Highly Efficient Au Nanocatalysts for Heterogeneous Continuous-Flow

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Highly Efficient Au Nanocatalysts for Heterogeneous Continuous-flow Reactions Using Hollow CeO2 Microspheres as a Functional Skeleton Sai Zhang, Huan Zhang, Ting Ni, and Xuetao Shen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04637 • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on February 26, 2018

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Industrial & Engineering Chemistry Research

Highly Efficient Au Nanocatalysts for Heterogeneous Continuous-flow Reactions Using Hollow CeO2 Microspheres as a Functional Skeleton Sai Zhang,*† Huan Zhang, ‡ Ting Ni‡ and Xuetao Shen‡ †

School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an, China,

740049 ‡

Frontier Institute of Science and Technology, Xi’an Jiaotong University, Xi’an, China, 710049

Email: [email protected]

ABSTRACT. The hollow Au/CeO2 as a highly efficient and stability heterogeneous catalysts are successfully used for continuous-flow reaction.

The hollow CeO2 microspheres with

thickness of 21 nm as a functional supports can greatly improve the catalytic activity and stability of the Au nanoparticles. The catalytic activity of hollow Au/CeO2 catalysts is 4.5 times higher than the Au catalysts without the hollow structure under the same reaction conditions for the reduction of 4-nitrophenol.

Meanwhile, the hollow structure also accelerates the mass

transfer of reactant for the continuous-flow system. A high turnover number of 65.7 mol h-1 molAu-1 is yielded for the hollow Au/CeO2 catalysts at the flow rate of 250 mL h-1 and a remarkable stability for continuous-flow 4-nitrophenol reduction reaction.

The present

continuous-flow system exhibits the huge potential in the automation treatment of pollutant 4nitrophenol.

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KEYWORDS: hollow CeO2 microspheres, heterogeneous catalysis, continuous-flow system, Au nanocatalysts


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1. INTRODUCTION Continuous-flow process represents a paradigm shift in the manufacture of organic catalysis, pharmaceutical synthesis, nanostructure production and wastewater treatment, which attracts extensive attention in both academia and industry.1-12 Compared with traditional batch reaction, the continuous-flow reaction has many advantages in efficiency, product quality, easy scale-up over batch processing and so on.13-15

Typically, the organic synthesis is realized with

continuous-flow systems often based on homogeneous catalysts.16

Although the reaction

efficiency is greatly improved compared with batch reaction, the separation of products or reuse of homogeneous catalysts is still a complicated process during the actual production. To develop a continuous-flow system based on heterogeneous catalysts has the potential to address the inconveniences of homogeneous systems.17 Due to the lower activity and/or selectivity, heterogeneous catalysts cannot simply replace homogeneous catalysts in most organic reactions.18

Generally, homogeneous catalysts are

catalyzed by molecular complexes in organic media or a mixture of organic solvent and water. Therefore, the homogeneous reaction system is easily to be flowed in the continuous-flow reactor. Different from the homogeneous catalysts, the immobilization/filling of heterogeneous catalysts in the reactor is also a challenge for continuous-flow system. Meanwhile, the flow of reaction liquid in the reactor should be considered after filled the heterogeneous catalysts, especially for the high flow velocity. Although the flowability can be well solved by designing the specific reactor or heterogeneous catalysts at high flow space, the catalytic efficiency is hardly to achieve due to these macroscopic structural characteristics of catalysts.19 Heterogeneous noble metal nanocatalysts exhibit outstanding catalytic activity in organic catalysis with batch reaction.20-36

