Pt/CeO₂ and Pt/CeSnO_x 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/CeO_x and Pt/CeSnO_x 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 CeSnO_x 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/CeO_x, whereas only the state Pt²⁺ of platinum could be detected in the Sn-modified catalyst Pt/CeSnO_x. Insertion of Sn ions into the Pt/CeO_x lattice destabilizes/reduces Pt⁴⁺ cations in the Pt/CeSnO_x 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/CeO_x. 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.