determined directly, sirice the rate-determining step changes to diffusion at the expected transition temperature.) As the temperature increases, the experimental Arrhenius activation energies decrease (Table VII). The final low value is 13.6 kcal. for the range 531’ to 616’ c. To test the k4 polymerization path concept, samples of primary distillate from runs a t 400’ C. were enclosed in bombs and heated to 500” C. for 0.5 to 3 hours. I n each instance extensive polymerizatiorl occurred. For calculation of activation energies in sequence reactions, assumption of a steady-state concentration for intermediates like the bitumen formed in step 1 is frequently a necessity. If the data for gas evolution in the temperature range from 331’ to 427’ C. from this research, and the data of Hubbard and Robinson for gas production alone in the range 400’ to 525’ C.. are treated in this manner, an apparent activation energy for gas production of 27 and 22 kcal. results. T h e data as plotted indicate excellent first-order kinetics for the gas production process. Most important in any sequence reaction is that such steady-state approximations not be used unless they in fact are demonstrated. Fortunately, the data of Zimmerley and of Hubbard and Robinson on bitumen production and disappearance are available. They enable specific values for bitumen concentration and correct activation energies to be
determined, thus resolving an apparent problem of different activation energies for kerogen decomposition. Acknowledgment
Appreciation is expressed to the Equity Oil Co. for support of this research and for the J. L. Dougan Fellowship for graduate research in the Fuels Engineering Department. literature Cited
(1) Allred, V. D., Nielson, G. I., Chem. Eng. Progr. Symp. Ser. 54, 160 (1965). (2) Cane, R. F., “Oil Shale and Cannel Coal,” Vol. 2, p. 592, Institute of Petroleum, London, 1951. ( 3 ) DiRicco, L., Barrick, P. L., Znd. Eng. Chem. 48, 1316 (1956). (4) DouEcan. J. L.. U. S. Patent 3.241.611 (March 22, 1966). ( 5 ) Hububard, A. D., Robinson, i V . E., ‘.Thermal Decomposition Study of Colorado Oil Shale,” U. S. Bur. Mines, Rept. Inbest. 47441 (1950). (6) Thomas, G. IV., “Effects of Overburden Pressure on Oil Shale during Underground Retorting,” Society of Petroleum Engineering AIME, Preprint SPE 1272 (October 1965). ( 7 ) Zimmerley, S. R., “Chemical Dynamics of the Transformation of the Organic Matter to Bitumen in Oil Shale,” M. S. thesis, University of Utah, 1923. RECEIVED for review May 11, 1966 ACCEPTEDOctober 31, 1966 Division of Fuel Chemistry, 151st Meeting, .4merican Chemical Society, Pittsburgh, Pa., March 1966.
CRYOGENIC IGNITION OF HYDROGEN AND OXYGEN WITH RANEY NICKEL W A R N E R B. L E E T h e Marquardt Gorp., Van .Vuys, Calif.
Low temperature hydrogen-oxygen ignition studies involving catalytic and/or pyrophoric powdered metals were performed. A significant feature was the utilization of Raney nickel, entrained as a dry powder in a stream of cryogenic hydrogen gas, to produce ignition upon contact with liquid or gaseous oxygen, Ignition delays were between 10 and 2 0 milliseconds. This is not a fundamental limit, but is rather characteristic of the technique employed. The activity of Raney nickel for this application was catalytic in nature and due exclusively to a layer of chemisorbed hydrogen. Raney nickel powder, stored under an atmosphere of hydrogen, should have indefinite shelf life in the activated state and produce ignitions a t any time upon contact with oxygen. Techniques are described for regenerating the activity of reacted, oxidized Raney nickel.
THE propellant system of liquid hydrogen and liquid oxygen is widely used for space and missile applications. A natural consequence of this widespread usage is the application of this system for a variety of auxiliary roles-i.e., reaction control, vernier thrusting, etc. Essential to these applications is a method of obtaining reliable ignitions of the cryogenic propellants under space conditions, preferably with multiple restart capability, )vith no external electrical power required for the ignition process itself. As part of an in-depth study of this area, the George C. Marshall Space Flight Center of the National Aeronautics and Space Administration sponsored the subject investigation, which was concerned with a feasibility study of oxygen-hydrogen ignition by the use ofpolvdered metals (Lee, 1965). Numerous theoretical and experimental studies over the past few decades have established that the reaction between gaseous hydrogen and oxygen f o l l o ~ s a branched-chain
mechanism. For the present purpose, the general nature of the reaction can be interpreted in terms of Reactions 1 to 12 (Duff, 1958; Lewis and von Elbe, 1961; Semenov, 1959).
