Effect of ceria surface facet on stability and reactivity of isolated platinum atoms

Well-defined surface structures and uniformity are key factors in exploring structure–activity relationships in heterogeneous catalysts. A modified atomic layer deposition method and three well-defined CeO₂ nanoshapes, octahedra with (111) surfaces, cubes exposing (100) facets, and rods with (100) and (110) surface facet terminations, were utilized to synthesize ultra-low loading Pt/CeO₂ catalysts and allow investigations on the influence of ceria surface facet on isolated Pt species under reducing conditions. A mild reduction temperature (150 °C) reduces the initial platinum ions present on the surfaces of the ceria support but preserves the isolated Pt atoms on all ceria surface facets. In contrast, a reduction temperature of 350 °C, reveals very different interactions between the initial single Pt atoms and the various ceria surface facets, leading to dissimilar and non-uniform Pt ensembles on the three ceria shapes. To isolate facet dependent Pt–CeO₂ interactions and avoid variations between Pt species, the Pt₁/CeO₂ catalysts after reduction at 150 °C were subjected to CO oxidation conditions. The isolated Pt atoms on the CeO₂ octahedra and cubes are less active in the CO oxidation reaction, compared with Pt on CeO₂ rods. In the case of Pt on the CeO₂ octahedra this is due to strongly bound CO blocking active sites together with a stable CeO₂(111) surface limiting the oxygen supply from the support. On the CeO₂ cubes, some Pt is not available for reaction and CO is bound strongly on the available Pt species. In addition, the Pt catalysts supported on the CeO₂ cubes are not stable with time on stream. The isolated Pt atoms on the CeO₂ rods are considerably more active under these conditions and this is due to a weaker Pt–CO bond strength and more facile reverse oxygen spillover from the defect-rich (110) surfaces of the rods due to the lower energy of oxygen vacancy formation on this CeO₂ surface. The Pt supported on the CeO₂ rods is also remarkably stable with time on stream. This work demonstrates the importance of using ultra-low loadings of active metal and well-defined oxide supports to isolate interactions between single metal atoms and oxide supports and determine the effects of the oxide support surface facet on the active metal at the atomic level.