Prior to the pulse experiments we pretreated the powder catalyst in a 10% O2/N2 atmosphere at 150 °C for 30 min (O150). Hence, we expect the catalyst, in particular the Ru nanoparticles (NPs), to be fully oxidized directly after the pretreatment. Previous X-ray absorption experiments on a 2.3wt.% Ru/-Al2O3 catalyst had indeed shown that this catalyst was completely oxidized after this treatment . The gases used for the pretreatment and in pulse experiments were supplied by MTI IndustrieGase AG (Ar 4.6, CO 4.7, O2 5.0, O2 5.0 (10%) / N2 5.0), Messer Industriegase GmbH (H2 6.0), and Campro Scientific (C 18 O2, purity 99%, 95 atom% 18 O).