Article pubs.acs.org/JPCC
Quantitative Analysis of Modulated Excitation X‑ray Absorption Spectra: Enhanced Precision of EXAFS Fitting Christian F. J. König,† Jeroen A. van Bokhoven,†,‡ Tilman J. Schildhauer,† and Maarten Nachtegaal*,† †
Paul Scherrer Institut, 5232 Villigen, Switzerland Institute for Chemical and Bioengineering, ETH Zürich, 8093 Zürich, Switzerland
‡
ABSTRACT: The sensitivity of X-ray absorption spectroscopy (XAS) can be increased by using the modulated excitation approach, where the sample is excited with a periodic external stimulation (e.g., changes in temperature, gas/liquid concentration, etc.) and the corresponding spectra are filtered with the frequency of the excitation. The resulting demodulated spectra contain similar spectral features as difference spectra but to much higher k values because of the isolation of the structural change that occurs with the same frequency as that of the excitation. Furthermore, multiple intermediate species that occur with a phase delay can uniquely be differentiated. We present a robust approach for the quantitative analysis of demodulated extended X-ray absorption fine structure (EXAFS) spectra. Simulations show that the proposed fitting approach resolves small changes in the sample’s structure with greatly enhanced precision. Experimentally, this is demonstrated by the formation of ruthenium oxide species upon partial oxidation of a Ru metal particle, which cannot be detected in a standard EXAFS experiment.
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INTRODUCTION Advances in X-ray instrumentation now allow recording extended X-ray absorption fine structure (EXAFS) spectra with a time resolution that matches the kinetics of chemical reactions, that is, in the millisecond to minute range.1−5 Analysis of the EXAFS region (starting about 100 eV above the absorption edge) yields the local geometric structure of the absorber atom, including the identity, number, and distance of nearest neighboring atoms up to a distance of ∼6 Å. The EXAFS region can be analytically described by a sum of scattering paths of the photoelectron between the absorber atom and neighboring atoms:6 χ (k ) =
∑
N ·S02·F(k) 2
kR
spectroscopy (XAS). This is, for example, the case in heterogeneous catalysts, where only a minor fraction of atoms contributes to catalytic conversion. A method that is able to quantify structural information only of the fraction of atoms that is involved in a reaction is therefore largely wanted. The fitting precision of experimental EXAFS data to theoretical models and experimental standards was examined by Li et al.7 By systematically varying the structural parameters, Li et al. mapped the difference between fit and experimental data for a range of experimental standards. They concluded that the fitting errors in a typical EXAFS fit for coordination numbers are generally
