Dynamic Mechanism of Heterogeneous Cata ysrs a
MELVIN A. COOK Department of Metallurgy, Universify of Utah, Solt Lake City, Utah
ALEX G. OBLAD Houdry laborafories, Houdry Process Corp., Marcus Hook, Pa.
A
theoretical linear variation of adsorption potential with surface coverage, called the chemisorption band, i s applied quantitatively in explaining the unusual rate law observed in activated adsorption. This chemisorption band, moreover, i s shown to account for catalytic orthopara conversion and should apply also in catalytic hydrogen-deuterium exchange. The more complicated catalytic reactions-e.g., catalytic cracking-are attributed to combination influences of the chemisorption band and active centers. A model i s proposed in which the catalyst surface
ECEKTLY, a model for chemisorption and catalytic action
R
was proposed based upon results obtained in a study of silica-alumina (16). This proposal &‘as as follows: A surface layer for various reasons (described later) is strained, and, theiFfore, reacts readily chemically with many substances, the adsorbate in effect creating the adsorption sites and thus reducing the stiain upon the surface. The release of the strain involve4 crystallographic changes in the catalyst surfacc atoms or ions. Desorption involves a reverse of this process, the catalyst reverting to its former state of strain. Catalysis is, therefore, looked upon as a dynamic reaction involving not only the adsoibate but the catalyst as well. This dynamic picture requires mobility of the catalyst surface. In developing this subject it is well to review briefly some of the work that has already appeared. Cook, Pack, and Oblad ( 4 ) applied the concept of “structural” I adsorption to IOK pressure “physical” adsorption. I n their paper, two types of surface strain were described. First-order (surface) strain ‘catalytic was defined as a surface or Reve r s i b l e strain induced over the whole Adsorpfion surface by the tendency of Band solids to eliminate or reduce t h e concentration of unbonded orbitals and/or disL-14.0 K.col. torted bonds on their crystalline surfaces. A4 second structural factor designated second-order (surface) strain was also mentioned and described as another type of strain associated with the
I I
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i s described as undergoing rapid fluctuations back and forth between its normal state and a chemically characterizable activated complex. This flip-flop activity i s attributed to a structural duality promoted by strain in acid catalysis, and a valence-structural duality also promoted by strain in hydrogenation-oxidation catalysis. This work provides deeper insight into processes of heterogeneous catalysis. The model emphasizes surface aspects of the catalyst, further study of which should open up new avenues of research in heterogeneous catalysis.
presence of impurities, structural influences of surface phase boundaries, and the presence of defects due to nonstoichiometry of lattice. On the basis of this more recent picture, the potential structural Lewis acid sites in silica-alumina deficrihed by RIilliken, Mills, and Oblad ( 1 6 ) are classified as secondorder strain sites, associated xith structural influences of surface phase boundaries. Active centers described extensively in the catalyst literature are second-order strain centers. I t is the purpose of this paper t o describe some addit,ional general features of solid surface dynamics associated with adsorption (chemisorption) and the strain centers mentioned, which the authors believe are fundamental to many types of heterogeneous catalyet’s. Chemisorption, Catalytic and Structural Adsorption
\
\ \
Potential B a r r i e r in Homogeneous Reoction
I ?
For Reversible Chemisorption
There ie now no doubt that a careful distinction must be made between what is currently knoM n as chemisorption and the type of ads o r p t i o n w h i c h m a j - he termed “catalytic adsorption.” (By catalytic. adsorption is meant that adporption which may result directly in the chemical rcaction for which the solidsurface is a catalyst.) I t is likely that catalytic adsorption is predominantly, if not almost invariably, cheinisorption, but the reverse ih by no means the case. While c h e m i s o r p t i o n m a y frequently involve the whole
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or a large part of the solid surface under some catalytic re-
-*i
.
