Measurements of Catalytic Efficiency of Silver for Oxygen Atoms and the O–O2 Diffusion Coefficient

Abstract

Measurements have been made of the heat-transfer rate to the stagnation point of catalytic spheres and cylinders placed in the slow flow of dissociated oxygen produced in a glow-discharge tube. The use of a rapid-response thin-film heat-transfer gauge permitted operation in a transient mode; that is, a step function of atoms was produced by suddenly turning on the rf discharge. Use of this technique leads to small error in the measurement of heat transfer. Operating conditions, range of pertinent parameters, as well as flow tube and probe diameters were selected to conform with the results of the theoretical analysis for this flow. Specifically, the Reynolds number based on diffusion Ua/D, where U is the flow velocity, a is the probe radius, and D the diffusion coefficient, was varied from about 0.2–0.8, and the catalytic parameter K = kwa/D, where kw is the effective speed of the surface reaction, took on values from about 0.3–1.7. The freestream atom concentration was determined by the familiar NO titration technique. Comparison of the measurements with the theoretical results has led to the determination of surface catalytic efficiency of silver oxide and oxygen atoms and the O–O2 diffusion coefficient. These values are γ = 0.15±15% and Dp = 200 cm2/sec·mm Hg±30%. Preliminary measurements of catalytic efficiency of other metals are also presented.