REACTION OF SO2 WITH STOICHIOMETRIC AND DEFECTIVE NIO(100) SURFACES

The interaction of SO2 with both stoichiometric and defective NiO(100) surfaces has been studied by x-ray photoemission spectroscopy, ultraviolet photoemission spectroscopy, Auger-electron spectroscopy, and low-energy electron diffraction. Surface defects are found to play an important role in chemisorption. SO2 interacts only weakly with the stoichiometric NiO(100) surface at room temperature. The adsorption that does occur yields adsorbates having direct S-O bonds (i.e., the S 2p core-level binding energy is 167 eV); the species is presumably associatively adsorbed SO2. Subsequent heating of the SO2-exposed surface to 570 K results in desorption of the adsorbate without any changes in the S 2p binding energy. In contrast, the interaction of SO2 with the defective, reduced NiO(100) surface is much stronger. At 111 K only associatively adsorbed SO2 is present. Upon heating to about 260 K, in addition to the original adsorbates with direct S-O bonds, adsorbates having a S 2p binding energy of 162 eV are observed; these correspond to sulfide bonds to surface metal cations, which indicates that some of the SO2 has completely dissociated. By comparing the results from stoichiometric and defective NiO(100) surfaces, active adsorption sites that involve oxygen vacancies and steps are suggested as an explanation for the dissociation at room temperature, and a thermally activated dissociation mechanism is proposed that involves adsorption at oxygen-vacancy sites only.