Rational Design of High-Performance Continuous-Flow Microreactors

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Rational Design of a High-Performance Continuous-Flow Microreactors based on Gold Nanoclusters and Graphene for Catalysis Yanbiao Liu, Xiang Liu, Shengnan Yang, Fang Li, Chensi Shen, Manhong Huang, Junjing Li, Ricca Rahman Nasaruddin, and Jianping Xie ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03858 • Publication Date (Web): 20 Sep 2018 Downloaded from http://pubs.acs.org on September 23, 2018

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ACS Sustainable Chemistry & Engineering

Rational Design of a HighHigh-Performance ContinuousContinuous-Flow Microreactors based on Gold Nanoclusters and Graphene for Catalysis Yanbiao Liua,c,d* , Xiang Liua, Shengnan Yanga, Fang Lia,c, Chensi Shena,c, Manhong Huanga,c, Junjing Li d, Ricca Rahman Nasaruddinb, Jianping Xieb*

ACS Sustainable Chemistry & Engineering Revision Submitted August 31st, 2018 a

Textile Pollution Controlling Engineering Center of Ministry of Environmental

Protection, College of Environmental Science and Engineering, Donghua University, 2999

North

Renmin

Road,

Shanghai

201620,

P.

R.

China.

E-mail:

[email protected]; Fax: +86 21 6779 2522; Tel: +86 21 6779 8752. b

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. c

Shanghai Institute of Pollution Control and Ecological Security, 1239 Siping Road,

Shanghai 200092, P. R. China. d

State Key Laboratory of Separation Membranes and Membrane Processes, Tianjin

Polytechnic University, 399 Binshuixi Avenue, Tianjin 300387, P. R. China.

1

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract In this work, we rationally designed a high-performance microreactor system for continuous-flow catalysis. The membrane consists of ultra-small gold nanoclusters (AuNCs) and two-dimensional graphene. The Au cores of the NCs act as catalysts, while their ligands have two functions: (1) protecting the Au cores to avoid agglomeration, and (2) providing a well-defined surfactant assembly to disperse graphene in aqueous solution. Hydrogenation of 4-nitrophenol (4-NP) was employed as model reaction to evaluate catalytic activity. The catalytic membrane microreactor demonstrated excellent catalytic activity and stability, where complete 4-NP conversion was readily achieved via a single pass through the membrane. This desirable performance was maintained over 12 h of continuous operation, although a certain amount of organic build-up on the membrane was observed. The catalytic membrane microreactor outperforms conventional batch reactors due to its improved mass transport. 4-NP-spiked real water samples were also completely converted. This study provides new insights for the rational design of membrane reactor for industrial applications.

Keywords: Gold nanoclusters; Reduced graphene oxide; Catalytic membrane microreactor; Hydrothermal; 4-Nitrophenol

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Introduction Recently, catalytic membrane microreactors have attracted much attention for their potential applications in a wide variety of fields.1-3 Most membrane microreactors adopt a continuous-flow design with catalysts either attached on, or embedded in, a support membrane. This allows separation and catalytic reactions to occur within a single unit. Such a design is amenable to automated processes and features low cost, easy operation, and up-scalability.4-6 Compared with conventional batch systems, catalytic membrane microreactors avoid post-separation of catalyst and improve mass transport performance.7,

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However, designing an effective catalytic

membrane microreactor with good catalytic and separation reaction performance remains a great challenge. Recently, gold (Au) nanoparticles have been widely applied as catalysts for various reactions due to their high catalytic activity.7, 9-14 The properties of Au nanoparticles are distinctly different from those of inert bulk Au.15-18 Numerous studies have suggested that the catalytic activity of Au is highly size-dependent, especially when the particle sizes are within the nanoscale range.19, 20 For instance, Link and El-Sayed reported that the surface plasmon resonance of Au nanoparticles can be tuned by changing their size.21 However, current preparation methods for supported Au catalysts (e.g., in situ reduction of Au salts and co-precipitation) often lead to broad nanoparticle size distributions because of the high surface energy of nanoscale Au and its poor affinity with support materials.22-24 These limitations can negatively impact the catalytic activity of Au.25 Alternatively, Au nanoclusters (NCs) with atomically precise compositions have the potential to address the above issues. At very tiny particle sizes (≤ 3 nm), molecular-like properties become apparent in these ultra-small AuNCs.26 Distinctive

