graphene composite hydrogels: Kinetics and performance

Journal Pre-proofs Full Length Article Catalytic reduction reactions over the silver ions embedded in polyacrylamide/graphene composite hydrogels: kinetics and performance Huawen Hu, Yuyuan Zhang, Yu Qiao, Dongchu Chen PII: DOI: Reference:

S0169-4332(19)33652-9 https://doi.org/10.1016/j.apsusc.2019.144835 APSUSC 144835

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

13 October 2019 15 November 2019 21 November 2019

Please cite this article as: H. Hu, Y. Zhang, Y. Qiao, D. Chen, Catalytic reduction reactions over the silver ions embedded in polyacrylamide/graphene composite hydrogels: kinetics and performance, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.144835

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© 2019 Published by Elsevier B.V.

Catalytic reduction reactions over the silver ions embedded in polyacrylamide/graphene composite hydrogels: kinetics and performance Huawen Hu,*a,b Yuyuan Zhang,a,c* Yu Qiao,c Dongchu Chena

a

School of Materials Science and Energy Engineering, Foshan University, Foshan,

Guangdong 528000, China; b

Guangdong Provincial Key Laboratory of New and Renewable Energy Research and

Development, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China; c

State Key Laboratory of Coal Combustion, Huazhong University of Science and

Technology, Wuhan, Hubei 430074, China

Corresponding Emails: [email protected]; [email protected]

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ABSTRACT Catalytic reduction of organic analytes has aroused substantial attention due to the significant role they have been playing in both fundamental and industrial fields. However, there still lacks a new catalytic mechanism since similar catalytic reduction pathways are frequently reported based on the literature review (e.g., the catalytic reactions generally controlled by the pseudo-first-order kinetics). In this contribution, the catalytic pathway is flexibly varied by tuning the microstructure of a ternary composite catalyst consisting of silver ions, graphene, and polyacrylamide. The pseudo-zero-order kinetics is, for the first time, realized using such metal ions-based catalysts. The in-depth structural characterizations and analyses of the catalysis pathway and performance are provided. Keywords: polyacrylamide; graphene; Ag+ ions; catalytic reduction reactions; pseudo-zero-order reaction kinetics

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1. Introduction

Metal (particularly Pt, Pd, Au, and Ag) nanoparticles (NPs) play a vital role in the catalytic transformation reactions, including Suzuki cross-coupling reactions [1], oxidation of methane [2], water oxidation [3], oxygen reduction reactions [4], hydrogen evolution [5], CO2 reduction [6], nitrophenol reduction [7], etc. [8]. Among them, the reduction of organic analytes is regarded as one of the foremost catalytic reactions, and substantial attention has been paid to the investigation of the catalyst structures, catalysis performance, and catalytic kinetics [9-23]. Notably, the catalytic transformation of toxic 4-nitrophenol (4NP) to value-added 4-aminophenol (4AP) has generated much interest in that 4AP shows many useful applications such as photographic developers, corrosion inhibitors, hair dyeing agents, and analgesic and antipyretic drugs [24-28]. In most cases involving catalytic reduction reactions over metal-based catalysts, the pseudo-first-order kinetics has been frequently achieved due to the stoichiometric proportion consideration [7, 13, 20, 21, 26, 29-35], including most recently reported Ag clusters stabilized on TiO2 nanocrystals [10], ultrafine Au NPs in a hyper-cross-linked

polymer

[7],

Pt-Au

NPs

supported

on

polydopamine-functionalized graphene [13], Ag NPs on chitosan-TiO2 composites [20], Au-Ag bimetallic NPs supported on LDH [21], Ag-hydrogel composites [26], etc. This pseudo-first-order kinetic pathway is routinely adopted for the kinetic analysis and the comparison of the catalytic activity with the reported catalysts [7, 16, 20, 21,

