ADSORPTION AND HEAT O F ADSORPTIOS O F AMhl0NI.i GAS ON hlETALLIC CATALYSTS* BY WALTER A. DEW A N D HCGH S . TAYLOR
In order that a more extensive knowledge might be acquired of the relationship existing between heat of activation, adsorption, heats of adsorption and catalytic actillty, measurements of the specific adsorption and the heats of adsorption of ammonia gas on copper, nickel and iron catalysts have been made. These determinations combined in one operat'ion three lines of investigation which had been previously carried out in this laboratory. Adsorption measurements have been made such as those of Taylor and Burns' while isotherms for ammonia and hydrogen corresponding to the investigations of Gauger and Taylor2 and integral and differential heats of adsorption were measured in the same manner as did Beebe and Taylor.s -4lthough it is known that substances which possess a great adsorbing power are not necessarily the best catalysts,l yet much emphasis has been placed upon the determination of specific adsorption of catalytic agents as a criterion for their activity in certain reactions and there seems to exist an intimate relationship between the specific adsorption and catalytic activity.j Assuming some relationship to exist between the adsorbent and the adsorbed gas it was thought that adsorption measurements upon various catalysts which cause the decomposition of ammonia might reveal some data of fundamental nature concerning the reaction, 2SH3 =
+
S? 3H2
That iron at elevated temperatures causes this decomposition to take place was first investigated by Itamsay and Young6 and, subsequently, more thoroughly by Beilby and Henderson' who, in addition, studied the action of ammonia on nickel, copper, silver, gold, and platinum, and various othei metals and alloys at temperatures lower than their melting points. The latter authors concluded that the decomposition of ammonia is due to a nitride formation occurring when excess ammonia is present. Although no measurements have been made of the specific adsorption of ammonia gas for any of the metals previously mentioned it has been found by Taylor and BurnsSthat iron adsorbs hydrogen gas to a certain extent and that nitrogen gas is not measurably adsorbed. Kickel and copper have been in* Contribution from the Lahoratory of Physical Chemistry, Princeton University, S. J. Taylor and Burns: J. Am. Chem. SOC.,4 3 , 1278 (1921). Gauger and Taylor: J. Am. Chern. Soc., 45, 920 (1923). Beehe and Taylor: J. Am. Chem. SOC.,46, L,S (1921). Benton: J. Am. Chem. Soc., 4 5 , 900 (1923). Summarised in 3rd Report of Committee on Contact Catalysis. Taylor: J. Phys Chem., 28, 897 (1924). Ramsay and Young: J. Chem. Soc.,4 5 , 88 (1884). Beilhy and Henderson: J. Chem. Sot., 79, 1245 (1901). 8 loc. cit.
2j 8
WALTER A. D E W AND HUGH S. TAYLOR
vestigated more thoroughly and the heats of adsorption of hydrogen determined by Reebe and Taylor' under various conditions. These metals also show no measurable adsorptive capacity toward nitrogen. In addition, a few adsorption measurements were made on a supported sodium catalyst. Catalysts from several groups of aetals were thus represented. Some work previously carried out in this laboratory by Dr. A. F. Benton on the adsorption of ammonia gas by a 50- j o mixture of iron-molybdenum has been incorporated also with these measurements. Of the metals examined, sodium is known to react with ammonia a t 300'4ooOC. to produce sodamide2 according to the equation 2Ka z?\THa = 2NaSH2 H2. Since it is a very reactive metal it might be expected that irreversible adsorptior, alone would be observed. At the temperature a t which the determination was made, however, reversible adsorption occurred. The remaining metals would be expected to show reversible adsorptions and this was found t o be the case. The relationship existing between the heat of activation and heat of adsorption has been clearly stated by C. hi. Hinshelwoods as follows: Eo = Et X' - X E, = the observed heat of activation where Et = the true heat of activation A' = the heat of desorption X = the heat of adsorption and the value for A' for hydrogen on catalytic nickel has been measured by Beebe and Taylor4 and also by Forestis and more recently by Fryling6 who extended the work to promoted nickel catalysts a t low pressures. Beebe and Taylor also measured the heat of adsorption of hydrogen on copper.
