Fabrication of High Efficient Silver Nanoparticle Catalyst Supported on

Nov 15, 2016 - Fabrication of High Efficient Silver Nanoparticle Catalyst Supported on ... Tianjin University, Tianjin 300354, People's Republic of Ch...
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Fabrication of High Efficient Silver Nanoparticle Catalyst Supported on Poly(glycidyl methacrylate)−Polyacrylamide Wenchao Zhang, Yan Sun, and Lin Zhang* Department of Biochemical Engineering and Key Laboratory of Systems Bioengineering (Ministry of Education), School of Chemical Engineering and Technology, Tianjin University, Tianjin 300354, People’s Republic of China S Supporting Information *

ABSTRACT: Fabrication of highly efficient silver nanoparticle (Ag NP) catalysts supported on polyacrylamide (PAM)modified poly(glycidyl methacrylate) (PGMA) microspheres was reported herein, for where PAM was used as the robust anchors because of its abundant amide groups. Well-dispersed Ag NPs with an average diameter of 9.7 nm were obtained on the PGMA−PAM microspheres (Ag NPs@PGMA−PAM). Excellent catalytic activity of Ag NPs@PGMA−PAM was observed in the reduction of 4-nitrophenol using sodium borohydride in water at room temperature, indicated by an activity parameter that was 6−1725 times higher than those reported in the literature. In addition, easy regulation on the size of Ag NPs was achieved through the adjustment on the concentration of the Ag precursor, AgNO3. Therefore, the synthetic method proposed herein was confirmed as being effective for the synthesis of the highly efficient catalyst Ag NPs@ PGMA−PAM. This would contribute to the preparation of highly efficient catalyst of supported noble metals and then facilitate their applications in environmental protection. poly(carboxylic acid)-modified carbon nanotubes,28 and polydopamine-modified PS substrate,29 were used as supports for Ag NPs. In our previous work,30 sulfhydryl-functionalized poly(glycidyl methacrylate) (PGMA) microspheres were used for the synthesis of monodispersed Ag NPs. Herein, polyacrylamide (PAM) with abundant amide groups, prepared by the surface-initiated atom transfer radical polymerization (ATRP),31,32 was considered as an efficient agent for the preparation of supported Ag NPs33 through the coordination of N, O atoms with Ag+.25 In addition, PGMA microspheres with solid core and exposed epoxy groups, were used for the preparation of various multifunctional polymers through chemical modifications.34 Therefore, in the present study, PAM polymer was grafted onto PGMA microspheres through ATRP to prepare hybrid PGMA−PAM microspheres. The in situ fabrication of well-dispersed Ag NPs was then performed in the outer layer of PGMA−PAM microspheres. The PAM layer was used as a medium for the generation and immobilization of Ag NPs in consideration of the coordination effect between amide groups of PAM and Ag+. The obtained Ag NPs@ PGMA−PAM composites were characterized by fourier transform infrared spectroscopy (FTIR), zeta potential

1. INTRODUCTION Metal particles in nanoscales have become a subject of intense interest in various fields including chemistry and physics during past decades.1−6 Silver nanoparticles (Ag NPs), because of their unique optical,7 antimicrobial,8,9 electronic,10−12 and catalytic properties,13,14 were extensively studied and recognized as one of the most promising catalysts in various applications.15−17 However, the application suffered from the aggregation of Ag NPs owing to van der Waals attraction and high surface energy,18 which decreased the catalytic activity. Depositing the Ag NPs on various supports to form colloidal composite particles was considered a promising strategy to resolve this problem. The synthesis of Ag NPs supported on various supports such as carbon materials,19−21 silica,22 and alumina23 has been widely studied. Immobilization of Ag NPs allowed Ag NPs to retain high stability and activity, and enabled easy and effective separation from the reaction system for reuse. For example, Liu et al.24 reported the synthesis of Ag NPs supported on spherical polyelectrolyte brushes, which were obtained through the polymerization of acrylic acid onto polystyrene latex cores. Murugan et al.25 fabricated Ag NPs immobilized on polymer-supported poly(styrene) (PS) functionalized with poly(vinylimidazole) (PS−PVIm−AgNPs) and used them as recoverable catalysts for the reduction of 4nitrophenol (4-NP). In addition, other polymers,26−29 such as a bottlebrush polymer prepared by grafting of poly(ethylene glycol) methacrylate onto a solid poly(styrene) core,27 © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

