Full Length ArticleFabrication of Ag nanoparticles supported on amino-functionalized peeled-watermelon structured silica-coated nano-Fe3O4 with enhanced catalytic activity for reduction of 4-nitrophenol
Graphical abstract
A novel peeled-watermelon-like magnetic catalyst (Fe3O4@SiO2-NH2-Ag) with excellent catalytic performance and magnetic recovery capacity was successfully synthesized for reduction of 4-nitrophenol. The peeled-watermelon-like structure of Fe3O4@SiO2-NH2-Ag can effectively inhibit the aggregation of Fe3O4 NPs and improve its stability.
Introduction
The environmental impact of contaminants discharged has been increasing, and p-nitrophenol (4-NP) is used to manufacture dyes, insecticides, and other organic chemicals, which is the widest contaminant in industrial wastewater [[1], [2], [3]]. This Aromatic nitro compounds may cause damage to the blood cells, central nervous system, kidneys, and liver, which will lead to a health hazard for humans and other living creatures [4,5]. Thus, it is eager to find an effective strategy for quickly and efficiently removing nitrophenols from industrial wastewater to solve environmental pollution. During the past few decades, a variety of approaches such as chemical methods, physical, biological processes, and electrochemical reactions have been developed to eliminate these pollutants [[6], [7], [8]]. In particular, using precious metal catalysts and NaBH4 reducing agents, 4-NP can be converted to 4-aminophenol (4-AP) under mild conditions without by-products, which can be used to produce corrosion inhibitors, antipyretics, and analgesic drugs [9]. In addition, the conversion of 4-NP to 4-AP can be conducted as a probe reaction to evaluate catalysts activity due to energy-saving, facile, simple operation.
In recent years, Nanoparticles (NPs) of precious metals (such as Au, Pt, Ag, or their alloys) has attracted extensive research interests in the removal of organic dyes in wastewater due to their excellent physicochemical properties [[10], [11], [12]]. Among these precious metal nanoparticles, silver(Ag) NPs have been extensively researched on removing organic dyes because of their unique physicochemical properties [13]. Which mainly include: (1) the catalytic competence can be easily upgraded by adjusting the particle size and shape of Ag NPs [14], (2) Ag NPs have catalytic activity under mild conditions or room temperature, (3) the cost of Ag NPs catalysts is lower; usually only 1/50 of that of Au or Pt [15], (4) Ag NPs catalysts are high application prospect due to their low toxicity [16]. Unfortunately, due to the high surface energy and Van der Waals force, silver NPs tend to agglomerate, change shape, and disrupt the surface state during preparation and catalysis, thereby unduly losing their intrinsic activity and selectivity [17]. To overcome the above problems, Ag NPs have been put onto various supports such as polymer nanospheres [18], magnetic materials [19], carbon materials [20], and SBA-15 [21]. Recently, magnetic nanoparticles Fe3O4 have attracted increasing attention as ideal catalyst supports because they can immobilize metal catalysts and be easily recycled using the magnet. The Fe3O4-Ag catalysts synthesized by Zhan et al. showed excellent catalytic activity in reducing 4-NP and 2-NA in the existence of NaBH4. Interestingly, the as-synthesized Fe3O4-Ag catalysts exhibited outstanding reusability even after eight cycles at room temperature [22]. However, Fe3O4 nanoparticles are easy to aggregate, oxidize, and corrode by acid, and their wide applications are limited due to the intense dipole-dipole interactions among particles [23]. A protective layer is needed to enhance its chemical stability, dispersibility, oxidation resistance, and functionalization. The surface modification of magnetic materials has been expanded, such as polymers [24], silica [25], carbon [26], and metals [27]. According to reports, using silicon dioxide as a protective layer to form a core-shell structure of Fe3O4@SiO2 is a good choice. The silica coating is a protective, non-toxic, and biocompatible outer silica layer, which can effectively inhibit the aggregation of Fe3O4 NPs groups in the liquid and enhance the stability [28,29]. Furthermore, abundant hydrophilic surfaces and silanol (Si/OH) groups with rich anchoring points can provide various surface modifications [30]. Zhang et al. succeeded in synthesizing highly branched Fe3O4@SiO2@Ag micro flowers through improved seed growth methods, which served as a highly efficient and recyclable catalyst for the catalytic reaction. The catalysts that demonstrate magnetic properties will be recycled and maintain a 93% conversion rate to reduce 4-NP and methylene blue (MB) for at least six cycles [31]. Sheibani et al. prepared Fe3O4@SiO2-Ag catalysts of safflower extract by the environmental method. The catalytic performance of catalysts was examined in mild medium. The apparent rate constants for the reaction of 4-NP, MO, and MB were calculated, being 0.756 min−1, 0.064 s−1, and 0.09 s−1, respectively [32]. Nevertheless, the silica layer does not have enough functional groups for stabilizing metal nanoparticles on the surface, and it must be modified before being used as support [33]. An important attractive feature is focused on amino functionalization because the amino group stabilizes the metal nanoparticles during the catalytic reaction to prevent aggregation [34]. 3-Aminopropyltriethoxysilane (APTES) can be grafted on the silica layer, and the amino group of APTES can be used as a stabilizer for supporting metal nanoparticles, thereby improving reusability [35]. Liu et al. prepared an amino-functionalized core-shell structured nanocatalysts Fe3O4@SiO2/APTES/Ru. Amino groups effectively immobilize Ru nanoparticles, promote their dispersion, and prevent them from agglomerating, indicating that Fe3O4@SiO2/APTES/Ru exhibits good reusability [36]. As far as we know, the particle size of Fe3O4 prepared at present is enormous, even exceeding 300 nm, and there are few reports of nano-sized Fe3O4. In addition, the prepared catalyst is easy to agglomerate, resulting in performance degradation after several cycles.
