A XPS investigation of SO2 adsorption on ceria-zirconia mixed-metal oxides

The interaction between ceria-zirconia mixed-metal oxide model catalysts and SO2 was studied using X-ray photoelectron spectroscopy (XPS). An in situ reaction cell was employed to expose the polycrystalline ceria-zirconia mixed-metal oxide catalyst wafers to various partial pressures of SO2 at elevated temperatures. Six compositions of CeO2/ZrO2 solid solutions were examined in this study, which include CeO2/ZrO2 molar ratios of 1:0, 9:1, 7:3, 5:5, 2:8, and 0:1. The samples were exposed to SO2 at temperatures from 298 to 873 K and pressures reaching 1000 Torr. XPS results show an increased adsorption of SO2 at either elevated temperatures or higher ceria mole fractions. Additionally, both a sulfate (SO4) and sulfite (SO3) surface species were identified. No significant change in sulfate-to-sulfite ratio as a function of temperature, exposure pressure, or composition was observed under the conditions of this study. However, surface hydroxyl functionalization was found to play an important role in the adsorption of sulfur dioxide on the materials studied.