+
+
+
Experimental carrying out the adsorption measurements the apparatus employed was entirely similar to that used and described by Pease and the temperatures employed were oo, I IO', 218', 305' and 444.6OC. which were attained by the use of ice and water, boiling toluene, naphthalene, acetanilide and sulphur respectively. The temperature regulation in the case of the determinations on iron-molybdenum was achieved by water a t 2 j°C. and IOOOC. and by an electrically heated air bath at the higher temperatures. Evacuation in this latter case was accomplished at 5oo°C. Evacuation at room temperature was employed for sodium, and a temperature of 218" was commonly used for copper, nickel and iron except where determinations were carried out at higher temperatures in which case the tube was evacuated a t the higher temperature.
METHODS OF ME.mmmmNT:-In
loc. cit. W. Titherley: J. Chem. Soc., 65, 504 (1894). 3 "The Kinetics of Chemical Change in Gaseous Systems," p. 178. 4 loc. cit. 6B. Foresti: Gam., 53, 487 (1923); 55, 185 ( r w 5 ) . Fryling: J. Phys. Chem., 30, 818 (1926). 1
2A.
ADSORPTIOS OF AMMONIA ON METALLIC CATALYSTS
279
Sitrogen was used as the reference gas by which the free space of the bulb was obtained, since it was found by Taylor and Burns to be unadsorbed. When heat of adsorption measurements were made a catalyst, tube exactly similar to the one employed by Beebe and Taylor, with an estimated capacity of about j o cc. nhen empty, was substituted for the catalyst tube previously used. Catalyst transfer from a separate reduction tube was accomplished in an atmosphere of inert gas. The heating coil used was made of platinumpalladium wire of 0.10mm. diameter and 44.3 cm. in length and had a resistance of 10.0oj ohms. The coil was insulated to prevent short circuiting as described by Reehe and Taylor. The leads, welded to the heating coil, were made of stout wire as short as possible and terminated in mercury cups by means of which connection was made to the calibration apparatus. Calibration v a s carried out esactly as described by Beebe and Taylor, the I O ohms heating coil being employed. Care was esercised to keep the leads and mercury cups well below the surface of the ice bath to prevent any loss of heat by conduction. For evacuation, the catalyst tube was heated by means of an air bath, the de Khotinsky joint being protected by a small lead tube wrapped about it several times, through which tube water circulated. In the case of copper, evacuation was carried out at Z O O O C . , but, 300’C. was employed for both nickel and iron. A Dewar flask filled with water and cracked ice was placed about the catalyst tube after evacuation was complete and all determinations of heats of adsorption were made a t oDC. Nitrogen was employed as a reference gas, since it was found by Beebe and Taylor to be unadsorbed and consequently a suitable gas by which thermal effects, due to compression of the gas entering the calorimeter and to cooling from room temperature, could be observed. Calibration measurements were carried out, however, with the catalyst chamber filled with ammonia gas. I t was found necessary also to introduce a correction factor for the effects noted above due to the fact that nitrogen is a diatomic gas and ammonia is a tetratomic gas. These corrections will be fully considered later.
