SOLUBILITY AS A COMPLICATING FACTOR I N ADSORPTION MEASUREMENTS AT GAS-SOLID INTERFACES B Y E. W. R. STEACIE
In adsorption experiments with gas-solid systems complications frequently arise on account of slow and sometimes irreversible effects which are superimposed upon the ordinary rapid process of adsorption. In a recent paper, Taylor’ postulates the existence of an activation energy in adsorption processes. This leads to the conclusion that, in certain cases, a high activation energy may be necessary, and adsorption will thus take place slowly. More than one kind of adsorption is therefore possible, and each type of adsorption process will proceed a t a rate depending upon its necessary activation energy. On this basis Taylor explains the slow and irreversible effects which often accompany the usual rapid adsorption. I n the present communication no attempt will be made to question the postulates of Taylor’s theory, or the possibility that it may be true in certain cases. On the other hand, an attempt will be made to show that Taylor’s hypothesis is not of general applicability, and that in most cases the slow and irreversible effects which accompany adsorption measurements must be explained on the basis of the known tendency of gases to dissolve in solids. This discussion will be mainly confined to the re-examination of the experimental evidence cited by Taylor in support of his hypothesis of an activation energy, The Solubility of Gases in Solids
It seeme to be the prevailing and erroneous opinion that the amount of a gas dissolved by a solid is invariably exceedingly small. Taylor, for example, states that in many of the cases which he discusses “the quantities of gas involved (in adsorption measurements) are of several orders higher magnitude than the known solubility of the gas in the substance concerned”. I t will be shown in a later section, however, that in almost all the cases cited by Taylor the solubility is quite large enough to account for the observed facts. We will therefore examine in detail the effects which would be produced in an experimental system if the adsorption equilibrium were complicated by the occurrence of solubility. The Rates of Adsorption and Solution The generally accepted view is that adsorption in a gas-solid system is a rapid process, equilibrium always being established within a few minutes. Solution, however, is usually a slow process since its rate is governed by the diffusion of the gas into the solid, and this is a slow process.* The actual rate J. Am. Chem. Soc., 53, 578 (1931). example, Johnson and Larose: J. Am. Chem. SOC.,46, 1377 (1924);49, 312 (192j);Lombard: Compt. rend., 177, 116 (1923);Richardson, pl’icol, and Parnell: Phil. Mag. (6), 8,I (1904);Deeming and Hendricks: J. Am. Chem. SOC.,45,2857 (1923).
* See for
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of solution of a gas in a solid has been measured in only a few cases. The system oxygen-silver , has been thoroughly investigL ,ed,' however, and will be treated as a typical system for purposes of discussion. At high temperatures in the neighbourhood of 8oo0C. equilibrium is attained instantly, but a t zoo°C. several days are required for equilibrium. At low temperatures where the extent to which oxygen is adsorbed is appreciable, the rate of solution is thus extremely small. When oxygen is brought into contact with silver a t low temperatures there is thus a rapid drop in pressure due to adsorption, followed by a slow solution of the gas in the metal, This would seem to be the obvious explanation of results such as those obtained by Benton and Elgin2 with oxygen and silver catalysts. I n their experiments at 110°C. about 415 of the gas was taken up rapidly, while the remainder required about 2 4 hours for equilibrium. Benton and Elgin ruled out the possibility of solution on the grounds that Sieverts3 found no nieasurable solubility of oxygen in silver below 4ooOC. This, however, has been disproved by Johnson4 and the author. The solubility of oxygen in silver a t 2ooOC. is about 0 . 