A POSSIBLE RIEC‘H-ISISlI FOR T H E L O K E K I S G O F T H E HEAT O F XCTIT’ATIOK O F =1 RE-dCTIOS B’L- X CATALYTIC SURF,1CE BY ROBERT E. BURK
Heat of Activation and Reaction Velocity bluch evidence has acciiiniilated in recent years to shoTT that the velocities of homogeneous chemical reactions at tlifferent temperatures are in accord with the empirical eqiiation tl Ink tlT = Q RT’, lirought forn-arc1ljydrrheniusl in 1889. ‘Q’ is the ‘heat of activation‘. which is found to lie the amount of energy molecules must possess, in excess of their average energy, in order to react. , . Kinet’ic analJws of several homogeneous hiniolecular gas reactions.’, have greatly clarified the nicchnnisms of these reflctions. antl have shown that their rates can he interpretetl on the h i s of simple therninl activation 1iy collision. This TI-oil; inclicates that for liiiiioleciilar rcactions, the niechanism of activation cannot tle1;entl upon the spccific nature of the reactants. antl that the rates of l)iiiio!ccul:ir reactions are govcrnetl siniplj- 11)- the availaldity of their heat of activation. i. e. 1))- the inagnitutlc of ‘Q’. There seems to lie consitleralile justification. therefore, for the Tien. that the rates of chemical reactions are tleterniinetl hy the niagnitutle of their heats of activation. I t ~ o u l t lseein t o folloir then that in order for a catalyst t o accelerate a cheniical reaction it must in general lon-er its heat of activation. This vien-, which TI-oultlnot, rule out s p c ~ i deffects, s.uch as the assistance of the walls in dissipating the excess energ)- of the nen-ly fornietl iiiolecules in reactions of the t)-pe A i A B=-AB, nor the effect of the change in the ort!er of the reaction on the surface.: appears t o he held liy various norkersfi. Severtheless, nothing very definite has lieen atlvuncetl t o explain the exact way in which the surface coultl lie expected to Ion-er the heat of activation. Thus H. S. Taylor‘ in the “Fourth Report of the (’onmiittee on Contact Cataly says that “our knon-ledge is almost neglipilile on the nature of the activation process induced in the reactant by its association with the catalyst.” The consensus of opinion seems t o lie that the attractive force h t n - e e n molecules varies inversely as the eighth or ninth pon-er of their distance apart. as calciilated by Deliye’ antl other?. On this hasis Langmuirg has calculated that the attractiT-e force of a surface, which enables molecules t o lie adsorbed, will decrease t o half its value in 0.3 x IO-^ cnis. This is about one seventh r’’
Hinshclw-ood :ind Hinshelvood and PricSh:ird: J. Chem. Soc..127. I L i F 2 (1925). CoIl~tnble:P ~ o cRoy. . SOC.108,3j,j I 192,i). ’ T a y l o r : J . Phys. Chem.. 30, 1 1 3 (1926). Dchye: Physik. Z.,21. 178 (1920). Langmuir: Trans. Faraday doc.. 17. 610 ( 1 9 2 2 ) .
‘
LOWERISG O F THE H E l T O F ACTIT-ITION
1135
the diameter of the monat,omic neon molecule,' and therefore probably not much more than this fraction of the diameter of any atom in the molecule adsorbed. The only possible exception would be the hydrogen atom, but it's minimuni diameter2 is I . O X 10.~.This would mean t'hat adsorption is a very localized matter indeed. and that the attractive force of the surface cannot tie very strong a t distances greater than the diameter of the atom actually attached in the adsorbed niolecule. Therefore, the possibility that the adsorbed moleczile CIS CI irhole may lie in a field of force n-hich, acting directly and not through the chain Tyould distort it in virtue of differences in 'polarity' or 'unsaturation' of different parts of the molecule would lie ruled out. Let us consider the thermal decomposition of a molecule X-B.For the tlecomposition to take place. A and B must he torn apart. If the decomposition is homogeneous antl nionomoleciilar the energy necessary t o separate &Ifrom B is directly the heat of activation, n-ha,tever the nature of the bond lietn-een A and B. If the clecomposition is liimolecular. there must still lie a partial separation of &Iantl B at the moment, of collision against some sort of attractive forces. The heat of activation per molecule is. however, less in this case. (The writer is aware that one cannot lie dogmatic as t o just n-hat happens at the moment of collision. hut it is difficult to see lion- something like this can tie avoided). Khatever the source of the energy of activation then, the net result is that n-hich ~~-oiilcl lie ohtained if -Iantl