reduction of aromatic nitroderivatives

European Polymer Journal 121 (2019) 109289

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Photocrosslinked hybrid composites with Ag, Au or Au-Ag NPs as visiblelight triggered photocatalysts for degradation/reduction of aromatic nitroderivatives Violeta Melinte, Lenuta Stroea, Tinca Buruiana, Andreea L. Chibac

T

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Polyaddition and Photochemistry Department, Petru Poni Institute of Macromolecular Chemistry, 41 A Grigore Ghica Voda Alley, 700487 Lasi, Romania

A R T I C LE I N FO

A B S T R A C T

Keywords: Noble metal nanoparticles Photopolymerized nanocomposite Photocatalysis

Flexible free-standing nanocomposite films embedding Ag, Au or Au-Ag nanoparticles were successfully synthesized by a simple one-pot process, namely photocrosslinking of organic matrix in tandem with photogeneration of noble metal nanoparticles, both initiated by Irgacure 819 photoinitiator. The achieved materials were characterized by UV-vis spectroscopy and transmission electron microscopy, in order to evaluate the morphology, size and shape of the prepared metallic catalysts. The photocatalytic activity of the obtained catalysts was evaluated following the photodegradation of 4-nitroaniline under ambient conditions and visible irradiation or the photoreduction of 4-nitroaniline to p-phenylendiamine in the presence of NaBH4. Further modification of the organic matrix by the inclusion of 3-acrylamidophenylboronic acid monomer, allowed us to prepare a catalyst with the hydrogen donor compound embedded in the polymer film. The photoreduction of 4nitroaniline to p-phenylenediamine occurred in the presence of the new photocatalyst without the addition of a reducing agent in the reaction mixture, although the reaction is slower than that accomplished in the presence of NaBH4.

1. Introduction In the past decades, metal nanoparticles have attracted considerable attention due to their interesting characteristics and enhanced physical and chemical properties as compared to the conventional bulk materials, reason for that they are widely employed in various application such as catalysis [1,2], sensors [3], biomedicine [4], optoelectronics [5] or solar cells manufacturing [6]. In particular, noble metal nanoparticles (NPs), such as gold (Au) or silver (Ag), and the materials incorporating them display distinctive optical properties and absorb visible light mainly due to the localized surface plasmon resonance (LSPR) effect [7,8] and UV light due to interband electron transitions [7,9]. This quality of metal NPs is intensively exploited in the design of efficient photocatalytic materials active in sunlight, which have already been used as efficient photocatalysts for a variety of reactions, from mineralization of organic pollutants [7,10,11] to fine organic reactions [7,12,13]. The great advantage of photocatalysis consisted in the direct conversion of light energy into chemical energy, thus reducing energy consumption or providing a green approach to combating environmental pollution, as these are some of the requirements of sustainable chemistry and green organic synthesis.

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An improvement in the activity of Ag/Au based photocatalysts can be achieved by increasing the electronic density on the surface of metal NPs, a possible way to attain this goal is to introduce the Ag and Au NPs into the same material to develop electronic interactions between Ag and Au. This synergistic effect between the two metals manifests when the Ag and Au NPs are in vicinity or even in contact with each other or they form structures of bimetallic NPs (randomly dispersed alloys, coreshell particles). The literature data already reported some studies regarding the improved activity of Ag-Au alloy nanostructures in comparison to monometallic Au or Ag based catalysts [14-17]. It is a challenge to develop catalytic systems highly active under visible light, easily separable and reusable that can be applied in fine organic reactions or in organic pollutants degradation. An important issue is the selection of the matrix for the immobilization of the active catalyst particles, so far there are investigated different supports such as polymers [10,18,19], cellulose [17], carbon materials [20], mesoporous materials [21], MOF materials [22] and so on. Among these, polymeric materials are very good candidates for this purpose since they are resistant to ultraviolet radiations and are stable in the environment [23], are chemically inert, have low prices, are easily available and due to their hydrophobic nature, the adsorption efficiency

Corresponding author. E-mail address: [email protected] (A.L. Chibac).

https://doi.org/10.1016/j.eurpolymj.2019.109289 Received 18 June 2019; Received in revised form 3 August 2019; Accepted 2 October 2019 Available online 03 October 2019 0014-3057/ © 2019 Elsevier Ltd. All rights reserved.

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same temperature up to the formation of a lithium alkoxide. Further, the maleic anhydride (3.95 g, 40 mmol) was added to the system and the reaction was kept at 0 °C for 90 min. Next, the reaction mixture was stirred for 24 h at room temperature and heated at 35–37 °C for another 24 h. The reaction mixture was neutralized with HCl 37% and the solution was filtered. After the removal of the solvent on a rotary evaporator, the monomer was dissolved in CH2Cl2 and the organic phase was washed 3 times with distilled water. The resulting extract was dried over anhydrous Na2SO4 and the solvent was removed under reduced pressure with a rotary evaporator at 30–35 °C, obtaining the Gly-COOH monomer as a viscous liquid. Yield: 85.1%. FTIR (KBr, cm−1): 3445 (OH); 2961 (C-H); 1724 (C] O); 1638 and 811 (CH2]C); 1165 (C-O). 1H NMR (CDCl3, δ ppm): 6.43–6.62 (eOCOeCH]CHeCOOH and CH2]CH); 6.08 (CH2]C in trans position relative to CH3 unit and CH2]CHe); 5.86 (CH2]CH); 5.57 (CH2]C in cis position relative to CH3 unit); 5.24 (eCHeCH2eOeCOe); 4.2–4.4 (eCHeCH2eOCOe); 1.89 (CH2]CeCH3). The preparation of the urethane dimethacrylate oligomer (M-1) was performed as previously reported [37]. Briefly, 5 g (5 mmol) PEG and 11.66 g (11.66 mmol) PTHF were degassed under vacuum for 2 h, after that 7.2 mL (33.33 mmol) IPDI was added and the mixture was stirred at 60 °C for 6 h in the presence of catalytic amount of dibutyltin dilaurate. Next, the system temperature was decreased to 40 °C, and 4.16 mL (33.33 mmol) HEMA were added together with THF solvent and the reaction continued for 10 h. After removal of the solvent and purification, the urethane dimethacrylate oligomer M-1 was collected as colourless viscous liquid. The preparation of 3-acrylamidophenylboronic acid monomer (M-2) was described into a previous paper [38].

