CH4 formation is characterized by extremely weak m/z = 15 pulses, with an initial increase in the first 6 pulses, up to 2•10 15 molecules gcat -1 per pulse. Subsequently, it slowly declines to < 1•10 15 molecules gcat -1 per pulse over the next 40 pulses, where it is hardly detectable any more, and then remains at this level. In the next two subsequences this behavior is repeated at even lower levels. At the end of the experiment, less than 0.03% of the incoming CO2 molecules are converted into CH4. Here it is interesting to note that the maximum of methane formation is reached only after 6 pulses, while the maximum consumption of CO2 and H2 is at the very beginning of the sequence. Apparently, during the first few pulses other side products or reaction intermediates are preferentially formed, most likely adsorbed carbon and hydrogen containing species.