Jan., 1962
REACTION BETWEEN URANIUM
~IYDRIDE AND AMMONIA AT R O O M TEMPERATURE 145
Hzas the neutral fragments, accounts for the largest peak in the tetrahydrothinphene spectrum; 208 kcal./mole i s calculated for the heat of formation of this ion, using the above process. If CzH4is
taken a8 the neutral fragment, AHf’(C2H4S) = 249 kcal./mole is obtained. A value of 224 kcal./ mole was obtained for the heat of formation of CzfIB+ in ethylene sulfide.s ‘Cf the value of AHf+ (CzHB) = 207 kcal./mole is chosen, it would indicate that the CZI&S* ion in the ethylene sulfide spectrum quite possibly retains its cyclic structure. m/e = 70.-This ion in the pyrrolidine spectrum can be due only to the CqHsNf ion. As a consequence of the abstraction of a hydrogen from the parent molecule ion, a value of 200 kcal./mole is calculated for AHf+(C4H8N). m/e = 71.-Ionization and dissociation of tetrahydrofuran to C*H70+and H is responsible for the ion a t mass 71; 161 kcal./mole is calculated for the heat of formation of this ion. m/e = 87.-Dissociation of the ionized tetrahydrothiophene to C & 3 + and H leads to a heat of formation of C4H7S+of226 kcal./mole. Table VI11 summarizes the “best values” of heats of formation for a number of ions whose values have not been reported previously, It is quite possible that CIHsO+, C3H,N+, C4H7S + and C&S+ retain a cyclic structure and, since each determination of the heats of formation of these ions was obtained from the parent molecule ion or the
parent minus a hydrogen ion, the values reported here possibly may differ somewhat from their straight chain counterparts. Combinations of these values of AH*+ with AHf(radica1) allows an estimation of the ioiiiaation potentials of the radicals. Table VI11 also summarizes a few of these values. HEATSOB FORNATION
TABLE VI11 IONIZATIOX POTENTIALS OF RADICALS
OF I O N S AND
Fragmenfi
Ai%+, kcal./mole
I.P. of radiortl (e.v.1
CaH4N GHaN CsH7N CaH60
255 256
7.6
CHS CHZS CzHaS GHa8
Ci”S CrH&3
225 193 271
230 248 207 226
190
.. .. .,
6.8 6 7
9.1
..
..
..
Acknowledgments.-The authors wish to thank Dr. R. N. McDonald for his aid in obtaining the purified azetidine sample with the spinning-band column. We also gratefully acknowledge the aid given by B. G. Hobrock in obtaining the data presented in Table I.
THE REACTION BETWEEN URANIUM HYDRIDE AND AiM11IONIA AT ROOM TEIYIPERATURE’ BY MOHAMMED ALEI University of California, Los Alamos Scientific Laboratory, Los Alamos, New Mexico Receiued August ii’, 1981
The reaction between NET, and UH3 a t room temperature has been found to proceed very rapidly to form essentially a monolayer o f nitride. Thereafter, the rate drops off rapidly according t o a logarithmic rate law. The kinetic data are quite well interpreted on the basis of the theory of P. T. Landsberg which postulates chemisorption of a reactant as the ratecontrolling atep.
work, the change in weight of a sample of U1J3exposed to NH, was used as a measure of the amount of reaction which had occurred. It is quite possible, therefore, that a reaction limited to the surface of the UH, particles might have gone unnoticed. Experimental
Introduction It has been known for some time2that UIT3reacts with NH, gas a t elevabed temperatures to produce mixtures of uranium nitrides. In the reference cited, no I-eaction was observed a t room temperature. At temperatures above 200”, however, a rapid reaction took place to form uranium nitride of composition between UNBand U7K3. A very stable nitride, U V , also was reported. It was formed by heating the higher nitrides of any composition to temperatures above 1300” i n vacuo. The purpose of the present work was to determine whether or not there might be sufficient reaction between UH, and ISHa a t room temperature to form a surface layer of nitride. In the previous
The apparatus was designed to permit measurement of the surface area and observation of the reaction with NH8 on a single sample of UIL. The UH, was prepareda in situ h? reaction of Nz with electropolished uranium metal a t -300 . Surface areas were determined by applying the B.E.T.4 method to Ny adsorption iaotherms measured a t liquid nitrogen temperature. The reaction with NH3 was studied by admitting a measured amount of pure XH8 into the system and circulating the gaa continuously through the URs sample a t room temperature (23’). Since preliminary experiments indicated that NIT3 was being consumed and Hz
( I ) This work performed under the auspices of the United States Atomic Energg Comniiasron. (2) R. E. Rundle, N. C Baenziger, A. S. Newton. A. H Daane, T. A. Butler, I. B. Jones, Pi. Tucker and P. Figard, Chemistry of Uranium, Collected Papws. TID-5290, Book 1, paper No. G, p. 53, August 1945
(3) J. J. Katz and E. Rabinowitch, “The Chemistry of Uranium,” 1st Edition, MoGraw-Hill Book Co., New York, N. Y.. 1951, pp. 186207. (4) S. Rrunauer, P. H. Emmett and E. Teller, J . Am. Chern. Soc., 60 309 (1935).
