The local structure of PdxCe1−xO2−x−δ solid solutions

Abstract

Pd_xCe_1−xO_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 Ce⁴⁺ ions by Pd²⁺ ions in Pd_xCe_1−xO_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 Pd²⁺ 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 A₁ vibrational mode of Pd²⁺ ions in [PdO₄] subunits. The binding energy of Pd3d_5/2 (E_b(Pd3d_5/2)) for the Pd²⁺ ion in the CeO₂ 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 Pd²⁺ ions with respect to the position of Ce⁴⁺ 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 PdO₄ 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_xCe_1−xO_2−x−δ solution leads to a decrease in the formation energy and to additional stabilization of palladium in the CeO₂ matrix. The formation of Pd_xCe_1−xO_2−x−δ solid solutions is accompanied by the dispersing effect caused by distortions of the fluorite structure induced by Pd²⁺ ions. The coprecipitation method, which allows Pd²⁺ 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_xCe_1−xO_2−x−δ solid solution.