Abstract
Colloidal ruthenium nanoparticles were prepared by chemical reduction of ruthenium trichloride (RuCl3) using sodium borohydrate (NaBH4) as reducing agent and sodium dodecyl sulfate (SDS) as a stabilizer. Size and size distribution of synthesized colloidal Ru nanoparticles were studied by varying different parameters such as molar ratio (MR) of SDS/RuCl3, NaBH4/RuCl3, effects of different stabilizers, and reducing agents. Prepared nanoparticles were characterized by transmission electron microscope (TEM) and dynamic light scattering (DLS). Stability of colloidal nanoparticles was detected by Turbiscan. Stable Ru nanoparticles were dispersed on γ-Al2O3 to prepare Ru/γ-Al2O3 catalyst. This catalyst was characterized by X-ray Diffraction (XRD) and transmission electron microscope (TEM).
1. Introduction
Metal nanoparticles have of great fundamental and practical interest due to their unique physical properties, chemical reactivity, and potential applications in electronics, catalysis, and biochemistry [1, 2]. Nanoparticles of many metals, such as gold, platinum, palladium, cobalt, silver [3], and rhodium [4], have been synthesized by different experimental techniques. However, the synthesis of ruthenium (Ru) nanoclusters is scarcely reported, despite the important technological role of ruthenium [5] as a catalyst and redox processes [6]. It also serves as electrocatalyst in the electrooxidation of methanol and CO, the core reaction that occurs in direct methanol fuel cells [7]. The catalytic activities of the Ru nanoparticles have been tested for partial oxidation of methane shows high activity and high CO selectivity [8].
Among the various techniques to obtain nanosized metal particles, the wet chemical method is probably the most popular due to its simplicity, low cost, and ability to produce large quantity. Chemical reduction of metal salts using various reducing agents in the presence of protecting agent is preferred due to the advantage of controllable size and shape of the particles [8]. Chemical reduction method have been carried out in the presence of a stabilizer such as linear polymers, ligands, surfactants, or heterogeneous supports, which prevents the nanoparticles from aggregating, allowing at the same time their isolation [9]. In order to synthesize uniformly distributed nanocatalyst, stability of the colloidal nanoparticles and the homogeneous dispersion over the support play the most important role.
In view of above literature, attempts had been made to synthesize uniformly distributed stable ruthenium by chemical reduction method using SDS as stabilizing agent. Different parameters which affect particle size and size distribution such as molar ratio (MR) of SDS/RuCl3, NaBH4/RuCl3, effects of different stabilizers, and reducing agents were studied systematically. Turbiscan had been used to monitor stabilized Ru nanoparticles and dispersions in the kinetic studies of their stability. Stable colloidal ruthenium nanoparticles were dispersed on γ-Al2O3 by mechanical stirring and characterized using XRD and TEM.
2. Experimental
2.1. Materials
Ruthenium trichloride (RuCl3·nH2O), sodium borohydrate (NaBH4, 95%), hydrazine hydrate (80%), sodium dodecyl sulphate (SDS), and cetyltrimethylammonium bromide (CTAB) were purchased from Finar Chemicals, India. Poly(N-vinyl-2-pyrrolidone) (PVP, average molecular weight 40,000) and Aerosol OT (AOT) were purchased from Heny Fine Chemicals, India. Gamma alumina powder of 98% purity was purchased from National Chemicals, India. Distilled water of pH 5.9 ± 0.2, conductivity 1.0 μS/cm (Millipore, Elix, India) was used throughout the experiments for preparing the aqueous solutions.
2.2. Preparation of Ruthenium Nanoparticles
Ruthenium nanoparticles were synthesized by the reduction of ruthenium trichloride in presence of reducing agents and stabilizing agents using water as solvent. RuCl3 solution (0.2 mM) was prepared by dissolving the known amount of RuCl3 in 50 mL distilled water under continuous stirring. Separately, known amount of SDS and NaBH4 was dissolved into 50 mL distilled water. Molar ratios of NaBH4/RuCl3 and SDS/RuCl3 were maintained at 30 and 20, respectively. Colloidal ruthenium nanoparticles were produced by gradual addition of prepared RuCl3 solution into the mixture of NaBH4 and SDS slowly under continuous stirring for 1 h [10].
2.3. Ru/γ-Al2O3 Catalyst Preparation
To prepare Ru/γ-Al2O3 catalyst, synthesized colloidal nanoparticles were collected by centrifugation and redispersed into the methanol by sonication (B. Braun Biotech International, Labsonic). 5 gm of γ-alumina was added in to the solution and the mixture was mechanically stirred at 6500 rpm using Ultraturax (IKA WERKE, GmBH & Co. KG) for 24 h at room temperature to form a homogeneous suspension. The mixture was washed with acetone and water to remove the organic material and dried at 100°C for 6 h. The Ru supported on γ-alumina was found in a powder with a dark brown color. The catalyst was calcinated at 300°C for 6 h in an oven. The catalyst was stored in moisture-free atmosphere.