o aoil
1'
0 70
TIME, MINUTES
Ihg 1 .
formtd, tho course of the reiictiori w a follomrd by ptriodicnlly measuring tho total m s w r c in fkr q a t r m and thcn trecaing out tho NIIs wit11 / i q d NI to mcntiiirc the prcssura of HX in tlic yysle~ri Witli tlicsr prc~niirrsniid thc ltriown volumc of tho w s l r ~ i i ,11 wa.9 pnmhlr t o cnlculato the rimouiils of NIIS nntl 111 in thr gn.i plmc M u function of t h o, Tlie gweb used $11 tliw work wrrc: ?;TI,, Hz, N t nnd 1Ie. Tho NII, u w c*o~iiiiiei r i ~ 1~11 l hydroua rnlltcriul further put ilied by diatillatioii fiom sodium. l h c HT, Nzand 110 were tnnk jiwes, l;lirificd by pxmgc ovrr hot rcduced Cu, hacariir
:/nd RI&(Cl
smoothly with timc. 111orcovcr, ~ u c ethe Isit(> 111i9 hccomc slow, replackg the gas p h a v T1-it.h piiw &I&does iiot restore the initial rapid rutv. T h ~ s clearly dcmonstrntcs t h u t ret ardatioii of tlle i r l ~ t ~ a l rapid rcactioii is uot due l o b~iild-~ip of I T a in t h c gas phase. It also is appnrcnt irnmcdjatolj. from Ihg. 1 and from the f:ml th:il the total yiussiire 111 tho systom iiicrcascs with tune tliat more than oil(' mole of 112 is formed for cmh mnlc of S H ? colisumed One may, in faot,, calculate for ew11 P O I I I I , nn oithrr of the curves in lhg. 1, the mmnlcs of' h 1 1 3 coiisumed and the mmoles of HsCoriiird in the same time interval. If a plot then is made of mmolcs H2 forrnccl us. niinoles NIIg con~~irned, n vrry good straight line is obtained. This iiidicates that the stoichiometry of the rcuct,ion was coubtarit dnring the\ time in which experiirienIx1 points wcro taken. Moreover, the slope nf thr lint: rcprcsciita the number of mmolcs of H, formed per nirriole of ?;HJ aonsumrd. The slopes obtained in wwn sepurate rxpeiiiiicrits are shoi.i.ri in Tahlc T. In all cases 1he valucs rcportcd U F O lrasl-syuureb sloprs with 95% cmlitlciicc lirnics. 'I'ARIA fiTOICIrlO\fCTnY
1
u&-KEJa
A'r
1 t L A L 1 ION
Hoo\f
r ,
1 k h I P h K A l UiRI?
Slop0
1 1 7 forincd -NIL consuincd
_ _Total JTI f o r i i i ~ l 'rotal VTT,
2 I 4 i 0.08 2.10 rk .14 2 13 + 1 0 2 09 f , I S 2 , 2 1 Zk . l o 2 03 f 1 0 1 9.2 f .I5
,)g
Results aud Discussion Surface Area of UH,. Surface areas were dctcrmincd on 20 different ssiiiiples of TJH,. In grncral, tlic surface areas fell into two c:itcgories. UHs sampleq, prcpnrcd hy h p l y uIlo\liing 1-12 to react to cornplot ion with urxiiiuin met a1 nt tcnipcrnt,iirrs of -300" or lower, had surface arouw raiigi~igfiom 0 3 to 0 A m */g. Samples prepared by hydriding thc r n r t 21, decoiiiposirrg the hydridc by piiiiipiiig, a i d theii rehydriding thc findy dividcd iuetal, all at -;300°, had siirftice arem 1angi11g irom 0 8 to 1 . I m.2/g. Duphcate determinations on thc samc saniple 01UIIa were in agrccmcnt to within 7%. To determine whothcr or iiot additional cycles of decomposing arid rp-forming wniild fiirthcr increase the surface area of UTI3,a. snmplc was subjected to l'oiir such e y d r s . Its surface awa was found to 0 8 ni.t/g., indicating that a practical uppcr h i t to the surfacc area of uII3 prcparcd in this way is -1 m.*/g. Assuming uniform spheres, this corresponds to particles -0.6 p in diameter. Stoichiometry of the UH3-NH3 Reaction.-A typical reaction between UH3 arid NH3 a t 23" is depicted in Fig. 1, which shows the composition of the gas phase as a function of time. The break at) 300 minutes reprcscnts pumping out the NH3-112 mixture present in thc gas phase at that time and introducing a frcsh sample of pure NIL. It is apparent from this figure that thc reaction is charact erized by a very rapid early stage follo~ed hy a s l o w r s t a g in which the rate decreases
Or TIIC
2
(I
coiisii~irt
!)
