Noncatalytic Electrodes for Solid‐Electrolyte Oxygen Sensors

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Abstract
The catalytic effects of various electrode materials on nonequilibrium oxygen measurements using solid‐electrolyte oxygen sensors have been investigated. Catalytically active electrodes can perturb the measurement of oxygen in nonequilibrium mixtures of oxygen and combustible gases. This paper reports the development of noncatalytic electrodes for accurate measurement of the free oxygen content in nonequilibrium gas mixtures containing and , , , or . The effects of Pt, Au, Ag, Ag deposited on Pt, and S‐ or Pb‐poisoned Pt electrodes on the measurement of oxygen in nonequilibrium gas mixtures were determined. These studies were made using solid‐electrolyte oxygen sensors of a new internal‐reference design as well as with traditional air‐reference sensors. Pt electrodes are catalytic in all of the gas mixtures studied. Both Ag electrodes and Ag vapor deposited on Pt were found to be noncatalytic to oxidation while Au electrodes were only slightly catalytic. Pt electrodes poisoned by S introduced as were also noncatalytic to oxidation until the sulfur desorbed at ∼800°C. The above electrodes were sufficiently catalytic to oxidation that they could not be used for quantitative nonequilibrium oxygen measurements when is present. Pt electrodes poisoned by Pb (introduced as ) produced the least perturbation of the nonequilibrium mixtures of oxygen with , , , or . The Pb‐poisoned Pt electrodes allow quantitative measurement of the bulk nonequilibrium oxygen concentrations at temperatures above 500°–550°C, depending on the combustible gas present. At lower temperatures, even the Pb‐poisoned electrodes are catalytic. This suggests that Pb may poison the platinum activity by decreasing the heat of adsorption of adsorbed reaction species. Sulfur‐ or Pb‐poisoned Pt or Pt with a thin layer (<200Å) of Ag had no adverse effect on the electrochemical pumping capabilities of Pt. Ag and Au electrodes performed poorly as electrochemical pump electrodes. The capability of solid‐electrolyte oxygen sensors to measure the partial pressure of gas phase oxygen at the electrode surface was used to investigate the catalytic reaction mechanism of oxidation on Pt electrodes partially poisoned by Pb. The results are best fit by an Eley‐Rideal mechanism where adsorbed reacts with gas phase . However, the data are not sufficiently definitive to rule out the Langmuir‐Hinshelwood mechanism where adsorbed O and react on the surface. The methods used demonstrate that solid‐electrolyte oxygen sensors can be useful experimental tools for identifying catalytic mechanisms.

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