18 The History of the BET Paper Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 17, 2015 | http://pubs.acs.org Publication Date: June 3, 1983 | doi: 10.1021/bk-1983-0222.ch018
EDWARD T E L L E R Hoover Institution, Stanford, CA 94305
In January, 1935, while working at University College in London, I got a letter from my friend, George Gamov, inviting me to join him as Professor of Physics at The George Washington University in Washington, D.C. In September of that year, my wife and I arrived in this country where we have lived ever since. The first six years in this country were quiet and productive years for me. I taught at the University, worked with George Gamov on some relatively early question of nuclear physics and worked with a great number of other people on the physics of molecules, including some properties of solids. Washington, D. C., then as now, had many laboratories. Quantum mechanics at that time was a relatively novel theory which in principle could solve all the questions of atomic and molecular interactions. However, many people who had obtained their degrees even a few years earlier were unfamiliar with i t . My first class on the subject at The George Washington University consisted of a dozen or so "older" students and was given in the late afternoon to accommodate their schedules. This schedule also happened to agree with my way of life at that time. I was hardly ever in bed before midnight, but much more pertinent, I was hardly ever out of bed before ten in the morning. Incidentally, during those years I never managed to visit the Capitol or any of the House or Senate buildings. Now that I reside on Stanford campus in California rather than just off Connecticut Avenue, I wish I could say the same thing. One of my students in these first classes was a fellow Hungarian, Stephen Brunauer. He worked in a place that was unofficially called "the fixed-nitrogen lab," more formally named the Bureau of Chemistry and Soils. More than once, I have been asked the flattering question: why have Hungarians played a disproportionately important role in the physical sciences? My standard answer is that this disproportion is the result of an optical illusion--Hungarians seem to be more visible. I have also offered the theory that 0097-6156/83/0222-0227$06.00/0 © 1983 American Chemical Society
In Heterogeneous Catalysis; Davis, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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they c a r r i e d away from Hungary the c h a r a c t e r i s t i c s of people who have escaped from a shipwreck—who have become more dedicated to l i f e having narrowly been saved. The most c o l o r f u l explanation was o f f e r e d by the most important Hungarian immigrant s c i e n t i s t : the aerodynamicist, Theodore von Kârmânn, who played the l e a d i n g r o l e i n the development of the United States a i r f o r c e i n World War I I . He gave away the " s e c r e t : " we are not Hungarians at a l l but Martians who are the vanguard of the planned conquest of earth and have s e t t l e d f i r s t i n a l i t t l e known region (Hungary) to assume a s u p e r f i c i a l l y human appearance. I p a r t i c u l a r l y l i k e t h i s explanation because Theodore von Kârmânn included i n t h i s unique f i f t h column not only s c i e n t i s t s but a l s o Zsa Zsa Gabor. I am a l s o glad that through the recent movie about an E ^ t r a T e r r e s t r i a l (whose i n i t i a l s I a l s o c a r r y ) von Kârmânn's theory has been given a l e s s i m p e r i a l i s t i c turn. One r e s u l t of the Hungarian i n v a s i o n was that two years a f t e r my a r r i v a l , Steve, of whom I saw q u i t e a l o t , t o l d me a p e c u l i a r f a c t . M u l t i l a y e r adsorption of gases had been s t u d i e d , and the dependence of the amount of the m a t e r i a l adsorbed on the pressure of the gas adsorbed looked s i m i l a r i n p r a c t i c a l l y a l l examples. Adsorption to a monolayer had been explained most s u c c e s s f u l l y by I r v i n g Langmuir. This very success may have d e f l e c t e d a t t e n t i o n from the study of m u l t i l a y e r adsorption. In monolayer adsorption, s a t u r a t i o n i s approached when the monolayer has been f i l l e d . But soon a f t e r t h i s has occurred, a d d i t i o n a l adsorption sets i n , thus producing a curve that has been c a l l e d S-shaped although i t i s necessary to l a y the S on i t s side i n order to reproduce the graph. Steve suggested that the adsorption of a d d i t i o n a l l a y e r s i s due to a f o r c e from the s o l i d a c t i n g over longer distances r a t h e r than the well-known contact f o r c e s which Langmuir used to e x p l a i n monolayer adsorption. A c t u a l l y , i n 1937 we knew q u i t e a l o t about the i n t e r a c t i o n s over longer d i s t a n c e s . Forces s u f f i c i e n t l y strong to e x p l a i n the m u l t i l a y e r adsorption appeared to be completely excluded. But Steve i n s i s t e d that the observations were there, and that h i s explanation would account f o r them. I f I d i d not b e l i e v e what he proposed, then I should suggest an a l t e r n a t i v e . At the time, my ignorance of adsorption was s u p e r l a t i v e . I did not even know what the word chemisorption meant. But Steve's challenge was e x c i t i n g and seemed j u s t i f i e d . The obvious i n i t i a l step i n m u l t i l a y e r adsorption would be t h a t , while the Langmuir adsorption depends on the i n t e r a c t i o n between the adsorbing surface and the gas molecules, the adsorption of a second l a y e r would depend p r i m a r i l y on the a t t r a c t i o n between molecules already adsorbed i n the f i r s t l a y e r and a d d i t i o n a l molecules of the second l a y e r . I t was, of course, e q u a l l y c l e a r that the adsorption of the t h i r d l a y e r
