1506
JOHN G. MALM,BERNARD WEINSTOCKAND E. ETJGENE WEAVER
Vol. e2
THE PREPARATION AXD PROPERTIES OF NpFfj; A COIMPARISON WITH PuF,,l B Y JOHN G. MALM,BERNBRD WEINSTOCIC
AND
E. EUGENE WEAVER
Argonne National Laboratory, Lemont, Illinois Received M a y IS, 1968 '
Introduction Neptunium hexafluoride is the volatile neptunium analog of uranium hexafluoride and of plutonium hexafluoride. NpF6 was first prepared and identified in 1946 by A. E. Florin2 on a microgram scale. Until recently no studies of NpF6 have been made due in part t o the small quan'tity of the element neptunium which has been available. Scientifically, the hexafluorides are an extremely fascinating group of compounds. Their high volatility is unusual in compounds of such high molecular weight. Each is the most volatile form of the element in question, being essentially a gas a t room temperature. This permits the measurement in the gas phase of such properties as the absorption spectra, Raman spectra and electron diffraction. I n addition, uranium, neptunium and plutonium hexafluorides represent the only gaseous compounds of the "rare earth" type of element in which the 5f electron shell is being filled. This paper will summarize the work that has been done at the Argonne National Laboratory with gram amounts of NpFe.
mm. During this pumping most of the HF is removed because its vapor pressure is of the order of several mm. a t this temperature. A second step is required to remove any traces of HF which may be t,rapped in the solid NpFs. While pumping, the solid NpF6 was allowed to warm slowly to room temperature and distilled to a U-tube which was immersed in a (302-trichloroethylene bath. The yield of NpF6 after purification was 0.800 g. or 91% of theoretical. A total of 1.750 g. of NpF6 was prepared by this method. The NpF6 prepared in this manner was shown to be pure by spectrographic analysis and by infrared spectra of the vapor. It should be pointed out here that NpF6, like P U F 6 and Ups, may be handled and stored indefinitely a t room temperature in quartz or Pyrex equipment with no sign of hydrolytic decomposition rovided that the glass has been thoroughly outgassed a n g the hexafluoride is completely free of HF. I n fact, any of these hexafluorides may be heated to 200' in quartz apparatus with little, if any, attack on the glass. Both NpFe and PuF6 are light sensitive compounds which photochemically decompose to give the tetrafluoride and Fz. When working in glass equipment care must be taken to avoid photodecomposition even with the intensities of light involved in ordinary room illumination. UF6 does not exhibit this behavior. Since the specific alpha activity of neptunium is about one-hundredth that of plutonium the hazards involved in handling NpF8 are correspondingly less than with PuF6 and rather minimal safety measures are required. Too, the difficulties arising from the self-destruction of PUFG~ Method of Preparation.-Neptunium hexafluoride can be (1.5% per day in the solid) by or-irradiation to give PuFl prepared by passing fluorine gas over NpF3 or NpF4 a t 500', and F2 have not arisen with NpFe. much in the same manner as in the preparation of UF6 Comparison of the Preparation of Actinide Hexafluorides. However, in this work, the NpFBwas prepared in a rather -UF6 can be prepared rapidly at 300" by the reaction of unique fluorination reactor (Fig. 1) which was designed for FZon UF,. While different type reactors were used, NpF6 use in the preparation of unstable hexafluorides such as was produced rapidly a t 500" and PuF6 rapidly a t 750'; PuF6. The unique feature of this fluorination system is the under the latter conditions AmF6 was not p r ~ d u c e d . ~This use of liquid fluorine which drips from the liquid nitrogen qualitative comparison shows that the actinide hexafluorides cooled condenser onto the heated charge (NpFd or PuF,). are produced with increasing difficulty as one proceeds This results in a rather high local concentration of Fz in through the series uranium, neptunium, plutonium and the hot zone and sets up violent convection currents which americium with the possibility that AmF6 cannot be sweep the volatile hexafluoride from the reaction zone to the prepared. This behavior is analo ous to the increascondenser. The details of the pre aration are given below. ingly negative value found for t%e IV-VI oxidation The starting materials were $F4 and Fz. The NpF4 potentials of these elements in aqueous solutions, which G. T. was prepared by heating a sample of dry Np(OH)4 in a Seeborg has suggested is partial evidence for an actinide stream of tank H F a t 600' for 5 hours. The fluorine gas s e r ~ e s . ~ was obtained from the Pennsylvania Salt Manufacturing Identification.-The similarity in volatility and Co. and analyzed greater than 99% Fz. An 0.783-g. sample of NpF, was placed in the nickel method of preparation between the neptunium sample dish which was then bolted in place on the reactor compound and uranium hexafluoride and pluwhich is shown in Fig. 1. The reactor was evacuated and tonium hexafluoride suggest that they have the then excess F2 added. Liquid nitrogen now was placed in same chemical composition. the reactor dewar to condense the Fz. The steady state FZ The similarity of the infrared spectrum of this pressure during the reaction was 450 mm. The furnace around the lower part of the reactor was brought slowly compound with UF6 and PuF6 is further evidence to 560' and maintained a t this temperature for 40 minutes. for the formula NpF6. The absence of spurious The furnace then was cooled to room temperature and the absorption bands in the infrared spectrum attests excess FZpumped off. The liquid nitrogen was removed from the reactor dewar and the reactor allowed to warm to to the purity of the compound used in these studies. The identification of this compound as NpF6 room temperature and the NpF6 distilled to the nickel-monel storage system. also has been established by X-ray powder photoThe NpF6 then was purified from H F in a two step procedure.* First, the NpFo was vaporized completely in a graphs taken in a thin-walled glass capillary. nickel storage container and then slowly condensed with a The pattern obtained showed the sample t o be solid COz-trichloroethylene bath ( -78'). The condensed isostructural with UFe6and PuF6. solid now was pumped on until the ion gauge read 10-6 Physical Properties.-Solid neptunium hexa(1) Based on work performed under the auspices of the U. 6. Atomic Energy Commission. (2) A. E. Florin, Memorandum, MUC-GTS-2165 (January 23, 1946). (3) B. Weinstock and J. G. Malm, J. Inorg. Nucl. Chem., 2, 380 (1956).
