524
L. E'. AUDRIETH .4ND WILLIAH L. JOLLY
CATALYTIC DECOMPOSITIOS OF HIGHLY CONCENTRATED HYDRAZISE BY RAKEY NICKEL' L. F. AUDRIETH A X D WILLIAM L. JOLLY
Noyes Laboratory of Cheinistqi, Gniuersily of Itlinois, Urbana, Illinois
Receiaed April 87, 1950
The susceptibility of dilute aqueous solutions of hydrazine to catalytic decomposition had been noted previously by Tanatar (9), Purgotti and Zanichelli (7), Gutbier and Neundlinger ( l ) , and Olireri-Mandala ( 5 ) , mho studied platinum as a catalyst, and by Irrera (2) and Kuhn (3), who investigat,ed Raney nickel. Recent interest in the possible use of hydrazine as a fuel made it desirable to undertake a more extended study of the stability of the highly concentrated material (above 90 per cent hydrazine) in contact with catalytically reactive materials and metals. Preliminary experiments were carried out by the authors to test the catalytic act'ivity of platinum, nickel, iron, cobalt, aluminum, copper, and silver. Only nickel and cobalt brought about rapid decomposition at room temperature. Since Raney nickel of reproducible quality mas available, its effect on the decomposition of highly concentrated hydrazine was subjected to a more intensive investigation. Additives such as hydrazine salts, water, ,and metallic salts with various anions were found to markedly influence the rate of decomposition. The decomposition reaction is essentially a dehydrogenation process leading to the formation of nitrogen and hydrogen with some ammonia. hX.LLTTICAL .LXD PROCEDURAL DETAILS
The decomposition of highly concentrated hydrazine was studied by measuring the rate of gas evolution. Reaction vessels of 4- to 5-ml. capacity were attached to a gas analysis apparatus, making it possible not only to measure accurately the volumes of evolved gases but also to effect their analysis at intervals for ammonia, hydrogen, and nitrogen. The reaction vessels were cleaned carefully before use by successive treatment with nitric acid or aqua regia, distilled water, hydrazine, and finally again with distilled water. The Raney nickel (stored under boiled distilled water) was transferred to the reaction vessel as a wet paste. Three milliliters of hydrazine solution was then pipetted into the vessel and the latter was connected immediately to the system. In some cases, an ampoule containing an additive was held magnetically in the space above the reaction vessel. This ampoule was allowed to drop into the reaction vessel after a preliminary study of the decomposition, without the additive, had been carried out. Unless otherwise specified, all experiments were carried out a t 25OC. The amounts of nickel were determined by analysis after completion of th? This paper is abstracted from a thesis presented by W. L. Jolly to the Graduate College of the University of Illinois in partial fulfillment of the requirements for the degree of Master of Science, June, 1949.
525
DECOMPOSITION OF HYDRAZINE BY RANEY NICKEL
runs, either by titration with a standardized potassium cyanide solution (8) or when additives such as copper were present by using the gravimetric dimethylglyoxime method. The highly concentrated hydrazine mas prepared by dehydration of the commercially available 85 per cent hydrazine hydrate with sodium hydroxide and subsequent distillation under reduced pressure. The hydrazine was analyzed by the iodate method (6) and was usually found to vary between 96 and 99 per cent in strength. TEE REACTION RATE
Preliminary experiments showed that the reaction rate for the decomposition of 90-95 per cent hydrazine with Raney nickel, based on the amount of gas evolved per minute, remained fairly constant with time. Some fluctuations did occur but the rate never varied by more than & 5 per cent from the mean over a 24-hr. period.2 TABLE 1
Decomposition of hydrazine by Raney nickel at 25°C PICKEL CATALYST
grams
0.0034 0,0042 0.0212 0.1200 0.1527
PATE d./VIi".
0.0503 0.0419 0.218 1.01 1.30
14.8 9.8 10.3 8.4 8.5
1' 1
PATE
-i 0.2146 0.2275 0,2750
0.5oOo
ml./min.
1.80 2.07 2.24 4.67
!
:
SPU-IFlC RATE
ml./min./r, nickel
8.4 9.1 8.1
Results of experiments using varying quantities of nickel catalyst demonstrate that the decomposition of hydrazine is a typical example of heterogeneous catalysis. The data given in table 1 for the "specific rate" (milliliters per minute per gram of nickel) show that the rate of gas evolution is directly proportional to the amount of Raney nickel employed, except in those cases where very small amounts of catalyst were used. The specific rate, excluding the first three values, averages 8.6 ml./min./g. nickel at 25OC. for 90-95 per cent hydrazine. Impurities normally present in commercial Raney nickel, such as aluminum and iron and their hydroxides, were found to have little effect on the specific reaction rate. Repeated digestion of the catalyst with sodium hydroxide solution was found to give a material whose activity remained somewhat more constant, in that the particular sample did not fluctuate more than 1.2 per cent from the mean over a 6-hr. period. I t was considered possible that the Raney nickel might adsorb or desorb hydrogen while in contact with hydrazinc. Analyses showed that the amount of adsorbed hydrogen dropped from 0.0025 to 0.0024mole per gram of nickel after treating Raney nickel with concentrated hydrazine for 2.5 h r . This change is significant only with respect t o the possible activity of the catalyst. The volume of desorbed hydrogen is negligible compared with the volumes of the gaseous decomposition products.