B n-ere separated liy a force sufficiently strong to overcome their mutual attraction. It seems inevitahle. therefore, that if one actually does apply a large enough force. the separation of A and B will lie accomplished, antl if the force is not sufficient t o effect the separation completely. it JT-ill nevertheless partially separate them antl thus diminish the heat of activation. I t is the purpose of the present paper t o offer a possible explanation of the mechanism by nhich the heats of activation can lie lon-eretl liy catalytic surfaces. Outline of the Theory I n view of t'he extremely short range of molecular attraction, emphasized above, it would seem that a surface could accomplish this partial separation of the atoms -1and B in the molecule A-B only if both crtouis are attached t o the surface, and the adsorbing atoms are so spaced that the distance between the points of maximum intensity in their attractive forces is not quite the same as the corresponding distance in the molecule -4-B. Atlsorption at two or more points under these conditions is the essential feature of the proposed mechanism, and the suggested condition necessary for a surface to lower the heat of activation. The diagram, Fig. I . , is supposed t o represent the spacing in the atoms in the surface of a solid which catalyzes the decomposition of the molecule A-B. An adsorbed atom is represented as connected with the adsorbing surface hy a clotted line. According to the theory, both A and B must be att,ached in I
Rankine: Proc. Roy. SOC.,83 -1,516 (1910). "Atomic Structure a n d Spectral Lincs," translatcd 1))- H. L. Broee. p. 213.
* Pommerfeld:
1136
ROBERT E . BURK
order for the decomposition of A-B to be catalyzed. The figure represents a momentary condition of the surface. There are three effective pairs i. e. 1-6,3-8 and 5-10. The surface atoms 2 , 4, j , 9, and 11 are 'out of action' through single adsorption. 1 2 is bare. A short time later, the doubly adsorbed molecules may have decomposed and the products partly evaporated as atoms, or, by combining with others of their kind from adjacent spaces. as molecules. Also, some of the single adsorbed molecules will have evaporated as such, thus offering new surface atom pairs with the proper spacing for effective double adsorption eg. '7-1. The effectiveness of the surface atom pair in diminishing the heat of actiration would depend upon the attractive force of each adsorbing atom for the respective atoms of the adsorbed inolecule, and therefore upon the nature of each of the four atoms concerned, and upon the orientation of the surface atoms. The distance apart of the pair of ,$ adsorbing surface atoms which would %, 5, '-i 'h /'' result in the maximum catalytic effective2 ' ? ness woultl depend upon the strengths of 6, '4\\ the adsorbing forces and their rate of 3 5 '3 decrease with distance, upon the distance apart of the centres of the attractive 4\ P forces concerned in the adsorbed molecule, /e %, ,,0 ,h ant1 upon the rate of decreaee in the heat 0' * 0' /O I/ ' z of activation as the two atoms of the adsorbed molecule are moved relative to FIG.I each other. In view of the very short range of molecular forces, pairs of surface atoms TI-ould be expected to be catalytically active only when their distances apart are very close to the distance of maximum effectiveness. This point js strongly emphasized as it accounts for the extreme variability of some catalytic surfaces. The arguments are equally valid for molecules attached to surfaces at more than two points. The remarlis of TI-. C.;Ilcc'. Lenisl at the Faraday Society discussion on catalysis, while they are less definite and are framed in the language of the radiation hypothesis, embody some of the features of the proposed theory.
J'..,
'\
~
P,'?
Evidence in Favour of the Theory It is very difficult to subject this theory to a conclusive test. However, CI priori theoretical considerations as outlined above render it very probable. It is also worthy of note that the multiple adsorption theory is capable of explaining the numerous diverse phenomena of catalytic behaviour in more definite terms than others which have been suggested. Some of these will be briefly discussed. That a molecule can be attached to a surface by more than one point was suggested by Hinshelwood2in connection with experiments on the thermal del
ITT. C. McC. Lewis: Tranq. Fnrndny Soc.. 17, 662 (1922). Hinshelwood and Burk: J. Chem. Soc., 127, I I 1 4 (1925).