Inspired by the above perspective, herein is a simple, highly effective, and reproducible approach to prepare Ag nanoparticles supported on amino-functionalized silica-coated magnetic peeled-watermelon-like catalysts (Fe3O4@SiO2-NH2-Ag) as highly catalytic active and easily recoverable catalysts. Firstly, spherical Fe3O4 magnetite particles with an average diameter of 20 nm are prepared by a simple microemulsion co-precipitation method. Secondly, the as-synthesized Fe3O4 particles are coated with silica forming a structure similar to peeled watermelon. Lastly, amino-functionalized Fe3O4@SiO2 uses APTES as an alteration reagent to stabilize the Ag NPs very small, almost monodisperse, and with a strong affinity to the surface. Amino-functionalized Fe3O4@SiO2-NH2-Ag shows excellent catalytic performance for the reaction of 4-NP. After the reaction is completed, the catalysts can be separated from the reaction medium by a magnet and can be reused at least 25 times without distinct degradation in the catalytic performance.
Section snippets
Materials and reagents
Cetyltrimethylammonium bromide (CTAB), n-octane, ethanol, p-nitrophenol (4-NP), and tetraethyl orthosilicate (TEOS) were obtained from Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin, China). Ferrous sulfate heptahydrate (FeSO4·7H2O), Ferric chloride hexahydrate (FeCl3·6H2O), and oleic acid were provided by Chengdu Jinshan Chemical Reagent Co., Ltd. (Sichuan province, China). Silver nitrate (AgNO3) and Sodium borohydride (NaBH4) were provided by Sinopharm Chemical Reagent Co., Ltd.
Preparation and characterization of catalyst
As illustrated in Fig. 1, the preparation of Fe3O4@SiO2-NH2-Ag requires four steps. Firstly, magnetic Fe3O4 nanoparticles were prepared by the microemulsion co-precipitation method with an average size of about 20 nm. CTAB was mechanically dispersed and stirred in deionized water in this step, and then n-octane was added. After vigorous mechanical stirring for 30 min, n-octane molecules entered the CTAB micelles, and the carbon chains of n-octane matched those of CTAB molecules to form larger
Conclusions
In summary, the novel silver nanoparticles supported on amino-modified magnetic peeled watermelon-like catalysts (Fe3O4@SiO2-NH2-Ag) with excellent catalytic performance and magnetic recovery capacity were prepared. The as-synthesized Fe3O4 nanoparticles were coated with silica to form the peeled-watermelon-like structure, which can effectively inhibit the aggregation of Fe3O4 NPs and improve the stability of catalysts. After being functionalized with amino groups, silver nanoparticles were
Author statement
This original research work is completed by all the authors together. Chun Wang: Investigation, Methodology, Validation and Writing - original draft; Lei Yang and Xiaofang Yuan: Formal analysis and Conceptualization; Wending Zhou: Project administration, Supervision and Writing - review & editing; Meisong Xu: Formal analysis and Writing - review & editing; Wanliang Yang: Formal analysis and Conceptualization, Methodology, Supervision and Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was financially supported by the Basic Research Program of Science & Technology Department of Guizhou Province [2020] 1Y055, the Opening Foundation of State Key Laboratory of Chemical Resource Engineering (CRE-2017-C-101), and the Science and Technology Project of Guizhou Province [2017] 5788-56.
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