Spontaneous synthesis of gold nanoparticles on gum arabic-modified iron oxide nanoparticles as a magnetically recoverable nanocatalyst
© Wu and Chen; licensee Springer. 2012
Received: 13 April 2012
Accepted: 19 June 2012
Published: 19 June 2012
A novel magnetically recoverable Au nanocatalyst was fabricated by spontaneous green synthesis of Au nanoparticles on the surface of gum arabic-modified Fe3O4 nanoparticles. A layer of Au nanoparticles with thickness of about 2 nm was deposited on the surface of gum arabic-modified Fe3O4 nanoparticles, because gum arabic acted as a reducing agent and a stabilizing agent simultaneously. The resultant magnetically recoverable Au nanocatalyst exhibited good catalytic activity for the reduction of 4-nitrophenol with sodium borohydride. The rate constants evaluated in terms of pseudo-first-order kinetic model increased with increase in the amount of Au nanocatalyst or decrease in the initial concentration of 4-nitrophenol. The kinetic data suggested that this catalytic reaction was diffusion-controlled, owing to the presence of gum arabic layer. In addition, this nanocatalyst exhibited good stability. Its activity had no significant decrease after five recycles. This work is useful for the development and application of magnetically recoverable Au nanocatalyst on the basis of green chemistry principles.
KeywordsGold nanoparticles Spontaneous green synthesis Magnetically recoverable catalyst Nitrophenol Catalytic reduction
Nanoparticles have been widely investigated in the fields of chemistry, physics, electronics, biology, and medicine due to their unique physical and chemical characteristics which are different from bulk materials [1, 2]. Among these researches, magnetic and noble metal nanoparticles (particularly Fe3O4 and Au) attracted considerable attention in the past decades. Magnetic nanoparticles have been extensively utilized in the recovery of metal ions and dyes, magnetic bioseparation, targeted therapy, drug delivery, and biological detection and imaging because magnetic separation technique possesses the advantages of rapidity, high efficiency, and cost-effectiveness [3–7]. Also, they have been shown to be highly efficient as supports in heterogeneous catalytic reactions owing to their high specific surface area and magnetically recoverability . On the other hand, Au nanoparticles not only exhibit unique optical and catalytic properties but also have excellent chemical stability and biocompatibility. These characteristics lead to many potential applications such as in optics, electrochemistry, catalysis, and biochemical sensing [9–12].
The composite nanoparticles of Fe3O4 and Au combine the functions of these two components and can be applied in the waste water treatment. As has been reported, some noble metal nanoparticles stabilized with surfactant or dendrimer are capable for catalytic reduction of aromatic nitro compounds or dyes [13–15] because they have higher Fermi potential and can be used as catalyst for many electron-transfer reactions [16, 17]. However, their recovery from such stabilizers-containing systems is not easy. To overcome this problem, using magnetic nanoparticles as their support is a superior strategy which makes them easy to recover. Thus, the composite nanoparticles of Fe3O4 and Au are expected to have great potential as magnetically recoverable catalyst for the treatment of waste water.
Various approaches for the synthesis of Fe3O4-Au composite nanoparticles have been reported, including the formation of Au shells on the polyelectrolyte modified Fe3O4 nanoparticles , reduction of Au ions on Fe3O4 nanoparticles with sodium citrate , electrostatic attraction of as-prepared Au nanoparticles onto Fe3O4/polypyrrole nanocomposites , thermal decomposition of Au(OOCCH3)3 on poly(vinylpyrrolidone)-modified Fe3O4 particles , and others [22, 23]. However, it is inevitable that most of these processes involved the use of surfactants and reducing agents which are usually harmful for the environment and ecosystem.
