The local structure of Pd sub(x)Ce sub(1-x)O sub(2-x- delta ) solid solutions

Pd sub(x)Ce sub(1-x)O sub(2-x- delta ) 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 super(4+) ions by Pd super(2+) ions in Pd sub(x)Ce sub(1-x)O sub(2-x- delta ) solid solutions. Palladium ions were shown to be in a near square planar environment with C sub(4v) symmetry, which is typical for Pd super(2+) ions. Such a near square planar environment was revealed by Raman spectroscopy due to the appearance of the band at omega = 187 cm super(-1), which corresponds to the A sub(1) vibrational mode of Pd super(2+) ions in [PdO sub(4)] subunits. The binding energy of Pd3d sub(5/2) (E sub(b)(Pd3d sub(5/2))) for the Pd super(2+) ion in the CeO sub(2) lattice is 1 eV higher than that of E sub(b)(Pd3d sub(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 super(2+) ions with respect to the position of Ce super(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 sub(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 sub(x)Ce sub(1-x)O sub(2-x- delta ) solution leads to a decrease in the formation energy and to additional stabilization of palladium in the CeO sub(2) matrix. The formation of Pd sub(x)Ce sub(1-x)O sub(2-x- delta ) solid solutions is accompanied by the dispersing effect caused by distortions of the fluorite structure induced by Pd super(2+) ions. The coprecipitation method, which allows Pd super(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 sub(x)Ce sub(1-x)O sub(2-x- delta ) solid solution.