3



changes in the physical chemical properties of Au particles in this size range can also be observed. AuNCs typically contain several to a few hundred Au atoms, which are stabilized by a typical organic ligand such as thiolate ligands. Recently, there has been extensive study of these sub-3 nm AuNCs due to their unique role in providing a “missing link” between single Au atoms and relatively large Au nanoparticles (> 3 nm).27-30 It is noteworthy that atomically precise AuNCs have well-defined sizes and compositions, and well-organized surfaces. Therefore, they provide an ideal platform for addressing existing problems associated with Au nanoparticles. For example, Nasaruddin et al. recently highlighted the importance of ligands in modulating the active sites of nano-Au catalysts, which could be used to direct their catalytic reaction pathway in solution.31 Furthermore, the support material used to host Au catalysts plays an important role in the design of high-performance catalytic systems.32 For example, Huang et al. designed a low-cost β-lactoglobulin fibril-supported Cu-Ag-Au alloy nanoparticle hybrid membrane with excellent catalytic activity.33 He et al. developed a highly effective fixed-bed reactor employing glass fibers supporting Au nanowires as catalytic fillers.34 Some synergistic effects between Au catalysts and their predesigned supports have also been reported.35 Among these support materials, carbonbased materials, such as carbon nanotubes and graphene, are promising “hosts” for nano-Au catalysts due to their rich surface chemistries, abundant reactive sites, large specific surface area, good stability and mechanical properties.36, 37 However, to the best of our knowledge, there are no reports on using combinations of two-dimensional materials with ultra-small and atomically-precise NCs for continuous flow catalytic membrane applications. In this study, we rationally designed a catalytic membrane microreactor system 4


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composed of a water-soluble thiolate-protected AuNC catalyst and reduced graphene oxide (rGO). In this design, both the AuNC catalyst and the graphene are indispensable. In particular, the Au atoms in the AuNCs serve as active catalytic centers. The organic ligands of AuNC serve as a stabilizer of Au atoms so that they can avoid agglomeration. In addition, these ligands can work as a hydrophobic surfactant assembly, which can facilitate the dispersion of hydrophobic graphene in aqueous solution (via hydrophobic interaction). The rGO serves as a catalyst support, as well as a membrane material. With these synergistic effects, a high-performance catalytic membrane microreactor can be fabricated by simple vacuum filtration, with AuNCs uniformly distributed throughout the membrane. The catalytic reactivity of the as-designed membrane reactor was evaluated using 4-nitrophenol reduction as a model reaction, due to its well-established characterization protocol.34 The catalytic performance of a continuous flow catalytic membrane was comparatively studied with a conventional batch reactor. The impacts of various key operational parameters on catalytic performance was systematically studied. Finally, three 4-NP-spiked real water samples were used to challenge the catalytic filter to evaluate its practical applicability.

Experimental section Chemicals and materials All chemicals were used without further purification. Graphene oxide was purchased from Suzhou Graphene Nanotech Co., Ltd. (China). Sodium hydroxide (NaOH, ≥96%) and N-Methyl-2-pyrrolidone (NMP, ≥99.5%) was obtained from Sinopharm Chemical Reagent Co., Ltd. Gold (III) chloride trihydrate (HAuCl4·3H2O, 5