Unfortunately, these nanocatalysts are difficult to apply


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directly in the continuous-flow system. Due to the nanoscale of catalysts, the tiny spaces between each nanocatalysts will greatly limit the flow of reaction liquid during the reaction. In order to increase the flow space, these nanocatalysts have to be prepared as the micron-sized particles through preforming and granulation technology. However, the surface of nanocatalysts will be close combined with each other during the preforming process. On this occasion, most of the nanocatalysts in the middle of micron-size particles will lose their catalytic activity sites, leading to the weak catalytic activity. The hollow nanomaterials can cause the reaction liquid to flow into the inside of micron-size particles through their hollow structure, which further results in the improvement reaction activity. Therefore, the hollow nanocatalysts exhibit the potential in structure as a possible heterogeneous catalysts for the continuous-flow system. Meanwhile, due to the enhanced heat and mass transfer, the nature active of nanocatalysts is another key factor for the continuous-flow system. As we all known that inclusion of noble metal nanocatalysts into oxide supports, such as CeO2,37 SiO238-39 and TiO2,40 will create new metal-oxides interfaces, causing a strong metal-supports interaction (SMSI).41-45 The supports not only stabilize noble metal nanocatalysts, but also modify the electronic structure of catalytic activity sites, which facilitates chemical reactions.46-48

Therefore, the catalytic activity of

nanocatalysts can be modulated by different oxide supports based on the SMSI. Herein, the hollow Au/CeO2 catalysts were successfully synthesized with SiO2 template and exhibited high catalytic performance in continuous-flow system. Ceria (CeO2) was selected as the supports due to the characteristic of strong lewis basic sites and good chemical, environmental and mechanical stability.49-53 It has found the following scale for the density of basic sites by CO2 chemisorption studies: CeO2 (3.23) > MgO (1.77) > ZrO2 (1.45) > 10% CeO2Al2O3 (0.44) > Al2O3 (0.18) > SiO2 (0.02) (expressed in µmol CO2 m-2).54-55 Obviously, CeO2


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exhibited the highest amount of surface basic sites, which may result in the highest SMSI. Due to the SMSI, the hollow CeO2 microspheres as a typical basic supports can greatly enhance the electronic density of supported Au nanocatalysts, which can further improve its catalytic activity and stability. Results indicated that Au nanocatalysts supported on CeO2 surface exhibited the obvious enhanced catalytic activity for the 4-nitrophenol reaction compared with it on SiO2 surface. Meanwhile, the hollow structure of Au/CeO2 nanocatalysts also accelerates the mass trasform of reactant for the continuous-flow system. When the hollow Au/CeO2 catalysts were prepared as the micron-size particles, the structural advantage was further highlighted under the same reaction condition. With the continuous-flow system, a high productivity (65.7 mol h-1 molAu-1) were achieved at a high flow rate of 250 mL h-1, which was 1.76 times higher than the value for batch synthesis (37.3 mol h-1 molAu-1) with the same amount of reactants. 2. EXPERIMENTAL SECTION 2.1. Synthesis of SiO2 microspheres. Ammonia (22.5 mL) was added into 90 mL solution with 50 mL water and 40 mL ethanol and stirred quickly. This mixed solvent named solution A. Tetraethyl orthosilicate (TEOS, 12.5 mL) was added into 100 mL of ethanol and this mixture was solution B. Then, the solution B was poured into A with stirring. After 4 h reaction, SiO2 microspheres was obtained by centrifugal separation and washed with ethanol 3 times. 2.2. Synthesis of SiO2@CeO2 microspheres.

The SiO2@CeO2 microspheres were

prepared by chemical precipitation method. SiO2 microspheres (200 mg) and Ce(NO3)3·6H2O (252 mg) were added into 60 mL ethanol (named as solution C). Another solution prepared by dissolving hexamethylenetetramine (HMT, 203 mg) in 40 mL water was added into solution C with stirring. After 2 h reaction at 75 oC, the precipitates were separated, washed and dried at 60 o

C for 12 h. The obtained SiO2@CeO2 microspheres exhibited the layer thickness of 21 nm.


