Adsorption and Reaction of SO₂ on Model Ce₁₋xZrxO₂(111) Catalysts

The interaction of SO 2 with ceria−zirconia model catalysts was studied using high-resolution synchrotron-based X-ray photoelectron spectroscopy (XPS). Epitaxial Ce 1 - x Zr x O 2 (111) ( x = 0.1 and 0.3) thin films (500−700 Å in thickness) were grown by oxygen-plasma-assisted molecular beam epitaxy on single-crystal Y-stabilized ZrO 2 (111). Slightly defective surfaces were achieved by vacuum annealing at 900 K, and highly defective surfaces with O vacancies were obtained by 1.5-keV Ne + sputtering. On the slightly defective Ce 0.9 Zr 0.1 O 1.95 (111) and Ce 0.7 Zr 0.3 O 1.95 (111) surfaces, the only products upon SO 2 adsorption at 300 K are SO 4 /SO 3 species, which gradually desorb from the surface between 300 and 900 K. SO 2 adsorption on the heavily reduced surfaces results in different behavior. A complex set of compounds is observed during adsorption and thermal conversion processes. The Ce δ+ states (δ ≤ 3) play a dominant role in the adsorption of SO 2 and cleavage of S−O bonds. The relative amount of sulfur-derived adsorbates depends on the defect concentration: the higher the Ce δ+ concentration, the larger the amount of formed atomic S. On Ce 0.9 Zr 0.1 O 1.50 (111) and Ce 0.7 Zr 0.3 O 1.50 (111) surfaces, sulfate, sulfite, and atomic sulfur species coexist at 300 K. The Zr cations increase the stability of the SO 4 /SO 3 groups on the oxide surface. Thermal annealing (for Ce 0.9 Zr 0.1 O 1.50 (111), >400 K; for Ce 0.7 Zr 0.3 O 1.50 (111), >700 K) leads to the formation of oxy-sulfides (M 2 O 2 S, M= Ce or Zr), which are converted from either sulfate or sulfite. The formation of the oxy-sulfides produces a substantial shift in the Zr 3d core levels.