Spatially resolved analysis of CO2 hydrogenation to higher hydrocarbons over alkali-metal promoted well-defined FexOyCz

A series of catalysts for CO2 hydrogenation to higher (C2+) hydrocarbons were prepared through controlled decomposition of iron (II) oxalate dihydrate impregnated with Li, Na, K, Rb or Cs carbonate. They consist of Fe3O4, Fe5C2, Fe3C and Fe. Their steady-state spatial phase distribution is, however, affected by the presence and the kind of the promoter. These structural characteristics determine catalyst activity and product selectivity. Fe3C seems to favor CH4 formation, while C2+-hydrocarbons are formed over Fe5C2. A positive relationship between the rate of C2+-hydrocarbons formation and the rate constant of dissociation of adsorbed CO2 was established, while an opposite dependence is valid for the rate of CO2 methanation. The rate constant of this reaction increases in the presence of alkali metal and with an increase in its atomic number. Therefore, CH4 is mainly formed over Rb- or Cs-doped catalysts through CO hydrogenation, while CO2 methanation prevails over the remaining catalysts.