CARBON-DIOXIDE HYDROGENATION OF FE-MN OXIDE CATALYST DOPED WITH RH AND LA

The hydrogenation of CO2 was investigated on a pure Fe-Mn oxide catalyst and on catalysts doped with Rh and La in a pulse microreactor. Temperature-programmed reduction of the catalysts showed that Rh addition led to a much faster and more complete reduction, by a probably catalytic effect of Rh, whereas the addition of La led to a slower and less complete reduction than in the case of the pure Fe-Mn catalyst. This was also shown by XPS investigation of the surfaces of the reduced catalysts. Additionally, XPS showed an enrichment of the surfaces with Mn on all three catalysts and a strong increase in La surface concentration in comparison to the bulk of the La-containing catalyst. The formation of a higher amount of OH species was observed on the La-containing catalyst than on the other ones after reduction. Three subsequent steps were distinguished during hydrogenation of CO2 pulses: a fast methanation connected with CO evolution, and two slower steps related to hydrogenation of carbonaceous surface species. The proportion of fast methanation was highest on Fe-Mn-Rh and absent on the Fe-Mn-La catalyst. A mechanism similar to the hydrogenation of CO combined with hydrogenolysis was postulated for the Fe-Mn and Fe-Mn-Rh catalysts. A different mechanism had to be assumed for the CO2 hydrogenation on the Fe-Mn-La catalyst.