The local structure of PdxCe1−xO2−x−δ solid solutionsElectronic supplementary information (ESI) available. See DOI: 10.1039/c4cp01033g

Pd x Ce 1− x O 2− x − δ solid solutions, which are highly efficient catalysts for the low-temperature oxidation of carbon monoxide, were examined using a set of structural (XRD-PDF, HRTEM, XRD) and spectral (XPS, Raman spectroscopy) methods in combination with quantum-chemical calculations. A comparison of the experimental results and pair distribution function (PDF) modeling data enabled reliable verification of the model of non-isomorphic substitution of Ce 4+ ions by Pd 2+ ions in Pd x Ce 1− x O 2− x − δ solid solutions. Palladium ions were shown to be in a near square planar environment with C 4v symmetry, which is typical for Pd 2+ ions. Such a near square planar environment was revealed by Raman spectroscopy due to the appearance of the band at ω = 187 cm −1 , which corresponds to the A 1 vibrational mode of Pd 2+ ions in [PdO 4 ] subunits. The binding energy of Pd3d 5/2 ( E b (Pd3d 5/2 )) for the Pd 2+ ion in the CeO 2 lattice is 1 eV higher than that of E b (Pd3d 5/2 ) for PdO oxide due to a decrease in the Pd-O distances and the formation of more ionic bonds because of the displacement of Pd 2+ ions with respect to the position of Ce 4+ ions in the fluorite structure. Five structural models of solid solutions are considered in this work. As demonstrated by the DFT calculations, the most realistic model is based on the displacement of palladium ions leading to a near square planar PdO 4 environment, which includes water molecules stabilizing the region of anion vacancies in their dissociated state as two hydroxyl groups. The introduction of water molecules in the composition of the Pd x Ce 1− x O 2− x − δ solution leads to a decrease in the formation energy and to additional stabilization of palladium in the CeO 2 matrix. The formation of Pd x Ce 1− x O 2− x − δ solid solutions is accompanied by the dispersing effect caused by distortions of the fluorite structure induced by Pd 2+ ions. The coprecipitation method, which allows Pd 2+ ions to be introduced at the stage of fluorite structure formation, was demonstrated to be the optimal method for the synthesis of a homogeneous Pd x Ce 1− x O 2− x − δ solid solution. In this paper, physical methods in combination with quantum chemistry calculations are used to study the local structure of Pd x Ce 1− x O 2− δ solid solutions.