Hydrogen : Hydrogen, Occlusion By Metals

Deville observed that platinum and iron become permeable to hydrogen at a red heat, and concluded that "metals and alloys have a certain porosity." Graham (1866-69) showed, however, that the penetration cannot be due to the porosity of the metal, since hydrogen is practically the only gas which exhibits the effect.

Graham filled a platinum bulb with hydrogen, and heated it in air. In half an hour 97 per cent, of the hydrogen had passed out, but no air entered, and a partial vacuum was produced inside the tube. Five hundred c.c. of hydrogen passed per sq. m. per minute through a platinum tube 1.1 mm. thick. Through a similar palladium tube the hydrogen began to escape at 100°; at a red heat 3993.2 c.c. of gas passed out per sq. m. per minute. No other gas, except ether vapour, penetrated the metal. Palladium in a glass tube was exposed to hydrogen at 90°-97° for three hours, and allowed to cool in the gas for ninety minutes. When the tube was heated by a flame, and the gas pumped off, the metal yielded 643 times its volume of gas. Upwards of 500 vols. of gas were given out at 245° in a vacuum.

Graham said that: "the whole phenomenon appears to be consistent with the solution of liquid hydrogen in the metal... It maybe allowed to speak of this as the power to occlude (to shut up) hydrogen, and the result as the occlusion of hydrogen by platinum." In 1869 he suggested that hydrogen was the vapour of an exceedingly volatile metal, hydrogenium. This hypothesis was disproved when solid hydrogen was shown to be a transparent, glassy solid, devoid of metallic properties.

Palladium charged with hydrogen is a strong reducing agent: it precipitates mercury from mercuric chloride solution, gives up hydrogen to chlorine and iodine in the dark, and reduces ferric to ferrous salts. Colloidal palladium takes up 2950 vols. of hydrogen.

Expt. 10. - The occlusion of hydrogen by palladium is exhibited by immersing two strips of palladium foil in dilute sulphuric acid and using them as electrodes. Oxygen is evolved from the anode, but no gas is evolved from the cathode until the metal becomes charged with hydrogen, when a stream of bubbles begins to come off. If the current is switched off, gas often continues to come off slowly from the cathode showing that the metal had become supersaturated with hydrogen It the current is then reversed, no gas comes from either electrode for a time The oxygen is combining with the occluded hydrogen in the one electrode and hydrogen is being occluded in the other. After a time gas comes off from both electrodes. The palladium strips bend, owing to the unequal expansion on absorption of hydrogen.

Troost and Hautefeuille (1874) pumped off hydrogen occluded in palladium, and measured the pressures during its removal at a given temperature. The first portions came off readily, but when 600 vols. of hydrogen were left to 1 vol. of palladium, the rest of the gas came off at a constant pressure, as does water vapour from a salt containing water of crystallisation. Hence these observers concluded that a definite hydride of palladium was present. Constant pressure intervals were observed at different temperatures.

The density of palladium is 12, hence the ratio of the weights of palladium and hydrogen in the metal which has occluded 633 vols. of hydrogen is 12:633 x 0.00009 = 12:0.057. The atomic weight of Pd is 106, hence the ratio of the atoms in palladium saturated with hydrogen is

corresponding with Pd₂H.

Roozeboom and Hoitsema (1895) repeated the investigations of the two French chemists. They considered that the pressure curves in the dissociation of the "palladium hydride," at temperatures between 0° and 190°, consisted of three parts:

Fig: Palladium and hydrogen curves

two rapidly ascending portions, joined by a nearly horizontal but slowly rising middle portion. At higher temperatures the flat part became appreciably shorter. It was less flat if palladium black was used instead of foil. The dotted curves give the results of Troost and Hautefeuille. The shapes of the curves were considered to speak against the existence of a definite compound; with certain reservations Roozeboom and Hoitsema thought they indicated the formation of solid solutions. The flat part, where the pressure is practically constant, indicates that two solid solutions must be present. Since the pressure depends only on the temperature, the Phase Rule gives F=1; C=2;

P=3, i.e., gas + 2 solids.

Roozeboom and Hoitsema pointed out that their hydrogen contained a little nitrogen, which would explain the upward slope of the curves: they did not consider their experiments sufficient to decide the question.

Holt, Edgar, and Firth in 1913 concluded that the hydrogen exists partly as a condensed layer on the surface, and partly dissolved in the interior of the metal, and not usually homogeneously distributed.

They found that palladium is normally inactive but becomes active as a result of: (a) oxidation by heating in air and reduction of the oxide film in hydrogen; (b) heating to 400° in hydrogen, followed by cooling in the latter; (c) heating to 400° in vacuo; the hydrogen must then be admitted as soon as cold, as the metal so activated soon loses its activity. In all cases, heating is necessary for the activation. The absorption of gas is at first rapid, then becomes increasingly slower. The rate of diffusion of hydrogen through palladium 0.3 mm. thick was 3288 c.c. per sq. m. per minute at 200°, and 5570 c.c. at 476°.

By pumping out a palladium tube saturated with hydrogen and surrounded with the gas, the pressure inside was reduced to zero at the ordinary temperature, whilst the pressure on the other side was 10.4 mm. At 140°, with two pumps working equally on both sides, the outer surface of the tube then lost 208 c.c. of gas, and the inside only 12 c.c. The hydrogen is not homogeneously distributed throughout the metal. The surface layer is easily removed by pumping; the gas in the interior is much more firmly held.

Gillespie and Hall (1926) took extra precautions to obtain states of true equilibrium, using very finely divided palladium and special heat treatment._ They obtained perfectly horizontal isotherms

Fig: Gillespie and Hall's results

and found evidence of two immiscible solid solutions, but at temperatures of 80°, 160° and 180° the one richer in hydrogen had practically the composition of a palladium hydride Pd₂H or Pd₄H₂, which is regarded as a definite compound, separating nearly pure at these higher temperatures, but at lower temperatures dissolving increasing amounts of hydrogen.

A. W. Porter (1918) remarked that different phenomena may be confused under the name "occlusion": (a) formation of a chemical compound; (b) simple solid solution, with or without chemical combination; (c) solid solution in contiguous phases (Hoitsema); (d) surface condensation under molecular forces, especially in pores; (e) inclusion of bubbles of gas.

Most metals in the finely-divided condition absorb small quantities of hydrogen, and metals prepared by electrolysis sometimes contain occluded hydrogen.

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