of organic molecules is highly increased [24]. For example, Farooqi’s group exploited these advantages and studied the influence of various parameters (catalyst concentration, temperature, pH s.o.) on the catalytic effectiveness of polymer microgels incorporating Au or Ag NPs. [25-29] A good choice for preparing catalysts containing noble metal NPs immobilized in/on polymer matrix is the in situ reduction of corresponding metal salts due to the simplicity of preparation process. More than that, simultaneous with the formation of noble metal nanoparticles via the reduction of metal salts, the polymerization process can take place. The most convenient technique to prepare catalytic materials in such manner is the photocuring technology (photopolymerization/ photocrosslinking and photogeneration of Ag/Au NPs) because is very simple, non-invasive and does not require expensive devices [30], finally resulting flexible free-standing polymer films. Nitroaromatic compounds (e.g. 4-nitroaniline, 2-nitroaniline, 4-nitrophenol) are used often in industrial production of textile and hair dyes, pharmaceutical drugs, pesticides, antioxidants, explosives [3133], but they also are dangerous and toxic to the environment due to the presence of the nitro units, and have been investigated as mutagens, carcinogens and teratogens, a part of them being listed in the environmental legislation [33,34]. Therefore, an important issue is to eliminate nitroaromatic derivatives from industrial and agricultural wastewaters or more convenient, to convert them in an aqueous medium to the corresponding aminoaromatic compounds through catalytic reduction [33], since the resulting amino products are less toxic than nitroaromatic ones and could find applications in different industrial fields. The photocatalytic reduction of nitroarenes process was also realized in the presence of polymer stabilized metal nanoparticles [35]. Although generally, the chemical reduction of nitroderivatives into aminoderivatives take place in the presence of a reducing agent (sodium borohydride) and a catalyst, their separation from the reaction mixture is usually a laborious process, interfering with the purification of the obtained amine [36]. In the present report, the development of new catalysts based on Ag, Au or Au-Ag NPs immobilized in a photocrosslinked matrix was performed, which are further used in the 4-nitroaniline mineralization or reduction in presence of NaBH4. In addition, an alternative catalyst (F2Au-Ag) that contains the hydrogen donor compound embedded in the polymer film was proposed, eliminating thus the addition of a reducing agent in the reaction mixture. Supplementary, the catalytic efficiency of F2-Au-Ag material in the reduction of dinitro- and trinitroderivatives was investigated, and also the reusability of the photocatalyst after several cycles was tested.

2.3. The in situ UV photogeneration of gold nanoparticles The hybrid composites were obtained via photopolymerization, by mixing the photopolymerizable monomers Gly-COOH, M-1 and M-2 into various ratios together with HAuCl4/AgNO3 (1 wt%) and Irgacure 819 (1 wt%) photoinitiator according to the mass percents listed in Table 1. The formulations were thoroughly mixed and to achieve a homogeneous composition, small amounts of chloroform were added during the mixing. Subsequently, the homogeneous compositions were cast in thin layers on glass slides and were exposed to UV irradiation for a period of 300 s, until tack-free coatings with the thickness around 0.25 mm were produced. The UV irradiation was performed using an Hg-Xe lamp (Hamamatsu Lightningcure Type LC8, Model L9588) with a light intensity of 15 mW cm−2.

2. Experimental 2.4. Measurements 2.1. Materials The 1H NMR measurements were performed on a Bruker 400 MHz spectrometer using deuterated chloroform-d3 as solvent. FTIR spectra were measured on a Bruker Vertex 70 spectrophotometer, by coating the monomers on the KBr plates. For the FTIR photopolymerization experiments, small amounts of monomer mixtures (detailed in Table 1) containing 1 wt% Irgacure 819 photoinitiator were placed on KBr pellets and the FTIR spectra of the uncured samples were recorded. The samples were then light cured for 300 s with Hg-Xe lamp (Hamamatsu Lightningcure Type LC8, Model L9588, light intensity of 15 mW cm−2) and the FTIR absorption spectra were registered after the irradiation. Triplicate specimens of each monomer mixtures were polymerized and analyzed. The conversion degree (DC %) was determined from the decrease of the peak area of the aliphatic C]C bond (1637 cm−1). This band intensity was normalized to that of the C]O signal located at 1720 cm−1. The UV absorption spectra were measured with a Specord 210 Plus Analytik Jena spectrophotometer in distilled water and in thin films. The elemental composition of the photopolymerized sample was

