Fe-Cu catalysts were applied in the production of high-calorie synthetic natural gas (HC-SNG), wherein a proper level of C-2-C-4 hydrocarbon selectivity must be secured. The Fe-Cu catalysts were activated by reduction under diluted CO gas before the reaction, and their catalytically active Fe phases changed according to the reduction temperature: Fe3O4 formed when reduced at 300 degrees C, carbon-deficient FeCx at 400 degrees C and Fe3C at 500 degrees C. Iron carbide catalysts achieved stronger CO adsorption and higher BET surface area than Fe3O4 catalysts, which resulted in higher CO conversion. The carbon-deficient FeCx, was metallike in its electron structure due to the low number of bonded carbons, and it was attributed to the highest CO and H-2 conversion of FC15-400R by providing H-2 activation ability. The C-n (n >= 2) selectivity or carbon chain growth of the hydrocarbons increased as the carburization degree of the active Fe phase increased, which was associated with an increase in the CO chemisorption strength. The impregnated Cu exerted little influence on the product selectivity, but it promoted hydrogen adsorption, thereby improving the paraffin-to-olefin ratio of the produced hydrocarbons. (C) 2016 Elsevier B.V. All rights reserved.