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, ultrav iolet photoemission spectroscopy, Auger-electron spectroscopy, and low -energy electron diffraction. Surface defects are found to play an imp ortant role in chemisorption. SO2 interacts only weakly with the stoic hiometric NiO(100) surface at room temperature. The adsorption that do es occur yields adsorbates having direct S-O bonds (i.e., the S 2p cor e-level binding energy is 167 eV); the species is presumably associati vely adsorbed SO2. Subsequent heating of the SO2-exposed surface to 57 0 K results in desorption of the adsorbate without any changes in the S 2p binding energy. In contrast, the interaction of SO2 with the defe ctive, reduced NiO(100) surface is much stronger. At 111 K only associ atively adsorbed SO2 is present. Upon heating to about 260 K, in addit ion to the original adsorbates with direct S-O bonds, adsorbates havin g a S 2p binding energy of 162 eV are observed; these correspond to su lfide bonds to surface metal cations, which indicates that some of the SO2 has completely dissociated. By comparing the results from stoichi ometric 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 disso ciation mechanism is proposed that involves adsorption at oxygen-vacan cy sites only.