(1.25•10 19 molecules gcat -1 per pulse), i.e., somewhat later than the CO2 pulses. Subsequently, they decay steadily over the rest of this subsequence (last pulses: 1.1•10 19 molecules gcat -1 per pulse). Also for H2, this is true both for the peak height (Figure ) and the integrated pulse intensity supporting our above conclusion (section 3.1) of constant H2 pulse shapes. This decay follows that of the Ar pulses and is therefore mainly due to the intensity decay of the incoming pulses. Therefore, consumption of H2 occurs mainly in the first 40 pulses of the subsequence. In the second and third subsequence, this general trend is repeated at a slightly lower level of the pulses. This results in an overall decay of the H2 pulses from the first maximum to the end of the experiment by about 22% (final pulse size 9.3•10 18 molecules gcat - 1 per pulse), i.e., identical to the decay of the Ar pulses.