A 9 EXCEPTIONAL ISOTHERRS AT 0' OF T H E SYSTEM: PALLADIUM-HY DROGES* B Y LOUIS J. GILLESPIE A S D dOHS €1. PERRY
The equilibrium between palladium and hydrogen appears to be well understood, as respects the region from oo to 180' or zooo and from no hydrogen to the atomic ratio H/Pd of about 0.6 or 0.7. Pressure-composition isothermals satisfying the demands of the phase rule, for the case in which univariant systems occur, have been given by Gillespie and Hall.' Their conclusion that a solution of hydrogen in palladium can be in equilibrium with another phase having the composition of Pd2H above about So', has recently been confirmed by the X-ray studies of Linde and Borelius? at IOO', I j O o and zooo. Three reservations should perhaps be made with regard to our understanding of the above-mentioned region. ( I ) Linde and Borelius suggest that there may be two crystalline forms of Pd2H. ( 2 ) They suggest that the course of the isotherms of Gillespie and Hall below 80°, which led these authors to believe that below 80' Pd2H is contaminated with an excess of dissolved hydrogen, may be due in reality to the undetected presence of the hydrogenrich phase of Graham (obtained by electrolysis). We may add, that such a solution effect is not in evidence in the otherwise rather similar pressurecomposition diagram of Valensi3 for chromium and nitrogen, in which system the compound C r S is found in equilibrium with a solid phase containing less nitrogen. The suggestion of Linde and Borelius is logically possible: and yet there is experimental evidence pointing to the conclusion of Gillespie and Hall, and there seems to be no experimental evidence pointing against it.4 (3) According to Wieland,j palladium containing analytically significant quantities of hydrogen is able to remove hydrogen from hydroquinone, producing quinone. If this were true, it would mean that all experimenters who have presented pressure-composition diagrams have been working with unstable systems, for the pressures observed by them are always measurable when appreciable quantities of hydrogen are present, and the pressure of hydrogen * Contribution from the Research 1.aboratory of Physicxl Chemistry, Massachusetts Institute of Technology, ,Xo. 269. Gillespie and Hall: J. Am. Chem. SOC.,48, 1207 (1926). Linde and Borelius: Ann. Physik, 84, 747 (1927). Valensi: Thesis, University of Paris, Les Presses rniversitaires de France, 49, Boulevard Saint-Michel, Paris V, 1929. Linde and Borelius point out that an equation log p =A,'T+B satisfying the data a t high temperatures also satisfies the data a t the lower temperatures, suggesting that there is no discontinuity in the nature of the solid phases. But this is also in accord with the belief of Gillespie and Hall, t h a t the hydrogen-rich phase in question is a solution of hydrogen in palladium, the composition of which approaches t h a t of P d 2 Ha t high temperatures nithout discontinuity. U'ieland: Ber., 45, 484 (1912).
3368
LOUIS J. GILLESPIE AND JOHN H. PERRY
at which it could be in equilibrium with hydroquinone and quinone in a reasonable molecular ratio would be of the order of IO-*^ atm., according to the electromotive-force measurements and calculations of Biilmann.6 But Gillespie and Liu' were unable to confirm the occurence of this reaction and concluded that no valid evidence exists that palladium can dehydrogenate hydroquinone when it contains an appreciable quantity of hydrogen. Gillespie and Hall stated that it happened twice that no change of curvature occurred in an isotherm, the isotherm rising as a continuous adsorption curve. They gave no data or curve to show the course of such a continuous curve, and indeed no such curve has been reported. The isotherm described below had been observed by us before their work, but the interpretation of it was not possible without the aid of their results.* I t appears to belong in the palladium-hydrogen diagram, partly as a broken line indicating a metastable condition. Experimental The technique was similar to that of Gillespie and Hall, except that the heat treatment was not used, its benefits not having been observed. Owing to the use of a capillary-tube manometer in connection with a meter stick, the pressure at 4 mm. and less of mercury has but a small accuracy. Palladium black was prepared in two ways: palladium A, from palladosammine chloride by the method of G ~ t b i e rpalladium ;~ B, by the method of Wielands5 In either case, the palladium black was placed in the apparatus and freed of oxygen by successively treating it with hydrogen and exhausting while hot with a mercury-vapor diffusion pump, before the addition of the measured quantities of hydrogen. Palladium A gave at oo an isotherm with an angle, the first ascending portion of the curve being followed by a line which for a short distance was horizontal and practically coincident with the isotherm of Gillespie and Hall.
TABLE I Date for the Exceptional Isotherm at CC.Hz/g. Pd 0.05 Pressure, mm. 0 . 3 CC. Hz/g. Pd 4 07 Pressure, mm. 6 . 0 Cc.Hn/g.Pd 9.79 Pressure, mm. 5 8 . 8 Cc.Hz/g.Pd 5.06 Pressure,mm. 7 . 5
oo
0.17
0.36
1.49
I
0.4 4.76
0.9
2.9 6.jo
2 . I
7.0 10.18 89.I 5.94 8.8
5 59 8.5 0.0j
0.3 6.66 10.4
10.5 0.17 0.4 7.75 15.9
.49
j.68 14.0
2.82
5.0
8.64
9.43 41.2 3.38
22.0
0.92
1.85
.3
2.6 9.34
I
8.ij 26.0
3.35
4.2
;4,2
4.7
9,94 110.3
6Biilmann: Ann. chim., 191 15, 109 ( 1 9 2 1 ) . Gillespie and Liu: J . Am. Chem. SOC.,53, (1931). Particularly, their identical results for palladium black prepared in two ways (method of Guthier, and method of Wieland). DGuthier: J. prakt. Chem., ( 2 ) 79, 235 irc,o9).