September 2, 2016 November 8, 2016 November 15, 2016 November 15, 2016 DOI: 10.1021/acs.iecr.6b03393 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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mmol), PMDETA (168 μL, 0.8 mmol), CuBr2 (8.9 mg, 0.04 mmol), and 8 mL of methanol/water (1:1, v/v) mixture were placed in a 25 mL vial. The mixture was sonicated for 5 min to ensure complete dissolution of the copper and purged with N2 for 30 min under magnetic stirring. Then CuBr (57.4 mg, 0.4 mmol) was added to the mixture under protection of N2 flow. After 30 min, the flask was sealed with a rubber stopper and polymerization was conducted at 30 °C for 8 h. Finally, the polymer was isolated via centrifugation at 5000 rpm for 5 min and the sediments were then washed at least three times each with ethanol and deionized water. The product was denoted as PGMA−PAM. 2.4. Synthesis of the Silver Composites. Synthesis of the silver composites was carried out by in situ chemical reduction of silver salt−polymeric mixture with NaBH4. In brief, 1 mL of AgNO3 aqueous solution with different concentration (2.5 × 10−3, 5.0 × 10−3, 7.5 × 10−3, 10.0 × 10−3 M) was introduced into 18 mL of 0.4 mg/mL PGMA−PAM dispersion solution with a final Ag concentration of 1.25 × 10−4, 2.50 × 10−4, 3.75 × 10−4, 5.00 × 10−4 M, respectively. The solution was then stirred for 30 min. Thereafter, 1 mL of 0.6 M freshly prepared ice-cold NaBH4 solution was added dropwise slowly to the reaction mixture with gentle stirring and the reaction was conducted for 3 h at room temperature (RT). After the reaction, the final products were stored at 4 °C or freeze-dried under vacuum at −40 °C for characterization. The obtained composites were denoted as Ag NPs@PGMA−PAM. The four composites obtained at different final Ag concentration were named as Ag 1, Ag 2, Ag 3, and Ag 4, respectively. 2.5. Catalytic Reduction of 4-NP. To investigate the catalytic activity of the developed Ag NPs@PGMA−PAM composites, the reduction of 4-NP by NaBH4 was carried out in batch experiments as follows. In a typical experiment, 45 mL of an aqueous solution of 4-NP (6.7 × 10−5 M) and NaBH4 (0.2 M) in a glass vessel was purged with N2 for 20 min to remove dissolved oxygen. Subsequently, 0.4 mL of dispersion consisting of Ag NPs@PGMA−PAM composites obtained in section 2.4 was rapidly injected into the mixture while stirring. Afterward, 3 mL of the suspension at given intervals was sampled and filtered through 0.22 μm membrane filters, and the UV−vis absorption spectra were recorded at RT. Two different modes were employed in the UV−vis testing process. For UV−vis absorption spectra, the scan range was 240−500 nm and the scan rate was 480 nm/min, while for time-dependent conversion of 4-NP, the monitoring wavelength was fixed at 400 nm. In all runs to be discussed here, the concentration of NaBH4 was much higher than that of 4-NP in the reaction system and was therefore considered as constant during the reaction, which allows the assumption of pseudo-first-order kinetics with respect to 4-NP. The apparent rate constant of the reaction was calculated by the decreasing concentration of 4-NP indicated by the reduction of the characteristic peak at 400 nm. The ratio of absorbance at time t (At) to that at t = 0 (A0) was used as the measure of the concentration of 4-NP at time t (Ct)/that at t = 0 (C0). The apparent rate constant in the reduction of 4-NP was then calculated by the following equation:

measurement, transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD), and ultraviolet−visible (UV−vis) spectroscopy. The catalytic activity of the resultant silver composites was then evaluated in the reduction of 4-NP.