+ M +2H + M Initiation + M +2 0 + M Initiation Hz + --+ 2 0 H Initiation HQ+ H + HO2 Initiation H + +OH + 0 Propagation 0 + H? +OH + H Propagation OH + Hz +HzO + H Propagation H + OH + M +H20 + M Termination 20H 2 + H2 Termination HS
(11
0 2
(2)
9
(3)
2 --f
(4)
0
0
0 2
0 2
VOL.
6
NO. 1
MARCH 1967
(5) (6)
(7) (8) (9) 59
2H
2HP
20
0 2
Termination
(10)
Termination
(11)
+M+HOz+ M HOz 2l/:Hq + Termination H f
0 2
0 2
( m
(12b)
I n this sequence of reactions, M denotes any inert molecule capable of stabilizing the indicated reaction. Reactions 5 and 6 represent the important chain-branching steps which lead to the extremely rapid chain reaction characteristics of the H r O 2 system. The chain-branching steps may be rapidly terminated by hydroxyl radicals (OH), or by the hydrogen or oxygen atoms striking the wall of the vessel and losing energy, as in Reactions 9 to 11. Termination reactions of the latter type may also occur by these same atoms or radicals striking hydrogen or oxygen molecules which are not in the activated energy state required to react and branch, thus losing their energy as by Reaction 12 or the reverse of Reactions 1 to 3. However, this sequence of reactions will not spontaneously initiate the chain process at low temperature, because of the high energy of activation associated with Reactions 1 to 4. One must therefore provide, by other means, atoms or free radicals to promote Reactions 5 to 7. Based on well known bond energy concepts, it is easier to prepare hydrogen atoms than oxygen atoms. Accordingly, the appropriate way to induce the chain process is by chemical processes which can readily generate hydrogen atoms. Reasons for Interest in Raney Nickel
Catalytic ignition in the Hz/Oz system is normally considered to operate through a mechanism of hydrogen chemisorptionthat is, the H? molecule is adsorbed so strongly to the surface of the catalyst that the H-H bond is partially or wholly ruptured, being replaced by H-M bonds, where M denotes the catalyst. I n schematic equation form, Hz(g)
+2M
+
H-H
1
1
(physical adsorption)
(13)
-M-M-
H---H
l 1 + 1
H
-M-M-----M-M-
+ H(g)
(free radical formation) (15)
The process of chemisorption is the sum of Reactions 13 and
14. Reaction 15, the generation of free H radicals, is shifted far to the left-Le., the concentration of free H atoms is very small. However, only small amounts of H atoms are necessary to trigger Reactions 5 to 7. Reaction 13, physical adsorption, normally occurs rapidly, even a t low temperatures. Chemisorption, however, is usually associated with an energy of activation. Thus, only that fraction of Hz molecules possessing this energy can undergo chemisorption. The fraction of molecules possessing a given energy decreases exponentially with the absolute temperature. At the cryogenic temperatures concerned in this study, this fraction may become so small that the kinetics of chemisorption will be too slow to initiate combustion in the desired time interval. This slow kinetic step could be eliminated if a preformed layer of chemisorbed hydrogen was provided on the catalyst surface. A chemisorbed layer of hydrogen can be 60
I&EC PRODUCT RESEARCH A N D DEVELOPMENT
deposited readily enough on a number of metal surfaces at ambient, or slightly higher, temperature. A question arises, however, concerning the feasibility of protecting this layer. If a metallic catalyst were sufficiently finely divided, it might behave as a pyrophoric metal powder-Le., the powdered metal would itself be oxidized, initiated by the heat release of the catalytic reaction, and would no longer, strictly speaking, be only a catalyst. Early in this study, it appeared that Raney nickel and similar materials satisfy these requirements. The term “Raney nickel” (Raney, 1927) refers t o a form of nickel obtained by leaching the aluminum from a nickel-aluminum alloy by reaction with sodium hydroxide. T h e resulting finely divided nickel powder contains an appreciable amount of chemisorbed hydrogen. Raney nickel has long been known to be a very active catalyst for many applications. I t has been utilized almost exclusively, however, in the field of organic chemistry, for a variety of hydrogenation reactions. I t is likely that this close association with specialized organic reactions has tended to obscure the fact that Raney nickel (and modifications thereof) is a very promising catalyst for inorganic reactions in which a source of active hydrogen is required. I n particular, it appears that past applications of this material have not been concerned with its properties as a source of hydrogen atoms at cryogenic temperatures. I n the form in which it is usually described in the literature, Raney nickel is stored under an inert carrier, such as ethanol, until use. I n addition to being an inconvenient arrangement for the present application, Raney nickel stored in this manner has a limited active life, generally a few weeks at most. I t was therefore necessary to develop techniques for handling and storing Raney nickel in another form and to verify the above postulated mechanism of the HY/O* ignitions obtained from Raney nickel powders. Ignition Mechanism and Storage Characteristics of Raney Nickel
The preceding considerations indicated the desirability of storing Raney nickel under an atmosphere of hydrogen, preferably in the form of the dry nickel powder under hydrogen gas at ambient conditions. This was done, with indications of a virtually indefinite storage life under these latter conditions.
