Applications of X-ray absorption fine structure (XAFS) spectroscopy in fuel science

X-ray absorption fine structure (XAFS) refers to the details of how x-rays are absorbed by an atom at energies near and above the core-level binding energies of that atom. XAFS spectra are especially sensitive to the formal oxidation state, coordination chemistry, and the distances, coordination number and species of the atoms immediately surrounding the selected element. Practically, XAFS can be divided into two regions, near-edge region and extended region. The near-edge XAFS, or x-ray absorption near edge structure (XANES), provides information on oxidation state, coordination symmetry, and can be used for identification of different species by fingerprinting. In contrast, extended XAFS, or EXAFS, measures the coordination structure including bond distance and coordination number of neighboring atoms. XAFS has been applied in many fields of research including fuel science. In this talk, two examples are given to illustrate the uniqueness of XAFS spectroscopy. In one example, catalyst deactivation in CeO ₂-Al ₂O ₃ supported Ni catalysts for hydrogen production from steam reforming of liquid hydrocarbons in the presence of sulfur impurities was studied by XANES for a better understanding of sulfur poisoning mechanism. XANES spectroscopy at sulfur K-edge (2472 eV), and carbon K-edge (284.2 eV) was used to identify the sulfur and carbon species in the used catalysts and provided new insight into sulfur poisoning mechanism in steam reforming catalysts. In the other example, structure of a PtNiFe/C cathode electrocatalyst used in PEM fuel cells was probed by EXAFS at Pt L3 edge (11,564 eV).