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When the amount of Ce(NO3)3·6H2O increased to 504 mg, the layer thickness increased to 38 nm. 2.3. Synthesis of Au/SiO2, Au/SiO2@CeO2 and hollow Au/CeO2 catalysts. Au/SiO2 microspheres with 3 wt% gold loading were prepared by a deposition-precipitation with some changes. 50 mg of the SiO2 microspheres (modification with 3-aminopropyl trimethoxysilane) was suspended in 5 mL HAuCl4·3H2O aqueous solution (0.62 mg mL-1) at room temperature for 1 h. After added 200 mg urea, the suspension was heated to 70 oC and kepted 2 h at this temperature under stirring. The suspension was cooled to room temperature and 5 mL NaBH4 ice solution (0.56 mg mL-1) was added for another 1 h reaction. Finally, the Au/SiO2 catalysts was washed 4 times with distilled water and dried overnight at 60 oC. The Au/SiO2@CeO2 catalysts were synthesized with the same procedure as Au/SiO2 microspheres with SiO2@CeO2 microspheres rather than SiO2 microspheres.

The hollow

Au/CeO2 catalysts were obtained by remove SiO2 core of Au/SiO2@CeO2 catalysts using 1 M NaOH solution overnight. All of these catalysts were processed into micron-size particles with 100 ~ 250 µm before used for reaction by through preforming and granulation technology. 2.4. Characterization.

The phase evolution of as-synthesized nanostructures was

monitored by powder X-ray diffraction (XRD) using Cu Kα radiation. Transmission electron microscopy (TEM) studies were conducted on a Hatchie HT-7700 transmission electron microscope with an accelerating voltage of 120 kV. The contents of Pd and Au were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis (Agilent 7500ce).


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2.5. General procedure for catalytic reduction of 4-Nitrophenol. The catalytic reduction was conducted in a standard quartz cell with a path length of 1 cm at room temperature. Typically, the Au catalytst (1.0 mL, 0.01 mmol/L Au catalyst) and 4-nitrophenol (1.0 mL, 0.1 mmol/L) were added. Immediately after further addition of NaBH4 (1.0 mL, 10 mmol/L), the absorption spectra were recorded by UV-vis spectrophotometer. The initial molar ratio of Au/4nitrophenol/NaBH4 was kept at 1/10/1000. 2.6. General procedure for the continuous-flow catalytic reduction of 4-Nitrophenol. A 40 mg hollow Au/CeO2 catalysts was mixed with 2 g silica sand (100~250 µm) . A certain amount of mixed catalysts enclose into a tube with a diameter of 4 mm. A solution with initial molar ratio of 4-Nitrophenol/NaBH4 of 1/20 enters into the tube with a controllable flow velocity at room temperature.

1 mL solution was took at given time and recorded by UV-vis

spectrophotometer. 3. RESULTS AND DISCUSSIONS The SiO2 microspheres were synthesized by the Stöber method as previous reports.56 As shown in Figure 1a, the obtained SiO2 microspheres were very uniform with an average diameter of 450 nm. Meanwhile, the SiO2 microspheres exhibited the amorphous structure from the XRD analysis (Figure 1d). The Au/SiO2 catalysts with 3 wt% Au loading could be prepared by a modified deposition-precipitation method. As shown in Figure 1b, Au nanoparticles with a average size of 5.9 ± 1.2 nm (Figure S1a) could be obversed and uniformly supported on the surface of SiO2 microspheres from the TEM image. As we all known that plenty of hydroxyl groups (-OH) were dispersed on the surface of SiO2 microspheres. The Ce3+ ions with positive charge can be easily absorbed on their surface and bound with hydroxyl groups. Therefore, the Ce(OH)3 precipitate could be formed on the


