July 9, 2007

Volume 85, Number 28

p. 7

Surface Science

Low-Cost Catalysis

Inexpensive MoS₂ mimics precious-metal catalyst

Mitch Jacoby

In work that could lead to economical substitutes for precious-metal catalysts, researchers in Denmark have produced hydrogen from water through a reaction catalyzed by a low-cost metal sulfide.

The unique surface properties of platinum, ruthenium, and other metals located in the same region of the periodic table endow those materials with the ability to catalyze numerous chemical reactions. They are widely used, for example, in automotive emissions cleanup and fuel-cell processes. Nonetheless, the metals' high cost has long motivated scientists to search for less expensive substitutes.

Using synthesis methods to control the size and morphology of single-layered, flat molybdenum disulfide nanoparticles, scientists at the Technical University of Denmark, in Lyngby, have demonstrated that the particles can catalyze the hydrogen evolution reaction (2H⁺ + 2e^– → H₂) in solution (Science 2007, 317, 100). They also have determined that this reaction occurs along the perimeter (edge) of the particles, a detail with both theoretical and practical value.

The gas-evolution reaction, which lies at the heart of solar-energy-driven hydrogen production via water splitting and runs in reverse in fuel cells, is a prime example of the type of reaction catalyzed by noble metals. Earlier theoretical work suggested that the edges of nanoparticulate MoS₂ could catalyze the reaction, but until now that prediction had not been verified conclusively.

To prepare the nanoparticles, postdoc Thomas F. Jaramillo, physics professor Ib Chorkendorff, and their coworkers used vapor deposition methods and heat treatments to react molybdenum with hydrogen sulfide on a gold surface. Through judicious choice of synthesis conditions, the team was able to control the extent of particle sintering and thereby systematically vary the particle size and the relative numbers of atoms that reside within the terrace (the open crystal face) and along the nearly atomically thin edge. After analyzing the MoS₂ samples using scanning tunneling microscopy, the team measured the particles' catalytic activity in electrochemical cells and determined that hydrogen evolution correlates linearly with the number of nanoparticle edge sites.

Chemical & Engineering News

ISSN 0009-2347

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