X-ray and kinetic study of the hydrogen reduction of γ-UO3

J. lnorg. Nucl. Chem., 1960, Vol. 14, pp. 55 to 64. Pergamon Press Ltd. Printed in Northern Ireland

X-RAY AND HYDROGEN

KINETIC STUDY OF THE REDUCTION OF ~,-UO/

K. J. NoTz and M. G. MENDEL Technical Divison, National Lead Company of Ohio Cincinnati, Ohio

(Received 16 February 1959; in revisedform 8 September 1959) Abstract--The hydrogen reduction of y-UO, to UO2 was studied by X-ray examination of quenched samples and kinetic observations over the temperature range, 450-550°C, and hydrogen partial pressures between ¼ and 1 atm. The data were interpreted in terms of three consecutive reactions: UO3 --~ UsOs+, U308+ -~ U3Os-, U808- ~ UO2. In both the first and third steps, two solid phases a r e present; it is postulated that the second step involves the homogeneous transition between the upper and lower limits of the U 3 0 , structure. The rate of reaction is directly proportional to surface area. Rate data obtained for the first and third steps of the reaction are expressed as a function of temperature and hydrogen partial pressure: Rate (per unit surface)= KP"exp(--E[RT). The pressure dependence exponent, n, is approximately 0.8. The activation energy, E, is 25-2 -4- 2"0 kcal/ mole for the reaction, UO3 --~ U308+, and 30-6 ± 1.8 kcal]mole for the reaction, U308- ~ UO2.

THE commercial processing of uranium ore concentrates involves the conversion of uranyl nitrate hexahydrate (UNH) to y-UOa (orange oxide) by thermal decomposition, followed by reduction of this oxide to UO~ with dissociated ammonia at elevated temperatures, tll The reduction reaction is of interest from both a practical standpoint and as a study related to the uranium-oxygen system. Although phase relationships in the uranium-oxygen system have not been fully defined, many of the oxides of multi-valenced uranium have been characterized, and their limits of homogeneous composition established with reasonable reliability. Only a brief summary of this system is given here; for more detailed descriptions, see KATZ and RABINOWITCH(2), GRONVOLD (8) a n d HOEKSTRA a n d SIEGEL (4) a m o n g others. The oxides, UO2, U4Os, UaO~, UsO18(UO~.8), UaOs and UOa, have been substantiated. The upper composition limit of cubic UOz increases with temperature; the composition range extends to UO21 at 500°C t31 and to UO2. 3 at 1160°C ~6~. Cubic U40 s extends over only a narrow domain. Tetragonal UaO~ is unstable above 500°C, disproportionating into U40 9 and an orthorhombic phase, t6~ The lower limit of orthorhombic UsO a has been reported as UOz., at 500°C by GRONVOLD (a), while HOEKSTRA e t al. tT) have found between the composition limits UO2.56 and * This paper is based on the work performed for the Atomic Energy Commission by the National Lead Company of Ohio. tx) D. S. ARNOLD, C. E. POLSON and E. S. NoE, Or. Metals 8, 637 (1956). 12) j. j. KATZ and E. I. RABINOWlTCFI, Radiochemical Studies: The Chemistry of Uranium (Edited by C. D. CORYELL and N. SUGARMANN)NNES, Plutonium Project Record, Div. VIII, Vol. 5, Chap. 11. McGrawHill, New York (1951). t3~ F. GRONVOLD, J. Inorg. NucL Chem. 1, 357 (1955). 14) H. R. HOEKSTRA and S. SIeGAI., Proceedings of the International Conference on The Peaceful Uses of Atomic Energy, Geneva, 1958. Paper No. A/Conf. 15/0/1548. Is~ W. BILTZ and H. MOLI.I~R, Z. Anorg. Chem. 163, 257 (1927).

