Unusual Redox Behavior of Doped Ceria Systems in H₂: Consequences for CO₂ Electrolysis

Ceria and doped ceria are well-known materials in heterogeneous catalysis due to their ability to catalyze numerous redox reactions. They are a promising choice for solid oxide-based CO₂ electrolysis cathodes due to their mixed ionic-electronic conducting nature. In this unprecedented study, we uncover the transformative role of hydrogen in CO₂ reduction reactions on ceria and doped ceria catalysts, which is pivotal for solid oxide-based electrolysis. Using operando Raman spectroscopy, online mass spectrometry, and optical imaging, we reveal that hydrogen critically influences the oxidation states of cerium sites, which are vital for CO₂ reduction. Whereas in both pure ceria and its Gd-doped version, H₂ amplifies oxygen defect formation, Pr-doping uniquely eliminates these defects and leads to an apparent oxidation of the catalyst in H₂ environments. First-principles calculations elucidate that hydrogen incorporation promotes the formation of oxygen vacancies in CeO_x and Ce{Gd}O_x but paradoxically stabilizes oxygen sites in Ce{Pr}O_x. Upon exposure to H₂, this leads to an extended ordered arrangement of oxygen atoms within the Ce{Pr}O_x lattice, which manifests itself as an apparent oxidation of the material. This breakthrough sheds new light on the intricate dynamics between hydrogen and doped ceria catalysts, opening up novel avenues for optimizing the CO₂ reduction processes.