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26, 29-33]. Moreover, almost all of the work, involving the metal-based catalytic reduction of organic analytes, considered the fabrication of the metal particles using metal ions-based precursors before the catalytic reduction reactions, and little work is devoted to directly using metal ions as the catalytic center for such reduction reactions. The novelty of the present work is herein described as the following aspects. First, silver ions were directly explored to catalyze the reduction of organic analytes, with the assistance of graphene oxide (GO) and polyacrylamide (PAM). Second, unlike most of the work describing that the metal-based catalytic reduction of analytes followed pseudo-first-order kinetics under the conditions that the reducing agent (e.g., NaBH4) was added in excess, the pseudo-first-order kinetics was not always applied to the Ag+-based catalytic reduction in our case. The conversion between the pseudo-zero-order and pseudo-first-order kinetics could be easily achieved by variation of the concentration of the GO nanosheets (within the PAM network) that were demonstrated to possess fascinating Ag+-carrying properties. The success of the present fabrication lied in the existence of the double cross-linking networks, as generated by PAM chains and graphene sheets. Such confining networks could inhibit the overgrowth of Ag NPs as in situ generated during the catalytic reduction reactions, as shown in Fig. S1. In-depth characterizations and catalytic reduction applications of a series of prepared samples were investigated and compared with recently reported metal-based catalysts. Profound kinetic analysis of such a catalytic reduction reaction over a PAM-based ternary composite catalyst with Ag+ ions and graphene sheets was 4

also presented.

2. Experimental

2.1. Materials

Graphite fine powder (extra-pure reagent) was supplied from Shanghai Chemical Reagents (Shanghai, China). All the chemicals, including acrylamide (AM), N, N'-methylene-bis(acrylamide) (MBA), and potassium persulfate (PPS), were A.R. grades and purchased from Tianjin Fuchen Chemical Reagent Factory (Tianjin, China). Silver nitrate of A.R. grade was provided by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All of the other chemicals involved in this study were analytical reagents and obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All of the chemicals were used as received unless otherwise stated.

2.2. Synthesis of GO nanosheets

GO was synthesized based on a modified Hummers’ method [36], with the procedures described as follows: fine graphite powder (1.0 g) and NaNO3 (0.5 g) were sufficiently mixed in concentrated H2SO4 (25 mL) in a flask (100 mL) equipped with an ice-water base. While the temperature was maintained around 5 oC, the mixture was magnetically stirred for 2 h. Then, KMnO4 (3.7 g) was added in small portions to avoid the temperature increase to above 20 oC. Upon completing the addition of KMnO4, the temperature was raised to 35 ± 2 oC, and concomitantly, continuous stirring was kept for 30 min. After the reaction had been completed, deionized (DI) 5

water (45 mL) was added into the mixture in a stepwise manner, and the reaction further proceeded by mixing H2O2 (3.5 mL, 30%) and DI water (27.5 mL). The coarse graphite oxide was finally produced and then separated from the reaction solution by centrifugation and filtration, followed by thoroughly washing with a copious amount of warm diluted HCl (3%, 100 mL) and DI water (100 mL). The washing steps had been repeated three times before drying treatment under vacuum for 24 h was implemented.

2.3. Preparation of PAM-based binary composites with GO

The prepared GO powder was homogeneously dispersed into water by sonication treatment, forming aqueous dispersions with different concentrations of GO (i.e., 1, 3, and 6 mg/mL). To a dispersion of GO (10 mL), the monomer AM (0.5 g) and the cross-linking agent MBA (3.5 mg) were dissolved. After the addition of the initiator PPS had triggered the free-radical polymerization of AM, it proceeded at 60 oC for 4 h, which led to the generation of the PAM-based composite hydrogels incorporating different concentrations of GO. The starting 1, 3, 6 mg/mL GO dispersions were corresponding to the finally prepared samples designated as PG1, PG3, and PG6, respectively. For comparison, the neat PAM hydrogel was also prepared according to the above procedures except that the initial 10 mL GO dispersion was replaced by 10 mL DI water. The prepared hydrogel samples were post-treated by lyophilization to maintain the porous structure of the hydrogel. The lyophilized samples were stored in a desiccator before use. 6

2.4. Preparation of Ag+ions-embedded PAM/GO composites

The prepared GO powder was homogeneously dispersed into water by sonication treatment, producing aqueous dispersions with different concentrations of GO (i.e., 1 mg/mL, 3 mg/mL, and 6 mg/mL). To a dispersion of GO (10 mL), the monomer AM (0.5 g) and the cross-linking agent MBA (3.5 mg) were dissolved. Separately, a 0.1 M silver nitrate solution was prepared. A portion of the prepared silver nitrate solution (2 mL, 0.1 M) was added into the GO dispersion mixed with AM and MBA. After the addition of the initiator PPS had triggered the free-radical polymerization of AM, it proceeded at 60 oC for 4 h, which led to the generation of PAM-based composite hydrogels embedded with different concentrations of GO and Ag+. The starting 1, 3, 6 mg/mL GO dispersions resulted in the production of the ternary composites designated as PG1-Ag+, PG3-Ag+, and PG6-Ag+, respectively. The prepared PAM-based ternary composite hydrogels were post-treated by lyophilization to maintain the porous structure of the hydrogel. The lyophilized samples were stored in a desiccator before use.