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surface of SiO2 microspheres with the OH- hydrolyzed form the HMT at 70 oC, resulting in a core-shell structure of SiO2@Ce(OH)3 microspheres. Then, the surface Ce(OH)3 layer could transfer to CeO2 layer during dryness process at 80 oC, due to the intrinsic crystallization tendency of Ce3+ oxidation to Ce4+ exposed in the air. Compared with the smooth surface of SiO2 microspheres, the obtained SiO2@CeO2 microspheres exhibited obvious rough surface, as shown in Figure 1c. The changed surface feature also demonstrated the successful formation of CeO2 layer on the surface of SiO2 microspheres. XRD analysis could further confirm the translation of Ce(OH)3 to CeO2 during the drying process. As shown in Figure 1d, peaks observed at 2θ of 28.6, 33.1, 47.4, 56.2, 58.8, 69.2 and 76.5° could attributed to the (111), (200), (220), (311), (222), (400) and (311) reflections of a cubic fluorite CeO2 (PDF # 34-0394). With the same deposition-precipitation method, Au nanoparticles with 3 wt% loading could also be successfully supported on the surface of SiO2@CeO2 microspheres. However, the Au nanoparticles were not observed on the surface of SiO2@CeO2 microspheres from the TEM image (Figure 1e). The SiO2 core of Au/SiO2@CeO2 catalysts could be easily removed by NaOH solution and formed hollow Au/CeO2 catalysts. The thickness of CeO2 shell was about 21 nm, as shown in Figure 1f. Meanwhile, Au nanoparticles with average size of 6.5 ± 1.3 nm (Figure S1b) could be observed on the surface of hollow Au/CeO2 catalysts after removal of SiO2 microspheres core, which could indirectly confirm the successful supported of Au nanoparticles on the surface of SiO2@CeO2 microspheres (Figure 1f). Meanwhile, the measured lattice spacing of the nanoparticle was 0.24 nm form the HRTEM (Figure 1g), which was consistent with 0.236 nm of Au (111) plane. The Au 4f7/2 peak of hollow Au/CeO2 catalysts exhibited the binding energy of 84.1 eV, indicating the metallic of supported Au nanoparticles (Figure 1h).


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Both of the Au/SiO2 and Au/SiO2@CeO2 catalysts were prepared to the micron-size particles with 100~250 µm before used for reaction through preforming and granulation technology. The loading of Au nanoparticles was 3 wt% for Au/SiO2 and Au/SiO2@CeO2 catalysts obtained from the ICP-OES testing.

The catalytic activity of Au nanoparticles

supported on different support surfaces was evaluated by 4-nitrophenol reduction in the presence of NaBH4 at room temperatures. As shown in Figure S2, the intensity of the characteristic absorption peak of 4-nitrophenol at 400 nm decreased quickly with the emergence of characteristic absorption of 4-aminophenol at around 300 nm. This phenomenon indicated that 4-nitrophenol was reduced to 4-aminophenol quickly with discoloration of the solution from bright yellow to colorless. Meanwhile, the Au/SiO2@CeO2 catalysts exhibited obvious higher catalytic activity than it with Au/SiO2 catalysts under the same reaction conditions, as shown in Figure 2a. The complete conversion of 4-nitrophenol to 4-aminophenol was obtained after 14 min reaction for the Au/SiO2@CeO2 catalysts. While, only 18% conversion of 4-nitrophenol was yielded for the Au/SiO2 catalysts. The decomposition kinetics is understood according to physical chemistry principles. Previous reposts has proved that the 4-nitrophenol reduction follows the Langmuir-Hinshelwood apparent first-order kinetics model due to an excess of NaBH4 in the reaction system.57-58 When the initial concentration (C0) of 4-nitrophenol is low, the simplified first-order model is as equation (1): ln C/C0 = -kt

(1)

Where k is the apparent first-order rate constant (min-1). The ln(C/C0) versus reduction time for the reduction of 4-nitrophenol over Au/SiO2 and Au/SiO2@CeO2 catalysts were shown in Figure 2b. It could be observed that k for Au/SiO2@CeO2 catalysts (k=0.3161 min-1) was 16.7


