Abstract
The influence of the form of the palladium catalyst used in catalytic flammable-gas sensing elements on their stability following exposure to high methane concentrations has been investigated. Carbon deposition was found to have an adverse effect on the response of some types of element. However, improvement in stability to coking effects can be achieved either by depositing palladium on a high surface-area support or by using a coprecipitation of palladium and thoria. For a series of Pd + ThO 2 elements having different Pd:ThO 2 ratios, the apparent hydrogen solubility was determined by a temperature-programmed technique. The solubility of hydrogen in palladium is known to increase with decreasing dispersion. Good correlation was obtained between the dispersion inferred from the solubility data and the rate of coking. Thus the role of thoria as a coke prevention agent is to decrease the palladium particle size, resulting in a lower rate of methane decomposition.
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