326
T H E JOURNAL OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
Vol. 14,No. 4
First Report of the Committee on Contact Catalysis’ By Wilder D. Bancroft CORNELLUNIVERSITY, ITAACA,NEW YORK
N certain cases of contact catalysis it is known that definite intermediate compounds are formed and Armstrong and Hilditch postulate the formation of ternary compounds. In other cases, it seems equally certain that no definite intermediate compounds are formed and that we are dealing with adsorption, which may mean formation of indefinite intermediate compounds. While it is very important to distinguish between these two types of contact catalysis and while this has not been done satisfactorily in the past, a determination of these points will not tell us why the reaction is accelerated, though i t will give us important information about the mechanism of the reaction. For catalysis to occur the reacting substances must be converted rapidly into active modifications or into active compounds. It has been suggested by W. C. McC. Lewis, Perrin, and others that the catalytic agents emit infra-red radiations and that these radiations cause the catalysis. These ingenious speculations seem quite inadequate, however, to account for the phenomena. It seems certain that in some way the catalytic agent activates one or more of the reacting substances, causing the opening and shutting of certain bonds or contravalences; but as yet we are not prepared to say which bonds or which contravalences are opened in any given case. This hypothesis makes possible a new, tentative explanation of the action of promoters. If the catalytic agent activates only one of the reacting substances or activates one chiefly, it is possible that the promoter activates or increases the activity of the other. If the given catalytic agent adsorbs or activates two substances in the wrong parts of the molecules, it may easily retard instead of accelerating the reaction; this case has been studied by Kruyt and van Duin. While we are still very much a t sea as to the way in which any given catalytic agent works, the theory of the poisoning of contact catalytic agents seems to be in very good shape. Any substance which retards, or prevents, the approach of the reacting substances to the catalytic agent will slow down or put an end to the reaction. Any strongly adsorbed solid, liquid or gas may therefore act as a catalytic poison. This should be distinguished from the cases in which the catalytic agent sinters together so as to have less surface or a more inactive surface. There is reason to suppose that a porous surface is essential and that a glazed surface is ordinarily quite inactive. While
the poisoning of a catalytic agent is usually a thing to be avoided, Rosenmund and Zetsche have made successful use of what they call partial poisoning to stop a reaction a t a given point. It is evident that different catalytic agents activate certain molecules in different ways because alcohol decomposes almost completely to ethylene and water in presence of alumina or kaolin and almost completely t o acetaldehyde and hydrogen in presence of pulverulent nickel. If we can determine what bonds or contravalences are opened by each catalytic agent we can probably predict the resulting reaction; but it is quite possible that we may have to reverse things and to deduce from the reaction products what bonds or contravalences were broken. Rideal has attempted to connect overvoltage with contact catalysis, the overvoltage being primarily a measure of the energy necessary to desorb hydrogen gas from a metai surface; the metals show no catalytic activity when the energy necessary for desorption exceeds that necessary for the activation of hydrogen in the gaseous state in the absence of a catalytic material. This does not seem a very satisfactory point of view; a more plausible line of attack would seem to be that platinum catalyzes the reaction between atomic and molecular hydrogen, but establishes an equilibrium with some monatomic hydrogen remaining. This accounts for the low overvoltage and the distinct activation. Lead, mercury, and zinc do not catalyze the reaction; but the equilibrium lies entirely a t the stage of molecular hydrogen. This accounts for the high overvoltage and the low catalytic action. The other metals come in between these two extremes and, a t higher temperatures, the activating power of nickel and copper for hydrogen becomes greater than that of platinum. This set of hypotheses accounts for agood many facts because devised for that purpose; but there is as yet no independent proof. According to this point of view there is no necessary relation between overvoltage and catalytic activity. The first depends upon the slowness with which equilibrium is reached and the second on the point a t which equilibrium is reached, two entirely different things. Experiments by Rupert and by Willstatter and WaldschmidtLeitz indicate that hydrogenation by means of platinum takes place better-perhaps only-in presence of oxygen. No adequate explanation has yet been offered for this phenomenon.
I
compound take place almost simultaneously, and of course a t equal rates when a “steady state” is reached. In this dynamic equilibrium the quantity of the intermediate substance remains constant under constant experimental conditions. The dynamic equilibrium may also be considered as a limiting case of periodicity, the length of the period being practically zero and the number of periods per unit time very great. Visible evidence of intermediate compound formation in contact catalysis is also obtainable in the oxidation or dehydrogenation of methanol in the presence of air and metallic copper. The copper gauze varies in color, showing constant changes in the state of oxidation. Bayley3 states that cobalt is oxidized by hydrogen peroxide and that this reacts with hydrogen peroxide, oxygen be>g evolved from both and the cobalt going back to a lower oxide. This is then oxidized t o cobalt peroxide and the same series of changes is continued. Oxide of cobalt therefore decomposes hydrogen peroxide catalytically. When cobalt oxide decomposes hypochlorites catalytically, Bayley considers that the cobalt oscillates between COSO:and C002. He says that Cosos has been shown to be the final product but he gives no reference. Hydrogen peroxide does not oxidize nickel oxide to a peroxide and
I
T has seemed that the best thing the committee could do in
its first report was to outline the present situation in regard to contact catalysis as clearly as possible. After that, it should be a relatively simple matter for the committee to determine what to do next. COMPOUNDS DEFINITEINTERMEDIATE There are two apparently distinct types of contact catalysis, one in which a definite chemical compound is formed, and the other when we have adsorption with activation. We will take up first the case in which definite chemical compounds are formed. In the catalysis of hydrogen peroxide by mercury, the intermediate formation of mercuric peroxide can be detected by the eye because there is an intermittent formation of a filma which then breaks down, only to form again. This periodic? phenomenon represents an extreme case. Under ordinary conditions the formation and decomposition of the intermediate 1 Report of Committee on Contact Catalysis of the Division of Chemistry and Chemical Technology of the National Research Council. Written by the chairman in collaboration with the other members of the committee: Messrs. H. Adkins, W. C. Bray, R. F. Chambers, C. G. Fink, E. P. Kohler, A. B. Lamb, E. E. Reid, and H. S. Taylor. Received February 4, 1922. 0 Bredig and von Antropoff, 2. Eleklrochem., 12 (leos), 681; von Antropoff, J . prakt. Chem., 121 77 (1908),273.
