The nature of the involved surface carbon species was discussed by Vasiliades et al. , by combining SSITKA and operando DRIFTS measurements. Here, three different carbon species were observed: highly reactive Cα consisting of adsorbed CO and CHx intermediates, less reactive Cβ consisting of CxHy intermediates, as well as an inert Cγ species, consisting of polymeric carbon, which contributes to deactivation and is only hydrogenated at higher temperatures. This supports our hypothesis, that Cβ is involved in the chain growth reaction, while Cα mainly contributes to the CH4 formation. The Cγ formation is irreversible under the present conditions and even slower than that of Cβ. Hence, the investigation of Cγ requires an adaption of the PTK method and the evaluation of consecutive periods before the limit cycle is achieved. However, we suspect a gradual conversion of Cβ to Cγ over time, as the decrease of reaction rate due to Cβ accumulation smoothly transitions to a deactivation in longer steady-state experiments. Elemental surface balance