EXAFS as a tool for catalyst characterization: a review of the ... - INT
EXAFS as a tool for catalyst characterization: a review of the ... - INT EXAFS as a tool for catalyst characterization: a review of the ... - INT
The X-ray absorption coefficient for an atom decreases as the X-ray energy increases. It displays discontinuities (absorption edges) as an incident photon is absorbed by the atom and electronic transitions from a core atomic level to unoccupied conduction states above the Fermi level take place. The photoelectron emitted in this process can be represented as a wave. If the excited atom is surrounded by other atoms, the outgoing wave scatters from the surrounding atoms, producing ingoing waves. These ingoing waves can constructively or destructively interfere with the outgoing waves. This interference produces the oscillatory behavior of the fine structure. The extended X-ray absorption fine structure (EXAFS) is the oscillation in the absorption coefficient on the high-energy side of X- ray absorption edges, ranging from 30 to about 1000eV above the edge. Stern, Sayers and Lytle related these fluctuations of the absorption coefficient to the atomic arrangement surrounding the absorbing atom (Stern, 1974; Stern et al., 1975; Lytle et al., 1975). In this paper, we will be concerned with neither the theory of EXAFS (Stern, 1974) nor the experimental techniques used to measure EXAFS (Lytle et al., 1975; Meitzner, 1998). The aim of this work is to review the mathematical procedure for treating the EXAFS spectrum in order to calculate the physical parameters. A final section presents one example of EXAFS application to catalysis. Data Analysis An X-ray absorption experiment involves the measurement of the total linear absorption coefficient, µ, as the photon energy is varied. In the case of a transmission experiment, ln I 0 /I = µ x (1) where I 0 and I are the photon intensities before and after the absorber of thickness x. A typical X-ray absorption spectrum can be separated into four parts (Bart and Vlaic, 1987) (Figure 1): a- pre-edge region; b- edge region; c- X-ray absorption near edge structure (XANES) region; d- extended X-ray absorption fine structure (EXAFS) region.
Figure 1: Regions of the X-ray absorption spectrum of metallic cobalt. In the first region, the absorption coefficient decreases as the energy increases due to transitions from other occupied levels of the same atom and of other atoms. A Victoreen law, µ(E) = C E -3 + D E -4 , or linear relation, µ(E) = A E + B, is used before the absorption edge to determine the constants. The absorption coefficient rises sharply at the edge, corresponding to the electron transitions to higher unoccupied levels. This region (b) is the absorption edge (within a range of a few electronvolts). Finally, the XANES and EXAFS zones are at the high-energy side of the absorption edge (c and d). XANES
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Figure 1: Regions <strong>of</strong> <strong>the</strong> X-ray absorption spectrum <strong>of</strong> metallic cobalt.<br />
In <strong>the</strong> first region, <strong>the</strong> absorption coefficient decre<strong>as</strong>es <strong>as</strong> <strong>the</strong> energy<br />
incre<strong>as</strong>es due to transitions from o<strong>the</strong>r occupied levels <strong>of</strong> <strong>the</strong> same<br />
atom and <strong>of</strong> o<strong>the</strong>r atoms. A Victoreen law, µ(E) = C E -3 + D E -4 , or<br />
linear relation, µ(E) = A E + B, is used be<strong>for</strong>e <strong>the</strong> absorption edge to<br />
determine <strong>the</strong> constants. The absorption coefficient rises sharply at<br />
<strong>the</strong> edge, corresponding to <strong>the</strong> electron transitions to higher<br />
unoccupied levels. This region (b) is <strong>the</strong> absorption edge (within a<br />
range <strong>of</strong> a few electronvolts). Finally, <strong>the</strong> XANES and <strong>EXAFS</strong> zones<br />
are at <strong>the</strong> high-energy side <strong>of</strong> <strong>the</strong> absorption edge (c and d). XANES
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