CATALYST PREPARATION :-Sodium was prepared by distilling the metal in vacuo and condensing it upon glass beads in the reaction tube. This was accomplished by drawing the tube out into two bulbs quite similar to an oldfashioned hour glass. Into the lower one was introduced a weighed piece of sodium, and the upper was filled with glass beads. The upper bulb was sealed to a capillary tube which was attached to the apparatus. A Toepler pump was used to evacuate the system as it was heated. Distillation was carried out a t 4oo-450°C. and a silvery mirror of sodium was deposited on the glass beads. The tube containing the catalyst was then sealed off and the weight of the sodium which had distilled was obtained. C o p p e i 2 was prepared from Kahlbaum’s “brown label” copper oxide ground to granules of about 2 mm. in size. Reduction was carried out a t 145-1jo°C. for a period of six days in a stream of pure dry hydrogen. The end point of the reduction was indicated when no increase was observed in the
2 80
WALTER A. DEW AND HUGH S. TAYLOR
weight of a calcium chloride tube through which the effluent hydrogen was allowed to pass for an hour. Copper A was prepared in the same manner as Copper 2 except that the temperature of reduction was maintained at 140-14~'c. for a period of twenty-two days. At the end of that time the reduction was still producing 0.6 mg. of water vapor per hour. Szckel 8 was prepared by igniting the pure nitrate in a casserole over a small flame, transfering this material to the reduction tube, calcining a t 300'C. and reducing in a stream of hydrogen as described by Gauger and Taylro for the unsupported catalyst which they used. h'ickel A was prepared in the same manner as Nickel 8 the reduction temperature being maintained at 285-3oo0C. Reduction was slow a t that temperature, for a t the end of fifty days 0.4 mgm. of water vapor was being formed per hour. Noticeable contraction of the mass was observed at the end of the reduction period. I r o n 1 was prepared by dissolving pure soft iron turnings in nitric acid. The resulting nitrate solution was filtered, evaporated to dryness and ignited slowly on a sand bath. Reduction was carried out in a stream of dry electrolytic hydrogen at a temperature of about 60o0c. for a period of sixteen days. Zron A was prepared exactly as Iron I except that the reduction temperature was maintained a t 440 460'C. At the end of thirty days no formation of water was observed and reduction was considered complete. Iron-molybdenum was prepared by dissolving 113 g. of soft iron in equal volumes of concentrated nitric acid and water, adding to this solution 2 2 7 g. of ammonium molybdate dissolved in water. The combined solution was evaporated to dryness, stirring constantly toward the end. It was then ignited on a sand bath to drive off excess nitric acid. Reduction was carried out in hydrogen at 445' for 2 0 hours, at 500' for I O hours, and at 530' for 26 hours. At the end of this time reduction was not absolutely complete, water being formed at the rate of 18 mg. per hour. PREPARATION OF GASES :--Nitrogen was obtained by passing tank nitrogen through a purifier, containing a solution of ammoniacal cuprous carbonate, to remove oxygen, washing with sulphuric acid, then passing over hot copper to remove traces of oxygen, and drying by passing through calcium chloride. Hydrogen was prepared electrolytically as described by Pease.' Ammonia was prepared by gently warming concentrated aqua ammocia in a large flask and passing the gas through drying towers filled with soda lime and potassium h y d >xide. Before introducing the gas into the measuring burette it was tested ior its solubility in water, portions of the gas being taken which were completely soluble in water. Experimental Results ADSORPTION DATA -In Table I are the values obtained for the adsorption of hydrogen and ammonia on the various catalysts at the several temperatures mentioned. The specific adsorption or the cc. of gas adsorbed by one gram of Pease: J. Am. Chem. SOC, 45, 1196(1923).
281
A D S O R P T I O S O F AMMOXIA O S METALLIC CATALYSTS
metal is noted instead of the volume adsorbed by one volume of catalyst. In order t o compare these values, therefore, with those given by other observers' it is only necessary t o multiply the value of the specific adsorption by the density of the metal catalyst to obtain the volume adsorbed by one volume of the metal. These values Rere checked so that all results were reproducible v,ith the sample examined.
TABLE I C.C.Gas required to fill bulb Reduced t o S . T. P. Metal
Keight
Sodium
1.7153
Copper
3 5 ,8282
Gas
16.8430
IO.7632
IO0
218'
-
-
-
S2
24.72 30.28 41.12
16 78 21.j8 23.48
13.36 16.71 17.38
27.69 42.13
19.92
15.75 17.43
14.41
27.62
12.84 12.8; 1 3 ,j6
10.19 10.40 10.58
8.99 9.19 9.08
1-2
SH3 Iron
I
7.89 8.16
Hz SH3 Kickel
0 0
Nz SH3
S?
17.72
Hz
17.80 19.60
SH3
15.j1
7.83 7.43 5.61 7.28
Specific Adsorption C.C.gas taken up by I gram of metal Sodium
Copper
0.0
XH3
sz. H, SHs
Sickel
S?
