NITROGENOUS CARBONS AS CATALYSTS
329
oxide, whereas the reduction with hydrogen takes place only in the presence of solid silver oxide; and (b) the reduction of silver oxide by carbon monoxide is practically complete under the proper conditions of sol formation, whereas the reduction of silver oxide by hydrogen is incomplete, giving hydrosols in which the particles are mixtures of silver oxide and silver. REFERENCES
Z. Elektrochem. 14, 49 (1908). (1) KOHLSCH~TTER: (2) WEISERAND ROY:J. Phys. Chem. 37, 1018 (1933).
THE EFFECT OF ACTIVE NITROGEN AND OF CERTAIN NITROGEN COMPOUNDS ON CATALYTIC PROPERTIES OF CARBOW PAUL F. BENTE
AND
JAMES H. WALTON
Department of Chemistry, University of Wisconsin, Madison, Wisconsin Received January 2, 1943
In a recent paper (1) the results of experiments using nitrogenous carbons as catalysts for several reactions were discussed. The catalysts were all prepared from purified materials containing nitrogen, a part of which remained present as nitrogen of constitution during carbonization and activation. Since the presence of the nitrogen seems to increase the catalytic activity of the carbons, it was of interest to determine more specifically the promoting effect of such nitrogen. The investigation of this problem is complicated by the fact that the activities of carbons prepared from various organic compounds vary markedly with the materials used. Thus it is impossible in comparing a nitrogenous carbon with a non-nitrogenous carbon to say how much of the difference in activities may arise from the effect of the different source materials and how much may be specifically due to the presence of the nitrogen. The purpose of this investigation was to obtain information concerning the promoting effect of nitrogen by preparing a nitrogenous carbon from an activated nitrogen-free sugar carbon the catalytic properties of which are known. A search of the literature pertaining to the catalytic properties of active carbons yielded only one reference concerning the enrichment of nitrogen in carbons. Honig (4)heated several commercial carbons in a nitric acid-sulfuric acid mixture, causing the nitrogen content of the carbons to increase from about 0.4 to 2.5 per cent. Resulting catalytic changes however, were not noted. In the following experiments the various methods employed by the present authors in chemically fixing nitrogen to the surface of activated carbon are described, together with the resulting changes in catalytic properties of the carbon catalysts so treated. 1 This investigation was financed by a grant from the Research Committee of the University of Wisconsin, Dean E. B. Fred, Chairman.
330
PAUL F. B E K T E AND JAMES H . WdLTON
I. E F F E C T O F ACTIVE N T R O G E N
Xethod of preparatzon An attempt mas made to activate non-nitrogenous lactose carbon by exposing it to active nitrogen prepared by passing an electric discharge through an atmosphere of nitrogen under reduced pressure (3). Of the several sources of excitation tried, the most satisfactory proved to be that obtained from an electrodeless discharge of high-frequency 6-meter radio waves generated by a portable diathermy apparatus. TT7hen the discharge was passed through a 2-liter Pyrex bulb filled with nitrogen at pressures varying between 0.1 and 10 mm., an afterglov appeared which was more pronounced near the lower limit of pressure. In the experiment 4.5 g. of lactose carbon (C), activated at 875"C., was placed in the bulb and exposed to active nitrogen for 12 to 16 operating hours. Fresh supplies of nitrogen, taken from a cylinder, nere bubbled through alkaline pyrogallol, and finally dried in toners of calcium chloride and phosphorus pentoxide. They were then admitted to the reaction vessel, which was cooled externally by a stream of air from an electric fan.