In Heterogeneous Catalysis; Davis, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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would depend on the molecules i n the second l a y e r , and that the same r e l a t i o n s h i p could be continued i n d e f i n i t e l y . Indeed, i f the pressure of the gas reached the vapor pressure of l i q u i f i e d gas at the temperature i n question, then the l a y e r s would b u i l d up s u c c e s s i v e l y with e s s e n t i a l l y equal occupation. The adsorption would continue i n t o a c t u a l condensation on the s u r f a c e , thus e s t a b l i s h i n g i n f i n i t e l y many adsorbed l a y e r s . I f we were d e a l i n g with a pressure below vapor pressure, than the amounts adsorbed i n the second, t h i r d and higher l a y e r s would form a geometric s e r i e s which could be e a s i l y summed. The f i r s t l a y e r would not form a term i n the geometric s e r i e s and would be more s t r o n g l y populated. Therefore, i t seemed that an explanation of m u l t i l a y e r adsorption was q u i t e p o s s i b l e . Furthermore, no new constants were needed i n order to e x p l a i n the experimental data. The two r e l e v a n t f a c t o r s are: the a t t r a c t i o n that the molecules experience i n the f i r s t l a y e r (which had already appeared i n Langmuir's equation); and the i n t e r a c t i o n between the various l a y e r s of adsorbed substance. This l a t t e r constant could be derived from the vapor pressure at each temperature. These ideas were simple and perhaps should have been discovered at an e a r l i e r time. But at t h i s p o i n t , Steve got deeply i n t e r e s t e d i n them. He suggested that the whole question should be discussed with Paul Emmett, the most experienced man i n surface chemistry who worked at the f i x e d n i t r o g e n laboratory. A l l the r i c h m a t e r i a l of observation of surface adsorption was a v a i l a b l e f o r the d i s c u s s i o n among the three of us. We soon n o t i c e d that the m u l t i l a y e r adsorption theory put the i n t e r p r e t a t i o n of adsorption curves on a more s o l i d f o o t i n g , thereby making i t p o s s i b l e to determine the area of the surfaces of s u b s t r a t e s , such as c h a r c o a l , more a c c u r a t e l y . Indeed, these surface areas could i n p r i n c i p l e be determined from the Langmuir equation, provided the i n t e r a c t i o n between the substrate and the adsorbed molecules was considerably stronger than the i n t e r a c t i o n between the adsorbed molecules themselves. I f one kept the pressure w e l l below the vapor pressure, then Langmuir's curve should apply and the adsorbed quantity would be d i r e c t l y p r o p o r t i o n a l to the s u r f a c e . The trouble was, however, that the surface of the substrate may not be q u i t e uniform. This would give r i s e to v a r i a t i o n i n the strength of adsorption. At low temperatures, the Langmuir s a t u r a t i o n could occur long before the buildup of f u r t h e r l a y e r s . But at the same low temperatures, even small d i f f e r e n c e s i n the i n t e r a c t i o n between the substrate and the various molecules at d i f f e r e n t l o c a t i o n s could s e r i o u s l y d i s t o r t the Langmuir curve. I f , on the other hand, the experiments were c a r r i e d out at higher temperatures, then the Langmuir s a t u r a t i o n i s l e s s
In Heterogeneous Catalysis; Davis, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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pronounced because even before the f i r s t l a y e r i s complete, the second l a y e r begins to get adsorbed. D i f f e r e n c e s i n the adsorption at various l o c a t i o n s would be l e s s pronounced i n the second than i n the f i r s t l a y e r , because the f i r s t l a y e r already would have had a smoothing e f f e c t on the surface c o n f i g u r a t i o n . The r e a l advantage of the m u l t i l a y e r theory i s , however, that i t can u t i l i z e both low and high temperature data and i n the end give much more r e l i a b l e values f o r surface areas. Of course, l i k e the Langmuir theory, the m u l t i l a y e r adsorption theory i s not u n i v e r s a l l y v a l i d . For i n s t a n c e , i f the surface has holes or c l e f t s , then only a l i m i t e d number of l a y e r s may be adsorbed. We were f u l l y aware of t h i s exception and t r i e d to s i f t the experimental m a t e r i a l a c c o r d i n g l y . Another d i f f i c u l t y was l e f t unanswered. The