Iron-comprising heterogeneous catalyst and a process for producing it, which comprises the steps of thermal decomposition of gaseous iron pentacarbonyl to give carbonyl iron powder having spherical primary particles, treatment of carbonyl iron powder with hydrogen, resulting in the metallic spherical primary particles at least partially forming agglomerates, contacting the agglomerates with iron pentacarbonyl, and thermal decomposition of the iron pentacarbonyl to give at least predominantly pore-free and void-free secondary particles.