~6~K. B. ALBERMANand J. S. ANDERSON,J. Chem. Soc. Suppl. 2, 5303 (1949). 171 H. R. HOEKSTRA, S. SIEGEl., L. H. F o c n s and J. J. KATZ, J. Phys. Chem. 59, 136 (1955). See also, S. SIEGEL,

Acta Cryst. 8, 617 (1955). 55

56

K.J. NOTZ and M. G. MENDEL

UO2.85 a n o t h e r o r t h o r h o m b i c phase, UO2. 6 (U5013), which is very similar to UaO s. T h e latter a u t h o r s also r e p o r t e d a metastable, f l - o r t h o r h o m b i c f o r m o f UzO 8 and a b o v e 400°C, a h e x a g o n a l f o r m o f UzO a. T h e " U ~ O s " phase r e p o r t e d b y RUNDLE et aL (81 c o r r e s p o n d s to U5013, m e n t i o n e d above. T h e region, U a O a - U O 3, has been observed as b o t h heterogeneous a n d h o m o g e n e o u s , d e p e n d i n g on the t h e r m a l history o f the sample. (~) U r a n i u m trioxide exists in at least five crystalline modifications a n d an a m o r p h o u s form. U s i n g the n o m e n c l a t u r e i n t r o d u c e d b y HOEKSXRA a n d SIEGELt4), these crystalline forms are ~ (hexagonal), fl ( u n k n o w n structure), 7 ( p r o b a b l y o r t h o r h o m b i c ) , t~ (cubic), a n d e ( u n k n o w n structure). T h e m o s t stable form, 7-UOz, has frequently been referred to as T y p e I I I , a n d is the f o r m used in the study r e p o r t e d here. I n p r e l i m i n a r y kinetic studies o f the h y d r o g e n r e d u c t i o n o f UO3, a variety o f rate types were observed, m a n y o f t h e m i n d e t e r m i n a t e b u t some o f which a p p r o x i m a t e d either linear, phase b o u n d a r y limited o r first o r d e r rates. This diversity o f results suggested t h a t the samples used differed in their physical properties, and t h a t gross particle effects were obscuring the true kinetics. Since a linear rate ( t h a t is, a z e r o - o r d e r reaction) was the least e n c u m b e r e d o f the various types observed, orange oxide samples which exhibited linear rates were used for the X - r a y a n d kinetic s t u d y r e p o r t e d here. MATERIALS AND METHODS 7'-UO3 was prepared by the thermal decomposition of purified uranyl nitrate hexahydrate. Rate data were obtained with sample D-3 ; the X-ray study was conducted with sample C-2. Both samples, as originally prepared, contained nitrate and water (see Table 1). These impurities were driven off TABLE 1.--COMPOSITIONAND

U content (%) H20 content ( ~ ) NO8 content ( ~ ) U308 content ( ~ ) Specific surface area* (m~/g) Mean particle sizet (~) Real density* (g/cm3)

PROPERTIES OF U O a SAMPLES

D-3

C-2

80'65 2"62 0.47 0-20 3"4 0.23 7.73

83-02 0"10 0"26 0-09 2-6 0-30 7-62

* Determined after driving off water and nitrate by heating at 500°C for 30 min. ~"Assuming that all particles exposed to gaseous contact are uniformly sized spheres or cubes, their mean diameter or edge length (d) is calculated as: p(SA) where p = real density SA ~ specific surface area. immediately prior to making a kinetic run or preparing a partially reduced sample by heating for 15 rain in the thermobalance at the temperature of the projected reduction, in an atmosphere of flowing helium. Both samples contained only trace amounts of cationic impurities. Reaction rates were determined with an automatic recording thermobalanee. The sample was suspended from a Gram-atic Model 1-910 balance and the weight was recorded continuously on a ts~ R. E. RUNDLE,N. C. BAENZIGER,A. S. WILSONand R. A. McDONALD,J. Amer. Chem. Soc. 70, 99 (1948).

