Sonochemical Reduction of Palladium Nanoparticles

Palladium (Pd) is well known for its catalytic functions but is also often used in materials research and the production of electronics, medicine, hydrogen purification, and other chemical applications. Via sonochemical route, mono-dispersed as well as aggregated palladium nano particles can be produced.

Ultrasonic Production of Palladium Nanoparticles

Nemamcha and Rehspringer have investigated the sonochemical production of dispersed and aggregated palladium nanoparticles. Therefore, a Pd(NO₃)₂ solution has been sonicated with the ultrasonic lab homogenizer UP100H in presence of ethylene glycol (EG) and polyvinylpyrrolidone (PVP).

Procedure

The samples were prepared as following:
For the samples, mixtures of 30mL of EG and 5·10^-6mol of PVP were preprared by magnetic stirring for 15 min. For the different samples, different amount of Pd(NO₃)₂ solution, 1.5mL and 2mL, were added. The sample mixtures were prepared with the ratio of 2·10^-3mol Pd(NO₃)₂ in sample (a) and 2.66·10^-3mol Pd(NO₃)₂ in sample (b). Both mixtures were sonicated in a 20mL vial using a probe-type ultrasonicator. Samples were taken after sonication times of 30, 60, 90, 120, 150, and 180 min.

The analysis of the experimental results shows that:

1. The sonochemical reduction of Pd(II) into Pd(0) depends on the sonication time.

2. The high PVP/Pd(II) molar ratio leads to the formation of monodispersed palladium particles having a rounded shape and a mean diameter of about 5nm.

3. However, the low PVP/Pd(II) molar ratio involves the obtaining of aggregates palladium nanoparticles with a large size distribution centered at 20nm.

The sonochemical route of reducing palladium (II) ions Pd(II) to palladium atoms Pd(0) can be assumed to be the following:

• (1) Water pyrolysis: H₂O → •OH+•H
• (2) Radical formation: RH (Reducing agent) + •OH(•H) → •R + H₂O(H₂)
• (3) Ions reduction: Pd(II) + reducing radicals (•H, •R) → Pd(0) + R•CHO + H+
• (4) Particle formation: nPd(0) → Pdn

–> Result: Depending on the PVP/Pd(II) ratio, dispersed or aggregated Pd_n were obtained.

Sonochemical reduction of Palladium: sample a (left) contains a high amount of PVP, sample b (right) a low amount of PVP. Sonication time with UP100H: 180 min. Sample a shows mono dispersed Pd nano particles, sample b aggregated Pd nano particles. [Nemamcha; Rehspringer 2008]

Analysis and Results

The UV-visible absorption analyses confirm the relation between the sonochemical reduction of palladium(II) ions to palladium(0) atoms and the retention time in the ultrasonic field. The reduction of palladium(II) ions to palladium(0) atoms progresses and can be completely achieved with increasing sonication time. The micrographs of transmission electron microscopy (TEM) show that:

1. 1. When a high amount of PVP is added, the sonochemical reduction of palladium ions leads to the formation of monodispersed palladium particles with spherical shape and a mean diameter of approx. 5nm.
2. 2. The use of a small quantity of PVP involves the obtaining of aggregates palladium nanoparticles. The dynamic light scattering (DLS) measurements reveal that the palladium nanoparticles aggregates have a large size distribution centered at 20nm.