RESUMO
The average structure and the local structure of nanocrystalline Rh-doped CeO2 catalysts, prepared using a co-precipitation method, were studied using a set of structural (PDF, HRTEM, XRD) and spectral (XPS, Raman spectroscopy) methods. The samples with Rh content less than 10 wt%, calcined at 450 °C, were homogeneous solid solutions. A comparison of the experimental results and Pair distribution function (PDF) modeling data showed that Rh3+ substitutes Ce4+ ions in the fluorite phase. Charge equilibrium is obtained by the oxygen vacancy for each Rh3+ cation introduced into the ceria cell. The solid solution demonstrated high catalytic activity in low-temperature CO oxidation (LTO CO). The solid solutions were stable only in a nanocrystalline state and decomposed upon thermal treatment. The calcination of the solid solution at T > 450 °C results in a decrease in the catalytic activity that is accompanied by Rh association in the subsurface area and strong distortion of the anionic subcell. At T = 800 °C α-Rh2O3 nanoparticles are formed on the surface of the fluorite phase. The XRD-detectable Rh oxide phases are formed after calcination at 1000 °C. However, some parts of Rh within the subsurface RhxCe1-xO2-δ solid solution remain and they preserve catalytic properties for low-temperature oxidation.
RESUMO
PdxCe1-xO2-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 PdxCe1-xO2-x-δ solid solutions. Palladium ions were shown to be in a near square planar environment with C4v 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 A1 vibrational mode of Pd(2+) ions in [PdO4] subunits. The binding energy of Pd3d5/2 (Eb(Pd3d5/2)) for the Pd(2+) ion in the CeO2 lattice is 1 eV higher than that of Eb(Pd3d5/2) 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 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 PdxCe1-xO2-x-δ solution leads to a decrease in the formation energy and to additional stabilization of palladium in the CeO2 matrix. The formation of PdxCe1-xO2-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 PdxCe1-xO2-x-δ solid solution.