2. EXPERIMENTAL SECTION 2.1. Materials. Glycidyl methacrylate (GMA), azobis(isobutyronitrile) (AIBN), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA) and ethylene glycol dimethacrylate (EDMA) were purchased from Alfa Aesar (Lancashire, UK). α-Bromoisobutyryl bromide (BiBB) was purchased from Sigma-Aldrich (Milwaukee, USA). Copper(I) bromide (CuBr), copper(II) bromide (CuBr2), triethylamine, and n-hexane were supplied by Fengchuan Chemical Reagent Co. Ltd. (Tianjin, China). Silver nitrate (AgNO3) was from Tianjin Yingda Rare Chemical Reagents Factory (Tianjin, China). 4-Nitrophenol (4NP) and sodium borohydride (NaBH4) were purchased from Guangfu Fine Chemical Research Institute (Tianjin, China). Acrylamide (AM), polyvinylpyrrolidone (PVP) K-30 and other reagents were purchased from Dingguo Biotechnology Co. (Beijing, China). All reagents were of analytical grade and used without further purification. 2.2. Synthesis of PGMA Microspheres. PGMA microspheres were obtained by a dispersion polymerization method35,36 with minor modification. Briefly, 8 g of GMA, 2.4 g of PVP K-30, and 0.16 g of AIBN were dissolved in 72 g of methanol in a 250 mL three-necked round-bottom flask using mild sonication. The solution was purged with N2 by stirring vigorously at 300 rpm for 30 min at room temperature (RT, 25 °C). Then, the mixture was heated to 75 °C and refluxed, incubating for 2 h with stirring. 160 μL of EDMA (cross-linker) was subsequently added to the solution, and the polymerization was kept going for another 12 h. After cooling, the product was collected and washed several times with ethanol and deionized water to remove any unreacted monomers or other organic matter. The resulting PGMA microspheres were stored in 20% (v/v) ethanol at 4 °C. 2.3. Grafting of PAM by ATRP onto PGMA Microspheres. The synthetic methods of PAM-functionalized PGMA microspheres are described in this section. 2.3.1. Ring Opening of PGMA by H2SO4. The PGMA microspheres (3 g) were dispersed in a 60 mL of 0.5 M H2SO4 aqueous solution for the ring opening reaction at 60 °C, 170 rpm for 3 h. The resultant microspheres were purified by several centrifugation/washing cycles in water until neutral. The microspheres were denoted as PGMA−OH. 2.3.2. Synthesis of Initiator-Modified PGMA Microspheres. In a typical example, 2 g of PGMA−OH was washed with nhexane to remove traces of water and then placed in a 100 mL three-neck flask containing 25 mL of n-hexane. A 1.2 mL aliquot of triethylamine was added and the solution mixture was left in an ice bath for 2 h. Then 1.5 mL of BiBB mixed with 5 mL of n-hexane was slowly dropped into the flask via the constant-pressure funnel in 30 min, and the reaction mixture was stirred overnight at 30 °C. The resulting solution was then centrifuged and washed three times each with n-hexane, ethanol, and water. The obtained initiator-functionalized PGMA microspheres were denoted as PGMA-Br. 2.3.3. Surface-Initiated ATRP of AM from PGMA-Br. In a typical ATRP of AM from initiator-modified PGMA, PGMA-Br microspheres (700 mg), the monomer AM (1421.6 mg, 20

