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August 5, 2002
Volume 80, Number 31
CENEAR 80 31 pp. 26-32
ISSN 0009-2347


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APPLYING THE NEW method to catalysts consisting of Pd6Ru6 nanoparticles on a porous silica support, the Cambridge team observed a distribution of particle sizes and noted that while some large particles reside outside of the silica framework, the active nanoparticles remain anchored in the pores. The researchers concluded (based on animations that can be viewed at http://www-hrem.msm.cam.ac.uk/research/CETP/ STEM_Tomo.html) that some of the particles coalesce, forming new particles that are too large to fit in the small pores [J. Phys. Chem. B, 105, 7882 (2001)].

TEM and electron tomography studies of catalysts also figure prominently in the research programs of Krijn P. de Jong, a professor of inorganic chemistry and catalysis at Utrecht University, in the Netherlands, and Abraham J. Koster of Utrecht's biology faculty. De Jong notes that electron tomography with nanometer resolution has been used in biology, at least since the 1960s. But in the past two to three years, the technique's popularity has grown significantly.

That's because what used to be a several-month-long operation involving manual specimen tilting, digitizing photographic images, and lengthy calculating now has been reduced drastically. According to de Jong, across-the-board instrument improvements and newly developed algorithms for computer-controlled specimen tilting make it possible today to record a 150-image tilt series in just 20 minutes.

Zeolites with pores in two size ranges are efficient catalysts for hexane isomerization and other processes. Micropores (less than 2 nm in diameter) provide reaction sites, and mesopores (up to 50 nm) serve as throughways along which reactants and products are shuttled, de Jong explains. Various postsynthesis procedures are used to prepare zeolites with the requisite mesopores. But verifying how well the methods work is difficult because conventional TEM and other methods provide only limited information.

VIDEO - CLICK ON IMAGE

In contrast, 3-D TEM techniques permit detailed characterization of the pores. By examining virtual slices through a reconstructed 3-D image of a zeolite crystal, de Jong and coworkers observed that some treatments produce mostly mesoporous cavities--holes--rather than channels that lead from the crystal's interior to its exterior as needed for shuttling reactants and products [Angew. Chem. Int. Ed., 40, 1102 (2001)]. By rotating and flipping a reconstructed 3-D crystal image (http://pubs.acs.org/cen/video/8031/8031sci1video.html), researchers can study the pores in great detail.

TOHOKU UNIVERSITY PHOTO
In related work, de Jong and coworkers observed that in SBA-15, a porous material, pores that appear straight and orderly in ordinary 2-D images turn out to be highly curved--even U shaped--when studied with 3-D TEM methods. (Animations can be viewed at (http://pubs.acs.org/cen/video/8031/
8031sci1video.html
.) The findings indicate that in diffusion studies of some materials, assumptions about path lengths have been significantly underestimated [Chem. Commun., 2002, 1632
].

Compared with X-ray crystallography, using electron crystallography to solve crystal structures is uncommon. But Osamu Terasaki, a physics professor at Tohoku University, in Japan, and coworkers solved the crystal structure of SSZ-48 using crystals that are far too small for X-ray analysis. The material is a complex, large-pore zeolite that contains a one-dimensional pore system that is circumscribed by 12 tetrahedral atoms [J. Phys. Chem. B, 103, 8245 (1999)].

Recently, Terasaki's group developed a method for solving 3-D structures of mesoporous silica crystals from high-resolution TEM images. They used the method to study Pt nanowires grown in the channels of MCM-41, MCM-48, and SBA-15 (three mesoporous silica materials) and showed that the nanowires' structures follow the geometry of each of the specimen's channels [Microsc. Microanal., 8, 35 (2002)].

Reconfiguring the insides of microscopes to accommodate reactors, spectrometers, and detectors can go a long way toward revealing chemical and materials secrets. But building out works, too. Northwestern University materials science and engineering professor Laurence D. Marks and coworkers have built up a complex vacuum system--connected to their microscope--that's used for preparing and characterizing materials and analyzing them with high-resolution TEM methods without removing the specimens from vacuum or other controlled atmospheres.

The group prepared novel boron nitride nanotubes by depositing B from an evaporation source and N ions from a plasma on a hot tungsten surface (C&EN, March 19, 2001, page 10), and they also have analyzed the structure of germanium films on silver and silver films on silicon crystals.

Decades' worth of innovations in transmission electron microscopy have transformed the lab-filling microscopes into powerful tools for chemistry research. Many chemists remain unfamiliar with their usefulness, but through the continued efforts of some microscopists, the situation may change.

"You have to be willing to take risks," DuPont's Gai says. "Unless you take courageous steps, science won't advance."

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