Nontrivial redox behavior of nanosized cobalt: new insights from ambient pressure X-ray photoelectron and absorption spectroscopies.

The reduction and oxidation of carbon-supported cobalt nanoparticles (3.50±0.22 nm) and a Co (0001) single crystal was investigated by ambient pressure X-ray photoelectron (APPES) and X-ray absorption (XAS) spectroscopies, applied in situ under 0.2 mbar hydrogen or oxygen atmospheres and at temperatures up to 620 K. It was found that cobalt nanoparticles are readily oxidized to a distinct CoO phase, which is significantly more stable to further oxidation or reduction compared to the thick oxide films formed on the Co(0001) crystal. The nontrivial size-dependence of redox behavior is followed by a difference in the electronic structure as suggested by theoretical simulations of the Co L-edge absorption spectra. In particular, contrary to the stable rocksalt and spinel phases that exist in the bulk oxides, cobalt nanoparticles contain a significant portion of metastable wurtzite-type CoO.