SURFACE PROTONATION AND ELECTROCHEMICAL ACTIVITY OF OXIDES IN AQUEOUS-SOLUTION

Measurements of the pH dependence of the surface-charge density on oxide particles are correlated with the pHdependence of the cyclic voltammograms and of the chemical activity of the oxide for electrolysis/fuel cell reactions. Studies of the pyrochlores Pb2M2-xPbxO7-y and the rutiles MO2 (M = Ru or Ir) and of the perovskite Sr1-xNbO3-δ show that (1) the oxygen-reduction reaction, found on the pyrochlores in alkaline solution, occurs by exchange of a surface OH- species at anO' site with an adsorbed O2- solution species, (2) the oxygen-evolution and chlorine-evolution reactions occur at a surfaceO- species made accessible by surface oxidation of a redox couple lying close to the top of the O2-:2p6 valence band, and (3)the hydrogen-evolution reaction occurs at the surface OH2 species bonded to cations with a surface redox couple lying close to the H+H level in solution. © 1990, American Chemical Society. All rights reserved.