SCIENCE & TECHNOLOGY
MINERS Ramaker (left) and Koningsberger find gold in discarded data.
HIDING IN THE BACKGROUND Ignored region of X-ray absorption spectrum now scrutinized for chemical information M I T C H J A C O B Y , C & E N CHICAGO
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MAGINE HOW DISAPPOINTING IT
would be to learn that youVe unintentionally been throwing away something valuable because it was tucked inside something else and you didn't know it was there—like free movie tickets hiding in a newspaper section that you never read. How much worse would it be to discover that something you've been discarding for years—deliberately—is valuable? That's exactly what's been happening in X-ray absorption studies for quite some time, according to Diederik C. (Diek) Koningsberger. "For 25 years I've been throwing away useful data," he says. "We've all been doing it." Fortunately, unlike the theater tickets, the lost data can be retrieved. Recently, scientists have begun extracting chemically significant information from data that's ordinarily overlooked and using it to understand the effect of materials used to support precious-metal catalysts. A longtime user of X-ray absorption methods to study catalytic materials, Koningsberger, a professor of inorganic chemistry and catalysis at the Debye Institute HTTP://PUBS.ACS.ORG/CEN
at Utrecht University, in the Netherlands, explains that data analysis required for techniques such as extended X-ray absorption fine structure (EXAFS) spectroscopy involves a considerable amount of number crunching. In EXAFS experiments (often pronounced ex-safs), which are carried out at synchrotron facilities, researchers collect data by measuring absorption of X-rays in the material under investigation while the X-ray wavelength (or photon energy) is scanned. But plotting absorption intensity versus energy isn't the most useful way to present the data. To derive coordination numbers and near-neighbor spacings, say for metal atoms in an oxide environment, scientists work up the raw data using Fourier transformations, background subtractions, smoothing functions, and other manipulations. When the processed data are plotted as signal intensity versus atomic separation (R), the curves typically include a large peak at a distance of a couple of angstroms, indicating the average spacing between a metal atom that's absorbing X-rays and the
smallest shell of metal atoms that surrounds the absorber atom. Sometimes second-, third-, and higher-order coordination shell spacings can also be read off the plot. First-shell interatomic spacings on the order of a few angstroms make sense physically. But peaks that pop up at half an angstrom, for example, are usually considered unimportant because they're too short to be real. David E. Ramaker, a theoretician who collaborates with Koningsberger on computational X-ray absorption work, notes that at first glance, these "low R" spectral features seem to be instrument artifacts or some kind of noise. Ramaker, who is a professor of chemistry at George Washington University, Washington, D.C., goes on to say that, traditionally, researchers remove the low-R features by deliberately choosing smoothing and background subtraction parameters that minimize them. "They're generally considered a nuisance," he says, chuckling, especially when the peaks don't seem to want to go away ONE OF THE FIRST clues that the low-R features might be real signals came a few years ago from studies of electrodes in electrochemical cells. William E. O'Grady, a staff scientist at the Naval Research Laboratory, Washington, D.C., and alongtime associate of Koningsberger and Ramaker, had made EXAFS measurements of carbon-supported platinum electrodes and found it difficult to rid the spectra of lowR features, which in some cases were very large. Teaming up to work on the data analysis, Koningsberger, Ramaker, and O'Grady determined that the low-R features were very real, and O'Grady and Ramaker subsequently proved that the features depend on the applied electrode potential. Suspecting that the neglected region of the spectrum holds information about the metal atoms' environment—an electric field set up by an applied voltage in the case of the electrode—Koningsberger and coworkers examined X-ray absorption data from supported metal catalysts to see if the low-R features in those spectra vary with changes in support properties. And indeed they do. "We find systematic changes in the 'atomic XAFS' {low-R peaks} as a function of support acidity," Koningsberger asserts. The idea of referring to the low-R region of the spectra as "atomic XAFS" comes from a group of scientists at the University of Warwick and the Daresbury Laboratory, in the U.K. Ramaker points C&EN
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SCIENCE & TECHNOLOGY out that in the late 1970s, the U.K. group indicated that oscillations in the X-ray absorption background could be due to interactions between photoejected electrons and the interatomic potential associated with the atom from which the