KH3
Iron
s> Hz ?;Ha
0.16
-
-
-
0.00
0.0
0.0
0.1jj
o 134
0.093
0.458
0.187
0.112
0.00
0.0
0.0
0.0
0.8jj
0.04j;
o loo
0.06j
0.0
0.0
0.0
0.0
o.ooj4
0.0028 0.0669
0.0195 0.0116 c ~ 3 6 0.0084 -
0.1jj
0.0
Taylor and Burns: J. Am. Chem. SOC.,43, 1277 ( 1 9 2 1 ) ; Gauger and Taylor: 45, 923 (1923):'Hempel and Thiele: Z. anorg. Chem., 11, 93 (1896); .\layer and Altmayer: Ber.,
41, 3062 (1908); Troost and Hautefeuille: Compt. rend., 80, ;88 (1875).
282
WALTER A. DEW AND HUGH S. TAYLOR
COPPER. Upon comparison of the values obtained for the adsorption of hydrogen by copper (namely 1.381 vols. per I vol. of copper) with the values obtained by Taylor and Burns (less than 0 . o j vols. per vol. of copper) it is seen that the amount adsorbed is some zoo times as great a t 110' and is more than 5 times as great as that observed by Pease who obtained 0 . 2 3 2 vol. per vol. of copper but is about one-third of that obtained by Beebe and Taylor, who found 3.8 vols. per vol. of copper. The comparatively large adsorption of hydrogen observed by copper is probably due to the fact that the temperature of reduction IYas low. The value obtained for the specific adsorption of ammonia on copper is large compared to the hydrogen value shoxing that the sample investigated possessed good adsorptive power for ammonia, the value being readily measurable at all temperatures employed. I n no case was there any indication of decomposition of the ammonia by the copper at the three temperatures employed as shown by the observation that the ammonia gas when pumped off was completely soluble in water. NICKEL. Sickel was observed to adsorb ammonia very strongly, giving a value which is nearly twice as large as that obtained for copper a t 0'. At 2 13') however, the initial volume adsorbed began to decrease slowly, indicating plainly that decomposition of the ammonia was taking place.* This decrease was not noticeable in the first five minutes, after which time it fell away until a constant value was reached. The gas which was pumped off the catalyst was tested qualitatively for hydrogen which was found to be present. At 305' the amount of decomposition of ammonia was slight and took several minutes before it became measurable, which was probably due to the small adsorption of the ammonia by the nickel. IRON. The values obtained for the adsorption of hydrogen were found to be small, checking closely with those obtained by Taylor and Burns.' Adsorption of ammonia was much larger than that of hydrogen but not so large as might be expected were total adsorptive capacity the only criterion of activity. This may be attributed to the high temperature a t which the iron was reduced and, as will be indicated later, this point of view is substantiated for Iron A on which heat of adsorption measurements were made. This sample showed adsorptions that were nearly as large as Nickel 8. These measurements indicate by contrast the importance of maintaining low temperatures in the preparation of unsupported catalytic materials if they are to have a high adsorptive capacity. I n making the measurements no decomposition of ammonia was observed at any temperature until 444.6OC. was reached. Considerable decomposition of the gas took place at this temperature as was indicated by collecting 9.8 C.C. of the desorbed gas of which only 4.1 cc. was soluble in water.
* Note:-It should be noted here that the initial value was taken as that of true adsorption instead of a value taken when the readlng of the burette became constant as was the case at lower temperatures. In taking the Initial value no doubt some error is involved since it is not known how rapidly the ammonia 1s decomposed at those temperatures. 1 Taylor and Burns: J. Am. Chem. SOC., 43, 1280 (1921).
