A Comprehensive Treatise on Inorganic and Theoretical Chemistry

142 INORGANIC AND THEORETICAL CHEMISTRY
absorbed by spongy platinum and capable of being oxidized in the cold by free
oxygen, evolves 9-5 CaIs. Platinum black prepared by reducing a soln. of a platinum
salt with formic acid, does not evolve any gas when heated to 500° or 600° in
vacuo, 62-255 grms. of the platinu mblack absorbed 0-0342 grm. hydrogen in the
cold, with evolution of 14-2 cals. per grm. of hydrogen absorbed, and formation
of two hydrides : in vacuo this loses 23 c.c. or 0-02 grm. hydrogen, and on passing
oxygen over it, the increase of weight is 0*0765 grm. with development of 51-6 Cals.
per 16 grms. oxygen, which is equivalent to 0-0091 grm. of hydrogen. Consequently,
0-0226 grm. of hydrogen, or nearly two-thirds of the gas, remains as a
hydride, which is not oxidized by oxygen in the cold. This hydride is decomposed
by gradually heating it to the temp, at which glass softens. The heat of formation
of the less stable hydride is -f-8-7 Cals., H=I grm., that of the more stable being
nearly double, or +17 Cals. The proportion of the total weight of hydrogen
absorbed to platinum=1 to 20, while in the more stable hydride it is 1 to 30.
Platinum black prepared by reducing platinum in alkaline soln. always contains
oxygen possibly as suboxide. The hydrogen absorbed is used partly in the reduction'of
the oxide, partly in the formation of hydride. The absorption of hydrogen
in this case is attended with the evolution of -f-12 Cals. per grm. of gas absorbed,
but a small quantity of water is formed at the same time. Of the hydrogen absorbed,
one-fifth is oxidized by oxygen in the cold, whilst four-fifths require a higher temperature.
H. Dobretsberger studied the effect of absorbed hydrogen on the highfrequency
resistance ; and G. Tammann, of cold-work. Observations on the heat
of absorption were made by P. A. Favre, but E. Rothe did not think the results
were accurate. H. S. Taylor, E. Waldburger, L. P. Hammett and A. E. Loreh,
and O. Schmidt studied the activation of hydrogen by platinum.
E. C. Auerswald, and C. Paal and C. Auerswald prepared a colloidal soln.
of platinum hydride by treating a colloidal soln. of platinum, protected by sodium
lysalbate, alternately with hydrogen and exposure to air. When the hydrosol
of platinum hydride is shaken with mercury, a hydrosol of platinum amalgam
and free hydrogen are formed. N. Bach studied the properties of suspensions of
platinized carbon. The state of the absorbed hydrogen was discussed by
H. Schroder. L. Mond and co-workers observed no evidence in favour of the
assumption that definite platinum hydrides, Pt30H3 or Pt30H2, are formed.
E. Bose concluded that the gas absorbed by the cathode is wholly or partially
dissociated into single atoms. It is inferred that the catalytic activity of platinum
in hydrogenation reactions is due to the accumulation of hydrogen ions at the
surface of the metal, and therefore the seat of the catalytic activity is in the neighbourhood
of the surface, and not in the metal itself. The subject was discussed
by E. Miiller and K. Schwabe, E. B. Maxted and G. J. Lewis, G. Bredig and
R. Allolio, P. Anderson, G. Vavon, F. Horton and A. C. JDavies, V. S. Sadikofif
and A. K. Mikhailoff, R. Koppen, T. Kariyone, M. C. Boswell and C. H. Bayley,
L. .Kandler and C. A. Knorr, M. Calvin, and G. Vavon. Y. Venkataramaiah and
M. V. N. Swamy noted that hydrogen is activated by diffusion through platinum.
I. Langmuir showed that probably the surface of the platinum, at press, below
1 bar, is covered with a layer of gas of the thickness of 1 atom or 1 molecule. This
layer is not removed at 360°, and there is very little more absorption when the press,
is raised to 200 bars, because layers thicker than a molecule are not formed. The
metal is saturated where a unimolecular layer is formed. On this assumption,
A. F. Benton said that the mol. vol. of platinum black is approximately 9*30, and
therefore, the vol. occupied by each platinum atom is 1-53 X 1O -23 c.c, and the
area assignable to each atom in a platinum surface is 6-2xl0~ 16 sq. cm. There
are then 1-6 XlO 15 atoms per sq. cm., and if the assumption be made that each
platinum atom on the surface holds 1 atom of adsorbed gas, there must be 0-80 XlO 15
mols. of hydrogen or carbon monoxide adsorbed per sq. cm. of surface. In other
words, 3-0 XlO -5 c.c. of gas is required to form 1 sq. cm. of such an adsorbed
layer. A. F. Benton observed an average of 37-3 c.c. of hydrogen per c.c. of