The effect of potassium was tested with unpromoted and K-promoted Fe-Al-O oxide and Fe5C2 carbide materials, formed during CO2 hydrogenation from Fe-Al-O spinel. Each one of the tested catalysts contained a single phase (oxide or carbide). 2 wt% potassium enhanced the RWGS rate of reaction on the oxide phase tenfold compared with the unpromoted oxide. This correlated with increase of Fe2+/Fe3+ ratio (XPS) determining oxygen vacancies as active sites for RWGS. Potassium suppressed the methanation rate on the carbide catalyst by a factor of five and increased the CO FTS rate to C2+, hydrocarbons by a factor of 1.4. EFTEM images and elemental profiles of unpromoted carbide nanocrystals measured after testing in CO hydrogenation displayed an amorphous surface layer enriched with oxygen. Potassium stabilized the surface iron atoms in reduced form. The observed effects of potassium on carbide phase were explained by a model implementing carbon vacancies for methanation and near-metallic iron atoms for FTS. (C) 2017 Elsevier Inc. All rights reserved.