dCt /dt = −kappCt

or ln(Ct /C0) = ln(A t /A 0) = −kappt B

DOI: 10.1021/acs.iecr.6b03393 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research where kapp represents the apparent rate constant, t represents the reaction time. Ct and C0 represent the concentration of 4NP at time t = 0 and time t, A0 and At represent the absorbance of 4-NP at time t = 0 and time t at 400 nm. Because kapp is calculated without consideration of the catalyst mass and the reaction volume, another parameter “activity parameter” k is calculated for the evaluation of catalytic activity,30,37 which is the ratio of the apparent rate constant kapp to the concentration of the added silver catalyst. 2.6. Characterization. FTIR measurements were carried out on a Bruker Tensor 27 spectrometer (Bruker, Germany) from 500 to 4000 cm−1 using KBr pellets. Zeta potential measurements were performed on Zetasizer Nano ZS (Malvern Instrument Ltd., UK) at RT. The size distribution of microspheres was determined using the Malvern Mastersizer 2000 (Malvern Instruments Ltd., UK). TEM analysis was performed with a JEM-2100F electron microscope (JEOL Ltd., Japan) operating at an accelerating voltage of 200 kV equipped with a microanalysis detector for EDS. The samples for TEM were obtained by dispersing a small drop of the suspension onto a carbon-film-covered copper grid. The particle size of Ag NPs was measured by image analysis software, Nano Measurer 1.2, and 300 Ag NPs were counted for each TEM image. The crystal structure of the synthesized silver composites were characterized by a D8 Focus X-ray diffractometer (Bruker, Germany) in the diffraction angle range 2θ = 30−80° at ambient temperature, using Cu Kα radiation at 40 kV and 40 mA. UV−vis spectra were performed at RT on a Lambda 35 UV−vis spectrophotometer (PerkinElmer, USA) using a quartz cell of 1 cm path length.

to introduce abundant amide groups. After the addition of AgNO3, Ag+ was absorbed to the grafted PAM through the electrostatic attraction38 from the negatively charged PGMA− PAM microspheres and the coordination of N, O atoms in amide groups. Ag nuclei were formed by subsequent reduction with the addition of NaBH4 at the nucleation stage, leading to the formation of Ag clusters. Growth of Ag crystal was then achieved, resulting in the formation of Ag NPs on PGMA− PAM microspheres (Scheme 1b). Meanwhile, the formed Ag NPs were stabilized by the amide groups in PAM layer. Zeta potentials of the microspheres are shown in Figure 1. An obvious decrease of zeta potential was observed from 3.2 ±

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of the Ag NPs@ PGMA−PAM Composites. The fabrication of Ag NPs@ PGMA−PAM composite was depicted in Scheme 1. PGMA microspheres were prepared using a dispersion polymerization method. A polymeric layer of PAM was then formed on PGMA microspheres through ATRP (Scheme 1a)

0.3 mV for PGMA to −37.0 ± 3.7 mV for PGMA−OH, as a result of the hydroxyl groups obtained in the ring opening reaction. A more negative charge was obtained in PGMA with Br atoms as end groups (−46.3 ± 2.9 mV), which was attributed to the hydroxyl groups and Br−. As compared with PGMA-Br, the absolute value of zeta potential decreased in PGMA−PAM (−10.1 ± 3.8 mV) owing to the polymerization of neutral acrylamide onto the modified PGMA. Therefore, the changes in surface chemistry of various microspheres, as demonstrated by the changes of zeta potential (Figure 1), indicated successful synthesis of PGMA−PAM. Moreover, the negative zeta potential indicated that the obtained PGMA− PAM microspheres were negatively charged. This was helpful for the following Ag loading. As shown in Figure 1, the positive zeta potential of PGMA indicated that PGMA microspheres were positively charged, which was unfavorable for the Ag+ adsorption due to electrostatic repulsion. In contrary, the PGMA−PAM microspheres, obtained after the modification of PAM layer, turned to be negatively charged, as indicated by the negative zeta potential. The electrostatic attraction of Ag+ by negatively charged PGMA−PAM microspheres could then facilitate the Ag loading38,39 and contribute to the increase of maximum Ag loading. The synthesis of PGMA−PAM was further demonstrated using FTIR, as shown in Figure 2. For PGMA, a sharp absorption peak at 1730 cm−1 was observed, corresponding to the characteristic stretches of carbonyl group.40 Two peaks at approximately 848, 908 cm−1 were attributed to the epoxy groups41 of PGMA. The characteristic band at 2950 cm−1 was assigned to the stretching of the methyl group in the PGMA blocks, confirming the successful fabrication of PGMA. After the coating of PAM on PGMA, a peak at 1665 cm−1 appeared,

Figure 1. Zeta potentials of PGMA, PGMA−OH, PGMA-Br, PGMA− PAM in aqueous solution.

