Better platinum catalysts take shape


Jun 06, 2007

A new form of platinum promises to boost the performance of catalysts such as those used to produce hydrogen for fuel cells, say researchers at Georgia Institute of Technology, Atlanta, Ga., and Xiamen University, Fujian Province, China, who collaborated in developing the material. The team for the first time has created platinum with a tetrahexahedral structure. Because of their 24 facets, the new nanocrystals reportedly can provide as much as four times more catalytic activity per unit area than existing commercial catalysts, which come in cubic, tetrahedral and octahedral forms.

This new shape for platinum catalyst nanoparticles greatly improves their activity. This work also demonstrates a new method for producing metallic nanocrystals with high energy surfaces,” says Zhong Lin Wang, professor in Georgia Tech’s School of Materials Science and Engineering. The structure remains stable at temperatures up to 800°C, allowing the materials to be reused again and again. He foresees production of hydrogen for fuel cells as the prime potential application for the catalysts.

The nanocrystals are made by electrodepositing polycrystalline platinum spheres about 750 nm in diameter onto a substrate of amorphous carbon. The spheres then are placed in an electrochemical cell with ascorbic and sulfuric acids and subjected to alternating positive and negative potentials at a rate of 10 Hz to 20 Hz. This converts the spheres to smaller nanocrystals in 10 minutes to 60 minutes. Catalyst output is around 100 mg per batch, says Wang.

“This electrochemical technique is vital to producing such tetrahexahedral platinum nanocrystals,” notes Shi-Gang Sun, professor in Xiamen University’s  College of Chemistry and Chemical Engineering. “The key… is to tune the voltage and the time period under which it is applied ... We can control the size with a high level of uniformity,” he says. “The technique used to produce the new platinum nanostructures may also have applications to other catalytic metals,” Sun adds.

While the nanocrystals have higher activity, they also are more than 20 times larger than existing catalysts — so they are less active per unit weight.

“We need to find a way to make these nanocrystals smaller while preserving the shape,” explains Wang. Tetrahexahedral nanoparticles with diameters under 8 nm are necessary to be competitive with commercial catalysts, he says.  Materials of that size should be produced within about two years. “If we can reduce the size through better control of processing conditions, we will have a catalytic system that would allow production of hydrogen with greater efficiency.”

The other challenge is to increase the batch sizes, notes Wang. He foresees output of a couple of hundred grams per batch within two to three years.