A particularly interesting observation is the appearance of the signal at 3676 cm -1 which we assign to surface OH groups. In a study by Schwarz et al., the interaction of water with atomically defined CoO(100), CoO(111) and Co3O4(111) was investigated. The authors showed that only very small amounts of OH groups are formed on CoO(111) at 400 K. These OH species were assigned to a small fraction of defect sites (for example at grain boundaries).