In Situ EXAFS Characterization of Nanoparticulate Catalysts
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Characterization of Nanoparticulate Catalysts John Evans, Anna Puig-Molina, and Moniek Tromp
Abstract X-ray absorption fine structure (XAFS) spectroscopy probes the structure and electronic properties of metal centers. Because it can be applied to noncrystalline materials, it is a key technique for probing nanoparticulate materials, such as colloidal and heterogeneous metal catalysts. The high brilliance of modern synchrotron radiation x-ray sources facilitates in situ studies, which provide direct structure–function relationships with both spatial and time resolution; this is especially effective when applied in combination with complementary techniques such as x-ray diffraction, mass spectrometry, and optical or vibrational spectroscopies. Tracking the particle formation of platinum-group metal catalysts, their behavior under reaction conditions, and the distribution of sites within a catalyst bed shows that this approach is essential for understanding the chemistry of these nanoparticles. Rather than behave as monolithic entities, nanoparticulate catalysts undergo rapid structural transformations induced by the gas environment and reaction conditions, and their lifetimes as catalysts depend on the reversibility of these changes.
Introduction X-ray absorption fine structure (XAFS) spectroscopy is a powerful characterization technique for determining the local structural and electronic properties of a material under investigation.1 The experiments rely on, as the name already suggests, highenergy x-rays, normally produced by synchrotrons. Laboratory equipment is available, however, that provides a much lower x-ray intensity and produces only relatively low-energy x-rays.2–5 In an x-ray absorption experiment, the sample absorbs a portion of the photons, depending on its absorption characteristics. If the energy of the x-ray photons is high enough to excite a core electron to an empty energy state or to the continuum, a strong increase in absorption (an absorption edge) is observed. The energy of the absorption edge is correlated with the binding energy of the core electron in an atom, making the technique elementspecific.
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The first part of the XAFS spectrum up to ~50 eV above the absorption edge is called the x-ray absorption near-edge spectrum. Xray absorption near-edge spectroscopy (XANES) probes empty energy states in the atom or molecule, providing detailed information on the electronic properties. It includes the absorption edge, which is a spectroscopically allowed dipole transition. The exact energy position of the absorption edge gives information on the oxidation state of the element, or the charge distribution within the molecule or material, under investigation. Other features observed in the XANES region are preedge peaks and multiple scattering features above the absorption edge. Pre-edge peaks originate from direct quadrupole transitions and/or transitions to dipole-allowed energy levels (of molecular orbitals) that have hybridized with other molecular orbitals in certain geometries; transiti
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