Scheme 1a

a

Reaction schematic of the synthesis of PGMA−PAM (a), and illustration on the preparation of Ag NPs@PGMA−PAM composites (b). C

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3b,c, confirming the formation of the PAM layer on PGMA microspheres through ATRP of acrylamide. This was consistent with the results of the zeta potential (Figure 1) and FTIR (Figure 2). Such layers were observed on all spheres, indicating that the PAM grafting was quite uniform (Figure 3b). Different from the neat and tidy surface of PGMA−PAM microspheres, small dark dots were observed on Ag NPs@PGMA−PAM composite (Figure 3c), indicating the formation of Ag NPs in the PAM layer of PGMA−PAM. From the enlarged TEM image of Ag NPs@PGMA−PAM composite (Figure 3d), well dispersed Ag NPs were observed evidently in the PAM layer of the PGMA−PAM spheres, which was further demonstrated in an enlarged SEM image of Ag NPs on PGMA−PAM (Figure S2 in Supporting Information). The size of Ag NPs was then measured from TEM images and the corresponding histograms were calculated by an image analysis software, Nano measurer 1.2, as shown in Figure 3e. The average diameter of Ag NPs (herein, [AgNO3] = 1.25 × 10−4 M) was 9.7 ± 2.7 nm based on the size distribution using over 300 NPs from different images. Therefore, PGMA−PAM was confirmed as a good supporting material for Ag NPs resulting from its availability, formability, chemical stability, and microstructure. Complete chemical composition of PGMA−PAM and Ag NPs@PGMA− PAM composite was examined using EDS spectra (Figure 4a,b). The signals for C, N, and O indicated successful preparation of PGMA−PAM (Figure 4a). The strong optical absorption peak was observed at approximate 3 keV in Figure 4b, which was typical for the absorption of metallic silver nanocrystallites,45 indicating the existence of Ag NPs in Ag NPs@PGMA−PAM composite. The XRD patterns were examined for the verification on the formation of Ag NPs, as shown in Figure 4c. The presence of a broad peak confirmed the amorphous nature of the PGMA and PGMA−PAM. For Ag NPs@PGMA−PAM, four diffraction peaks at 2θ angles of 37.9, 44.2, 64.4, and 77.2 were observed, corresponding to the (111), (200), (220), and (311) crystalline planes of sliver, respectively46,47 (JCPDS No. 4-783). This confirmed the existence of Ag NPs in the crystalline state, and

Figure 2. FTIR spectra of PGMA and PGMA−PAM.

indicating the stretching of the carbonyl group42 in PGMA and amide groups.43 Furthermore, an obvious characteristic peak of −OH and N−H overlapping between 3288 and 3609 cm−1 was observed. These results demonstrated that PAM was successfully grafted onto PGMA microspheres. The obtained PGMA−PAM was then used to prepare supported Ag NPs using NaBH4 as a reducing agent. After the addition of NaBH4, the milky solution of PGMA−PAM became light yellow over time, indicating the formation of Ag NPs on PGMA−PAM microspheres. A layer of absorbed borohydride anions on the surface of Ag NPs made the nanoparticles separate and prevented them from agglomerating/coagulating,44 contributing to the formation of a stable colloid. TEM images of PGMA, PGMA−PAM, and Ag NPs@ PGMA−PAM composite are shown in Figure 3. The obtained PGMA microspheres were monodispersed, spherical with a smooth surface (Figure 3a). An average diameter of 1.275 μm with narrow polydispersity of PGMA microspheres was also determined by a Mastersizer 2000 at RT in water (Figure S1 in Supporting Information). After the modification, an obvious polymer layer with a thickness of ca. 112 nm is shown in Figure

Figure 3. TEM images of PGMA (a), PGMA−PAM (b), and Ag NPs@PGMA−PAM composite (c). Enlarged TEM image of Ag NPs@PGMA− PAM composite (d) and the corresponding size distribution of Ag NPs (e), when the concentration of AgNO3 was 1.25 × 10−4 M. D

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Figure 4. EDS spectra of PGMA−PAM (a), Ag NPs@PGMA−PAM composite (b), and power XRD patterns of PGMA, PGMA−PAM, and Ag NPs@PGMA−PAM composite (c).