Reactavziy of actave nitrogen with carbon The carbon treated by the above method was found to contain 1.21 per rent Kjeldahl nitrogen. During the course of the preparation of this synthetic nitrogenous carbon, it was noted that the walls of the Pyrex bulb were darkened by a brown film which collected on the inside. Qualitative examination showed that this material mas paracyanogen. Apparently the active nitrogen reacts with the rather inert carbon to form cyanogen gas, 11hich polymerizes to form paracyanogen. Since there might be doubt as to whether the nitrogen is actually present as a surface complex or merely as adsorbed cyanogen (or paracyanogen), an experiment was performed in nhich a sample of the same lactose carbon v a s treated at 300°C. with a stream of cyanogen gas (ca. 1.5 liters). The system xas finally flushed out with nitrogen. On analysis the resulting carbon showed 1.52 per cent nitiogen content. It is possible, therefore, that the nitrogen present in the carbon after treatment vith active nitrogen might be present as adsorbed cyanogen.
Catalytic propertzes of the carbons To determine the changes in catalytic propertie? brought about by treatment with active nitrogen and cyanogen, the catalytic properties of the carbons were compared with those of the untreated activated lactose carbon. The rcactions studied were the decomposition of hydrogen peroxide and the oxidation of hydroquinone and of potassium urate by molecular oxygen, the methods and techniques being the same as those recently described (1). The data are given in part 1of table 1. The concentrations of reactants are included in the footnotes for the table. From table 1 it is apparent that for the reaction involving the decomposition
NITROGENOUS CARBONS A S CATALYSTS
331
of hydrogen peroxide the treatment of the carbon with active nitrogen causes inhibition. The cyanogen treatment has little effect, resembling the lack of inhibitory action of 0.01 N potassium cyanide towards lactose carbon (I). For the reaction in which hydroquinone is oxidized, the carbons treated as above continued to show poor activity. Towards the oxidation of potassium urate, the catalytic behavior parallels that for the decomposition of hydrogen peroxide, the active nitrogen treatment causing inhibition and the cyanogen treatment causing little change. Since in two of the catalytic reactions the carbon treated with active nitrogen differs noticeably from the carbon treated with cyanogen, it is likely that the nitrogen of constitution differs in the two samples. In the case of the cyanogen treatment the nitrogen may be present as adsorbed cyanogen, while that which is fixed from active nitrogen probably exists as a surface complex. 11. USE OF AMMONIA
In the destructive distillation of nitrogenous organic materials to form a char, ammonia and other basic gases are liberated. Since the resulting char contains nitrogen of constitution which is stable even in a vacuum at high temp.ratures, it was of interest to determine whether a nitrogen-free carbon would react with ammonia a t elevated temperatures. In view of the fact that the active nitrogen showed chemical reactivity with the carbon, it is possible that the thermally decomposing ammonia might form intermediates capable of reacting chemically with the carbon.
Methods used Two 3-g. samples of an 875°C. activated lactose carbon (D), heated respectively to 625°C. and 81OOC. in a quartz tube, were treated with a stream of ammonia gas for about 3 hr. Before removal of the carbon for study, the system was flushed out with nitrogen to expel undecomposed ammonia. Two samples of an 875°C. activated lactose carbon (B) were also heated a t 400OC. and 625°C. for 2 and 1 hr., respectively, in a steel bomb containing ammonia a t a pressure of about 3(30 lb. per in? To determine what changes would arise merely from a similar temperature treatment, a sample of the pure lactose carbon (D) was heated for 3 hr. a t 625°C. in a stream of pure nitrogen. After it was ascertained that the ammonia treatment of carbons was very I n this connection the following experiment is of interest. Ash-free gelatin, when immersed in liquid ammonia, undergoes an exothermic reaction. Upon evaporation of the ammonia, a rubbery mass which turned brittle was obtained. This was charred and activated in moist oxygen, as were the carbons discussed in a preceding paper (1). The activated carbon contained 5.84 per cent nitrogen. The catalytic activity of the carbon was tested in the usual manner with hydrogen peroxide. It was found to be considerably more active than the gelatin carbon already described, though not as active as the hexamethylenetetramine carbon. For 0.013-g. samples the half-life was 42.5 sec., while for 0.100-g. portions i t was about 17 see. When 0.W1 N potassium cyanide was also present, the half-lives were increased to 78 and 30 sec., respectively.
