For the H2 reactant, the pulses are much less intense than the Ar or CO2 signals (Figure ), which in addition to a low sensitivity factor also points to a reactive consumption of H2 (see section 3.2). The H2 pulse shapes are significantly narrower than those of the CO2 related signals (Figure and), which is mainly due to the lower mass of H2, leading to a faster diffusion through the catalyst bed. Furthermore, contributions from pulse broadening due to slow adsorptiondesorption processes while passing through the catalyst bed must be low, which is consistent with expectations for the lifetime of adsorbed hydrogen under present reaction conditions (190 °C) . Here it is important to note that due to the very narrow H2 pulse shape, gas phase H2 is only available in the catalyst bed in the beginning of the CO2 pulses. Hence, only in this phase it can act as reactant.