The local structure of Pd_xCe_1−xO_2−x−δsolid solutions

Abstract

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.

An algorithm for removing charging effects from X-ray photoelectron spectra of nanoscaled non-conductive materials

“…Fitting the restored Pd3d spectrum obtained for 5%Pd/CeO 2 -25SnO 2 produced a single doublet with E b (Pd3d 5/2 ) = 338.0 eV (Fig. 11b), which corresponds to a solid solution of Pd 2+ ions in the fluorite phase, which is in agreement with our previous findings [33,[35][36].…”

“…It can be seen that the restored spectrum is virtually identical to the Pd3d spectrum obtained using the CN technique. The Pd3d 5/2 line position corresponded to Pd 2+ ions in the fluorite phase, which is in agreement with our previous studies [33,35,36]. Moreover, this treatment allowed us to allocate an additional palladium state, which corresponds to PdO x clusters on the surface of the solid solution Pd x Ce 1-x O 2-x-δ [51].…”

“…We used simulation of Sn3d 5/2 and Pd3d photoelectron lines spectra collection with the assumption of non-constant surface charging. [36]. Characteristic values of the binding energies for tetravalent palladium are in the range above 338.6 eV [49].…”

“…et al, which has been previously used for the analysis of X-ray photoelectron spectra of different catalysts, including reducible oxides [31][32][33][34][35][36], was used in this study. This software allows one to perform all standard procedures for XP-spectra processing: subtraction of K α3,4 satellites, Fourier smoothing, subtraction of the background caused by inelastically scattered electrons on different models, and the decomposition of XP-spectra to singlet or doublet lines described by pseudo-Voigt or Doniach-Sunjic distributions [37].…”

Oxide-based nanomaterials for fuel cell catalysis: the interplay between supported single Pt atoms and particles

“…12d and Pd 2+ species is that Pt 2+ is preferentially stabilized at the oxygen pockets at the surface, whereas Pd can be stabilized both at the surface and in the bulk in a similar square planar arrangement. 94 This property facilitates the diffusion of Pd 2+ species into the bulk upon annealing. In sharp contrast to (Fig.…”