332
PAUL F. BENTE AND JAMES H. WALTOX
effective, another sample of activated lact,ose carbon (E) mas treated with a stream of ammonia a t 875°C. for 5 hr.
Reaction of ammonia with carbon On Kjeldahl analysis all of the ammonia-treated carbons showed nitrogen content, the carbon containing the most nitrogen being the one treated at 875°C. at atmospheric pressure (see table 1). Since it vas possible that some of t,he nitrogen might be present as adsorbed ammonia, samples were heated in a vacuum at 400°C. for 30 min. For the carbon treated a t 810"C., Kjeldahl analysis before and after this gave 3.38 and 3.34 per cent nitrogen, indicating that the nitrogen was not present as removable ammonia. Similar results m r e obtained for the carbon treated with ammonia at 625°C. and atmospheric pressure. The carbons treated n-ith ammonia under pressure 11-ereallowed to cool in the bomb and therefore cont,ained much adsorbed ammonia in addition to nitrogen of constitution. Adsorbed ammonia was largely removed by evacuating for 90 min. a t 100°C., after which the carbons no longer smelled of ammonia. These vere not treated in a vacuum a t 400°C., because the catalytic properties were not considered sufficiently modified, relative to those treated a t atmospheric pressure, to warrant further attention.
The catalytic properties of the carbons The ammonia-treated carbons xere then tested for catalyt'ic activity in the three reactions mentioned above. The data are summarized in parts 2, 3, and 4 of table 1. The reactivities in decomposing hydrogen peroxide are also included in figure I, from which it is evident that the carbons have been great,ly promoted in activity. The carbon treated a t 875°C. showed such great promotion that it could not be included in the figure. For the carbon t'reat'ed at 810°C. the promotion amounted to a 25-fold increase in activity after the reaction had proceeded for 1 min. Since the carbon heated in a stream of nitrogen a t 625°C. showed slight loss of catalytic activity towards hydrogen peroxide, the promotion observed cannot be due to a mere temperature effect. The pH of the double-distilled Tyater suspension of t'he two carbons (0.3 g. in 15 ml.) xhich aere treated with ammonia a t atmospheric pressure a t 625" and 810°C. vas 8.9 when measured by a glass electrode. The value is not unusually high for carbons treated a t high temperatures, but since the ammonia treatment raised the pH by more than one unit, samples of the untreated carbon and of the carbon treat,ed a t 625°C. xere tested in hydrogen peroxide solutions buffered to a pH of 8.45 by monopotassium acid phosphate and sodium hydroxide. It was found that the buffered solutions gave reactions of slightly higher velocities than the unbuffered solutions. However, since the relative promoting effect of the ammonia-treated carbon remained unchanged, the change in catalytic activity by ammonia treatment is not caused by a pH increase due to the presence of ammonia. The addition of ammonium hydroxide to a system containing untreated lactose carbon and hydrogen peroxide in the usual concentrations caused a mild increase in activity, but this did not a t all approach
333
NITROGENOUS CARBONS AS CATALYSTS
the activity of the carbon promoted by the high-temperature ammonia treatments. Consequently the promoting effect cannot be due to the presence of ammonium ions liberated from the surface of the carbon. This evidence may all be taken as further proof that the nitrogen in the carbon is not present simply as adsorbed ammonia. TABLE 1 The catalytic properties o j activated lactose carbons treated with active nitrogen and several gaseous nitrogen compounds
-
CATALYTIC ACIIVITY' TOWARDS ~
%R CENT TBEATYENI OB CATALYST
:JELDAEL IITROGEN
Decomposition of H204
Hali- or tenth-life
___
Part 2: using 875%. lactose carbon (D)
Part 3: using 875°C. lactose carbon (B)
Part 4: using 875°C. lactose carbon (E)
o
Oxygen
bsorbed i, 1 hr.
kidation potassium urate5 Ox gen rsorged in 15 mm.
ml.
ml.