526
L. F. AUDRIETH AND WILLI.4hl L. JOLLY
Decomposition of hydrazine was also studied over a range of temperatures from 1°C. to 49.7"C. Temperatures and actual measured rates corrected to standard conditions are given in table 2. These data lead to a value of 17.1 kcal./mole for the experimental activation energy. THE NATURE OF THE REACTION
If it is assumed that ammonia, nitrogen, and hydrogen are the only decomposition products, the overall stoichiometry can be defined by means of a variable, which may be designated as X for the generalized equation:3 (3
+ X)N*H*= 4NH3 + (1 + X ) S * + 2XH2
X may assume any value from zero to infinity. I t is possible to evaluate X for a particular experiment by establishing a relationship between the moles decomposed and/or formed of two of the substances appearing in the above TABLE 2 Deconiposition of hydrazine b y R a n e y nickel as a j u n c l i o n oJ lentperalure ( 3 rnl. hvdrttzine: 0.2822 r . nickel cntalvsti RATE (COFXECTED)
mi.iniin.
4.41 12.23 21.88
equation. The analytical ratios which were employed and the values of X ohtained therefrom are given in the following summary: ANALYTICAL RELATIONSHIPS
(a;
3 3
NzH,
=
Kz + H I SpHi (c)
5 Kz
=
4 __ 3 + X
-
EXPERIYESTAL VALUES FOP
x
14.7, 13.5
+
1__-3 5 3 X
2x l + X
I
+
11.1
3-0
Results for the three different procedures for evaluating the nature of the reaction are only qualitatively consistent. It would appear that the major reaction involves a dehydrogenation of hydrazine. Such a process could be initiated (a) by the adsorbed hydrogen on the catalyst surface and ( b ) by adsorption of This generalized equation is a composite of two reirctions which c:in conceivably occur: and
DECOMPOSITIOX OF HYDRAZISE BY RANEY EiICKEL
527
hydrazine molecules on the active surface, followed by a rupture of the X-X or the S-H bonds. Intermediate formation of several radical species may be.
MOLARITY OF HYDRAZINE
FIG.I . Effect
of hydrazine concentration on the rate of decomposition by Raoey nickel
assumed in either case. These possibilities may be represented by the following equat>ions:
& 7
H
+ EzHd I
+ KzH3
(a)
NH3
H 7
N*H4
+ NHz
+ SZH3
I
(b)
2SHz
EFFECT OF CHANGE IK THE NATURE OF THE SOLUTION
Eflect of water: The specific activity of Raney nickel was determined for a series of hydrazine-n-ater mist,ures ranging from 0.52 to 30.0 ;Irl with respect to hydrazine. These data are depicted graphically in figure 1. Hydrazine is much more susceptible to decomposition by Raneynickel inthe dilute range (8 & 2 .If) than as the highly concentrated material. :lddition of hydra2in.e salts: Hydrazine salts may be espected to behave as acids in highly concentrated hydrazine. It was found that the specific rate increased as the concentration of hydrazonium ion, introduced as SZH,.H?SO,, was increased (see table 3). This accelerating affect was only temporary, however. Rates of gas c\-olution fell off with time, in some instances to a value less than would be expected for pure hydrazine.
528
L. F. AUDRIETH .4SD \YILLI.iM L. JOLLY
Solutions of hydrazine salts in anhydrous hydrazine react with the Raney nickel catalyst. The solutions take on a purple color identical with that of the trihydrazine nirkel(I1) complexes. It is probable that the observed deceleration was due to a dissolving action of the acid solutions with consequent malformation of the nickel surface, or possibly to a specific poisoning effect of the catalyst by the sulfate ion. CHASGE I N SATURE O F THE CATALTBT
By addzfzon qf reductble metal salts: Weighed amounts of reducible metal salts were held magnetically in ampoules over hydrazine samples undergoing decomposition with Raney nickel. After measuring the rate of decomposition of the hydrazine with Raney nickel alone, the ampoule \vas allowed t o drop into the solution. When it appeared that the initial reaction due to reduction of the added metal compound was complete, the rate \vas again measured. h T.\BLE 3 Effect o!" the h y d m z o i i i u i i i i o n o n the tlecoiuposilioii uf h ydrnriiie b g I2ane!l nickel ~ O S C E S T R A T I O S OF ADDED S&.HISOI*
I
ISlTI*L SPECIFIC R 4 T E
Y
tu/,
I
0.01 0.1 1
rmin. 6 . rti
