to Figure . Typical for the CO methanation, an overshoot in CH4 formation can be observed after the switch from CO/H2 to pure H2, caused by the hydrogenation of reactive carbon species remaining on the catalyst surface. The general temporal progression of the CH4 formation is comparable to the CO2 methanation. As for CO2 methanation the last 10 s of the pseudosteady-state were averaged for the comparison to steady-state experiment. In Figure the CH4 formation rate over TOS for steady-state operation as well as for dynamic operation with two different period lengths (240 s and 120 s) is depicted. The overall decrease in activity after 12 h TOS of the three cases is approximately the same with 42 to 44 %, which is significantly higher than for CO2 methanation, and which was also discovered in early studies . This is probably due to stronger coke formation by the Boudouard reaction, which is negligible during the CO2 methanation. It is known from early studies that the deactivation of Nickel catalysts in CO methanation typically follows a rate law for the activity, which is first order in both the activity itself and the CO partial pressure (cf. Eq. 9). The initially lower deactivation rate may be due to hydrogenation of carbon species on the catalyst surface during the H2-rich half-period, which would not be reactivated under steadystate CO methanation conditions. As these species are therefore not removed during steadystate, they continue to block the adsorption sites for H2 and may even form less reactive or inert carbon deposits. The deactivation of Nickel catalysts in CO methanation were extensively studied by show that the carbon-catalyst interaction are complex and indeed reversible under reaction conditions for most of the formed carbon .