4. Parallel to the initial build-up of these reactive intermediates, also little reactive or nonreactive adsorbed species are formed, which is responsible for most of the CO2 consumption. These species and the active sites for their formation must be different from the adsorbed reaction intermediates and the active sites for their formation, otherwise the rapid increase of the amount of non-reactive species on the surface would reduce the activity for CH4 formation much faster than observed experimentally. Finally, the fact that similar trends are observed also in the subsequent subsequences, indicates that part of the adsorbed species can desorb in the absence of H2/CO2/Ar pulses, pointing to reversible adsorption of these species on a time scale of 5 min.