3-(acryloyloxy)-2-hydroxypropylmethacrylate, maleic anhydride, lithium hydride, polyethylene glycol (Mw = 1000 g/mol), isophorone diisocyanate (IPDI), 2-hydroxyethyl methacrylate (HEMA), gold (III) chloride trihydrate (HAuCl4 3H2O), silver nitrate (AgNO3), dibutyltin dilaurate, Irgacure 819, 4-nitroaniline, 2,6-dinitrophenol, picric acid and sodium borohydride (NaBH4) were used as received (from Sigma Aldrich Chemical Co.). Polytetrahydrofuran (PTHF, Mn = 1000 g/mol) was purchased from BASF (Ludwigshafen, Germany). 2.2. Synthesis of glycerol-derived monomer (Gly-COOH) The photopolymerizable Gly-COOH monomer containing carboxyl sequences was synthesized according to the reaction pathway depicted in Scheme 1. Thus, 40 mmol (7.5 mL) 3-(acryloyloxy)-2-hydroxypropylmethacrylate were dissolved in 50 mL tetrahydrofuran and the solution was cooled to 0 °C. To this solution, 0.33 g (40 mmol) lithium hydride was added and the stirring was continued for 30 min, at the 2

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Scheme 1. Synthesis of glycerol-derived monomer (Gly-COOH).

photodegradation, the only difference being that sodium borohydride (final concentration in the reaction mixture 50 mM) was added to 4-NA solution as a reducing agent (H donor compound). Furthermore, the F2-Au-Ag polymeric film which contains a hydrogen donor derivative incorporated inside the polymer matrix was also tested in the reduction of 4-NA in aqueous solutions in ambient conditions, under ambient light or under visible irradiation. Also, it was investigated the effect of variation of noble metal nanoparticles or boronic acid content in polymeric films on the catalytic efficiency in the reduction of 4-NA in aqueous solutions under visible irradiation. Additionally, F2-Au-Ag was tested as catalyst for the photoreduction of 2,6-dinitrophenol (dinitro-derivative) and picric acid (trinitro-derivative), in aqueous solutions in ambient conditions, under visible irradiation. The reusability of F2-Au-Ag catalyst was tested by using the same film in five successive cycles for 4-nitroaniline photoreduction (aqueous solution), and then the film was stored for 48 h in ambient conditions (air, ambient light and room temperature) and used again in the sixth photoreduction cycle of 4-nitroaniline. Progress of all reduction reactions was monitored by UV-vis spectrophotometry, as previously described.

Table 1 Type and quantity (wt. %) of the materials used for the synthesis of hybrid composites (each formulation contains 1 wt% Irg819). Sample

Gly-COOH

M-1

M-2

HAuCl4

AgNO3

F1 F2 F1-Ag F1-Au F1-Au-Ag F2-Au-Ag

50 40 50 50 50 40

50 40 50 50 50 40

– 20 – – – 20

0 0 0 1 1 1

0 0 1 0 1 1

analyzed using an environmental scanning electron microscope QUANTA200 coupled with an energy dispersive X-ray spectroscope (ESEM/EDX). The film cross-section was examined in low vacuum mode operating at 20 kV using an LFD detector. The X-Ray photoelectron spectroscopy (XPS) investigations were made on a Physical Electronics PHI-5000 Versa Probe XPS system with a monochromatic Al Kα radiation (hν = 1486.6 eV) source for excitation and a photoelectron take off angle of 45° from the surface. Transmission electron microscopy (TEM) analyses were performed using a HITACHI T7700 microscope operated at 120 kV in high-resolution mode. For these measurements, the photopolymerizable compositions containing the nanoparticles precursors were directly deposited onto the copper grid and were irradiated with UV light in the same conditions as the models for the UV study, further being dried at 50 °C into a vacuum oven for 24 h.

3. Results and discussion 3.1. Materials characterization The initial goal of this work was to in situ photogenerate gold, silver or gold-silver nanoparticles inside UV-curable hybrid coatings as well as to investigate the influence exerted by the organic matrix on the subsequent properties of the achieved materials. Thus, UV-curable compositions incorporating the photopolymerizable monomers Gly-COOH, M-1 and M-2, taken in different proportions (data given in Table 1), together with 1 wt% Irgacure 819 as photoinitiator and 1 wt% nanoparticle precursors (HAuCl4 and/or AgNO3) were prepared, a general reaction pathway being illustrated in Scheme 2. The photopolymerization efficiency, expressed through the extent of (meth)acrylate units photoconversion, represents a very important material feature that is directly correlated to the material characteristics and implicitly to the potential applicability. In order to monitor the photocuring process of the organic coatings, the behaviour of each mixture was examined under identical UV curing conditions and the double bond conversion degree was quantified from the FTIR spectra, by measuring the diminish in the intensity of the double bond absorption band at 1637 cm−1, as a function of the C]O peak located at 1720 cm−1. Upon irradiation with UV light (lamp intensity 15 mWcm−2) for 300 s, the area of C]C double bond absorption peak start to decrease in the FTIR spectra (Fig. 1a), and therefore, the final double bond conversion could reach high ratio within few minutes of irradiation. The conversion degree attained for the proposed formulations during 5 min of photocuring varied between 79.6 and 88.2% (Fig. 1b). Noble metal nanoparticles (silver, gold or silver/gold) have been prepared by photogeneration from the respective metal salts in tandem with the photocrosslinking process, during the UV irradiation of liquid