2.5. Post-reduction treatment of PG1-Ag+

The typical PG1-Ag+sample was adopted as the target to be post-treated with a 0.8 M hydrazine hydrate solution. This post-treatment was performed by immersing the PG1-Ag+ hydrogel sample in the hydrazine hydrate solution for 6 h, and then it was collected for the washing treatment with a copious amount of DI water and then for the lyophilization processing, resulting in the sample designated as PG1-Ag. After 7

lyophilization, the PG1-Ag sample was also further oven-dried at 60 oC for 24 h, and the resulting sample was named as PG1-Ag (dried).

2.6. Characterizations

X-ray diffraction (XRD) patterns of the prepared samples were recorded using a Bruker D8 Advance X-ray diffractometer (Bruker AXS, Karlsruhe, Germany). The characterizations of the structural functionalities of the sample were studied by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy using an IRAffinity-1S spectrometer (Shimadzu Co., Kyoto, Japan). The scanning electron microscopy (SEM) images and elemental mapping images of the freeze-dried PG1-Ag+, PG3-Ag+, and PG6-Ag+ samples, as well as the PG1-Ag+ sample after subject to the catalytic reduction reaction, were obtained with a field-emission scanning electron microscope (JEOL JSM-6335F, Japan) equipped with an elemental analyzer. The lyophilization processing was carried out by the following procedures: the PAM-based hydrogel samples were pre-frozen in liquid nitrogen, and then lyophilized at the temperature of -60 oC and the pressure of 0.026 mbar for 48 h using a Heto FD8 freeze dryer. The real-time ultraviolet/visible (UV/Vis) spectroscopic monitoring of the catalytic reaction was conducted using a Lambda 25-Perkin Elmer UV-vis spectrophotometer (Massachusetts, USA). Differential scanning calorimetry (DSC) was employed to measure the glass transition behavior of various prepared samples using a DSC-200 F3 calorimeter (Netzsch Group, Selb, Germany).

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2.7. Investigation of the catalysis performance

The catalysis performance was studied by the real-time monitoring of the characteristic UV/Vis absorption peak that changed with the catalytic reduction reaction. The characteristic UV/Vis absorption at 400 nm was used to monitor the catalytic reduction of 4NP, while the characteristic UV/Vis absorption band was altered to 662 nm as far as methylene blue (MB) was concerned. The dependence of the absorbance at 400 nm for 4NP (or 662 nm for MB) on time was recorded. For a single run of the catalytic reduction of 4NP, a 4NP solution (0.2 mM, 2.5 mL) was added to the cuvette which was pre-loaded with the prepared hydrogel sample (30 mg), and the spectroscopic test on monitoring the catalytic reaction was started immediately upon the addition of the NaBH4 powder (5.0 mg). The plots of UV/Vis absorption at 400 nm or 662 nm with reaction time was obtained from the UV/Vis spectrophotometer. The spectroscopic measurement of the catalytic reduction of MB was conducted according to the same procedure as that of 4NP. The durability test was conducted by successively varying the solution with fresh ones after each cycle, and a total of five cycles was considered.

2. Results and Discussion

2.1. Structural analysis

The success in the fabrication of GO nanosheets is the prerequisite for the subsequent effective water-based free-radical polymerization, as demonstrated by the 9

UV/Vis and FTIR spectra, the photo image of an aqueous dispersion of GO, the XRD pattern, the SEM, TEM and AFM images, and the height profiles based on the AFM (Fig. S2). The unique carrying ability of GO is unravelled by varying and fixing the GO and Ag+ concentrations in the mixed solution containing the monomer before the free-radical polymerization, respectively (Fig. 1 and Fig. S1). Results prove that the higher content of GO results in an apparent increase in the content of the Ag+ ions embedded in the PAM/GO matrix. The all-water solution-based fabrication process is facile, fast, clean and environmentally friendly, and the free-radical polymerization, cross-linking reaction, incorporation of GO, and inclusion of Ag+ can be achieved in a one-step manner. The polymerization leads to the formation of a polymer network structure that strongly immobilizes the GO nanosheets, impeding their aggregation even after the violent catalytic reduction with NaBH4 or the post-treatment with hydrazine hydrate. This result implies that the robust composite hydrogel is likely to impart high stability and durability. The hydrogel-based catalyst with a bulk structure also helps to achieve easy recycling and regeneration after the catalytic reaction in comparison with the commonly-used powdery catalysts [32, 33].

