Structure of the catalytically active copper–ceria interfacial perimeter

Cu/CeO₂ catalysts are highly active for the low-temperature water–gas shift—a core reaction in syngas chemistry for tuning the H2/CO/CO₂ proportions in feed streams—but the direct identification and quantitative description of the active sites remain challenging. Here we report that the active copper clusters consist of a bottom layer of mainly Cu⁺ atoms bonded on the oxygen vacancies (O_v) of ceria, in a form of Cu⁺–O_v–Ce³⁺, and a top layer of Cu0 atoms coordinated with the underlying Cu⁺ atoms. This atomic structure model is based on directly observing copper clusters dispersed on ceria by a combination of scanning transmission electron microscopy and electron energy loss spectroscopy, in situ probing of the interfacial copper–ceria bonding environment by infrared spectroscopy and rationalization by density functional theory calculations. These results, together with reaction kinetics, reveal that the reaction occurs at the copper–ceria interfacial perimeter via a site cooperation mechanism: the Cu⁺ site chemically adsorbs CO whereas the neighboring O_v–Ce³⁺ site dissociatively activates H₂O.