Structural defects working as active oxygen-reduction sites in partially oxidized Ta-carbonitride core-shell particles probed by using surface-sensitive conversion-electron-yield x-ray absorption spectroscopy

We analyzed the local structure of the surface Ta-oxide phase of TaCN / Ta 2 O 5 core-shell particles that have a high oxygen reduction activity by using surface-sensitive conversion-electron-yield x-ray absorption spectroscopy, suppressing the contribution from the TaCN cores. The radial structure analysis revealed that the catalytically-active Ta 2 O 5 phase in the TaCN / Ta 2 O 5 particle surface contains oxygen-vacancy defects with shorter Ta-O bonds leading to the slight expansion of the first Ta-O shell. Such oxygen defects are likely responsible for the oxygen reduction capability by creating electronically favorable oxygen adsorption sites and electron conduction pathways.