Determining the Co valence, particularly in Co-based nanocatalysts is a longstanding experimental challenge. In this paper, we utilize in situ electron energy-loss spectroscopy and first-principles density functional theory calculations to distinguish between metallic Co, Co3O4, as well as CoO. More specifically, differences in the OK- and CoL-edges are utilized to determine the Co valence in different Co-oxide particles. We will further demonstrate that while the metallic CoL3/L2-ratio equals that of partially reduced Co3O4, the near-edge fine-structure of the metallic CoL-edge exhibits additional features not present in any Co-oxide. The origin of these features will be discussed. Based on our experimental and theoretical results, we will propose a fitting method to distinguish metallic Co from Co-oxides.

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