INDUSTRIAL AND ENGINEERING CHEMISTRY
170
least one lower polymerized hydrocarbon of the same chemical constitution. The theory put forward enables us to explain most of the problems connected with rubber industry. Detailed discussion of the various problems may be found in the work
Vol. 19, KO. 1
of Hauser and Mark previously mentioned.2 The purpose of this paper is merely to show that systematic x-ray research has enabled us to view rubber structure much closer than before and to give an explanation which in all respects is in full accordance with the experimental facts.
Catalytic Oxidation of Ammonia' By Wilfred W. Scott a n d William D. Leech UNIVERSITY
OF
SOUTHERN CALIFORNIA,
LOS
ANGELES,CALIF.
H E R E are in America in operation and under conThe material for the converter tube was carefully studied, struction nine synthetic ammonia plantsS2 Four of because of the tendency of ammonia to decompose in the these are small, ranging from one to four tons of presence of certain materials, especially at the working anhydrous ammonia per day. The other five produce from temperature of 690" to 800' C. The most satisfactory ten to twenty-seven tons per day. The larger plants are converter tubes used in previous experiments were made located near Syracuse, New York; Niagara Falls, New York; of aluminum,6 clay,6 chromium-vanadium,' ~ i l i c a or , ~ porceSeattle, Washington ; Charleston, West Virginia; and Terre lain.s Silica was used in the present work because of its Haute, Indiana. The nine plants have a total estimated desirable properties, in regard to decomposition of ammonia, capacity of 103 tons per day. During 1925, five of the above and because of its general utility in an analytical laboratory. The dimensions of the tube plants produced 13,140 tons. were: inside diameter 30 It is estimated that during mm., length 60 cm. The 1926 the plants operating Cobalt oxide has been used in preparation of a large inside surface was glazed will produce about 22,000 number of catalysts, by promotion with twenty differand had a uniform bore. tons, or an average of apent metals. Their efficiency, as applied t o large-scale T h e f l o w m e t e r s were p r o x i m a t e l y sixty tons of production, has been determined a n d recorded. standardized by a gasometer synthetic ammonia per day. Complete curves are presented for catalysts made of the American Meter ComThe comparative scarcity from cobalt promoted with bismuth a n d with alupany with an accuracy of 0.1 of platinum and the heavy minum. per cent. From these caldemands upon it for use as The melting points of t h e cobalt promoters bear a reculations a curve was oba catalytic agent s u g g e s t lation t o their catalytic activity. Metallic oxides havtained, and thereafter flow that relief from any source ing a melting point below t h a t of cobalt promote acwas read directly. would be welcome. The tivity and those melting above act as poisons. The catalyst was placed authors have observed in a vertical tube, about t h a t c o b a l t , similarly to 15 cm. from lower end of p l a t i n u m , f o r m s many complex compounds with a special affinity for ammonia. furnace. This permitted a quick exit of the outlet gases Over two thousand cobalt-ammonia compounds are known. and a reasonably quick heating of the inlet gases, thus It was believed that cobalt oxide, a weak base, could be maintaining a minimum decomposition in both cases. The promoted so that i t might be used commercially as a catalyst catalyst was held in a porcelain Gooch crucible, 27 111111. in the oxidation of ammonia to nitric acid. Cobalt, at room in diameter, and was made secure by packing asbestos temperature, it is claimed, has both the hexagonal closest firmly around it and between the walls. This prevented packed grouping and the face-centered cubic arrangement.3 the by-passing of any gas, and caused the gas mixture to flow through the catalyst and out through the perforated Apparatus bottom of the crucible. The amount of catalyst