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times higher than it calculated from Au/SiO2 catalysts (k=0.0189 min-1). Therefore, both the reaction rate and k proved that Au nanoparticles supported on CeO2 surface exhibited obviously enhanced catalytic activity for the 4-nitrophenol reduction compared with it on SiO2 surface. Due to the similar average size and same loading of Au nanoparticles, the enhanced catalytic activity of Au/SiO2@CeO2 derived from the strong metal-support interaction between Au and CeO2. The mechanism of reduction of 4-nitrophenol to 4-aminophenol by NaBH4 in the presence of Au nanoparticles was discussed in terms of the Langmuir-Hinshelwood (LH) model, as illustrated in Figure 3.58-59 Firstly, borohydride ions adsorb on the surface of Au nanoparticles and transfer a surface-hydrogen species to its surface. Secondly, 4-nitrophenol molecules are adsorbed on these surface-hydrogen species. Both of the two processes are reversible and can be modeled by a Langmuir isotherm. The reduction of 4-nitrophenol by surface-hydrogen species is considered as the rate-determining step. Compared with SiO2 supports, the stronger basicity of CeO2 support resulted in the enhanced electronic density on the Au nanoparticles, which was beneficial to the formation of active surface-hydrogen species.59-61 Therefore, the density of surface-hydrogen species could be obviously increased due to the strong SMSI between Au and CeO2 supports, resulting in the improvement catalytic activity of Au nanoparticles. Meanwhile, the mass transfer of reactants could also affect the reaction activity for the heterogeneous catalytic reaction, especially for micron-sized heterogeneous catalysts.

The

hollow microspheres supports are beneficial to mass transfer of reactants compared with solid structure, which can improve the catalytic reaction activity.62-64

In order to confirm the

advantage of structure for hollow Au/CeO2 catalysts, the 4-nitrophenol reduction was also employed to expose their catalytic performances. As shown in Figure 4a, the hollow Au/CeO2


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catalysts with 6.1 wt% Au loading (obtained form the ICP-OES analysis) exhibited obviously enhanced catalytic activity compared with Au/SiO2@CeO2 catalysts with the same amount of Au under the same reaction conditions. The 100% conversion of 4-nitrophenol was yielded only after 3.5 min reaction, which was 5.1 times shorter than Au/SiO2@CeO2 catalysts. Due to the same supports and reaction conditions, the enhanced catalytic activity of Au nanoparticles could be attributed to the accelerated mass transfer. In order to further confirm the assumption, the hollow Au/CeO2 catalysts with 38 nm of CeO2 shell (Au/CeO2-38) were also synthesized with the same process (Figure S3). The Au loading was 6.6 wt% obtained from the ICP-OES analysis. The average size of Au nanoparticles for hollow Au/CeO2-38 catalysts was 5.8 ± 1.7 nm (Figure S3c), which was similar with the size of Au for the Au/CeO2-21 catalysts (6.5 ± 1.3 nm). Meanwhile, the Au4f7/2 for the hollow Au/CeO2-38 catalysts also exhibited the binding energy of 84.2 eV (Figure S4). Therefore, the Au/CeO2-21 and Au/CeO2-38 catalysts exhibited the similar Au loading, average size and binding energy of the supported Au nanoparticles. However, the 100% conversion of 4-nitrophenol was yielded after 6 min reaction for the hollow Au/CeO2-38 catalysts under the same reaction conditions, which was also obvious longer than the hollow Au/CeO2-21 catalysts (3.5 min) (Figure 4a). The reduced catalytic activity can be attributed to the thicker CeO2 shell, which could lead to the slower mass transfer of reactants in the hollow Au/CeO2-38 catalysts compared with hollow Au/CeO2-21 catalysts. Therefore, the hollow structure of Au/CeO2 catalysts can promote more effective mass transfer of reactants, which is also suitable for the continuous-flow reactions. The catalytic stability is also another important factor to evaluate the practical properties for the present heterogeneous catalysts. As shown in Figure 4b, the hollow Au/CeO2 catalysts exhibited the excellent catalytic stability with almost no deactivation even after 8 times