t*
action conditions, catalytic adsorption may involve only a small fraction of the surface corresponding to the catalytically active centers. Even in cases where it has not yet been possible to demonfitrate that active centers are responsible for the catalytic reaction, or even where evidence points to the possibility that the whole surface may be catalytically active, there remains a sharp distinction between catalytic adsorption and chemisorption. This situation is strikingly illustrated in the treatment by Rideal and Trapnell (18) of the low temperature para-hydrogen conversion on evaporated tungsten films. According to these authors, the first chemisorption (at e = 0) of hydrogen on tungsten has a heat of adsorption of about 14 kea]./ mole, but drops steadily as the surface coverage, 0, increases to 3 kcal./mole as e approaches unity. If it is assumed with Rides1 and Trapnell that the activation energy for the (chemical) orthopara conversion is simply that associated with desorption, the distinction here between chemisorption and catalytic adsorption is illustrated by Figure 1. It should be realized in considering the significance of a progressively decreasing adsorption potential that the first-order strain as well as the second-order strain factors are always superimposed on the normal primary and secondary bonding potentials. The potential associated with chemisorption in the sense of present usage is therefore the resultant effect of the normal binding forces and the structural strain contribution. Since the latter forces are variable, depending on the magnitude and extent of strain, they tend t o cause spreading of the otherwise discrete or partially banded (if the surface is truly heterogeneous) energy levels. The resultant effect is the chemisorption band. Clearly, only the chemisorbed hydrogen toward the top of the band is reversible and, therefore, catalytic adsorption. The sum of the free energy changes in each step must be the same as the free energy change in the homogeneous reaction. Moreover, in order that the chemisorption band may remain in steady state during reaction, the reactants' energy level must be substantially above the top of the chemisorption band so that reactants may adsorb a t the same rate that products are removed. The driving force of the reaction is thereby determined, in accordance with thermodynamic requirements, by the over-all free energy change. It is commonly thought that heterogeneous catalysis involves primarily active centers. Even in cases where these centers are the most prominent feature of catalysis, the first-order strain factors are not unimportant. However, the possibility should not be overlooked that in some cases the first-order strain factors may become of primary importance-Le., rate determining. This effect may be characterized by an activation energy for evaporation from the surface increasing with temperature, since the steady state corresponding to reversible adsorption would be established at lower and lower levels of the well as the temperature increases. (Consideration here must be given to how the adsorption potential changes with temperature.) Whether or not the surface adsorption well (illustrated in Figure 1) associated here with structural adsorption will be a determining factor even in reactions in which the chemisorption band is important willdepend, of course, on the total depth of the well, yo, and on the value of 0 at the steady-state condition illustrated in Figure 1. If, for example, the adsorption potential a t maximum surface coverage, de,,,. ), were large a t temperatures below a certain value, T', the chemisorption band would be filled even a t quite low pressures. On the other hand, if yo, the adsorption potential corresponding to the bottom of the band, were small, the chemisorption band would remain practically empty during the reaction, a t least a t low pressures. In either case, AFT should not change appreciably with T . In the intermediate case between these two extremes, a considerable variation of the activation energy with temperature should be expected. This case should turn out t o be the most prominent in heterogeneous catalysis. July 1953
There is evidence, as illustrated in the following paragraph, that the equation y(e) = y,(i
- e'/€)
(1)
or, more simply, the equation y(e) = yo
+
(la)
suggested by Cook, Pack, and Oblad ( 4 )for the adsorption potential in structural adsorption may apply approximately in chemisorption on a homogeneous surface arid even over short ranges on a heterogeneous surface. Here y(0) is the adsorption potential -AFad., a t coverage 8, yo that at 0 = 0, 8' the coverage corresponding to the end of chemisorption (perhaps generally unity in, chemisorption), and E in (Equation 1) and a' in (Equation la} are appropriate constants. Equation l a will be used in this paper. Deviations from Equation l a might be expected a t 0's in the neighborhood of zero and unity due to heterogeneities, points of discontinuity due to heterogeneities, and configurational entropy. First-Order Strain and Rate of Activated Adsorption The example of catalytjc para-hydrogen conversion illustrated in Figure 1 represents perhaps one of the simplest heterogeneous catalytic chemical processes. Since it is possible that the adsorption process itself may be rate determining in various catalytic processes, it is desirable to discuss the process of activated adsorption from the viewpoint of the structural (first-order strain) factors. Extensive studies of the rate law in activated adsorption summarized by Taylor and Thon ( 6 , 19, 25, 25) show that activated adsorption generally follows the empirical (isothermal) rate law