1 5 C.C. a t N.T.P. per I O grams of silver. This is about ;yoof the total amount taken up in Benton and Elgin's measurements at' 110°C.. From the form of the solubility temperature curve, the solubility should be much greater a t 110' than a t 200'. It would therefore appear that practically all the gas which was taken up slowly can be accounted for on the basis of solubility, and that the assumption of an activation energy and a second type of adsorption is not necessary in this case. Slow effects can similarly be explained in other systems cited by Taylor where it has been definitely established that the gas is appreciably soluble in the solid concerned. Such cases include the systems, palladium-hydrogen: platinum-hydrogenJ6copper-carbon monoxide, and nickel-carbon monoxide.' There is but one known case in which slow complicated adsorption effects have been observed with nitrogen, namely in the system nitrogen-iron.* I t is highly significant that a large number of investigators have shown that iron is the only metal which will dissolve nitrogeng to an appreciable extent. Rate of Solution and Extent of Surface It may be objected that a t the temperatures a t which many catalysts are used, say 100' to zooo, the rate of solution of the gas in the solid is much Steacie and Johnson: Pror. Roy. Yo(.., 112A, jq2 (1926). J. Am. Chem. Sor., 48, 3 0 2 ; (1926). 3 Z . physik. Chem.. 60, 179 (190;). Proc. Roy. So?.. 112A, 542 (1926). Seumann and Streintz: JVied. . h i . , 46, 4 3 1 (1892); Hoitsema: 2. physik. Chem., 17, I (18x5); Holt, Edgar, and Firth: 82, 513 (1913);Siwerts: 88, 105 (1914);Holt: Proc. Roy. Soc., 90A, 226 (1914). Seumarin and Ytreintz: \Vied .41111., 45, 4 5 1 (1892); hlond, Ramsay, and Shields: Phil. Trans., 1Y6A. 6 5 ; (1S95); Sicverts: 2 . Iihysik. Chern., 60, 129 (190X); Gutbier and Maisch: (I
everts and Krumhhaar: Ver., 43, 893 (1910). and Mayrhofer: Z. Elektroi,hemie, 35, 590 (1929). Graham: Proc. Ro?-. Sor., 15,502 (1b66);Seumann: Stahl untl Eisen, 34, z j z ; Jurisch: Dnktorarbeit, Leipig ( 1 9 1 2 ) .
* Frankentnirger
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slower than the observed slow “adsorption” effect. The rate of solution of a gas in a solid plate is, however, inversely proportional to the thickness of the plate. The rate of solution in finely divided catalysts, while still slow, will be great enough to account for the observed effects. At very low temperatures it is probable that solubility equilibrium will never be attained. The amount of gas dissolved in any experimentally feasible time interval will then depend on the fineness of subdivision of the solid, i.e. on its surface. It follows that a t low temperatures only the outside layers of the solid will be saturated, and the apparent solubility will cease to be dependent on the mass of the solid and will depend primarily on its surface. Measurements carried out with varying surface-volume ratios will therefore no longer distinguish between adsorption and solubility.
The Variation of Sorption with Temperature In a few cases the solubility of gases in solids decreases with increasing temperature, for example the solubility of hydrogen in palladium, titanium, and vanadium.’ I n the majority of cases, however, the solubility increases as the temperature increases. I n the case of oxygen and silver the solubility passes through a minimum a t 400OC. and commences to rise again a t low temperatures. This is the only known case of such behaviour, but few systems have been thoroughly investigated, and there is some theoretical justification for the assumption that minima would be obtained in other systems if accurate low temperature measurements were made.2 Adsorption, on the other hand, usually decreases as the temperature increases. It follows that the variation in the total sorption by a catalyst as the temperature varies may be complicated, and maxima or minima may occur. Similarly, since definite heat effects accompany adsorption and solution, composite heats of sorption may be obtained. The peculiar types of adsorption-temperature curves obtained with platinum and h y d r ~ g e n ,palladium ~ and h y d r ~ g e n ,and ~ copper and hydrogen: are apparently due to composite effects of this sort. The fact that the adsorption of oxygen on gold increases with increasing temperature6 cannot, however, be explained in this way since it has been shown by Johnson and Toole’ that oxygen is not appreciably soluble in gold. I n this particular case some mechanism of the kind postulated by Taylor may be responsible for the observed behaviour. ‘Kirschfeld and Sieverts: Z. physik. Chem., 145A, 227 (1929); Z. Elektrochemie, 36, 123 (1930). Steacie and Johnson: Proc. Roy. Soc., 117A,662 (1928). a De Hemptinne: Z. physik. Chem., 27, 429 (1898). Guthier, Gebhardt, and Ottenstein: Ber., 46, 1453 (1913). Harris: unpublished, quoted from Taylor’s paper. Benton and Elgin: J. Am. Chem. Soc., 49, 2426 (1927). Unpublished work.