8
Phit. Mug., [51 I (18791,126.
327
THE JOURNAL OF INDUSTRIAL AND ENGINEERING CHEMIXTRY
April, 1922
is therefore not decomposed catalytically by it.4 If nickel peroxide is made independently, it reacts with hydrogen peroxide just as the corresponding cobalt salt does. These experiments seem t o prove fairly conclusively that one stage in the catalytic decomposition of hydrogen peroxide by cobalt oxide i s the formation of cobalt peroxide. It would be interesting to see whether a change of temperature or of some other factor would make the oxide of nickel behave like the oxide of cobalt, or vice versa. Hydrated ferric oxide and hydrated zinc oxide do not react with hydrogen peroxide, and consequently Bayley does not consider them as peroxides. It would be interesting to know the action of hydrogen peroxide on passive iron. In the catalytic oxidation of carbon monoxide by mixed oxides of cobalt, manganese, etc., it is usually believed that there is alternate (or simultaneous) reduction and oxidation of the catalyst. The oxygen carrier oxidizes the carbon monoxide and is itself reoxidized by the oxygen of the air. While this is undoubtedly the case, no measurements have yet been published, showing the two end stages between which the catalyst oscillates.6 This is known, however, for the case of the catalytic oxidation of alcohol by air in presence of osmium tetroxides because this reaction can be run in two stages. Osmium tetroxide will oxidize alcohol in the absence of air, and air will oxidize the dioxide to the tetroxide. When carbon monoxide is activated by metallic copper in an alkaline solution, Hofmann’ considers that there is intermediate formation of CUZO~ or CUOZ. Armstrong and Hilditch* consider that the catalytic action of ferro-ferric oxide on carbon monoxide and water is due to the alternate reduction of the ferro-ferric oxide to ferrous oxide or iron by the carbon monoxide, and oxidation of these compounds by water. The acceleration of the reverse reaction by ferroferric oxide can be accounted for in a similar manner, reduction by hydrogen and oxidation by carbon dioxide. At a suitable temperature the iron oxide causes the reaction to proceed practically to equilibrium. Metallic copper accelerates the reaction between carbon dioxide and hydrogen; but even a t its optimum temperature does not cause more than 50 to 70 per cent of the possible amount of chemical change. Armstrong and Hilditch account for this on the assumption that copper tends to produce an equilibrium,
+
4cuc12 = 2CuzClz 2c12 2CuzClr 0 2 = 2cu0.cuc12 ~CUO.CUCIZ 4HC1 = 4CuClz 2Hz0
++
+
There seems to be no evidence that cupric chloride breaks down into cuprous chloride and chlorine a t the temperature employed, so we have another formulation: 4CtlClz 0 2 = 2CUO.CUClz 2C12 ~ C U O . C U C ~4HC1 Z = 4C~Clz 2H20
++
++
This seems more reasonable than the others; but we do not know whether this, or any, basic cupric chloride is formed. If we start with copper sulfate as catalytic agent, which may be done, the whole thing becomes hopelessly complicated. There is really no definite knowledge in regard to the intermediate compound or compounds a t all. Mr. Bray points out that this alternative suggestion, that oxygen can displace chlorine from solid cupric chloride, may be considered as a border-line case. In a crystal of cupric chloride the cupric chloride “molecules” are combined with each other. A single oxygen molecule might be taken up and two molecules of chlorine be given off without the formation of a definite new molecule, such as the basic cupric chloride suggested. On the other hand, as anhydrous copper sulfate is really a good catalyst the mechanism may be that hydrochloric acid molecules are taken up by the copper salt (chloride or sulfate) and that the oxygen then attacks the hydrogen of the added hydrochloric acid, setting free chlorine. Mr. Bray endorses the statement that there is as yet no real evidence of the formation of a definite intermediate compound. Mr. Taylor writes that he does not now think that the Deacon chlorine process involves intermediate compounds a t all. He expects that experiments now being prepared a t Princeton will show that the copper halides probably adsorb hydrochloric acid gas and sulfur dioxide-possibly oxygen. This indefiniteness in regard to the nature of the intermediate compound is a serious defect in much of the work on contact catalysis. Sabatier12believes that the catalytic action of alumina on alcohol is due to the intermediate formation of an instable compound of alumina and water, while HendersonIs assumes the intermediate formation of aluminium ethylate. Neither man offers any experimental evidence in support of his belief, and neither one discusses why kaolin should act practically as well as alumina The formation of a definite kaolin ethylate CO H20eHC02H. seems improbable. Sabatier assumes definitely the existence The formation of graphite is usually preceded by the forma- of intermediate compounds of metal and hydrogen in hydrotion of a carbide. The conversion of acetic acid into acetoneg genation and dehydrogenation processes. H e says : by passing the vapor over heated