a JEOL JSM-6700F field emission scanning electron microscopy (FESEM) and a JEOL 2100 transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) analysis was performed on a VG ESCALab 220i XL system (Thermo VG Scientific Ltd., UK) under high vacuum (1 × 10-9 Torr). The binding energies of the identified elements were calibrated by the C 1s (C-C bond) at 284.5 eV. The measurements of contact angles of the membranes were carried out on a Contact Angle Goniometer (Rame Hart), and DI-H2O was used as the probe liquid to evaluate the hydrophilicity of the membrane surface. The gold concentration of the hybrid membrane was further determined by an Agilent 7700x inductively coupled plasma mass spectrometer (ICP-MS, Santa Clara, CA). 2.3. Evaluation of the catalytic performance of the membranes. The catalytic performance of the as-fabricated membranes was evaluated using the conversion of 4nitrophenol (4-NP) to 4-aminophenol in the presence of Au nanocatalysts (Figure S2 in Supplementary Information). All the experiments were conducted with a commercial polycarbonate Whatman filtration casing (D = 47 mm; Piscataway, NJ). After sealing the filtration casing and priming with DI-H2O, a Masterflex L/S digital peristaltic pump (Vernon Hills, IL) was used to pump DI-H2O through the membrane for rinsing and calibration purposes. After that, an aqueous solution of 4-NP (0.15 mmol/L) was pumped into the filter to achieve the adsorptive saturation at a flow rate of 1.0 mL/min. Subsequently, various 6

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amount of freshly prepared NaBH4 solution was passed through the membrane together with the 4-NP to induce the catalytic hydrogenation reaction. Effluents were collected and analyzed using a Shimadzu UV-1800 UV-vis spectrometer (Singapore). The corresponding residence time τ and the rate constant k were calculated according to the recent reports.[30, 31] As a comparison, the control experiments were performed in a conventional batch reactor system using the as-fabricated catalytic membranes and 10 mL of aqueous solution of 4-NP (0.15 mmol/L) and NaBH4 (56 mmol/L). The conversion of 4-NP was calculated via a pseudo-first-order kinetics (Equation 1) considering the large excess of NaBH4 in the reaction solution. ஼

ln ቀ ೟ ቁ = −݇t ஼

(1)



where Ct and C0 are the concentrations of 4-NP at the time t and the initial stage, respectively.

3. RESULTS AND DISCUSSION 3.1. Characterizations of the hybrid membranes. Figure 1(a) and (b) compare the field emission scanning electron microscopy (FESEM) images of the HNO3-treated CNT membranes before and after the loading of AuNCs. Both samples show uniform porous microstructure with pore size of ~100 nm. Meanwhile, no obvious difference was observed in these two samples, which suggests that the loading of AuNCs did not affect much the CNTs’ surface morphology. This could be attributed to the ultrasmall size of the as-synthesized AuNCs. The combination of AuNCs with CNTs was further examined by transmission electron microscopy (TEM). The diameter of the multiwall CNTs is 15 ± 3 nm, as shown in Figure 1(c). As shown in Figure 1(d), the AuNCs are uniformly distributed on the surface of the CNTs, and the NCs have an average diameter of 1.5 ± 0.4 nm (please refer to Figure S3 in Supplementary Information for more details). This data provides supportive evidence for the 7

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successful loading of AuNCs onto CNTs. It should be noted that, due to the support effect of CNTs, the size of the AuNCs on the CNT surface was slightly bigger than those reported NCs in solution.[34]

Figure 1. Representative FESEM and TEM images of (a, c) HNO3-CNTs and (b, d) AuNCs/CNTs membranes.

It is well known that the pristine CNTs have a super-hydrophobic surface, and they can be hardly dispersed in DI-H2O even under strong ultrasonic treatment.[39] Although the treatment with HNO3 introduced a certain amount of oxy-functional groups (e.g., carboxylic and hydroxyl groups) on the CNT surface,[40] which has significantly decreased the water contact angle from 132.2 ± 1.5° to 60.5 ± 7.0°; the as-treated HNO3-CNTs can only be dispersed well in DI-H2O at a very low concentration of ≤0.1 mg/mL (Figure S4 in Supplementary Information). Interestingly, when replacing DI-H2O with the as-prepared aqueous AuNC 8