varied in Before selecting the materials to be used, a thorough weight from 8 to 11 grams, with a depth of 3 cm. I n the survey of literature was made to ascertain the equipment preparation of the catalyst the fine screenings were rejected used by previous workers. Physical properties which might and large granules crushed, which gave a material of uniform interfere were most carefully studied. As representative granules varying from 1.5 to 3 mm. in size. By using a cataof the complete equipment necessary, reference is made to lyst of this texture minimum resistance to gas passage was the sketch given by Scott4 and to that of Piggot.6 The obtained and sufficient time to complete the catalytic process apparatus used by Scott was an adaptation of a very expen- was allowed. The temperature of the catalyst was found by sive equipment, and gave results with nearly the same ac- placing the thermocouple near the crucible holding the matecuracy. His apparatus in general was adopted. Briefly, rial. The couple was not permanently fixed within the this consisted of a heating unit, converter tube, pyrometer, converter tube, but was used from time to time in checking mixing chamber with pressure gage, air chamber, ammonia the temperature. The temperature varied from 690" to chamber with exit flowmeters and controlled safety traps, 800' C.; 750" C. gave the best results. a needle reducing valve, and a chamber in which the exit The volume of ammonia-air gas passing through the congases were absorbed. verter varied from 5 to 6.5 liters per minute, the average being The heating unit was a n electrically controlled Hoskin a little less than 6 liters per minute. The ratio of ammonia furnace in which the temperature could be raised to the de- gas to air was maintained as nearly constant as possible. sired degree. This lay between 1 to 10.5 and 1 to 11, which is near the 1 Received July 19, 1926. theoretical value required. 2 MacDowell. Chem. Met. E n g . , 33, 9 (1926). The percentage of conversion was determined by the Gail-
T
8 4
5
Wyckoff, "Crystal Structure," Chemical Catalog Co., New York,1924. THIS JOURNAL, 16, 74 (1924). J . A m . Chem. Soc., 43, 2034 (1921).
0
7
Parsons, THIS JOURNAL, 11, 541 (1919). Killeffer, I b i d . , 17, 789 (1925).
ILVDUSTRIAL ALVDENGINEERING CHEMISTRY
January, 1927
lards method, a summary of which is given by Scott.9 The inlet and exit gases were sampled in evacuation bulbs, the samples being taken as nearly simultaneously as possible. After weighing, the gases were absorbed in water and the solutions titrated with 0.05 N standard reagents, with methyl red as an indicator. The ratio between outlet and inlet nitrogen was the percentage of conversion, which was the measure of efficiency recorded in all the data herein presented. KOsamples of gases were taken until the catalyst had functioned for some time. Two hours were set as a minimum, with the remaining samples distributed over a period 6 to 48 hours of continuous run. Preparation of the Catalyst The preparation of the catalyst was considered a very important preliminary step in the oxidation of ammonia to nitric acid. The fourth reportlo of the Committee on Contact Catalysis set forth very conclusive evidence that not only the mode of preparation, but the temperature, duration, source, and combination, all have a marked influence on the activity and physical properties of’ a catalyst. Consequently, a very careful survey of the literature was conducted with the purpose of selecting the best method for preparing the catalysts, bearing in mind the “factors influencing activity” as suggested by Merrill and Scalione. l1 Apparently the physical structure, the contact surface, and free space are factors which manifest their presence by altering the conversion. Benton,12 in his study on the absorption of gases by oxide catalyst, observed that “the absorption by cupric oxide prepared by ignition of metallic copper (Table VIII) or evaporated copper nitrate (Table I X ) is very much smaller