barium carbonate presumably This conception of the way in which catalytic action occurs involves the intermediate formation of barium acetate. In [temporary formation of intermediate compounds] is supported the Deacon chlorine process, where a mixture of hydrochloric by the experimental confirmation of the predictions which one acid and oxygen is passed over heated cupric chloride, Hurterlo can make. If the metallic catalytic agents, nickel, copper, assumes the existence of an intermediate compound and he is etc., tend to react with hydrogen, forming intermediate products probably right in this. People are not agreed, however, whether analogous to the hydrides, these metals must also be able to remove hydrogen from substances which can give off hydrogen. the intermediate compound is cuprous chloride, basic cupric In other words, these metals should act also as dehydrogenating chloride, or both.” The reaction as usually written is: catalysts. It was this line of thought which caused Sabatier and Senderens to try these metals as catalyzers for the dehydro4CuCl2 = 2cuzc12 2c12 genation of alcohols into hydrogen and aldehydes or ketones. 2C~zClz 4HC1 0 2 = 4C~C12 2H20 It was found that nickel was less suitable than copper for this Since it is not probable that the second reaction takes place purpose because of its tendency to form with carbon monoxide an intermediate compound which was instable a t the temperain one stage, we may write : ture of the reaction and which gave rise to a secondary decomposition of the aldehydes and ketones. 4 Work done at Cornell since this report was written indicates that
+
+
+
+
+
Bayley is wrong in this statement. The whole subject is now being studied. 5 Mr. Bray believes t h a t i t is useless to try to decide whether, a t the dynamic equilibrium, a molecule of oxygen at the surface of the catalyst actually changes some of a lower oxide to a higher (and the reverse change with carbon monoxide) or whether the oxygen is merely held on the surface in an active condition ready to combine with carbon monoxide. In either case all the processes are taking place simultaneously. GHofmann, Ber., 46 (19121, 3329; 46 (19131, 1657, 2854;46 (1915), 1585. 7 Ber., 61 (1918),1334, 1526; 6SB (1920),2165. 8 Prnc. Roy. Soc. London, 97A (1920),266. 9 Squibb, J . A m . Chem. Soc., 17 (1895), 187. 10 J . SOC.Chem. I n d . , 2 (1883), 106. 1 1 Woker, “Die Katalyse,” 1910, 474.
This seems to satisfy the scientific criteria. The hypothesis is used to predict new results and the new results are obtained. The experiments of Taylor and Burns,14however, show that no compounds analogous to the hydrides are formed with nickel and copper.16 The flaw in Sabatier’s reasoning is that he did “Die Katalyse in der organischen Chemie,” 1914, 240. “Catalysis in Industrial Chemistry,” 1919, 4. 14 J . Am. Chem. Soc., 43 (1921), 1273. 16 Mr. Bray considers t h a t the “tendency” of a metal like nickel to combine with hydrogen may exist even when a definite compound does not form, He believes that this is the correct explanation of adsorption a t higher temperatures. 12
13
328
T H E JOURNAL OF INDUSTRIAL A N D ENGINEERING CHE-MISTRY
not consider the possibility of some other hypothesis leading to the same conclusion. Armstrong and HilditchlB believe that catalytic change is always effected by the intermediate formation of definite chemical compounds, although they do not postulate in general the formation of intermediate compounds of well-defined stability. They consider that these are not to be expected in the course of catalytic change, the specific action of various catalysts probably depending largely on the suitable instability of the intermediate complex for the purposes of the action-in other words, the change of the compounds which they have studied depends on the coincidence that the catalyst and the initial substance will interact to form a compound sufficiently instable under the conditions prevailing to decompose (or interact with other constituents of the system), giving new products. Armstrong and Hilditch say, in regard to these intermediate compounds that The physicochemical data from which our present deductions are made do not afford any evidence as to the precise stoichiometrical relation, subsisting between catalyst and organic compound. The latter is not necessarily simple, for example, an atom of nickel in temporary union with an organic group containing-one ethylenic linkage: it may be more probably of the nature of a “coordinated” compound comparable to nickel carbonyl, in which the “subsidiary valences” of the interactants play the chief part; or again, the combination might be still less definite and analogous to the adsorption of dye by fibers, in which case the composition of the unstable complex might vary with physical conditions such as temperature or concentration. By “intermediate compound” in the present series of papers we mean an association between catalyst and the other compounds due to the same “chemical” forces which operate in more ordinary chemical actions, but we are not able to offer any opinion as to the precise character of the compounds thus transiently produced.