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solution (low concentration and the major component is still DI-H2O), a much higher concentration of HNO3-CNTs can be well dispersed in solution (e.g., 10-folds increase to ≥1.0 mg/mL; Figure S5 in Supplementary Information). The enhanced solubility of CNTs in the presence of AuNCs could be attributed to two factors: electrostatic attraction and hydrophobic interaction between AuNCs and HNO3CNTs. To investigate the detailed interactions between the AuNCs and CNTs, we measured the zeta potential of the samples. The data suggest that both the AuNC solution (ζ = -30.9 ± 1.5 mV) and the aqueous dispersion of HNO3-CNTs (ζ = -35.9 ± 0.9 mV) carried negative charges, indicating an electrostatic repulsion rather than attraction exist between the two parties. On the other hand, it should be noted that our AuNCs are stabilized by a number of thiolate ligands, which possess some hydrophobic segments (e.g., alkyl chain or aromatic group) that could facilitate the hydrophobic interactions.[34,

38, 41]

Therefore, if the

hydrophobic interaction between AuNCs and HNO3-CNTs dominates, the hydrophobic alkyl chain of the thiolate ligands (i.e., MHA) of AuNCs could attach to the hydrophobic aromatic moieties of the surface of HNO3-CNTs, which could further increase the hydrophilicity of the hybrid membranes. This hypothesis was verified by the water contact angle measurement, where the water contact angle was indeed decreased further from 60.5 ± 7.0° of HNO3-CNTs to 30.9 ± 10.5° of AuNCs/CNTs hybrid membranes. To further verify the effects of hydrophobic interactions between the thiolate ligands on the surface of AuNCs and the aromatic moieties of CNTs, we prepared AuNCs protected by another thiolate ligand carrying the hydrophobic benzene ring (e.g., 4-mercaptobenzoic acid, p-MBA). The data also suggest that the HNO3-CNTs can be uniformly distributed in the as-prepared Au25(p-MBA)18 NC solution under similar experimental conditions (Figure S6 in Supplementary Information). Therefore, the hydrophobic interaction between the AuNCs and CNTs may play a key role to promote the dispersion of CNTs in aqueous solution. In particular, the ligands of AuNCs 9

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could not only serve as the protecting ligand to protect the AuNCs from agglomeration, but also serve as good surfactants (or surfactant assembly) to facilitate the dispersion of CNTs in aqueous solution. The possible interactions between the AuNCs and CNTs are illustrated in Scheme 1. It should be mentioned that a recent study also highlighted the important role of thiolate ligands of AuNCs on their catalytic activity in solution.[42] In particular, this study suggests that the Au25(SR)18 NCs bearing a shorter alkyl chain (in SR) showed a better accessibility (to the substrates) and higher catalytic activity in solution.

Scheme 1. Schematic illustration of the proposed mechanism of the as-designed hybrid catalysts.

We further used X-ray photoelectron spectroscopy (XPS) to obtain the chemical composition of the as-fabricated hybrid membrane. As shown in Figure 2a, the XPS data clearly indicate the presence of Au 4f, S 2p, C 1s, O 1s, and Na 1s, while the HNO3-CNTs 10

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membrane only shows the signal of C 1s and O 1s. In particular, the S 2p was from the thiolate ligand of the AuNCs (i.e., MHA) and Na 1s was from the excess amount of NaBH4 used to reduce the Au salt to AuNCs that are adsorbed onto the membrane surface. These data unambiguously confirm the successful embedding of Au25(MHA)18 NCs inside the membrane. More specifically, the high-resolution scan of Au 4f over a small energy window (Figure 2b) indicates the Au 4f5/2 and Au 4f7/2 peaks at 88.4 eV and 84.7 eV, respectively, which could be further deconvoluted into Au(I) (Au 4f5/2 peak at 88.4 eV and Au 4f7/2 peak at 84.7 eV) and Au(0) (Au 4f5/2 peak at 87.4 eV and Au 4f7/2 peak at 84.2 eV). This data suggests that the oxidation states of Au in Au25(MHA)18 NCs are between 0 and 1, which agrees well with the reported XPS data of thiolate-protected AuNCs.[6, 34] The atomic ratio of C and Au on the hybrid membrane surface was ~80.9% and 0.52%, respectively, according to the XPS data. These numbers are also consistent with the theoretical molar ratio of Au/C, which was calculated to be 0.6%. Figure 2c shows the high-resolution scan of C 1s. The data indicates the existence of C sp2 (284.5 eV), C sp3 (285.0 eV), C-OH (286.0 eV), O=C-OH (288.4 eV), and π-π* transition (291.4 eV, HOMO–LUMO). The –OH group centered at 286.0 eV was originated from the nitric acid treatment, and the –COOH group was either from the protecting ligands of AuNCs or from the nitric acid treatment of CNT surface. The relatively low intensity peak at 291.4 eV is due to the π-π* shake-up interactions.[43] This minor peak is the signature of any high-quality undisturbed sp2 graphitic material (i.e., electrically conductive). The high-resolution scan of O 1s (Figure 2d) can be further deconvoluted into components of C-O (531.5 eV), C-OH (533.1 eV), and Na KLL (535.9 eV, sodium Auger peak). In addition, the energy-dispersive spectroscopy (EDS) spectra and elemental mapping also confirm the presence of the Au element on the AuNCs/CNTs hybrid membrane, again suggesting the successful loading of AuNCs onto the CNTs (Figure S7 in Supplementary Information). 11