Fig. 1. Simultaneous in situ conversion of Ag+ ions to Ag NPs and catalytic reduction 10

reaction based on two kinds of PAM-based composite catalysts differing significantly in the GO concentration and consequently the content of Ag+ ions.

The pure PAM exhibits a broad XRD band at approximately 22o (Fig. 2), confirming the amorphous nature of PAM due to the disordered polymer chain alignment, a characteristic of polymeric hydrogels. The incorporation of GO does not gravely alter the crystal structure since similar XRD patterns are observed among the neat PAM and PAM-based binary composites with GO at different concentrations (i.e., PG1, PG3, and PG6). Even for the PG6 sample with the highest loading of GO nanosheets, the XRD pattern is still close to that for neat PAM, albeit with a slight difference. Concerning the three ternary composites with Ag+, notable differences from neat PAM and binary composites are observed in the XRD profiles, with a few weak peaks that emerge in the range of 26 to 40o. These peaks can be assigned to oxides of silver and Ag+ ions, and no peaks can be indexed to metallic Ag. These results reveal that Ag+ ions are effectively embedded into the PAM/GO matrix and undergo partial oxidation during the lyophilization. During the catalytic reduction process, these species are probably converted to metallic Ag. To confirm this, the typical sample (PG1-Ag+) is post-treated with hydrazine hydrate, and the metallic Ag NPs indeed appear for the PG1-Ag sample (Fig. 3). The characteristic XRD peaks at approximately 38.4, 44.4, 64.6 and 77.6o can be assigned to the (111), (200), (220) and (311) crystallographic planes of the face-centered cubic (fcc) Ag, respectively, as evidenced by the JCPDS data. The further oven-drying treatment of the lyophilized 11

PG1-Ag sample changes the crystal structure of the formed Ag. This change can be manifested by the variation of the characteristic XRD bands associated with fcc Ag. The enhancement of these characteristic XRD bands elucidates that the oven-drying processing can lead to the growth of Ag NPs due to the destruction of the double cross-linking networks by the heat. The double cross-linking networks are helpful for stabilizing the Ag NPs and imparting high stability and durability to the composite catalyst. The Scherer equation [37], D = Kλ/(Bcosθ), is employed to estimate the average crystallite size of the formed Ag NPs within the PAM/graphene matrix (where D, K, λ, B and θ represent the average crystallite size, the Scherrer coefficient, the wavelength of the radiation, the full-width at half-maximum (FWHM), and the Bragg angle, respectively). The mean crystallite size of Ag NPs in the PG1-Ag sample is calculated to be 10 nm, and after the oven-drying processing, the size is increased to 11.5 nm for the PG1-Ag (dried) sample as a result of the heat-induced destruction of the polymer network structure.

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PG6-Ag+

Intensity (arb. units)

PG3-Ag+ PG1-Ag+

PG6 PG3 PG1 PAM

10

20

30

40

50

60

70

80

2θ (Deg.) Fig. 2. XRD patterns of various prepared samples, including neat PAM, PAM-based binary composites (PG1, PG3, and PG6) with different concentrations of GO, and the PAM-based ternary composites (PG1-Ag+, PG3-Ag+ and PG6-Ag+) with different concentrations of GO and ensuing loadings of Ag+ ions.

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Intensity (arb. units)

PG1-Ag (dried)

PG1-Ag

PG1-Ag+

PG1

30

40

(311)

20

(220)

10

(200)

(111)

PDF#04-0783 fcc Ag

50

60

70

80

2θ (Deg.) Fig. 3. XRD patterns of the prepared PG1, PG1-Ag+, PG1-Ag and PG1-Ag (dried) samples; the JCPDS card (no. 04-0783) are also referenced for comparison, and the FWHM of the (111) diffraction peak is adopted for the calculation.

Many FTIR absorption peaks reveal the functionalities on the PAM molecular chains (e.g., at 3331, 3187, 1651, 1607, 1318 and 1114 cm-1), as marked in the panel of Fig. 4. The incorporation of GO nanosheets into the PAM matrix makes shifts of some characteristic absorption bands, especially the one assigned to the asymmetric stretching vibration (νas) of the -NH2 groups [38]. The low content of GO cannot make a significant shift of this absorption as generated by the νas of –NH2, and almost the same peak location is found for neat PAM and PG1. Nonetheless, this typical peak 14