2.5. Photocatalytic activity measurements First, the photocatalytic activity of the polymeric films (F1-Ag, F1Au and F1-Au-Ag) incorporating Ag, Au or Au-Ag NPs was evaluated following the photodegradation of 4-nitroaniline (4-NA) chosen as model reaction, in aqueous solutions under ambient conditions and visible irradiation. 100 mL of aqueous solution of 4-NA (10−4 M) containing the hybrid films F1-Ag/F1-Au/F1-Au-Ag (1 g) was irradiated with a visible light source (Xe lamp, λ = 400–800 nm, light intensity 8.24 W/m2, Hamamatsu Lightningcure Type LC8, Model L9588) during certain time intervals (between 0 and 200 min) under constant stirring. The progress of the photodegradation reaction was monitored on a UVvis spectrophotometer (Perkin Elmer Lambda 2), at different times, 1.5 mL of the irradiated solution being collected and analyzed. To obtain information about the kinetics of 4-NA photodegradation, the correlation between ln(C0/Ct) and reaction time for all catalysts was examined. From the slope of the lines, the apparent rate constant (k) of the reaction for different catalysts was obtained. Control experiments were also performed, the 4-NA solution without polymeric films being irradiated for 200 min with visible light, or the 4-NA solution with F1Au-Ag catalyst was kept in the dark for 200 min and finally, the solutions were evaluated by UV-vis spectroscopy. Secondly, F1-Au-Ag film was tested as catalyst in reducing 4-nitroaniline to p-phenylendiamine, in aqueous solutions under ambient light illumination or under visible light irradiation. The experimental procedure was almost the same as in the case of 4-NA 3

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Scheme 2. Representation of photopolymerization process in tandem with the photogeneration of Au/Ag NPs.

formulations deposited as thin films. The formation of nanoparticles was confirmed by UV-vis spectroscopy through the appearance of specific surface plasmon resonance (SPR) bands (Fig. 2), this technique being easily available and widely used for characterization of noble metal nanoparticles (size and shape) [39]. However, for all proposed formulations, we must take into account the fact that the photogeneration of metal nanoparticles is produced in thin films, and therefore their degrees of freedom are reduced, since within a few seconds from the irradiation start, the double bonds conversion degree is around 50%, fact that limits the possibilities of migration and recombination of metal nanoparticles inside the organic network. This phenomenon is more evident in the organic matrices containing units capable to stabilize through electrostatic, hydrogen or covalent bonding

the new arising nanoparticles [40]. In the present study, the UV spectrum of the synthesized Ag NPs showed a single SPR absorption band located at around 419 nm, while that registered for the Au-containing material display an absorption band with λmax at around 566 nm. The absorbance spectrum of F1-AuAg composite contain two peaks ascribed to the SPR of Au and Ag NPs, namely the peak occurring at λmax of about 437 nm is due to Ag NPs, while the other occurring at λmax ~ 525 nm is given by the Au NPs. The formation of two absorption peaks in the case of F1-Au-Ag composite suggested the presence of a physical mixture of the nanoparticles [41,42], although their small bathochromic and respectively hypsochromic shift comparative to the individual nanoparticles may be an indicator of some electronic interaction between the formed

Fig. 1. Diminish of the double bond absorption band at 1637 cm−1 for F1 formulation after 300 s of UV irradiation (a) and the conversion degree measured for the proposed experimental formulations (b). 4

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were investigated by transmission electron microscopy (TEM), as illustrated in Fig. 4. The silver nanoparticles developed in F1 matrix (Fig. 4a) displays mostly a spherical shape with sizes between 5 and 10 nm, denoting thus a narrow dimensional distribution that sustains the sharp absorption maximum achieved in the UV absorption spectrum. In comparison, the gold nanoparticles generated in F1 formulation (Fig. 4b) are larger, with dimensions between 10 and 20 nm, and moreover, besides spherical shapes, some hexagonal nanoparticles can be distinguished. TEM investigation of the simultaneously photogenerated gold and silver nanoparticles in F1 matrix (Fig. 4c) illustrates the formation of metal nanoparticles of various shapes (spherical, triangular, rhomboidal) and sizes (6–25 nm). The visual findings corroborated with the previously discussed UV data confirm the formation of a mixture of monometallic nanoparticles, although some core-shell structures can be observed (magnified in Fig. 4c). This outcome is also supported by the UV spectra, since core-shell nanoparticles would likewise exhibit two absorption peaks [45]. The modification of the organic matrix by introducing M-2 monomer into the F2 formulation influenced to a large extent the formation of silver/gold nanoparticles, as can be observed in Fig. 4d. The achieved nanoparticles are mainly spherical, have a large dimensional variation and are much crowded than those generated in F1 formulation, suggesting that the presence of phenylboronic acid moieties significantly transformed the nucleation pattern. As in the case of F1-AuAg composite, a clear delimitation of the 2 types of metal is hard to be done, however o better interconnectivity between the two metals is more probable.

Fig. 2. UV-vis spectra of Au, Ag or Au-Ag NPs embedded in photopolymerized matrices.

nanoparticles. As for the case of F2-Au-Ag formulation, a broad absorption band with a single maximum at about λmax = 528 nm can be observed, indicating the formation of Au-Ag nanostructures in alloy form. However, the broadened absorption band may arise as a result of the formation of nanoparticles with larger sizes and high polydispersity, having in mind that the localized surface plasmon resonance of metallic NPs is strongly influenced by their shape, size and composition [43]. The EDX spectrum of F2-Au-Ag hybrid composite in fracture is illustrated in Fig. 3a, confirming the elemental composition of the photocrosslinked samples where Au and Ag elements give the peaks at about 2.2, and 3.0 keV respectively, beside the peaks given by the elements composing the organic matrix (C, O, B and N). Likewise, the XPS spectrum of F2-Au-Ag sample reflecting the chemical composition near the surface (Fig. 3b), revealed the presence of peaks corresponding to Au 4f (87.6 eV), B 1s (190 eV), C 1s (284.6), Ag 3d (367.5 and 373.5 eV), N 1s (399 eV) and O 1s (531 eV) elements. In the case of Ag element, the gap between the 3d5/2 and 3d3/2 peaks of Ag (D = 6.0 eV) have exactly the same value as for the zero-valent Ag, demonstrating thus that Ag atoms are in zero valence state on the surface of the photocrosslinked F2-Au-Ag film [44]. Supplementary, the size distribution and the shape of noble metal nanoparticles photogenerated inside the photopolymerized templates

