Tuning the f Band for Enhanced Surface Redox in Strained Rare Earth Oxides

Reducible rare earth oxides (REO_2-x) are essential in catalysis due to their 4f band-governed surface redox properties, which influence crucial reactions such as hydrogen dissociation and water formation. However, correlating the 4f band structure with catalytic activity has been a long-standing challenge due to the complexities of manipulating and characterizing 4f electrons. Here, we demonstrate that tensile strain effectively modulates the 4f electronic structure, narrowing the band gap and activating surface oxygen, leading to enhanced redox activity. Using atomically flat ceria ultrathin films under up to a 7% biaxial strain range, we observed a five-fold increase in surface reaction kinetics via time-resolved ambient-pressure X-ray photoelectron spectroscopy. Complementary density functional theory calculations reveal that the tensile strain reduces energy barriers for key catalytic steps by narrowing the 4f-2p band gap. These findings highlight the RE 4f electronic structure as a critical descriptor for catalysis and demonstrate the utility of atomically flat model systems.