a x 2.8 2 3 2 5 2.5
2 7
If, at the coiiclusioii of rz rnnm ~ r i n p r r : ~ t iiicrw~-~tiori experiment, thc gas phase is yiiiiipeil (ill and t he solid product heated to -Y0Oo 111 thc c ~ l o ~ d s p t e I ~ i1,1 1s l o ~ i i dthat a 11~cn~11r:lhl~ ~111ou1il 01 112 is Iiheratcd This H q it: not r(I&oibed on cooliiig thc solid 10 ~ ' o o ~Icnipcraluir ii If OIIC addt: tliis a m o u n l UT 1-18 to the total I L lihrralctl during the room-tempcmtnrc rcnrtion 011 the same inatc~inl and coynptms tliis sum tt it11 the total NTL cnnsiimcd during the room-trmperaturc ~ r t w ion, 1 0 1 1 oblaiiis ~ the ratios listcd in tho socond coluiiii of 'I'i~bleI. wrlirr, the t h e e known nitritks of urailium LW U S Z , UzNa and TJK. The forinatlon or each of these componuds by iwction of UITJ with XIJis iiidicalcd by the equations lwlow. I Iz formedRoartion
+ 2KH3 --+ US2 + 4.5€1* + 3NH3 + + 7.5142
UH3 2UH3 UH3
UJ3
+ NH.9 + UN + 3H2
S l I s consumed
2.25 2.50 3 00
The expcrimcntally obtained ratios thus iiidicatc. that the room temperature rmction betmen UHa and KH3 generally is forming UX,, U2S3 or nitrides intermediate between these compositions. Furthcrmore, the process of heating these nitrides to -300" in the presence of excess UH3 is tending to bring about coiiversian to a lower nitride, c.g.,
+
31TKz UI-1, +2u&;3 1.5& 1THa -e 3ux 1- 1.5112
1-1N3
+
The observed room-tempesnture stoichiometry thus, in general, agrees with the fitoi~hiometryfor formation of known nitrides. There appear, hocvever, to be some real differences in stoichiometry of reactions proceeding 011 diff erent UHB samples. The reaeon for this is not understood. Kinetics of the UHB-NH8 Reaction.-Data of the type shown in Fig. 1 are fit very well by a kinetic expression of the form
where k and to = constants t = time [H,] = mmoles Hzin gas phase a t time t [SH,] := mmoles KH3 in gas phase at time t [NH8Io-= mmoles NB, originally introduced
The data of Fig. 1 and similar data obtained in t1q-o other experiments are plotted in the above form in Fig. 2 . I n each case, the surface area of the UE, sample was measured before its reaction with
O
0
L
'
0.10
'
0,eO
'
0.8.2
'
0.40
109
[
'
0.50
'
' 0.70
0.60
M0( ] . It t i t ,
I _
0.80
0.QO
1.00
1.10
I
Fig. 2.