0.7
14.5
1.21
0.7
6.0
1.52
None
13.4
None. . . . . . . . . . . . . . . Stream of nitrogen for 3 hr. a t 625°C. Stream of ammonia for3hr. at625'C. Stream of ammonia for 3 hr. at 810°C.
0.0
0.7
6.2
0.0
0.75
8.1
1.07
1.6
5.8
None. . . . . . . . . . . . . . Ammonia in a bomt a t 400°C.. . . . . . . . . Ammonia in a boml a t 625°C.. ........
0.0
2.0
14.1
1.0
3.4
8.5
None. . . . . . . . . . . . . . Stream of ammonia for5 hr. at 875OC.
0.0
minvles
Part 1 : using 875°C. lactose carbon (C)
'idation hydro?,unone:
None. . . . . . . . . . . . . . . Exposure t o active nitrogen for 16 hr.. . . . . . . . . . . . . . . . Stream of cyanogen a t 300°C . . . . . . . . . . .
0.0
Ti/io
= 14.5
12.9
3.38
0.5
4.00
__
___
* 0.300 g. of catalyst and 15 ml. of
solution were used for all runs. t The 15 ml. of solution contained 50 ml. of available oxygen. f The hydroquinone concentration was 0.50 molar. 8 The concentration was 12.5 g. of uric acid and 15.6 g. of potassium hydroxide per liter.
For the hydroquinone oxidation, table 1 shows that the ammonia-treated carbons have nearly doubled in activity, though the over-all reactivity is still not very great. The oxidation of potassium urate was not significantly influenced, except for the carbon treated a t 810°C., which showed doubled activity. The carbon treated under pressure a t 4OOOC. showed only two-thirds of the
334
PAUL F. BEKTE AKD JAMES H. W.4LTON
normal activity. As in the experiment with the decomposition of hydrogen peroxide, the heat treatment with nitrogen at 625OC. failed to bring about significant changes in the catalytic properties for these two reactions. 40
30
3
-I
0
b 5
PO
:
0
i" IC
0
TIMEIN MINUTES FIG.1. Decomposition of hydrogen peroxide by lactose carbons treated with nitrogencontaining gases. 15 ml. of hydrogen peroside, containing 50 ml. of available oxygen, was treated with 0.300-g. samples of carbon, Curve Curve Curve Curve Curve Curve Curve Curve Curve Curve
__
I . .. , . , , , . ./Lactosecarbon D treated with ammonia at 810°C. I I . ,, . , . . . . . Lactose carbon B treated with nitrous oside a t 810°C. III., . . . . , . . Lactose carbon D treated with ammonia a t 625°C. IV. . , , . . . , .!Lactose carbon B treated with ammonia in a bomb at 625°C. and evacuated at 100°C. 1'. . . . . , . . . . . iImtcse carbon B treated with ammonia in a bomb a t 400°C. and evacuated at 100°C. V I . . . . . . . . . . Lactose carbon B untreated V I I . . . . . . . . Lactom carbon B treated with nitric oxide at 810'C. V I I I . . . . . . . . Lact,ose carbon D untreated IS.. . . . . . . . Lactose carbon B treated with nitrogen a t 625°C. X . . . . . , . . . Lactose carbon B treated with nitrous oxide a t 400°C.