In the past decade, the green chemistry which aims to reduce or eliminate hazardous substances in the design, development, and implementation of chemical processes and products is becoming more and more important [24, 25]. So a lot of routes obeying the green chemistry principles have been developed for the synthesis of Au nanostructures by replacing toxic chemicals with environment-friendly materials [26–30]. Gum arabic (GA) is a polysaccharide which consists plenty of amino acids and hydroxyl groups on the polymer chains. It is known that these functional groups could reduce metal ions to metal nanoparticles through the oxidation mechanism [31, 32]. Recently, we synthesized Au nanoparticles successfully by using GA as both reducing and stabilizing agent in the absence of other additives . In our earlier work, we have also modified the surface of Fe3O4 nanoparticles with GA as a novel magnetic nano-adsorbent for the removal of heavy metal ions. Thus, it seems interesting and meaningful to fabricate the Au-Fe3O4 composite nanoparticles by the in situ green synthesis of Au nanoparticles on the GA-modified Fe3O4 nanoparticles. So, in this work, we proposed a new green route to fabricate Au-Fe3O4 composite nanoparticles by in situ synthesis of Au nanoparticles on GA-modified Fe3O4 nanoparticles. Furthermore, it is known that nitrophenols are the typical aromatic nitro pollutants in industrial and agricultural wastewaters. A lot of methods have been developed for their removal, including adsorption , microbial degradation , photocatalytic degradation , microwave-assisted catalytic oxidation , electro-Fenton method , electrocoagulation , and electrochemical treatment . Because Au nanoparticles can serve as the electron relay between 4-nitrophenolate ion (oxidant) and BH4− (reductant) for the catalytic reduction of 4-nitrophenol (4-NP) with sodium borohydride [41, 42], the catalytic capability and performance of the resultant Au-Fe3O4 composite nanoparticles were demonstrated by investigating the catalytic reduction of 4-NP with sodium borohydride.
Ferric chloride, 6-hydrate was purchased from J. T. Baker Chemical Company (Phillipsburg, NJ, USA). Ferrous chloride tetrahydrate, gum arabic, and sodium borohydride were obtained from Fluka (Fluka Chemical Corporation, Buchs, Switzerland). Ammonium hydroxide (29.6%) was supplied by TEDIA Company (Fairfield, OH, USA). Hydrogen tetrachloroaurate and 4-nitrophenol were purchased from Alfa Aesar (Ward Hill, MA, USA). All chemicals were of guaranteed or analytical grade reagents, commercially available, and used without further purification. The water used throughout this work was the reagent-grade water produced by Milli-Q SP ultra-pure-water purification system of Nihon Millipore Ltd., Tokyo, Japan.
GA-modified magnetic nanoparticles (GA-MNP) were prepared according to our previous work . Firstly, iron oxide magnetic nanoparticles (MNP) were prepared by coprecipitation of Fe2+ and Fe3+ ions with ammonia solution and then followed by thermal treatment. The ferric and ferrous chlorides (with molar ratio of 2:1) were dissolved in water at a concentration of 0.3 M iron ions. Chemical precipitation was achieved by adding NH4OH solution (29.6%) at 25°C under vigorous stirring. During the reaction process, the pH was maintained at about 10. The precipitates were heated at 80°C for 30 min, then washed several times with water and ethanol, and finally dried in a vacuum oven at 70°C. Secondly, for the surface modification with GA, 100 mL of GA solution (10 mg/mL) was mixed with 1.0 g of MNP in a stoppered bottle. The reaction mixture was sonicated for 20 min, then mixed on a vortex mixer for 5 min, and was sonicated again for another 10 min. The product GA-MNP were recovered magnetically from the reaction mixture by using a permanent magnet with a surface magnetization of 6,000 G, then washed three times with 100 mL of distilled water, and finally dried in an air oven at 50°C for 24 h and stored in a stoppered bottle for further use.
