Using an in house developed in situ magnetometer the passivation in diluted oxygen or pure carbon dioxide of a platinum promoted and silica supported cobalt catalyst was studied. No passivation of cobalt was observed after treatment in CO2, neither at 30 degrees C nor at 150 degrees C. The magnetic cobalt metal phase content remained constant during these treatments and decreased in the subsequent exposure to air. The catalyst exposed to 1% O-2 in N-2 was stable in air at 30 degrees C However, no long-term stability was observed. The magnetic measurements during passivation in 1% O-2 indicate partial oxidation of the nanoparticles in form of a CoO layer. The degree of reduction decreased from 89% to 51%, which corresponds to a theoretical oxide layer thickness of 1.3 nm surrounding the metallic core with a diameter of 9.8 nm. Upon re-reduction full recovery of the metal phase was obtained. The re-reduction occurred at significantly lower temperatures than the reduction of the freshly prepared catalyst or the re-reduction of a reduced catalyst after exposure to air without any passivation. (C) 2016 Elsevier B.V. All rights reserved.