Consequently, the relative content of 16 O in these states increases quickly with increasing stability and hence with increasing desorption temperature, which results in a rapid increase of C 16 O2 formation from these species in the TPD on the expenses of C Finally, we also detected the evolution of water (H2 16 O) in this experiment in a broad peak, whose shape closely resembles that for C 16 O evolution. Considering the high desorption temperature this must also stem from decomposition of adspecies, possibly in processes that also result in CO evolution. It is interesting to note that water formation is much more pronounced in this TPD experiment than in that performed after H2/CO2 pulsing. Apparently, the large amounts of H2 in the pulses lead to adsorbed products that upon thermal activation mainly form H2 rather than H2O, while the surface species either present already before CO2 pulsing or formed upon pure CO2 pulsing, by reaction with hydrogen present on the surface, mainly decompose to H2O. Such decomposition pathway would be expected, e.g., for thermal activation of hydroxyl species, while H2 formation may be more characteristic for decomposition of species such as adsorbed bicarbonates, formates of CHx species.