2.4. Characterization
The sizes of nanoparticles were measured using DLS (Malvern Zetasizer, Nano ZS-90, UK). Morphology of the nanoparticles was observed by TEM analysis (Philips Tecnai—20, Holland) operating at 200 kV provides 0.27 nm point resolution. Nanoparticles stability was analyzed using Turbiscan Classic MA 2000 at light rays of 880 nm wavelength (Formulaction, France). Phase composition of Ru/γ-Al2O3 catalyst was observed by X-ray diffraction (Philips, X’ Pert-MPD, Holland).
3. Results and Discussion
3.1. Effect of SDS∕RuCl3 Molar Ratio
Generally, anionic surfactant, SDS, was used as stabilizer to prevent the growth and aggregation of nanoparticles. SDS/RuCl3 molar ratio was changed from 1–40 keeping RuCl3 concentration at 0.2 mM and 30 molar ratio (MR) of NaBH4/RuCl3 (Figure 1). The size of the particles decreased with increasing MR of SDS/RuCl3 up to 20. Hydrodynamic diameter of the colloidal Ru nanoparticles, which was formed due to aggregation of nanocrystals inside micelles was found to be 90 nm at MR = 1 and 20 nm at MR = 20. Above 20 MR, size of the particles increased with increasing the MR of SDS∕RuCl3. Actually larger size of the ruthenium nanoparticles was produced at lower concentration of SDS because of higher rate of agglomeration due to insufficient amount of stabilizing agent in the system [11]. The increase of SDS/RuCl3 MR, increased the concentration of SDS. High concentration of surfactant increased viscosity, the increase in viscosity led to reduce rate of surfactant migration or reduced rate of diffusion speed of micelles and decreased the electrostatic repulsion, there by promoting the particle agglomeration [12, 13].
3.2. Effect of NaBH4∕RuCl3 Molar Ratio
Sodium borohydrate was used as reducing agent for the synthesis of Ru nanoparticles. The effect of NaBH4 concentration was studied by varying MR of NaBH4/RuCl3 (10–30), keeping other parameters constant RuCl3 = 0.2 mM and 20 MR of SDS/RuCl3 (Figure 2).
At lower value of MR of NaBH4/RuCl3 (MR = 10), larger size of nanoparticles was observed by DLS due to the insufficient reduction of RuCl3. However, with increasing the MR of NaBH4/RuCl3 from 15 to 30, narrow peaks were obtained, suggesting that Ru nanoparticles were produced with smaller size. Liu et al. [14] also found that lower concentration of NaBH4 produced boron hydroxide through hydrolysis of NaBH4. This boron hydroxide was absorbed to the Ru nanoparticles, reducing the electron density of surface and causing aggregation of Ru nanoparticles which resulted larger nanoparticles size. On the other hand, higher concentration of NaBH4 increased the concentration of boron hydroxide which formed thick BH4− layer preventing the boron hydroxide from absorbing into the surface of Ru nanoparticles, resulting in well-dispersed smaller nanoparticles.
3.3. Effect of Different Types of Stabilizing Agents on Particles Size
In order to know the effect of different types of stabilizing agent like PVP, SDS, CTAB, and AOT on the size of Ru nanoparticles, at constant (RuCl3) = 0.2 mM, 20 MR of Surfactant∕RuCl3 and 30 MR of NaBH4∕RuCl3. It was observed that the smallest particle size (Figure 3) was obtained for PVP (20 nm) and SDS (25 nm) and the particle size was significantly smaller than AOT and CTAB. Actually PVP would act as stabilizing as well as a reducing agent, which resulted in the lowest particle size of ruthenium. For cationic surfactant (CTAB), Ru nanoparticles were attracted by the positive charge of the surfactant, hence agglomerated near the outside of micelle which resulted larger nanoparticles [15, 16].
TEM image and DLS histogram of SDS stabilized colloidal Ru nanoparticles were shown in Figure 4. TEM showed that average particle size was 3–25 nm, lower than that of the particle size (20–70 nm) obtained by DLS. Actually in TEM only nanoparticles without surfactant layers were visible, this resulted lower in particle size.
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3.4. Stability of Ruthenium Nanoparticles
The nanoparticles were stabilized due to attractive and repulsive electrostatic forces created by stabilizing agents present in the system. Nanoparticles stability was analyzed using transmission and back scattering (BS) profiles, scanning the colloidal sample by light rays of 880 nm wavelength using Turbiscan. It was observed that BS profiles at different times for SDS and PVP at 20 MR (Figure 5) were superimposing which indicated that the structure and average size of the Ru nanoparticles were not changing up to 24 h.
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3.5. Characterization of Ru∕γ-Al2O3 Catalyst
XRD diagram (Figure 6) showed that the diffraction peak at , which was exactly consistent with the value (2.07 Å) of ruthenium metal [16]. The size of the Ru cluster was calculated by the Debye Scherrer formula [17] which was found to be 15 nm, nearly close to the average diameter observed by TEM analysis of Ru/γ-Al2O3 catalyst as shown in Figure 7.
4. Conclusion
SDS-stabilized ruthenium nanoparticles were synthesized by proper selection of stabilizer, reducing agent and optimizing SDS/RuCl3 MR at 20, NaBH4/RuCl3 MR at 30. Stabilized ruthenium nanoparticles were dispersed on γ-alumina by mechanical stirring to obtain uniformly distributed supported catalyst. The size of the nanoparticles obtained from XRD was consistent with TEM data.