Pt/CeO2 and Pt/CeSnOx catalysts for low-temperature CO oxidation prepared by plasma-arc technique

We applied a method of plasma arc synthesis to study effects of modification of the fluorite phase of ceria by tin ions. By sputtering active components (Pt, Ce, Sn) together with carbon from a graphite electrode in a helium ambient we prepared samples of complex highly defective composite PtCeC and PtCeSnC oxide particles stabilized in a matrix of carbon. Subsequent high-temperature annealing of the samples in oxygen removes the carbon matrix and causes the formation of active catalysts Pt/CeOx and Pt/CeSnOx for CO oxidation. In the presence of Sn, X-Ray Diffraction (XRD) and High-Resolution Transmission Electron Microscopy (HRTEM) show formation of a mixed phase CeSnOx and stabilization of more dispersed species with a fluorite-type structure. These factors are essential for the observed high activity and thermic stability of the catalyst modified by Sn. X-Ray Photoelectron Spectroscopy (XPS) reveals the presence of both Pt2+ and Pt4+ ions in the catalyst Pt/CeOx, whereas only the state Pt2+ of platinum could be detected in the Sn-modified catalyst Pt/CeSnOx. Insertion of Sn ions into the Pt/CeOx lattice destabilizes/reduces Pt4+ cations in the Pt/CeSnOx catalyst and induces formation of strikingly high concentration (up to 50% at.) of lattice Ce3+ ions. Our DFT calculations corroborate destabilization of Pt4+ ions by incorporation of cationic Sn in Pt/CeOx. The presented results show that modification of the fluorite lattice of ceria by tin induces substantial amount of mobile reactive oxygen partly due to affecting geometric parameters of ceria by tin ions

Pt/CeO2 and Pt/CeSnOx Catalysts for Low-Temperature CO Oxidation Prepared by Plasma-Arc Technique

We applied a method of plasma arc synthesis to study effects of modification of the fluorite phase of ceria by tin ions. By sputtering active components (Pt, Ce, Sn) together with carbon from a graphite electrode in a helium ambient we prepared samples of complex highly defective composite PtCeC and PtCeSnC oxide particles stabilized in a matrix of carbon. Subsequent high-temperature annealing of the samples in oxygen removes the carbon matrix and causes the formation of active catalysts Pt/CeOx and Pt/CeSnOx for CO oxidation. In the presence of Sn, X-Ray Diffraction (XRD) and High-Resolution Transmission Electron Microscopy (HRTEM) show formation of a mixed phase CeSnOx and stabilization of more dispersed species with a fluorite-type structure. These factors are essential for the observed high activity and thermic stability of the catalyst modified by Sn. X-Ray Photoelectron Spectroscopy (XPS) reveals the presence of both Pt2+ and Pt4+ ions in the catalyst Pt/CeOx, whereas only the state Pt2+ of platinum could be detected in the Sn-modified catalyst Pt/CeSnOx. Insertion of Sn ions into the Pt/CeOx lattice destabilizes/reduces Pt4+ cations in the Pt/CeSnOx catalyst and induces formation of strikingly high concentration (up to 50% at.) of lattice Ce3+ ions. Our DFT calculations corroborate destabilization of Pt4+ ions by incorporation of cationic Sn in Pt/CeOx. The presented results show that modification of the fluorite lattice of ceria by tin induces substantial amount of mobile reactive oxygen partly due to affecting geometric parameters of ceria by tin ions

Structural insight into strong Pt-CeO 2 interaction: from single Pt atoms to PtOx clusters

Pt-CeO2 nanocomposites were obtained by coprecipitation, varying the Pt loading over a wide range of 1-30 wt %. The samples were calcined in air at 450-1000 °C. The Pt-CeO2 nanocomposites were investigated by a set of structural (X-ray diffraction, extended X-ray absorption fine structure (EXAFS), pair distribution function (PDF), and transmission electron microscopy) and spectroscopic (X-ray photoelectron spectroscopy and Raman spectroscopy) methods. Over the whole range of Pt loading, the main species were Pt2+ and Pt4+. They were localized either in a single-atom state or in the form of PtOx clusters on the ceria surface. The joint PDF and EXAFS modeling based on the combination of [Pt2+O4] single-atom and Pt3O4 structural fragments allowed us to propose the local structure of the PtOx clusters. The formation of such surface structures is associated with a distorted ceria surface on the Pt-CeO2 nanocomposites. We assume that the close arrangement of platinum ions in the PtOx clusters could be responsible for the effective redox properties of the samples

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

Transformation of a Pt–CeO2 mechanical mixture of pulsed‐laser‐ablated nanoparticles to a highly active catalyst for carbon monoxide oxidation

The pulsed laser ablation (PLA) in alcohol and water media was employed to prepare Pt and CeO2 PLA‐nanoparticles of different sizes and degrees of defectiveness. Interactions of metallic platinum and ceria particles were studied using the thermal activation of Pt–CeO2 mechanical mixtures in the CO+O2 reaction medium or O2 atmosphere. The thermal activation resulted in oxidized Pt2+/Pt4+ states of platinum in the surface solid solutions PtCeOx and/or PtOx clusters. Catalysts formed after calcination of the PLA‐ablated Pt–CeO2 mixtures in oxygen at 450–600 °C revealed CO conversion at very low temperatures up to 70 % depending on the conditions of PLA particles preparation and thermal activation of Pt–CeO2 mechanical mixture