LOWERING O F THE HEAT O F ACTIT-ATIOS
1137
composition of ammonia, where it mas found that the quartz surface as different temperatures showed capricious variability. It was also found in these experiments that added hydrogen sometimes retarded the reaction and sometimes failed to do so. =Idkins" 2.3.4 has been led by numerous experiments to the conclusion that the spacing of the surface affects the reactivity of the adsorbed molecule, but is not definite as to the esact mechanism by which this is brought about. Langmuirj has also conqidered the spacing of surface atoms t o be capable of affecting the velocities of chemical reactions taking place upon them, but his apparent conception is quite different from that advanced in this paper, and supplements it, The observation of Gauger antl Taylor6 that the adsorptive capacity of nickel for hydrogen is different at 25' from the value at 305' can be interpreted as evidence for multiple adsorption. The adsorptive capacity of copper for carbon monoxide7 also varies with temperature. This evidence would be conclusive if the same effects n-ere observed on single faces of crystals compoqed of one kind of atom only. On whole crystals (crystal powders), a certain number of saturation limits woulcl be expected corresponding t o the number of types (strengths) of force fields offered by the individual atoms of the crystal. =1 definite number of saturation limits would also be expected from the various possible types of multiple antl single adsorption. An investigation of the number of saturation limits on crystal powders at different temperatures should therefore furnish convincing data as regards multiple adsorption.
Promoter Action Promoter action, which has been one of the least iintlerstood of catalytic phenomena, receives a ready interpretation on the basis of multiple adsorption. The proposed mechanism of promoter action will be illustrated by the decomposition of hydrogen peroxide by glass wool,^ promoted by CuSO4 and l l n S 0 4 . On the glass surface alone, one can imagine H2Os to be doubly adsorbed and the attraction of the surface for the oxygen atoms of the HZO? to be strong, but that for the hydrogen atoms weak (or vice-versa). K h e n the copper or manganese salts are added they are adsorbed on the surface, and offer points of strong attraction for whichever atom of the H 2 0 2that is weakly adsorbed on the glass, thus offering. in conjunction with the glass, effective pairs for the catalysis of the HZO? decomposition. The action of the promoter was quite marked as the following table shows. l d k i n s antl Sisson: J. .hn. Cheni. Soc.. 45, 8c9 (1923). Adkins a n d S k s o n : J. -hn.Chem. Soc., 46. 130 (1924). Aldkinsand Lazier: J. -1m. Chem. Soc., 46, 2291 (1924). See also Adkins' remarks in the Fourth Report of the Committee on Contact Catalysis: (LOC.Cit.). 6 Langmuir: (Loc. Cit. p. 618). Gaugrr and Taylor: J. -1m. Chem. Soc., 4 5 , 920 f1923). Taylor: "Fourth Report . . . ." (Loc. C i t . ) . Elissafoff: Z. Elelitrochem., 21, 3 5 2 (1915 ) .
1138
ROBERT E. BCRK
Conc. of CuSOl millimols ’litre,
Amount of glass wool (grams). 0.5
Relative rate.
1.54
0.5
0.06 0.67
3.IO
0.;
0.68
0
1.54 Conc. of ?tlnS04
0
0.I O
0
2.3
0 . 2
2.3
0.0085 0.0860
0 . 2
0
0.0052
The glass wool in these experiments was digested in order to remove the alkali. Also even more marked promoter action was obtained when Jena Glass wool was used, so that it could scarcely be a question of the formation of a third compound with marked catalytic properties. The catalytic action should have dropped off from a maximum as the glass wool became saturated with C u S 0 4 , on the basis of a multiple adsorption mechanism. Elissafoff did not investigate this point thoroughly but 3.1 millimols of CuSOi per litre (see the table above), produced no added promoting effect over 1.53millimols per litre of soft glass n-001. I n separate adsorption experiments with Jena glass wool, saturation was reached at a concentration of 50 millimols of CuS04 per litre. The reaction appeared to be monomolecular on the surface. Again this evidence is not absolute proof, but the explanation of promoted reactions of this type along the lines of that described above is quite definite and plausible, and the alternatives vague. Poisons and Active Centres There has been abundant evidence t o demonstrate the following phenomena, among others, with regard to catalytic poisons. (a) There are cases where a poison will stop catalytic action n-ithout stopping adsorption to a proportionate degree. For example Pease’ has found that mercury poisons a copper catalyst for the hydrogenation of ethylene, with a marked reduction in the adsorption of hydrogen, but only a slight reduction in the adsorption of ethylene. The rate of this reaction on a copper catalyst was cut down 8gC; by 0.5 C.C. of CO, whereas this catalyst was capable of adsorbing j C.C. of CO, I C.C.of H2 and 2 C.C. of ethylene.? (b) Some catalysts can be poisoned with respect t o one reaction but not with respect to another. Thus T’avon and Husson3 found that with 3 gms. of platinum black and j gms. of diethyl ketone in j o C.C. of acetic acid, 2 5 C.C.of hydrogen were ‘fixed’ in four minutes. On addition of 2 . 3 milligrams of CS2 the action stopped. Pease: J -1ni. Chem Sac , 45. 2296 (1923). *Pease and S t e n a l t : J. .Im. Chem. Sac. 47, 1235 (192j). Vavoii and Husson: Compt. rend , 175, 277 (1922).
