Redox and Catalytic Properties of Rh_<i>x</i>Ce_1–<i>x</i>O_2−δ Solid Solution

In this work, a detailed study of the redox properties of solid solution Rh_<i>x</i>Ce_1–<i>x</i>O_2−δ in correlation with its catalytic activity in CO oxidation reaction was carried out. The ex situ X-ray photoelectron spectroscopy technique was applied to follow the charging states of the elements on the surface during the redox treatments at a temperature range of 25–450 °C. The results were compared with the data of temperature-programmed reduction by CO. The dissolution of rhodium in the ceria bulk considerably increased the mobility of CeO₂ lattice oxygen, with redox transitions Ce⁴⁺ ↔ Ce³⁺ and Rh³⁺ ↔ Rh_<i>n</i>^δ+ observed already at low temperatures (below 150 °C). The reduced rhodium clusters (Rh_<i>n</i>^δ+) formed during the reduction treatment significantly improved the catalytic activity of the Rh_<i>x</i>Ce_1–<i>x</i>O_2−δ solid solution. The small size of the rhodium clusters (Rh_<i>n</i>^δ+) and high defectiveness of the fluorite phase provided the reversibility of Rh_<i>n</i>^δ+/CeO₂ ↔ Rh_<i>x</i>Ce_1–<i>x</i>O_2−δ transitions upon redox treatment, resulting in the high reproducibility of the CO conversion curves in the temperature-programmed reaction CO + O₂. The homogeneous solid solution was stable up to 800 °C. Above this temperature, the CeO₂ volume was depleted of Rh³⁺ ions because of their partial segregation into the surface and/or subsurface layers with the formation of Rh₂O₃. For these inhomogeneous samples, the oxygen mobility was considerably lower, while the redox transitions, Ce⁴⁺ ↔ Ce³⁺ and Rh³⁺ ↔ Rh_<i>n</i>^δ+, required higher temperatures