≥49% Au basis), 6-mercaptohexanoic acid (MHA, 90%), 4-mercaptobenzoic acid (MBA, 99%) and sodium borohydride (NaBH4, ≥98%) were purchased from SigmaAldrich. 4-nitrophenol (4-NP, ≥99%) was purchased from Alfar Aesar. Ultra-pure water produced from a Milli-Q Direct 8 purification system was used for the preparation of AuNCs. Other aqueous solutions were prepared using deionized water (DI-H2O) with a resistivity of 18.2 MΩ·cm. Regenerated cellulose dialysis tubing with a molecular weight cut-off (MWCO) of 3.5 kDa was purchased from Fisher Scientific. Fabrication of the catalytic membrane microreactor Au25(MHA)18 was synthesized according to our reported NaOH-mediated NaBH4-reduction protocol.38 The as-synthesized AuNCs were purified by dialysis for 3 h at 0 °C. Their corresponding UV-Vis absorption spectra were in good agreement with reported results,39 indicating the successful synthesis of thiolate-protected AuNCs (Figure S1, Supporting Information). Reduced graphene oxide (rGO) with different reduction extent was synthesized via a hydrothermal route40 and labeled as xh-rGO (x = 3, 4, 5, 10, and 20 h) depending on the hydrothermal time. In a typical procedure for the fabrication of 4hrGO, 2.0 mg/mL of GO was dispersed in DI-H2O under ultrasonic irradiation for 15 min, and then the mixture was transferred into a 50 mL Teflon autoclave, sealed, and heated at 180 °C for 4 h. Various hybrid membranes with different amount of AuNCs loading (atomic concentration of Au ranging from 5 to 20 µmol) were fabricated and comparatively studied. In a typical process, 5 mg of 4h-rGO was added into 10 mL of AuNC and 10 mL of DI-H2O mixed solution. After ultrasonication for 45 min, the homogeneous dispersion of rGO and AuNC was vacuum-filtered onto a 5 µm JMWP PTFE 6


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membrane (Merck, Germany). Finally, the catalytic membranes were further washed with 100 mL of DI-H2O to remove extra solvents. To evaluate the effect of AuNCs on catalytic performance, pure rGO membranes were prepared by dispersing the same amount of 4h-rGO in 20 mL NMP and vacuum filtrating it onto a PTFE support. Characterizations The morphologies of the as-fabricated catalytic membranes were characterized by a Hitachi S-4800 field emission scanning electron microscopy (FESEM, Japan), a JEM-2100 transmission electron microscopy (TEM, Japan), and an ARM200F scanning transmission electron microscope (STEM, Japan). The STEM was equipped with a high angle annular dark field detector and a spherical aberration correction system. Samples for TEM and STEM analysis were prepared by dropping a homogeneous dispersion solution of rGO and AuNCs onto a lacey-carbon film supported by a 300-mesh copper TEM grid and operated in a N2 glovebox. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Scientific Escalab 250Xi under high vacuum (1 × 10−9 Torr). The binding energies of the identified elements were internally referenced to the C 1s peak at 284.8 eV. The contact angles of the hybrid membranes were measured with a Ramé-Hart Model 400 contact angle goniometer (Succasunna, NJ) to evaluate the effect of AuNCs loading on the wettability of the hybrid membrane. The zeta potentials of rGO and AuNCs were determined by a ZEN 3600 zeta potential analyzer (Malvern, UK). The gold concentration of the effluent was determined by an Agilent 7700× inductively coupled plasma mass spectrometer (ICP-MS, Santa Clara, CA). Catalytic reduction of 4-nitrophenol The catalytic hydrogenation reaction of 4-nitrophenol (4-NP) to 4-aminophenol was chosen as a model reaction. All the continuous flow catalytic processes were 7



conducted with a polycarbonate Whatman filter casing (D = 47 mm; Piscataway, NJ). The flow rate was controlled by a 4-channel digital peristaltic pump (Ismatec REGLO, Wertheim, Germany). The membrane was first wet with DI-H2O for rinsing and calibration purposes. Then, an aqueous solution of 4-NP was pumped into the membrane to achieve sorption saturation to eliminate the contribution of adsorption. Subsequently, certain amount of freshly prepared NaBH4 was passed through the membrane together with 4-NP to induce the hydrogenation reaction (Figure S2, Supporting Information). Aliquots of effluent samples were collected and characterized by UV-Vis spectrometry (Shimadzu UV-2600, Japan) to evaluate the activity of the catalytic membrane microreactor. To compare the corresponding reaction kinetics, control experiments were carried out in a conventional batch reactor using the same catalytic membranes. Given that there was a large excess of NaBH4 in the medium, pseudo-first-order kinetics were used to simulate the reaction kinetics of 4-NP conversion.

