Structural transformation and redox chemistry of Pd/CeO2 during SO2-induced sulfurization: an in situ XAFS study

Abstract

In situ X-ray absorption fine structure (XAFS) spectroscopy was employed to elucidate the structural evolution of Pd/CeO₂ catalysts during SO₂-induced sulfurization. Linear combination fitting of Ce L₃-edge XANES spectra quantified the Ce⁴⁺ → Ce³⁺ transformation, revealing fundamentally different sulfurization mechanisms for bare versus Pd-promoted CeO₂. At 500 °C, bare CeO₂ exhibited surface-limited sulfurization with Ce³⁺/(Ce³⁺ + Ce⁴⁺) reaching 0.18, while Pd/CeO₂ achieved 0.68, demonstrating bulk oxygen participation. Temperature-dependent measurements (200–500 °C) confirmed thermal activation of the sulfurization process, with higher temperatures enabling deeper lattice penetration. Complementary Pd L₃-edge XANES revealed that Pd maintained its oxidized state throughout SO₂ exposure, excluding PdS formation. S K-edge analysis confirmed exclusive SO₄²⁻ formation via direct oxidation without intermediate species. These findings establish that Pd catalyzes oxygen mobility within the CeO₂ lattice, transforming sulfurization from a surface-confined to a bulk-accessible process while preserving the fluorite structure. The resulting cerium sulfate oxide (Ce₂O₂SO₄) exhibits enhanced stability against re-oxidation in Pd/CeO₂, contrasting with the partial reversibility observed for bare CeO₂.