fluoride as observed in glass is bright orange in (4) G. T. Seaborg, "The Actinide Elements," NNES IV-ICA, Seaborg and Kats, editors, McGraw-Hill Book CO., New York, N. Y., 1954, Chap. 17. ( 5 ) J. L. Hoard and J. D. Stroupe, "X-Ray Crystal Structure of UFB," Cornell Report A-1296 (1944).
Dee., 1958
1507
PREPARATION AND PROPERTIES OF 1\TPF6 To Vacuum
color. The vapor is colorless. The melting point was determined to be 54.4' in a thin walled Pyrex X-ray capillary. For this determination the capillary was strapped t o the bulb of a thermometer, immersed in a clear mineral oil-bath. and was observed with a microscope. PuFs melts a t 50.803; UF6 melts a t 64". The vapor pressure of NpF, between 0" and 77" was determined using a quartz "sickle" gauge. The details of this work and a discussion of the results will be given in another publication.6 The preliminary results are represented by the equations 2892 log p solid (mm.) = - T - 2.699 log T 18.4813
+
log p liquid (mm.) =
- 1913 - 2.347 log T f 14.6125
The equation for the liquid extrapolates t o a boiling point of 55.2". NpF6 is slightly more volatile than PuF6 which boils a t 62.2'. The vapor pressures are plotted in Fig. 2. A comparison of some of the physical properties of UF6, NpFs and PuFs is shown in Table I. TABLE I PHYSICAL PROPERTIES NPFB
UFs
Color of solid Color of vapor M.p., "C. B.p., "C. Vapor pressure a t 0", mm. Vapor pressure a t 25', mm.
PuFs
white colorless
orange colorless
dark brown brown
64.0 56 5 17.5
54.4 55.2 20.8
50.8 62.2 17.G
111 . 9
126.8
104.9
Infrared Spectrum.-The infrared spectrum of NpF6 vapor' is similar to PuF6 and UFs. Therefore the molecular structure of NpFs is taken as a regular octahedron, point group o h . Of the fundamental frequencies tabulated in Table I1 only v3 is observed directly. The other fundamental frequencies are calculated from observed combination bands. Although Raman spectra have been obtained for UF6,8it has not been possible to obtain Raman spectra for NpF6 and PuF6 because of photochemical decomposition. TABLE I1 FUNDAMEXTAL VIBRATIONFREQUENCIES Designation
Spectral activity
UF6
NpFe
PuFa
l'1
Raman Raman Infrared Infrared Raman Inactive
668 532 626 189 202 144
648 528 624 200 206 164
628 523 615 203 211
1'2 l'3 l'4 l'6
l'6
1i1
Visible and Near-Infrared Spectra.-The absorption spectrag were photographed over the (6) E. E. Weaver, B. Weinstock and J. G. Malm (to be published). (7) J . G. Malm, B. Weinstock and H. H. Claassen, J . Chem. Phvs., 28, 2192 (1955). (8) H. H. Claassen. B. Weinstock and J. G. RIalm, ibid., 26, 426 (1950). (9) J. K. Brody, F. 9. Tornkins, S. Fred and H. H. Claassen,
RI.
unpublished work.
I
Fig. 1.-Reactor
,
for neptunium hexafluoride.