In an earlier paper the study of the hydrogenation of liquid fats in presence of nickel led Armstrong and Hilditchl’ to believe that there is formed a ternary, unspecified, and instable complex of nickel, unsaturated fat, and hydrogen, which breaks down into a saturated, or more saturated, fat and nickel. The nickel is believed to attach itself in some way to the unsaturated carbons, -CH : CH--. They also consider that the interchanges take place in an electrolytic circuit in which the interacting substances are necessarily all comprised. The hydrogen and the fatty oil are both to be thought of as coupled with the nickel and the interaction as brought about through the inclusion in the system of the electrolyte required to establish a conducFing circuit. Both hydrogen and the fatty oil are to be regarded as having some affinity for the catalyst; and the rate of change is to be considered as determined by the rate at which the less active hydrogen enters into productive association with the catalyst. Dehydrogenation is believed to be due to the reverse formation of the nickel-unsaturated compound-hydrogen complex. I n the partial hydrogenation of ethyl oleate, ethyl stearate is formed18 and also ethyl esters of isomeric forms of ordinary’ oleic acid, these being most probably ethyl elaidate and an ethyl “iso-oleate” derived from A1lzrz oleic acid. The appearance of the elaidic ester, the stereoisomeric form of ordinary oleic ester, is readily explained by the assumed equilibrated action between the catalytic nickel and the ethylenic linkage, for on re-formation of the constituents during the balanced action it is plain that either or both stereoisomeric ethylenic derivatives might result. The production of the isomeric form of oleic acid takes place t o a greater extent the higher the temperature of hydrogenation, and is also more marked a t equal temperatures, with palladium, generally a more vigorous catalyst than nickel. Though Armstrong and Hilditch believe firmly in the formation of definite intermediate compounds, their conception of
.
PYOC.Roy. SOL.London, ?SA (lgZO), 37. I b i d . , 96A (1919),137, 322. 18 Moore, J . SOC.Chem. Ind.,38 (1919),320.
an intermediate compound is so elastic and their knowledge of the compositions of the assumed intermediate compounds is so limited, that there would probably be no difficulty in rewriting all their results in terms of adsorption.
-
INDEFINITE INTERMEDIATE COMPOUNDS
While there are undoubtedly many cases of contact catalysis involving the formation of definite intermediate compounds, it is equally certain now that many others do not come under this head. Mention has already been made that no hydrides of nickel and copper are formed under the conditions favorable to catalytic action. It seems improbable that it would be so difficult to make carbon tetrachloride if the chlorine which is adsorbed by charcoal’@and thereby made active consisted of a compound of carbon and chlorine. Oxygen adsorbed by charcoal will oxidize ethyl alcohol to acetic acid20 and ethylene to carbon dioxide and water, reactions which are certainly not due to any known oxide of carbon. It is very important that we should decide in each given case whether a definite intermediate compound is formed and, if so, what compound. Only in this way can we escape from the haziness which mars so much of the work on catalysis. The view of Armstrong and Hilditch is practically the same as that of Mr. Bray, except that he prefers to speak of indefinite chemical compounds instead of definite ones and he considers , these indefinite compounds as identical with adsorbed material wherever adsorption is something more than capillary condensation. While he agrees with the statement that the results of Armstrong and Hilditch could be rewritten in terms of adsorption, he would prefer to translate many results now expressed in terms of adsorption back into the language of indefinite compounds, Mr. Bray would like to see emphasized the differences between what he calls the two classes of adsorption: (1) when the amount of material adsorbed is relatively small and not more than one molecule in thickness; and (2) when the adsorption takes place on a porous material a t a relatively low temperature, in which case capillary forces come into play and a relatively large amount of material is adsorbed. He believes that it will be difficult a t times to distinguish between definite and indefinite compounds. When we are not dealing with contact catalysis, it is usually easy to decide whether or not two substances, A and B, react to form a definite compound. Thus oxygen will react with solid calcium to form a definite oxide but not with platinum or with carbon a t relatively low temperature. If it were not possible to bring more than a few molecules of oxygen to the surface of one of these elements, Mr. Bray believes “that it would be almost impossible t o decide whether or not a definite compound is formed. A single atom of oxygen on a surface of calcium or platinum would be held by the same kind of bond (or valence force) in the two cases and it is only after the introduction of much more oxygen that the difference appears. The molecules of the definite compound calcium oxide arrange themselves as required in that compound, and we may say that the Ca-Ca bonds are broken. On the other hand, Pt-Pt or C-C bonds are not broken and the action of the oxygen is restricted to the surface. In the latter case we have adsorption.” There are two points on which Mr. Bray lays no stress. It is of course possible that oxygen may be adsorbed to a varying extent as molecular oxygen without necessarily forming a substance analogous to a peroxide because it may be held by different bonds or contravalences. When the Bt-Pt or C-C bonds are broken, as in the formation of a definite chemical compound, we get two phases, each of constant composition, in case the definite compound is a solid. In the corresponding case of the indefinite compound, the system behaves like a phase of continuously varying concentration.
16
17
Vol. 14, No. 4
19
P
Damoiseau, Compt. rend., 73 (1876),60. Calvert, J . Chem. SOC.,20 (18671,293.