’
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The Adsorption Isotherm The fundamental characteristic of the usual adsorption isotherm is the large amount of gas which is taken up a t low pressures. I n the case of solubility, however, Henry’s law usually holds. A small amount of solubility, therefore, will exert little influence on the total sorption a t low pressures, but will have a much larger effect a t high pressures. In consequence the fundamental form of the adsorption isotherm will not be greatly influenced by the presence of complicating effects due to solubility. The main effect of such solubility will be to mask the presence of a definite saturation limit to adsorption, and to cause a slow steady rise in sorption as the pressure is increased. This is probably the cause of some of the apparent exceptions to Langmuir’s theory of adsorption. Hysteresis Effects Since solubility is normally a slow process, it follows that the rapid heating and cooling of a solid in the presence of a gas may give rise to apparent hysteresis effects. In consequence the total sorption may be dependent upon the history of the system. This will be referred to again in connection with the system hydrogen-nickel. The hysteresis effects obtained with copper and hydrogen,’ and oxygen and silver are undoubtedly of this type. “Irreversible Adsorption” On account of the slow rate of solution of gases in solids, the dissolved gas can be removed from the solid only by long continued evacuation a t a fairly high temperature. This has led to much inaccurate work in which gases taken up by solids could not be recovered by evacuation. Such gas was said to be “irreversibly adsorbed”. It is of course apparent that any investigation in which the adsorbed or dissolved gas cannot be completely recovered will lead to erroneous results. This point has been emphasized by Johnson2 and the author and by Bircumshaw. Such irreversible adsorption is direct evidence that solubility is a complicating factor. Another cause of irreversible adsorption lies in the methods used in the preparation of many catalysts, or in their chemical composition. If, for example, a metallic catalyst is prepared by the reduction of an oxide with hydrogen, the resulting metal will contain hydrogen which will not be removed by evacuation a t a low temperature. If adsorption measurements are now made with oxygen, there will of course be a permanent loss of oxygen by reaction with dissolved hydrogen to form water. All adsorption measurements in which irreversible phenomena occur should therefore be regarded with suspicion.3 Harris: Master’s Thesis, University of Virginia (1924), quoted from Taylor’s paper. (1926). 3 Phil. Mag., ( 7 ) , 1, 510 (1926).
1
* Proc. Roy. SOC., 112A, 542
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Carbon is a notorious offender in this respect and no confidence should be placed in measurements in which there is a permanent loss of hydrogen or 0xygen.l Special Cases Three or four of the experimental systems on which Taylor laid particular emphasis will now be considered in some detail. ( A ) Hydrogen and Nickel. Benton and Whitez observed that with hydrogen and nickel different amounts of adsorption occurred depending on whether the metal was kept exclusively a t low temperatures, or was saturated at high temperatures and then cooled. This is in agreement with the idea that solubility is the cause of complications. The rate of solution is negligible a t low temperatures, and nickel which is cooled rapidly will thus contain all the hydrogen which was dissolved a t high temperatures.3 The data of Benton and White on the rate at which equilibrium is attained also support the assumption of solubility. Thus a t very low temperatures ( - 183') equilibrium is rapidly established. At such temperatures any dissolved hydrogen will remain in solution, while no more will dissolve on account of the immeasurably slow rate of solution. The whole process is therefore one of adsorption and the rate is consequently high. At higher temperatures Benton and White found that equilibrium was established fairly rapidly at very low pressures, slowly at moderate pressures, and somewhat less slowly a t high pressures. At such temperatures the rate of solution will be appreciable and adsorption will be complicated by the resulting solubility. On account of the different forms of the adsorption and solubility isotherms, solubility will be inappreciable at very low pressures. Adsorption will therefore be the predominat'ing process and equilibrium will be rapidly established. At higher pressures the solubility becomes more pronounced and the rate of the whole process is much slower. The rate of solution in nickel will, by analogy with the oxygen-silver system, be proportional to the square root of the pressure. Hence a t high pressures equilibrium will be established somewhat more rapidly than a t intermediate pressures. The nickel-hydrogen system, which has been cited as furnishing the best evidence for the activation energy theory, is t'hus excellently explained on the basis of solubility. ( B ) Hydrogen and Oxide Catalysts. Taylor cites the results of Garner and Kingman," who found that hydrogen adsorbed at low pressures on ZnO-CrZOo catalysts was given off on heating to I O O - I ~ and O ~ ,uas then readsorbed in 2 0 or 30 minutes. In the first place, adsorpt,ion measurements with hydrogen on an oxide should be regarded with a certain amount of suspicion. since there is always 1 See particularly I m n y and Hulett: J. Am. Chem. Sur., 42, I & (1920); Rhead and JYheeIer: J. Chem. SOC., 101, 8 3 1 (1912); 103,461 (1913). 2 .J. Am. Chem. ?or., 5 2 , 2 3 2 5 (1930). 3 Hydrogen has heen shown I O h e moderately soluble in nickel \)y Sieverts. Z. phyaik. Chem., 77, 611 (1911). ' S a t u r e , 126,352 (1930).