X-Ray and kinetic study of the hydrogen reduction of ~'-UO,~

57

Schaevitz-modified Brown recorder which sensed the beam deflexion by means of a linear variable differential transformer mounted on the balance. Recorded weights were accurate to -z0.5 mg and had a maximum range of 100 mg. The reaction tube was of quartz, 11 cm in diameter by 43 cm long, heated by a vertically-mounted tube furnace, and controlled isothermally by a West Model JS Stepless Controller. Reaction temperatures were determined with a calibrated thermocouple mounted just below the sample pan. The temperature during a run was constant within ± I°C, and the estimated accuracy was ±5°C. For rate determinations, a 1.600 g sample of D-3 orange oxide screened to --150, +200 mesh size was placed in a 200-mesh platinum gauze basket 5.5 cm in diameter, forming a layer about 0.2 mm thick. After the loaded basket was suspended in the preheated reaction chamber, helium was allowed to flow for 15 min before admitting hydrogen. During this time temperature equilibrium was attained and the nitrate and water content of the UO3 was driven off. The original sample weight was therefore corrected to give the true UO3 weight, 1-552 g. The calculated weight loss for reduction to UO2 is 87 rag. The observed losses were 86-87 mg. A total gas flow of 41/min was used; hydrogen partial pressures of less than one atmosphere were obtained by dilution with helium. Cylinder hydrogen was used directly; the helium was oxygen-gettered and dried by passage over hot UO~, Drierite, and magnesium perchlorate. The X-ray study was conducted with sample C-2 because the D-3 material was no longer available. The C-2 oxide was selected because its reduction curves were very similar to those of D-3. Partially reduced samples were prepared in the thermobalance by reducing to the desired compositions (as calculated after correcting for loss of water and nitrate) at about 400°C with He at a low partial pressure. These mild reduction conditions were necessary in order to halt the reaction at the desired degree of conversion. Samples were quenched in a helium atmosphere by cooling the reaction tube with an air stream. Visual examination of the quenched samples showed uniform colouring throughout the entire bed, thus indicating homogeneous reaction. The composition of the products was determined accurately by calcining a weighed portion to U308 (30 min at 950°C in air). The product composition determined in this manner agreed with the calculated value within 0-02 moles of oxygen and the estimated accuracy was ±0.01 moles of oxygen per mole uranium. X-ray diffraction patterns were obtained by means of a Norelco unit, using nickel filtered Cu radiation and a 114.6 mm camera. KINETICS Investigation of external physical factors that might be expected to affect the observed reduction rate gave the following results: Sample size. F o r 0.5-4 g samples, the rate of reaction per g r a m m e of sample a n d the time required to reach any given fraction of conversion were i n d e p e n d e n t of the sample size. F o r larger samples, heat given off by the exothermic reaction temporarily upset isothermal conditions. Bed depth. Variation of sample bed depth u n d e r 1 m m thickness had no effect on the reaction rate. Gasflow rate. At 1 a t m pressure, increasing the hydrogen flow from 1 to 4 l/rain had n o effect o n the reaction rate. Particle size. F o r a sample of 4 m2/g surface area (equivalent to a mean particle size of 0 . 2 / 0 , identical reaction rates were obtained for gross particle size cuts of 0-5, 5-10, 10-60 a n d 0-105/~. F r o m the above observations, it was concluded that the reduction rates per g r a m m e of U O 3 observed for sample D-3 u n d e r the test conditions employed were i n d e p e n d e n t of sample size, bed depth, gas flow rate and particle size. Since surface area is a p r i m a r y variable in a gas-solid reaction, this factor was investigated considering (1) variations in the surface area of the UO3 starting material, a n d (2) the change in surface area as the reduction progressed. Surface areas were determined by the BET method, nitrogen being used as the adsorbate. F o r orange oxide samples prepared in the l a b o r a t o r y u n d e r carefully controlled

58

K.J.

NOTZ a n d M . G . MENDEL

conditions from a single U N H feed, where the specific surface area of the products was varied by altering the denitration conditions, the reduction rate per gramme was directly proportional to the specific surface area, as shown in Fig. 1.