Results and discussion Characterizations of the catalytic membrane microreactor The combination of AuNC with rGO was first characterized by transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). As shown in Figures 1a and b, with an average diameter of 1.7 ± 0.2 nm, these AuNCs are uniformly distributed on the surfaces of rGO nanosheets. No aggregations of NCs were observed. All these data provide evidence that AuNCs were successfully loaded onto the rGO. Meanwhile, the presence of AuNCs among layers of rGO sheets, together with intrinsic graphene corrugation, defects, and/or voids, may provide nanochannels that are permeable to fluids (Figure S3, Supporting Information). STEM 8


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characterization of the catalytic membrane samples was performed. As displayed in Figure 1c, ultra-small AuNCs with average size of 1.7±0.2 nm can be observed. Detailed energy dispersive X-ray (EDX) elemental mapping of the as-synthesized material at a resolution of 5 nm shows the coexistence of Au, C, and S signals throughout the rGO support (Figure 1d). Each AuNC consisted of a bright core corresponding to the reported structure of Au25SR18 NCs (SR denotes the thiolate ligand), i.e., an Au13 icosahedral core capped by six −SR−Au−SR−Au−SR−motifs. It is also can be observed that the Au and S mappings both show strong intensities at the Au core and clear decreases in intensity at the edge regions, in accordance with the core-shell structural characteristics of AuNCs. Figure S4a (Supporting Information) shows a photograph of the as-fabricated flexible catalytic membrane microreactor on a PTFE support, which was quite thin (2.0 ± 0.2 µm for 5 mg rGO, and 4.8 ± 0.5 for 10 mg rGO; Figure S5, Supporting Information). The membrane surface was smooth, uniform, and black in color. Examination by field emission scanning electron microscopy (FESEM) identified a typical, crumpled, sheet-like microstructure of the graphene-based membrane, but no AuNCs were observed (Figure S4b, Supporting Information). Also, no differences were evident between the rGO membrane and the AuNC-rGO hybrid membrane (Figure S6, Supporting Information). This data suggests that AuNC loading did not significantly affect the surface morphology of rGO due to the ultra-small size of the as-synthesized AuNCs.

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Figure 1. TEM images (a and b) of the AuNC/4h-rGO catalytic membrane microreactor; high-resolution STEM image (c), and EDS mapping (d) of a single AuNC on rGO.

It is well-known that GO nanosheets are negatively charged due to being heavily decorated with oxygen-containing groups such as hydroxyl, carbonyl and carboxyl groups.41 It was also expected that these oxy-functional groups in GO could be partially or completely reduced via hydrothermal treatment in a “water-only” system.42 Overheated supercritical water can serve as an effective reducing agent for GO. When the hydrothermal reaction was conducted for less than 4 h, the products could be well-dispersed in DI-H2O under ultrasonic treatment. This data suggests that 10


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only limited reduction was achieved within this period, and the remaining oxyfunctional groups could facilitate its dispersion in DI-H2O. Although a uniform membrane can still be fabricated by vacuum filtration of such dispersion solution, the membrane stability is very poor. Once in contact with water, the hydrophilic nanosheets tend to peel off the substrate and deteriorate the membrane integrity. As expected, 0.25 mg/mL of rGO (with 4 h or longer hydrothermal treatment) was dispersed poorly in DI-H2O even under 2 h continuous ultrasonication (Figure S7a, Supporting Information). This data provides evidence of successful reduction of the oxy-functional groups of GO. Meanwhile, a pure GO membrane displayed a typical brown color, while an rGO membrane with 4 h hydrothermal treatment showed a totally different black color. This color difference also provides evidence of successful GO conversion during the hydrothermal process. Interestingly, when replacing the DI-H2O with a certain amount of the as-produced AuNC solution, a 1.0 mg/mL or even higher concentration of the 4h-rGO could be uniformly dispersed in aqueous AuNC solution (Figure S8, Supporting Information). It is noteworthy that the major component of the AuNC solution was still DI-H2O with an Au concentration of only 2 mmol/L) or the flow rate (>2 mL/min) could also lead to incomplete conversion of 418