where I is the time, a and constants (actually temperature dependent). This law has a simple explanation in the concept of structural adsorption involving first-order strain. In the absolute reaction rate theory, the activated complex is treated, except for one degree of vibrational freedom (regarded as a degree of translational freedom), as a normal molecule. I n heterogeneous catalysis this complex, designated here RS, may be regarded essentially as a normal molecular species adsorbed on t h e catalytic surface. The adsorption potential of RS as well as that* of the chemisorbed product, P, itself should, therefore, follow Equation 1 with B representing the total coverage both by t h e RT adsorbate and the chemisorbed product, P. In other words, the adsorption potential of the activated complex should depend on the magnitude of first-order strain just as for any other adsorbate. If AF: ip the activation energy for chemisorption on a homogeneous surface, AFS(O), the activation energy a t e coverage should then be given according to Equation la by the relation AFT(0) = A8'2
+ aft'
(3)
where a' is expected to be primarily temperature independent. Hence, the rate equation for chemisorption may be written as follows:
which leads to Equation 2 with a and
o(
defined as follows:
(4) and a =
or'/RT
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Heref(R) is some function of the activity of the reactant (adsorbate) in the gas phase. Perhaps f will also be a function of e, particularly as 0 approaches unity; but in any event, it will involve 0 (or 1 - e ) to a power probably no greater than unity. Since the activation energy, which involves e directly, occurs as an exponent, the e dependence off may be disregarded in the same sense that A in the Arrhenius equation may be taken to be temperature independent. (Becording to the absolute reaction rate theory, 9 should be proportional to T . ) Figure 2 illustrates the
f
Chemiwrption Band‘ (Irnpresasd b y ‘P-Band‘)
I
i hsmisorp,lion Band
Figure
2.
Configuration Coordlnafa Potential Energy Surfaces Activated Adsorption
in
potential energy curve for activated adsorption according to the concepts of structural adsorption. The activated complex band or transition state band arises from the filling of the surface, not necessarily with RS molecules but primarily by the chemisorption product. Thus AF: is the minimum activation energy, realized only when e = 0, and A F I ( O j increases linearly with 0, the coverage being primarily by the chemisorbed product rather than by an appreciable concentration of the R t molecules. Not only does this explain the form of the observed rate law (Equation a), but Equation 4 is apparently also a satisfactory definition of a. This is illustrated in Figure 3 by plotting log a versus 1/T for three examples discussed by Taylor and Thon-namely, hydrogen on zinc oxide (81). hydrogen on chromic oxide (a), and nitrogen on iron ( 7 ) . For hydrogen on chromic oxide, f is within experimental error proportional to the hydrogen pressure. While Equation 4 appears to give a satisfactory explanation of the temperature dependence of a in these examples, in other examples chemisorption is complicated by what Taylor and Thon call an “initial massive rapid adsorption” and by other recognizable factors, and discontinuities are found, therefore, in the log a versus 1 / T curve, as in many ordinary log rate versus 1 / T curves. According t o the structural adsorption concepts 01’ should be roughly independent of T , and 01 therefore should vary as 1/T. While the data listed by Taylor and Thon show anomalous variations of a: with T , when other recognized anomalies discussed by these authors are accounted for, this description of a appears essentially correct. If the concepts of adsorption described in this paper are valid, it is expected that Equation 2 will apply generally in activated adsorption, due cognizance being taken of the influence of the discontinuities in some y versus e curves where real heterogeneities occur. The simplicity and quantitative nature of this explanation for the complicated rate law in activated adsorption must be considered as strong evidence for the concepts of structural adsorption.
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Catalytic Conditions There is a well-knorvn tendency for crystals of one type t o impose their structure on those of another structure a t the interface between them. This effect gives rise to local strain centers a t the interfaces between the crystals. The presence of strain of this sort, spoken of here as second-order strain, does not in itself give rise to catalytic activity, although it perhaps alviags influences locally the strength of the chemisorption bond. Such strain centers can hardly ever involve more than a small part of the total surface. It is probably only when the second-order strain centers involve a suitable chemical duality, however, that they become catalytically active centers. Therefore, the possible nature of this duality, under what conditions it may arise, and how it may influence the activation energy of a chemical reaction n 111 be considered. First, it may be well to illustrate the possible importance of first-order strain even in catalytic reactions occurring only at active centers. I n a reaction such as para-hydrogen conversion or the hydrogen-deuterium exchange, it is primarily only necessary for chemisorption and re-evaporation to take place, the breaking of the hydrogen and deuterium bonds being all that is required of the catalyst. I n other reactions, such as hydrogenation of ethylenr, much more drastic changes must occur. I n such cases, the very complexity of the reaction euggests a type of specificity of interaction with the surface characteristic only of chemical reactions which may occur a t active centers. The importance of a catalyst in such cases will be primarily to make one type of reaction much more favorable than all others, or to allow a particular complex reaction to occur that otherwise would involve too much entropy of activation. I n the hydrogenation of ethylene, it is believed that the evidence favors overwhelmingly the chemisorption of both ethylene and hydrogen (9, 84). On an uncovered surface, the total adsorption potential is about 65 kcal. ( 8 ) or more ( 1 ) for the sum of the chemisorption potentials of hydrogen and ethylene. However, as the surface fills (primarily with ethylene)
I
I
1.6
1.8
I
2.0
I 2 ,
2.2
1
2.4
2.6
“T Figure 3.