hypothesis that the second l a y e r i s exposed to the same f o r c e s as an a d d i t i o n a l l a y e r i n the l i q u i f i e d form of the adsorbed gas i s not v a l i d unless the f i r s t l a y e r i s already r a t h e r completely developed. S i m i l a r l y , one may have doubts about the t h i r d l a y e r being adsorbed on the second. We were, aware of t h i s d i f f i c u l t y , but while we mentioned i t , we did not r e s o l v e i t . A c t u a l l y , our proposed adsorption equation turned out to be r a t h e r more p r a c t i c a l than I had expected. A measure of i t s success i s that the names of the authors have been p r a c t i c a l l y f o r g o t t e n , and the equation bears the h o n o r i f i c t i t l e of BET. The whole adventure was, f o r me, one of u n d i l u t e d pleasure, not only on account of the success but a l s o on account of the harmonious cooperation which l e d to the o r i g i n a l p u b l i c a t i o n . T h i s i s not q u i t e the end of the s t o r y , however. About t h i r t e e n years l a t e r , a young colleague, B i l l McMillan, and I f i n a l l y returned to the question that the BET theory f a i l e d to answer: To what extent can the assumption that the second l a y e r i s b u i l t up as though the f i r s t l a y e r were complete (and the t h i r d as though the second were complete, and s i m i l a r l y a l l the other l a y e r s ) be j u s t i f i e d ? The answer was, of course, that the assumption was not justified. Because of the incompleteness of the l a y e r s , changes i n the BET equation were necessary. A c t u a l l y , i t turned out that the required changes are small and can be expressed i n a s u r p r i s i n g l y simple form. The changes turned out to be the same as the changes that would have a r i s e n i f Steve Brunauer's o r i g i n a l hypothesis had been c o r r e c t . They are s i m i l a r i n nature to those that would be produced i f the substrate i n t e r a c t e d with the d i s t a n t gas molecules according to a law where t h i s i n t e r a c t i o n decreased with the i n v e r s e t h i r d power of the distance of the molecule from the s u r f a c e . While these changes were i n the d i r e c t i o n of the o r i g i n a l Brunauer suggestion, these corresponding imaginary f o r c e s would be so small that i n themselves, they would not begin to e x p l a i n the presence of the m u l t i l a y e r s . The t r u l y amusing (and a t the same time somewhat
In Heterogeneous Catalysis; Davis, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 17, 2015 | http://pubs.acs.org Publication Date: June 3, 1983 | doi: 10.1021/bk-1983-0222.ch018
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disappointing) point comes with a comparison of the r e s u l t s of the two s t u d i e s . The theory of the van der Waals f o r c e s p r e d i c t s that the substrate i n t e r a c t s with d i s t a n t molecules, producing a p o t e n t i a l energy that decreases with the i n v e r s e t h i r d power with the distance from the s u r f a c e . Therefore, the changes that B i l l McMillan and I had to introduce d i d nothing more than point up an a d d i t i o n a l i n t e r a c t i o n that behaves i n the same way as the van der Waals i n t e r a c t i o n . S t i l l t h i s i n t e r a c t i o n i s d i s t i n c t from the van der Waals i n t e r a c t i o n . D i f f e r e n t substrates have d i f f e r e n t i n t e r a c t i o n s i n the van der Waals i n t e r a c t i o n , whereas the term that B i l l and I found i s n e c e s s a r i l y the same f o r a l l m u l t i l a y e r s of a given gas no matter on what substrate they were adsorbed. T h i s l a t e r work was incomparably more d i f f i c u l t than the o r i g i n a l development of the BET theory. I t was a l s o incomparably l e s s p r a c t i c a l . Indeed, i t s e f f e c t on the explanation of surface adsorption phenomena was p r a c t i c a l l y n i l . But i n t h i s case as i n many others, s c i e n t i f i c work turned out to be a pure pleasure. Whether t h i s pleasure i s derived from the ease of the work, or i t s abundant p r a c t i c a l r e s u l t s , or from the s a t i s f a c t i o n of answering a d i f f i c u l t complex question i n r e l a t i v e l y few words does not g r e a t l y matter. T h i s most p o s i t i v e aspect points up the s i m i l a r i t y of science and a r t . In n e i t h e r f i e l d can work and pleasure be d i s t i n g u i s h e d . On almost the l a s t day of 1979, Steve Brunauer, Paul Emmett and I (together with our wives) got together to c e l e b r a t e our own paper with a most q u i e t but even more enjoyable dinner. T h i s was done at the suggestion and expense of a f r i e n d of the BET theory. I had j u s t returned a few hours e a r l i e r from Taiwan. Instead of going to bed to s u f f e r with j e t l a g , I met my two old f r i e n d s whom I had not seen f o r many years. I am happy to r e p o r t that having a happy time with f r i e n d s who share pleasant memories of the past seems to be an e x c e l l e n t cure f o r jet lag. RECEIVED December 22, 1982
In Heterogeneous Catalysis; Davis, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