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NP under the given experimental conditions (Figure 3d). The effect of 4-NP concentration can be explained by the mass transport limitation of the current catalytic membrane system. Meanwhile, the effect of flow rate can be explained by decreased residence time within the catalytic membrane causing insufficient contact between 4NP and the active sites.

Figure 4. Reductive flux of the AuNC/4h-rGO membrane with various concentrations of 4-NP and amounts of Au at flow rates of (a) 1.0 mL/min and (b) 2.0 mL/min.

The reductive flux can be calculated by normalizing the conversion efficiency with the effective catalytic membrane area and time. For each 4-NP concentration and Au loading combination, the steady-state reductive flux was measured. The numbers of molecules reduced per unit time and per unit effective catalytic membrane area were calculated and summarized in a 3D plot (Figure 4). As shown in the figure, the reductive flux of the catalytic membrane increased with 4-NP concentrations ranging from 0.5 to 2.0 mmol/L at a given Au concentration and flow rate. The corresponding reductive flux at 2.0 mL/min is almost two times higher than that at 1.0 mL/min, suggesting a mass transport-limited catalytic process under the given conditions. At a higher flow rate, more substrate molecules could pass through the catalytic membrane 19



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within the same reaction period.

Table 1. Comparison of the normalized catalytic activity of the present catalytic reactor with the reported continuous-flow systems. Entry

4-NP

Catalytic

Membrane

Flow Rate

Normalized

(mM)

membrane

Area (cm2)

(mL/min)

Activity

Ref.

(mmol/h/m2/g) 1

0.5

Au/CNT

7.1

1.0

2.2 × 104

37

2

0.1

Au/β-

12.6

2.5

6.1 × 104

32

12.6

2.5

7.4 × 104

32

0.78

0.06

2.3 × 104

33

78.5

9

8.1 × 104

34

7.1

2.0

1.2 × 105

this

Lactoglobulin 3

0.1

Pd/βLactoglobulin

4

0.28

Cu-Ag-Au/βLactoglobulin

5

20.0

Au/ Glass fiber

6

1.5

Au/rGO

work

To evaluate the catalytic performance of the as-designed catalytic membrane, we further compared the catalyst-normalized catalytic activity with few state-of-the-art catalytic membrane systems for the same model reaction. As summarized in Table 1, the present continuous flow system exhibited the highest normalized catalytic activity of 1.2×105 mmol/h/m2/g, which is 1.5-5.5 times higher than those of the reported 20