of initial rapid reaction and compares this amount of reaction with the surface a~rea.of the UH3 sample,
it is quite conceivable that the rapid process represents reaction between IVH3 and bare UH, surface to form essentially a monolayer of nitride. Sub"3. Direct logarithmic rate laws of the type observed sequent reaction then involves chemisorption of here have been encountered frequently in studies NH, on a nitride surface as the rate-determining of the growth of oxide films on meta1s.j Theoreti- step, If such an interpretation is valid, the intercal mechanisms which lead to such laws generally cept should, in each case, be proportional to the postulate a reaction whose rate is determined by total surface area of the UEIBsample. That this is diffusion through thin films, pores or cavities. A so is shown by the constancy of the rat,io, intercept/ notable exception is the treatment of Landsberg,6 V,, in Table 11. The intercepts, slopes, and values who derives a direct' logarithmic rate law assuming of to are, in each case, the best values determined by that chemisorption of a react)ant is rate controlling. the method of least squares. The limits indicated In attem1pting t o decide which of the above mech- represent the standard deviation in each case. anisms might conceivably be operative in the Tho quantity V , is the volume of Nz required to present work, it would be important to have an esti- form a monolayer on the surface in the B.E.T. surmate of the thickness of nitride films produced by face area determination and is therefore a measure the observed reaction of NH3 with UH3. In the of Che total surface area of the UH:, sample. initial rapid reaction (first IO min.) for the study TAB&& I1 shown in Fig. 1,-0.2 mmole of XH3was consumed. Assuming a product of composition UN2, this XH3 CORRELATION OF S U R r A C E AREA WITH KINETIC DATAIN U&-N& REriCTION AT R O O M TEMPERATCR~ consumption corresponds to formatlionof 0.1 mmole or 27 mg.of UKZ. The X-ray density2 of UK2 is Experiment 1 2 3 1.06 0.46 0 71 11.73 g . / ~ m . ~ Hence, . t'he volume of nitride V,, cc. Ns S:T.P. Specific surface formed is 27 >c 10-3/11.73 = 2.3 X IOvs area. u, m.a/g. 0.30 0.99 0.44 The data obtained in Fig. 1 were taken on 4.7 g. [NHalo 0.979 1.044 1.137 of UHBhaving a surface area of 1 sq.m./g. Hence €lope 0 . 3 6 4 h 0.008 0 . 1 5 3 I 0 . 0 1 4 0.281 1 0 . 0 2 5 0.251 zk 0.006 0,078 & 0.007 0.134i 0.011 the total surface area of the solid is 4.7 X lo4 em2. Int'ercept o 23.8 f 7 , 3 29.7 4 ~ 7 . 7 1 6 . 4 4 1.5 Assuming t8hatthe volume of nitride is uniformly tIntercept/ V, 0.24 0.17 0.19 distributed over this area, the average thickness of Slope/ VIrt INI-hlo 0.30 0.34 0 38 the nitride film is 2.3 X 10-a/4.7 X IO4 cm., or loIW€Ialnu'/Q 18.6 15.5 17.0 5 k. Since the lattice constant for the face-centered When the theory of Landsberg is applied to the cubic UX2 is a. -. 5.3 A2, such films of nitride can data in Fig. 2, the slope of the line should be given, be at most, a few atom-layers thick, With films as thin as t'hese, it seems very unlikely in each case, by the expression that a diffusion process would be rate controlling. Ic'Vrn(NH2)Q Slope = One thus i s led to the conclusion that the reaction observed here is one whose rate is controlled by where chemisorpt'ion of NHa on a nitride film. In this k' = a constant connection!,it should be noted that the lines in Fig. b = av. no. of sites invalidated by itdsorpt,ion of it single 2 do not extrapolate to the origin. This indicates KH, molecule that the init,ial reaction is much more rapid than dictated by the logarithmic law which applies to If b is constant from one preparation of UH, t,o ~, be the data between 10 and 300 min. If one uses the another, then the ratio, ~ l o p e / V , [ S H ~ ]should intercepts of these lines as a measure of the amount constant. Table II shows that this condition is met very well. (5) U. R. Evans, "The Corroeion and Oxidation of Metals," EdFinally, in Landsberg's theory applied to the ward Arnold Ltd., London, 1960, pp. 829-837. present problem, to is given by (6) P. T. Landsberg, J . Chern. Phus., 28, 1079 (1955).
14s
A. PKBLCR AXD W.E. WALLACE
where C’ = a constant = effective area of contact between an NH3 molecule and the surface upon collision sn = sites per unit area a t time zero
a
We already have assumed b constant and found that this correlates well with the observed slopes. Moreover, u ought to be very nearly the crosssectional area of an NHa molecule and independent of the UH, surface. One thus would conclude that to should be proportional to l,/[il;H3]0so. From the data in Table 11, it can be seen that whereas the slopes and intercepts vary dircctly with toid surface area (as meas-
Vol. 66
ured by Vm), to varies in an inverse fashion with specific surface area. This suggests that so is directly related to the specific surface area, (r. I n fact, if one arbitrarily assumes so cy ~ ‘ 1 2 , then the product to[XH3]0a l l 2 should be constant for all experiments. The bottom line in Table I1 shows that the constancy is indeed quite good. This suggests that the number of sites per unit area is inversely proportional to the square root of the average particle diameter. Acknowledgment.-The author wishes to express his appreciation to Dr. J. F. Lemons of this Laboratory for his active interest and helpful direction in this work. He also wishes to thank Dr. Walton P. Ellis of this Laboratory for helpful discussions concerning the interpretation of some of the results of this work.
