GUEST AUTHOR John M. Thomas
University College of North Wales Bangor, U. K.
T e x t b o o k Errors,
29
The Existence of Endothermic Adsorption
It, is often1 explicitly stated, hut far. more f r e q u e d y implied, that adsorptiou is always an exothermic process. This is tant,amount t o saying that adsorption is always acrompanied by a decrease of heat oont,ent or that an endot,hermic adsorption cannot. occur. Such conclusions are helied both by observed fact and by moderu views on the role of adsorption in heterogeneous catalysis. I n the following discussion, for convenience, it is t,acit,lyassumed that by adsorption is meant adsxpt,ion from t,he gas phase on a solid adsorhent. The just,ification generally produced for t,he statement that adsorptious are always exothermic is a thermodvnamic one: Since the Gihbs free energy, -.. F,. must decrease for any spontaneous process. and since adsorpt,iou is one suvh proress which is always accompauied by a decrease in entropy (the number of degrees of freedom in the adsorbed state being less than the number in the gaseous st,atc) t,hen,arcording to
dissociative chemisorption of molecular hydrogen on an iron surface contaminated with sulfide ions is endothermic (3',4). There are a t least three other cases of chemisorptions where endothermicity is strongly suspected. ( I ) Trapuell has suggested ( 6 ) that, very oft,en, where chemisorpt,ion 011 clean evaporat,ed metal films is uot observed a t temperat,ures up to O0C, the chemisorption is, in fact,, endothermic. Typical examples are molerular hydrogen on copper, silver, gold, and cadmium; molecular oxygen on gold, and carboll monoxide on zinc (6). (2) Zwiet,ering has noted (6) that the exchange reartion Hz+ 1). F? 2 HI) (2)
AH must always be negat,ive; i.e., adsorption is always exot,hermic or, as is customary t,o express it in this field, the heat of adsorpt,ion i s , ositive. I:or t,he vast, majority of adxxptions no error is introduced hy employing this argument. It may well be that t,he argumeut applies invariably t o physical adsorptions where it can be shown, using equations derived from stat,istical mechanics, that even when the adsorbed species is supermobile, the entropy of a species in the adsorbed st,ate is always less than t,he eutropy in the gaseous state (1). I n view of this, it is not. surprising that endothermic physical adsorption has never been reported.
and thr para-ortho hydrogen conversion arc both catalyzed hy strongly dehydrated aluminium oxide a t temperatures as low as -80°C. The fact that in neither rase was there measurable adsorption of hydrogen (111 the catalyst has been iuterpreted (3') as evidence for endothermic dissociative chemisorption of hydrogen. (3) deBaer, in reviewing the energetics of endot,herniir: chemisorpt,ion ( S ) , concludes that for both the silver-oxygen and the copper-oxygeu systems the high value of the work function of the metal makes it likely that the adsorptiou of O Zions, for which there is evidence, is end~t~hermic. Detailed potential energy diagrams d e p i h n g endothermic. chemisorpt,ion for many of the above-mentioned systems are already available ( 3 ) . However, the following discussion may prove useful; its prime aim is to account for the absence of endothermic physical adsorptions and the existence of endothermir chemisorptions.
Endothermic Chemirorptions
Positive Entropy Changes Can Occur
AF
=
AH - TAS
(1)
Vor certain chemisorpt,ions, however, bhere is incont,rovert~ibleevidence t,hat endothermic processes have been observed. I n the dissociative chemisorption of molecular hydrogen 011 glass, a process which requires considerable activation, the heat of adsorptiou is approximately - 15 kcal mole-' (2, 5). Again, the heat of adsorptiou of hydrogen ou a metal surface is drastically reduced, and may even become negative (AH positjive), if the surface is contaminated; e.g., the Suggestions of material suitable for this column, and guest eolunms suitable for publication directly, are eagerly solicited. The," should be sent with as many debails ns possible, and particulitrly with refcronccs to modern textbooks, to Karol J. 31ysclsj lkpartment of Chemistry, University of Southern California, Los Angeles 7, California. ' &nee the purpose of this column is to prevent the spread and conti~ruationof orrors and not the evaluation of individual texts, the sourco of rrrors discusqed will not be cited. To be presented tho error must occur in at lcast two independent stand:&d books. 138
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Journal of Chemical Education
The ent,ropy change is always negative when physical adsorption occurs, since this is essentially a condensation process analogous t o liquefaction on the absorbent. This is so because the number of degrees of freedom of the adsorbed species is less than the numher it possessed prior t o adsorption, and because the ent,ropy of the absorbent is unaltered, since there is no rhemical interaction. (This may not he entirely correct-see belolv.) Chemisorption differs from physical adsorpt,ion in that. it is not merely a surfare condensat,ion but a surface reaction involving the rupture and creation of chemical bonds. I11 the same way t,hat some chemical reactions are endothermic, so also it may be expert,ed that some surface reactions (chemisorptions) ran be endothermic. I n endothermic adsorptio~the entropy of adsorption not only must be positive, but the TAS term must also exceed numerically the AP term as shown in equatioii
