Catalytic Activity of Palladium
|Palladium is a powerful catalyser in each of its various physical varieties.|
Catalytic Activity of Compact Palladium
|Hydrogen combines with oxygen in the presence of palladium foil at 280° C., yielding water; on the other hand, hydrogenised palladium is without action on carbon monoxide or dioxide at the same temperature. |
Ammonia is oxidised to oxides of nitrogen by means of oxygen in the presence of palladium foil heated to redness; and, as has already been pointed out, palladium foil saturated with hydrogen effects the reduction of ferric salts, chlorine water, iodine water, etc., to ferrous salts, hydrochloric acid, and hydriodic acid respectively. Hydrocarbons are oxidised to carbon dioxide and water when passed with air over palladium wire heated to redness. In the absence of air they are decomposed, yielding a deposit of carbon. After a time the palladium becomes brittle, and its surface, seen through a lens, resembles coke.
Catalytic Activity of Finely Divided Palladium
|Palladium may be obtained in a finely divided condition particularly suitable for catalytic action by pouring a solution of palladous chloride on to asbestos, adding a few drops of alcohol and igniting. By repeating the process until the asbestos increases some 6 per cent, in weight in consequence of the palladium, a very efficient catalyser is produced. It will effect the oxidation of hydrogen in the cold, although, be it remarked, hydrogen is the only combustible gas that admits of oxidation in this way without being heated. At bright red heat and in excess of air palladised asbestos will effect the complete oxidation of all hydrocarbons to carbon dioxide and water as efficiently as copper oxide. The employment of palladised asbestos has therefore been recommended for gas analysis. The paraffins are the most difficult to oxidise; acetylene and carbon monoxide come next, whilst the olefines are the most easily oxidised. Of the members of the same homologous series of hydrocarbons the lower are the more stable and offer greater resistance to oxidation. |
The lowest temperatures at which oxidation of various substances was effected by palladised asbestos in the presence of air are given in the following table:
|° C.||° C.|
|Alcohol vapour||150||Carbon monoxide||290|
Palladised copper oxide has also been used 6 for a similar purpose, and for other catalytic reactions palladised metals have been tried. For example, the oxidation of sodium hypophosphite solution takes place according to the equation
NaH2PO2 + H2O = NaH2PO3 + H2
through the agency of palladised copper or platinum, gaseous hydrogen being evolved. A further oxidation to sodium phosphate also takes place, thus:
NaH2PO3 + H2O = NaH2PO4 + H2.
The palladium deposit is found, however, to slowly lose its catalytic activity in a solution of the hypophosphite. Neither palladium foil nor palladium wire will exhibit catalytic activity in this reaction.
The amount of hypophosphite oxidised per unit time is proportional to the weight of catalyst present. Sodium hydroxide, hydrochloric and sulphuric acids, each retard the reaction, whilst the addition of sodium hydrogen carbonate either is without action or slightly enhances the velocity of reaction.
Pallacious Salts likewise possess catalytic properties. For example, they accelerate the oxidation of oxalic acid by persulphates, and, to a less extent, by nitric acid. This is attributable to the alternate formation of a palladic salt by the oxidiser and its reduction by the oxalic acid. Ammonia is likewise oxidised by persulphates in the presence of a palladous salt.