Oxygen Vacancy-Controlled Reactivity of Hydroxyls on an FeO(111) Monolayer Film

The reactivity of surface hydroxyls on FeO (111) monolayer films on Pt(111) with different oxygen vacancy concentrations has been investigated by means of X-ray photoelectron spectroscopy, thermal desorption spectroscopy, low energy electron diffraction, and density functional theory calculations. Surface hydroxyls on the FeO(111) monolayer films undergo two types of surface reactions: one type is surface reactions to form H2O and create oxygen vacancies; the other is surface reactions to form H-2. Surface reactions to form H2O and create oxygen vacancies are preferred for surface hydroxyls on the stoichiometric FeO (111) monolayer film but get suppressed with the increasing of the oxygen vacancy concentration on the FeO(111) rnonolayer film. On the FeO0.67(111) monolayer film, surface hydroxyls prefer surface reactions to form H-2. The accompanying DFT calculation results demonstrate that the thermodynamically favorable reaction between two OH(a) switches from the surface reaction to form H2O and oxygen vacancies on the stoichiometric FeO(111) monolayer film to the surface reaction to form H-2 on the FeO0.75(111) monolayer film. These results reveal a novel concept of oxygen vacancy-controlled reactivity of surface hydroxyls in which the thermodynamically favorable reactions switch from reactions to form H2O and oxygen vacancies on the stoichiometric FeO(111) monolayer film to those to form H-2 on the partially reduced FeO0.75(111) monolayer film. The interplay between oxygen vacancies and surface hydroxyls that both exert great influence on the physical chemistry and reactivity of oxide surface will greatly deepen the fundamental understanding of the relevant heterogeneous catalytic reaction systems involving transitional metal oxides.