The interaction of SO2 with stoichiometric and defective NiO(100) surfaces was studied by using X-ray photo-emission spectroscopic, ultra-violet photo-emission spectroscopic, Auger electron spectroscopic, and low-energy electron diffraction methods. It was found that surface defects played an important role in chemisorption. The SO2 interacted only weakly with the stoichiometric NiO(100) surface at room temperature. Any adsorption that did occur yielded adsorbates which had direct S-O bonds (the S 2p core-level binding energy was 167eV). The species was presumed to be associatively adsorbed SO2. Subsequent heating of the SO2-exposed surface to 570K resulted in desorption of the adsorbate, without any change occurring in the S 2p binding energy. On the other hand, interaction of SO2 with the defective reduced NiO(100) surface was much stronger. At 111K, only associatively adsorbed SO2 was present. Upon heating to about 260K, adsorbates which had an S 2p binding energy of 162eV were observed, in addition to the original adsorbates with direct S-O bonds. The former corresponded to sulfide bonds to surface metal cations, and indicated that some of the SO2 had completely dissociated. Upon comparing the results for stoichiometric and defective NiO(100) surfaces, it was suggested that active adsorption sites which involved O vacancies and steps explained the room temperature dissociation, while a thermally activated dissociation mechanism was proposed which involved adsorption only at O-vacancy sites.

X.Li, V.E.Henrich: Physical Review B, 1993, 48[23], 17486-92