ADSORPTIOS O F AhlMOXIA ON METALLIC CATALYSTS
283
Adsorption Isotherm of S H B o n Copper at 118":-Since copper mas found not to decompose ammonia at 218' it was decided to obtain an isotherm curve which iyould indicate the nature of the ammonia adsorption at 218". The following diagram, Fig. I , indicates very clearly the relationships which are found to exist between the pressure and the volume adsorbed. It will be observed that there is strong adsorption a t lorn pressures. At 400 nim. the adsorption has reached 3.4 cc., whilst an additional 400 mm. pressure results only indicates approaching saturation . in the adsorption of a further 0 . 6 ~ ~This of the surface. I
I I
/.50T4'C/?M1 AT P I 6 *C
OF
ZOO
400 FIG.I
600
Isotherms f o r .YHB on S i c k e l at 110" and O°C:-The accompanying curves, Figs. 2 and 3 indicate the variation of the adsorption of ammonia gas with pressure at the two temperatures. I t may be observed that the S H I curve on nickel a t I I O " ~ .is very similar to that obtained on copper which has just been described. At o°C., however, the adsorption of ammonia a t the lower pressures is quite marked but there is little evidence of approaching saturation even at the highest pressures studied. IROY-MOLYBDENUM. The results of the measurements of adsorption on this catalyst sample may be briefly summarized in a table. TABLE I1 Catalyst
w t in grams
Gas
cc. Gas required to fill bulb a t S T. P. 2 j"
Iron Molybdenum
Izj
H1 SHI
69 3 0 357.4
Specific Adsorption 25"
1000
jj 2
-
o
0032
2
30
IO00
o o
-
284
WALTER A. DEW AND HUGH S. TAYLOR
Benton says: “No solution of hydrogen or chemical reaction occurred a t since equilibrium was always quickly reached and the curves coincide for introducing and withdrawing the gas, all the hydrogen being recovered. The same is true at 100’ except that 0.37 cc. of hydrogen were not recovered and therefore presumably reacted with the unreduced oxide. 2 jo
tSO1HERM AT //O’C, OF 4MMON/A ON NiCK€L
FIG.2
“If any hydrogen is adsorbed at z j” or 100’it must be roughly proportional to the pressure. If it be assumed, however, that no hydrogen is nd372.2 X -- 7 j . z cc. and the 2i3 volume of gas required to fill this space at z j o would be 68.9 cc. Actually, however, 69.3 cc. of hydrogen a t z j owere needed. Hence the minimum adsorption of hydrogen a t 2 5 ’ was 0.40 cc. which is the order of magnitude ob-
sorbed at rooo the free space in the bulb is
j 5.2
tained by Taylor and Burns for hydrogen on iron. “In any case, since the volume adsorbed cannot possibly vary inversely as the absolute temperature, these calculations show that Fe-Mo adsorbs scarcely any hydrogen at 2 5 ’ and 100’ and, therefore, hydrogen determinations can be used to determine the free space in the bulb, especially as the ammonia adsorptions are large.” The isotherm given, Fig. 4, indicates that the amount of ammonia adsorbed at low pressures is very large. Heats of Adsorptzon:--A slight deviation from the method used by Beebe m d Taylor was necessitated by the fact that nitrogen, a diatomic gas, was
ADSORPTION O F AMMONIA ON METALLIC CATALYSTS
285
employed to determine the heat effects previously mentioned, whereas ammonia is a tetratomic gas. This can probably be illustrated best by an example or typical calculation as in the case of NICKEL A. Determinations of Free Space and Heat Effects employing the average of Runs I and 2 .
FIG.3 2
I
Maximum Temperature. . . . . 0 . 2 I O t, Initial . . , . , ,o ,160
Maximum Temperature. . . . . . o . zoo >l Initial . . . . . . 0.Ijj
0bserved " Rise. . . . o . o jo Observed " Rise. . . . . .o . o jj Average Observed Temperature Rise, . . . . . . . . . . . . . . .o.o53Oin 3 minutes Observed Temperature Rise ....................... 1st minute correction., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2nd
"
3rd
"
'I
,!
..................................... .........................
........
0.053 .O.OOI 0.002 0.002
True Temperature Rise. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Average 1-olume S 2Introduced.. . . . . . . . . . . . . . . . . . . . . . . 3 9 . 9 3 cc. N. T. P. 3101. Heat of S H I = 0 . 5 2 0 ~ X I j = 8 . 8 4 , , , , A-2 = 0.2438 x 28 = 6 . 8 2 >1
Amount of heat X2would liberate in cooling from ZI.IOC. to oo is given thus: 39.93 6.82 S 2 heat capacity = __ X -x 2?,400
13. jj
2 1 . IO0 = 0 . 0 1 9 O .
where 13 ; 5 calories = Heat capacity of Calorimeter. Total Temperature Rise = o o j 8 " S 2heat capacity = o 019. Hence, by subtraction, the heat effect due to compression = 0.039'.
