PROMOTER ACTION IN CATALYSIS

PROMOTER ACTION I N CATALYSIS BY ROBERT NORTON PEASE AND HUGH STOTT TAYLOR

It was recognized early in the development of the field of catalysis that catalysts were very susceptible to inhibition or “poisoning” by impurities originally present in the catalyst or absorbed from the reactants. Thus, Faraday found that the catalytic combination of hydrogen and oxygen in presence of clean platinum voltameter electrodes was markedly inhibited by such diverse substances as carbon monoxide and tapgrease. The fact was further emphasized by the early difficulties experienced in the technical application of the contact sulphuric acid process due to the poisoning of the platinum catalyst by As, Sb, P and Pb compounds present in the converter gases. On the other hand, it has more recently been shown that in certain instances, a foreign substance is able t o render the catalyst considerably more active. Such substances are called “promoters” in the patents of the Badische Anilin und Soda Fabrik. The term “promoter action” may be assigned to the corresponding phenomenon. An extract from a Badische patent specification, on improvements in the preparation of hydrogen-rich gases from water-gas and steam, which deals with the function of promoters, may be of interest. The extract runs as follows: “In the researches on the production of hydrogen from mixtures of carbon monoxide and steam, according to the equation : CO

+ HzO

COa

+ Hz

we have found that the power of catalytic agents generally can be improved by the presence of certain bodies which may be termed PROMOTERS. We have found, for instance, that the activity of the catalytic agents, especially those consisting of or containing iron, or cobalt, or oxides thereof, and also the catalytic activity of other metals or oxides even such as, by themselves, are less active, can be greatly increased by the addition of certain compounds or bodies to which, as afore-

Robert Norton Pease and Hugh Stoft Taylor

242

said, may be given the name, promoters. Thus, the activity of catalytic agents consisting of or containing iron, nickel or cobalt, or oxides thereof, can be greatly increased by the addition of oxygen compounds of chromium, thorium, uranium, beryllium, antimony and the like. Further, a catalytic agent consisting of or containing iron in admixture with less than its weight of nickel, particularly after working for a long time, yields better results than does either iron or nickel when employed by itself.. . . . In many cases, particularly when using catalysts of weak activity, we prefer to employ as promoter a compound which differs considerably from the catalytic agent, in particular with respect to valency, chemical basicity and capability of reduction.. . . . The contact masses containing iron as catalytic agent, and a smaller quantity of nickel, as above described as promoter, bring about rapid and farreaching conversion without the simultaneous formation of methane, even when a comparatively low temperature is employed, and, as compared with pure nickel, are further characterized (especially when suitable oxides or oxy-compounds are employed as binding agents, or as promoters) by possessing greater stability and less sensitiveness to deleterious influences such as, for instance, fortuitous increase of temperature and impurities in the gas mixture.” I n attempting a definition of promoter action a t this time, it seems best to make the tetm rather more comprehensive than the above quotation would suggest and t o include under it all those cases in which a mixture of two or more substances i s capable of producing a greater catalytic eflect than can be accounted for on the supposition that each substance in the mixture acts independently and in proportion to the amount present. As far as the study of the literature has revealed, the term “promoter” was first applied in the patents of the Badische Anilin und Soda Fabrik on ammonia synthesis, to substances, themselves inert catalytically, which are able to increase the activity of a moderately good catalyst by admixture with it. Cases of promoter action, however, as here understood, were 1

Brit. patent 19,249,8/16/10,C.A., 6, 1346.

Promoter Action in Catalysis

243

well known prior to this time, as will emerge in the following discussion. Among such, many cases are known in which a mixture of two or more substances-each by itself a moderately good catalyst-is unusually active. It is to such cases that the definition given above more directly applies. Nevertheless, it is reasonable to regard examples in which one or more of the components of the catalyst are by themselves relatively inert as limiting cases of the general rule, particularly, as in many cases t o be cited later the efficiency of the promoter by itself as catalyst is at present unknown. When more than one of the components are themselves catalysts a difficulty presents itself in choosing between “promoter” and “promoted” analogous to the difficulty in distinguishing between solvent and solute in liquid mixtures. On the basis of the available data, i t is not possible to designate one component as catalyst and the others as promoters even though that may be legitimate particularly in cases in which one component preponderates. The distinction will, therefore, be made between “activatioa” of a catalyst by a substance relatively inert catalytically, or by a small quantity of a relatively active substance, and “co-actiuation” of a number of catalysts each by the rest. Thus in homogeneous catalysis, neutral salt action is a clear case of simple activation, since the salt by itself is without noticeable effect. I n the synthesis of ammonia, the iron-molybdenum catalyst furnishes an interesting example of co-activation. Iron and molybdenum are both catalysts for the reaction. A mixture of equal parts of the two, however, is a much superior catalyst to either one alone. Information on the subject of promoter action is widely scattered through the literature and is in general meagre, being largely limited, in the field’ of heterogeneous catzlysis, t o simple statements in patent specifications that certain substances are able to activate some particular catalyst. While it is known that in the industries the field has been developed to a considerable extent, almost no useful information on the subject has found its way into the literature. The purpose of this paper is to collect what information is available and to