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Intensity (a.u.)

[email protected] filter HNO3-CNT filter

C 1s

Au 4f7/2

(a)

(b)

Au 4f Au 4f5/2

O 1s

Na 1s

Au 4d

O KLL

Au 4f S 2p

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800

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0

95

90

85

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2

C sp

C 1s

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(c)

C-O

O 1s

(d)

C-OH

Na KLL

3

C sp

C-OH π-π*

295

O=C-OH

290

285

280

Binding Energy (eV)

540

535

530

525

Binding Energy (eV)

Figure 2. (a) Survey XPS spectra of the AuNCs/CNTs membrane (with 10 µmol Au loading) and high-resolution XPS spectra of (b) Au 4f, (c) C 1s, and (d) O 1s.

3.2. Comparison of the catalytic activity of batch reactor with the catalytic membrane. The catalytic activity of the as-fabricated membranes was evaluated using the conversion of 4-nitrophenol (4-NP) into 4-aminophenol as a model catalytic reaction in the presence of AuNC catalysts. Figure 3a shows the time-dependent UV-vis absorption spectra of the conversion of 4-NP with the as-fabricated AuNCs/CNTs catalytic membrane (with 10 µmol Au loading) in a conventional batch reactor system (Figure S8 in Supplementary Information). The intensity of the characteristic absorption peak at 400 nm, associated with 4NP, was rapidly decreased with time due to the continuous consumption of the substrate (4NP). Correspondingly, the characteristic absorption peak at 300 nm, associated with 4aminophenol, was emerged and increased with time. The linear relationship between ln(C/C0) 12

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of 4-NP and the reaction time (min) confirms the pseudo-first-order kinetics of our catalytic reaction in the batch system, and the reaction constant (k) was calculated to be 0.20 min-1 (Figure 3b). This value is similar as the reported data in a graphene-supported Au nanocatalyst system with a specific rate constant of 0.19 min-1.[44] However, it is much lower than a recent reported value of 3.9 min-1 using a pure Au25(MHA)18 as the catalyst in a typical batch reactor.[45] The difference could be attributed to the changes of the reaction volume (10 mL vs. 1.5 mL), substrate concentration (0.15 mmol/L vs. 1.2 mmol/L), NaBH4 amount (or the excess times; ×250 vs. ×20,000), and more importantly to the mass transfer rate (heterogeneous vs. homogeneous) in the two different systems. As a comparison, we also carried out the catalytic reaction using the hybrid AuNCs/CNTs membranes synthesized by vacuum filtration of their homogeneous dispersions on top of a commercial 5 µm PTFE support membrane (Figure S9 in Supplementary Information). To eliminate the contribution from physical adsorption of 4-NP by the as-fabricated membrane filters, the adsorption breakthrough of 4-NP was achieved before the addition of NaBH4 for both the hybrid filter and the HNO3-CNTs filter. The breakthrough curves for [4-NP]in of 0.15 mmol/L onto the filters are shown in Figure 3c. In the first few (<5) minutes, only limited 4-NP was detected in the effluent for both filters, indicating a quantitative sorption within the residence time of τ ≤ 3.0 sec. The rapid 4-NP sorption can be due to its relatively strong π-π interaction with the sp2-conjugated CNT walls. However, the limited thickness or the thin layer of the hybrid filter (35 ± 5 µm for the catalytic (or CNT) layer and 75 ± 8 µm for the PTFE support layer, Figure S10 in Supplementary Information) would lead to a relatively low absolute sorption capacity of 4-NP, and in turn 4-NP breakthrough occurred at ~2 h at a flow rate of 1.0 mL/min for both filters. To evaluate the reactivity of the continuous catalytic membrane filter, a 200 mL aqueous solution of 0.15 mmol/L 4-NP with freshly prepared 28.13 mmol/L NaBH4 was prepared (N2 13

