The atomic AXAFS and Δμ XANES techniques as applied to heterogeneous catalysis and electrocatalysis

Abstract

X-Ray absorption spectroscopy (XAFS) is an attractive in situ and in operando technique. In recent years, the more conventional extended X-ray absorption fine structure (EXAFS) data analysis technique has been complemented by two newer analysis methods: the ‘atomic’ XAFS (AXAFS) technique, which analyzes the scattering from the absorber atom itself, and the Δμ XANES technique, which uses a difference method to isolate the changes in the X-ray absorption near edge structure (XANES) due to adsorbates on a metal surface. With AXAFS it is possible to follow the electronic effect a support has on a metal particle; with Δμ XANES it is possible to determine the adsorbate, the specific adsorption sites and adsorbate coverage on a metal catalyst. This unprecedented new information helps a great deal to unravel the complex kinetic mechanisms operating in working reactors or fuel cell systems. The fundamental principles and methodology for applying the AXAFS and Δμ XANES techniques are given here, and then specific applications are summarized, including H adsorption on supported Pt in the gas phase, water activation at a Pt cathode and methanol oxidation at a Pt anode in an electrochemical cell, sulfur oxidation on Pt, and oxygen reduction on a Au/SnO_x cathode. Finally, the future outlook for time and/or space resolved applications of these techniques is contemplated.