Therefore, we here investigated three different pre-oxidized catalysts that were prepared by different oxidation procedures. These procedures include Oact deposition by exposing a prereduced catalyst to either a H2/CO2/Ar multipulse sequence or to O2/Ar pulses, in addition to using a freshly calcined (O150) catalyst. Comparison with a similar experiment performed on a reduced Ru/Al2O3 catalyst, which was reported previously , provides additional information on the role of surface oxidation in these processes. Correspondingly, the consumption of C 18 O2, which is given by the difference between the pulsed and measured C 18 O2 signals (pulsed signal intensities are indicated by the dots in Figure ) decreased accordingly during this sequence. As described in the Experimental, the number of C 18 O2 molecules in the incoming C 18 O2/Ar pulses was calculated from the measured Ar pulses, assuming a constant ratio between Ar and C 18 O2 species in the supply tank and hence in the pulses. Furthermore, it was calibrated such that at the end of the sequence, where we assume steady-state conditions, the consumption of C 18 shown in Figure . The C 16 O2 signal (Figure , Figure ) initially increased steeply to reach a maximum after 10 pulses (see inset Figure ) and then decreased continuously in an approximately exponential way, losing 50% of its maximum intensity after about 80 pulses.