Co2C, an emerging catalyst for the conversion of syngas to oxygenates, shows support-sensitive behavior that has not yet been fully explained. Here, we characterize Co catalysts modified with ZnO atomic layer deposition on SiO2, carbon, CeO2, and Al2O3 supports. We find that under syngas conditions, ZnO-promoted Co transforms into Co2C on SiO2, carbon, and CeO2, but not on Al2O3. Moreover, the support affects the extent of carburization: while the SiO2-supported catalyst carburizes completely, carbon- and CeO2-supported catalysts show incomplete conversion of Co to Co2C. These three catalysts also exhibit different oxygenate selectivities. In contrast, the modified Al2O3-supported catalyst retains the Fischer-Tropsch catalytic properties of metallic Co. By depositing increasing amounts of Al2O3 by ALD on the SiO2 support, decreasing Co2C formation and oxygenate selectivity occurs. In-situ XANES reveals that Al2O3 prevents Co2C formation by enabling the ZnO to restructure into ZnAl2O4 during reduction. Thus, in addition to modifying the active catalyst phase, the promoter can also strongly interact with the support, significantly impacting catalyst performance.