Application of Equation 4 as a Definition of a in Equation 2
the adsorption potential decreases and a chemisorption band is formed ( 1 ) . If a homogeneous surface were being dealt withLe., if no active sites or second-order strain centers were involved, t.he activation energy niight then be represented by AF: corresponding to surmounting a particular barrier of the type illustrated by the upper barrier in Figure 4, which would be high because of the low probability of reactants coming together in the appropriate manner Tyithout any particular directing influence of ordinary surface sites, This barrier would be so high, in fact, that the gas should even evaporate off the surface as CzHa(g) I&($)unreacted. Hovever, on appropriate active sites, which undertake to direct the path of the reactants, the reaction might proceed over a much more conservative path indicated by the solid
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line and the activation energy, A F f . It has been suggested, on the other hand, that the evaporation of chemisorbed ethane is the rate determining step and that the activation energy is simply the adsorption potential of ethane as illustrated by the lower hatched region and dashed curve in Figure 4. This can be disproved, however, when the (experimental) chemisorption band is taken into consideration. Besides a few specific clear examples such as the acid sites on cracking catalysts ( 1 ) and the poisoning effects of arsenic trioxide on nickel and platinum in hydrogen overvoltage studies by Bockris ( d ) , the evidence for active centers lies in the wellrecognized poisoning effects of mere traces of substances characterized primarily as Lewis bases with free electron pairs and free valence electrons. Perhaps the greatest difficulty in recognizing active centers is the fact that evidence is frequently found for poisons which are not specific to active centers but merely compete with reactants for the entire surface and thereby influence the chemisorption band. Furthermore, a linear relationship between the amount of poison on a surface and the decrease in catalytic activity on one hand, and the amount of poison required to eliminate or reduce to a certain low value the activity as a function of initial activity do not necessarily mean that the poison is merely competing with reactants for the whole surface. For these reasons, it should be realized that the chemisorption band is important even when active centers are involved for i t will determine the energy level of the basin ahead of the pass over which the reaction must proceed to reach the product configuration. The nature of the mountain pass, however, except for a few simple reactions such as para-hydrogen conversion and various simple exchange reactions which may depend primarily or even exclusively on the chemisorption band, probably always involves the characteristics of the active centers which are specific in their interactions with reactants, and, therefore, must always be characterized chemically (14,16).
Flip-Flop Kinetics in Cracking Catalysts The chemical characterization of solid acid catalysts has been amply treated by Millikin, Mills, and Oblad but it may be of value to illustrate qualitatively the energy band diagram implied by their work, and discuss their mechanism as an illustration of the general principles discussed in this paper. The energy diagram for the cracking of a hydrocarbon is illustrated in Figure 5. The straight thermal cracking process occurs over the maximum barrier with an activation energy, AF$. For the reaction over a catalyst, consider first the adsorption branch, which involves adsorption of the hydrocarbon on the catalyst surface with the alumina primarily in the minimum energy (but surface strained) six-coordinated state. The adsorption potential and the adsorption band width are probably small in this case, perhaps even corresponding to merely weak structural adsorption with no chemisorption-Le., no direct contribution of primary chemical bonds-and the top of the band will thus lie close to the energy level of the reactants. On a pure silica surface, the only effect of the adsorbent would be a possible slight lowering of the activation energy of cracking which, because of a slightly greater adsorption potential for the activated complex than for the reactants, is presumed t o be small or negligible. The activated state on the catalyst with active potential acid sites is considered to be the carbonium ion adsorbed on a negatively charged fourcoordination site, formed by adsorption of a n olefin or aromatic hydrocarbon or by extraction of a hydride ion (H-) from the incoming paraffin or naphthene hydrocarbon. I n the case of the latter substances the reaction proposed will be essentially the same whether the hydride ion is transferred to an unsaturated hydrocarbon or held by the catalyst. Such sites are promoted by means of the tetrahedral (cristobalite) silica, which tends to impose its structure on the alumina micelles at the surface between the silica and alumina micelles. This effect is, by means of July 1953
0
15
c 0 E
>
0
30
h
46
Homogeneous Surface on Less Active Sites
60
~~
or
~~~~
Configuration Coordinate
Figure 4. Suggested Model for Hydrogenation of Ethylene on Nickel Catalyst Near Room Temperature N e a r room temperature Intermediate energy levels merely suggestive
second-order strain, to reduce the energy level of the activated complex, which i t will do in proportion to the magnitude of the strain. The activation energy will be a minimum for active sites with maximum strain, corresponding to a composition of about 30% alumina and 70y0silica incorporated by a technique which gives a maximum strain of these secondary-strain centersLe. , by incorporating the silica and alumina under conditions under which the alumina is four-coordinated and then removing the stabilizing base (ammonia), the loss of which allows the alumina to revert t o the six-coordinated state under highly strained conditions.