CR17STAL STRUCTURES OF SOME LANTHANIDE IIYDRIDES’ BY A. PEBLER A I ~ DW.E. WALLACE Dcpartment of Chemistry, University of Pittsburgh, Pittsburgh i3, Pa. Received Auguat 26, 1961
Hydrides of Pr, Nd, Sin, Tb, Dp, Ho, Er, Tm, Lu and Y have been formed and examined using X-ray diffraction techniques. The dihydrides of the several metals always were observed to be cubic. Appreciable contraction of the lattice occurs when PrH2, NdHl or SmHzabsorbs extra hydrogen. The PrHz and NdHz phases remain cubic to the highest. h.ydrogen concentration attainable, whereas the Sm-H system undergoes a transformation such that the Sm ion cores are in a cph arrangement for hydrogen concentrations exceeding that corresponding to the formula SmHl.se. Similar transformations are observed for the several heavy lanthanides studied and for yttrium, the range of stability of the dihydride being, however, considerably less than that for the alloys based on SmHs. The dependence of the lattice spacing on composition is discussed in terms of the presumed electronic nature of the lanthanide hydrides. The observed contraction of the dihydride lattice is consistent with the notion (suggested by their electrical and magnetic behavior) that the lanthanide hydrides are essentially saline in nature and the hydride ion is formed by absorption of electrons from the conduction band of the metal.
Introduction For many years it has been known that when the lanthanide metals are exposed to an atmosphere of hydrogen at elevated temperatures, reaction occurs and hydrogen is incorporated in the solid, forming what usually are termed “hydrides.” Several investigations have been carried out for the purpose of elucidating the structural features of these hydrides. Using conventional X-ray diffraction techniques the arrangement of the metal atoms or ions has been established in several cases and in one instance (the Ce-H system) the hydrogen atoms or ions were located using neutron diffraction data. To date attention has been focussed largely on the light lanthaiiides La, Ce, Pr, Nd and Sm. I n the elemental state Ce is fcc, Pr and Nd are cph, La is cph with a doubled c spacing and Sm is rhombohedral. However, each of these is observed2 to form a dihydride MeHz in which the arrangeinent of the metal ion cores is fee.+' These dihydrides have been found to possess the ability to (1) This work w3s assisted by a oontrart with the U. 6. Atomic Energy Commis.ion. (2) A. Rossi, iVature, 133, 174 (1934). (3) B. Dreyfuss-Alain, Compt. r e n d , 236, 540, 1295 (1952); 236, 1265 (1953); 237, 806 (1953). (4) R. N. R. Mulford and C. E. Holley, J . P h y s . Chem., 69, 1222 (195 5 ) . (5) C.E. Holley, 61’ al. *hid 59, 1226 (1955). (6) K. Dialer and W. Rothe, 2. Elektrochem., 69, 970 (1955). (7) JJ Stalinski. B i d acad polon. SCI. 3, 613 (1955)
absorb considerable additional hydrogen without alteration of the structure of the metallic matrix. Also, it was ascertained that to a limited extent hydrogen can be removed from the dihydride. It thus is clear that the phase based on the dihydride stoichiometry embraces a considerable range of composition, which in La-H and Ce-H extends6$’ to the trihydride composition MeH,. Neutron diffraction work on Ce& showed5 the H’s to reside in the tetrahedral interstices; thus this material possesses the fluorite structure. A similar study of a sample of composition represented by the formula CeH2.7 showed6 that all the tetrahedral interstices were filled and the additional H’s were randomly distributed in the octahedral interstices. Although direct supporting experimental evidence is lacking, i t generally is believed that (1) the situation is similar for the La, Pr and Nd dihydrides, with and without additional hvdrogen, and (2) in those hydrides in which H/Ce < 2.0, the H’s are distributed randomly over the tetrahedral sites. In 1956 Sturdy and Mulford conducted what appears to have been the first and only Investigation of a heavy lanthanide-hydrogen system--the Gd-H system.* They found by X-ray diffraction, which of course detects only the metal ion (8) G. E. Sturdy and R. N. R. Mulford J . Am Chem SOC.7 8 , 1083 (19.56I .