is shifted from 3331 for PAM to 3336 cm-1 for PG3 with an increased GO content relative to PG1, and, to a more significant extent, to 3346 cm-1 for PG6 with the highest content of GO. Such hypsochromic shifts can be owing to the hydrogen bonding interactions between GO nanosheets and PAM chains [39-41]. The hydrogen bonding endows the polymer composite with another physical cross-linking network except for the cross-linked polymer chain network (Fig. S1). The double cross-linking networks provide a confinement structure for stabilizing the Ag NPs as formed in the catalytic induction period (Fig. 1), an initial time interval where no catalytic conversion reaction seemingly occurs [42]. Embedding Ag+ into the PAM/GO matrix makes a redshift of the peak related to the νas of -NH2, with a marked shift extent for the PG6-Ag+ sample (3333 cm-1) relative to the corresponding PG6 sample (3346 cm-1). The result implies that the embedded Ag+ disrupts the hydrogen bonding interactions between the PAM chains and GO nanosheets due to the interactions of Ag+ with the functional groups on the GO nanosheets. For PG3-Ag+, the absorption is red-shifted from 3336 (PG3) to 3334 cm-1, with a lesser extent as compared to the PG6-Ag+ vs. PG6. This result can be due to the lower content of GO and hence the fewer active sites that are accessible for the interactions with Ag+. Of interest is an opposite trend found for the PG1-Ag+ sample with the lowest content of GO. A hypsochromic shift from 3328 (PG1) to 3334 cm-1 is observed; the low content of GO within the PAM matrix cannot provide sufficient functionalities to interact with Ag+ ions, which turn to interact with PAM chains. The FTIR absorption peak can also indicate the existence of silver composition at approximately 820 cm-1 originating 15

from the Ag-O stretching mode caused by multiphonon processes [41].

n (O-H)

1117

1043

822

1319

d (C-H)

1648

3187

PG1-Ag+

1047

1117 1319

1603

d (N-H)

1601

n as(N-H)

1648

n s(N-H)

819

n (C=O)

n (C-N) 3334

n (C-O)

1317 1318

1119

1318

1114

1607

1116

1606

1651

3187

3331

1319

1605

1651

3187

3328

1116

1604

1647

1116

1605

1648

1319

1649

3336

PAM

3187

3346

PG1

r(-NH2)

3187

PG3

3187

PG6

3334

Transmittance (arb. units)

n (Ag-O)

d (O-H)

3187

+

3333

PG3-Ag

3424

PG6-Ag

+

4000 3500 3000 2500 2000 1500 1000

Wavenumber (cm-1) Fig. 4. FTIR spectra of various prepared samples including neat PAM, PAM-based binary composites with GO (PG1, PG3, and PG6), and PAM-based ternary composites with GO and Ag+ ions (PG1-Ag+, PG3-Ag+ and PG6-Ag+).

The FTIR spectra of the PG1-Ag and PG1-Ag (dried) samples are also compared (Fig. 5(a-c)). After the conversion of Ag+ ions into Ag NPs, one can detect a noticeable redshift in the position of the FTIR peak indexed to the νas of -NH2 (from 3334 cm-1 for PG1-Ag+ to 3321 and 3295 cm-1 for PG1-Ag and PG1-Ag (dried), respectively, see Fig. 5(b)). This finding implies that the as-formed Ag NPs cover the 16

active sites on the GO nanosheets, thereby reducing their number and consequently weakening the hydrogen bonding interactions with PAM chains. The larger the crystallite sizes of Ag NPs (10 nm for the PG1-Ag sample vs. 11.5 nm for the PG1-Ag (dried) sample), the larger the extent of the redshift, since more active sites on the GO nanosheets become inaccessible when covered. Apart from this characteristic peak due to the νas of -NH2, another FTIR absorption peak associated with the amide I (C=O) stretching vibration also shifts to some extent as a result of the reduction of Ag+ to Ag NPs (Fig. 5(c)).

Fig. 5. a-c) FTIR spectra of the PG1-Ag+ sample before and after the reduction treatment by hydrazine hydrate over a broad wavenumber range (a), a selected range of 3500~3000 cm-1 (b), and a selected range of 1750~1500 cm-1 (c).

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The weakening of the hydrogen bonding interactions between the polymer chains and GO nanosheets after including Ag+ can also be revealed by the DSC analysis (Fig. 6). The glass transition temperatures are steeply decreased after the loading of Ag+ in the PAM/graphene matrix regardless of the contents of GO; this is due to the weakened hydrogen bonding interactions as caused by Ag+, revealing that the polymer chain segments become freer from the hydrogen bonding confinement effect and thus the glass transition temperatures are lowered, e.g., 60.8o (PG1) vs. 57.9o (PG1-Ag+) and 61.1o (PG6) vs. 57.1o (PG6-Ag+).

