Unraveling the Dynamic Nanoscale Reducibility (Ce 4+ → Ce 3+ ) of CeO x –Ru in Hydrogen Activation

The interaction of molecular hydrogen with ceria is of important relevance for heterogeneous catalysis related to green chemistry and renewable energy. Here, the complex structural transformations of a well‐defined cerium oxide model catalyst are followed in situ and in real time when exposed to a reactive H 2 environment. By using electron spectromicroscopy and diffraction with chemical and structural sensitivities, it is demonstrated that the transition from CeO 2 to crystalline Ce 2 O 3 occurs through a mixture of transient, coexisting phases on the nanoscale. The findings establish a clear relationship between structure and functionality for hydrogen dissociation over ceria(111), bearing profound implications on the nature of the reduction (Ce 4+ → Ce 3+ ) and mechanism for H 2 scission.