April, 1922
THE JOURNAL OF INDUSTRIAL AND ENGIR’EERING CHEMISTRY
329
Perrin.26 RidealZ6has come out in favor of this view and it has been criticized severely by L a n g m ~ i r , with * ~ a rebuttal by Lewis.28 It apparently is not claimed that these infra-red radiations act effectively a t finite distances. It is believed that they act only on adsorbed material. Even with this limitation the hypothesis has difficulties before it. Zsigmondyn9showed years ago that alumina and salts of aluminium do not absorb infra-red rays. This means that they do not emit these rays and that consequently alumina should be a very bad catalytic agent, whereas it is an admirable one for some purposes. Of course it is possible that alumina absorbs infra-red rays very strongly a t the temperatures a t which it is effective catalytically; but that should have been shown in advance by the proponents OF this hypothesis. It seems scarcely possible that the absorption of infra-red radiation by the reacting substance a t the surface of the catalyst can vary in nature with the state of subdivision of the catalyst a t a given temperature. I t is therefore difficult to see how, on this hypothesis, the dehydration products of amyl alcohola3can vary with the age of the catalytic agent. It is perhaps unreasonable to insist upon it; but one would really like to know which lines or bands in the infra-red give rise to the different products which are obtained when ethyl acetate, for instance, is passed over different catalytic agents. TyndallaObas pointed out that gum arabic is exceedingly opaque to the infra-red rays. Accofding to the hypothesis, ACTIVATION addition of gum arabic to a solution of methyl acetate ought to be a t least as effective in promoting hydrolysis as adding hydrochloric Since reaction velocity is a function of the concentration of acid. Somebody will certainly have to try this experiment. the reacting substances, it was natural to ascribe the catalysis It is quite extraordinary that a theory like this should get a of oxyhydrogen gas by platinum to the increased concentration. We know now that this is not the whole truth, because it was found scientific standing with apparently nothing back of it. If we eliminate radiation, the only remaining possibilities seem during the war that oxyhydrogen gas was quite stable under to be that we may have more effective collisions between molepressures of two thousand atmospheres. This shows that the catalytic action of platinum black is due to something more than cules or that we may have the conversion of one or more of the an increase in concentration. The marked difference in the reacting substances into an active form. According to the kinetic theory the reaction velocity is probehavior of platinum and charcoal towards oxyhydrogen gas is portional to the number of collisions between possibly reacting another argument in favor of the action of a catalytic agent being specific. Patrick’s silica gel has a surface of 2.5 X lo6sq. cm. molecules; but it does not follow a t all that two molecules react per gram and is an excellent adsorbent; but it does not catalyze every time they collide If a large number of collisions is necesmany reactions which are accelerated by platinum or charcoal.21 sary on an average before a pair of molecules react, anything It does, however, accelerate the reaction between nitric oxide and which would make these collisions more helpful might increase oxygen; and it is very effective in the synthesis of amines. The the reaction velocity enormously. The first question is then prediction was madezz that it would work well with the mixed whether there is any evidence of ineffective collisions. This matvapors of alcohol and acetic acid a t 100’ because this reaction ter has been studied by Strutt,*1 who comes to the conclusion takes place a t a measurable rate in the vapor phase in the ab- that a molecule of ozone reacts every time it strikes a molecule sence of any catalyst and consequently the effect of increased of silver oxide; but that a molecule of active nitrogen collides concentration should be extremely marked. This prediction with a molecule of copper oxide five hundred times on an average has been verified by Reid a t the Johns Hopkins University; before they react, while two molecules of ozone a t 100” collide but there is apparently a specific effect because Reid obtained a on an average 6 X 10 times before they react. Without in76 per cent yield of ethyl acetate over quite a long period of sisting on the absolute accuracy of these figures, there is evidently time, although the theoretical equilibrium corresponds to 67 plenty of margin for an increase in reaction velocity with ozone a t 100” if one could produce more effective collisions. Langper cent conversion. mui1-3~finds that 15 per cent of all oxygen molecules a t a pressure Increased concentration a t the surface of the catalytic agent of not over 5 bars striking a tungsten filament a t 2770” K. react cannot, of course, account for cases in which a substance decomposes in one way in the presence of one catalytic agent and with it to form tungstic oxide, WOS. This coefficient increases a t higher temperatures, and a t 3300” K. about 50 per cent of in another way in the presence of a second. A typical case of all the oxygen molecules which strike the filament react with it this is the breaking down of alcohol into ethylene and water in presence of alumina and into acetaldehyde and hydrogen in to form WOa. Since there are three atoms of oxygen in WO3 and only two in the oxygen molecule, Langmuir considers that a t presence of pulverulent metals. Senderensz3 finds that amyl least one-half of the tungsten surface must be covered a t 3300’ K. alcohol gives varying dehydration products with varying age with oxygen in some form. of the same catalytic agent. This certainly is not exclusively a It is possible that a catalytic agent may cause one molecule to concentration effect. W. C. McC. Lewisz4believes that all catalysis is due to infra25 Ann. fihys., [SI 11 (1919),5. 28 Tvans. A m . Electrochem. SOC., 86 (1919),195;J . Chem. Soc., 117 (19201, red radiation, and a similar view has been put forward by Waiving these two points, there is substantial agreement between the viewpoint from which the bulk of the report is written and that held by Mr. Bray. The difference is in the stress laid on different sides of the problem. It is admitted that we may have definite compounds, indefinite compounds, and capillary condensation. Mr. Bray wishes to emphasize the difference between the last two, perhaps because of the differencesbetween adsorption by carbon and adsorption by silica. For reasons to be given in the next few paragraphs, the chairman of the committee feels that the capillary condensation plays so small a part in the interesting cases of contact catalysis that it may be neglected, especially since the theory of it presents no difficulties. On the other hand the failure to distinguish as sharply as possible between definite and indefinite intermediate compounds has apparently been one of the reasons for the retarded development of a theory of contact catalysis. I t is in order to emphasize this distinction that the term “adsorption” has been used in many cases in preference to the term “indefinite compound.’’ The difference is psychological and not chemical. In so far as the reaction takes place in or a t the surface of the catalytic agent without formation of definite intermediate compounds we are dealing with adsorption and we must now consider the general case of adsorption with activation or activation in consequence of adsorption.
