CATALYSIS OF THE OXYGEN REDUCTION ON W18O49 ELECTRODES BY OH- INDUCED SURFACE-STATES - A STUDY BASED ON XPS UPS AND ELECTROMOTIVE-FORCE MEASUREMENTS

The in situ formation of a surface layer on W18O49 ceramics under open-circuit conditions in aqueous solutions of different pH has been investigated employing x-ray and UV radiation stimulated photoelectron spectroscopic surface analytical techniques (XPS/UPS). The utility and the effectiveness of these techniques to the investigation of in situ formed surface layers are clearly demonstrated by the dependence of the spectra on immersion time. The spectra of the emersed surfaces give evidence that OH- ions in solution adsorb at the electrode surface and donate their electrons to the surface cations. This is observed as a reduction of the tungsten valency to a W5d1 electron state located in the conduction band which is, for the first time, directly revealed by UPS. This surf ace cluster state catalyzes the reduction reaction of dissolved oxygen and is essential for the formation of adsorbed superoxide radicals. In a model based on combined electromotive force measurements we postulate that the activity of the OH ions in the solution determines the activity of adsorbed superoxide ions via donor-acceptor reactions whereby the surface tungsten cation functions as an electron transmitter from OH- to the adsorbing oxygen molecule. This coupling of the OH ion activity in the solution with the activity of adsorbed superoxide is proposed as the fundamental mechanism which explains the remarkable stability and reversibility of the open-circuit electrical potential observed on W18O49 and LaxWO3 oxygen electrodes.