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Occlusion of Hydrogen by Palladium

Palladium possesses to a remarkable extent the power of absorbing or occluding hydrogen, and numerous researches have been carried out by different investigators with a view to determining the condition of the occluded gas, and the influence it has upon the properties of palladium.

The absorption of hydrogen by palladium foil is readily shown by passing an electric current through acidulated water, using a platinum anode, but a plate of palladium, just previously heated to redness, as cathode. By using a narrow, vertical glass cell an image of the apparatus may be thrown on to a screen. Oxygen gas is evolved from the anode, but no gas evolution appears at the cathode, until the palladium has become saturated with gas, after which point hydrogen is evolved.

The first detailed researches on the subject are those of Graham, who experimented with thin palladium foil. He observed that palladium which has been ignited in vacuo absorbs large quantities of hydrogen at ordinary temperatures, rapidly yielding a large portion of the gas up again upon being placed in a vacuum, and slowly yielding it when exposed to air. The gas is rapidly and almost completely evolved in vacuo at 100° C. In experiments in which the foil was heated in hydrogen and allowed to cool in the gas, the metal absorbed 643 times its own volume of hydrogen, whilst in later experiments palladium wires absorbed over 900 times their volume of hydrogen. The occluded hydrogen was termed by Graham hydrogenium.

At its melting-point molten palladium absorbs less hydrogen than the solid metal, and consequently there is no "spitting" on solidification in an atmosphere of hydrogen.

Sieverts has studied the absorption of hydrogen from 138° to 821° C. and with varying pressures of gas ranging from 1 mm. to 760 mm. He observed that the amount of hydrogen absorbed per unit weight of palladium is nearly proportional to the square root of the hydrogen pressure.

The experimental results may be expressed by the equation: Lp = k1p½ + k2p,

in which Lp is the quantity of hydrogen absorbed at pressure p, and k1, k2 are constants depending upon the temperature.

The expression is taken to represent that hydrogen molecules are in equilibrium with hydrogen atoms both in solution in the palladium and in the gaseous phase. Henry's Law appears to hold both for molecules and for atoms.

Graham showed that the density of palladium during hydrogenation undergoes a change, becoming considerably less, the volume of the whole undergoing appreciable expansion. In an experiment with palladium wire, which was made to absorb 936 times its volume of hydrogen, the density fell from 12.38 to 11.79, from which the density of the hydrogenium was estimated to be 1.708.

This change in volume of palladium during hydrogenation may be made the subject of a pretty lecture experiment. Two plates of palladium foil are varnished each on one side, and immersed in a cell containing acidulated water. On passing an electric current through, the electrode serving as cathode absorbs hydrogen on one side, expands and curls up, varnished side inwards, whilst the anode remains perpendicular. On reversing the current, the curled plate gradually straightens out, whilst the other plate begins to curl.

By using a narrow, vertical glass cell, the image may be thrown on to a screen, yielding a very effective demonstration.

Other physical properties of palladium are considerably modified by the absorption of hydrogen. For example, the electric conductivity falls continuously with increase of occluded hydrogen, becoming half its original value when the metal is saturated with the gas. At first, up to 40 volumes of hydrogen to one of palladium, the diminution in conductivity is directly proportional to the amount of occluded hydrogen; it then diminishes asymptotically until 600 volumes of gas have been absorbed, corresponding, according to Wolf, to the formation of a dihydride, PdH2, after which the further absorption of hydrogen results in a linear diminution in the conductivity once more.

The magnetic susceptibility of palladium is also diminished by hydrogen absorption, and if the diminution continues uniformly - which has not as yet been experimentally demonstrated - then palladium saturated with hydrogen should actually be diamagnetic. The tensile strength of palladium is likewise reduced. Alloying elements exert a very important influence upon the occlusion of hydrogen by palladium. The absorptive power of commercial palladium is from 10 to 20 per cent, less than that of the pure metal, a fact that is attributed to the presence of small quantities of platinum and ruthenium.

