A heterogeneous single-atom catalyst (SAC) based on a tiny Cu loading (0.05 wt.%) supported on CeO 2 À TiO 2 (Ce/Ti = 0.18) shows very high Cu mass-normalized CO oxidation activity, 100 % selectivity for CO 2 between 60 and 100 °C and essentially no deactivation during 17 h on stream in the preferential oxidation of CO (PROX) at 120 °C. Based on state of the art, this catalyst is among the most active and PROX-selective catalysts. Employing a combination of ex-situ and operando methods, we infer that the isolated Cu single sites are incorporated with redox-active Cu 2 + À OÀ Ce 4 + .Cu + À &À Ce 3 + moieties connected by labile oxygen and are exposed on the surface of highly dispersed ceria. This high dispersion is promoted by TiO 2 which itself does not participate in redox steps during PROX, as Ti ions remain essentially tetravalent while Cu and Ce undergo reversible redox shuttles.

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