STRUCTURAL EFFECTS IN ELECTROCATALYSIS - OXYGEN REDUCTION ON PLATINUMLOW-INDEX SINGLE-CRYSTAL SURFACES IN PERCHLORIC-ACID SOLUTIONS

Oxygen reduction has been studied in 0.1 M HClO4 on the platinum low i ndex surfaces, employing a hanging meniscus rotating-disk technique. A special cleaning procedure has been developed to remove impurities fr om the surface prior to the oxygen reduction measurements. Oxygen redu ction was found to be sensitive to the crystallographic orientation of the platinum electrode surface. The activity for oxygen reduction dis cerned from the half-wave potential decreases in the sequence (110) > (111) > (100). The reaction proceeds on all well-ordered low index pla nes with exchange of four electrons per O-2 molecule. Tafel slopes of -120 mV per decade at high current densities and -60 mV/per decade at low current densities were found for all three planes at room temperat ure. Oxygen-containing species chemisorbed on platinum appear to be th e cause of the change in the Tafel slope.