The second TPD measurement was performed with a surface that had been exposed to a C 18 O2 multipulse sequence, after O150, CO pulse reduction, and O2/Ar re-oxidation, as it was described in section 3.3.2 (Figure ). Here we are mainly interested in the distribution of the CO2 isotopologues during desorption. Figure shows the desorption curves of C 16 O2, C 16 O 18 O, and C 18 O2, and also those of H2O, C 16 O, C 18 O. The integrated signal intensities are presented in Figure , the peak temperatures and number of molecules in the respective peaks are summarized in Table and Table . Most important for the purpose of this measurement, the spectra of the different CO2 isotopologues differ in shape, i.e., the distribution of the different species over different states is not equilibrated, but dominated by kinetic effects. C 18 O2 evolution (2.02•10 19 molecules gcat -1 ) clearly exhibits a distinct lowtemperature peak with a maximum at 286 °C, which is not present for C 16 O2 and C 16 O 18 O. This is followed by a decaying desorption up to the maximum temperature maximum. Such kind of low-temperature desorption was also not observed in the measurement shown in Figure , indicating that i) pure CO2 pulsing, without H2 in the gas, leads to the formation of other and partly less stable species than H2/CO2 pulsing, and that ii) these species must be created during C 18 O2 pulsing and cannot result from preceding C 16 O pulse reduction or were present on the pristine catalyst surface. On the other hand, desorption of C 16 O2 (4.14•10 19 molecules gcat -1 ) is dominated by a pronounced peak with a maximum at 450 °C, which coincides with the temperature maximum in the measurement. Therefore, as discussed before, C 16 O2 desorption is expected to continue at higher temperatures, resembling our findings after H2/CO2 pulsing (Figure ). It is important to note that the onset for C 16 O2 evolution after C 18 O2 pulsing is higher than that observed for CO2 evolution from a H2/CO2 pretreated catalyst (see Figure ), further supporting that oxygen exchange is less facile for the more stable adsorbed species. ) and negligible C 16 O2 formation. At least qualitatively, this seems to agree with the experimental data. For more strongly bound species oxygen exchange is increasingly hindered, which reduces the amount of 18 O in these states during C 18 O2 pulsing.