The fact that the oxidation state of the Ru nanoparticles plays no significant role for the oxygen exchange behavior with C 18 O2 indicates that this process proceeds mainly on the oxide support surface. This agrees with an earlier report by Guerrero et al. . Note that based on previous studies Ru exist mainly in a metallic state under CO2/H2 mixture at temperatures between 190 and 350°C . For a final confirmation, we performed a similar oxygen exchange experiment on pure (Ru-free), O150 pretreated -Al2O3. This test indeed resulted in an oxygen exchange behavior very similar to that obtained for the Ru/Al2O3 catalysts (see Figures and). Only the concentration of exchangeable O atoms seems to be significantly lower than on the catalyst. Here we have to keep in mind, however, that the specific surface area of the pure -Al2O3 (16.67 m 2 g -1 ) is only 1 /6 of that of the catalyst, which should be accounted for. If we normalize per surface area of the oxide, the mass normalized number of exchangeable oxygen (1.7110 20 O atoms goxide -1 , Table ) converts into 1.0310 19 O atoms moxide -2 . Furthermore, if the -Al2O3 is also active for O exchange, we have to consider also the material (20 mg) used for dilution of the catalyst (5 mg) in the pulse experiments, which increases the available Al2O3 surface area in those experiments to 166.7 m 2 gcat -1 . Using the average amount of about 2010 20 O atoms gcat -1 for the exchangeable oxygen in those experiments (see Table ) for comparison, we obtain a surface area normalized value of 1.210 19 O atoms moxide -2 , as compared to 1.0310 19 O atoms moxide -2 for the pure -Al2O3 support Hence, there seems to be no significant difference in the surface concentration of exchangeable oxygen, confirming that the Ru nanoparticles are not important for the O exchange process.