Structural transformation and redox chemistry of Pd/CeO 2 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/CeO 2 catalysts during SO 2 -induced sulfurization. Linear combination fitting of Ce L 3 -edge XANES spectra quantified the Ce 4+ → Ce 3+ transformation, revealing fundamentally different sulfurization mechanisms for bare versus Pd-promoted CeO 2 . At 500°C, bare CeO 2 exhibited surface-limited sulfurization with Ce 3+ /(Ce 3+ +Ce 4+ ) reaching 0.18, while Pd/CeO 2 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 L 3 -edge XANES revealed that Pd maintained its oxidized state throughout SO 2 exposure, excluding PdS formation. S K-edge analysis confirmed exclusive SO 4 2-formation via direct oxidation without intermediate species. These findings establish that Pd catalyzes oxygen mobility within the CeO 2 lattice, transforming sulfurization from a surface-confined to a bulk-accessible process while preserving the fluorite structure. The resulting cerium sulfate oxide (Ce 2 O 2 SO 4 ) exhibits enhanced stability against re-oxidation in Pd/CeO 2 , contrasting with the partial reversibility observed for bare CeO 2 .