,

Robert Norton Pease and Hugh Stott Taylor

244

draw attention to promoter action as a promising field for investigation. Some experimental work on promoter action is at present being carried forward in this laboratory. It is not intended that the present communication should deal intensively with the causes of promoter action. Rather is the aim t o collect examples of shch action already t o b e found in the existing literature. At the same time, it is to be understood that several cases of promoter action have been exhaustively investigated. Especially is this true in the case of neutral salt action in ester hydrolysis and in similar reactions of this type. I n order the better to systematize the material available, the literature had been analyzed with respect to different reactions and differing types of reactions. Thus, the examples of promoter action in the case of a particular reaction have been collected together and are summarized in the succeeding sections. Owing to their more general technical importance, cases of promoter action in heterogeneous catalysis are first discussed. Promoter Action in Catalytic Hydrogenation Processes A number of cases of promoter action are known in connection with catalytic hydrogenation processes. The standard catalysts for hydrogenation are the platinum group metals, especially Pd and Pt, which work at room temperatures in some cases, and Ni, Co, Cu and Fe, the activity of these metals decreasing roughly in the order named. Ipatiew appears to have been among the first t o detect promoter action in a heterogeneous system and this quite by accident. He found' that in the presence of CuO and in a copper tube, amylene was only one-third converted into isopentane by hydrogen at 2 0 0 atm. and 300° C in 2 8 hours; whereas with CuO in an iron tube under the same conditions complete conversion was effected in 1 2 hours. He has repeated this experiment with the same results and has found the same relation to hold in the reactions: 1

Ber. deutsch. chem. Ges., 43, 3387 (1910).


245

tetrahydrobenzene -+ hexahydrobenzene tetrahydrotoluene + hexahydrotoluene as well as for the hydrogenation of pinene and carvene. Badische patents' state that for general hydrogenation, such as the hydrogenation of fats, of phenol to cyclohexanol, nitro-benzene to aniline, and oxides of carbon to hydrocarbons, Fe, Ni, Co or Cu may be used as catalysts and may be improved by addition of one or more of the following-often in quantities of less than I percent: oxides or oxygen-containing salts of the alkaline earth or rare earth metals or Be, Mn, Mg, U, V, Nb, Ta, Cr, Ti, or B or difficultly soluble phosphates, tungstates or selenates of the alkaline earths (or Li) or compounds of F, Te and Sb, or elementary T e or Sb. An example states that 40 parts NiC03 and I part of ammonium tellurite, ignited and reduced, will hydrogenate cotton-seed oil at 100'. It would be interesting to know what the velocity of hydrogenation was a t this temperature since the working temperature of Ni alone is normally nearer 200'. Another Badische patent' for the catalytic reduction of nitro-aromatic compounds to the corresponding amino compounds states that the copper used as catalyst may be considerably improved by addition of Zn, Ag, MgO, A1203or sodium silicate. Satisfactory catalysts are : Cu-Zn ; pumice impregnated with Cu, Ag and Mg nitrates, ignited and reduced; or 130 g pumice, 24 g CuCO3, 3 g ZnCOs and 2 0 g commercial water-glass, ignited and reduced. It may be pointed out that for this reaction, too active a catalyst is undesirable since further hydrogenation of the ring may take place or the compound be completely broken up. Thus, Sabatier and Senderens3found that Ni, which is a more active hydrogenation catalyst than Cu, tended to split off NH3 or even t o form CH, at 300'. Hence the use of Cu. Dewar and Liebman state' that for the catalytic hydrogenaBrit. patent 2306, 1/28/14, C. A., IO, 287; German patent 282,782, 12/12/13,Ibid., 9, 2461. Brit. patent 5,692,4/15/15,Ibid., IO, 2510. 3 Comptes rendus, 133, 321 (1901). U. S. patents 1,268,692,C. A. 12, 1841;1,275,405,Ibid., 12, 2139.

246

Robert Norton Pease and HNgh Stott Taylor

tion of fats, Ni, Co and Cu can be activated if two or three of them are mixed together or with finely divided Pt, Pd, Ag or AgzO. Another advantage claimed is that the oxides of the metals may be reduced in the fat at ordinary hydrogenation temperatures ( I 80-200 "), whereas Ni oxide alone requires a temperature in the neighborhood of 250' for reduction-a temperature which results in some decomposition of the fat. A solution of Cu and Ni nitrates corresponding to I O percent Cu and 90 percent Ni after evaporation and ignition, yields an oxide which can be reduced at 190' C and the product will hydrogefiate cotton-seed oil rapidly at the same temperature. That a mixture of two catalysts is not in all cases superior to either one alone is indicated by some interesting experiments by Paal and Windischl on catalytic hydrogenation, using as catalysts either platinum or palladium deposited on various other metals. Of the metals used as supports, Ni, Co, Fe and Cu are by themselves hydrogenation catalysts working at temperatures in the neighborhood of zoo', Ni being the most active and Cu and Fe the least, whereas Pt and Pd will work a t room temperature. It was found that deposition on Ni and Co had no appreciable effect on the activity of the Pt or Pd while deposition on Fe or Cu rendered the Pt or Pd almost completely inactive even at somewhat increased temperatures and pressures. Thus, those catalysts whose activity is most widely different from the Pt and Pd appear actually to act as poisons. Experiments a t considerably higher temperatures to determine whether there was any improvement in the activity of the F e or Cu catalysts were not made but would be of great interest. Of the other metals employed as supports, all but magnesium completely inhibited the action of the Pt and Pd. On the other hand, the activity with Mg was very high. The activity of magnesium by itself is unknown, but by analogy with effects here produced by Ni and Co would appear to be rather great and is worthy of investigation. 1Ber. deutsch. chem. Ges., 44, 1013(1911); 46, 4010 (1913).