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was purged into the solution to remove any O2 residue before the filtration) and passed through the membrane filter using a peristaltic pump. As shown in Figure 3d, during the passing through the hybrid filter (with 10 µmol Au loading), the effluent color immediately changed from dark yellow to colorless. This phenomenon maintained until all the influent solution was consumed (i.e., over ~3 h), indicating the excellent catalytic activity and stability of the hybrid membrane filter. After checking the UV-vis absorption spectra of the influent and effluent (over ~3 h continuous filtration) samples, the data indicates that the characteristic absorption peak of 4-NP at 400 nm was not observed and the characteristic absorption peak of 4-aminophenol at 300 nm was emerged instead. The hydraulic residence time within the hybrid filter took only ≤3.0 sec at a flow rate of 1.0 mL/min, although simply increasing the flow rate can also accelerate the filtration process. However, a control experiment using the HNO3-CNTs alone filter (i.e., without the AuNC catalysts) only demonstrated negligible changes in the characteristic absorption of 4-NP, confirming the important role of the AuNCs in the catalytic membrane. Compared with the conventional batch system where the catalytic performance was predominantly controlled by the diffusion process of the reactants (as shown in Figure 3a or the reported data[45]), the evidently enhanced catalytic kinetics in our catalytic membrane can be ascribed to the convectivelyenhanced mass transfer during the filtration. This is another important feature of the catalytic membrane. Additionally, in the batch reactor, the Au catalysts tend to aggregate due to their high surface energy, which might also block some active sites, leading to the decrease of the catalytic activity over the reactions. However, in the current design, the AuNC catalysts were well-protected by the thiolate ligands, and they were uniformly distributed throughout the CNTs matrix, which could further enhance their stability in the catalytic membrane, especially over a long run of catalytic reactions.

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(b)

Influent 1 min 2 min 3 min 4 min 5 min 7 min 9 min 12 min 15 min 20 min

2

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60 40 HNO3-CNT

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[email protected]

20 0

[email protected]

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Figure 3. (a) Time-dependent UV-vis absorption spectra from 280 to 500 nm, and (b) the rate kinetics for the conversion of 4-NP in a conventional batch reactor using the AuNCs/CNTs hybrid membrane. (c) Breakthrough curves of 4-NP only (without the addition of any NaBH4; the data was obtained at the wavelength of 317 nm). (d) Comparison of the UV-vis absorption spectra before and after passing through the AuNCs/CNTs hybrid membrane and the HNO3-CNTs membrane.

We also investigated the effect of various parameters on the catalytic performance of the hybrid membrane filters. As indicated in Figure 4a, the Au loading amount is an important parameter. Only 53 ± 4% of 4-NP could be catalytically converted if the Au loading was 5 µmol in the catalytic membrane. However, once the Au loading was increased to ≥10 µmol, a complete conversion of 4-NP could be achieved. This trend can be explained by the better accessibility of the 4-NP to the Au catalysts at a higher Au loading. Meanwhile, the substrate (i.e., 4-NP) concentration (Figure 4b) and flow rate (Figure 4c) could also affect the catalytic performance of the hybrid membrane filters. Increasing the substrate concentration (≥1.0 mmol/L) or the flow rate (≥2.0 mL/min) would lead to an incomplete conversion of 4-NP (or 15

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decreased catalytic performance) at the given experimental conditions. The effect of substrate concentration suggests that the mass transport limitation could be caused by the relatively limited catalytic active centers (or limited Au catalysts) inside the membrane when compared to the substrates. The flow rate effect can be ascribed to the decreased residence time when the reactants were passing through the hybrid filter with an increased flow rate, and the substrates might not have sufficient time to contact with the AuNC catalysts. This trend was in well accordance with a reported catalytic membrane system employing the protein fibrils loaded with alloyed nanoparticles for the same model catalytic reaction.[30] Furthermore, given that the concentration of NaBH4 has greatly exceeded that of the 4-NP in the catalytic system, the catalytic kinetics can be considered as a pseudo-first-order, which was further confirmed by the similar effluent absorption spectra, regardless of the NaBH4 amount (Figure 4d).