than those observed for precipitated cupric oxide.” Rudisill and Engelder’3 observed a variation in the combination of the salt depending on whether i t was sulfate, nitrate, or carbonate ion. Chemically pure nitrates of the metals were selected as the sources of the catalysts in the present work. Consideration was given to the fact that we must have a “cheap material that can be prepared for use in a very simple way.”l4 The catalysts consisted of three series of cobalt oxide with varying amounts of promoters. I n group I, 1 per cent the metallic oxide promoter was used in each case in group 11,3 per cent; and in group 111,9 per cent. The sample was first slowly ignited in air, to find the amount of oxide formed under these conditions. With this as a basis, the amount of nitrates required was mixed to give the percentage of mixture desired. T o the mixture of nitrates was added enough distilled water to take up the crystals. The mixture was then slowly evaporated, with occasional stirring to secure a uniform catalyst. It was finally calcined for 6 hours a t 230” C. I n this study, no carriers were used. The slow drying secured by calcining a t a low temperature produced a catalyst having a desirable physical structure, securing a t the same time a maximum number of active absorbing centers. Data
I n the preliminary adjustment for each catalyst, properties peculiar to each catalyst were noted. Some required very rapid passage of the gas; others, a much slower rate. The chromium-promoted catalyst represented the extreme for THIS JOURNAL, 11, 745 (1919). “Standard Methods of Chemical Analysis,” D. Van Nostrand Co.,
0
1925 ‘0
11 12
13
J . P h y s . Chem., 30, 145 (1926) J . A m . Chem SOC., 43, 1982 (1921). I b i d , 45, 887 (1923) J . Phrs. Chem., 30, 1 (1926)
Schmidt, Tech -BIdller. 15, 241 (1925)
171
fast-moving gas. By increasing the usual flow about threefold, a fairly active catalyst was obtained. With this catalyst a t normal flow (10 to 13 cm. per cc.), practically no conversion or activity was observed. Some gave conversion a t considerably lower temperature than usual (6.50’ to 800” C.). The catalyst promoted with vanadium gave the greatest activity a t the lowest temperature. However, it failed to improve as it approached the normal working temperature. The relative volume of gas was another factor of interest. The catalyst promoted with bismuth converted varied quantities with an equal degree of efficiency. I n the determination of the optimum range, the authors were guided by the color of the exit gas. “Smoke” indicated ammonium nitrate, showing that ammonia was “by-passing” or was not completely converted by the catalyst. The brown, deep ruddy gas, indicative of NO2 gas, was interpreted as good activity on the part of the catalysts. The gages were set for this optimum range and held a t that point until samples were taken, or adjustment appeared advisable. From the samples the data in Table I were obtained. Table I-Cobalt CATALYST
Oxide Promoted with One Per c e n t of Metal Oxide, Arraneed in Order of Activitv CONVERSION CONVERSION CATALYST Per cent Per cent
Cobalt with: Bismuth Beryllium Lead Copper Cobalt Aluminum Zinc Mercury Cerium Iron
89.0 83.5 81.5 79.3 79.3 78.6 78.3 77.5 75.1 74.9
Cobalt with: Nickel Gold Chromium Silver Magnesium Uranium Cadmium Boron Tungsten Vanadium
74.0 73.7 71.5 70.0 68.0
65.0 63.0 62.0 60.0 57.3
Volume of ammonia-air mixture through converter tube, 5-6 5 liters per minute Ratio of ammonia-air mixture, 1:lO.S t o 1 11 Temperature of catalyst, 690’ to 800’ C. Weight of catalyst, 10 grams
The catalysts consist of cobalt oxide promoted with one per cent of the metallic oxide. Bismuth, beryllium, and lead act as promoters for cobalt oxide, increasing its activity from about 3 per cent to 12 per cent. Copper, aluminum, zinc, mercury, cerium, iron, and nickel show little effect i n either direction. It is quite evident that vanadium, tungsten, boron, cadmium, uranium, and magnesium show poisoning. The latter group gave dense white “smoke,” which indicated ammonia passing without oxidation. Table 11-Three