247

The results for deposition on Pd are given below. Cottonseed oil was the substance hydrogenated. The catalyst was placed in a shaking vessel connected with a burette containing hydrogen. 2-5 g of oil and 2 g of catalyst were placed in the vessel, connection to the gas burette made and the diminution in volume of hydrogen read off. The experiments were carried out a t room temperature and pressure, the results being for the first hour of the run. The'efficiency of Pd by itself is not given but is said t o be approximately that obtained with Ni, Co and Mg. Pd Deposited on

Weight, Pd-mg Weight, metal-g Hz absorbed-cc

I-I -I I-I Cu F e

Ni

Co

I

Mg1

1 I 1

Zn *A1 Ag Sn Pd

- -I-I-

60 6 0 30 60 60 60 60 60 2 2 2 2 _.---____-_ 2 3 58.470.4

2

1

104

20

2

2

4 0.6

I&

60 60 6 0 2

o

The palladium and platinum were deposited from HC1 solutions of their respective chlorides. I n certain cases, the inhibition noted may be due to this method of preparation. Thus, in the case of silver, a thin film of AgCl might form a protective coating over the catalyst. However, apart from this, there is evidently a specific effect of support metal on catalyst which is a function of the catalytic activity of the support. A further investigation of this case should yield valuable results in connection with promoter action. The above investigation was suggested by Ipatiew's observations on promoter action in hydrogenation of an ethylenic linkage (9. v.). Dehydrogenation of Methyl Alcohol Hochstetter has found2 that higher yields of formaldehyde may be obtained from methyl alcohol, in the presence of oxygen, if a catalyst consisting of a mixture of Ag and Cu or one of the Pt group metals is used in place of any one of these metals separately. Two reactions may occur when 1

Mg powder was used for the first experiment and ribbon for the second. U. S. patents 1,100,076, C. A.,8, 2770; I,I 10,289,Ibid., 8, 3618.


248

CH30H and air are led over the catalyst. HCHO may be formed by oxidation : CH30H 0 HCHO HzO or by dehydrogenation : CHaOH + HCHO Hz. The latter reaction may proceed further to give HCHO + CO Hz. Pt or Cu deposited on asbestos are energetic catalysts for the complete dehydrogenation of CH30H to give CO and Hz acting at 95 O and 300-400 O, respectively, although at lower temperatures (200-300 ") the reaction proceeds to give more HCHO in presence of Cu, according to Sabatier.2 Kuznetzov3 was able to obtain maximum yields of 50 percent and 70 percent with Cu and Ag, respectively, and found that the amount of further decomposition to CO and 'Hz depended on the temperature, as Sabatier states. It appears, therefore, that the function of the mixed catalyst is either to accelerate the oxidation reaction (or primary dehydrogenation) or t o depress the complete dehydrogenation reaction. Further information as to the course of this conversion and the effect of mixed catalysts on the relative velocities of these reactions would be of interest. Hochstetter emphasizes the fact that " * * such an association of metals as will allow the vapors to contact with each of the metals as individuals'' is to be used. He believes that the increased yields (he claims IOO percent conversion) in the presence of two metals are to be explained by the fact that one metal catalyses more particularly the oxidation reaction and the other the dehydrogenation reaction. By causing the two reactions to proceed simultaneously, a more complete conversion is secured. He cites especially as suitable catalysts, a copper tube containing metallic silver; and silver associated with a Pt group .metal, particularly rhodium.

+ e

+

+

+

2

Orlov: Jour. Russ. Phys. Chem. SOC.,39, 855, 1023; C. A . , La Catalyse en Chimie organique, 161 (1913). Jour. Russ. Phys. Chem. SOC.,45, 557; C. A., 7,3126.

2,

263, 1692.


