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

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.