Lanthanide oxides (Sm2O3, La2O3, especially CeO2) were more effective for ammonia synthesis when used as the support of ruthenium catalysts rather than when used as the dopant. The activity of Ru/lanthana was superior to other metal oxide-supported ones such as Ru/MgO. The activity, Ru dispersion, BET surface area, and the amount of hydrogen consumption of Ru/CeO2 were influenced remarkably by hydrogen treatment, suggesting the SMSI phenomena. Partially reduced CeO2 donates electrons to Ru atoms, forming the active sites.
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Niwa et al. (1996) studied this question.