Fig. 6. a) DSC thermograms of various prepared samples, including pure PAM, PAM-based binary composites with GO (PG1, PG3, and PG6), and PAM-based ternary composites with GO and Ag+ ions (PG1-Ag+, PG3-Ag+ and PG6-Ag+). b) Comparison of the glass transition temperatures (Tg) among these samples.

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The above results obtained via FTIR tests have revealed the small content of GO in the PG1, without sufficient binding sites for Ag+. To further confirm these results, the elemental mapping technique is then employed (Fig. 7). The low concentration of Ag+ ions in the PG1-Ag+ sample is further detected, in stark contrast to those in PG3-Ag+ and PG6-Ag+. While both the PG3-Ag+ and PG6-Ag+ samples present continuous bright green spots which tightly pile together to form a green surface, especially for PG6-Ag+, only a few sparse green spots can be seen in the Ag elemental mapping image of PG1-Ag+. These results conclusively demonstrate that the higher the GO concentration, the larger the quantities of the embedded Ag+ ions, implying the unique carrying ability of GO nanosheets.

Fig. 7. Elemental mapping images of the prepared PG1-Ag+ (a), PG3-Ag+ (b), PG6-Ag+ (c) samples; scale bars in all of the corresponding SEM mages are 10 μm.

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2.2. Investigation of the catalytic kinetics and performance

After the Ag+ ions are concomitantly in situ reduced to the Ag NPs during the catalytic reaction, the activation energy of the catalytic reduction of 4NP and MB is dramatically lowered, enabling the reaction to proceed rapidly [43]. As expected, an induction period is observed before the catalytic reduction reaction due to the need to convert the Ag+ ions and oxides of silver into the metallic Ag NPs, which can be reflected by the dependence of UV/Vis absorption on time (Fig. 8). The gradual lowering of the peak around 400 nm (indexed to 4NP), along with the progressive promotion of the peak around 300 nm (corresponding to 4AP), indicates the effective catalytic transformation of 4NP to 4AP over the PG6-Ag+ sample (Fig. 8(a)). This catalytic reaction is also evidenced to be a clean transformation reaction without the generation of any side products based on the clear isosbestic points found at 279 and 318 nm (Fig. 8(a)) [14, 16, 30, 43, 44]. The Lambert-Beer law facilitates the change the UV/Vis absorption of 4NP to its concentration [45, 46], and the dependences of Ct/C0 on time are then depicted (Fig. 8(b)), where Ct and C0 represent the 4NP concentration at time t and the initial 4NP concentration before the catalytic reaction is started, respectively. The sluggish reaction kinetics is detected for the PAM and PG6 samples used as the typical catalysts, indicating limited catalytic activity. The induction periods probed in both the reaction systems with PG1-Ag+ and PG6-Ag+ account for around 20 s. Although there are still controversial arguments on the origin of the induction period [42], the induction process in our case is well correlated with the conversion of Ag+ ions into 20

Ag NPs.

Fig. 8. a) UV/Vis spectra captured for monitoring the reduction of 4NP catalyzed by PG6-Ag+. (b) The plots of time-dependent 4NP concentration are automatically recorded for the reaction systems with PAM, PG6, PG1-Ag+, and PG6-Ag+. (c) Logarithmic transformation of the plots of Ct/C0 vs. time to those of ln(Ct/C0) vs. time. (d) Normalized apparent rate constant at different cycles of the reduction of 4NP to 4AP using the PG6-Ag+ catalyst.

Most importantly, discrepant catalytic reaction kinetics is noted between the reaction systems with PG1-Ag+ and PG6-Ag+. For the catalytic reaction system with PG1-Ag+, the linear dependence of Ct/C0 on time, with the correlation coefficient R2 value obtained as 0.983, reveals that the pseudo-zero-order kinetics dictates the 21