249

An interesting case of promoter action, in which a hydrogenation and a dehydration catalyst appear each to activate the other when the two are used together to carry out a reaction which would ordinarily involve a number of successive steps, has been noted by Ipatiew. The reactions taking place are, hydrogenation of a carbonyl group, dehydration to give an ethylenic linkage and, finally, hydrogenation to give the saturated hydrocarbon. He found that camphor was hydrogenated in presence of NiO a t 320-350' to borneol. This in turn could be dehydrated at 350-360" in presence of A1203 to camphene, which could then be 'easily hydrogenated at 240' to give camphane. By using a mixture of NiO and A1203 in presence of hydrogen, camphor could be converted directly into isocamphane a t 200' or less. The steps involved are: H

/

HzC-C-

1

H3c+cH3

c-c-c I

CHz

Hz

\

1

=

0

- I - Ji-'1 -CH2

- CH

-CH2

-CHOH

-CH

- CHa

CH3

Camphor

Borneol

Camphene

Camphane

Similarly, fenchone2 may be hydrogenated to give fencheno1 in presence of NiO at 240°, but the dehydration of the latter and subsequent hydrogenation of the fenchene are very difficult to effect directly at all. Yet, in presence of a mixture of A1203 and NiO, fenchone can be directly converted to fenchane a t 215" C.

Promoter Action in Ammonia Synthesis The Badische Company has patented a great number of catalysts for use in ammonia synthesis. One patent covers3 Ber. deutsch. chem. Ges., 45, 3205 (1912). Jour. Russ. Phys. Chem. SOC.,44, 1695; C. A., 7, 1170. Brit. patent 19,249, 8/16/10, Ibid., 6, 1346.

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Robert Norto% Pease and Hugh Stott Taylor

250

'

the use of Fe, Ni or Co with Mg, Al, Zr, V, Ta, Cr, Mn, Mo, W, U, Th, Nb, the alkali, alkaline earth or rare earth metals or their compounds. Another1 covers the use of mixtures of metals or their compounds from different groups or subgroups of the periodic table. According to Maxted,2 Fe activated by KOH is probably used in the Badische plants. The use of the nitrides of the alkali metals with other metals such as Ni has been patented by the General Chemical C ~ m p a n y . According ~ to the patent claims, these are particularly active catalysts working a t 500-550' C and 30-100 atmospheres as against 2 0 0 atmospheres used in the German plants. I n an example, a mixture of Co (59 parts) and Na (69 parts) is treated with NHB a t 300' C and the resulting nitride used as a catalyst for ammonia synthesis at 520-540' and 80-90 atmospheres. An interesting suggestion has been made as to the function of the mixed catalyst in ammonia synthesis. I n the case of the iron-molybdenum catalyst, which is used t o a considerable extent, iron is known to be a good absorbing agent for H2 and molybdenum for N2. It is possible that the mixture of the two owes its superiority to the bringing together of the two in increased concentrations by means of the mixed catalyst. Indeed, the Badische Company has patented the use of a catalyst which shall consist of one substance capable of absorbing H2 and another capable of absorbing N2. Palladium-molybdenum is cited as an example. While there is no direct evidence to support this assumption, the fact that ammonia can be prepared by alternately passing H2 and N2 over a catalyst consisting of Mo alone or a mixture containing Mo, in which the absorption of Nz by Mo would seem to be one step, lends support to the hypothesis. 1

Brit. patent 26,167,12/14/11,C. A., 7, 1958. Jour. SOC.Chem. Ind., 36, 777 (1917). U.S. patents 1,151,537, 1,159,365, 1,143,366, e. g., C. A., 9 , 2 2 9 5 . Brit. patent 21,151,9/25/11,Ibid., 7, 1083. German patent 265,294,5/3/12,Ibid., 8,408.

,


2.5 I

Promoter Action in Ammonia Oxidation In the catalytic oxidation of ammonia, Maxtedl has published curves indicating the relative efficiencies of iron alone and admixed with other metals as promoters. The percentage compositions are not stated nor are any data available t o show 100

90

60

70

60

50

40

30

the efficiencies of the promoters by themselves. It seems probable, however, that the mixtures are those of maximum activity in each case and that the promoters, with the exception of Cu, are not particularly active by themselves since their 1

Jour. SOC.Chem. Ind., 36, 777 (1917).

252


use has not been patented. An examination of the accompanying curves (Fig. I ) indicates that Bi, W and Cu (also T h and Ce) render the iron considerably more active while Ca and Mn have the opposite effect. Addition of Pb gives a very flat curve, that is, the yield of nitric acid, though low in any case, is not appreciably affected by the rate of passage of the gases over the catalyst. The Fe-Sb mixture was found to give high initial results (as would be expected from the close relationship of Sb to S i ) but to approximate to the value for iron alone after a short time, probably because of the slow oxidation of the Sb and volatilization of the oxide. KOH had little effect on the activity of the iron. Maxted and Ridsdalel claim that tubes of Fe, Ni, Co, Cu or Pt alone or coated with the corresponding metals, by treating with a solution of Cu, W, Ce, Th, Bi or Pb nitrate and reducing, are satisfactory catalysts. The above-mentioned metals may also be “coupled” with Cu or Ag. They have also patented2 the use of a mixture of iron oxide, oxide of a metal electro-negative t o Fe, which will be reduced but not fused under working conditions, such as Cu, and the oxide of an alkaline earth metal. A mixture of Pe, Cu and Ca nitrates is precipitated by NaOH or Na2C03and the precipitate molded, dried, calcined and used as a catalyst. Some of the catalysts given in the Badische patents for oxidation of ammonia3 are mixtures of substances from any two of the following groups: oxides of metals of the iron group oxides of the rare earth metals Bi oxide. One such was presumably in use at the plant a t Oppau. Examples:

Fe and La or U t oxides Fe and Bi oxides.

lBrit. patent 126,083, 12/4/16, C. A . , 13, 2 2 5 9 . 2 Brit. patent 10781, 7/26/15, Ibid., 13, 1625. Brit. patent 13,848, 6 / 1 8 / 1 4 , Ibid., 9, 3338.