21
Taylor, Trans. A m . Electrochem. Soc., 36 (1919), 150.
z2 Bancroft, I b i d . , S I (1920), 31.
Comfit. rend., 171 (1920), 916. J. Chem. Soc., 106 (19141,2380; 107 (1915), 233; 109 (1916),55, 61, 796; 111 (1917),457, 1086; 113 (1918). 411; 117 (1920),623; ”System of Physical Chemistry,” 5 (1919), 138. 23 2*
1288. Criticized by Taylor, THISJOURNAL, 13 (1921), 75. 27 J. A m . Chem. Soc., 42 (1920), 2190. 28 Ibid., 43 (1921),15. 18 Dznglcr’s polytech. J . , 37 (1893),17. 80 “Fragments of Science: Radiant Heat and Its Relations.” 8 1 Proc. Roy. SOC. London, 87A (1912),302. a2 J. A m . Chem. SOC., 36 (1913),105; 38 (1916), 2270.
330
THE JOURNAL OF INDUSTRIAL AND ENGINEERING CHEMISTRY
strike another amidships instead of head-on and may thereby increase the effectiveness of the collisions. It is not impossible that part a t least of the effect of solvents on reaction velocity may be due to some such thing as this. If we adopt Langmuir’s views of oriented adsorption, all sorts of things become possible. If ethyl acetate, for instance, attaches itself to one adsorbent by the methyl group, to another by the ethyl group, and to a third by the carboxyl group, it might very well be that bombardment of the captive molecule by free ones might lead to very different reaction products in the three cases. Such a suggestion is of very little value, however, unless it can be made definite. We do not know as yet whether ethyl acetate is actually adsorbed in one way by nickel, in another way by thoria, and in a third way by titania, nor do we know whether the difference in the manner of adsorption, assuming it to occur, is of such a nature as to account for the differences in the reaction products. It may well be that some entirely different hypothesis will prove necessary. The general problem of increasing the effectiveness of molecular collisions is distinctly an important one which has not been studied a t all. Instead of more effective collisions between unchanged portions of the reacting molecules, we may have a partial conversion of one or more of the reacting substances into active forms either through the rupture of normal valences or of contravalences, opening up fields of force, as Balya3puts it. I n order to account for the change of reaction velocity with change of temperature in the case of such reactions as the inversion of cane sugar, Arrheniuss4 fell back on the hypothesis which had developed into the electrolytic dissociation theory and postulated the existence of active and inactive molecules of cane sugar. If the number of active molecules is very small relative to the inactive ones, a slight displacement of the equilibrium will account for a large percentage increase in the reaction velocity. This hypothesis did not prove fruitful and no progress was made in determining what was the difference between the active and inactive molecules. Since the majority of reactions have temperature coefficients larger than can be accounted for on the kinetic theory, this meant that practically all substances occurred in two forms, the active form always being relatively low in concentration. This was SO unsatisfactory that most persons have preferred to accept the temperature coefficient as a n empirical fact without attempting any theoretical explanation. Baly and K r ~ l l consider a~~ that the change to an active form consists in opening the condensed systems of force lines. Although the term residual affinity has frequently been used to explain many chemical processes, no satisfactory and connected explanation has been brought forward as to the nature and origin of this property. It is generally accepted that the formation of compounds such as hydrates and double salts are due t o the secondary valencies of the atoms of the compounds concerned, and that every elementary atom possesses these secondary valencies to a greater or lesser extent. In any compound that is formed by virtue of the primary valencies of its atoms only, the secondary valencies are unsaturated. Every atom must therefore be the center of a field of force, the nature and strength of which depends on the nature of the secondary valencies in each case. In addition to the unsaturation of the secondary valencies, we must add the unsaturation of the primary valencies, when this is known to exist. Clearly therefore such atoms in a compound must be the center of a field of force, the lines of which radiate in every direction. Xow the independent existence of the several fields of force in any one molecule must be a metastable condition, for the line of force of the several fields must condense together with the escape of free energy. The results of this condensation will be the production of a closed system of force lines, and the free affinities of the molecule will be considerably reduced. When the condensation of the force lines has occurred, it is not necessary that the whole of the free affinities should disappear, for aa
J . Chem. SOC.,101 (1912),1469,1475.
2.physik. Chem., 4 (1889), 226. 81 J . Chem. SOC.,101 (1912), 1469. 84
Vol. 14, No. 4
this will only take place when there is a perfect equality between those of opposite type within the molecule. I n those cases where there is not a perfect equality there will naturally be left a balance of affinity, and it is this balance which should be defined by the term “residual affinity,” and this term should mean that amount of affinity left uncompensated after the maximum condensation between the force fields has occurred. This residual affinity of a molecule may be of positive or negative, or, as more usually called, basic or acid type, according to the conditions obtaining within the molecule. Again it is evident that the chemical activity of a molecule must depend on the free affinity which exists, and it is a necessary deduction that the condensing together of the lines of force must result in a decrease of chemical reactivity, and, indeed, it would seem to follow that the true chemical affinity of any molecule cannot be exhibited until the condensed systems of force lines within each molecule have been unlocked or opened by some means. Baly and have applied this conception of condensed systems of force lines around the molecules to chemical action and to catalysis.
