Friday, May 04, 2007

Better Catalysts for Fuel Cells

Nanoparticles with a completely new shape may lead to cheaper catalysts that could make many experimental-energy technologies more practical.

By Kevin Bullis

Nano geometry: This 24-sided platinum nanoparticle could lead to cheaper alternative energy. Credit: Zhong Lin Wang, Georgia Tech

New nanoparticles with a totally original shape, made by researchers at Georgia Tech, in Atlanta, and Xiamen University, in China, and described in the current issue of Science, could lead to cheaper catalysts for making and using alternative fuels. The 24-sided platinum nanoparticles have surfaces that show up to four times greater catalytic activity compared with commercial catalysts.

If researchers can make even smaller nanoparticles with this same efficient shape, it could significantly reduce the amount of platinum used. Reducing the amount of this expensive metal--it currently sells for about $1,300 per ounce--would make applications such as fuel cells more affordable. Reducing the cost of platinum catalysts could also be critical in other applications, such as synthesizing alternative fuels and converting waste materials like carbon dioxide into useful products. (See "Making Gasoline from Carbon Dioxide.")

The new work is important, says Francesco Stellacci, professor of materials science and engineering at MIT, because it involves platinum, which he says is "by far the most interesting metal" for catalysis. The work could also advance the basic understanding of how changing the shape of particles affects catalysis, he says.

To make the nanoparticles, the Georgia Tech and Xiamen researchers began with relatively large platinum particles scattered on a carbon surface. They then applied an oscillating voltage, which induces alternating chemical reactions that determine where platinum atoms will accumulate and where they won't. For example, at positive voltages, oxygen atoms can infiltrate some areas of these nanoparticles, dislodging platinum atoms. At the same time, a layer of platinum oxide forms on other parts of the nanoparticle, protecting them. The resulting 24-sided shapes, called tetrahexahedra, were the first such shapes formed artificially in metals, says Zhong Lin Wang, professor of materials science and engineering at Georgia Tech.

The multifaceted shape made by the researchers has many high-energy areas in which more atoms are unstable and reactive than in conventional platinum nanoparticles. The researchers showed that these surfaces, compared with the surfaces of commercial platinum nanoparticles, catalyzed reactions at a much higher rate.

The current work is only a step toward the goal of making cheaper catalysts. Alexis Bell, professor of chemical engineering at the University of California, Berkeley, says that while the work is interesting because it addresses one of the particular challenges of creating catalysts--controlling the surface structure--the new nanoparticles are in fact not small enough. Existing commercial platinum catalysts can be less than five nanometers wide. The Georgia Tech and Xiamen researchers made particles between 50 and 200 nanometers. Being larger, the new type of nanoparticles have a larger proportion of the expensive platinum locked beneath the surface, where it can't serve to catalyze reactions. As a result, for now, the new nanoparticles are actually worse catalysts than are commercial catalysts available today.

According to Wang, the goal is ultimately to use the new nanoparticles and the methods for making them to help find ways of transforming much cheaper materials into useful catalysts. If that can be done, some technologies limited to the lab bench today could be applied to meeting growing worldwide energy needs.

Indeed, notes Daniel Feldheim, professor of chemistry and biochemistry at the University of Colorado at Boulder, in a commentary accompanying the Science article, researchers have long known that changing particle shape and size can make even seemingly inert materials such as gold into valuable catalysts. The methods used by the Georgia Tech and Xiamen researchers, Feldheim says, provide a new level of control that could lead to improved mixed-metal and metal-oxide catalysts, which are cheaper than precious metals such as platinum.