t ' , ion State Bond' ordinatad Alumina)
Reo
'Adsorption Bond'
6- Coordinated Alumina Stata
Configuration Coordinate
Figure 5.
Qualitative Energy Level Diagram for Catalytic Cracking
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It is essential in this catalytic cracking process that the potential acid state, even though raised considerably by strain, should remain the lower energy state with respect t o the four-coordinated (Lewis base) state, in order to force thereaction in the direction of the chemisorption process in which the hydrocarbon exists on the surface as a carbonium ion (the activated complex or transition state). Yet, the potential acid state must be close enough to the basic state that the barrier will be small and thus account for the high reaction velocity. This is the role of second-order
suggest a type of chemical characterization apparently worth qualitative considerations a t this time and which may suggest crucial experiments of the type utilized in the chemical characterizations of acid catalysts. The basic suggestions in the proposed model are: 1. The active centers of hydrogenation-oxidation catalysts always involve a flip-flop duality in which the catalyst may exist in two valence states, the energy levels of which are as close together as feasible still allowing one valence state (corresponding to the transition state) t o be somewhat higher than the normal valence state. 2. The direction of reactions on the catalytic surface and the efficiency of the flip-flop quality will be determined prominently, if not generally, by combinations of first-order strain and secondorder strain effects in similar manner to those illustrated in Figures 4 and 5 for the hydrogenation of ethylene and in structural acid catalysts.
Homogeneous Reaction
'Transition
State Band‘ or Bonded d-Shell electron Band ( g e n e r a l l y empty
* Y E
w
’Chemisorption Band‘ (unbonded d-shell ( o l r a y s occupied
Configuration Coordinote
Figure 6. Proposed General Model for Hydrogenation-Oxidation Catalysis strain-i.e., to raise the level of this six-coordinated state so that the four-coordinated (salt) state will become accessible under thermal activation. By virtue of high surface mobility and this particular arrangement between the six- and four-coordinated states, the surface is able t o execute rapid “flip-flop” transitions. B y means of the activation energy delivered t o the Burface, say, by a hydrocarbon molecule striking it, the potential acid site by itself or with the help of an unsaturated hydrocarbon is able to extract the hydride ion from the hydrocarbon and thus flip into fhe transition state. I n passing over this state, however, the products are immediately released and the surface then flops back into its slightly more stable (six-coordinated) state. This essential flip-flop may easily be eliminated by a Lewis base with an adsorption potential great enough to lower the fourcoordinated (structural adsorption) state (illustrating clearly the influence of trace amounts of poisons of this sort-e.g., basic heterocyclic nitrogen compounds, alkali metal, and certain sulfur and oxygen compounds) by lowering of strain potential on the activated state, thus dropping it t o a nonaccessible energy level.
