Hydrogen spillover monitored by resonant photoemission spectroscopy

► Hydrogen spillover leads to net transfer of hydrogen atoms on Pt/CeO2(1 1 1). ► Hydrogen spillover gives rise to reversible changes of cerium oxidation state. ► Reduction of ceria is reversible in the temperature region between 150 and 350 K. ► Reduction of ceria is irreversible in the temperature region between 350 and 700 K.