The CO2 pulses are significantly broader than the Ar pulses (Figure ), and the maximum is shifted to later times. Considering the comparable molecular weight of both species, this must be due to more pronounced contributions from adsorption -desorption processes, reflecting a stronger interaction of CO2 with the catalyst surface than Ar . Also for CO2, the pulse shapes do not change during the sequence, while the pulse intensity decreases slightly less than the decay of the Ar pulses (Figures and). Note that this decay refers from the maximum intensity after a few pulses to the end of the sequence, and not from the first pulse to the last pulse. This way, we correct for the considerable consumption of CO2 in the first pulses The width and shape of the m/z = 16 and m/z = 28 pulses largely resemble those of the CO2 pulses (Figure ), which is consistent with a picture that they mainly arise from fragmentation of CO2 in the mass spectrometer rather than from CO and CH4 species present in the gas phase. This assignment, which is also supported by the fact that their intensity decay resembles that of the CO2 pulses, will be discussed further in the next section.