Per c e n t Metallic Oxide w i t h Cobalt Oxide
CATALYST
CONVERSION Per cent
Volume of ammonia-air mixture through converter tube, 5-6.5 litera per minute Ratio of ammonia-air mixture. 1:10.5 to 1:11 Temperature of catalyst, 690’ to 800’ C. Weight of catalyst, 10 grams
The catalysts in’Table I1 consist of 97 per cent of cobalt oxide, and 3 per cent of a metal oxide. It is very interesting to note that, of the eight metals selected, seven increased the activity of cobalt. Zinc alone failed to do this. The selection for the third group contained all of Table I1 except zinc, which had apparently failed from the beginning. Its inhibitory effect was proportional to the amount present. The active catalysts in Tables I and I1 were studied carefully to ascertain if there might be some common property related to their activity as promoters for cobalt. Tin was added to the new list, because of certain properties thought
I S D USTRIAL -4XD ELVGI-YEERING CHEMISTRY
172
to be characteristic of a promoter for cobalt in the above experiments. Inasmuch as Table I1 showed a general increase over the results of Table I, i t is interesting to note the results in Table 111, in which 9 per cent of the promoter is used. N n e per cent stannous oxide raised the activity of the cobalt catalyst 6 per cent, a point which must have some bearing on the properties of a promoter. Table JII-Nine
Per c e n t Metallic Oxide w i t h Cobalt Oxide CONVERSION Per cent
CATALYST3 Cobalt with: Aluminum Bismuth Beryllium Lead Tin Copper Iron Nickel
94.8
89.1 88.2 85.6
84.0 82.6 78.7 73.5
Volume of ammonia-air mixture through converter tube, 5-6.5 .. per minute Ratio of ammonia-air mixture, 1:10.5 t o 1:11 Temperature of catalyst, 690" to 800' C. Weight of catalyst, 10 grams
liters
I
I
I
I
I
I
are N2 and S20,indicating a low activity, due, probably, to an unbalanced absorption ratio. Effect of Moisture on Catalysts*
I n the work by Scott and Leech on oxidation of ammonia by catalysis, a difference in percentage conversion was obtained, depending upon whether moisture, free air, and air that had been bubbled through water was used. Table IV shows the results obtained with some of the better catalysts under different moisture conditions. When the results of forty determinations were tabulated, the outstanding features were as follows: Only two out of the group of catalysts showed a marked increase when moisture was used. Cobalt-beryllium (98 per cent to 2 per cent) gave an increase of as much as 19 per cent on the same adjustment upon the addition of 0.97 grams of water per cubic foot of air. The experiment was made by leaving the adjustments just as they were and snitching the air from the drier to the humidifier. Table IV
I n Table 111, arranged in the order of activity of the catalyst, we find taking first place aluminum, which was second in Table I1 and sixth in Table I. Table I11 presents a very interesting state of affairs. I n this group, the first case of "sintering" develops. Cobalt with lead oxide remained active for 2 hours, just sufficient time for sampling. The sample was obtained, but there was doubt as to its accuracy. Of the remaining oxides, only two continued to increase the percentage conversion, aluminum and beryllium. Copper, iron, nickel, and bismuth reached their peak in the vicinity of the range of Table 11. The curves in Figure 1 are plotted from the results of the most promising catalysts of Group 111. As is noted, bismuth is lower a t 9 per cent than a t 3 per cent. Because of the fairly high conversion obtained, it was deemed advisable to increase the proportion of bismuth still more. Two catalysts were tested, one containing 25 per cent bismuth to 75 per cent cobalt, and a second containing equal amounts of the two. The 50-50 catalyst sintered. It was concluded that the range of greatest activity of bismuth as a promoter to cobalt lay between 1 and 9 per cent.