that support property in catalytic activity At one time, the conventional thinking in heterogeneous catalysis had been that catalyst supports merely provide inert substrates upon which to disperse catalytically active metals into ultrafine particles. But several researchers, including Northwestern University's WolfAbsorption Intensity gang M. H. Sachtler, Yale Univer.04 0.50 Background removal Pt-Pt sity's Gary L. Haller, and others, • Conventional have shown that catalytic activity • Optimal in many reactions increases • Insufficient markedly with increasing support 0.25 acidity. (Zeolite acidity is easily modified via ion-exchange reactions and other methods.) Koningsberger observed the 100 200 300 1 2 same effect. In studies conductPhoton energy, eV Interatomic spacing, Â ed in collaboration with the Utrecht group, Jeffrey T. Miller, NUMBER CRUNCHING Processing raw X-ray absorption spectra (left) with careful a research scientist at BP Chemattention to background signals can uncover chemical information in the form of atomic icals (formerly Amoco), prepared X-ray absorption fine structure (atomic XAFS). From processed XAFS data (right), a large number of L-zeolite specinteratomic spacings (Pt-Pt) in a platinum specimen and other types of information imens that varied systematically have long been determined. But subtle details of electronic and other properties can be from strongly acidic to strongly extracted from the atomic XAFS region using new data analysis procedures. basic. Catalysts supported on the acidic materials were found to exhibit activities for some reactions that electrons were ejected (the absorber photoelectron may scatter against elecwere three orders of magnitude larger atom). The effect was shown more directly trons in a deep valence electron shell on than the same reactions carried out on in the mid-1990s byjohnj. Rehr, a physics the same Pt atom—hence the term "atombasic zeolites. Koningsberger and Raprofessor at the University of Washingic XAFS."These scattering events give rise maker then observed that the intensity to the spectral features that are observed ton, Seattle. of the atomic XAFS signal tracked the at distances much smaller than typical inBut the connection between this phecatalytic activity. teratomic spacings. nomenon and chemistry wasn't obvious. In fact, it wasn't until the late 1990s—some Koningsberger and Ramaker stress that ARMED WITH THESE and related find20 years after the U.K. group described because the signal arises from interactions ings, Koningsberger, Ramaker, and the effect—that connections to chemistry with valence electrons, this region of the coworkers studied hydrogenolysis and were made by O'Grady, Ramaker, and spectrum bears chemical information and isomerization reactions of neopentane responds to subde changes in environment. Koningsberger. and hydrogénation of "You don't need to Elaborating on the chemical significance tetralin on supported change the geometry of the atomic XAFS signal, Ramaker explatinum catalysts. or structure to see a plains that in the familiar X-ray absorption Once again, catalytic change in atomic scenario (the one responsible for large-R activity was found to XAFS," Ramaker says. EXAFS), X-ray photons from a synchrodepend on support tron impinge upon a specimen and eject Just change the charge acidity and correlate on platinum slightly or core-level electrons. The outgoing photowith atomic XAFS inmodify a support maelectrons can be thought of as waves, he tensity \J. CataL, 203, terial sitting next to says, not unlike waves generated by tossing 7 (2001)}. These effects platinum, and the efa stone into a body of water. can now be quantitafect can be seen in the tively described using THE WAVES EMANATE and spread from low-R region. Rehr's widely used the absorber atom and can scatter against And there's the concomputer program. neighboring atoms—or more specifically, nection to catalysis. AfFrom the combined against electrons centered on neighborter concluding from experimental and ing atoms—subject to certain energy conthe electrochemical computational study, straints. A portion of the outgoing waves cell studies that the the group reports that bounce back and interact with the ablow-R features hold interactions between sorber atom, causing modulations in the chemically relevant inthe metal and its support change the ionabsorption cross section (the probability formation, Koningsberger and coworkers ization potential of Pt's valence electrons of absorbing X-rays). T h e modulation were motivated to examine their catalysis and cause charge rearrangement. The elecshows up as the characteristic wiggles, the data for correlations between atomic tronic shuffling takes place between Pt 6s fine structure, in X-ray absorption spectra. XAFS and acidity of the zeolite supports orbitals in catalyst particles and oxygen In the same way that an electron (elecbecause of the emerging importance of tron wave) can be scattered by electrons centered on neighboring atoms at a distance of a couple of angstroms or more, so, too, the outgoing wave can be scattered by other electrons on the same absorber atom. In platinum, for example, a core-level Pt
"Right now it's a bit of an art to extract the atomic XAFS. It's in everybody's data, and it's been there for years. But we need to develop methods... to make it easy for anyone to do it."