Experimental Arrangement. Nine samples were withdrawn from a common source of Raney nickel-methanol sludge, placed in separate flasks, and covered with methanol. The original Raney nickel for these studies was prepared by procedures similar to those of Adkins and Billica (19481 and Pavlic and Adkins (1946). The flasks were simultaneously evacuated to dryness by using a standard laboratory vacuum pump. Hydrogen gas was admitted simultaneously to the nine flasks, to approximately atmospheric pressure. These flasks were then set aside, to serve as long-term storage tests. At various times throughout the remainder of the investigation, the flasks were opened and tested for pyrophoricity in ambient air. Eight flasks were opened after storage times of 1 to 159 days. The ninth was opened after 1 year and 7 days of storage. Qualitatively, each sample was immediately pyrophoric upon contact with air, with no observable decrease in activity with storage time. Some of these samples, including the one stored for 159 days, were used to obtain successful cyrogenic Hz/Oz ignitions. According to the theoretical model described above, there should be no reasonable limit to the shelf life of Raney nickrl powder stored under these conditions. I t was next necessary to demonstrate that the pyrophoricity of the Raney nickel used in these tests was due to a chemisorbed layer of hydrogen, rather than simply being a result of the
small particle size of the nickel powder. This was accomplished by the follov ing simple experiment. Two samples were withdrawn from a common batch of Raney nickel stored under methanol; this parent batch of nickel was shown to be pyrophoric just before the samples were withdrawn. The two samples were evacuated to dryness for an extended time using a standard laboratory mechanical vacuum pump, then back-filled with ambient nitrogen gas. After a short time, both samples were exposed to the air for a few seconds with no reaction observed. The same two samples were then returned to the vacuum desiccator and re-evacuated, followed by back-filling with ambient hydrogen gas. Two days later, these samples were exposed to air and found to be pyrophoric. Similar results were obtained on other occasions.
If the sample of desorbed Raney nickel is left exposed to the air for more than a few seconds, oxidation of the nickel powders will proceed to compleiion, and pyrophoricity is not regained merely by re-evacuation and exposure to hydrogen. This oxidation of the desorbed powder by air is not to be confused with the pyrophoric reaction which occurs when normal Raney nickel is exposed to air. In the latter case the reaction occurs within a few milliseconds, a t most, and is extremely intense. The former is a gradual reaction, requiring 10 seconds or more (depending on sample size and configuration) for completion, with a detectable but mild heat release. Additional verification came from tests in which reacted, oxidized Raney nickel (now in the form of nickel oxide) was regenerated by reducing under hydrogen at about 300" C. and 1 atm. Upon completion of the reduction, the nickel residue n a s desorbed a t approximately 1 micron and 300" C. for one to a few days. FolloLving hydrogen chemisorption a t ambient conditions on the reduced nickel powder, the powder was again made active, as evidenced by its pyrophoricity. The pyrophoricity observed in every case in which this was done was qualitatively milder than that observed for dried Raney niche1 samples which had not been oxidized to completion in air. T h e oxidized nickel powders reactivated by the above procedure \+ere mildly incandescent directly on exposure to air, but with no direct flamevisible. With samples which had not been previously oxidized, copious, visible flame surrounding the powders persisted for one to a few seconds. This phenomenon was doubtless due primarily to the larger amount
of chemisorbed hydrogen present on previously unoxidized nickel particles. Similar vigorous evolution of flame occurred with samples which had been subjected to multiple desorption and readsorption of hydrogen prior to exposure to air. This difference in the degree of pyrophoricity is presumably due to certain irreversible sintering effects accompanying the large heat release associated with the initial exposure to atmospheric oxygen, or perhaps to incomplete reduction. Experimental Ignition Apparatus
