Surface Combustion

As has been seen, slow combustion of gases may be enormously accelerated by contact with catalysing surfaces. The rate may be so increased as to cause the evolution of light and intense heat, but without any visible flame. All such phenomena are grouped together under the term surface combustion.

Davy, in 1818, called attention to the fact that a spiral of platinum wire, when plunged in a warm condition into a mixture of coal gas and air, bccomes incandescent and may cause the gases to burst into flame. By holding a piece of platinised asbestos, after warming in the Bunsen flame, in a stream of coal gas and air, the surface of the asbestos glows brightly, but as a rule no flame appears. If, however, the asbestos is held over a jet of hydrogen, sufficient heat is generated to ignite the gas.

A pretty lecture experiment consists in suspending a spiral of platinum wire in a beaker over some methyl alcohol. The latter is ignited, and when the spiral has become warm a card, punctured with small holes, is laid over the top of the beaker. This extinguishes the flames, but sufficient air enters to allow some of the alcohol to burn on the surface of the platinum, which glows at red heat.

The reaction, when once started, is self-supporting, and may be represented as follows:

(i) CH₃OH + O = HCHO + H₂O;
(ii) HCHO + O = HCOOH;
(iii) HCOOH + O = H₂O + CO₂.

By suitably adjusting the apparatus formaldehyde may be made the most important product, and advantage has been taken of this to construct the well-known formaldehyde lamp, in which a mixture of methyl alcohol vapour and air is passed over a warmed platinum wire.

Finely divided platinum, on account of its enormous surface area, is very reactive, and a small quantity introduced into electrolytic gas may cause instant explosion.

Surface combustion phenomena are not confined to platinum or the noble metals. A warmed iron wire may be raised to incandescence by surrounding it with an atmosphere of air and coal gas.

It is now established that:

• The property of accelerating gaseous combustion at temperatures below ignition point is shared by all substances irrespective of their chemical composition.
• Whilst at lower temperatures there exist very marked differences in the catalysing powers of various solids, at high temperatures these disappear.

At bright incandescence all solids behave alike.

Bone has applied the phenomena to industrial problems, mixtures of coal gas and air being caused to burn on the surface of porous firebrick, generating enormous heat, but with no visible flame.