Silver and gold have an interesting influence. Addition of silver to palladium at first increases the solubility of hydrogen despite the fact that hydrogen is insoluble in pure silver. The maximum solubility is reached with 40 per cent, of silver, after which it falls. At 138° C. an alloy containing 40 per cent, of silver and 60 per cent, of palladium absorbs four times as much hydrogen as pure palladium. With 70 per cent, of silver the solubility of hydrogen is reduced to zero.

Gold behaves in an analogous manner, at first increasing and then decreasing the power of occluding hydrogen. In both cases the solubility of hydrogen is proportional to the square root of the pressure, and diminishes with rise of temperature.

Platinum in all proportions causes a reduction in the solubility of hydrogen in palladium, and although the solubility is, as in the preceding alloys, proportional to the square root of the gaseous pressure, it does not diminish with rise of temperature - on the contrary, it increases.

The question now arises as to the condition of the hydrogen occluded by the metal. Is it chemically combined to yield a hydride or is it merely absorbed?

Troost and Hautefeuille considered that a hydride of formula Pd2H was formed, and based their conclusions upon the results of a study of the tensions of hydrogen disengaged at. various temperatures from hydrogenated palladium.

Graham, on the other hand, considered it "probable that the hydrogen enters palladium in the physical condition of liquid."

This theory of a mechanical absorption of hydrogen by palladium receives support from the researches of Hoitsema, who studied the variation in tension of hydrogen with the amount absorbed by the metal at different temperatures. If a definite compound is formed, then by reducing the volume of hydrogen in contact with the palladium no increase in pressure should be observed, only an increase in the amount of compound formed.

hydrogen absorption by palladium
Hydrogen absorption by palladium
On plotting the hydrogen absorbed against the hydrogen pressure, the curves for different temperatures were found to assume the shapes indicated in Fig., and consisted of three portions, namely, a preliminary rise of pressure with hydrogen content, followed by an almost stationary pressure with rise of absorbed hydrogen, and finally a further rise in pressure.

At first glance the horizontal portion of each curve appears to indicate the formation of a compound, but this is negatived by the fact that the length of the central portion not only diminishes with rise of temperature, but it does not end at the same concentration of hydrogen (A in the figure) as would otherwise be expected. It seems highly improbable, therefore, in view of these results, that a definite chemical compound of hydrogen and palladium is produced.

In further support of this may be cited the observation of Sieverts, that the quantity of hydrogen absorbed by unit weight of palladium is a function of the pressure and temperature only, and is quite independent of the superficial area of the metal. This would indicate that the absorption of hydrogen is an example of true solution rather than of definite chemical combination.

Occluded hydrogen is more reactive chemically than the normal gas. Hydrogenated palladium precipitates mercury and mercurous chloride from an aqueous solution of the dichloride, without any evolution of hydrogen. It reduces ferric salts to ferrous; potassium ferricyanide to ferrocyanide; chlorine water to hydrochloric acid; iodine water to hydriodic acid; chromates to chromic salts; eerie to cerous salts; whilst cupric, stannic, arsenic, manganic, vanadic, and molybdic compounds are also partially reduced.

Ferric salts and potassium ferricyanide are completely reduced by charged palladium foil or wire, and the reduction may be carried out quantitatively if required for analytical purposes.

The reduction of a ferric salt to the ferrous condition may be demonstrated very effectively by an experiment described by Newth. A piece of palladium foil is charged with hydrogen by first heating to redness and then making it the cathode in a cell containing acidulated water, through which an electric current is passing. When saturated with hydrogen the foil is withdrawn and immersed momentarily in a solution of ferric chloride. It is then dipped into a dilute solution о potassium ferricyanide, when the reduced ferrous chloride betrays its presence by yielding an immediate blue coloration.

It is interesting, in view of the foregoing reactions, to note that Hoitsema calculates that the hydrogen occluded by palladium, though at first it appears to be represented by H2, yet above 100° C. it appears to be monatomic; and more recently Winkelmann has been led to the conclusion that hydrogen diffusing through palladium at high temperatures is dissociated. Although in both cases higher temperatures than atmospheric are postulated, the conclusions are interesting.

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