Finally, in Figure we plotted the relative conversions of CO2 and H2 to CH4 and H2O molecules over the entire sequence, which were derived from the data in Figure . The number of generated CH4 molecules per pulsed CO2 molecule was approx. 0.05 in the beginning and then it decreased slightly to < 0.01 during the sequence. This is much less than the total consumption of CO2, which was about 60% in the first two pulses of the first subsequence, and then slowly decreased to 2% over the rest of the subsequence. In the beginning of the second and third subsequence, 40% of the CO2 pulsed was consumed in the initial pulses, then the conversion decayed to 2% and 0.1%, respectively. Obviously, the pristine surface, before exposure to H2/CO2/Ar pulses, is highly reactive for the build-up of adsorbed carbon and hydrogen containing adsorbed species, which we attribute to a high availability of very reactive sites. After saturation of these sites, further build-up of little reactive adsorbed species is rather slow. CH4 formation, in contrast, increased over more pulses (about 8). Most simply, this can be explained in a picture where reactive carbon-containing adsorbed intermediates have to accumulate initially, which can then react with pulsed H2. Based on spectroscopic and theoretical studies, these adspecies may be formates, bicarbonates, CO, or alkyl groups, which can act as intermediates of the CO2 methanation reaction and are sequentially hydrogenated to finally form CH4 . Adsorbed bicarbonates were concluded as reaction intermediates for CO2 methanation over Ru/Al2O3 catalysts at 260 °C in a recent transient, isotopic exchange FTIR study . This also means that the active sites for the formation of stable adsorbed side products are not identical with the active sites and intermediates for CH4 formation, since the latter sites are largely occupied after the first few pulses. One possibility would be sites on the support for the rather stable adsorbed species and interface sites for the reaction intermediates . Furthermore, we postulate that the formation of H2O is dominated by processes such as the reactive formation of adsorbed species from CO2, reaction or background reactions, rather than by CO2 methanation, which contributes only 2 H2O per CH4 product.