Flip-Flop in Hydrogenation-Oxidation Catalysis Keys t o the chemical characterization of the active sites which seem to exist on heterogeneous oxidation-hydrogenation catalysts are meager. Correlations with d-shell activity ( 6 ) ,the parallelisms between magnetic and catalytic properties of solids found in the work of Selwood (SO), and an apparent correspondence between semiconductor properties of various metal oxides and SUIfides and catalytic activity, in light of the above flip-flop ideas, 1460
The assumption of a valence duality implies, of course, that the active sites on metal catalysts must be associated with something like metal oxide micelles or micelles of other metallic compoundp or impurities. The fact that metal catalysts are effective through active centers is sufficient evidence itself to justify the concept that impurity centers may exist in these catalysts. The poisoning of platinum and nickel electrodes in hydrogen overvoltage by arsenic trioxide as described by Bockris (2)is also strong evidence for this view. The role of active d-shell electrons may be t o provide the essential valence duality. In other words, d-shell activity is, in fact, the equivalent of valence dualitjr-a flip-flop between a nonbonding state and a bonding state. The purpose of the impurity center in this mechanism is the same as that of silica in the structural acid (alumina-silica) catalyst-Le., to provide a second-order strain center that will promote the tendency of the underlying, normally unbonding, d-shell electrons to a bonding state under the added influence of thermal energy or to promote a transition from one valence state to another. The transition state or activated complex is the state in which the d-electron is acting as a bonding electron. On the unstrained surface this state, however, is usually too far above the normal state for the transition state to be reached by thermal fluctuations. The situation is here visualized t o be essentially the same as that depicted in Figure 5 , for the structural acid catalysts, and in Figure 6 for hydrogenation-oxidation catalysis. There is an essential difference between the chemisorption (or structural adsorption) band and the transition state band of this theory. The former is one created by the occupation of a certain fraction, 0, of the surface sites. The latter is essentially an empty band representing states of the activated complex under varying degrees of promotion. This applies equally t o the transition state band of the structural acid catalysts. In other words, the transition state band in acid and hydrogenation-oxidation catalysts is probably affected only very little by first-order strain and in this sense differs from the transition state band in the activated adsorption illustrated in Figure 2. Semiconductors possess two fundamental characteristics important according t o this theory in hydrogenation-oxidation catalysis. These are high mobility of one or more of the constituent particles, particularly in the surface phase ( I O , l 7 ) , and the valence duality which the theory contemplates for the active centers. I t is, therefore, possible that a close relationship may be found between the semiconduction and catalytic properties of metal oxides. However, this correlation may not be a simple and direct one. For instance, i t may turn out that the best n-type semiconductors are the best hydrogenation-dehydrogenation catalysts, whereas the best p-type semiconductors may be the best oxidation catalysts, direct correlations being expected only if the type of semiconduction and the type of catalysis desired are first considered. I n any event, the substances which lead to semiconduction are also good hydrogenation or oxidation catalysts, although the conditions which lead t o optimum behavior in
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catalysis may not be the same as those which make for optimum semiconduction. The postulated valence duality may be due in many cases t o d-shell activity, although there are other ways in which this might arise, for example, b y merely thermal breaking of the s, p valence bonds. The implication is t h a t i t is not d-shell activity in itself but rather valence duality which is fundamental in catalysis of this sort. A particularly good example indicating that the hydrogenationoxidation type catalysts may involve a duality (flip-flop) kinetics fundamentally like t h a t described for the acid cracking catalysts is found in the work of Simard (99) on vanadium oxide catalysts. Simard showed t h a t the vanadium in a n unpromoted oxide catalyst is in V + + + +and V + + + + +valence states associated with VZOSand VzOr.34. The relative proportions of these oxides are dependent on the hydrocarbon concentration and time of operation, suggesting the establishment of a steady state composition during reaction. The oxides VzOl and VzOa were found to be inactive catalytically. v 2 0 4 . 3 4 was found to have an octahedral structure, while the structure of V206is tetrahedral. As in the silica-alumina six-, four-duality the transformation of V204.84 t o Vz06is relatively simple; relatively little structural change and vanadium atom movement is required t o effect the transition from one structure to the other. “The difference between V206-like and Vz04.a4-likestructures is believed sufficiently small t h a t transform$ions may occur with only slight movements of atoms.” One of the best examples of a reversible dynamic system is the aluminup chloride system. It is well known t h a t the complex, which is here regarded as the activated complex, involves the following reaction:
R‘
+ HCl + (AlCla)
R + (AlCI4)-
(R- is an olefin, R + a carbonium ion). Here, the promoter and the olefin react t o form a carbonium ion complex which decomposes at the moment of desorption. This, it is believed, is an important aspect of aluminum chloride (halide) chemistry. Finally, a few remarks concerning surface dynamics and observable surface changes in heterogeneous catalytic reactions seem pertinent t o this discussion. Hindin, Mills, and Oblad (18) followed the exchange of oxygen between water-enriched 0 1 8 and a silica-alumina cracking catalyst under conditions closely simulatr ing commercial operation-450” C., water contact time, 10 seconds. They found t h a t 50 t o 100% of all oxygen of the silicaalumina structure undergoes exchange in a 20-minute run. The exchange of surface oxygen, which at 100’ C. was found by Mills and Hindin (16) to be complete in 15 minutes, must be instantaneous at 450: C. These results demonstrate conclusively the high mobility which the present theory visualizes for surface species in the flip-flop kinetic processes.