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atoms that reside at the particle-support interface. The researchers add that both effects alter Pt's electronic structure, which in turn affects the catalytic properties of Pt surface atoms by varying bond strength and bond order to reaction intermediates. The bottom line, they say, is that catalyst-support interactions, which are controllable, alter a catalyst's electronic and catalytic properties and can be probed via atomic XAFS. The next step, according to Koningsberger, is to use the new handle on catalytic activity to improve catalyst performance. Tetralin hydrogénation and other reactions are being investigated by the Utrecht group and their chemical industry collaborators as part of an effort to develop catalysts for preparing clean diesel fuels. Reducing the concentration of sulfur and aromatic compounds in diesel fuels lowers the level of soot in the exhaust. A commercial platinum-palladium catalyst is currently available for preparing clean diesel fuels. One of its key features is fairly high tolerance to sulfur poisoning. But according to Koningsberger, the system's high activity is driven by the support's
high acidity, which ultimately cracks some of the desired product, limiting the yield. A more promising route to clean fuels may result, in part, from atomic XAFS studies that offer new insights into the nature of catalyst-support interactions. The researchers suggest that suitable additives may be used to tune the catalytic system's electronic properties while lowering its acidity Investigations of this type are underway "For some catalytic reactions, you'd like to have high-lying Pt d orbitals and for others, low-lying d orbitals," Koningsberger says. "Ifwe understand the interactions between the support and catalyst, then we can manipulate the support, tune-up our Pt particles just so, and prepare tailor-made catalysts." Koningsberger and Ramaker acknowledge that it may take a while for the atomic XAFS concept to catch on in the catalysis community "Right now it's a bit of an art to extract the atomic XAFS," Ramaker says. "It's in everybody's data, and it's been there for years. But we need to develop methods for quantitative atomic XAFS analysis to make it easy for anyone
to do it." At this point, that's the bottleneck, Ramaker says. Bruce C. Gates, a professor of chemical engineering and materials science at the University of California, Davis, remarks HYDROGENATION Atomic XAFS may lead to tailormade catalysts Pt/H2 350 °C
Decalin
Tetralin
that EXAFS spectroscopy is an exquisite method for determining structures of highly dispersed catalysts such as supported metals, even in the functioning state (C&EN, Aug. 6,2001, page 33). Gates adds that "the Ramaker-Koningsberger group has recognized how to mine new information from EXAFS spectra—atomic XAFS — signaling a promising new approach to investigating interactions ofmetal clusters {and other types of catalysts} with supports and adsorbed ligands." •
University of Southern California Celebrating the Twenty Fifth Anniversary of the
Loker Hydrocarbon Research Institute T r e n d s in Hydrocarbon Chemistry for the New Century A Kimbrough Research Symposium
March 14-15, 2002 SPEAKERS
R o b e r t H. G r u b b s C a l i f o r n i a I n s t i t u t e of T e c h n o l o g y G e o r g e A. O l a h U n i v e r s i t y of S o u t h e r n C a l i f o r n i a L e o A. P a q u e t t e The Ohio State University R o y A. P e r i a n a U n i v e r s i t y of S o u t h e r n C a l i f o r n i a G. K. S u r y a B r a k à s h U n i v e r s i t y of S o u t h e r n C a l i f o r n i a Horst Prihzbach U n i v e r s i t y of F r e i b u r g L a w r e n c e T\ S c o t t B o s t o n College K. B a r r y S h a r p l e s s The Scripps Research Institute J a y S. Siegel U n i v e r s i t y of California, S a n D i e g o G a b o r A. S o m ô r j a i U n i v e r s i t y of C a l i f o r n i a , B e r k e l e y P e t e r J. S t a n g T h e U n i v e r s i t y of U t a h J. F r a s e r S t o d d a r t U n i v e r s i t y of C a l i f o r n i a , Los A n g e l e s For registration and information: Phone: (213)740-5974; Fax: (213)740-5087; e-mail:
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