’


253

Other Badische patents’ give metals of the iron group with Bi or its compounds. Thus, 45 parts Fe or Mn nitrate with I part Bi nitrate, precipitated with NH3, dried and heated, will give,a 90 percent yield of oxides of nitrogen at 700’ C. Again it is stated that Bi or its compounds plus Ti, Si or MgO or that Pt group metals deposited on carriers and activated by Bi may be used. Te or its compounds may be substitued for Bi or its compounds. Other examples given are granular copper oxide impregnated with ammonium telluride and one or more of Ag, Au and the Pt group metals with tellurium or its compounds.2 A later patent covers the use of lead or its compounds as promoter^.^ In the United States, platinum gauze containing iridium is used as catalyst for this reaction. The function of the iridium is merely to increase the mechanical strength of the gauze, the activity of the platinum not being appreciably altered by i t s presence in small quantities.4 Contact Sulphuric Acid Process Experiments have been conducted to discover cheap substitutes for platinum catalysts in the contact sulphuric acid process. Burnt iron pyrites appears to be the only one that has been used on a large scale. According to RideaI and Taylor,5 oxides of Cu, V, U, Cr, Ni or Co incorporated with oxides of Al, Be, Zr or Zn have been suggested. It is of interest that arsenic oxide, which acts as a poison to Pt catalyst, is itself a fair catalyst for the reaction, working at a somewhat higher temperature. In other words, a mixture of these two catalysts, working separately at somewhat different temperatures, is by no means better than the platinum alone. Whether or not it is superior t o the arsenic oxide alone, Ibid.,

German patents 283,824, 4/15/14, c. A., 9, 2577; 287,009, 5/24/14, 1583. Brit. patents 7,651, 5/21/15, Ibid., IO, 2 9 7 1 ; 13,297, 5 / 2 1 / 1 5 , Ibid., 11,

IO,

528.

Brit. patent 13,298,5/21/15. Parsons: Jour. Ind. Chem. Eng., 11, 549 (1919); see, however, Perley: Ibid., 12, 1 1 (1920). 6 “Catalysis in Theory and Practice,” p. 87 (1919).


254

is not stated. The problem of the behavior of mixed solid catalysts will be seen to be complicated by the difficulty of estimating the surface of each exposed and by the fact that they may work alone a t quite different temperatures. A case similar to that described above will be found under “Catalytic Hydrogenation.’ ’ Preferential Combustion of Carbon Monoxide In Presence of Hydrogen Harger and Terry1 have found that carbon monoxide present in hydrogen may be preferentially oxidized in the presence of certain catalysts, the hydrogen being practically untouched provided the temperature is not too high. The process consists in leading Hz containing small quantities of CO and a little more than enough oxygen to combine with the CO over the catalyst a t suitable temperatures and rates of flow. I n the examples given, the catalysts are the mixed oxides of Fe, Cr and Ce, or Th, or both Fe, Cr, A1 and Ce Fe, Bi and Ce. The following experimental results are contained in the patent specifications. A catalyst is prepared by dissolving in water: 194 parts ferric nitrate 5 parts ammonium bichromate I part thorium nitrate.

The solution is evaporated and the residue calcined a t a low temperature. Hydrogen containing I . 5 percent CO with air was passed over this catalyst a t different rates of flow and the C o n in the effluent gases determined. I O cc of catalyst were used. Typ.

C

I

Rate Lit/Hour 10.5

15 .o I2 .o

9.0 9.0 1

Brit. patent 127,609, 4/28/17.

Percent

coz


25.5

The CO was completely removed in all but the last experiment a t the lowest temperature. The mixed oxides named above have been found to be the most satisfactory. The working temperature of iron oxide alone is about 300' C. Rideal and Taylor1 have applied the above process to the determination of small quantities of CO in hydrogen. They find that whereas iron oxide alone is catalytically active a t 300°, addition of chromium oxide lowers the working temperature to about 2 5 0 ° , while a mixture of 97 parts iron oxide, 2 . 5 parts chromium oxide and 0 . 5 parts cerium oxide gives a catalyst working a t I 80 '.

The Water Gas Reaction Promoter action has been studied in connection with the water gas reaction carried out catalytically a t low temperatures for the removal of carbon monoxide from hydrogen. The reaction involved is: CO f H2O

-

COz

+ Hz.