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recycling. The Au nanoparticles can be isolated by the minimum unit of hollow CeO2 supports, which can further stabilize it during the reaction process. Both of the enhanced catalytic activity and excellent stability obtained from the structure of hollow supports make the hollow Au/CeO2 catalysts with great potential application in continuous-flow reaction. As we all known that 4-nitrophenol is a common organic pollutant in wastewater. However, the reduced product was 4-aminophenol, which is a potent intermediate for the manufacture of many drugs. Therefore, the reduction of 4-nitrophenol with continuous-flow system has a great significance for industrial application. As shown in Figure 5a, the 4-nitrophenol and NaBH4 solutions were pumped into a continuous-flow reactor loaded with hollow Au/CeO2 catalysts. Then, the 4-nitrophenol can quickly reduce to 4-aminophenol along with the reaction liquid flowing in the reactor. This process will realize the full automatic treatment of 4-nitrophenol wastewater. The hollow Au/CeO2 catalysts had explored the possibility to produce a continuousflow system for the reduction of 4-nitrophenol from the above analysis. Hollow Au/CeO2 catalysts with a diameter of 100~250 µm was mixed with silica sand (100~250 µm), and then filled in the common reaction tube with inner diameter of 3 mm, as shown in Figure 5a. The micron-sized catalysts and silica particles can not only increase the flowing space, but also enhance the perturbation of the reaction fluid. In our experiments, this microreactor can bear a maximum flow rate of 500 mL h-1. The handling capacity of hollow Au/CeO2 catalysts was tested by employing 2 g silica sand mixed with 20 mg hollow Au/CeO2 catalysts. With the initial concentrations of 4-nitrophenol (2.5 mmol/L) and NaBH4 (50 mmol/L), the conversions of 4-nitrophenol were summarized in Figure 5b. At a flow velocity of 50 mL h-1, the achieved conversion of 4-nitrophenol could reach 98.6%. Increasing the flow velocities of the reaction solution could decrease the conversion of


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4-nitrophenol (Figure 5b). For example, the conversions of 4-nitrophenol decreased from 68.8% to 53.4% and then 37.9% with flow rates of 150 mL h-1, 250 mL h-1 and 350 mL h-1, respectively. Such phenomenon could be attributed to the shortened duration of the contact time between catalysts and reactants. To further illustrate the productivity of hollow Au/CeO2 catalysts integrated with continuous-flow systems, calculated by multiplying the conversion and the amount of reactants flowing through the catalysts in unit time, are used to characterize the efficiency of the catalytic system. Although the conversion of 4-nitrophenol could reach the highest value of 98.6% at the flow velocity of 50 mL h-1, the calculated productivity of hollow Au/CeO2 in the continuousflow reactor was the smallest value of 9.71 mol h-1 molAu-1 (Figure 5c). By increasing the flow rates, the productivity clearly showed two distinct regions: the initial productivity quickly increased from 24.3 to 65.7 mol h-1 molAu-1 with the increasing flow velocity from 50 to 250 mL h-1, and the immediately maintained at the value about 65.7 mol h-1 molAu-1 when the flow velocities were higher than 250 mL h-1 (Figure 5c).

In contrast, the productivity of the

corresponding batch synthesis was 37.3 mol h-1 molAu-1, which was 1.8 times lower than the productivity of continuous-flow reaction with a flow velocity of 250 mL h-1.

The high

productivity of hollow Au/CeO2 catalysts integrated into a continuous-flow system unambiguously illustrated the advantages compared with the traditional batch reaction. The metal bleaching of heterogeneous catalysts is almost inevitable during the reaction. Different from the batch reaction, the bleached metal active center will be lost with the fluid reaction solution rather than in the reaction system. Therefore, the stability of heterogeneous continuous flowing system is more difficult than the batch reaction. In order to further assess the stability of the hollow Au/CeO2 catalysts, long-time flow reaction was tested with a flow