After a fundamental understanding of the reaction mechanism had been gained, experimental ignitions a t cryogenic conditions were obtained utilizing dry Raney nickel powder entrained in a stream of cold hydrogen gas. A schematic of the test arrangement is shown in Figure 1. T h e salient features of this arrangement are as follows: The ignition device was located within a double Dewar arrangement. T h e outer Dewar contained liquid nitrogen, which chilled and liquefied the oxygen stream, chilled the nitrogen purge, and also served as a prechill for the hydrogen stream. The latter was then directed into the inner Dewar, which was further cooled by the flow of cold gaseous helium obtained from a commercial liquid helium Dewar. The temperature inside the inner Dewar, as measured by the copperconstantan thermocouple, T.C. 2 (see Figure 2), was controlled by adjusting the rate of addition of cold helium from the liquid helium source. This technique permitted a convenient control of temperature in this region over the desired range. T h e propellants were obtained from commercial gas cylinders and used without further purification. Hydrogen flow rates were determined from the gas rotameter, F I . A threeway solenoid valve, V I ,diverted the hydrogen flow to the bypass line except when an ignition test was being performed. The fluid resistance of the bypass line was adjusted by the needle valve, V2, to match that of the injection line. This provided a smooth flow transition when the three-way solenoid valve was activated to the ignition mode. The catalyst fluidizer, charged with dry Raney nickel powder, was located downstream of the three-way solenoid valve. In this device the hydrogen stream entrained a small amount of Raney nickel powder. Approximately 100 mg. of Raney nickel, on the average, were entrained each ignition. No effort was expended on determining the minimum, but some ignitions were obtained under conditions in which it was certain that a far smaller amount was
VENT
VENl FROM I e DEWAR
IGNITER,
h
(111 Figure 1 .
-
I
Schematic diagram of ignition assembly VOL. 6
NO.
1
MARCH 1967
61
Experimental Ignition Results
r
OH FROM LN2bRE.CHI
Figure 2.
Detail of igniter
injected. The contents of the fluidizer were protected by check valves, Q and CS, or, in certain tests, by burst diaphragms. The hydrogen line downstream of the fluidizer was purged and precbilled by a portion of the hydrogen bypass flow, by activation of the two-way solenoid valve, Va. T h e hydrogen line was then passed into the outer Dewar for prechill. Flow rates in the oxygen system were measured by the gas rotameter, Fs. The gaseous oxygen flow was vented to the bypass line by the three-way solenoid valve, Va, except when flow to the injector was desired. The fluid resistance of the oxygen bypass line was also adjusted to match that of the injection line by means of the needle valve, VS. The gaseous oxygen was liquefied by passing through a coil immersed in liquid nitrogen in the outer Dewar, and then flowed as a liquid to the injector. The precooled, gaseous nitrogen purge normally flowed continually, except during ignition periods. Provisions were made, by means of valves V7and VS,to purge each propellant system with nitrogen. Propellant flows were diverted to the bypass lines by manual operation of the switches controlling valves V I and V , promptly after each ignition. The range of nominal oxidizer-fuel ratios studied was from 0.5 to 6.0 (mass basis). The oxygen flow rate varied from 0.28 to 2.22 grams per minute, while the hydrogen varied from 0.57 to 0.37 gram per minute. Output data, consisting of thermocouple and pressure transducer traces, were recorded on a Tektronix Type 564 four-channel storage oscilloscope and photographed- by a Tektronix C-12 oscilloscope camera. T h e electrical noise associated with the activation of the solenoid valves produced a pip on the oscilloscope track, providing the basis for computing ignition delay times. The delay due to gas lead time was determined by blank runs in which only one gas was flowing. The gas lead time represents the time interval between the valve "on" pip and the pressure transducer response. This lead time (approximately 20 milliseconds) was subtracted from the over-all ignition delay time. Two types of instrumentation were used for ignition sensing: two unshielded 0.005-inch Pt/Pt-IO% Rh thermocouples (T.C. 3 and T.C. 4 of Figure 2) and a Kistler 601-H quartz crystal p:essure transducer. T h e quartz transducer pickup was mounted directly over the injector. The response time of the pressure transducer plus amplifier was of the order of 10 microseconds, and was therefore negligible for the purpose of this program. It was determined that the response time of the 0.005-inch Pt/Pt-lO% R h thermocouples plus amplifier could not have been greater than about 5 milliseconds and was probably considerably less. 62
