Selective oxidation

When a mixture of oxidisable substances is so treated that certain of the constituents only are oxidised to the more or less complete exclusion of the remainder, the process is known as selective oxidation. In cases of gaseous combustible mixtures, the passage over a catalyst frequently effects the selective combustion of one constituent. Thus, for example, when a mixture of oxygen, hydrogen, carbon monoxide, methane, and nitrogen is passed over spongy platinum at 177° C. the hydrogen and carbon monoxide are oxidised, but not the methane. This is termed selective oxidation, and is the basis of Hempel's method of analysing certain gaseous mixtures. The selective oxidation of carbon monoxide in excess of hydrogen by passage over catalysts at suitable temperatures has been studied by Rideal, who shows that the oxide of copper (operative at 110° C.) and the oxides of iron and chromium (operative at 250° to 350° C.) are active in inducing the oxidation of the monoxide, although in no case is the selective oxidation complete. Thus, for example, when mixtures of carbon monoxide, hydrogen, and oxygen, in the proportions 9, 14, and 77 per cent, respectively, were passed through iron oxide catalyst at 220° C., the mean value for the ratio of monoxide to hydrogen burnt was

CO/H₂ = 4.32,

despite the preponderating proportion of hydrogen in the original mixture.

The rates of oxidation of the two gases between 100° and 400° C. are connected with their partial pressures by the following expression:



where C₁ and C₂ are the concentrations of carbon monoxide and hydrogen respectively in the original mixture, K being a constant.

Elevation of temperature effects a decrease in the apparent selective character of the reaction, more hydrogen in proportion being oxidised.

Up to the present, reactions of this type have not been very thoroughly studied, but would appear to offer a useful field for fruitful research.

An interesting case of selective oxidation in solution is given by Jones, who has studied the absorption spectrum of a solution of uranous bromide in a mixture of water and methyl alcohol. Addition of potassium perchlorate effects an alteration in the absorption bands in such a manner as to show that the portion of the uranous salt combined with the methyl alcohol has undergone no change, whilst that united with the water has become oxidised.