Figure 5. TEM images of Ag NPs supported on PGMA−PAM microspheres prepared with different concentration of AgNO3: 2.50 × 10−4 M (a), 3.75 × 10−4 M (c), and 5.00 × 10−4 M (e) (the volume of reaction solution was 20 mL). The size distribution of Ag NPs calculated from images a and c are shown in panels b and d, respectively. UV−Visible spectra of the supernatant after the formation of Ag NPs on PGMA−PAM microspheres are shown in panel f. The average diameter of Ag NPs in panels b and d was determined based on the size distribution using over 300 NPs from different TEM images.

adjustment on the relative concentration of AgNO3. Then the preparation of Ag NPs@PGMA−PAM at various AgNO3 concentrations was investigated. The size and morphology of obtained Ag NPs were examined from the TEM images of Ag NPs@PGMA−PAM composites, as shown in Figure 3d,e and Figure 5. Herein, the obtained Ag NPs@PGMA−PAM at the

further confirmed that Ag NPs were successfully immobilized on the PGMA−PAM. 3.2. Regulation on the Size of Ag NPs. Because the composites were fabricated through the electrostatic attraction and the coordination of Ag+ ions with PAM, the regulation on the size of Ag NPs was expected to be realized through the E

DOI: 10.1021/acs.iecr.6b03393 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research AgNO3 concentration of 1.25 × 10−4 M, 2.50 × 10−4 M, 3.75 × 10−4 M, and 5.00 × 10−4 M was denoted as Ag 1, Ag 2, Ag 3, and Ag 4, respectively. In the presence of low concentration of AgNO3 in reaction solution, 1.25 × 10−4 M, well dispersed Ag NPs with an average diameter of 9.7 ± 2.7 nm was obtained (Figures 3d,e). When the concentration of AgNO3 increased to 2.50 × 10−4 M (Figure 5a) or 3.75 × 10−4 M (Figure 5c), significant increase of loading amount of Ag NPs was observed. Meanwhile, significant increase of the particle size of Ag NPs was also observed. For example, an average diameter of 16.7 ± 3.3 nm was observed at the AgNO3 concentration of 3.75 × 10−4 M (Figure 5c,d). However, with further increase of AgNO3 in the reaction solution (e.g., 5.00 × 10−4 M), unbounded Ag NPs and partial aggregation were observed in the TEM image (Figure 5e). Moreover, an absorption peak at 400 nm was observed in the UV−vis spectrum of supernatant after the centrifugation of reaction solution ([AgNO3] = 5.00 × 10−4 M), indicating the existence of free Ag NPs in the reaction solution (Figure 5f). These results suggested that the amount of AgNO3 should be optimized. Otherwise, it was unable to absorb all Ag+ ions once the amount of AgNO3 exceeded the loading capacity of PGMA−PAM microspheres. In an appropriate concentration range, the size of Ag NPs on the PGMA−PAM spheres could be easily regulated through adjustment of the concentration of the precursor Ag+ ions, because different Ag precursor concentration versus the same active sites in PGMA−PAM microspheres resulted in different local concentrations of Ag precursor, and thus the different growth of Ag crystal. 3.3. Catalytic Activities of Ag NPs@PGMA−PAM Composites. Metallic silver composites were demonstrated as excellent catalysts. Immobilization on a solid support was useful to make Ag NPs as a recyclable catalyst. However, the catalytic activities should be examined. Herein, the catalytic activities of obtained silver composites were evaluated using the reduction of 4-NP to 4-aminophenol (4-AP) by NaBH4 in aqueous solution. A characteristic maximum absorption at 317 nm was observed for 4-NP, as shown in Figure 6a (dot line). A redshift to 400 nm was immediately observed after the addition of NaBH4 due to the formation of 4-nitrophenolate ions (Figure 6a, black line). The solution was very stable, and the absorption was maintained even after 12 h without the catalyst. On the contrary, once the silver composites were introduced into the solution as catalyst, the reduction of 4-NP was promoted, as indicated by the decrease in the absorbance at 400 nm (Figure 6a). Meanwhile, a new peak at 300 nm appeared and increased with reaction time. This peak could be indexed to the characteristic absorbance peak of 4-AP, confirming the reduction of 4-NP to 4-AP. This was also confirmed by the gradual change of solution color from yellow to colorless in several minutes.30 In addition, the isosbestic points at 247, 281, and 313 nm were displayed, indicating the generation of the sole product, 4-AP, in the reaction.24,48 In the catalytic reaction, a good linear fitting of ln(At/A0) versus the reaction time t was obtained, indicating that pseudofirst-order kinetics could be used to calculate the kinetic rate constant (Figure 6b). The apparent rate constant kapp was then estimated from the slope of the straight line in Figure 6b, as shown in Table 1. An increase of kapp was observed from 5.76 × 10−3 s−1 for Ag 1 (1.25 × 10−4 M Ag+) to 11.05 × 10−3 s−1 for Ag 3 (3.75 × 10−4 M Ag+), accomplished by the decrease on the reaction time, indicating that a higher reaction rate was