3.2. Photocatalytic activity Catalytic materials represents a continuously extending field, the major target being the development of catalysts that will favour chemical transformations carried out in mild conditions, at room temperature, using nontoxic chemicals and “green” solvents, without toxic waste production, in the end leading to synthesis or degradation methods that are economically feasible and with a minimal environmental impact. The above synthesized materials have a great potential to work as photocatalysts for some chemical reactions performed in visible light at ambient temperature due to the surface plasmon resonance effect [10,46] of the noble metal nanoparticles entrapped in the polymeric films, in the literature are presented some reports on Ag nanoparticles and Au nanoparticles stabilized in various polymeric systems used for catalytic applications [47,48]. In order to evaluate the

Fig. 3. EDX pattern (a) and XPS spectrum (b) for F2-Au-Ag nanocomposite sample. 5

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Fig. 4. TEM images of Ag (a) and Au nanoparticles (b) dispersed in F1 matrix. Au-Ag nanoparticles photogenerated in F1 (c) and F2 (d) photocrosslinked networks.

photocatalytic activity of our hybrid polymeric films (F1-Ag, F1-Au, F1Au-Ag, F2-Au-Ag), the behaviour of 4-nitroaniline (4-NA) aqueous solution (10−4 M) under visible irradiation in their presence was followed by UV-vis spectroscopy, this technique is suitable to monitor reaction kinetics in the field of nanocatalysis [39]. Particularly, the catalytic reduction of 4-nitroaniline to p-phenylenediamine is envisaged due to the importance of the resulting reagent in different industries, mainly in the manufacture of textile dyes and pigments, and also in the fabrication of colorants for hair dye products. Additionally, taking into consideration that 4-NA is highly toxic and can often be found in waste waters, it is also essential to find a convenient way to eliminate this compound from the environment.

evolution of 4-nitroaniline concentration as a function of the irradiation time, in the presence of F1 catalysts (Fig. 5b). The photodegradation kinetics of 4-NA were studied by correlating the ln(C0/Ct) and the reaction time for the prepared materials (F1-Ag, F1-Au and F1-Au-Ag). The experimental data of the photoreactions exhibited pseudo firstorder kinetics along the investigated time intervals for all catalysts. It can be easily noticed that in the presence of F1-Ag catalyst, the 4-NA degradation efficiency (95%) and degradation rate (k = 17.2 × 10−3 min−1, R2 = 0.914) are better than in presence of F1-Au (76%, k = 7.7 × 10−3 min−1, R2 = 0.971) after 200 min of visible irradiation. This finding can be mainly attributed to the smaller size of Ag NPs generated in the polymer matrix compared to Au NPs, since it is well-known that the catalytic performance depends on the surface area, which increases with the decrease in the NPs sizes [49,50]. From Fig. 5b it is obvious that the best results for 4-NA photodecomposition were obtained when F1-Au-Ag was used as catalyst, a degradation degree of 95% with k = 24.2 × 10−3 min−1, R2 = 0.992 after only 120 min of irradiation being achieved. The enhanced catalytic activity for F1-Au-Ag hybrid material, incorporating a mixture of monometallic nanoparticles and some core-shell structures (Au and Ag NPs), as compared to those with one single type of nanoparticles (F1-Au and F1-Ag) can be ascribed to the synergic effect between Ag and Au nanoparticles [51] located in the vicinity or even in contact with each other, as observed in the TEM images. This effect could induce electronic interactions between Ag and Au and increase the electron density

3.2.1. Use of hybrid materials as catalysts for 4-nitroaniline photodegradation In the time dependent UV-vis spectra for the catalytic transformation of 4-NA in the presence of hybrid catalysts based on the F1 formulation (F1-Ag, F1-Au and F1-Au-Ag), a gradual diminution of the absorption bands characteristic to nitroderivative (382 nm and 230 nm) with increasing the irradiation time was observed, indicating the progressive photodegradation of 4-NA, as exemplified in Fig. 5a for F1-AuAg. Since the absorbance of 4-NA is proportional to its concentration in the reaction medium, the ratio At/A0 = Ct/C0 (At - absorbance at any time “t”, A0 - absorbance at t = 0, Ct – 4-NA concentration at any time “t”, C0 is the 4-NA initial concentration) was employed to follow the 6

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Fig. 5. UV-vis absorption spectra of 4-NA aqueous solution during visible light irradiation in the presence of F1-Au-Ag film (a); temporal evolution of photodegradation efficiencies (Ct/C0) of 4-NA in different conditions: under visible irradiation without any hybrid film, in the presence of F1-Ag, F1-Au, F1-Au-Ag films, and also in the dark in the presence of F1-Au-Ag (b).

on the active surface, improving thus the catalytic activity [52]. For comparison, two control experiments were done: the first one was conducted in the dark in presence of F1-Au-Ag catalyst, the other experimental conditions being kept identical, and second, the 4-NA solution was irradiated in the absence of the polymer composite. In both cases, the 4-nitroaniline solutions are very slightly degraded (Fig. 5b), confirming thus the cumulative effect of all components on the catalytic performance. Therefore, the proposed hybrid materials, namely F1-Ag, F1-Au and F1-Au-Ag can be used as catalysts in 4-NA mineralization reaction for water purification.