NITROGENOUS CARBOKS AS CATALYSTS
335
The effect of 0.001 N potassium cyanide on the catalytic activity of the lactose carbon treated with ammonia a t 875°C. for 5 hr. vas tested in the decomposition of hydrogen peroxide. When 0.100-g. samples of the carbon were used, the time of half-life of the reaction was increased from 58 to 143 see. This indicates that the nitrogen complexes formed are easily poisoned by the cyanide. It does not, however, enable one to determine xhether or not the ash content plays a part in the very active nitrogen complexes poisoned by the cyanide, as it has been assumed by Warburg (8).* 111. USE OF OXIDES O F XITROGES
Method and chemical reactivity Since the thermally decomposing ammonia was found to react with the carbon to promote its catalytic activity, similar experiments were carried out with nitric oxide a t 810°C. and nitrous oxide a t 400" and 810°C. In each run about 8 liters of the pure gas was slowly passed through a bed of 3 g. of 875°C. lactose carbon (B) over a period of 3 hr. Kjeldahl analyses of the carbons so treated showed that the carbon exposed to nitric oxide contained 1.8 per cent of nitrogen, and that treated with nitrous oxide a t 400°C. contained no nitrogen, while that treated a t 810°C. showed only 0.1 per cent. Riese (6), in using active carbon to remove small amounts of nitric oxide and nitrogen dioxide from gases, claims that the removal is not a simple adsorption process but involves an interaction with the carbon to form carbon dioxide and nitrogen gas. This confirms Shah's more detailed study of the reduction of nitrous oxide and nitric oxide by activated carbon (7) to form molecular nitrogen and a surface oxide Jyhich further decomposes to give oxides of carbon. However, the present authors found that in the nitric oxide treatment of the carbon a t 810°C. a significant amount of nitrogen was also fixed to the carbon surface. For the treatment with nitrous oxide a t 4OO0C.the resulting carbon was nitrogen-free, while for the trial a t 810°C. a trace of nitrogen was detected. Since Briner, Miner, and Rothen (2) have shown that above 700°C. the decomposition of nitrous oxide may produce nitrogen and nitric oxide instead of oxygen, and since the present authors found that carbon treated with nitric oxide a t 810°C. can fix nitrogen to its surface, the presence of 0.1 per cent nitrogen in the carbon treated with nitrous oxide a t 810°C. can be attributed to the effect of the nitric oxide formed.
T h e catalytic properties The catalytic propertieq of the carbons treated with oxides of nitrogen were tested for the decomposition of hydrogen peroxide. The data are given graphically as a part of figure 1, which shows that the carbon treated with nitric oxide a t 810°C. decreased in activity, while that treated with nitrous oxide a t 400°C. possessed even less activity. The sample treated with nitrous oxide a t 810°C. showed an enormous increase in catalytic activity, being surpassed only by the
336
PAUL
F. BENTE AND JAMES
H. W.4LTOS
carbons treated with ammonia at 810" and 875°C. This shows that nitrous oxide, if used under the proper conditions, is also a good activating agent for carbon. The inhibitory effect of 0.001 LV potassium cyanide on the lactose carbon treated xith nitrous oxide at 810°C. was tested in the decomposition of hydrogen peroxide. When 0.100-g. samples of the carbon were used, the addit'ion of tire potassium cyanide caused the half-life to increase from 40 to 79 min. The promoting effect of nitrous oxide is of special interest here, since it shows that the presence of nitrogen is probably not the only factor involved in increasing the catalytic activity of this carbon. Since only 0.1 per cent nitrogen was found present, the great inhibitory effect of the potassium cyanide is more difficult to explain on the basis of its specificity for ash-cai-bon-nitrogen complexes, unless one assumes that the nitrogen in this case is all attached t o the ash-carbon complexes and thus creates extremely active centers. The other alternative, which seems more likely to t,he authors, is that the nitrous oxide ran be more effective than molecular oxygen in oxidizing the carbon to form surface oxides rrhich are of such great catalytic activity that they are also sensitive t o potassium cyanide.