Figure 6. Time-dependent UV−vis spectra of the reaction solution in the presence of Ag 1 (a), and plots of ln(At/A0) against the reaction time for the reduction of 4-NP (b).

obtained with a larger amount of Ag NPs. However, in order to evaluate the intrinsic catalytic activities and exclude the disturbance induced by the amount of catalyst, an “activity parameter” k was calculated (see section 2.5), which could also be used to compare the catalytic activities of the currently developed Ag NPs-containing composites with those of other Ag NPs-based hybrid catalysts. Herein, the catalyst loading was determined by the amount of AgNO3 added into the reaction solution. The k value for Ag 1 was 4.80 × 10−2 L·mg−1·s−1. This was 6−1725 times higher than that of Ag NPs on different supports22,30,49−53 reported in the literature, confirming the high activity of Ag NPs@PGMA−PAM composites obtained in the present study. Compared with our previous study,30 the k value for Ag 1 was 10 times higher than that of Ag NPs on sulfhydryl-functionalized PGMA (4.38 × 10−3 L·mg−1·s−1), resulting from the modification of PAM layers. The comparison demonstrated PGMA−PAM as a better support for Ag NPs than sulfhydryl-functionalized PGMA. With the assistance of PGMA−PAM support, enhanced surface area of Ag NPs (as a result of reduced size) and improved electron transfer to 4NP54 were obtained, which was considered as the main reason for the enhanced catalytic activity and the contribution of this work. However, a decrease of k values was observed for Ag 2 (3.60 × 10−2 L·mg−1·s−1) and Ag 3 (3.07 × 10−2 L·mg−1·s−1), although higher kapp values were obtained. The decrease of k with increasing Ag+ concentration could be attributed to more embedment of Ag NPs inside the PAM layer at higher Ag+ concentration.55 In addition, the increased size of Ag NPs (Figure 5) resulted in decreased specific surface area. Therefore, the adsorption of Ag+ by the electrostatic attraction from negatively charged PGMA−PAM microspheres F

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Industrial & Engineering Chemistry Research Table 1. Comparison of Rate Constants of Various Supported Silver Catalysts for the Reduction of 4-NP catalyst support PGMA−PAM 1 PGMA−PAM 2 PGMA−PAM 3 polystyrene microspheres sulfhydryl-functionalized PGMA graphene oxide-Fe3O4 SiO2 nanosilica (KCC-1) polyaniline nanofibers Fe3O4@C

size (nm) 9.7 ± 2.7 14.2 ± 3.0 16.7 ± 3.3 30 16.9 ± 3.15 9−20 19 ± 4 4 20−100

T (°C)