2 86
WALTER A . DEW AND HUGH S. TAYLOR
To find the NH3 equivalent of the nitrogen value for heat capacity of the 8.84 gas, we have, 0.019' X - = NH3 equivalent, or 0 . 0 2 4 ~ , d uto e heat capac6.82 ity of the ammonia. Hence the total effect due to introducing 39.93 ccs. NH3 is 0.024 0.039 = 0.063Oc. The remaining calculations are similar to those of Beebe and Taylor.
+
FIG.4
Copper:-This catalyst mas prepared as previously described in order to reproduce the sample upon which adsorption measurements alone mere made. It is of interest to note the extent of this reproducibility. The following table reveals that the average volume of ammonia taken up by the sample was 1i.3; CC. which gives a specific adsorption of 0.4958 cc. per gram for copper A as compared with 0,4577 cc. per gram for copper 2 . This shows a reproducibility of about 6 per cent. The results obtained for the heat of adsorption on copper may be summarized most conveniently in two tables, the first showing determinations of the integral heats of adsorption and the second showing heats of adsorption a t various partial pressures.
TABLEI11 Integral Heat of Adsorption Copper A. Weight 35 03 grams Determination
C.C.
adsorbed
Temp. Rise
(0,760) I
Ii.50
0.455
2
17
0
2j
449
The heat capacity of the calorimeter was found to be 13.24 calories.
7>71= 7,720
ADSORPTION O F AMMONIA ON METALLIC CATALYSTS
TABLE IV (Q at Partial Pressure).
Differential Heat of Adsorption. Determination
Pressure in m.m.
cc. adsorbed (0,760)
178.5
9.53 7.60
I99
'55
7.00
192
9 . I3
Temp. Rise
0.291 0.224 0.218 0.266
x 9,056 8,743 9,236 8,663
Kzckel A:-The catalyst used here was prepared in order that it might be as nearly as possible of the same activity as the first sample on which adsorption measurements were made. Though greatest care was exercised the latter catalyst was about 14 per cent less active in ammonia adsorption than the first. One run was made to determine the integral heat of adsorption of hydrogen on this catalyst. A value of 14,960 calories was obtained which checks very well with those obtained by Beebe and Taylor. The results of the integral heat of adsorption measurements of ammonia on nickel may be tabulated as follows:
TABLE V Integral Heat of Adsorption of Ammonia on Kickel Determination
C.C.
adsorbed
Temperature Rise
(0,760)
21.19 .oo
I
2
11,220
11,570
21
The heat capacity of the catalyst and calorimeter was 13.75 calories
TABLE VI Differential Heats of Adsorption on Nickel A Determination
Initial Pressure
I
0
2
0
'83.5 198
3 3A 4 4d
5 SA
5B SC 6 6A 6B
Final Pressure
0
210
210
is8
0
141
141
7 59
0
62.5 212
465 0
89 224.5 511.5
6 2 .j 212
465 7 59 89 224.5 5'1.5 756
C.C. Vol. gas Adsorbed (0,760)
11.90 13.94 11.79 6.82 10.03 5.06 6.50 3.77 3.40 3.46 7.73 5.03 3.90 2 54
6C T is the highest temperature observed plus corrections. T,is the initial temperature. '
T-To
0.445 0.446 0.412 0 . I70
0.360 0.186 0.240 0.116 0,087 0.067 0.282
0.085 0.089 0.058
11~520 9,854 10,760 71677 I 1,060 11,320
288
WALTER A. D E W A S D HUGH S. TAYLOR
It will be observed that the heat of adsorption of ammonia at low pressures is about 11,300 calories and gradually decreases to about .ijooo calories as the pressure increases. Iron A . In order that a catalyst of greater adsorptive capacity might be obtained, the reduction temperature was maintained as low as possible. The sample examined weighed 49.35 grams and showed a specific adsorption of about 0.5 cc. per gram which is about three times as large as that of iron I at 0°C. It is approximately thirty per cent less, however, than the specific adsorption of Kickel A a t o°C. The heat capacity of the catalyst and calorimeter in these measurements was I 7.98 calories. ?io determinations were made of the integral heat of adsorption. Table TI1 gives the differential heats of adsorption.
