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CAUGHT IN THE ACT
Atomic resolution technique probes catalysts during reaction
Adding an analytical technique to catalysis researchers' toolbox, scientists in Denmark have demonstrated an electron microscopy procedure capable of probing catalytic materials with atomic resolution under conditions approaching those of industrial reactions. The new method was used to elucidate the chemical function of low-concentration additives known as catalyst promoters and may be useful in enhancing catalyst performance.
Decades of instrumentation and method development have provided investigators with an assortment of powerful laboratory tools useful for measuring and deducing structure, bonding, chemical state, and other properties of heterogeneous catalysts in exquisite detail. But many techniques work best under carefully controlled conditions--temperature and pressure, for example--that are very different from real-world catalytic reaction environments.
|TOPSØE TEAM From left to right: Haldor Topsøe, Jakob B. Wagner, Poul L. Hansen, Søren Dahl, Thomas W. Hansen, and Jacobsen.
This nagging question about the scientific relevance of data recorded under unrealistic conditions has driven researchers to try to develop in situ techniques designed to catch catalysts in the act of facilitating chemical reactions.
The small number of available in situ probes has just been augmented by Poul L. Hansen, Claus J. H. Jacobsen, and coworkers at Lyngby-based catalyst manufacturer Haldor Topsøe. The group has designed a transmission electron microscope (TEM) cell that can hold catalyst specimens at high temperature and in high pressures of reactive gases while probing the material's structure with angstrom-level resolution [Science, 294, 1508 (2001)].
Using the newly designed TEM facility and procedure, the Topsøe scientists uncovered new details concerning the location, chemical state, and function of barium used to promote a boron nitride-supported ruthenium catalyst. Used for ammonia synthesis, the catalyst's activity increases by some 2.5 orders of magnitude with the addition of just a small amount of barium.
Under reaction conditions, two barium phases grow on ruthenium particles in the highly active form of the catalyst, the group reports. Combining the microscopy work with computational studies, the researchers identify the promoter phase as the one characterized by very thin patches of barium oxide that form near particle edges at certain types of crystal sites.
"We're extremely excited about the results," exclaims Research Director Henrik Topsøe. For years, the company has been working to develop in situ methods. And its efforts have provided insight into the dynamics and structure of catalysts during reactions. "But in the absence of the atomically resolved images, interpretations have often remained ambiguous," Topsøe notes.
University of Washington chemistry professor Charles T. Campbell, who wrote a commentary on the work for the same issue of Science, praises the company's efforts in fundamental catalysis research. In situ analysis is essential for understanding the nature of working catalysts, he says. "The Topsøe group shows that conventional TEM, carried out in high vacuum, fails miserably in this respect because the surface structure of the catalyst is completely different in vacuum than it is during reaction."
"Using the term 'promoter' just shows how little we know about the function of these additives," remarks Robert J. Davis, a University of Virginia chemical engineering associate professor who has studied ammonia synthesis catalysts using X-ray absorption and other methods. But with new techniques that can reveal atomic-level details of promoters, scientists will be able "to optimize existing catalysts and design new catalytic materials," he says.
UP CLOSE An atomic resolution micrograph recorded under reaction conditions (left) and an accompanying structure diagram show the location of barium (gold), oxygen (red), and ruthenium (green) atoms in a Ru ammonia synthesis catalyst that is made highly active by using barium as a promoter.
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