On the Activity of Platinum Catalysts in Solution: I . Effects of Thermal Treatment and Chemical Etching on the Pt‐O/Hydrogen Specific Reaction Rate

Abstract

The effects of thermal treatment and chemical etching of platinum on the specific rate of the chemical reaction of chemisorbed oxygen with hydrogen were determined. The hydrogen was present in electrochemically clean and in the derma of the metal. On successive thermal treatments of bright Pt beads, which were heated to the melting point and then slowly recrystallized under high temperatures, the specific rate varied randomly from trial to trial. Where heating was more uniform, and the cooling rate slower, reaction of hydrogen with Pt‐O was usually faster. Repeated aqua regia etching of a given Pt bead caused monotonie improvement until a rate between 0.014 and 0.021 amp/cm2 was attained. Rates on Pt wire electrodes, which probably differed from the flame‐formed Pt beads both in average crystallite size and in number of defects (created by the drawing process and only partially removed by subsequent annealing), were highly variable, but considerably lower than on beads. Surfaces whose activity for the Pt‐O/ hydrogen reaction differed manyfold showed no differences in anodic charging curves nor did electrochemical rates of water oxidation at +0.617 and +0.587v (NHE) or reduction of hydrogen ions at +0.300v differ. It appears that many electrochemical reactions are insensitive to these differences in surface condition. Because of its great sensitivity to catalyst pretreatments, the Pt‐O/ hydrogen reaction is identified as a “demanding reaction,” especially useful for identifying and examining certain variables affecting contact catalysis in solution. Since impurity sorption is one such variable, the rate of this reaction is one of the most responsive cleanliness tests available.