l & E C P R O D U C T RESEARCH AND DEVELOPMENT
Aspects other than application of this modified form of Raneynickel have been discussed in detail (Lee, 1965). A total of 115 recorded ignitions were obtained, as well as many unrecorded ones-with a few exceptions, all at cryogenic conditions. Table I presents data obtained with gaseous propellant ignition, the temperature of each propellant being in the vicinity of 90" K. The ignitions obtained a t a mixture ratio of 6.0 produced a clearly defined ignition pressure wave, which showed u p as a vertical pip on the oscilloscope trace of the transducer output. Ignitions obtained a t a mixture ratio of 1.5, however, were usually characterized by the absence of a pressure wave and occurred near the exit of the exhaust duct, precluding accurate sensing by the thermocouples. This factor is responsible for much of the scatter in the dsta of Table I . Figure 3 illustrates the form in which the ignition data were obtained from the oscilloscope trace. T h e pressure wave associated with the ignition process is clearly visible in this figure. The thermocouple trace shown is that for T.C. 3 (Figure 2) ; T.C. 4 was not operative in this run. Tests were also conducted under conditions in which the catalyst fluidizer was immersed in liquid nitrogen during the run to ensure that the catalyst powder itself was a t a cryogenic temperature upon contact with oxygen. Some of these tests are shown in Table 11. I n these runs, the temperature of both propellants and all injector hardware was also a t liquid nitrogen temperature (77' K.). Table I11 presents some of the data obtained using liquid helium cooling of the hydrogen stream.
,
t,
TIME, 200 n r e i / c ~ !
Figure 3.
Table 1.
Oscilloscope ignition trace
Ignition Tests with Gaseous Propellants
(Test conditions. Propellants and ignition hardware near 90' K. Oxygon lead) I&-
"."," r&s
Mlr-
Corrected Ignition Timc, Msec. Remarks 191 6.0 20 Normal ignition 192 6.0 30 Normal ignition 193 6.0 120 Normal ienition 194 4.0 530 violent ignition 209 1.5 600 Mild ignitionb 210 1.5 500 Mildb 214 1.5 Not measured Rapid ignitiond 215 1.5 400 Mild' 216 1 . 5 Not measured Mildb 217 1.5 Not measured Mildb 218 1.5 Not measured Mildk 219 1.5 10-20 msec. to initiation Exhaust duct exit of reaction; 40 msec. blocked to detonation pulse a Nominal oxidize-fuel mars retio. Primary ignition occuned downsneom of I C ~ S O ~ , Run No.
turd
Ratios
Table II. Ignition Tests with liquid Oxygen (Test conditions. Propellants, ignition hardware, and catalyst near 77 K. Liquid oxygen lead) zgnrtion MixRun ture Corrected Ignition ho. Ratioa T i m e , Msec. Remarks
236 237 238 239 240 241 242 243 244 245 246
6.0 6.0 6.0 6.0
60 Not measuredb Not measuredb 400
Rapid ignition
6.0
75 Not measured* 60 15 Not measuredb Not measured* 10
Rapid ignition Normal ignition Normal Normal Normal Normal
6.0
6.0 6.0 6.0 6.0
6.0
.Yominal oxidizer-fuel mass ratio. instrumentation dij%ulties. a
Table 111.
Jnni--
b
Unrecorded delay times due to
Ignition Tests with Reduced Temperature Hydrogen
(Test conditions. ,%-
Sparking occurred prior to ignition
Liquid oxygen and injection hardware below 77 K . Liquid oxygen lead)
266
HP Temp., K. 70
Mixturf Ratio" 6.0
Corrected Ignition T i me , M s e c . 500
267 268 269 270 271
79 71 63 57 64
6.0 6.0 6,0 6.0 6.0
Not 1neasur.ed 300 400 500 540
272
63
6.0
520
273 274
64
6.0 6.0 4.0
130 70 600
tion Run .To,
275 a
63 64
I
I
100
200
I 300
400
500
600
700
HYDROGEN PRESSURE, p, nwHg
Figure 4. nickel
Hydrogen adsorption isotherms on Raney
Remarks
Strong delayed ignition Normal Normal Normal Normal N B purge left on, strong ignition Prior ignition audible Normal Normal Strong delayed ignition
n'ominal oxidizer-fuel mass ratio.