There exists in the literature a large amount of interesting material corroborating the ideas proposed in this paper, particularly the ideas concerning surface mobility. No attempt will be made t o summarize this evidence, except to mention the fact discussed by Handforth and Tilley (11) and Hoog (IS)that the platinum-rhodium gauze in ammonia oxidation and the silver gauze in methanol oxidation t o formaldehyde undergo remarkable surface erosion during use as catalysts.
literature Cited (1) Beeck, O., Discussions Faraday SOC.,No. 8, 120 (1950). (2) Bockris, J. O’M., and Conway, B. E., Trans. Faraday SOC.,45, 989 (1949). (3) Burwell, R. L., and Taylor, H. S., J . Am. Chem. SOC.,58, 697 (1936). (4) Cook, M. A., Pack, D. H., and Oblad, A. G., J . Chem. Phys., 19. 367 11951). (5) Dowden, D. A.,‘ Chemistry & Industry, 1949, p. 320; [ J . Chem. SOC., 1950, p. 242. (6) Elovich, S. Yu., and Zhabrova, G. M., Zhur. Fiz. K h i m . , 13, 1761, 1775 (1939). (7) Emmett. P. H.. and Brunauer, S.. J . Am. Chem. SOC.,56, 35 (1934). (8) Eyring, H., Colburn, C. B., and Zwolinski, B. J., Discussions Faraday SOC.,No. 8 , 39 (1950). (9) Farkas, A., Trans. Faraday Soc., 35, 941 (1939). (10) Garner, W. E., Discussions Faraday SOC.,No. 8 , 211 (1950). (11) Handforth, S. L., and Tilley, J. N., IND.ENG.CHEM.,26, 1287 (1934). (12) Hindin, S. G., Mills, G. A., and Oblad, A. G., to be published. (13) Hoog, H., Chemistry & I n d u s t r y , 1951, p. 872. (14) Huttig, G. F., Discussions Faraday SOC., No. 8 , 2 1 6 (1950). (15) Milliken, Jr., T. H., Mills, G. A,, and Oblad, A. G., Discussions Faraday Soc., No. 8, 279 (1950); Advances in Catalysis, 11, 199 (1960). (16) Mills, G. A., and Hindin, S. G., J . Am. Chem. SOC., 72, 5549 (1950). (17) Mott, N. F., and Gurney, R. W., “Electronic Processes in Ionic Crystals,” 2nd ed., England, Oxford (Clarendon) Press, 1946. (18) Rideal. E. K.. and Tramell. B. M. W.. Discussions Faradau Soc.. ‘ No. 8, 114‘(1950). (19) Rozinski. S.. and Zeldovich. Y . . Acta Phvsicochirn. U.R.S.S., 1. 554, 595 (1934). (20) Selwood, P. W., and Lyons, L., Discussions Faraday SOC., No. 8, 222 (1950). (21) Sickman, D. V., and Taylor, H. S., J . Am. Chem. SOC.,54, 602 (1932). (22) Simard, G. L., presented a t the Conference on Catalysis, Am. Assoc. Advance Sei., Colby, N. H., June 20, 1951. (23) Taylor, H. A,, and Thon, N., J . Am. Chem. SOC.,74,4169 (1942). (24) Twigg, G. H., Discussions Faraday SOC.,No. 8, 152 (1950). (25) Zeldovich, Y., Acta Phusicochim. U.R.S.S., 1, No. 3/4, 449 (1934). -
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RECEIVED for review Deoember 18, 1952.
ACCBPTED May 2, 1953,
4.
July 1953
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