HIDING IN THE BACKGROUND
Ignored region of X-ray absorption spectrum now scrutinized for chemical
information
MITCH JACOBY, C&EN CHICAGO
Imagine how disappointing it would be to learn that you've
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 MINERS Ramaker (left) and
inorganic chemistry and catalysis at the Debye Koningsberger find gold in discarded
Institute at Utrecht University, in the Netherlands, data.
explains that data analysis required for techniques such PHOTO BY MITCH JACOBY
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 Xray 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 a longtime associate of
Koningsberger and Ramaker, had made EXAFS
measurements of carbon-supported platinum electrodes
and found it difficult to rid the spectra of low-R 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
NUMBER CRUNCHING Processing a function of support acidity," Koningsberger asserts.
raw X-ray absorption spectra (top) The idea of referring to the low-R region of the spectra as
with careful attention to background "atomic XAFS" comes from a group of scientists at the
signals
can
uncover
chemical University of Warwick and the Daresbury Laboratory, in
information in the form of atomic X- the U.K. Ramaker points out that in the late 1970s, the
ray absorption fine structure (atomic U.K. group indicated that oscillations in the X-ray
XAFS). From processed XAFS data absorption background could be due to interactions
(bottom), interatomic spacings (Pt-Pt) between photoejected electrons and the interatomic
in a platinum specimen and other potential associated with the atom from which the
types of information have long been electrons were ejected (the absorber atom). The effect was
determined. But subtle details of shown more directly in the mid-1990s by John J. Rehr, a
electronic and other properties can be physics professor at the University of Washington, Seattle.
extracted from the atomic XAFS But the connection between this phenomenon and
region using new data analysis chemistry wasn't obvious. In fact, it wasn't until the late
1990s--some 20 years after the U.K. group described the
procedures.
effect--that connections to chemistry were made by
O'Grady, Ramaker, and Koningsberger.
Elaborating on the chemical significance of the atomic XAFS signal, Ramaker explains that in the familiar
X-ray absorption scenario (the one responsible for large-R EXAFS), X-ray photons from a synchrotron
impinge upon a specimen and eject core-level electrons. The outgoing photoelectrons can be thought of as
waves, he says, not unlike waves generated by tossing a stone into a body of water.
THE WAVES EMANATE and spread from the absorber atom and can scatter against neighboring atoms-or more specifically, against electrons centered on neighboring atoms--subject to certain energy constraints.
A portion of the outgoing waves bounce back and interact with the absorber atom, causing modulations in
the absorption cross section (the probability of absorbing X-rays). The modulation shows up as the
characteristic wiggles, the fine structure, in X-ray absorption spectra.
In the same way that an electron (electron 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 photoelectron may scatter
against electrons in a deep valence electron shell on the same Pt atom--hence the term "atomic XAFS."
These scattering events give rise to the spectral features that are observed at distances much smaller than
typical interatomic spacings.
Koningsberger and Ramaker stress that because the signal arises from interactions with valence electrons,
this region of the spectrum bears chemical information and responds to subtle changes in environment.
"You don't need to change the geometry or structure to see a change in atomic XAFS," Ramaker says. Just
change the charge on platinum slightly or modify a support material sitting next to platinum, and the effect
can be seen in the low-R region.
And there's the connection to catalysis. After concluding from the electrochemical cell studies that the lowR features hold chemically relevant information, Koningsberger and coworkers were motivated to examine
their catalysis data for correlations between atomic XAFS and acidity of the zeolite supports because of the
emerging importance of 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 Wolfgang M. H. Sachtler, Yale University's Gary
L. Haller, and others, have shown that catalytic activity in many reactions increases markedly with
increasing support acidity. (Zeolite acidity is easily modified via ion-exchange reactions and other
methods.)
Koningsberger observed the same effect. In studies conducted in collaboration with the Utrecht group,
Jeffrey T. Miller, a research scientist at BP Chemicals (formerly Amoco), prepared a large number of Lzeolite specimens that varied systematically from strongly acidic to strongly basic. Catalysts supported on
the acidic materials were found to exhibit activities for some reactions that were three orders of magnitude
larger than the same reactions carried out on basic zeolites. Koningsberger and Ramaker then observed that
the intensity of the atomic XAFS signal tracked the catalytic activity.
ARMED WITH THESE and related findings, Koningsberger,
Ramaker, and coworkers studied hydrogenolysis and
isomerization reactions of neopentane and hydrogenation of
tetralin on supported platinum catalysts. Once again, catalytic
activity was found to depend on support acidity and correlate
with atomic XAFS intensity [J. Catal., 203, 7 (2001)]. These
effects can now be quantitatively described using Rehr's
widely used computer program.
From the combined experimental and computational study, the
group reports that interactions between the metal and its
support change the ionization potential of Pt's valence electrons and cause charge rearrangement. The
electronic shuffling takes place between Pt 6s orbitals in catalyst particles and oxygen 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 hydrogenation 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 under way.
"For some catalytic reactions, you'd like to have high-lying Pt d orbitals and for others, low-lying d
orbitals," Koningsberger says. "If we 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 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 of metal
clusters [and other types of catalysts] with supports and adsorbed ligands."