1.5 - -

~ 1.0

~ 0.5 0~"

I

I

I

I

0

1

2

3

4

SURFACE AREA (rn2/g) FIG 1.--Relationship between surface area and reduction rate of UO3. Reduction rates determined with 1.60 g samples at a temperature of 500°C a n d a hydrogen pressure of I atm.

The specific surface area of a given sample was shown to remain essentially constant during reduction. For orange oxide C-2 with a surface area of 2.6m/g, a partially reduced sample of composition UOz. 81 had an area of 2.1, and for composition UO~.~a the area was 2.2 m2/g. For another orange oxide with .75 m~/g area, the partially reduced products had areas of 5.0 and 5-2 m2/g at compositions UO2.81 and UO2.41, respectively. In both cases, the area decrease at composition UO2.sl was

7

-_

o

I

2OO

400

600

800

TIME (see)

FIG. 2.--Some o f the types o f observed UOs reduction curves.

Curve 1--Pronounced two-step reaction. C u r v e 2 - - N o induction period; contains 2.5 per cent sulphate. Curve 3--Pronounced three-step reaction. C u r v e 4--Extreme induction period; contains 0-5 per cent sulphate. Curve 5--Phase-boundary type. Reduction conditions: Temperature, 500°C; Hydrogen pressure, 1 atm.

about five times greater than the change calculated on a molar volume basis, indicating that some sintering had occurred. The small increase in area of the low oxygen compositions suggested that a limited amount of particle breakdown had occurred. The reduction curves of orange oxides exhibit a variety of shapes; Fig. 2 depicts

X - R a y a n d kinetic s t u d y o f the h y d r o g e n reduction o f y - U O a

59

some of the varieties obtained. Both the manner of denitration of U N H and the use of additives in ppm amounts affect the shape of the curve and the over-aU rate of reaction. Generally, those samples that reduced the fastest exhibited linear rates ~oo

a

~

t

I

I

400

800 "rIME (sec)

] 200

80

i ,° z~ 4o a_ 20

FIG. 3 . - - T y p i c a l

1600

r e d u c t i o n curves o f o r a n g e o x i d e D - 3 .

Curve Curve Curve Curve Curve

1--1 atm 2--1 atm 3---~ atm 4--¼ atm 5--½ atm

H2, Hz, Hz, H~, Hz,

552°C 504°C 504°C 504°C 452°C.

and one or two relatively sudden rate changes. Samples of this type were considered to be more nearly representative of the true reaction kinetics, and the deviations of other types ascribed to particular physical characteristics of those types, such as degree and nature of agglomeration and distribution of particle and crystallite sizes. Typical reduction curves for D-3 orange oxide are shown in Fig. 3. These curves may be generally described as follows, in terms of increasing degree of conversion: UOa-UOz. a: increasing rate UOz.9-UO2. 7: constant rate UO2.7-UO2.6: decreasing rate, with a linear region for the more slowly reacting samples UO2.6-UO2.1_~.2: constant rate UO~.I_2.~-UO2: decreasing rate Kinetically, the reduction was considered to consist of three stepwise reactions: (1) the conversion of UO z to ~-~U~.7 (2) the conversion of ~-~UO2.7 to ~ U O 2 . 6 (3) the conversion of ~UO2.6 to UO 2. The accelerating portion of the first step reflects the approach to a steady state. The tailing during the last part of the third step may be caused by either nonuniform crystalhte size or poor gas accessibility to the centers of some gross particles--it does not seem to be a systematic function of the degree of conversion. The linear regions of the first and third steps, as specified above, were used to determine reaction rates for those steps, in milligrams of weight loss per see per square metre of surface area, and are tabulated in Table 2. Rates are not given for the second step, UO2.7 ~ UO2.s, since the short duration of this reaction, particularly at the higher temperatures, did not permit the accurate determination of rates.

