Sorption of oxygen from solution by noble metals: I. Bright platinum

Abstract

Platinum electrodes in the form of beads or wires were permitted to sorb oxygen from O 2-saturation H 2SO 4 solutions under steady-state conditions of constant current, constant potential, and open circuit. True areas were determined from constant current anodic pulses, and the amount of charge associated with the sorbed oxygen was determined from the measured transition times of potential-time traces obtained with constant current, cathodic stripping pulses. Two kinds of adsorption sites are identified with respect to the diffuculty with which the sorbed oxygen is removed by a cathodic stripping pulse. The strong adsorption sites are located in the skin of the metal (first 1 or 2 atom layers of the metal) and the sorbed oxygen on these sites corresponds to the so-called dermasorbed oxygen. Weak adsorption sites are located on the metal surface, and the sorbed oxygen on these sites is called adsorbed oxygen. Oxygen which is dissolved in the interior of the metal beyond the dermasorb region is called absorbed oxygen. The amount of oxygen sorbed is a function of the potential. Below 800 mV, sorbed oxygen could not be detected. The first oxygen to be sorbed appears above 800 mV on the surface adsorption sites but dermasorbed oxygen is not detected until potentials greater than 1000 mV are reached. Under open-circuit conditions, the maximum amount of surface adsorbed oxygen present at steady state (potential = 1060 mV) in O 2-saturated acid solution is 30% of a monolayer. A complete layer of adsorbed oxygen (Pt—O) may be obtained with anodization at potentials of about 1600 mV. At higher potentials (>2000 mV), the Pt—O sites may be converted to PtO 2 sites until a maximum of a monolayer of PtO 2 is formed. Under open-circuit conditions, the PtO 2 sites in the presence of Pt decompose to Pt—O sites. The equivalent of many complete layers of oxygen may be dissolved in the interior of the metal with anodic polarization, and this absorbed oxygen may replace any dermasorbed oxygen removed.