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velocity of 50 mL h-1. As shown in Figure 5d, the hollow Au/CeO2 catalysts showed the remarkably stable catalytic activity. The conversion of 4-nitrophenol could maintain at the range of 97.2% and 99.3% during this 15 h reaction. Meanwhile, the Au ions was not detected in the reaction solution by the ICP-OES analysis. Therefore, the almost unchanged conversion proved the excellent catalytic stability of hollow Au/CeO2 catalysts for the continuous flowing reaction. 4. Conclusions In summary, the hollow Au/CeO2 catalysts as heterogeneous catalysts for continuous-flow system have been successfully synthesized by SiO2 as template. The excellent catalytic activity of micron-sized hollow Au/CeO2 catalysts for the 4-nitrophenol reduction can be attributed to the enhanced electronic density of Au nanoparticles obtained from the SMSI and the accelerated mass transfer of reactants obtained from the huge hollow structure. The catalytic activity of hollow Au/CeO2 catalysts is 4.5 and 22.4 times higher than it of Au/SiO2@CeO2 and Au/SiO2 catalysts under the same reaction conditions, respectively (Table S1). The catalytic stability can be attributed to the inlaid Au nanoparticles into the minimal until of CeO2 layer. Hollow Au/CeO2 catalysts exhibit a high catalytic activity, productivity, and stability for continuousflow 4-nitrophenol reduction reaction compared with traditional batch reaction. The current continuous-flow systems integrated with hollow CeO2 catalysts have the potential for practical applications and can be extended to many important chemical reactions. ASSOCIATED CONTENT Supporting Information.

This file provides more detailed information regarding the size

dissociation of Au nanoparticles for the Au/SiO2 and Au/SiO2@CeO2 catalysts, general timedependent UV-vis spectral changes of the 4-NP reduction, TEM images of Au/SiO2@CeO2-38


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and hollow Au/CeO2-38 catalysts, and summary of the catalytic performance of various Au catalysts. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest ACKNOWLEDGMENT We acknowledge the financial support the China Postdoctoral Science Foundation Grant 2017M620453. We acknowledge the help of Prof. Yongquan Qu and the technical supports for experiments from Frontier Institute of Science and Technology.


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Figure 1.

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TEM images of (a) SiO2 microspheres, (b) Au/SiO2 catalysts, (c) SiO2@CeO2

microspheres, (e) Au/SiO2@CeO2 catalysts and (f) hollow Au/CeO2 catalysts.

(d) XRD

spectrum of SiO2 and CeO2@SiO2 microspheres, respectively. (g) HRTEM image and (h) XPS analysis of hollow Au/CeO2 catalysts.


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Figure 2.

(a) Catalytic activity of the reduction of 4-nitrophenol with Au/SiO2 and

Au/SiO2@CeO2 catalysts. Reaction condition: Au catalytst (1.0 mL, 0.01 mmol/L of Au), 4nitrophenol (1.0 mL, 0.1 mmol/L) and NaBH4 (1.0 mL, 10 mmol/L). (b) Plot of ln(C/C0) versus time for Au/SiO2 and Au/SiO2@CeO2 catalysts.


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Figure 3.

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Mechanistic model (Langmuir-Hinshelwood mechanism) of the reduction of 4-

nitrophenol by borohydride in the presence of Au nanoparticles embedded on the CeO2 surface.


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Figure 4. (a) Catalytic activity of the reduction of 4-nitrophenol by various Au catalysts. (b) The catalytic stability of hollow Au/CeO2 catalysts. Reaction condition: Au catalytst (1.0 mL, 0.01 mmol/L of Au), 4-nitrophenol (1.0 mL, 0.1 mmol/L) and NaBH4 (1.0 mL, 10 mmol/L).


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Figure 5.

The continuous-flow reaction of hollow Au/CeO2 catalysts.

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(a) Schematic

representation of the continuous-flow system of 4-nitrophenol reduction reaction by hollow Au/CeO2 catalysts. (b) plots of conversion of 4-nitrophenol at various flow rates for continuousflow, (c) plots of productivity of 4-aminophenol at various flow rates for continuous-flow and (d) plots of conversion of 4-nitrophenol catalyzed by hollow Au/CeO2 catalysts with the flow velocity of 50 mL h-1.


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Highly Efficient Au Nanocatalysts for Heterogeneous Continuous-Flow

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