catalytic reaction. By contrast, for the system with PG6-Ag+, the linear fitting cannot be applied to the plot of Ct/C0 vs. time (Fig. 8(b)). We then apply the logarithmic transformation of the plots of Ct/C0 vs. time to the variation of ln(Ct/C0) with time (Fig. 8(c)). Conversely, the linear fitting (with the R2 value obtained as 0.972) can be applied to the plot of ln(Ct/C0) vs. time as for the system with PG6-Ag+ (indicative of the pseudo-first-order reaction kinetics) but is invalid for the system with PG1-Ag+. Therefore, the concentration of Ag+ might play a pivotal role in governing the catalytic reaction kinetics. Concerning the system with PG1-Ag+, the number of embedded Ag+ ions is demonstrated to be small as a result of the limited incorporation of GO nanosheets. The low concentration of Ag+ makes the catalytic reaction independent of the 4NP concentration. The reducing reagent added in excess causes the catalytic reduction reaction to be also independent of its concentration. The small number of catalytic sites provided by the metal NPs imparts the catalytic reaction to being strongly associated with the accessible catalytic sites provided by the metal NPs. Taken together, the reaction system with PG1-Ag+ follows the pseudo-zero-order kinetics. On the other hand, the system with the PG6-Ag+ sample contains much more Ag+ ions relative to the PG1-Ag+ sample, and thus a considerably increased quantity of the Ag NPs can be formed in the catalytic induction period, which provides abundant catalytic sites for 4NP. The sufficient catalytic sites turn the catalytic reaction to rely on the 4NP concentration. As a result, the reaction becomes radically dictated by the pseudo-first-order kinetics for the system with PG6-Ag+. To further prove these results, MB is adopted as another organic analyte under the same reaction 22

conditions as those for 4NP. The catalytic reduction of MB results in the formation of leucomethylene blue (LMB), and the blue-color (MB) to colorless (LMB) conversion facilitates the monitoring of the catalytic reaction both by the naked eye and by the UV/Vis spectroscopy (Fig. 9). Likewise, the catalytic reduction of MB over PG1-Ag+ and PG6-Ag+ exhibits similar kinetics to the catalytic reduction of 4NP, i.e., the pseudo-zero-order and pseudo-first-order kinetics indexed to the catalytic reaction systems with PG1-Ag+ and PG6-Ag+, respectively. These results corroborate the above analysis that the metal NPs-based catalytic reduction reactions do not always follow the pseudo-first-order kinetics even at an excessive amount of reducing agents, and the kinetics of the catalytic reactions turns out to be pseudo-zero-order under the low catalyst content conditions. One can also note that the UV/Vis lines are not smooth and vibrations on them can be observed along the catalytic reaction, generating much noise; this is caused by the violent catalytic reduction reactions, with gases produced. The release of the gases out of the reaction solution brings O2 into the solution, and the dissolved O2 can react with the reaction product, e.g., 4AP, in the presence of the same catalyst to form 4-nitrophenolate [42].

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Fig. 9. a) The plots of Ct/C0 as a function of reaction time for the systems with PAM, PG6, PG1-Ag+ and PG6-Ag+ catalysts are automatically recorded. (b) Logarithmic transformation of the plots of Ct/C0 vs. time to those of ln(Ct/C0) vs. time.

For the PG6-Ag+-catalyzed reaction system with 4NP, the apparent rate constant 24

k1 is estimated to be 0.0247 s-1 (or 1.482 min-1) from the slope of the linear plot fitted from the dependence of ln(Ct/C0) on time, which is larger than most of the values recently reported for metal NPs-based catalysts [7, 13, 16, 20, 21, 26, 29-33], as summarized in Table S1. To calculate the normalized apparent rate constant by the effective catalyst weight, we also dried the wet PG6-Ag+ hydrogel sample to a constant weight that was measured to be 1.8 mg. The rate constant normalized by the PG6-Ag+ catalyst weight was summarized in Table S1 (0.823 min-1 mg-1 or 0.014 s-1 mg-1), which exceeds the corresponding values for many recently reported catalytic reaction systems with the metal-based catalysts. The durability of the PG6-Ag+ catalyst is also examined by five-run repeated usage of the catalytic reduction of 4NP, and the results are presented in Fig. 8(d). All of the kn values (n=2,3,4,5) are close to that of k1, revealing the robustness of the PG6-Ag+ catalyst owing to the double cross-linking networks constructed by PAM chains and graphene, which stabilizes the as-grown Ag NPs (Fig. S1). The underlying mechanism of the Ag+-enabled catalytic reaction is further analyzed through the investigation of the structure of the PG1-Ag+ sample recycled after the first-cycle catalytic reduction reaction, and the results are provided in Fig. S3, S4, and S5. XRD patterns reveal the effective conversion of Ag+ ions into Ag NPs, as shown in Fig. S3. According to the Scherer equation, the average crystallite size of the Ag NPs formed after the catalytic reaction is estimated to be 9.9 nm, almost the same as that of the Ag NPs in the PG1-Ag sample prepared by the post-treatment of PG-Ag+ with hydrazine hydrate. The results imply that the chemical reduction of 25

