The catalytic activity of metals supported on oxides depends on their charge and oxidation state. Yet, the determination of the degree of charge transfer at the interface remains elusive. Here, by combining density functional theory and first-principles
molecular dynamics on Pt single atoms deposited on the CeO2 (100) surface, we show that the common representation of a static metal charge is oversimplified. Instead, we identify several well-defined charge states that are dynamically intercon-
nected and thus coexist. The origin of this new class of strong metal–support interactions is the relative position of the Ce(4f) levels with respect to those of the noble metal, allowing electron injection to (or recovery from) the support. This process is
phonon-assisted, as the Ce(4f) levels adjust by surface atom displacement, and appears for other metals (Ni) and supports (TiO2). Our dynamic model explains the unique reactivity found for activated single Pt atoms on ceria able to perform CO oxida-
tion, meeting the Department of Energy 150 °C challenge for emissions.