Palladous Oxide, PdO

Palladous Oxide, PdO, was obtained by Berzelius by strongly heating palladium amalgam, by precipitating palladous salt solutions with alkali, as well as by fusing metallic palladium with alkalis.

Palladous oxide is also obtained in the anhydrous condition by heating palladous nitrate to a high temperature, but the simplest and easiest method consists in hydrolysing a faintly acid solution of the nitrate by boiling with water.

When palladium is heated to dull redness in oxygen palladous oxide is formed. But it is difficult to oxidise the metal completely by this method owing to the sintering of the metal, which renders oxidation slow. Palladium sponge is most suitable for the purpose, and should be heated in pure oxygen at temperatures rising slowly from 700° C. to 840° C.

The colour of palladous oxide varies according to its method of preparation. Prepared by the action of alkali upon palladous salts in aqueous solution, it has a yellowish brown colour which becomes darker on prolonged boiling. The product obtained by the hydrolytic decomposition of the nitrate is almost black, and it may well be that the extent of hydration is responsible for these changes.

Air-dried specimens contain rather more water than corresponds to one molecule; when heated on the water-bath the residual water corresponds to about two-thirds of a molecule, but the last traces are not removed even at 500° C.

The freshly precipitated oxide is readily soluble in dilute acids.

It is reduced by hydrogen in the cold, evolving so much heat that it becomes incandescent.

The dissociation of palladous oxide has been studied by Wohler, who shows that it decomposes when heated, yielding palladium and oxygen, the reaction being reversible:

PdO ⇔ Pd + O.

The dissociation pressure of palladium monoxide at 877° C. reaches 760 mm.