Iron carbide phase formation during carburization of alpha-Fe catalysts were studied using in situ XRD, where the effect of varying carbon chemical potentials of CO/syngas/C2H4/C2H2 gases and potassium promoter on the carburization behaviors was studied. The actual carburization capability of these gases follows an order of 2% CO/He < 2% C2H4/He < 2% CO/8% H-2/He < 2% C2H2/He, which does not correlate well with the theoretical carbon chemical potential. In addition, the carburization rate and the formation of carbon-rich iron carbides are favored by potassium under CO or syngas, while inhibited by potassium under C2H4 or C2H2. Temperature programmed carburization together with pulse experiments and DFT calculations indicate that the potassium promoter could weaken the adsorption ability of the C2H4 and C2H2 and thus reduce their carburization capabilities. (C) 2019 Elsevier Inc. All rights reserved.