Unusual Hydrogen-mediated Oxidation in Doped Ceria Systems: Operando Insights

Ceria and doped-ceria are well-known material in heterogeneous catalysis due to their ability to catalyse are large number of redox reactions. They are a promising choice for solid oxide-based CO2 electrolysis cathodes due to their mixed ionic-electronic conducting nature. Gd and Pr doping is known to significantly improve both electrochemical and thermochemical reactions involving CO2 reduction. H2 in the feed stream was found to play a critical and unusual role in these reactions. This study systematically investigates the hydrogen’s impact on CO2 reduction within CeOx, Ce{Pr}Ox, and Ce{Gd}Ox. Operando Raman spectroscopy, combined with online mass spectrometry and optical imaging, revealed that CO production is linked to the reduction of Ce⁴⁺ to Ce³⁺ sites under bias. While dopants typically enhanced oxygen defect formation, Ce{Pr}Oₓ eliminated oxygen defects and tends to oxidize in H₂. First-principles calculations showed that hydrogen incorporation could stabilize or destabilize oxygen vacancies in the catalyst. It promotes reduction in CeOₓ and Ce{Gd}Oₓ, but counterintuitively stabilizes oxygen sites in Ce{Pr}Oₓ, leading to its oxidation. This study reports the first ever system which oxidized upon exposure to H2.