[Ag] (mg·L−1)

RT RT RT RT RT RT 25 20 RT 25

0.12 0.24 0.36 0.54 0.90 8.09 1.10 5.98 13.50 611.30

kapp (s−1) −3

5.76 × 10 8.64 × 10−3 11.05 × 10−3 3.49 × 10−3 3.94 × 10−3 2.67 × 10−2 2.73 × 10−3 1.00 × 10−2 2.14 × 10−2 1.70 × 10−2

k (L·mg−1s−1) 4.80 3.60 3.07 6.46 4.38 3.30 2.53 1.67 1.58 2.78

× × × × × × × × × ×

10−2 10−2 10−2 10−3 10−3 10−3 10−3 10−3 10−3 10−5

ref this work this work this work 50 30 51 22 49 52 53

concentration of AgNO3. The obtained silver composites exhibited excellent catalytic properties for the reduction of 4NP with NaBH4, as indicated by a k value of 4.80 × 10−2 L· mg−1·s−1, which was 6−1725 times higher than that of various supported Ag NPs reported in literature. The enhanced catalytic activity was attributed to the enhanced surface area of Ag NPs (as a result of reduced size) and the improved electron transfer to 4-NP with the assistance of PGMA−PAM support. Furthermore, the catalysts can be easily recovered for reuse by simple centrifugation, which is preferred in terms of cost and environmental protection. This would facilitate the application of silver composite catalysts in environmental protection. Moreover, the controllable fabrication of Ag NPs@ PGMA−PAM composites proposed herein would provide a potential platform for the fabrication of other noble metal catalysts, such as gold nanoparticles, and facilitate the applications of various noble metal-based catalytic reactions.

(Figure 1) and the coordination from N, O atoms in amide groups were considered important for the formation and regulation of supported Ag NPs. Similarly, the adsorption of Au+ using amine groups has been reported in the literature.56 The as-proposed synthetic method was then proposed to be used for the synthesis of gold nanoparticles, and would provide a potential platform for the fabrication of other noble metal catalysts. Finally, the reuse of Ag NPs@PGMA−PAM composites was examined. This was achieved by simple centrifugation separation (Figure S3 in Supporting Information), which is preferred in terms of cost and environmental protection. Moreover, a conversion efficiency of higher than 94% was observed after running 8 successive cycles, as shown in Figure 7. A well maintained structure and morphology of Ag NPs@

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b03393. Size distribution of PGMA microspheres in aqueous solution (Figure S1); enlarged SEM image of Ag NPs@ PGMA−PAM composite (Figure S2); photo images of Ag NPs@PGMA−PAM solution and the residue after centrifugation (Figure S3); TEM image of Ag NPs on PGMA−PAM after a storage for 12 months (Figure S4) (PDF)

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Figure 7. Conversion of 4-NP in eight successive cycles with Ag NPs@PGMA−PAM composite.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] Tel.: +86-22-27404981. Fax: +86-22-27403389.

PGMA−PAM composite was also observed after storage at RT for more than 12 months, as shown in the TEM image in Figure S4 (Supporting Information). Therefore, excellent stability and reusability of Ag NPs@PGMA−PAM composite could be concluded.

ORCID

Lin Zhang: 0000-0002-8601-0175 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (Nos. 21236005, 91534119 and 21621004), the Natural Science Foundation of Tianjin (13JCZDJC31100), and the Innovation Foundation of Tianjin University.

4. CONCLUSIONS An effective strategy for the fabrication of well-dispersed Ag NPs supported on PGMA−PAM was proposed. PGMA microspheres were prepared and modified by PAM through ATRP of acrylamide, which was abundant with amide groups and acted as anchors for the growth of the Ag NPs. Spherical Ag NPs with a diameter of 9.7 ± 2.7 nm were synthesized. Easy regulation on the size of Ag NPs developed on the PGMA− PAM spheres was achieved through the adjustment on the

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REFERENCES

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DOI: 10.1021/acs.iecr.6b03393 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX