Types of Palladium
|Crystalline Palladium was obtained by Joly as the result of heating palladium ribbon, dusted over with finely divided topaz, to redness by means of an electric current. The topaz apparently decomposes, evolving fluorine, which attacks the palladium, yielding a fluoride, which in turn dissociates, leaving a residue of crystalline metal. The crystals resemble those of platinum obtained in a similar manner both in colour and lustre, and appear also to be isomorphous with them.|
|Palladium Hydrosol or Colloidal Palladium is readily prepared by the reduction of the chloride with acrolein or with hydrazine hydrate, in either case in the presence of an extract of Iceland moss or in contact with sodium lysalbinate or protalbinate, gum acacia, or with lanolin in a precisely similar manner to platinum, the function of the organic additions, which are protective colloids, being to increase the stability of the colloidal phase. |
Colloidal palladium may also be prepared by Bredig's method, which consists in sparking between palladium electrodes under ice-cooled water containing a little sodium hydroxide (approximately 0.04 grams NaOH per litre).
Palladium hydrosol is a brown or brownish black liquid in which the metallic particles are exceedingly small, as evidenced by the fact that a small portion will pass through a collodion filter that will retain dilute haemoglobin solutions. The colour of the filtrate is practically the same as that of the original liquid proportionately diluted, although it sometimes exhibits a slightly redder cast. Well-defined catalytic activity is manifested by dilute solutions containing only 0.0005 per cent, of palladium, and such solutions have a detectable colour in layers of upwards of one centimetre in thickness.
Colloidal palladium catalytically assists the decomposition of aqueous solutions of hydrogen peroxide, oxygen being evolved. The reaction is monomolecular, and the influence of as minute a quantity of palladium as one gram atom in 26,000,000 litres of solution can be detected with N/60NaOH and N/10H2O2 solution. The reaction is accelerated by the presence of caustic soda, the optimum concentration of which is about N/16. In acid solution the peroxide decomposes very slowly.
Colloidal palladium assists the combination of hydrogen and oxygen to form water. When excess of hydrogen is present the velocity of the reaction is greater than when the gases are in the exact proportion necessary to form water. On the other hand, excess of oxygen retards the velocity. In this respect colloidal palladium differs from colloidal platinum, the maximum velocity in this latter case being reached with the volume ratio: H:О = = 2:1.
It is reasonable to suppose that the great adsorptive power of palladium for hydrogen is the cause of this peculiarity.
By treating the pure hydrosol with hydrogen and measuring the amount of gas absorbed it has been observed that even with the same sample of colloidal palladium a considerable variation occurs, the amount varying from 926 to 2952 volumes of hydrogen per unit volume of metal. On heating the solution only a portion of the absorbed hydrogen is evolved. Why the whole of the gas is not liberated is not clear.
The activity of hydrogen is increased by absorption into palladium hydrosol. This was first observed in connection with the reduction of nitrobenzol in alcoholic solution, a quantity of aniline being produced by passing hydrogen through the solution after the addition of some palladium hydrosol. This is the more interesting inasmuch as neither palladium black nor palladium foil shares this property, and further, because aniline is a poison for catalysers, at any rate in small quantities, as Bredig first showed, although in larger amounts it exerts an accelerating action.
Comparison of the efficiencies of different colloidal metals may be established by determining the relative volumes of hydrogen used for reduction by them in unit time. This was effected by conducting hydrogen into a flask containing
10 c.c. metallic hydrosol
2 grams nitrobenzol 10 c.c. alcohol
and maintaining at 70° C.
The results indicate that there is but little to choose between platinum and palladium, but that iridium exerts a much less powerful activating influence, whilst that of osmium is very small. The actual figures obtained are:
|Used 1 c.c. of||C.c. of hydrogen per hour.|
The activity varies largely with the state of the colloid, as is to be expected. Old hydrosols appear to work more efficiently than samples freshly made. The temperature, too, exerts a most important influence.
In a similar manner to the foregoing, namely, by passing hydrogen gas through an alcoholic solution of the substance to be reduced, in the presence of palladium hydrosol, many other organic compounds have been exposed to the action of hydrogen. Of these not the least important are unsaturated vegetable oils such as linseed, olive, castor and cot ton-seed, which are completely reduced, hydrogenated or hardened, yielding white solid oils or fats. These oils are now hydrogenated on a commercial scale, but finely divided nickel or its oxide is usually employed as catalyst.
Like that of platinum, palladium hydrosol is affected by poisons, hydrogen sulphide, mercuric chloride, hydrogen cyanide, and hydrogen arsenide exerting a marked retarding influence. For example, the addition of N/107 iodine to N/13200 palladium reduces the velocity of decomposition of hydrogen peroxide by colloidal palladium by 25 per cent. Hydrogen sulphide probably retards the reaction by converting the palladium into sulphide.
Colloidal palladium exerts a remarkable absorptive power on acetylene, some 5000 times its volume of the gas being occluded. The reaction continues for several days, proceeding rapidly at first, and then gradually slowing down. The absorption is facilitated by slight increase in pressure, and also by raising the temperature. Upon exposure to air, only part of the acetylene is evolved, and only a small amount is absorbed again upon further exposure to acetylene. It is generally assumed that polymerides or condensation products are formed.