I
Vol. 19, No. 1
j too
Figure 1
JNo further study was given beryllium, its expense being considered too high for commercial use. Of all the catalysts studied, cobalt-aluminum gave the highest conversion. The proportions were vaned in a series of mixtures. The complete story, as noted in the curve (Figure 1) shows that the percentage of conversion is proportional to the promoter aluminum, up to 9 per cent. It follows a high level up to 60-40, slowly decreasing a t first, and then rapidly. The products of high aluminum content
CATALYST
CONVERSION USING D~~ Per cent
MOISTURE CONTENT Grams per cu. f t . air
91.8 92.9
0.63 0.7 0.76
Aluminum, 9; cobalt, 91 70
94.8 93.3
0.44
Beryllium, 2 ; cobalt, 98 %
67.7
Bismuth, 3; cobalt, 97 %
0.77 0.72 1.00 0.95
CONVERSION WITH
MOISTURE Per cent
94.6 96.2
96.7 87 87.3 86.6 86.7 80.9
Cobalt-bismuth (97 per cent to 3 per cent) gave an increase of 6.2 per cent on the same adjustment, by using 0.7 gram of water per cubic foot of air. General Discussion
If the application of one or two of the superior catalysts can be determined, perhaps a better understanding of the feasibility of the use of base metallic oxide catalysts can be obtained. Considering the cobalt-bismuth catalyst, where 3 per cent bismuth oxide was used, a 10-gram catalyst handling 5.6 liters of the ammonia-air mixture per minute converted 91.8 per cent to the oxides of nitrogen, as determined over an 8-hour period. Calculating this on the basis of a pound of catalyst per 24-hour day and on the basis of a 50 per cent concentration (the nitric acid obtained by the direct oxidation varies from 50 to 55 per cent) , the result is as follows: One pound of cobalt-bismuth catalyst, in 24 hours, will oxidize approximately 60 pounds of anhydrous ammonia, giving 400 pounds of 50 per cent nitric acid. Similarly, 1 pound of the cobalt-aluminum catalyst, in which cobalt oxide is promoted with 9 per cent aluminum oxide, a 10-gram catalyst, handling 5.6 liters per minute, will oxidize 94.8 per cent. Calculating this on the basis of a 1-pound catalyst in the same proportion it would oxidize 60 pounds of anhydrous ammonia to 421 pounds of 50 per cent nitric acid per day of 24 hours. As is shown in Table I, extreme purity of the catalyst is not necessary, nickel and iron, common impurities in cobalt, do not harm. With the cobalt-aluminum mixture the common impurities are also harmless. The ammonia used was furnished by the Pacific Ammonia Company, of Seattle, Washington, The amount of poison materials that could be handled by either catalyst was not determined, but the ease of reworking the catalyst is a t least advantageous from this viewpoint. An effort was made to learn whether activity of the catalyst
* The experimental work on the effect of moisture was done by I.. L. Sutherland, with Mr. Scott.
January, 1927
INDUSTRIAL A N D ENGINEERING CHEMISTRY
bore any relation to crystal structure. Cobalt a t room temperature can exist as a hexagonal and as a face-centered cubic crystal. The working temperature undoubtedly affects the structure to some extent. I n Table I, where cobalt is promoted with 1 per cent of the metal oxide, the crystal structures of the metals are fairly well distributed, no type predominating in any section. Description of the structure of the oxide crystals was not sufficiently complete in available literature for any substantial comparison to be made. There was considered the possibility of the unit edge of the crystal exerting some influence upon the direction of the reaction or upon the relative activity, but the scattering data prevented any definiteness. With the available data, no grouping as to length of the unit edge appeared. Short and long occurred in all sections of the table. A grouping was observed when the melting points of the metals were compared. All the metals which increased activity have melting points lower than cobalt. That of iron, however, lies only slightly above that of cobalt. The metals with high melting points were consist,ent in their activity, giving signs of actually poisoning the cobalt. Cadmium and magnesium were exceptions. I n spite of low melting points they showed marked effect in decreasing the activity of cobalt. Theory of Catalysis
I n the process of the work, constant vigilance was maintained for any clue which might have a bearing upon the theory of catalysis, especially as conditioned in contact catalytic processes. Nearly every catalyst used showed some peculiarity specifk to i t alone. Operation factors, such as velocity of gas, optimum temperature, physical structure of catalysts, were the variables chiefly encountered. It is certain that physical structure is important, especially at the temperature of this reaction. Ruggedness of the catalysts is an absolute necessity. This is demonstrated when cobalt is promoted with either lead or bismuth. Small quantities accelerate the activity and continue to increase the efficiency until the quantity of promoter substances approaches 3 per cent. Above this amount, ihe efficiency slowly decreased, followed by sintering, showing that a small quantity associated in the interfaces of the cobalt crystal assist probably in the diffusion and vaporization of the atom along the active edges and centers of the catalysts.