The following table will give an idea of the effect of adding various promoters t o the iron oxide catalyst. The results are for a water gas containing 25 percent CO and 75 percent Hz, steam having been introduced to make the steam-H2 ratio 2 : I . The term space velocity is used to express the ratio of cc gas per hour to cc apparent volume of catalyst and may be taken, together with the quantity of carbon monoxide converted, as a measure of the activity of the catalyst. The following catalysts were employed after ignition : ( I ) Spathic iron ore ignited a t 600' C. (2) 85-Fe nitrate and 15-Cr nitrate (3) 40-Fe nitrate, 5-Cr nitrate and 5-Ni nitrate (4) 1g4-Fe nitrate, 5-(NH4)2Cr207 and I-Thorium nitrate. All tests were made a t 440-450' C. The Analyst, 44, 89 (1919).

'


256

Catalyst

Gas flow-lit/hour Apparent volume, catalyst-cc Space velocity Percent COZin effluent

(1)

(4)

(2)

3.1

21.5

5

3

620

7000

20.2

24.5

8 2

4000 25.2

16 3 5300 25 .o

With iron oxide alone, the oxidation was incomplete a t the relatively small space velocity of 620. Addition of approximately 3 percent of chromic oxide and 0 . 5 percent thorium oxide resulted in complete conversion at a space velocity of 53oo-over eight times as great. I n the results given above, the comparatively great effect produced by so small quantities of promoters becomes somewhat more explicable if one takes into account the fact that the catalysts were prepared by evaporation of a solution of the soluble salts of the metals and subsequent ignition. In the evaporation, the promoters, being present in small quantities, will only begin to deposit toward the end of the evaporation while the iron oxide begins to come down fairly early as a flocculent precipitate. This forms a pasty mass on the surface of which it is to be expected that the promoter salts will be slowly deposited. The mass is stirred during the evaporation. The promoter will therefore presumably be largely concentrated on the surface of the iron oxide finally obtained, where the gas reaction is subsequently to take place. An experimental test of the validity of this view-point would not be without interest. The Ba,dische Company have made comprehensive claims in the case of this reaction also. Thus, one patent1 states t h a t suitable catalysts are mixtures of Fe, Ni or Co oxides with Cr, Th, Ce, U, Be or Sb oxides; Fe, Ni and Cr oxides; Fe, Cr and T h oxides; Zn, Pb, Cu, V, Mn or Ti oxides with a promoter; and oxides of the following pairs of metals: Zn-Cr, Pb-W, Cu-Zr, Mn-Cr, Ti-Sb, V-Cr and Ce-Cr. 1

Brit. patent 27,963, 12/4/13, C. A.,

IO,

97.


257

The Incandescent Mantle Auer von Welsbach has found that additions of small quantities of ceria to the thoria used as refractory in incandescent mantles considerably increased the light emissivity, the latter reaching a maximum for the mixture, 99. I percent thoria, 0 . 9 percent ceria. If one may consider that the thoria acts in part as a catalyst for the oxidation of the gas, then it would not appear improbable that the ceria played the r61e of a promoter. It has usually been assumed that the thoria acts simply as a radiator and that the ceria is possibly an oxygen carrier. From the point of view of catalysis, we are dealing here with a case of surface combustion. The mantle is a catalyst for the oxidation of illuminating gas. I n the light of this, the increased emissivity of a mantle containing ceria is due in part at least to More rapid combustion, which results in the mantle attaining a higher temperature. In general, .ceria appears to be especially useful as a promoter in oxidation reactions. A direct determination of the catalytic activity in this reaction, of thoria, alone and containing small quantities of ceria, would be of interest. Discovery of a maximum in catalytic activity corresponding to the maximum in light production would go a long way toward explaining what is at present a very puzzling phenomenon. As already mentioned, 0 . 5 percent of ceria has a marked accelerative influence on the catalytic preferential combustion of CO in presence of Hz by the iron-chromium oxide catalyst. It is not known whether or not there is in this case a maximum activity corresponding to a small amount of ceria. A study of this point would be of interest. Hydrolysis-The

Twitehell Reagent

The Twitchell reagent for the saponification of fats furnishes a very interesting example of what may properly be considered as intra-molecular promoter action. It has been known for a long time that sulphuric acid is a catalyst for the saponification of fats. If the fat is mixed

258


with 2-3 percent HzS04, heated for a short time at 1 5 0 O and then boiled with water, hydrolysis takes place. The preliminary heating is essential and is supposed to lead to the formation of stearo-sulphonic acid, which then acts as a catalyst for the reaction. Twitchell has found that by introducing a benzene or naphthalene ring into this compound, thus forming benzene(or naphthalene-) stearo sulphonic acid, a much superior catalyst is obtained. This substance is much more stable than stearo-sulphonic acid, not being hydrolyzed even by boiling water, when the latter is very largely hydrolyzed. It is more soluble in both fat and water and is highly ionized, which property an efficient catalyst for hydrolysis must possess. About 0 . 5 to I . o percent of this catalyst is sufficient. The benzene or naphthalene ring may be regarded as a sort of promoter which has been added to the stearo-sulphonic acid molecule.