60

K.J. Norz and M. G. MENDEL

The temperature dependence of the first and third reaction steps was determined by least-squares treatment of Arrhenius plots. The activation energies were 25.2 42.0 kcal and 30.6 4- 1.8 kcal for the first and third steps, respectively, at a hydrogen pressure of one atmosphere. The activation energy values obtained at the lower hydrogen pressures were within these 95 per cent confidence intervals. TABLE 2.--REDUCTION RATES OF U O a , SAMPLED-3

Rate (mg/sec per m2 of UO3 surface) First step UO~ "-~ U308 +

Third step: UaO8- ---"UO2

552°C 1 atm ½atm ¼atm

0'256 0'160 0"079

0'141 0'075 0.041

530°C 1 atm

0"172

0"0761

504°C 1 atm ½atm ¼atm

0'103 0"057 0.033

0"0438 0-0252 0.0129

479°C 1 atm

0'0643

0-0221

452°C 1 atm ½atm ¼atm

0'0302 0"0175 0-0107

0.01056 0.00622 0.00381

The pressure dependence of both reaction rates was expressed as an exponential function of the hydrogen partial pressure, Rate ~ P'~. The value of n was determined by means of log-log plots, and was found to be 0.83 for the first step and 0.89 for the third step at 550°C, both values decreasing slightly at 500 °, and then decreasing to 0.75 at 450°C. X-RAY STUDIES Diffraction data were obtained for fourteen samples prepared from C-2 oxide, covering the composition range UO z to UO 2. The X-ray data, which are summarized in Table 3, suggest that 7-UO3 is reduced stepwise, first to a UzOs+ phase, then to a UzOs_ composition, and finally to UO2. The only phases detected at room temperature were 7-UO3, orthorhombie UzO S and a cubic phase. Hexagonal U30 s, fl-UaOs, UsOa3 and U30 ~ were not observed. The X-ray data indicate that the cubic phase was UOz, and not U40 9 or UO2+, throughout its entire range of observation. Arguments against the formation of

X-Ray and kinetic study of the hydrogenreduction of 7-UO3

61

U409 as a stable intermediate are also provided by the observation of the U30 s phase at compositions UO2.~.3 and UO2.13 and by the absence of a rate change at composition UO2.25 during reduction. However, the possibility of a UO2+ phase cannot be ruled out. TABLE 3.--X-RAY DIFFRACTION DATA Phases

Composition* Cubic

U30s

7-U03 x

U02.99

x

x

UO2.~s

x

UO2.s4 UOs.7~ U02.~4 UO,.7o U02.63 UO~.ss U0~.56 U0~.33 U02.~3 U02.~3 U02.o2

x

x x x x x x

x x

(Very weak)

(Veryweak)

x

× (Weak)

x

× ×

x x x

(Veryweak)

x × ×

× indicates t h a t the phase is present. * A s determined by calcination to UaO s. I" Starting material sample C-2. The a p p a r e n t oxygen content of this sample also includes nitrate a n d water.

Since X-ray diffraction is not very sensitive for minor components, the limits of detection of UO 2, UaOs and y-UO3 in admixtures with uranium oxides were determined to provide a basis for correcting the observed compositions where a minor phase was first ot last detected. These lower limits were: 8 % y-UO a in U30 s, 2 % UO z in U30 s and 9 % U30 s in UO2, for samples crystallized at about 500°. Application of these factors to the data in Table 3 corrects the limits of composition of two-phase mixtures as follows: lower limit of y-UO 3 and U30 a, UO2.72; upper limit of U308 and UO2, UO2.59; lower limit for U308 and UO~, UO2.0s. The early appearance of U30 s at a composition corresponding to only 3 % UaO 8 suggests that kernels of U30 s are initially formed on the surface of the UO 3. It also indicates that, under reduction conditions, there is no appreciable region of solid solution of U308 in ~,-UO3 DISCUSSION

On the basis of the X-ray and kinetic data, it is postulated that the hydrogen reduction of ~-UO z proceeds in three consecutive reactions: UO3 ~ U3Os+

(1)

U308+ ---+U308_

(2) (3)

