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

Area of Science:

• Materials Science
• Catalysis
• Solid-State Chemistry

Background:

• Palladium-cerium oxide solid solutions (PdₓCe₁₋ₓO₂₋ₓ/₂) are highly effective catalysts for low-temperature carbon monoxide oxidation.
• Understanding the precise structural and electronic state of palladium within the cerium oxide matrix is crucial for optimizing catalytic performance.

Purpose of the Study:

• To elucidate the structural and chemical state of palladium in PdₓCe₁₋ₓO₂₋ₓ/₂ solid solutions.
• To verify the non-isomorphic substitution model of Ce⁴⁺ by Pd²⁺ ions.
• To identify the optimal synthesis method for homogeneous PdₓCe₁₋ₓO₂₋ₓ/₂ solid solutions.

Main Methods:

• X-ray diffraction (XRD) with pair distribution function (PDF) analysis.
• High-resolution transmission electron microscopy (HRTEM).
• X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy.
• Density functional theory (DFT) quantum-chemical calculations.

Main Results:

• Experimental and PDF modeling confirmed non-isomorphic substitution of Ce⁴⁺ by Pd²⁺ ions, forming PdO₄ subunits with a near square planar environment (C₄v symmetry).
• Raman spectroscopy identified a characteristic band at 187 cm⁻¹ corresponding to the A₁ vibrational mode of Pd²⁺ in [PdO₄].
• DFT calculations supported a model where Pd²⁺ displacement stabilizes the structure, with water molecules aiding anion vacancy stabilization via hydroxyl groups, lowering formation energy.

Conclusions:

• The PdₓCe₁₋ₓO₂₋ₓ/₂ solid solutions exhibit a unique Pd²⁺ local environment crucial for catalytic activity.
• The coprecipitation method is optimal for synthesizing homogeneous PdₓCe₁₋ₓO₂₋ₓ/₂ solid solutions due to effective Pd²⁺ incorporation during fluorite structure formation.
• The study provides a detailed structural and electronic understanding of these advanced catalytic materials.