It is evident that the condensing together of the lines due to the force fields round the component atoms of a molecule must result in an enormous decrease in the reactivity of the molecule, and, in fact, it may be said that such a condenged system cannot react unless i t previously be opened or unlocked by some means. For example, the well-known cases described by Baker, when pure, dry substances, such as ammonia and ydrogen chloride, lime and carbon dioxide, do not react toge her are doubtless due to the fact that the force fields of the molecules are so condensed together that no reaction takes place when they are brought together. The presence of water is required in order t o open these systems sufficiently for the reaction to proceed, the merest trace of water being enough t o catalyze the whole reaction. Again, the converse cases of the vapors of ammonium chloride and mercurous chloride may be explained in the same way, for these molecules evidently possess their force fields so condensed that increase of temperature alone is not sufficient to open them, and the vapor densities correspond with those of the undissociated molecules. The presence of water, however, opens the condensed fields sufficiently for the molecules to dissociate under the influence of higher temperatures. The general phenomenon of catalysis is capable of explanation on these lines, for a catalytic agent may be defined as one which opens the condensed system of the reacting substance or substances so that their chemical reactivity is enhanced. h’egative catalysis is equally capable of explanation, for a negative catalyst is simply a type of substance which tends to close up the condensed systems, and thus conteract the action of the solvent. Specific evidences of such closing of the molecular systems have been observed from absorption spectra, as will again be mentioned. It follows from the above that any chemical reaction must take place in a t least two stages. First, the reacting substances must have their condensed systems opened up, and, secondly, these opened systems will react together to give the expected compound. It is clear that these stages should be possible of observation, and that, in the event of their taking place, they should evidence themselves in some way. There is no doubt that the explanation of the color changes of the aromatic aminoaldehydes and ketones when treated with alcoholic hydrogen chloride are due to these stages in the reaction. When, for example, o-aminobenzaldehyde is dissolved in alcohol, the condensed system is partly opened, owing to its penetration by the.force lines due to the residual affinity of the alcohol. On the addition of hydrogen chloride, the final product is, of course, the hydrochloride; but the base in the form in which it exists in alcoholic solution does not itself react with the acid to give the salt. It passes through an intermediate phase when it is opened up to a more complete stage, and it is this intermediate phase that reacts with more acid to form the salt. The intermediate phase has a yellow or red color with a characteristic absorption bands? as has already been described.
t
Experiments with the nitroanisols dissolved in alcohol and in concentrated sulfuric acid showed the occurrence of a stage in the sulfuric acid solution preliminary to the sulfonation. The authors consider that this is an intermediate formation of CHaOCdHa.H2SO4. They consider that any molecular trans86
87
J . Chcm. Soc., 101 (l912), 1475. Baly and Marsden, J . Chem. SOC.,98 (1908).2108.
April, 1922
THE JOURNAL OF INDUXTRIAL AND ENGINEERING CHEMIXTRY
formation whatsoever may be looked upon simply as a rearrangement of the electrons due to the potential gradient within the condensed systems of force lines of the molecule being sufficiently steep. In a discussion of catalysis in homogeneous systems, Dhars8 says: “If active and inactive molecules exist, it appears reasonable to expect that a catalyst or light simply acts in shifting this equilibrium. Plainly if sufficient catalyst were added to change a relatively large amount of inactive into active molecules or vice versa, it would follow that the temperature coefficient would be smaller than when the reaction is not catalyzed.” This proved to be true e ~ p e r i m e n t a l l y .I~n~the oxidation of quinine sulfate by chromic acid, the temperature coefficient is 1.87 in the dark and 1.04 in the light. With iodine and potassium oxalate, the temperature coefficient is 6.73 in the dark and 3.4 in diffused light. Mercuric chloride and potassium oxalate showed a temperature coefficient of 2.2 in the dark and 1.19 in the light. The conception of Baly that a catalytic agent activates the reacting substances by opening up fields of force covers the ground admirably as a purely formal statement. The trouble is that it is as yet too vague to be of any value as a working hypothesis, though it is undoubtedly the best starting point. By combining Baly’s hypothesis with a modified form of Langmuir’s theory of adsorption, we may get something really worth while. Langmuir considers that the particles of a crystal of any solid mass are held together by chemical forces and therefore constitute a giant molecule. An adsorbed gas will be held chemically by the unsaturated valences a t the surface of the solid. He says: In general the law of multiple combining proportions will apply. Thus each metal atom of the surface will be capable of holding a definite integral number (such as one or two of atoms of the gas or possibly each two atoms of metal may hold one atom of gas). The atoms held on the surface in this way will form a part of the solid body, being a real continuation of the space lattice of the solid. This layer of atoms (or molecules) on the surface may be said to be adsorbed. The surface of the metal is thus looked upon as a sort of checker-board containing a definite number, NO, of spaces per square centimeter. The number of elementary spaces, NO, is probably usually equal to the number of metal atoms on the surface. But this is not essential, for we can imagine cases in which each atom holds, for example, two adsorbed atoms or molecules, so that we should then have twice as many elementary spaces. as metal atoms on the surface. These so-called compounds are not of the ordinary type and no definite formulas can be written for them. In the case of the adsorption of argon by charcoal, we should have to write CzAry where x varies with the mass of charcoal and y with its surface and also with the pressure and temperature. These substances should therefore be called indefinite intermediate compounds. Starting from this point of view, L a n g m ~ i has r ~ ~developed a theory of contact catalysis, which however deals primarily with the form of the reaction velocity equations and does not throw much light on the real problem of contact catalysis. This defect could be remedied if the theory were developed to such an extent as to show that an ester was adsorbed in different ways by different catalytic agents and that the reaction took place a t the point of the molecule where the ester tied on to the adsorbent, forming the indefinite compound. This is a perfectly normal development because Langmuir now postulates that hydrogen is taken up-or a t least is given off-as monatomic and not as molecular hydrogen. PROMOTERS This general treatment of contact catalysis as a problem in activation has the advantage of suggesting a possible explanation for the behavior of promoters. Rideal and Taylor41 say: 88 89 40 41
J. Chem. SOL, 111 (19171, 746. Cf.Cox, Ibid., 119 (1921), 142;Baly and Barker, Ibid., l i 9 (1921), 653. J . A m . Chem. SOC.,88 (1916), 2286. “Catalysis in Theory and Practice,” 1919, 31.
