Hydrogen : Hydrogen, Catalytic Combustion

Although oxygen and hydrogen gases do not react at the ordinary temperature, a jet of hydrogen is inflamed if directed on a little platinum sponge. The same effect is produced by a bundle of fine platinum wires which are perfectly clean; these become red-hot and then kindle the hydrogen (Dobereiner, 1823). It is not shown by metals such as iron or copper, and in this case, therefore, the platinum exerts a specific catalytic action.

Döbereiner's lamp

Fig: Döbereiner's lamp

is a small hydrogen generator, composed of a glass tube immersed in dilute sulphuric acid, with a stopcock and jet at the top. A piece of zinc hangs inside the tube, and the gas generated displaces the acid until it is no longer in contact with the zinc, when action ceases. Opposite the jet is a sponge of fine platinum wire enclosed in a brass tube, and when the tap is opened the stream of hydrogen ignites. The activity of the platinum rapidly falls off, but it may be renewed by boiling the metal in nitric acid, when impurities from the hydrogen which cause the loss of activity are removed.

Faraday (1833) observed that the combination of a mixture of hydrogen and oxygen can also be brought about by a piece of clean platinum foil - in some cases the gas explodes. There are two theories to account for this catalytic activity of platinum:

Faraday considered that both the gases formed a condensed film on the metal surface as a result of the action of surface-forces of the solid. Under the high pressure existing in this film the gases entered into reaction. It is in fact known that pressure enhances the activity of gases. Thus, Beketoff found that hydrogen gas displaces silver and mercury from solutions of their salts under 100 atm. pressure.
De la Rive and Marcet (1828) believed that chemical compounds, unstable oxides, are formed as superficial layers on the metal. These react with the hydrogen in a cyclic manner, the metal being alternately oxidised and reduced: 2Pt + O₂ = 2PtO; 2PtO + 2H₂ = 2Pt + 2H₂O. There is some evidence for the formation of superficial oxide films.

According to Faraday, platinum does not catalyse the union of hydrogen and chlorine. He also found that the catalytic activity of a clean platinum surface was arrested when a small quantity of carbon monoxide was added to the mixture of hydrogen and oxygen, but the metal recovered its activity when brought into a gas mixture free from carbon monoxide. The presence of traces of hydrogen sulphide in the gas "poisoned" the platinum so that it did not become active again until it had been boiled in concentrated sulphuric acid.

Graham in 1868 suggested that gas films formed by adsorption on metals might contain the gas molecules orientated in a particular direction, so that the same part of the molecule would always be in contact with the metal, and the other part exposed as a film to the gas space.

Later investigations, notably by Langmuir, indicate that the adsorbed layer is unimolecular in thickness, that it is generally orientated, and that "poisoning" is due to the formation of films of molecules which prevent the adsorption of gases otherwise capable of reacting on a clean surface. In some cases (e.g., with carbon monoxide) these films may evaporate again in a pure gas. Langmuir supposes that the adsorbed molecules are held by attractive forces analogous to residual affinity, which originate in the atoms of metal lying on the surface, these possessing uncompensated attractions, whereas the metal atoms inside the mass have no residual attractions, since they are completely surrounded by other atoms. He looks upon a metal surface as something like a chess-board, the black spaces being metal atoms and the white spaces the spaces between the atoms. Molecules of gas are held by the spaces occupied by metal atoms. The catalytic action may take place by interaction between molecules or atoms held on adjacent surface atoms of metal, or between an adsorbed film and the atoms of the solid, or directly as a result of a collision between a gas molecule and a molecule or atom held on the surface. Reaction between hydrogen and oxygen, he considers, occurs between adjacent adsorbed atoms, that between carbon monoxide and oxygen between oxygen atoms, formed from adsorbed oxygen molecules, and colliding carbon monoxide molecules. The products of reaction then evaporate from the surface. In the case of adsorption of gases on salt crystals, Haber (1914) considers that the molecules are held by electrical forces from the positive and negative ions of the salt in the surface of the crystal.

Protein crystallography
Chemistry guide
Hydrogen
-H-
<<<	---