UaOs---~ U02

62

K . J . NOTZ and M. G. MENDEL

The first reaction is the diphasic transformation of ~,-UOa to orthorhombic UsOs+, the upper composition limit of U3Oa. The second reaction is the homogeneous transition from the upper to the lower composition limits of the UaO s structure. The third reaction is the conversion of UaO a_ to a cubic structure. The above reaction sequence requires that either equilibrium or a steady state be attained in the solid phase. This is possible if the rate of reaction is restricted at the surface of the solid, permitting oxygen to diffuse through the solid to the surface as fast as it is abstracted. Rate data, supported by the diffraction data, provide direct evidence that the reduction reaction is limited by the rate of surface reaction. In the simple case where solid A reacts with a gas to yield solid B, the conversion versus time curve is linear only* if the reaction rate is limited by a surface process such as adsorption, chemisorption or chemical reaction at active sites. Other rate-limiting mechanisms (diffusion, nucleation and growth, solid-solid phase interface reaction) yield parabolic, cubic, logarithmic, sigmoidal or other non-linear rate expressions. The various rate laws have been discussed by other authors.(6,9,1°1 In the more complex case, where solid A reacts with a gas to yield first solid B, then solid C and finally solid D, each reaction may be considered independently if each goes to completion in sequence, which the diffraction data indicate occurs in this case. The first reaction, UO3---~ UaOs+, is linear after an initial period of accelerating rate. This induction period probably indicates the time required to form a complete layer of UaOs+ around each UO 3 crystallite, although it might also be caused by the time required for hydrogen to displace helium from all the micropores. The rate of the second reaction, U308+---~ U3Os_, could not be quantitatively defined because of its short duration. However, during some of the slower reductions, this step did exhibit a linear portion. The third reaction, U30 s_ --,- UO~, is linear except for the final part of the conversion. This tailing might be caused by external physical factors, as mentioned before, or by the formation of a continuous series of non-stoicheiometric cubic oxides in the range UO2.1-UO2. Phase data are in agreement with the postulated three-step reaction sequence. The non-solubility of UaO 8 in UO s, at least under some conditions, has been reported, tz~ Bmxz and MOLL~cs~ obtained compositions of the type UaOs+ by decomposing UO 3 at various oxygen pressures. The U30 a_ structure has been established.t3,5,7,TM The upper limit of the cubic UO~ structure is about UOz. x at the temperatures at which reduction was carried out33~ Other reduction studies of uranium oxides also indicate that reduction occurs in a sequence of reactions. GRONVOLI)(alfound that the reduction of UaOs with hydrogen at 330°C yielded orthorhombic UO2.56. ARONSONand CLAYTON(12} concluded that U40 9 was first reduced to UO~+x (0.06 3> x > 0.10) and thento UO2, at temperatures of 400-600°C. DnMARCO and M~NDELt13~ observed that "high surface area" UO 3 * Exceptions can be devised, but they depend upon very special properties: the solid must be in the form of thin platelets, or the reaction must proceed preferentially in one direction through a uniform crosssection. Neither of these exceptions applies in this case. t,~ S. ARONSON, R. B. Root, JR., and J. B~LL~, J. Chem. Phys. 27, 137 (1957). cxo~F. E. MASSOTHand W. E. HENSEL, JR., J. Phys. Chem. 63, 697 (19591. ~xt~ H. HERINO and P. PERIO, Bull. Soc. Chim. Ft. 351 (19521. ~a~ S. ARONSO~ and J. C. CLAYTON,J. lnorg. NucL Chem. 7, 384 (19581. tls~ R. E. DEMARCO and M. G. MENDEL,J. Phys. Chem. 64, 132 (1960).