Siocatives The drying of oils, such as linseed oil, when exposed to air is an oxidation process, and it is generally agreed that the action is auto-catalytic, that is, a catalyst appears to be formed in the oil during the reaction. This is shown by the fact that the velocity “constant” increases markedly in the initial stages of the reaction. The auto-catalyst is probably a peroxide of the oil. It has been found that addition of various substances, known as “siccatives,” such as salts and especially organic salts soluble in the oil, accelerates the oxidation considerably. Mn, Pb, Zn, Co, V and W are suitable metals employed in the form of salts such as the borates and especially soaps such as lead oleate, manganese rosinate and cobalt linoleate. The inorganic salts, being insoluble in the oil, are much less effective and in some cases the solid particles appear to inhibit the reaction by assisting in the decomposition of the auto-catalytic peroxides. It is a matter of uncertainty whether the siccative itself acts as a catalyst or is in reality a promoter which accelerates


259

the reaction by hastening the formation of the auto-catalyst or by giving it greater stability. Ingle and Mackeyl have come to the following conclusions with regard to the action of different siccatives: I. When present as a soap soluble in the oil, those metals which exist in more than one state of oxidation act as driers, provided the salts of the lower oxides are the more stable. 2 . Metals which form a number of oxides are the more active. It was found, however, that sodium and silver, which are univalent, were more active than this fact would lead one to expect. Promoter Action in Homogeneous Catalysis Several well defined cases of promoter action in Bomogeneous liquid systems have been investigated. Few problems in catalysis have been more thoroughly studied than that of ester hydrolysis by hydrogen-ion catalysts and certainly no case of promoter action has received so much attention as has the accelerative action of neutral salts of strong acids on the catalytic effect of the corresponding acid. It is beyond the scope of this paper to describe in detail the work that has been done or to summarize the literature. (For such a summary one may consult Lewis : System of Physical Chemistry, Vol. I, p.423 et seq. (I~IS).) The reaction RCOORl -I- HzO RCOOH f RlOH is known to be catalyzed by hydrogen-ion, If, in addition to a strong acid, a neutral salt of that acid is added to the reaction mixture, it is found that the velocity of the reaction is increased rather than diminished as would be expected from the fact that according to tlie law of mass action, the dissociation of the acid (and therefore the hydrogen-ion concentration) is repressed by addition of the ion possessed in common by both acid and salt. As the neutral salt by itself has no appreciable effect on the hydrolysis, this is clearly a case of 1

Jour. SOC.Chem. Ind., 35, 454 (1916); 36, 317

(1917).


2 60

simple activation-which is, however, not as simple as it sounds if one may judge by the amount that has been published on the subject. A number of cases of co-activation in homogeneous liquid systems have been observed. Bredig and Brown1 have found that in the catalytic oxidation of aniline by sulphuric acid according to the equation

+

+

( C B H ~ N H Z ) ~ S 28H2S04-+ O~ (NH4)2S04 28SO2

+ 1zCOZ+3HzO,

CuS04 and Hg2S04are together more active than their separate catalytic activities would lead one to expect. The progress of the reaction was followed by means of the volume of gas evolved, the above equation being quantitative-for very dilute aniline solutions. I n Fig. z are given

Fig. 2 Oxidation of aniline sulphate by conc. H&04 in presence of various catalysts. Temp. 275 C (CaH5NHz)2S04 = 0.0556 g; CuSOn = 0.3746 g; HgzS04 = 0.14g; H2S04 44.0 cc

curves representing total volume of gas evolved against time. Curves I and I1 are for HgzS04 and CuS04, respectively, alone. Curve I11 gives the value calculated for the two used together on the assumption that they act independently, 1

Zeit. phys. Chem., 46,

502

(1903).


261

that is, that their effects would be simply additive and Curve IV gives the observed results for the mixed catalyst. It is clear that the observed velocity for the mixed catalyst is greater than that calculated; that is, the two catalysts have activated one another. Bredig and Brown consider that a reciprocal oxidation and reduction of Cu and Hg may be the cause of these unusual results. As no further results for other pairs of salts are given, it is not possible to say whether either one of these two is particularly susceptible to activation or what relation holds between the acceleration noted and the proportion of salts present. Price1 has very thoroughly studied the effects of catalysts on the reaction KzSzOs

+ zKI e2KzS04 +

12.

He has found that the effects of CuS04 and FeS04 together are more than additive. Below are given the accelerations observed and those calculated on the assumption that the effects were additive. N / 8 o KzSsOs Conc. Catalysts-Each Molar

N/4o K I

.

.

Acceleration ~

0bserved

Calculated

525 238

190 99

I22

51 27

/

I I 6000 I /3 2000 I /64000 I I 2 8000

/

64

It will be noted that the observed accelerations are considerably greater than the calculated. FeS04 alone is rather active; CuS04 is only slightly so. MnSO1 and ZnSOr, which are also only slightly active, each gave results with FeS04 which were less than additive. With CuS04 they gave additive results. Similar results were obtained by Brode2 in the reaction HzOz

+ 2HI

Zeit. phys. Chem., 27, 499 (1893). Ibid., 37, 257 (1903).

+

zH~O

12.