Discussion These experiments on the fixation of nitrogen to the surface of carbon show that the presence of nitrogen may cause widely varying effects. S'ariation in activity might arise from different forins of carbon-nitrogen surface complexes, from different spacings in the resulting surface latt'ice, and from Variations in the amount of surface affected. Nothing definite can be said, hovierer, concerning the structure of these carbon-nitrogen surface complexes. Since experiments shoTr that nitrogen introduced into a non-nitrogenous carbon may promote catalytic activity, it is also probable that the nitrogen in carbons derived from organic nitrogen compounds accounts for a t least a part of the usual high catalytic activity. Indeed, artificially prepared nitrogenous carbons are not generally as active as those made from materials such as hex+methylenetetramine, gelatin, and glucosazone. However, the carbon treated xith ammonia a t 875°C. for 5 hr. was nearly equal in activity t o the carbon prepared from gelatin, the second most active type of normal carbon prepared. Using 0.1-g. portions of carbon on hydrogen peroxide samples of the usual concentration, the half-lives of the reactions for the two catalysts were: gelatin carbon, TI/*= 50 see.; lactose carbon treated TTith ammonia a t 875OC., Tip = 58 see. It is evident that the nitrogen here does not merely function as a disrupting agent during the formation of carbons, as suggested by Rideal and Wright ( 5 ) , for the treatment of a carbon with ammonia could not so materially change the surface area of the carbon. Rather, the catalytic activity of a nitrogenous carbon is a function of the promoting effect of t & nitrogen. In general, the nitrogen enrichment of a carbon by treatment with ammonia had a promoting effect for the three reactions studied. The authors believe that this promoting effect is definitely related to the similar high activit,y of carbons prepared iron organic
NITROGENOUS CARBONS AS CATALYSTS
337
nitrogen compounds, since the catalytic properties are similar. The charring of organic nitrogen compounds also produces some ammonia, so that for both types of carbon the atmosphere during formation was similar. In general, when the nitrogen is being attached to the carbon surface, the situation may vary in several ways, as follows: An association may occur between the nitrogen and the ash content in the carbon or between the nitrogen and the carbon surface which consists chiefly of an oxide layer. The relatively high percentages of nitrogen fixed compared to the per cent ash content shoiv that the latter always occurs, while the inhibitory effect of potassium cyanide, if it be specific for ash-carbon-nitrogen complexes of high catalytic activity, shows that the former may also occur, a t least when ammonia is used as a source of nitrogen. When the nitrogen becomes attached to the surface of the carbon, it may cover up or destroy active points or it may create new active points or desirable space increments on the surface. Thus the effect of the nitrogen may be to retard or promote the catalytic activity of the carbon. Experimentally both of these variations in activity were realized. SUMMARY
Activated lactose carbon acquired a significant amount of nitrogen content by treatment u9ith active nitrogen and with ammonia and nitric oxide a t high temperatures. Treatment with nitrogen or nitrous oxide under similar conditions did not cause nitrogen fixation. The catalytic activities of the carbons were tested for the decomposition of hydrogen peroxide, the oxidation of hydroquinone, and the oxidation of potassium urate. The carbon treated with active nitrogen and with nitric oxide showed a decrease in catalytic activity. Ammonia treatment caused a promoted activity which could equal the activity of carbons prepared from organic nitrogen compounds. The promoting effect is not due to the presence of ammonia. Treatment with nitrous oxide a t high temperatures also caused great promotion without significant nitrogen fixation to the carbon. The results are discussed from the point of view of the promoting action of the nitrogen. REFERENCES (1) BENTE, P. F . , AND WALTON,J. H . : J. Phys. Chem. 47, 133 (1913). (2) BRINER, E., MINER,CH., A N D ROTHEIF, A , : Helv. Chim. Acta 9,409 (1926). (3) GI.OCKLER, G., AND LIXD,S. C.: The Electrochemistrv of Gases and other Dielectrics, Chapter XII. John \Tileg and Sons, Inc., New York (1939). (4) HONIG,P.: Kolloidchem. Beihefte. 22, 345 (1926). ( 5 ) RIDEAL, E. K., ANDWRIGHT, W. &I.: J . Chem. Sac. 1926,1813. (6) RIESE,W.:Brennstoff-Chem. 20, 301 (1939); 21, 25 (1940). (7) SHAH,M. S.: J. Chem. Sac. 1929, 2661. (8) WARBURG, O., et al.: Biochem. Z.119, 134 (1921); 136, 266 (1923); 146, 461 (1924).
