Abstract
Nanostructured Pt-CeO2 films with low Pt loading show high activity and stability as anode catalysts in proton-exchange membrane fuel cells. Under electrochemical conditions, the noble metal in the catalyst films can be reversibly converted between two chemical states, an atomically dispersed Pt2 + species and subnanometer Pt particles.
The nature of these states and the mechanism of their interconversion have been investigated combining surface science and electrochemical experiments. The local structure of the Pt2 + species, their stability, and reactivity were studied by means of synchrotron radiation photoelectron spectroscopy and resonant photoemission spectroscopy under ultrahigh vacuum conditions in combination with density functional modeling.
We employed surface science-based model systems of different complexity to probe the reactivity of the atomically dispersed Pt2 + species in the absence of other species such as Pt4 +, metallic Pt, or oxygen vacancies. It was found that the conversion of Pt2 + to subnanometer Pt particles is triggered by a redox coupling with Ce3 + centers generated through the formation of oxygen vacancies or by charge transfer between the metal and the support.
These findings characterize the Pt-CeO2 material as a structurally highly dynamic catalyst which attains its high stability from the ability to adapt to the changes in the operation conditions.