2. The narrow pulse width of CH4, comparable to that of H2 and much narrower than that of CO2, indicates that CH4 is formed from freshly pulsed H2 and carbon containing adsorbed species such as bicarbonates or CHx,ad. These carbon containing ad-species, which can be considered as intermediates in the CO2 methanation reaction, are either formed in the initial phase of the H2/CO2/Ar pulse or were present already on the catalyst surface, e.g., upon reactive formation in preceding pulses. In the latter case they must be sufficiently stable survive the time between subsequent pulses.