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catalytic membrane systems. These results suggest that the AuNCs are excellent catalyst and the combination of AuNC with rGO is a promising choice for potential industrial applications. Meanwhile, for a fixed 4-NP concentration, the reductive flux is dependent on the Au amount and flow rate. For a lower 4-NP concentration, complete 4-NP conversion can be achieved for all cases. For example, for 0.5 mmol/L 4-NP and 1.0 mL/min flow rate, a stable reductive flux of 42.5 ± 0.2 mmol/h/m2 can be obtained regardless of Au loading. For higher 4-NP concentrations, complete 4-NP conversion can only be achieved with increased Au loadings. This finding indicates a shift from a mass transfer-limited regime to an active site-limited regime. As can be seen in Figure 4a, the reductive fluxes for 2.0 mmol/L 4-NP were 105.2 and 170.0 mmol/h/m2, respectively, at Au loadings of 5 and 20 µmol. A similar trend was also observed when the flow rate was increased to 2.0 mL/min, as shown in Figure 4b. Such high catalytic performance maintained over 12 h of continuous operation indicates good stability and activity of the membrane (Figure S13, Supporting Information). After operation for 12 h, FESEM images of the hybrid catalytic membrane showed some organic build-up on the membrane surface that might eventually foul the membrane by blocking the nanochannels necessary for fluid permeation (Figure S14, Supporting Information). However, only minimal negative effects of fouling on permeability or catalytic activity were observed during the operation. Further, we also determined the effluent Au concentration via an inductively coupled plasma mass spectrometer (ICPMS) to evaluate the stability of AuNCs. Results suggest that desorption indeed occurs but at a very limited rate. The effluent Au concentration over 3 h and 6 h continuous filtration of 1.0 mmol/L 4-NP at a flow rate of 1.0 mL/min was determined to be only 8.1±2.1 and 3.6±1.8 µg/L, respectively, which accounts for only 0.41% and 0.18% of 21



the initial 10 µmol Au loading. This data suggests that negligible Au leakage and excellent stability of AuNCs of the as-designed catalytic filter. Desirable stability of the AuNC catalysts within the catalytic filter is crucial for their further applications in the industry sector. Catalytic performance in real water samples To further evaluate the performance of the catalytic membrane system in real water, three water samples (Table S2, Supporting Information) were collected from a local tap, a local river, and industrial dyeing wastewater (alkalidecrement wastewater, diluted 1000 times), respectively. Then solid impurities were removed through a simple physical filtration process using filter paper, followed by spiking 1.0 mM 4-NP and 250-fold NaBH4 (compared with 4-NP) before filtration. As shown in Figure 5, the catalytic kinetics in all three real water samples display similar behavior to that in DI-H2O, and complete 4-NP conversion can be achieved in all cases by a single pass through the membrane reactor. It is of note that all three real water samples have totally different compositions and characteristics. These results suggest that the proposed catalytic membrane system has excellent catalytic activity.

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Figure 5. UV-Vis absorption spectra three 4-NP-spiked real water samples before and after passing through the AuNC/4h-rGO membrane. Experimental conditions: 10 µmol Au loading, 1.0 mL/min flow rate, 1.0 mM 4-NP, and 250-fold excess NaBH4.

Conclusions In conclusion, we have rationally designed a gold-graphene catalytic membrane for continuous flow reduction of 4-NP. In this design, ultra-small Au nanoclusters serve as catalytically active sites (Au atoms) and as a surfactant assembly (organic ligand) to facilitate catalysis and graphene dispersion in aqueous solution. Such highperformance hybrid catalytic membranes can be fabricated with simple vacuum filtration technology. The results demonstrate that this continuous flow catalytic system has better catalytic kinetics than a conventional batch system. This design strategy has potential for the fabrication of advanced catalytic membrane systems for use in industrial applications.

Conflicts of interest There are no conflicts to declare.

Acknowledgements This work was supported by the National Key Research and Development Program of China (No. 2018YFF0215703 and No. 2016YFC0400501), the Shanghai Pujiang Program (No. 18PJ1400400), Natural Science Foundation of Shanghai, China (No. 18ZR1401000), and the State Key Laboratory of Separation Membranes and Membrane Processes (Tianjin Polytechnic University, No. M2-201709). Y.L. thanks 23



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Donghua University for the start-up grant (No. 113-07-005710).

Supporting Information There are 2 tables and 14 figures in the SI section. Supplementary spectra (UV-vis, ESI-MS), FESEM characterizations and photographs are provided in SI. In SI section we also provide the atomic concentration of C and O of rGO filters changed with hydrothermal time and the characteristics of three real water samples.

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For Table of Contents Use Only

Synopsis: A high-performance microreactor system consists of ultra-small and atomically precise gold nanoclusters and two-dimensional graphene for continuousflow catalysis was rationally designed.

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Rational Design of High-Performance Continuous-Flow Microreactors

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