X-Ray and kinetic study of the hydrogen reduction of y-UOs

63

reduced first to UOs. . and then to UOs. The oxidation of UO s was also found to occur as a two-reaction sequence: UO s--+ U80¢ and U 3 0 ¢ ~ UsOs. (9) The rate of hydrogen reduction of y-UO3 should therefore be considered in terms of the three sequential reactions that have been postulated. The rate of each individual reaction may be expressed by the general equation, Rate (in mg/sec per m s) = KP" exp (--E/RT) where K = proportionality constant, P = partial pressure of hydrogen, n = pressure dependence exponent, E = activation energy. The values of the above constants for the reactions, UO 3 ~ UaOs+ and UaO s_ --+ UOs, are tabulated below: Reaction

UOs--)'Ua08+

U30.- ~ UO,

K

1"12 × lip 0"8 25"2

15"8 × 106 0'8 30.6

n

E

The values of n and E reported by ARONSON and CLAYTON(IS) and DEMARCO and MENDELtlal and the values for K(mg/sec per m s) calculated from their data are listed below, for comparison with the above constants: ARONSON and CLAYTON (O40 s

UOs) Sample

B

C

Surface Area(m'/~ K

1'75 0"266 × 106 0.7 25

0"45 0"162 × liP 0-7 25

n

E

DEMARCO and MENDEL (surface area, 26 mS/g) Reaction

K n

E

UOs'-+UO,.. 1"81

×

0"6 27"2

106

U02.. --* UO, 3620 × 106 0.4 39.6

In a heterogeneous reaction, the surface characteristics of the solid may greatly affect the rate of reaction. Since surface characteristics are notoriously variable, the preceding constants show relatively good agreement for corresponding reactions, with three exceptions. These are: (1) the high activation energy for UOs. . - - ~ U O v

64

K . J . N o r z and M. G. MENDEL

(2) the very large (20,000-fold) variation in K for the reaction leading to UO~ and (3) the low values of n obtained by DEMARCO and MENDEL. No explanation is apparent for the large activation energy. It may have resulted from impurities, tm The large variation in the proportionality constant, K, which includes the factor, number of active sites per unit area, can be explained qualitatively. The U409 samples of ARONSON and CLAYTON were prepared by a sintering and annealing technique which would tend to decrease the number of active sites per unit area, thus decreasing K. DEMARCO and MENDEL started with an active, possibly metastable form of UO s, which on reduction to UO~.56 at low temperatures could easily yield a highly disoriented product with a high concentration of active sites. In the present work, the most stable form of UOa was used as a starting material, but the intermediate oxide was formed in situ and not annealed. Thus, an intermediate value would be expected for K, as was observed. Another factor is the structure of the source material. U40 9 was prepared from cubic UO~. The other oxides were derived from orthorhombic structures. The lower values of n were obtained by DEMARCO and MENDEL at 300-400°C, as compared to 400-600°C by ARONSON and CLAYTON. In the present work, a corresponding variation of n with temperature was noted, the value decreasing from about 0.85 at 550°C to about 0.75 at 450°C. The fractional value of n, and its variation with temperature, are also evidence that the reduction reaction is limited by the rate of surface reaction. LANGMUIR'Streatment shows that as the fraction of available sites occupied increases from zero to unity, n decreases from one to zero. tts~ At lower temperatures, where the reaction rate is slower, a greater fraction of the available sites should be occupied, thereby causing n to decrease, as observed. Based on ROBERTS' work, t18~it is probable that rate of chemisorption of hydrogen on the solid surface is the actual rate limiting step among the various surface processes. For a UO2 surface, he finds that chemisorption occurs at measurable rates above 400°C and that the activation energy is about 35 kcal. He concludes that his "results are compatible with hydrogen adsorption taking place on oxygen sites." Acknowledgements--The authors wish to acknowledge the services of F. FORD for the preparation of X-ray diffraction patterns and Dr. M. T. KELLEY of Oak Ridge National Laboratories for determinations of specific surface area. The recording thermobalance was designed and constructed by J. A. WILLIAMSON. tt4) G. PARRAVANO,J. Amer. Chem. Soc. 74, 1194 (1952). tls) S. GLASSTONE, Textbook of Physical Chemistry (2nd Ed.) p. 1120. Van Nostrand, New York (1946). ttel L. E. J. ROaERTS, J. Chem. Soc. 3939 (1955).