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Figure 4. The effect of (a) Au loading, (b) 4-NP concentration, (c) flow rate, and (d) NaBH4 excess times on the conversion of 4-NP using the AuNCs/CNTs catalytic membranes.

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We further used FESEM to monitor the evolution of the morphology of the assynthesized hybrid membrane filter after ~3 h continuous operation. As shown in Figure S11 (Supplementary Information), although a large amount of precipitates and/or polydispersed nanoparticles were seen on the membrane surface, which could be polyphenol or the accumulation of the 4-NP/4-aminophenol on the CNT sidewalls via strong π-π interactions, we didn’t observed obvious catalytic reactivity deterioration in our reactor. However, the buildup of these precipitates/polymers might cause some adverse effects on the catalytic performance by increasing the resistance to the mass transfer of the reactants and blocking a certain amount of reactive centers of the AuNC catalysts. This issue can be addressed by applying certain engineering methods. For example, a physical/chemical washing of the CNT filter with an acid-ethanol solution (e.g., mixture of 50% ethanol with 50% 1.0 M HCl) could be used to remove the major buildup on the filter. Additionally, based on the ICP-MS data, the effluent Au concentration (for the 10 µmol Au loading hybrid filter) over a 3-h continuous operation was determined to be 7.58 ± 2.44 µg/L, which accounts for ~0.15% of the initial Au loading on the hybrid filter. This data suggests negligible Au leakage during the catalytic reactions inside the membrane. It should also be noted that the initial Au loading concentration in the membrane (determined from the ICP-MS data) was 2.47 µmol, which is ~25% of the theoretical value (i.e., 10 µmol). The difference in Au concentration might be due to the ultrasmall size (<2 nm) of the AuNCs. Therefore, compared to the relatively larger pore size of the HNO3-CNTs matrix (i.e., ~100 nm in diameter), a certain amount of Au loss was inevitable during the fabrication of the hybrid membrane filter via the vacuum filtration. Furthermore, both the XPS (Figure S12 in Supplementary Information) and ICP-MS data (e.g., the Au concentration was determined to be 2.35 µmol for the hybrid filter after a continuous catalytic reaction) for the hybrid membrane filters before and after catalytic reaction for ~3 h suggest that only <5% Au loss was observed, further highlighting the 17

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desirable stability of the as-designed catalytic membrane. The high stability of the AuNC catalysts inside the membrane could be crucial for their further applications in the industry sector.

4. CONCLUSIONS In summary, the catalytic reactivity of ultrasmall AuNCs immobilized on CNTs network has been systematically investigated. In the current design, both the AuNCs and CNTs are indispensable for the fabrication of high-performance catalytic membrane. In particular, the Au core of the NCs serves as good catalyst for a certain catalytic reaction, and the ligand of the NCs plays an important role as good surfactant to effectively dissolve the CNTs in aqueous solution, and as organic molecule to protect the Au core (and catalytic centers) to avoid agglomeration. The hybrid AuNCs/CNTs can then be easily fabricated into a homogeneous 3D self-support membrane by vacuum filtration. The catalytic performance of the as-designed membrane in the conversion of 4-nitrophenol to 4-aminophenol was comparatively studied. The continuous flow catalytic system showed an enhanced catalytic kinetics as compared to the conventional batch reactor. The good catalytic activity of the ultrasmall AuNC catalysts combined with the high aspect-ratio, mechanically stable, and electrically conductive CNTs allows for the rational design of various continuous flow catalytic membranes for potential industrial applications.

Supporting Information Experimental details for schematic illustration of the experimental set-up, digital photos for HNO3-CNTs dispersion results and materials characterizations; this material is available free of charge via the Internet at http://pubs.acs.org. 18

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Corresponding Author [email protected] and [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Acknowledgments This invited contribution is part of the I&EC Research special issue for the 2017 Class of Influential Researchers. Part of this work is financially supported by Ministry of Education, Singapore, under the grant R-279-000-481-112. Y. Liu thanks the start-up grant from Donghua University. We thank Professor Choon Nam Ong (NUS Environmental Research Institute, NERI) for his great help on this project. We also thank Ms. Per Poh Geok for her kind help on ICP-MS measurement and Mr. Zhang Hong for the XPS data analysis.

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