173
Wyckoff states that, in cobalt-iron alloys, the mixtures are homogeneous in all proportions. The cobalt crystal becomes apparent only when the alloy consists of more than 98 per cent cobalt. From Tables I to I11 it is seen that a catalyst with 1 per cent iron produces no effect over that of cobalt alone. When the catalyst is promoted with 3 per cent iron, its efficiency is increased 9 per cent. As previously noted, this is the range where the cobalt crystal is changing. When the iron content is increased to 9 per cent, the efficiency drops to practically the same as cobalt. Thus, while alteration of the crystal structure is being effected, a maximum change is occurring in the active edges and active points, developing the highest percentage conversion. As the iron increases, the crystals of the catalyst become evenly uniform and consequently less active, as can be observed. The conversion stops a t N2and NzO,suggesting that now the weak chemical affinity of the active edges and points have become fixed by the presence of increased amounts of iron. The cobalt-aluminum catalyst is interesting, first, because it gave the highest efficiency of all the catalysts tested. Secondly, its activity, as is noted in Figure 1, is uniformly proportional to the quantity of aluminum oxide present, until it has reached the peak. It follows a high level, dropping slowly a t first, then very rapidly, to the phase where cobalt becomes the promoter, Among a large number of papers treating absorption, not a few treat of the sorption power of a l u m i n ~ m , ’also ~ with respect to its dehydration reaction and its sorption of water vapor over a range of temperatures varying from 200’ to 800” C.,I6 which is just within the range of this experiment, as measured by the pyrometer (690’ to 800” C.). It is a very common practice to use the blast flame to dehydrate alumina; consequently, when water vapor is added to the gas mixture the decrease of t h e conversion factor may be due to the hydration of a portion of the active edges and centers of the catalysts. K h y is it that water of the reaction is not absorbed similarly a t all times? Possibly the introduction of a common molecule disturbs the equilibrium of desorption, affecting the direction and increasing the probability of the formation of the water, forcing a slight shortage of nascent oxygen in the immediate zone of action, thus permitting the molecules of nitrogen to form. 16 16
.
Sabatier, “Catalysis in Organic Chemistry,” Van Nostrand Co 1922. J . Phus. Chem , 30, 2 (1926).
An Improved Method of Organic Microcombustion’ By George Kemmerer and L. T. Hallett CHEMICAL
LABORATORY, UNIVERSITY
OF WISCONSIN,
MADISON, WIS.
Improvements in Pregl’s microcombustion method are described which make it practicable f o r the accurate determination of carbon and hydrogen where only small samples are available. The use of a sealable microabsorption tube and a special electric furnace accounts for the greater accuracy. S THE scientific study of the lakes of WisconsinlJ,* and the northwestern lakes of the United States6 has progressed, more accurate methods of chemical analysis have become necessary. This is especially true in the analysis of the residues from the soft water lakes of northern Wisconsin,’ where a very large volume of water must be evaporated or a small sample used. The determi-
A
Received August 30, 1926. Birge a n d Juday, Wisconsin Geological a n d Natural History Survey, Bull. 22. 3 J u d a y , T r a n ~ Wisco?isin . A c a d . Sci., 16 ( l ) ,17 (1909). 4 Domogalla, Juday, and Peterson, “Forms of Nitrogen Found in Certain Lake Waters,” J . Bioi. Chem., 63, 269 (1925). 3 Kemmerer, Bovard, a n d Boorman, Bur. Fisheries, Bull. 39, Document 944 (1923-24;. 1
2
nation of carbon and hydrogen by ordinary combustion methods from the small residue obtained by evaporating 1 to 6 liters of water is not practicable. The PreglG microcombustion method, with gas furnace and apparatus as described by him, gave fairly satisfactory results, but the method seemed capable of improvement. I n the first place, the temperature of the gas furnace could not be accurately controlled and a temperature high enough to decompose the carbonates of calcium and magnesium could not be obtained. T o overcome this an electric furnace was built in sections to take the place of the various heating units of the Pregl furnace.
* “Quantitative Organic Micro Analysis,” translated
by Fyleman, p. 15.
