PNNL

Abstract

The interaction of SO2 with ceria-zirconia model catalysts was studied using high-resolution synchrotron-based X-ray photoelectron spectroscopy (XPS). Epitaxial Ce1 xZrxO2(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 yttria-stabilized ZrO2(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 Ce0·9Zr0·1O1·95 (111) and Ce0·7Zr0·3O1.95 (111) surfaces, the only product upon SO2 adsorption at 300 K is sulfate, in the form of Ce(SO4) 2, which gradually desorbs from the surface between 300 and 900 K. SO2 adsorption on the heavily reduced surfaces results in a different behavior. A complex set of compounds is observed during adsorption and thermal conversion processes. The Ce3? states play a dominant role in the adsorption of SO2 and cleavage of S-O bonds. The relative amount of sulfur-derived adsorbates depends on the defect concentration: the higher the Ce3? concentration, the larger the amount of chemisorbed species. On Ce0·9Zr0·1O1.50 (111) and Ce0·7Zr0·3O1.50 (111) surfaces, SO4 (in the form of Ce2(SO4)3), SO3, and atomic sulfur species coexist at 300 K. Thermal annealing (for Ce0·9Zr0·1O1.50 (111), > 400 K; for Ce0·7Zr0·3O1.50 (111), > 700 K) leads to the formation of an oxy-sulfide (Ce2O2S), which is converted from either SO4 or SO3.