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the possibility of a reaction between the hydrogen and the oxide, or between hydrogen and oxygen dissolved in the oxide.’ However, even if the measurements are trustwort’hy, the behaviour is simply explained by the assumption of a combined adsorption and solubility. At low temperatures adsorption will predominate on account of the very slow rate of solution. As the temperature is raised the adsorption decreases in the usual way, adsorption equilibrium is rapidly attained, and some gas is desorbed. This is then followed by a slow solution of the gas in the oxide, since the solubility usually increases with increasing temperature. The experiments of \17illiamson2on the adsorption of hydrogen by manganous oxide, in which “irreversible adsorption” was observed, are to be suspected in common with all experiments in which complete desorption is not obtained. (C) Hydrogen and Quarfz. According to Taylor, the re-investigation of the hydrogen-quartz system is necessary in order to ascertain if complicating effects occur due to solubility. This would hardly seem to be necessary in view of the large number of workers who have found that hydrogen is soluble in quartz to a small but definite extent, and will diffuse3 through it. Conclusions It may therefore be concluded that the experimental evidence strongly favours the assumption that the slow effects accompanying adsorption processes are due to the solubility of the gas in the solid concerned. It should again be emphasized that the mere determination of the surfacevolume ratio will not settle the question at very low temperatures. At such temperatures solubility equilibrium is never reached, only the outside layers of the solid are saturated, and the apparent (non-equilibrium) solubility is dependent on the extent of the surface. I t is regrettable that the majority of investigators who are interested in adsorption processes are inclined to overlook entirely the large amount of available experimental data on the solubility of gases in solids.
summary The experimental evidence cited by Taylor in support of the hypothesis of an activation energy for adsorption processes has been reexamined. It is concluded that irreversible adsorption, and slow effects accompanying adsorption processes may be simply explained on the basis of existing data regarding the solubility of gases in solids. Physicnl ChemirtrU. Laboratory, M c M Z l / n i w r s i t ! j , .liontrenl, Carindo. The solubility ot osygen in lower asides at temperatures a t which the higher oxides are unstahle has been prored h y LeBlanc: Ann., 6.480 (1816), and by LeBlanc and Sachse: Z. Elektrochemie, 32, 20 (1926). * Cnpuhlished, quoted from Taylor’s paper. See for example: Steacie and Johnson: Proc. Roy. SOC., 117A,662 (1928); Bodenstein and Kranendierk: Sernst Festschrift, 1912, 100; Kustner: Ann. Physik, 46 1095 (1915); IYiliinms andFerguson: J . .4m. Chem. Soc., 44, 2160 (1922);46, 6 3 j (1924);Vil1ard:Cornpt. remi., 130, 1752 (1900); Berthelot: 140, 821 (1905); Richardson and Richardson: Phil. Map., 22, 704 (1911); Slayer: Phys. Rev., (2) 6, 2 8 3 (1915).
