Ag–SiO2 nanocomposites with plum-pudding structure as catalyst for hydrogenation of 4-nitrophenol
Introduction
As efficient and low cost effective noble metal nanoparticles (NPs), Ag NPs are considered as one of the most promising functional materials in electronic, chemical, bio-logical and catalytic field [1], [2], [3], [4], [5]. However, the practical use of Ag NPs is extremely hindered by their severe aggregation during catalytic process, which unavoidably gives rise to the decrease in active surface area and further degradation of performance under long term operation [6], [7], [8]. One way to improve the stability of the metal NPs is to stabilize them by the protection of SiO2 during the catalytic process, which is very resistant to coagulation, even at high-volume fractions [9], [10]. Recently, Mulvaney et al. [11], [12] and Matijević et al. [13] reported that noble metal particles such as Ag and Au could be significantly protected by coating with silica shells. However, some disadvantages of this method is that these silica-coating procedures usually involved a multistep process, and pre-modification of the NPs surface with silanecoupling agent is needed, which is time consuming and expensive [11], [12], [13]. In order to overcome these disadvantages, Yang et al. developed a reverse micelle system using water-in-oil (W/O) microemulsion as templates to synthesize hybrid NPs [14]. The microemulsion method has many advantages, such as simplicity of operation, facile controlling of the properties of the metal NPs by experimental conditions [15], [16], [17], [18], [19], [20]. The mono-core/shell structure Ag/SiO2 with an Ag core size of 5–8 nm was synthesized within reverse micelles by various groups [21], [22]. Asher et al. fabricated monodisperse SiO2 spheres containing several dispersed Ag NPs, but the particle size of SiO2 was as big as 100 nm [23].
4-nitrophenol (4-NP) is one of the most refractory substances present in industrial wastewaters, which have carcinogenic character and high toxicity [24], [25]. A lot of methods have been developed for its removal, including adsorption, microbial degradation, photocatalytic degradation electrochemical treatment, catalytic reduction and so on [26], [27]. Among these techniques, the reduction of 4-NP to 4-aminophenol (4-AP) using catalyst is considered to be the most efficient, green, and economical approach to dispose 4-NP. Moreover, 4-AP is an important intermediate for the manufacture of analgesic and antipyretic drugs [28], [29], [30], [31]. Thus, the catalysts are the predominant factor for the hydrogenation of 4-NP [32], [33], [34], [35], [36], [37], [38], [39]. Up to now, lots of catalysts have been tested for 4-NP reduction, including noble metals Ag [3], [4], [5], [6], [7], [8], Au [26], Pt [32], AuPt [33], [34], Pd [35] and non-noble metals Co [27], Cu [28], [29], Fe–Ni [36], Ni [37], CuNi [39], among which most of the catalysts with high performance is still remained a considerable challenge. Therefore, development of highly effective and stable catalyst for the reduction of 4-NP in aqueous solutions under mild condition is highly desirable.
Herein, using the modified reversed micelle method, we reported a simple one-pot synthesis of the plum-pudding-like Ag–SiO2 nanocomposites (NCs). Several ultrafine Ag NPs with a size of ~2 nm were well dispersed in each SiO2 nanosphere (~28 nm). The Ag–SiO2 NCs with “plum-pudding” structure exhibited highly effective catalytic property for hydrogenation of 4-NP at room temperature.
Section snippets
Chemicals
All chemicals were commercially available and used without further purification as follows: 4-Nitrophenol (4-NP, Aladdin, 99%), 3-nitrophenol (3-NP, Aladdin, 98%), 2-nitrophenol (2-NP, Aladdin, 99%), sodium borohydride (NaBH4, Sigma-Aldrich, 99%), silver nitrate (AgNO3, Shanghai Shenbo Chemical Co. Ltd, ≥99.8%), tetraethoxysilane (TEOS, Sigma-Aldrich, 98%), methanol (CH3OH, Tianjin Fuchen Chemical Reagent, 99.5%), polyethylene glycolmono-4-nonylphenyl ether n≈5 (NP-5, Tokyo Chemical Industry
Characterization of the samples
The XRD patterns of Ag–SiO2 NCs, Ag NPs supported on SiO2, free Ag NPs and SiO2 are presented in Fig. 1, respectively. The strong and broad peaks in the range of 2θ=15–30° can be assigned to amorphous SiO2. However, For Ag–SiO2 NCs, Ag NPs supported on SiO2, free Ag NPs, the diffraction peaks (2θ) at the 38.1°, 44.3°, 64.4°, 77.5° are corresponded to (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes of Ag, respectively, which can be indexed undisputedly to cubic Ag (JCPDS no. 04-0783).
The broad view
Conclusions
The plum-pudding-like Ag–SiO2 nanocomposites have been synthesized successfully via a facile reversed micelle technique. The characterized results showed that several ultrafine Ag nanoparticles (~2 nm) are well dispersed in a SiO2 nanoshperes with a diameter of 28 nm. Ag–SiO2 nanocomposites with plum-pudding-like structure exhibited a good catalytic activity and stability for hydrogenation of 4-NP. This simple synthetic method can be extended to other metallic systems in more applications.
Acknowledgment
This work was financially supported by National Natural Science Foundation of China (No. 21463012), Open Project Program of Key Laboratory of Functional Small Organic Molecule, Ministry of Education, Jiangxi Normal University (No. KLFS-KF-201427), Scientific Research Foundation of Graduate School of Jiangxi Normal University (YJS2014056), and the Sponsored Program for Cultivating Youths of Outstanding Ability in Jiangxi Normal University.
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