Utah Researchers Find Strong Correlation Between Size of Catalyst Particles and Their Electronic Structure and Activity; Potential For Less Expensive, More Efficient Catalysts

7 November 2009

CO oxidation activity (left axis, solid squares) compared with shifts in the Pd 3d binding energy, relative to expectations from smooth bulk scaling (right axis, open circles), as a function of cluster size. Source: Kaden et al. Click to enlarge.

University of Utah chemists demonstrated the link between the size of catalyst particles on a solid surface, their electronic properties and their ability to speed chemical reactions. Senior author Professor Scott Anderson says it is the first demonstration of a strong correlation between the size and activity of a catalyst on a metal surface and the electronic properties of the catalyst.

The study, published in the 6 November issue of the journal Science, is a step toward the goal of designing less expensive, more efficient catalysts to increase energy production, reduce greenhouse gas emissions and manufacture a wide variety of goods from medicines to gasoline.

The researchers used palladium particles of specific sizes that were deposited on titanium dioxide and used to convert carbon monoxide into carbon dioxide for the model catalyst study.

Previous experiments documented that electronic and chemical properties of a catalyst are affected by the size of catalyst particles floating in a gas. But those isolated catalyst particles are quite different than catalysts that are mounted on a metal oxide surface—the way the catalyst metal is supported in industrial catalysts.

Also, past experiments with catalysts mounted on a surface often included a wide variety of particle sizes. Those experiments failed to detect how the catalyst’s chemical activity and electronic properties vary depending with the size of individual particles.

One of the big uncertainties in catalysis is that no one really understands what size particles of the catalyst actually make a chemical reaction happen. If we could understand what factors control activity in catalysts, then we could make better and less expensive catalysts.

Most catalysts are expensive noble metals like gold or palladium or platinum. Say in a gold catalyst, most of the metal is in the form of large particles, but those large particles are inactive and only nanoparticles with about 10 atoms are active. That means more than 90 percent of gold in the catalyst isn’t doing anything. If you could make a catalyst with only the right size particles, you could save 90 percent of the cost or more.

There’s a huge amount of interest in learning how to make catalysts out of much less expensive base metals like copper, nickel and zinc. And the way you are going to do that is by ‘tuning’ their chemical properties, which means tuning the electronic properties because the electrons control the chemistry.

...take a metal that is not catalytically active and, when you reduce it to the appropriate size [particles], it can become catalytic. That’s the focus of our work—to try to identify and understand what sizes of metal particles are active as catalysts and why they are active as catalysts.

—Professor Scott Anderson, senior author

As the size of a catalyst metal particle is reduced into the nanoscale, its properties initially remain the same as a larger particle, Anderson says. But when the size is smaller than about 10 nanometers—containing about 10,000 atoms of catalyst—the movements of electrons in the metal are confined, so their inherent energies are increased.

When there are fewer than about 100 atoms in catalyst particles, the size variations also result in fluctuations in the electronic structure of the catalyst atoms. Those fluctuations strongly affect the particles’ ability to act as a catalyst, Anderson says.

Anderson conducted the study with chemistry doctoral students Bill Kaden and William Kunkel, and with former doctoral student Tianpin Wu. Kaden was first author.

Resources

• William E. Kaden, Tianpin Wu, William A. Kunkel, Scott L. Anderson (2009) Electronic Structure Controls Reactivity of Size-Selected Pd Clusters Adsorbed on TiO₂ Surfaces. Science Vol. 326. no. 5954, pp. 826 - 829 doi: 10.1126/science.1180297