331
Thus far no theory put forward to account for the acceleration of reaction by minute quantities of promoters added t o the main catalyst is completely satisfactory. A possible mechanism, which, however, has received no experimental test, may be advanced by considering the case of ammonia synthesis from mixtyres of nitrogen and hydrogen. Reduced iron is an available contact substance, the activity of which may be regarded as due to the simultaneous formation of the compounds, hydride and nitride, with subsequent rearrangement to give ammonia and unchanged iron. Or, maybe, the activity of the iron is due t o simultaneous adsorption of the two gases. The particular mechanism of the catalysis is unimportant for the present considerations. Now such bodies as molybdenum, tungsten, and uranium have been proposed, among others, as promoters of the activity of iron. It is conceivable that these act by adjusting the ratio in which the elementary gases are adsorbed by or temporarily combined with the catalytic material to give a ratio of reactive nitrogen and hydrogen more nearly that required for the synthesis, namely, one of nitrogen to three of hydrogen. From the nature of the materials suggested as promoters, it would seem that they are in the main nitride-forming materials,4* which on the above assumption of mechanism would lead to the conclusion that the original iron tended to adsorb or form an intermediate compound with a greater proportion of hydrogen to nitrogen than required by the stoichiometric ratio. The catalytic activity of reduced iron as a hydrogenation agent would tend to confirm this viewpoint. In reference to this suggested mechanism it must be emphasized, however, that in such examples of “promotion” as require only minute quantities of added promoter the activity is more difficult t o understand. With the case of the ammonia synthesis, the promoters are added in marked concentrations. It is difficult to realize, however, that 0.5 per cent of ceria or a concentration of one molecule of ceria among two hundred molecules of iron oxide, in the example cited above in reference to catalytic hydrogen production, can so far “redress the balance” of adsorption or combination as to produce the marked increases in activity of which it is capable. It is obvious that in this phase of the problem there lies an exceedingly fascinating field for scientific investigation, with the added advantages that, being practically virgin territory, the harvest to be gained therefrom should be rich and abundant. Instead of the promoter changing the ratio of adsorption, it may be that the cdalytic agent activates only one of the reacting agents or activates one chiefly, and that the promoter activates the other. Thus it may be, in the ammonia synthesis, that iron activates the hydrogen chiefly so that we have hydrogenation of the nitrogen. The molybdenum may tend to activate the nitrogen giving rise to nitridation of hydrogen, or it may increase the activation of the nitrogen. Such a state of things is not impossible theoretically. When a dye reacts with oxygen under the influence of light, the light may make the oxygen active and the activated oxygen may oxidize the dye, or it may be that the light makes the dye active and that the activated dye reduces the oxygen. It is easy t o decide this question by seeing whether the effective light corresponds to an absorption band for the dye or for the oxygen. In many cases it is the dye that is made active.43 4 2 The dissociation equilibrium of iron nitride has recently been studied by Noyes and Smith, J . A m . Chem. SOC.,43 (1921), 475. ‘ 8 Depierre and Clouet. J . SOC. Dyers Colourists, 1 (1885), 246; Dufton, Ibid., 10 (1895), 92; Bredig and Pemsel, Beibt., 23 (1899), 796.
(To be continued) I t the first meeting of the recently formed Interdepartmental Petroleum Specifications Committee, held in the oilices of the Bureau of Mines in Washington, D. C., February 27, 1922, consideration was given to certain changes in specifications for lubricating oils of various grades, which were advocated by refiners on the Pacific Coast. Some of the changes suggested were agreed to by the Committee. The changes have been submitted to the Federal Specifications Board, and it is hoped t o incorporate them in the technical paper containing revised specifications for petroleum products, to be published shortly by the Bureau of Mines. The Committee postponed the discussion of proposed changes in specifications for transformer oil, aviation gasoline, kerosenes, and signal oil.