3.2.2. 4-nitroaniline photoreduction in presence of polymeric films In order to prepare materials that can be used as catalysts in organic synthesis, the composite with best results in 4-NA photodegradation, namely F1-Au-Ag, was selected and tested as catalyst for the obtaining of p-phenylenediamine by the reduction of 4-nitroaniline. Initially, the reaction was realized by the conventional procedure with sodium borohydride as reducing agent and F1-Au-Ag as catalyst. The reaction mixture was irradiated with visible light and the disappearance of the typical absorption peak of 4-NA at 380 nm in the UV-vis spectra in only 15 min, together with an the increase of the absorption band intensity at 240 nm and the appearance of a new band at ~300 nm (Fig. 6) indicated the formation of p-phenylenediamine [34,53]. Moreover, the formation of isosbestic points at approximately 226 nm and 263 nm is a proof that the photocatalytic reduction of 4-nitroaniline lead only to the p-phenylenediamine derivative. The p-phenylenediamine yield in the presence of F1-AuAg + NaBH4 system under visible illumination was 94.7% and the apparent rate constant for the reduction of 4-NA was k = 38.9 × 10−4 s−1 (R2 = 0.957), the value having the same order of magnitude as other polymer-catalyst systems reported in literature [26,28]. Upon the addition of sodium borohydride in 4-nitroaniline solution, the hydride ions in NaBH4 are adsorbed onto the F1-Au-Ag catalyst by the noble nanoparticles and formed the H-[F1-Au-Ag]-H species [34,54], which further induced the reduction of 4-NA to p-phenylenediamine. In the absence of the reducing agent, the only phenomenon observed is the photodegradation of 4-NA, as can be noticed by comparing Fig. 5a with Fig. 6. The same reaction was conducted in ambient light without irradiating the solution with visible light. In these conditions the reaction required a longer time, and after 25 min, the p-phenylenediamine yield was 93.3% with an apparent rate constant k = 17.7 × 10−4 s−1 (R2 = 0.989). Thus, it seems that the visible irradiation enhances the catalytic activity of F1-Au-Ag due to the fact that Au and Ag NPs

Fig. 6. UV-vis absorption spectra of 4-nitroaniline aqueous solution during visible light irradiation in the presence of F1-Au-Ag film and NaBH4 reducing agent.

strongly absorb visible light through the surface plasmon resonance effect [10,46]. The reaction medium in the abovementioned reduction processes has a basic nature since, in aqueous solution, sodium borohydride splits into borohydride anion and sodium cation. However, the literature data also reported some cases when the reduction reaction of 4-NA is conducted in acidic medium [55,56], reason for that we have also tried to carried out the reduction of 4-NA to p-phenylendiamine in acid conditions. More than that, in the composition of the catalyst a H donor compound was incorporated, namely boronic acid derivative M-2 (20 wt%), finally resulting the F2-Au-Ag catalyst, where the boronic acid derivative acts as reducing agent in the photocatalytic reduction. Only in water this behaves as a mild Lewis acid, the reducing activity being mostly correlated to formation of hydroxyboronate anion. In a suitable environment and appropriate surrounding chemical groups, its Lewis acid character may be strengthened so that it works as a strong hydrogen donor [57,58]. A great advantage given by the incorporation of the reducing agent into the polymer film consist in the elimination of his adding in the reaction mixture, simplifying thus the purification process of the resulting amine. The behaviour of 4-nitroaniline aqueous solution in the presence of 7

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Fig. 7. UV-vis absorption spectra of 4-nitroaniline aqueous solution during visible light irradiation in the presence of F2-Au-Ag film (a); temporal evolution of 4-NA reduction efficiencies (Ct/C0) in different conditions: in presence of F1-Au-Ag (film) + NaBH4 (reducing agent), and in presence of F2-Au-Ag film, under visible irradiation or in normal ambient (b).

in high energy electrons at the NP surface [7]. Furthermore, if they are distributed in the neighbourhood of boronic acid and positively charged amine units, the energetic electrons can interact with the amine protons, which represent the hydrogen source for the formation of H-[AuAg]-H species, required in the catalytic cycle. The 4-nitroaniline from aqueous solution can be adsorbed on the boronate surface from the polymer matrix by intermolecular interaction [53], and the formed H[Au-Ag]-H species leads to the reduction of the nitro group, finally resulting p-phenylenediamine. The experimental data of the photoreaction exhibit a pseudo firstorder kinetics along the interval from 0 to 120 min for F2-Au-Ag catalyst, the value of the apparent rate constant is k = 34.35 × 10−3 min−1 (R2 = 0.994) (or k = 5.73 × 10−4 s−1). This reaction is slower than the reaction in which sodium borohydride was used as reducing agent (94.7% after 15 min, k = 38.9 × 10−4 s−1), as can be clearly noticed from Fig. 7b, but the inconvenience of adding the reducing agent to the reaction mixture was eliminated.