L O W E R I S G OE THE HEAT O F ACTIVATIO?;
1139
On adding 2 gms. of piperonal 2 j C.C.of hydrogen were fixed in four minutes. This action was stopped by the addition of 1.4 mgs. more of CS?.The catalyst was still able to reduce nitrobenzene, but the addition of more CS2 stopped even this action. Hydrogen exerts a strong retarding effect upon the thermal decomposition of ammonia on platinum,l and but a slight retarding effect upon the thermal decomposition of formic acid on platinum? and hydriodic acid on p l a t i n ~ m . ~ The rate of interaction of H ? and COOon platinum js proportional to the pressure of € I 2 up to high pressure^,^ whereas the rate of the reaction passes through a sharp maximum as the pressure of CO? is increased to the same value. These phenomena, together with certain other evidence, have led various investigators to the conclusion that catalytic reactions take place on ‘active centres’ of the surface which have been considered to be ‘small areas’ of unusually high activity.j Taylor has considered them to be ‘peaks’ of highly unsaturated surface atoms.6 Vhile it seems certain that some of this evidence points to the existence of active centres, e.g. that of Pease and Stewart and of Hinshelwood and Prichard, some of it can be interpreted in other ways, In the case of the copper catalyst poisoned by mercury. the amount n-as sufficient to form a monomolecular layer. The surface is now really mercury, which can be imagined to adsorb both ethylene and hydrogen without necessarily offering effective atom pairs for the activation of either. Poisoning data could be imagined, on the basis of multiple adsorption, which would give the same impression as some of those cited above, even on plane crystal surfaces. Thus consider the activation of a long molecule t o be brought about by multiple adsorption of two points with several atoms intervening. The surface could now be completely blocked as regards this activation by the adsorption of relatively few molecules of say CS?if they are permanently held, and are distributed a t random over the surface. The surface could then still adsorb considerable quantities of short molecules like hydrogen. or molecules which could be adsorbed in a vertical position, and have a small ‘head’, like nitrobenzene. However. crystal lattices would not be expected t o have very high catalytic activity, unless a frequent spacing of suitable atoms happened to be just that required for effective multiple adsorption. The metals used in the reactions on hot wires consist of minute crystals embedded in vitreous materials.’ Therefore, it seems likely that Tyhen the evidence points definitely to reaction centres in these cases, that the reaction is taking place either on the vitreous Hinshelwood and Biirk: J. Chem. Soc.. 127, 1105 (1925). Tingey and Hinshrln-ood: J. Chem. Soc., 121, 1668 (1922). Hinshelvood and Burk: J. Chem. Soc., 127, 2896 (1925:. Hinshelwood and Pritchard: J. Chem. Soc., 127, 806 (19.j). Constable: Lot. Cit. Taylor: ,+. Cit . Beilby: Aggregation and Flow of Solids“ (1921).
1140
ROBERT E . B r R K
portion, or at the borders of the crystals (one atom adsorbed on the crystal and the other on the vitreous portion), or both. The other well known phenomena of catalytic behaviour have been interpreted in the light of molecules stretched through multiple adsorption, e.g. the variability of catalytic surfaces, change in catalytic activity with heat treatment, roughening of catalytic surfaces, the lack of correspondence beb e e n the amount of adsorption and activation, varying saturation capacity of the surface for different substances, specific activation of the case of reactions which can proceed in more than one way etc. Detailed applications of the theory in these cases will not be communicated a t present, since the fact that this theory is capable of explaining them does not necessarily mean that the theory is the real cause of the effects. However, these phenomena would seem to follow necessarily from multiple adsorption under the specified conditions, whereas it is not clear \Thy they should from the less definite theories of contact catalysis. The writer will attempt some experiinents n-hich he hopes will yield more direct evidence for the assuinptions of the theory. Physzccd che~?!ldl~/ LnlKJicctory, Ballzol attd T r l m t ~C'o'lcgcs, Oxford.
