The CH3 signal is by far the smallest in this measurement. Nevertheless, the typical pulse shape can be identified. Both the initial and final CH3 pulses are relatively narrow, with the general pulse shape closely following that of the H2 signal (Figure ). This indicates that CH4 is formed from the interaction of H2 with carbon containing adsorbed species that are either formed in the initial phase of the H2/CO2/Ar pulse or that were present already on the catalyst surface, e.g., upon reactive formation in preceding pulses. In the latter case these carbon containing ad-species must represent relatively stable intermediates in the CO2 methanation reaction.