For in situ green synthesis of Au nanoparticles on GA-MNP, hydrogen tetrachloroaurate (0.3 mM) was dissolved in an aqueous solution of GA-MNP (1 mg/mL) at first. Then, the solution was stirred gently at 55°C for 8 h to yield Au nanoparticles. The Au nanoparticles-loaded GA-MNP (GAAu-MNP) were recovered magnetically by a permanent magnet, then washed three times with distilled water, and finally dried in a vacuum chamber at room temperature. Since it was found that all Au(III) ions were reduced completely, the loading of Au nanoparticles on GA-MNP could be calculated to be 0.059 mg/g. The ultraviolet–visible (UV–VIS) absorption spectra of the resultant colloid solutions were monitored by a JASCO model V-570 ultraviolet–visible near-infrared spectrophotometer (JASCO Inc., Easton, MD, USA). The particle size was determined by transmission electron microscopy (TEM) on a Hitachi model HF-2000 field emission transmission electron microscope with a resolution of 0.1 nm. The high-resolution TEM image was observed by a JEOL model JEM-2100 F electron microscope (JEOL Ltd., Tokyo, Japan) operated at 200 kV. The samples for TEM analysis were obtained by depositing a drop of colloid solution onto a 200-mesh Formvar-covered copper grid and evaporating in a vacuum chamber at room temperature. An X-ray diffraction (XRD) measurement was performed on a Shimadzu model RX-III X-ray diffractometer (Shimadzu Corporation, Canby, OR, USA) at 40 kV and 30 mA with CuKα radiation (λ = 0.1542 nm). The samples for XRD analysis were washed twice with water, collected by centrifugation, and dried in a vacuum chamber overnight.
For the catalytic reduction of 4-NP with NaBH4, typically, 95 mL of aqueous solution containing NaBH4 and 4-NP was prepared at first. Then, 5 mL of aqueous solution containing GAAu-MNP (5 mg of GA-MNP and 0.295 mg of Au) was added to start the reaction. The yellow color of the solution gradually vanished, indicating the reduction of 4-NP. The variation of 4-NP concentration with time was monitored spectrophotometrically at a wavelength of 400 nm. The reaction temperature was controlled at 30°C. Also, the initial concentration ratio of NaBH4 to 4-NP was fixed at 100 so that the concentration of NaBH4 could be considered as a constant during the reaction. After the reaction, GAAu-MNP were collected by using a permanent magnet, washed two times with deionized water, and then reused for recycling in the experiment to examine the reusability.
Results and discussion
Pseudo-first-order rate constants for the reduction of 4-NP with NaBH 4 in the presence of GAAu-MNP
102 × [Au]0
104 × [4-NP]
102 × k (min-1)
The rate constant was proportional to the diffusion coefficient, but the diffusion coefficient was inversely proportional to the concentration of 4-NP . So, the effect of initial 4-NP concentration could be explained by the Smoluchowski expression. Also, the NaBH4-reduction of 4-NP by the GAAu-MNP developed in this work could be recognized as diffusion-controlled. This was similar to those observed in the reduction of 4-NP by the dendrimer-encapsulated metal (Ah, Pt, and Pd) nanoparticles  and the chitosan-stabilized Au nanoparticles . As for the diffusion-controlled mechanism, it could be attributed to the GA layer on the surface of Fe3O4 nanoparticles where Au nanoparticles were in situ synthesized and stabilized.
According to the above findings, the green method for the synthesis of Au nanoparticles on the surface of magnetic nanoparticles has been developed successfully. Also, the resultant magnetically recoverable Au nanocatalyst was demonstrated to possess good catalytic property.
A novel magnetically recoverable Au nanocatalyst was fabricated by in situ formation of Au nanoparticles on the surface of GA-modified Fe3O4 nanoparticles with GA as a reducing and stabilizing agent simultaneously. The resultant Au nanoparticles formed a 2 nm-thick layer on the surface of GA-modified Fe3O4 nanoparticles. Their catalytic ability was demonstrated by the study on the reduction of 4-NP to 4-AP with NaBH4. The reduction reaction followed the pseudo-first-order kinetics. The corresponding rate constants increased with the increasing amount of Au nanocatalyst but decreased while the initial 4-NP concentration increased, suggesting that the reaction was diffusion-controlled, owing to the presence of GA layer. Furthermore, the activity of magnetically recoverable Au nanocatalyst had no significant decrease after five recycles, revealing its good stability. Such a product is expected to be useful in the waste water treatment.
D-HC (PhD) is a distinguished professor of Chemical Engineering Department at National Cheng Kung University (Taiwan). C-CW received his PhD in Chemical Engineering at National Cheng Kung University (Taiwan) in 2010 and now works as a researcher in Eternal Chemical Co., Ltd. (Taiwan).
This work was performed under the auspices of the National Science Council of the Republic of China, under contract number NSC 97-2221-E-006-119-MY3, to which the authors wish to express their thanks.
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