Robert Norton Pease and Hugh Stott Tayloi

262

FeS04 was found to be a good catalyst for the reaction and CuSO4 a rather poor one. A mixture of the two was found to-be extremely active as Traube’ had previously reported. The latter suggested its use in testing for HzOzin the presence of acids with starch-KI. This probably represents the first observation of promoter action. Enzyme Action Cases of promoter action are to be found in the field of enzyme action. A number of these, taken from Bayliss: will be given. “The Nature of Enzyme Action” (1g14), Effront has found that amylase, which is present in malt, and converts starch into maltose, was rendered considerably more active by the addition of asparagine (p. 103). Mendel and Blood have found that papain was activated by HCN and H2S. No others of a number of substances tried had a similar effect. This is puzzling since the only distinctive property which these two possess in common, beside a low hydrogen-ion concentration, is their reducing action, which has no apparent connection with the hydrolysis of proteins. The action of laccase, an oxidizing enzyme present in the pancreatic juice, is accelerated by the presence of manganese salts and, in general, manganese and iron have a decided influence on the action of various oxidases. Since one or other of these substances is always present in the ash of these enzymes, Bayliss suggests that the enzyme may simply be a means of rendering these metals extremely active and that addition of the salts to the enzyme merely increases the concentration of the former present in the active state. Indeed, Dony-Henault has been able to prepare artificial laccase by precipitation with alcohol of a solution of gum arabic, manganese formate and acid sodium carbonate, which is about as active as true laccase. Enzymes have been found t o be especially susceptible 1

Ber. deutsch. chem. Ges., 17,1062 (1884).

Provvloter Action in Catalysis

263

t o electrolytes as would be expected because of their colloidal nature (p. 1 0 2 ) . Thus, Cole has found that ptyalin was much more active in the digestion of starch in the presence of acids in low concentrations and of neutral salts. Somewhat similar results were obtained with invertase. Again, trypsin is activated by low hydroxide-ion concentrations while practically inert in neutral or acid solution (p. 87). It is possible that the auto-accelerative property of invertase noted by Victor Henri (p. 88) is due to the formation of acids during the reaction. As mentioned above, acids activate invertase and it does not appear improbable that this may be the explanation. The fact that Kullgren has observed the formation of acids in the inversion of cane sugar at 100’ is significant. Such an example of auto-catalysis, if authentic, is to be distinguished from simple auto-catalysis, in which a reactant or product has a direct catalytic effect on the reaction. Here the specific effect of the product is on the catalyst. Bayliss recognizes this, but is inclined to include both under the term “auto-catalysis,” From the point of view of promoter action such a term as “auto-activation” or “auto-promotion” would be more appropriate.

Co-Enzymes Certain enzymes furnish us with a special limiting case of promoter action which has no analogue in other fields of katalysis. It is sometimes observed that an enzyme is completely inactive in the absence of certain substances called co-enzymes” or is made up of two or more components, each by itself inactive. Thus, Harden and Young (p. 124) have found that if zymase be dialyzed under pressure, the colloid left on the gelatin is inert, but becomes active again on addition of a portion of the filtrate, which is also inactive by itself. It was further found that the filtrate contained two substances, both of which were required to activate the non-dialyzed portion. One of these was soluble inorganic phosphate, which is known to accelerate the action of the original yeast juice. The nature of the other substance is not known, but its ex(6

264

Robert Nortovl Pease and Hugh Stott Taylor

istence is proven by the fact that phosphates alone are not able to bring about the activation. Magnus has obtained similar results with extract of liver, which contain lypase. Loevenhart has shown that bile salts possess all the properties of the co-enzyme in this case and can probably be so designated. Many other examples of this phenomenon might be cited in which the co-enzyme is an inorganic salt. Thus Bierri, Giaja and V. Henri have found that dialyzed pancreatic juice, which is inactive, can be rendered active by addition of chloride or bromide-ion.

............ As to the explanation of the phenomena here grouped together, it can only be pointed out that since catalysis itself is as yet unformulated one cannot at present expect to elucidate promoter action. If the intermediate compound theory of catalysis were the correct one it might be assumed that the promoter was in reality a catalyst for the formation or decomposition of the intermediate compound, thus hastening reaction. If the explanation of catalysis is to be found in the surface condensation or adsorption theory, that is, if catalysis depends upon certain surf ace characteristics of the catalyst, the catalytic influence of a mixture of substances would not be expected to be consistently additive, for this is not the case with the properties of mixtures in general. It seems likely that a study of promoter action may lead to at least a partial solution of the problem of catalysis. By its means it is possible to alter the properties of a catalyst as gradually as desired while keeping its general nature qualitatively the same. Some uncertainty must attach to deductions arrived a t by comparison of measurements on totally distinct substances used as catalysts. Necessary differences in manner of preparation alone will result in this. On the other hand, a series of catalysts containing varying quantities of promoter can be made up and used under conditions which will at least vary in a regular manner even though they may not be precisely the same in each case.

Promoter Actiovl in Catalysis

265

Summary In this paper, promoter action in catalysis has been defined and a distinction made between two types which have been termed “activation” (of a catalyst by a substance relatively inert catalytically) and “co-activation” (of two or more catalytically active substances each by the others). This is followed by a number of examples from heterogeneous and homogeneous catalysis and from enzyme action. Princeton, N . J . January, 1920

PROMOTER ACTION IN CATALYSIS

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