Artificially Tuning the Population of Ce ³⁺ on the Cerium Oxide Surface toward Model Catalytic Reactions

Cerium oxide materials have long been attractive catalysts across a wide range of applications, yet fundamental understanding of their surfaces under in situ conditions remains incomplete. Using near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), we demonstrate that Ce3+ species on cerium foil surfaces are tunable under various in situ conditions, including ultrahigh vacuum (UHV), 1 mbar O2, H2, and CO2 atmospheres. This delicate balance between Ce3+ and Ce4+ enables artificial manipulation of Ce3+ concentration and oxygen vacancies (Ovs) and results in automatic recovery of Ce3+ sites upon return to UHV. Interestingly, CO2 activation on the cerium oxide surface is facile and can be modulated by Ovs via in situ annealing, while Ce3+ species remain preserved after CO2 removal. In summary, our work provides insights into the reaction mechanisms of commonly used model catalysts and informs the rational design of high-performance materials.