|Platinum possesses considerable catalytic activity, even in the compact state, for thin platinum foil or wire when freshly heated in air glows upon insertion in a current of coal gas, and causes ignition if a jet of hydrogen gas is allowed to impinge upon it.|
But it is when platinum is in a finely divided condition that its catalytic activities are most pronounced.
That in such circumstances the metal can act catalytically in assisting chemical reactions to take place has been known for many years. Thus the reduction of nitric oxide to ammonia by means of hydrogen in the presence of platinum was observed by Kuhlmann in 1838; the combination of iodine with hydrogen, by Corenwinder in 1852; the conversion of hydrocyanic acid into methylamine, by Debus in 1862; and the production of ethane from acetylene, by de Wilde in 1874.
It is frequently convenient to employ finely divided platinum in the form of a deposit upon other substances such as asbestos.
Paal and Windisch prepared platinised metals such as nickel, copper, etc., by shaking the metal in the form of powder with an aqueous solution of chlor-platinic acid, a deposit of platinum amounting to about 2.5 per cent, of the weight of the base metal being aimed at. The product was washed with water, alcohol, and ether, in succession, and dried in vacuo.
It was found that nickel has no effect upon the catalytic activity of the platinum; aluminium, cobalt, and bismuth reduce its activity appreciably; whilst copper, zinc, silver, tin, and iron inhibit its activity altogether.
Platinised asbestos has been used to catalytically accelerate many chemical reactions, such as, for example, the union of oxygen and sulphur dioxide to form the trioxide. This reaction proceeds almost to completion in presence of excess of air, and in 1898 and 1899 several patents were issued in respect of this process.
Dilute solutions of potassium permanganate are reduced by finely divided platinum, potassium hydroxide and hydrated manganese dioxide being produced. Thus:
2KMnO4 + H2O = 2KOH + Mn2O7
Mn2O7 + 4H2O = 2Mn(OH)4 + 3O.
In a similar manner the reduction or hydrogenation of numerous organic substances such as cinnamic acids, etc., has been effected with finely divided platinum.