Colloidal palladium oxidises solutions of sodium dihydrogen hypo-phosphite to phosphite and phosphate in the same manner as the electrolytieally deposited metal and palladium black, but the activity of the colloidal metal is greater.
Colloidal palladium converts mercury and mercuric oxide into colloidal solution, and thereby loses its own catalytic activity. It is suggested that possibly a hydrosol of palladium amalgam is formed.
Palladium hydrosol may be obtained in the solid condition by concentration of the aqueous solution, containing a protective colloid such as gum acacia, by exposure over sulphuric acid in vacuo. It has also been prepared by passage of hydrogen through a solution of palladium chloride at 60° C. in the presence of sodium protalbate. After dialysis the solution is evaporated on the water-bath, and dried as before in vacuo over sulphuric acid.
The solid hydrosol readily dissolves in water again to yield a colloidal solution. When warmed to 60° or 110° C. in hydrogen the solid hydrosol absorbs the gas, but yields it up again when raised to 130° or 140° C. in an atmosphere of carbon dioxide, at which temperature the protective colloid (sodium lysalbate) is still undecomposed.
|Spongy Palladium is readily obtained by heating ammonium chlor-palladite, (NH4)2PdCl4, in hydrogen. It is then in a particularly suitable condition for exhibiting catalytic activity. Graham 2 prepared it by ignition of the cyanide, and Berry by reduction of the chloride with sodium formate in hot aqueous solution. |
When spongy palladium is exposed to an atmosphere of hydrogen gas at temperatures ranging from - 50° C. upwards it occludes hydrogen, the mass becoming pyrophoric. The amount of occluded gas steadily decreases from 917 volumes to one of palladium at - 50° C. to a minimum of 661 volumes at + 20° C. It then rises again to 754 volumes at 105° C.
At very low temperatures, such as those of liquid air, palladium sponge has an extremely large capacity for hydrogen, and palladium sponge at - 190° C. has therefore been recommended as a convenient substance for removing the last traces of hydrogen from other gases.
The chemical activity of the absorbed hydrogen is considerably enhanced. For example, the hydrogenated sponge when placed for twenty-four hours in the dark at ordinary temperatures in dilute solutions of ferric salts reduces them to the ferrous condition. In a similar manner potassium ferricyanide is reduced to ferrocyanide; chlorine water to hydrochloric acid; and iodine water to hydriodic acid.
Graham also found palladium sponge to exhibit a selective absorption for alcohol in preference to water, a power not manifested either by platinum sponge or by spongy iron. For example, palladium sponge was left in contact with a mixture of alcohol and water in a sealed tube for two days. At the expiration of this time the supernatant liquid was removed and found to contain less alcohol, whilst the portion retained by the palladium contained more alcohol in proportion than the original mixture.
|Palladium Black is not pure palladium, but an indefinite mixture in a very high state of fine subdivision, obtained by precipitation from solutions of palladium salts with reducing agents 6 such as sodium formate. |
Obtained by reduction with sodium formate, Mond found palladium black to contain 1.65 per cent, of oxygen, which, however, cannot be removed in vacuo even at a dull red heat, and must therefore be estimated by reduction with hydrogen and weighing the water formed.
Dried at 100° C. palladium black contains 0.72 per cent, of water, and hence, assuming the oxygen exists as palladium monoxide, the composition of the black is as follows:
When ignited in oxygen palladium black absorbs the gas up to a red heat, yielding a brownish black substance that does not yield up its gas at red heat in vacuo, the inference being that the free metal is thereby converted into the monoxide. When exposed to hydrogen 1100 volumes of gas are absorbed, the bulk being occluded, the remainder serving to reduce the monoxide. Of the occluded hydrogen, 92 per cent, can be slowly regained by pumping at the ordinary temperature, and practically all the remainder upon heating to 444° C.
The heat evolved on the occlusion of hydrogen by palladium black is nearly the same as that evolved by platinum black under similar conditions. The heat of occlusion remains constant for the different fractions of hydrogen occluded, namely, 4640 calories per gram of hydrogen, or 4370 calories if the external work done by the atmosphere is allowed for.
Palladium black when heated in hydrogen readily absorbs the gas which it evolves on heating, one volume of the metal evolving 674 times its volume of hydrogen.
When suspended in water palladium black absorbs even more hydrogen, namely, in one case 1204 times its own volume. Under similar conditions it likewise absorbs acetylene slowly; when suspended in 60 per cent, alcohol it has a pronounced adsorptive action, as also when suspended in an aqueous solution of sodium protalbinate. The absorbed acetylene exhibits an enhanced chemical activity. The curves obtained by plotting the rate of absorption of hydrogen by platinum black are smooth, indicating that only one allotropic form of palladium is present.
Palladium black catalytically assists the oxidation of sodium dihydrogen hypophosphite solution to sodium phosphite, thus:
NaH2PO2 + H2O = NaH2PO3 + H2,
hydrogen gas being evolved. The reaction proceeds even further, the phosphite being oxidised to phosphate:
NaH2PO3 + H2O = NaH2PO4 + H2.
Palladium black slowly absorbs 36 times its volume of carbon monoxide at ordinary temperature, and appears to form a chemical compound with it, although efforts to isolate such have proved abortive. It can hardly be purely mechanical absorption of the gas, since other gases such as hydrogen do not effect its elimination. Upon heating to 520° C. the carbon monoxide is suddenly liberated.