3369
EXCEPTIONAL ISOTHERM O F PALLADIUM-HYDROGEN
Palladium B gave the quite different results shown in Table I. After the first series of additions of hydrogen, all the hydrogen was removed by heating and pumping and a second series of additions of hydrogen was made. In view of the great steepness of the curve at about I O cc. per g., the two series are in good agreement. If these results are plotted, and a smooth curve drawn without regard to the isotherm of Gillespie and Hall, the best curve from o to 4 mm. is close to the best corresponding curve through their data, but lies a little to the right (so that the two isotherms would intersect). But the experimental accuracy is not sufficient to establish a difference of this sort. One line is capable of
:f:l 9 100
90I
80 -
To 640m
b I I
701
'
60 -
d
I
,
,
20
30
,
20
i),
10
OO
10
40
50
FIG.I The exceptional and the equilibrium isotherms for P d and H J a t oo. The inset shows on an enlarged scale the observations taken near the origin.
representing both sets of data with an exceptional error as large as 0.8 mm. Although a better degree of consistency would be maintained in any single experiment with the technique used, it is probable that the experimental error was as large as this.I0 I n constructing Fig. I , the best curve was therefore drawn for the data of Table I together with the data of Gillespie and Hall, and it is represented above 4 mm. by a broken line. It is believed that the same line should represent all data below about 4 mm., and that above this pressure the system is in a metastable condition, as indicated by the broken line. The rest of the isotherm of Gillespie and Hall is also shown for comparison, as well as a dotted line in the region between, as a basis for the followin, discussion. Positive evidence that the error was uf this order has been obtained by Gillespie and Ambrose from a comparison of observed and calculated heat of reaction. J. Phys. Chem., 35, 3105 (1931).
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LOUIS J . GILLESPIE AND JOHN H. PERRY
Discussion Linde and Borelius state that under certain circumstances a (third) phase is formed, an apparent continuous extension of the hydrogen solution (easily obtained a t small concentrations) up to a concentration greater than in Pd,H. The curve for such an extension would be somewhat like the dotted line in Fig. I. But, if we accept the broken line as the true extension of the curve into the metastable region, it, is impossible that any curve such as the dotted line should be at oo a proper extension of the curve from o to 4 mm., where the word “extension” is used in the same sense. At other temperatures we have no data for broken-line extensions, and from the appearance of the isot,herms of Gillespie and Hall it seems probable that such extensions would lose at higher temperatures the characteristic curvature of the broken line of Fig. I , and thus the characteristic difference between the broken and dotted line of Fig. I might largely be lost at higher temperatures. Yet the experiments at: higher temperatures, particularly those of Lambert and Gates,” seem to prove clearly that isotherms resembling the dotted line, and obtained by various experimenters,12 cannot be regarded as extensions analogous to the broken-line extension. For, Lambert and Gates obtained hysteresis loops by successive addition and subtraction of hydrogen. The occurrence of hysteresis loops clearly indicates that, the solids are indifferent to a significant excess or deficit of hydrogen in the gas phase in contact’ with the solids. In such a case the pressure curve tells us nothing definite about the nature or composition of the solid phases. The statement of Linde and Borelius appears therefore to need qualification as regards the composition, up to which the extension has been observed. True extensions of the curve for the weaker hydrogen solution have not been shown at temperatures other than oo,and in this case the highest concentr:ition reached was far from that of PdJI, being in fact more nearly that corresponding to 0.9 j atoms of hydrogen per atom of palladium. The Characteristic shape, noted by Hoitsema, of curves in the palladiumhydrogen system such as the dotted line of Fig. I is evidently due to the presence of two solid phases in contact with a gas phase, the pressure of which gas can be changed considerably without provoking reasonably rapid changes in the s01ids.l~ Summary An exceptional pressure-composition isotherm has been observed at oo for the system palladium-hydrogen, which appears to represent an extension of the first rising isotherm into a metastable region. The highest pressure observed was I I O mm. of mercury, a t about 0.9j atoms of hydrogen per atom of palladium. .1 diagram is given showing the relation of this isotherm to the equilibrium isotherm at 0’. “Lambert and Gates: Proc. Roy. Sor., 108A, 456 i p , j : . ‘?Hoitsema (and Roozeboom): Z. physik. Chem., 17. I ( 1 8 q j i ; IIolt, Edgtir and Firth: 82,513 (1913);Lambert and Gates: loc. cit.; Sieverts: 2. physik. Chem., 88, 4jr ( r ? r q ) . I 3 The possihilitv that 111 this system at a given temperature two solid phases might be in equilibrium wit6 ;as at more th”m one pressure, is excluded by the phase rule.