As can be seen, corrected ignition times of 10 to 20 msec. were obtained at cryogenic conditions under favorable conditions. This is of the same order as certain limiting correction terms, primarily the hydrogen gas lead time. The figure of 10 msec. does not, therefore, appear to represent an absolute limit, but rather, the smallest time increment in which the ignition process could be subdivided by the instrumentation in use in this study. Under nonoptimum conditions, apparent ignition delays of several hundred milliseconds were observed on occasions. This appeared to be a result primarily of poor mixing in the ignition zone, and was at times amplified by the low flow rates employed. If proper precautions were taken to prevent contact with air, at no time in this study did the Raney nickel powder fail to display catalytic activity (as evidenced by pyrophoricity) upon contact with liquid oxygen. The pvrophoricity was determined by visual observation through a prism mounted above the exhaust duct. T h e lowest hydrogen temperature at which an ignition was recorded was 57' K. Again, this does not appear to represent a fundamental limit, but rather reflects limitations imposed by the specific apparatus and experimental conditions involved. In particular, the specific laboratory injector utilized, together with the small flow rates involved, tended to place limitations on how low zhe hydrogen gas temperature might be in the immediate vicinity of the injector. The liquid oxygen, flowing in the central tube of the coaxial injector, may approach
its freezing point (approximately 55' K.) should the temperature of the surrounding hydrogen be substantially colder. Gas Adsorption Studies
Adsorption measurements were conducted in a metal and glass volumetric apparatus. Valving was accomplished by Veeco bellows vacuum valves and high vacuum stopcocks, Gas pressures during a run were measured with a Meriam manometer of estimated accuracy i . O . 1 mm. The surface area of two different batches of Raney nickel powder was determined by means of standard nitrogenadsorption techniques (Brunauer et al., 1938). The value obtained for each sample was approximately 50 sq. meters per gram. The samples were desorbed at ca 1 micron at 220' and 350' C., respectively, for about 24 hours. ,4s an indication of the quantity of hydrogen bound on the surface of the Raney nickel particles, Figure 4 presents hydrogen adsorption isotherms at three temperatures. At each data point in Figure 4, an initial rapid adsorption was observed, followed by a slow uptake of a relatively small additional amount of hydrogen, which lasted from several hours to 2 or 3 days. The isotherm at 299' K. is representative of the conditions under which the Raney nickel was utilized during most of this study. The same sample of Raney nickel was used in one of the nitrogen BET runs plus all of the hydrogen isotherms. Each was preceded by desorption in the range 315' to 350' C. for about 24 hours. The volume of hydrogen adsorbed at 299' K. appears rather large in relation to measured surface area when compared to other published results on this system (Mars et al., 1961). One possible source of error would be the presence of chemisorbed oxygen on the nickel surface. The latter could conceivably have occurred during the initial desorption of the Raney nickel-methanol prior to the nitrogen adsorption isotherm. A specific prereduction process was not performed on the Raney nickel; the sample was, however, exposed to ambient temperature hydrogen at reduced pressure for several hours, prior to the isotherm a t 299' K. There is evidence (Ponec et al., 1961, 1965) that hydrogen does not react with chemisorbed oxygen on nickel at 78' K., but does at 273' K. It is therefore likely that the pre-exposure to ambient hydrogen VOL. 6
NO. 1
MARCH 1967
63
would remove a large fraction of any hypothetical chemisorbed oxygen. Conclusions
Dry Raney nickel powder, stored under an atmosphere of hydrogen, is capable of reliably producing ignition of cold gaseous hydrogen (down to at least about 60’ K.) and liquid oxygen. The catalytic activity of Raney nickel for low temperature H2/02 ignition is due solely to a pre-existing layer of chemisorbed hydrogen, which produces ignitions by one or both of two main paths. The first is based on the fact that the chemisorbed hydrogen has a small but finite tendency to desorb, producing an extremely small concentration of hydrogen atoms in the vicinity of the nickel surface. This is the same as Reaction 15. This very small concentration of free hydrogen atoms may react with oxygen molecules to initiate the combustion process by Reactions 5 to 7. Alternatively, the oxygen molecules may react directly with the