FIG.5
TABLE F'II Differential Heats of Adsorption in Iron A Determin:ition
IX
16
IB IC
97.5 368
Final Pressure 16 97.5 368 756
0
16
I
2
2A 2R 2c
Initial Pressure 0
16 102.5
368
102.5
368 756
cc. Vol. gas adsorbed (0,760) 8.05 5.61 6.84 5.61
T-To 0.320 0.152 0 .I
gj
9)'2i
5.02
0 .I22
8,113 15,580 9,548 9,788
4.43
0.088
8,008
7.56 5.99
0.113
x 16,010 10,910
0.202 0. I32
At low pressures it will be seen that the heat of adsorption of ammonia on iron is about 16,000 calories, a value much larger than that obtained in the case of nickel. Employing the value of 38.22 cc. for free space obtained by using nitrogen it is possible to obtain an isotherm curve (Fig. 5 ) for ammonia on iron at 0°C. It may be noticed that the curve obtained is quite similar to the isotherm of ammonia on nickel a t the same temperature.
ADSORPTION OF AMMONIA ON METALLIC CATALYSTS
289
Discussion of Results 411 the catalysts studied have been shown to possess adsorptive capacity for ammonia at the temperatures studied and the adsorption has been shown to be reversible in the lower range of temperatures employed. At the higher temperatures decomposition of ammonia occurs. From the known reactivity of ammonia with sodium to yield sodamidel and from the presence of nitrogen in samples of nickel and iron which have been used to decompose ammonia, we may conclude that the ammonia is attached to the metal surfacez by a metal-nitrogen linkage. At low temperatures the ammonia may be removed intact. As the temperature is raised, however, internal rearrangements in the adsorbed ammonia molecule may occur. In the case of sodium, this leads to a definite compound, NaNH,, with elimination of one atom of hydrogen from each ammonia molecule. I n the case of the other metals such compounds are not identifiable but it is found that the hydrogen comes off in such cases prior to the nitrogen, leaving the metal charged with this latter, not however in definite stoichiometric proportions. Assuming that the whole surface is not uniformly active and that it is the fraction of the surface having the strongest adsorptive capacity which is the most active catalytically, it is of interest to note that the heats of adsorption at very low partial pressures stand in the order ( I ) iron, 16,000calories; ( 2 ) nickel, 11,300calories; ( 3 ) copper, 8 , j o o calories. It is these values which are to be inserted for X in the expression Eo = E t A’ - x previously given. This would indicate that the apparent heat of activation, Eo, would be most markedly affected Iiy the heat of adsorption in the case of iron, least in the case of copper. There is, however, another point of view in this respect. The higher heat of adsorption of ammonia on iron means that it is possible t o raise an adsorbed molecule to a much higher temperature on an active iron spot than on an active copper spot,. There is thus offered a greater probability of interaction between the metal atom and the animonia molecule in the case of iron than in the case of copper. I t is possible that the higher activity of iron as a catalyst for ammonia decomposition is to tie nssociated with this observation. Expressed otherwise, there is the possibility of inelastic collisions between impinging ammonia molecules and the metal surface at much higher temperatures in the case of iron than in the case of copper, which is equivalent to saying that the ‘active mass’ of ammonia is greatest in contact with iron a t the temperature in question.
+
Summary Values for the specific adsorption of ammonia and hydrogen gases were determined for sodium, copper, nickel, iron, and a j o - 5 0 mixture of iron molybdenum a t various temperatures. (I)
Titherley: J. Chem. Soc., 6 5 , j o q (1894). Beilby and Henderson: J. Chem. SOC.,79, 1 2 5 0 (1901); Fowler: 79,288 (1901);White and Kirschbaum: J. Am. Chem. Soc., 28, 1347 (1906)
2 90
WALTER A . DEW AND HUGH 5. TAYLOR
(2) Adsorption isotherm curves were determined for ammonia on copper at ZI~', on nickel at I IO' and oo, on iron molybdenum at 25' and on iron a t 0'. (3) Integral heats of adsorption for ammonia gas at o'C. on copper, and nickel have been determined. (4) Differential heats of adsorption for ammonia gas a t o'C. on copper, nickel, and iron have been tabulated. ( 5 ) The effect of temperature on the preparation of unsupported active catalyst samples has been emphasized. ( 6 ) An apparent relationship between heat of adsorption and catalytic activity has been presented in the case of the reaction 2NH3 = N P 3Hz.
+