F2-Au-Ag catalyst under visible light irradiation was monitored. As can be observed from Fig. 7a, the absorption band from 380 nm is reduced with the increase of the irradiation time in tandem with the enhance in the intensity of the absorption band at 240 nm and the appearance of a new band at ~300 nm, demonstrating that the photoreduction of 4-NA to p-phenylenediamine takes place under these conditions [34,53], without the addition of any reducing agent in the reaction mixture. After 120 min of visible irradiation in the presence of F2-Au-Ag film, the conversion degree of 4-NA to p-phenylenediamine is almost 98%. The possible mechanism for the formation of p-phenylenediamine only in the presence of F2-Au-Ag hybrid film is illustrated in Scheme 3. In water, the boronic acid sequence from our polymer matrix became a stronger Lewis acid due to its interaction with amine functions from the network (present in M1 compound), which created thus a higher positive charge in the surrounding of the boron atom [57]. By visible irradiation, the conduction electrons of the Au-Ag NPs alloy embedded in the polymer network gain the irradiation energy, resulting

Scheme 3. Possible mechanism for the reduction of 4-NA in the presence of F2-Au-Ag film under visible irradiation. 8

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In another experiment, the comportment of 4-nitroaniline aqueous solution only in the presence of F2-Au-Ag and ambient light was also investigated, without using a supplementary source of visible radiation. Under these conditions, after the same time (120 min), the conversion degree of 4-NA to p-phenylenediamine is only 48% (Fig. 7b) and apparent rate constant is k = 6.42 × 10−3 min−1 (R2 = 0.9357) (or k = 1.07 × 10−4 s−1). The reaction rate is about five times lower than the rate of the reaction conducted under irradiation with visible light (k = 34.35 × 10−3 min−1). These results support the proposed mechanism for 4-NA reduction, clearly indicating that the SPR effect of AuAg NPs alloy included in the polymer matrix play a major role in the reduction mechanism. Under visible irradiation, Au-Ag alloy nanostructures can couple the light flux to the conduction electrons of noble metal NPs, and the excited electrons at the NP surface can migrate to the protonated amine in close proximity of the Au-Ag alloy, diminishing its interaction with boron atom and becoming a hydrogen donor. Also, the excited electrons and enhanced electric fields of the Au-Ag alloy can convert the light energy to chemical energy [7], thus the reaction rate is higher when we used an irradiation source.

Table 2 Values of apparent rate constant (k) for reduction of 4-nitroaniline under visible irradiation in presence of F2-Au-Ag film containing various quantity of noble metal NPs (catalyst) or boronic acid derivative (M-2, reducing agent). HAuCl4 (wt. %)

AgNO3 (wt. %)

M-2 (wt. %)

k (s−1)

Regression coefficient (R2)

1 2 3 4 1 1

1 2 3 4 1 1

20 20 20 20 30 40

5.73 × 10−4 10.3 × 10−4 12.0 × 10−4 12.6 × 10−4 8.73 × 10−4 11.1 × 10−4

0.994 0.9984 0.9995 0.998 0.9909 0.9898

Similarly, the 4-NA photoreduction was realized under visible irradiation and in presence of F2-Au-Ag film containing different concentrations of boronic acid derivative (M-2), the reducing agent component from our polymeric film, the content of Au-Ag NPs being kept constant (1 wt%). We increase the M-2 concentration from 20 wt% to 30 wt% and 40 wt%, but at this value the polymeric films become fragile and our catalyst can no longer be reused, as was our purpose when design this material. The catalytic efficiency is improved by the increase boronic acid content in the polymeric film (Fig. 8b) and the values of apparent rate constants are k = 8.73 × 10−4 s−1 for 30 wt% M-2 and k = 11.1 × 10−4 s−1 for 40 wt% M-2. This finding also indicates that we can optimize our catalyst material by introducing a higher content of boronic acid derivative in the polymeric film, but also we have to keep the flexibility of the material in order to be use multiple times.

3.2.3. Effect of variation of noble metal nanoparticles or boronic acid content in polymeric films on the catalytic activity Also, it was studied the effect of the noble metal nanoparticles content in polymeric films on the 4-NA photoreduction efficiency, by increasing the feeding ratio of noble metal precursors (HAuCl4, AgNO3) in the hybrid material from 1 wt% to 4 wt% and keeping the rest of the polymer matrix the same. From Fig. 8a can be observed that the reduction efficiency of 4-NA under visible irradiation in presence of F2Au-Ag film containing different concentrations of noble metal nanoparticles as catalyst (1 wt% Au, 1 wt% Ag; 2 wt% Au, 2 wt% Ag; 3 wt% Au, 3 wt% Ag; 4 wt% Au, 4 wt% Ag) is improved by higher amounts of Au-Ag NPs introduced in the polymeric film. The apparent rate constants (k) were calculated and given in Table 2, and their values increase by rising Au-Ag NPs content in the polymeric film, due to fact that more Au-Ag NPs in the system provides a larger surface area for adsorption of reactant [19,59]. The value of apparent rate constant is almost doubled when we increase the amount of noble metal nanoparticles in the polymeric film to 2 wt% Au and 2 wt% Ag (k = 10.3 × 10−4 s−1). After this concentration the increasing of k value isn’t so remarkable, the value obtained for 4 wt% Au-Ag (k = 12.6 × 10−4 s−1) being almost the same as that for 3 wt% Au-Ag (k = 12.0 × 10−4 s−1). So, the 2 wt% Au − 2 wt% Ag content in the polymeric film may be the optimum concentration in order to achieve hybrid polymeric catalyst with high catalytic efficiency from economic point of view.