I
I
C. G. FRYE, H. 1,. PICKERING AND H. C. ECKSTROM Vol. 62 tioned is that of a regular octahedron. The elec- ture and even up to 200” the rate of thermal tronic configurations aside from closed shells are decomposition of PuF6 is not measurable. How5f0,5f and 5f 2. The assignment of the configuration ever, at 280” PUFBrapidly decomposes t o give 1508
5f2 rather than 6d2 to PuFs was made on the basis of paramagnetic susceptibility For PuFB an anomalously low and temperature independent susceptibility was observed and explained on a model based on the spin pairing of the two “f” electrons. Such pairing cannot occur with “d” electrons until at least two electrons have gone into the d shell with their spins unpaired. Recently, Griffith and Orgelll have treated this model in detail theoretically and show it to be a reasonable one. The paramagnetic susceptibility of NpFs has also been measured12 and its magnitude and behavior with temperature can be explained on the basis of its possessing one 5f electron. Chemical Properties.-As pointed out earlier in this paper, the increasing difficulty that is experienced in the preparation of the hexafluoride as one proceeds through the series uranium, neptunium and plutonium is a qualitative measure of their decreasing stability. UF6 has been shown t o be an extremely stable compound toward dissociation into Fz and a lower f l ~ 0 r i d e . l ~At room tempera(10) D. M. Gruen, J. G. Malm and B. Weinstock, J . Chem. P h p . , 24, 905 (1956). (11) J. 9. Griffith and L. E. Orgel, ibid., 26, 988 (1957). (12) B. Weinstock and J. G. Malm, ibid.,27, 594 (1957). (13) J. J. Kats and E. Rabinowitch, “The Cheniistry of Uranium.” NNES, VIII-5, p. 412.
PuFl and F2. The equilibrium constant for the reaction PuF~ $. FI +P u F ~
has been measured a t various temperatures and PuFB has been found to be substantially decomposed in all instance^.^^'^ It is a curious fact that in spite of its thermodynamic instability and reactivity PUFBcan be stored for long periods of time at room temperature with no measurable thermal decomposition. NpFa has been shown to be more stable toward thermal decomposition than PuFB. A 212-mg. sample of NpFe at a pressure of about 900 mm. was heated a t 560” for three hours in a nickel reaction vessel and showed no evidence of thermal decomposition. BrFa converts uranium compounds quantitatively to UF6. However PuFB reacts instantly with BrF3to produce BrF3. I. Sheft’s has shown that NpF3 reacts with BrF3 to produce NpF4 and that NpQz reacts with BrF3 to produce NpF4. Preliminary experiments have shown the NpF6 reacts very slowly with pure BrF3 to form a non-volatile product, presumably NpF4. Like UFBand PuFB, NpF6 is hydrolyzed vigorously in water to give neptunyl ion, Np02++. (14) A. E. Florin, el al., J . Inorg. Nurl. Chem., 8, 368 (1956). (15) ANL-4709, p. 65, December, 1951.
CATALYST KINETICS I N THE HYDROCARBON SYNTHESIS REACTION BY C. G. FRYE, H. L. PICKERING AND H. C. ECKSTROM Pan American Petroleum Corporation, Tulsa, Oklahoma Received M a y 16, I068
I n studying the reaction kinetics of hydrogenation of carbon monoxide over an iron base catalyst to form hydrocarbons and oxygenated hydrocarbons, it was observed that changes in operating conditions changed the surface quality of the catalyst and that this change was reversible. When the operating conditions were changed, a corresponding change in the reaction rate occurred. However the reaction rate continued to change until it approached a steady-state value some time later. Thus, the observed kinehcs is a combination of the ordinary reaction kinetics and the kinetics associated with change in catalyst surface quality. The ordinary reaction kinetics for the reaction may be determined by relating the original steadystate value of the reaction rate with the extrapolated ‘Lzero-time”value after a change in the operating conditions. The catalyst kinetics (change in catalyst surface quality) ma be associated with the time-dependent reaction rate. This phenomenon observed for this particular heterogeneous cata&tic system may occur in other systems as well. I n such cases, if the change in catalyst surface quality is not taken into account, the kinetic observations will yield results either greater or less than those which should be associated with the “true” reaction kinetics.
Introduction The synthesis of hydrocarbons and oxygenated hydrocarbons from carbon monoxide and hydrogen over iron base catalysts has been the subject of much investigation; many of the publications have been reviewed by Storch, e t al.1 The reaction kinetics and mechanism of the reaction for this synthesis process have not as yet been satisfactorily determined. Reaction kinetics normally are studied by evaluating the effects of operation variables such as pressure, temperature and reactant concentrations in terms of the active center and activated adsorption (1) H. H. Storch, N. Golumbic and R. B. Anderson, “The FischerTropsch and Related Syntheses,” John Wiley and Sons, Ina., New York, N. Y., 1951, pp. 464-593.
theories of Taylor, Eyring and others. This type of study presupposes a catalyst system in which the catalytic properties, or catalyst surface quality, do not change with changes in operation variables. It is further assumed that product fouling effects and other irreversible catalyst changes are negligible during the time interval required for any given set of experiments, If the catalytic properties are reversibly changed by operating conditions during the time interval required for any measurements, then the observed kinetics become the combined effects of both catalyst kinetics and reaction kinetics. Boudart2 has discussed a basis for catalyst surface quality variation with reaction conditions. (2) M. Boudart. J . Am. Chem. SOC.,74, 1531 (1952).
