Art. VII.—The Temperature of Combustion of Methane in the Presence of Palladiumised Asbestos
[Read before the Canterbury Philosophical Society, 7th June, 1905.]
A Considerable doubt appears to envelop the question as to the temperature at which methane combines with oxygen in the presence of palladiumised asbestos. Winkler states that the action scarcely takes place under a red heat. Phillips (Chem. Soc. Jour., 66, 2, 194) gives 404–414°C.; Hempel, 200°; whilst Richardt has quite recently determined 500°C. as being the temperature at which an appreciable oxidation takes place. It was to throw light on this question that this set of experiments has been carried out.
Scope of Work.
1. Determination of the temperature of combustion of methane and oxygen when in proper volume for complete combustion.
2. The influence of a change of rate at which the gases were passed over the palladiumised asbestos.
3. The influence of a change in the proportion of the gases.
4. The effect on the temperature of combustion of methane when varying proportions of hydrogen were added.
5. A brief inquiry into the question as to how far the catalytic action of the palladium is due to a superficial oxidation.
Apparatus and Method.
Pure methane was prepared by the method of Parker and Tribe from zinc-copper couple and methyl iodide; and the oxygen and hydrogen by the electrolysis of pure dilute sulphuric acid.
A heavy iron vessel containing lead, shielded with asbestos walls, was used as a bath, and gave extremely satisfactory
results, for rarely did one degree of variation occur in the course of an experiment. All temperatures were measured with a platinum resistance thermometer.
The method adopted was to pass the gas through a capillary containing the asbestos, the tube being kept at the requisite temperature by the lead bath. The gas was then allowed to take up a constant temperature, and carbon-dioxide sought for by absorption in potassium-hydrate.
With regard to testing the oxidation of the metal, the method was altered. Oxygen was slowly passed through the capillary; the apparatus was then swept out with nitrogen, and methane passed through. It was anticipated that, if an oxidation had taken place, traces of carbon-dioxide would be found.
The conclusions arrived at may be summarised as follows:—
1. The temperature of oxidation of pure methane and oxygen is about 520–546 C., very close to that observed recently by Richardt.
2. An increase in the rate at which the gas is sent over appears to cause a decided increase in the temperature of combustion. Since every particle of gas was in contact with the heated asbestos for over a second, this increase cannot be attributed to insufficient heating. A decrease in the rate caused very little alteration in the temperature of oxidation, showing that increased facilities for combination in no way help to overcome the natural retardation observed by Mallard and Le Chatelier, and by Richardt.
3. A variation in the proportions of the gases causes a decided change in the temperature of oxidation, for a well-defined minimum is obtained when the gases are present in proper volumes for total combustion. 1 vol. CH4 : 2 of O2 gave as temperature of combustion 520–546°C.; 1 vol. CH4:1 of O2 gave as temperature of combustion 565–585°C.; 1 vol. CH4: 3 of O2 gave as temperature of combustion 620°C. The curve connecting volumes and temperatures is appended.
4. The addition of hydrogen, even in large quantities, does not cause the methane to burn at a lower temperature. Consequently the usual method of fractional combustion of hydrogen in gas-analyses should give reliable results, provided the temperature does not rise above 500° C. It is interesting to note that the action of the catalyser does not appear to be aided by the intense heat generated by the combustion of the hydrogen. Perhaps the most reasonable explanation of this is that the palladium, which is catalytically active not only towards a methane-and-oxygen mixture but also towards a hydrogen-and-oxygen mixture, acts in the presence of these three gases in such a way as to confine its energy as a catalyser to that direction in which it is the
more active-namely, to aiding the union of hydrogen and oxygen.
5. The catalytic action of the palladium does not appear to be due to a previous oxidation of the metal.
6. In no case has anything approaching complete combustion been observed, although at times the gas was 150° above its temperature of combustion. This is directly contrary to the work of Phillips, but in good accord with the recently published work of Richardt.
In conclusion, I feel that I cannot close this paper without expressing my deep gratitude to Dr. W. P. Evans for his kindly assistance and encouragement, and also to Dr. C. C. Farr for the loan of electrical apparatus.