chemisorbed hydrogen atoms to produce free radical species, such as OH, which could then desorb and trigger the combustion reaction by a similar process. In either case the ignition reaction is postulated as occurring in the immediate vicinity of the nickel surface. The magnitude of the hydrogen atom concentration existing in bulk hydrogen gas which had been exposed to, or swept over, active Raney nickel powder was not investigated. Under the experimental conditions prevailing, there was no indication that the ignition process occurred in the bulk gas phase. The heat release associated with these reactions, which occur either on or very near the nickel surface, is absorbed primarily by the nickel particles. The latter, because of their small thermal mass, quickly attain a temperature sufficient to promote the direct oxidation reaction between elemental nickel and oxygen-i.e., the nickel powders are pyrophoric. Although this pyrophoricity may be useful in promoting an ignition, it definitely represents a secondary process resulting
from the primary catalytic reaction involving the chemisorbed hydrogen. Upon removal of the latter, with no change in particle size or configuration, the nickel is no longer pyrophoric, This modified form of Raney nickel may possess unique advantages, for a variety of applications, as opposed to the conventional form which is stored under an inert carrier such as ethanol. One advantage is virtually indefinite shelf life in the activated state. Another is that the catalytic activity may be regenerated in many cases by the proper sequence of desorption and readsorption of hydrogen. This material is worthy of consideration in various applications requiring a source of hydrogen atoms a t low temperature. Acknowledgment
The author is grateful for the helpful suggestions of R . E. Pecsar and the assistance of R. F. Gilman. Literature Cited
Adkins. H.. Billica. H. R.. J . A m . Chem. Snc. 70. 695 11948) Brunauer, S., Emmett, P.’H., Teller, E.; Zbrd.,’GO, 309 (ij38). Duff, R. E., J . Chem. Phys. 28,1193 (1958). Lee, LV. B., “Feasibility Study of Oxygen-Hydropen Powdered Metal Ignition,” Marquardt Corp. iikpt. 25,173 (September 1965). Lewis, B., Elbe, G. von, “Combustion, Flames and Explosions of Gases,” 2nd ed., Academic Press, New York, 1961. Mars, P., Scholten, J. J. F., Zwietering, P., Actes Congr. Intern, Catalyse, 2e, Paris, 1960, Vol. 1, p. 1245, Editions Technip, Paris, 1961. Pavlic, A. A., Adkins, H., J . Am. Chem. Soc. 68, 1471 (1946). Ponec, V., Knor, Z., Actes Congr. Intern. Catalyse, 2e, Paris, 1960, Vol. 1, p. 195, Editions Technip, Paris, 1961. Ponec, V., Knor, Z., Cerny, S., 3rd International Congress on Catalysis, 1964, Vol. 1, p. 353, North-Holland, Amsterdam, 1965. Raney, M., U. S. Patent 1,628,190 (1927). Semenov, N. N., “Some Problems of Chemical Kinetics and Reactivity,” Vol. 2, Pergamon Press, New York, 1959. RECEIVED for review March 14, 1966 ACCEPTED December 2, 1966 Work performed as part of National Aeronautics and Space Administration Contract No. NAS 8-1 1250.
DETONATION AND PROPAGATION OF LEAD AZIDE I N COMPACTED POWDER TRAINS L E W I S B. J O H N S O N , J R . University of Virginia, Charlottesuille, Vu. 22907
and propagation of the exothermic decomposition of the explosive inorganic azides have been studied widely in the past decade. Bowden and coworkers a t Cambridge have emphasized the slow, thermal-decomposition approach (7, 3-5). Their earlier work used the measured rate of gas emission and its variation with time a t different temperatures to arrive at empirical theories explaining solid product nucleation. Later, optical microscopy was used to study the formation of nuclei of solid products, the chief disadvantage being that the nuclei became visible only at advanced stages of the reaction. More recently, transmission electron microscopy was used to observe nucleation at earlier stages. HE INITIATION
64
l & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
Cook and coworkers (2) have pursued the electron microscopy approach, especially with lead azide, showing that the autocatalytic stage of the decomposition appeared to occur by a semiconductor mechanism similar to the photochemical process in silver halide photographic emulsions. The growing lead specks seemed to be the catalyst. Most of these investigations were carried out with carefully prepared pure crystals or single crystals of the explosive azides. Commercially prepared lead azide, used to prepare various types of fast-delay detonation cords, is neither very pure nor very carefully prepared, except as regards safety. I t appeared worthwhile to determine the extent to which basic studies could be applied to the behavior of the commercial material.