PG1-Ag+ is independent of the kinds of reducing agent employed. The double confinement networks in the PAM/graphene matrix contribute to the stabilization of the formed Ag NPs, thus prohibiting the overgrowth of Ag NPs during the catalytic reduction reaction. The Ag+ ions-to-Ag NPs conversion and the hydrogen-bonding interactions can also be revealed by FTIR characterizations (Fig. S4), and apparent shifts of the characteristic absorption bands (especially those resulting from the variations of the -NH2 group and amide I, as shown in Fig. S4(b) and S4(c), respectively) are observed. This result is consistent with the analyses of the FTIR spectra shown in Fig. 4 and 5. Elemental mapping analysis is further employed, and the result is provided in Fig. S5. After the catalysis process, the signals from the silver element are enhanced; nevertheless, they are still lower as compared to those for the PG3-Ag+ and PG6-Ag+ samples. The slight gain in the intensity of the Ag signals after the catalytic reduction reaction is likely attributed to the reduction of Ag+ and consequently the growth of the electrically conducting Ag NPs.

4. Conclusions

In summary, several significant findings have been presented in this paper. First, the synergistic functions of the ternary components (namely Ag+ ions, GO nanosheets, and PAM) are demonstrated. Individually, the Ag+ ions are the precursor of Ag nanocatalysts for the catalytic reduction reactions. GO nanosheets possess the fascinating Ag+-carrying performance, and their content is directly correlated with the Ag+ concentration. The PAM network allows the final PAM-based composite catalyst 26

to be self-standing, far superior to the traditional powdery catalyst in recycling and reuse. The combination of PAM and graphene nanosheets also creates the double cross-linking network that can stably immobilize the Ag nanocatalysts as in situ formed during the catalytic reduction reaction. Second, it is not necessary to convert the Ag+ embedded in the PAM/GO composite to metallic silver before the catalytic reduction reactions. Third, the frequently-reported pseudo-first-order kinetics does not exclusively apply to the catalytic reduction reaction in our case, even at an excessive amount of the reducing agent. The catalytic reaction begins to follow the pseudo-zero-order kinetics under low Ag+ concentration conditions. Fourth, a highly efficient and durable catalytic reduction of 4NP and MB with the Ag+-embedded PAM/GO composite hydrogel is also achieved.

Acknowledgements

We gratefully appreciate the National Natural Science Foundation of China (51702050), the Featured Innovation Project of the Department of Education of Guangdong Province (2017KTSCX188), the Open Research Foundation of Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development (Y807s31001), the Foundation of State Key Laboratory of Coal Combustion (FSKLCCA1808), and the Open Research Foundation of Guangdong Provincial Key Laboratory of Industrial Surfactant (GDLS-01-2019).

27

Appendix A. Supplementary materials

Supplementary information associated with this article can be found in the online version at http://, including Fig. S1 depicted for the schematic illumination of the microstructure of the Ag+ ions-embedded PAM/GO composite and its structural change when directly employed as a catalyst for the chemical reduction of organic analytes, Fig. S2 presented for the demonstration of the success in the fabrication of homogeneous water dispersions of single-layer GO nanosheets through many characterization analyses such as UV/Vis and FTIR absorption spectra, XRD patterns, and SEM, TEM and AFM images, and Fig. S3 (XRD patterns), Fig. S4 (FTIR spectra), and Fig. S5 (elemental mapping images) provided for the comparison of the microstructures of the PG1-Ag+ sample before and after the catalytic reduction process.

References

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31

Author Contributions Contribution

Author

Conceptualization

Huawen Hu

Formal analysis

Huawen Hu, and Yuyuan Zhang;

Funding acquisition

Huawen Hu, and Yuyuan Zhang;

Project administration

Huawen Hu

Resources

Yu Qiao, and Dongchu Chen

Writing—original draft

Huawen Hu Huawen Hu, Yuyuan Zhang, Yu Qiao, and

Writing—review and editing Dongchu Chen

32

The authors declare that there is no conflict of interest.

33

1.

Ag+ ions embedded in a polyacrylamide/graphene composite.

2.

The fascinating carrying property of graphene oxide for the Ag+ ions.

3.

Governing the catalytic reduction kinetics by the Ag+ ion content.

4.

Pseudo-zero-order kinetics of catalytic reduction reactions was achieved.

5.

Efficient and durable catalytic reduction of organic analytes over the metal ions.

34