(1). This conclusion may prove difficult to explain physically in view of the popular not,ion that the entropy of adsorption is always negative. The fallacy of the last statement, and the reality of positive values for t,he entropy of adsorption, are brought out by envisaging the following hypot,hetical system which is useful for heuristic purposes: A molecule, Az, is dissociat,ively chemisorbed on the surface of a solid, M; consider strength of the M-A bond equal to half that of t,he A-A bond. If the adsorbed atoms, A , have complete two-dimensional mobility, it would then follow that a positive entropy change, A S , would result, corresponding to a net gain of one dcg~eeof freedom, and the free energy change accompanying this thermally neutral process would he equal solely t,othe TAS term. Posit,ive values for the entropy of adsorption are more likely to originat,e, however, not in the increase in entropy of the species adsorbed-it is only in rather exceptional cases, such as the above hypothetical example, that the entropy of the adsorbed species will in fact increase-hut in t,he change in entropy of the adsorbent itfelf. Thus, during chemisorption even if, as is likely, the entropy of the species adsorbed decreases on adsorption, (due largely to reduction in the number of degrees of freedom) this decrease may be exceeded by a concomitant increase in the entropy of the absorbent. This, in turn, will lead to endothermic rhemisorption if the TAS term is greater numerically t,han the AF t,erm. The possibilit,y of endothermic chemisorption was not considered early in the growth of surface chemistry, probably because there has been a persistent tendency until very recently to at,tribute the observed total entropy of adsorption t,o the adsorbate only, i.e., t o ignore entropy changes emanating from chemical changes in the surface condition of the adsorbent. This const,itut,es a serious omission. We recall the early work of Bangham and his as~ociat~es, whose invest,igations showed (7, 8) that, upon reduction of the surface free energy of charcoal and coal by the chemisorpt,ion of gases, t,he solid expands by amounts of the order of 0.5% in one dimension. Such dimensional changes will, of course, contribute t o the observed total entropy of adsorption. More recently, for other systems, Zettlemoyer (9) has emphasized that the contribution of the entropy change which the adsorbent itself undergoes on adsorption can be exceedingly large. Another import,ant demonstration that a solid surface is significantly altered during adsorption is contained in the work of Yates (10, 11). The latter showed that when nonpolar gases such as argon, krypton, and oxygen are adsorbed on porous silica glass a t liquid air temperatures, expansion of the adsorbent occurs. I t is, in fact, estimated (I 1) that the expansion occurring when a monolayer of argon is adsorbed a t 90°K corresponds t o an entropy change of roughly 4 cal deg-' mole-'. These results are remarkable since the adsorption is a physical one, as evidenced by the small heat of adsorption (AH = -2.23 kcal mole-' a t half coverage for argon a t liquid air temperatures). So even for physical adsorption it is not absolutely correct, to state that the adsorbent is inert and undergoes no
entropy though .. changes, - the ensuing - error is likely to be very small. The endotherniic or negative heat of adsorption discussed above should not be confused with negative net heat of adsorption which was first observed (G, 13) as early as 1927. By net heat of adsorption is meant the difference between the heat of adsorption and the latent heat of condensation. Graham (14) has discussed fully the reasons why negative net. heat of adsorption occurs in terms of the entropy of the adsorbed phase. Summarizing, it may be said that in all adsorptions entropy changes due t o surface-structural changes in the adsorbent itself must be considered along with the entropy changes of the adsorbate. In physical adsorption, the cont,ribution of the first of these two entropy changes t o the total entropy change is always likely to be insignificant compared t o the rontribution of the second (though further investigations alone can establish this unambiguously). I n chemisorption, however, t,here is ample evidence already available to show that t,he entropy change due to wrface structural changes in the adsorbent contributes significantly t o the total entropy of adsorption. Whenever the positive entropy change due to adsorbent surface changes exceeds numerirally the negative entropy change due t o loss of freedom of the adsorbate, the net entropy of adsorption is posit,ive. Failure t,o recognize this has led to serious error when using equation (1) to arrive a t the sign of the heat of chemisorption. This, in essence, is why endothermir rhemisorption has been discounted until very recently, though its existenre should be considered as normal as the existence of endothermic solution. Acknowledgment
I wish to express my gratihde to Professor Karol J. Mysels for advice in the preparation of this paper; to 1'. H. Emmett for a useful suggestion; and to 1'1.0fessor St,anley Peat, F.R.S. for his interest in this work. Literature Cited (1) KEMBALI,, C., "Advances in Catalysis," Vol. 11, p. 233,
1950. (2) LEHR,J. J., AND DEBOER, J. H., Z.Pk&. (1933).
Chew., BZZ, 423
13) DEBoER,J. H., "Advances in Catalreis." Vol. IS, 1,. 472,
1957.
(4) DEBOER, J. H., "Advances in Catalysi~,"Vol. VIII, p. 17, 1956 -~ (5) TRAI'NEI~I., B. M. W., "Chemisorption," Butters-ort,h, London (1955), p. 61. P., ~~npublished work cited in references (-i) (6) ZWIETERIKG, ~
~
and (4). (7) BANGHAM, I). H.; FAKHOTRY, N., AS" MOHAMED, A. F . , Pmc. RO],.SOC.,A147, 152 (1034). (8) BASOHAM, I). H., AND MAGGS, F. A. P., "Conference on the Ultril-Fine Structure of Coals and Cakes," p. 118. 1944
(London: British Coal Research Assaeia.tition Committee). (!I) ~ETTLEMOYER, A. C., Chem. Reus., 59, 042 (1959). (10) Yams, I). J. C., Pme. Roy. Soe.. AZZ4, 526 (1954). (11) YATES,11. J. C., "Advanc~sin C ~ t k l y s i ~ ,Vol. " IX, p. 481,
1957
(12) Coo~mcm,A. S., J . Am. Chem. Soc., 49, 708 (1927). (13) KEYEB, F. (:., A K D MARSHAL, hl. J., J . - 4 l n Chem. Soc., 49, 156 (192i). (14) GRAHAM, I)., J . P h ~ s Chew., . 60, 1022 (1956).
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