3.2.4. Photoreduction of dinitro- and trinitro-derivatives in presence of polymeric catalyst Furthermore, the catalytic efficiency of F2-Au-Ag film was checked in the reduction reactions of another two nitro-derivatives, 2,6-dinitrophenol and picric acid, with two and three nitro units, respectively, in the structure. The reactions were performed in the same conditions as in the case of 4-NA, namely 10−4 M aqueous solution of nitro-derivative in the presence of F2-Au-Ag polymeric film and under visible irradiation. Following the UV-vis spectra of 2,6-dinitrophenol as a function of irradiation time (Fig. 9a) it can be noticed that the absorption band at 430 nm characteristic to nitro units decreased as the reaction progressed, together with the formation of a new band at 295 nm, indicating thus the conversion of 2,6-dinitrophenol to 2,6diaminophenol. The reaction is complete after 150 min of visible irradiation, the conversion degree being almost 100%. Regarding the reduction of picric acid to 2,4,6-triaminophenol, the phototransformation

Fig. 8. Temporal evolution of 4-NA reduction efficiencies (Ct/C0) under visible irradiation in presence of F2-Au-Ag film containing different concentrations of catalyst - noble metal nanoparticles (1 wt% Au, 1 wt% Ag; 2 wt% Au, 2 wt% Ag; 3 wt% Au, 3 wt% Ag; 4 wt% Au, 4 wt% Ag) (a) or different concentrations of reducing agent - boronic acid derivative (20 wt% M-2, 30 wt% M-2, 40 wt% M-2) (b). 9

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Fig. 9. UV-vis absorption spectra of 2,6-dinitrophenol aqueous solution during visible light irradiation in the presence of F2-Au-Ag film (a); temporal evolution of reduction efficiencies (Ct/C0) of different nitro-derivatives in presence of F2-Au-Ag catalyst and under visible irradiation.

the photoreduction of 4-NA. Surprisingly, the photocatalytic efficiency increased to 92.7%. It is worth noticing that the recycling process of F2Au-Ag consists only in maintaining the polymer film under ambient conditions (air, ambient light and room temperature). 4. Conclusions Photocatalysts based on Ag, Au or Au-Ag nanoparticles supported on photocrosslinked organic matrix were prepared via a simple, green and efficient method, avoiding the use of organic solvents and the formation of harmful volatile organic compounds. Also, the photogeneration of the noble metal nanoparticles in the organic environment prevent their release during the testing experiments and additionally allow an easy recovery of the catalytic material. The photocatalytic degradation of 4-nitroaniline under visible irradiation in aqueous solution displayed an improved catalytic activity for F1-Au-Ag hybrid material comparatively with F1-Au and F1-Ag catalysts, behaviour that can be related to the synergic effect between Ag and Au nanoparticles located in the vicinity or even in contact with each other. The catalytic reduction of 4-nitroaniline to p-phenylenediamine in the presence of F1-Au-Ag + NaBH4 system had an apparent rate constant of k = 38.9 × 10−4 s−1 (R2 = 0.957), while for F2-Au-Ag photocatalyst, the value of the apparent rate constant is k = 5.73 × 10−4 s−1 (R2 = 0.994). The second process is slower than that catalyzed by NaBH4, but in this case, the adding of a reducing agent to the reaction mixture was eliminated. Moreover, the catalyst can be reused repeatedly up to 6 cycles with no major decrease of the catalytic efficiency. The proposed catalytic system needs to be improved, but however it is a promising material having many economic advantages (stable, easily available, cost effective and reusable).

Fig. 10. Photoreduction efficiency of the F2-Au-Ag film on the 4-nitroaniline aqueous solution after 6 cycles under visible light irradiation (120 min). Cycle 6: after five successive cycles, F2-Au-Ag film is left 48 h under ambient conditions and a new cycle proceeds.

required longer reaction time, so after 200 min of visible irradiation the conversion degree is 92.1%. The corresponding ln(C0/Ct) vs. time photoreduction curves for 2,6dinitrophenol and picric acid in the presence of F2-Au-Ag polymeric film (unshown here) demonstrated that photoreactions exhibited pseudo first-order kinetics along the investigated intervals, while the apparent rate constant were k = 16.52 × 10−3 min−1 (R2 = 0.901) for 2,6-dinitrophenol and k = 12.31 × 10−3 min−1 (R2 = 0.993) for picric acid. It can be concluded that F2-Au-Ag can be used as catalyst/reducing agent in the photoreduction reactions of nitro group to amine units for various compounds, but the reaction time growths with the increase in the number of the nitro groups of the employed nitroderivative, as can be observed from Fig. 9b. The rate constants decreased in the order 4-nitroaniline (k = 34.35 × 10−3 min−1), 2,6-dinitrophenol −3 (k = 16.52 × 10 min−1) and picric acid (k = 12.31 × 10−3 min−1), this parameter depending also on the number of nitro units of the compound used in the reduction reaction.

Acknowledgments This work was supported by a grant of Ministry of Research and Innovation, CNCSIS-UEFISCDI, project number PN-III-P1-1.1-TE-20161390 (34/02.05.2018), within PNCDI III, Romania.

3.2.5. Reuse of the photocatalyst in the photoreduction process The great advantage of the heterogeneous photocatalysis consists in the reusability of the catalyst. So, in the next experiments, the F2-Au-Ag reusability was studied in the photoreduction of 4-nitroaniline aqueous solution (10−4 M) and the obtained results are given in Fig. 10. It can be easily observed that the F2-Au-Ag photocatalyst can be reused for several times, in a set of six cycles, but a slight loss of the photocatalytic efficiency over the five successive cycles can be evidenced, decreasing from 97.9% (cycle 1) to 83.6% (cycle 5). Subsequently, the used F2-Au-Ag film was stored for 48 h under ambient conditions, and after this time the photocatalyst was reused for

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