Highly enhanced concentration and stability of reactive Ce ³⁺ on doped CeO ₂ surface revealed in operando

Trivalent cerium ions in CeO ₂ are the key active species in a wide range of catalytic and electro-catalytic reactions. We employed ambient pressure X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy to quantify simultaneously the concentration of the reactive Ce ³⁺ species on the surface and in the bulk of Sm-doped CeO ₂(100) in hundreds of millitorr of H ₂-H ₂O gas mixtures. Under relatively oxidizing conditions, when the bulk cerium is almost entirely in the 4+ oxidation state, the surface concentration of the reduced Ce ³⁺ species can be over 180 times the bulk concentration. Furthermore, in stark contrast to the bulk, the surface's 3+ oxidation state is also highly stable, with concentration almost independent of temperature and oxygen partial pressure. Our thermodynamic measurements reveal that the difference between the bulk and surface partial molar entropies plays a key role in this stabilization. The high concentration and stability of reactive surface Ce ³⁺ over wide ranges of temperature and oxygen partial pressure may be responsible for the high activity of doped ceria in many pollution-control and energy-conversion reactions, under conditions at which Ce ³⁺ is not abundant in the bulk.