Kinetics of the reduction of U4O9 in hydrogen

J. Inorg. Nucl. Chem., 1958, Vol. 7, pp. 384 to 391. PergamonPress Ltd., London

KINETICS OF THE REDUCTION OF U~O9 IN HYDROGEN S. ARONSON a n d J. C. CLAYTON Bettis Plant, Westinghouse Electric Corporation, Pittsburgh, Pennsylvania

(Received 14 April 1958) Abstract--The reduction of U409 powders in hydrogen was studied at temperatures of 400-600°C and at hydrogen pressures of 20-500 m m Hg. Under these conditions U 4 0 ~is reduced to UO~.02 ±0-0g. X-ray diffraction studies showed that during reduction U409 is converted into the fluorite structure of uranium dioxide. X-ray data on U409 samples reduced at 470°C indicate that the initial reduction product is non-stoicheiometric UO2+, (0.10 > x > 0.06). The rate data were analysed on the assumption that the reaction rate is controlled at the solid-gas interface, i.e., the particle surface. Rate constants and activation energies were calculated. The rate constant k fits an empirical expression of the form k = Kf(p)e -l~lltT, where E is the activation energy, f(p) is an undetermined function of pressure and K is a proportionality constant. The term f(p) is approximately equal to /0o'7; however, the pressure dependence decreases with increasing pressure. A possible mechanism for the surface reaction is discussed. The calculated activation energy for the reduction of U4Oa is 25 ± 3 kcal/mole.

IN the composition range UO2-UO2.25 of the uranium-oxygen system, two thermodynamically stable phases exist, uranium dioxide and U40 9. GRONVOLDhas shown that oxygen is soluble in the uranium dioxide lattice at high temperatures. ~1) According to GRONVOLO,the boundary of the uranium dioxide phase increases from about UO2.oz at 400°C to about UOa.20 at 1000°C. The U409 phase has a relatively narrow range of non-stoicheiometric stability at these temperatures. The uranium lattices in UO~ and U40 9 are similar. UO~ has a fluorite structure with a lattice parameter of 5.470 A for the unit cell (U4Os). The nature of the oxygen lattice in UaO9 has not been established. The lattice parameter of the unit cell (U409), on the assumption that the structure is cubic, is 5.440 A. Several workers have studied the oxidation of UO 2 in air and oxygen.~2,3~ At temperatures of 100-350°C, UO z is o~dized to a tetragonal phase, U307, or, under some conditions, to U40 9. At temperatures above 250°C, UsO7 is oxidized in a second step to UzO s. The rate of oxidation of UO~ to UzO 7 or UaO 9 is controlled by the diffusion of oxygen through the oxide lattice. Oxidation of U307 to UsOs is autocatalytic, indicative of a nucleation and growth process. The present work is a kinetic study of the reduction of U40 9 powder by gaseous hydrogen. The reduction was studied at temperatures of 400-600°C and at pressures of 20-500 mm Hg. EXPERIMENTAL

Apparatus The recording vacuum balance, X-ray and surface area equipment used in this study have already been described in connexion with the kinetic study of the oxidation of UOz. ~z~ I1~ F. GRONVOLD,J. Inorg. NucL Chem. 1, 357 (1955). ~2~j. S. ANDERSON, L. E. J. ROBFRTSand E. A. HARPER,J. Chem. Soc. 3946 (1955). ~a~S. ARONSON, R. B. ROOF and J. B~LLF,J. Chem. Phys. 27, 137 (1957). 384

Kinetics of the reduction of U~Ogin hydrogen

385

Procedure

Before each run, the U~O9 sample (weighing approximately 0-5 g) was evacuated to pressures of 10-~ to 10-8 mm and was flushed several times with helium. It was then heated to the reaction temperature in 300 mm of helium gas. The system was evacuated to 10-3 mm and hydrogen gas was admitted. The use of helium essentially eliminated the slow oxidation of the sample which occurred on heating in vacuo (10-2-10 -3 mm). Pressures up to 400 mm were measured by a Wallace and Tieman absolute pressure manometer; a mercury manometer was used for the measurement of higher pressures. The compressed hydrogen was purified by passing it over palladized asbestos at 400°C and then over magnesium perchlorate. The compressed helium was dried over magnesium perchlorate. Materials

Three U40 9 powders were used in this study. Two of thepowders were prepared from the same uranium dioxide source. The uranium dioxide was prepared from UO3 obtained by pyrolysis of Mallinckrodt Chemical Works uranyl nitrate hexahydrate at 300°C. The UO 3 was ball-milled and reduced to UO~ by heating in hydrogen at 500°C. The two U40 9 powders were prepared by controlled oxidation of this UO 2 preparation to UOz.z5 in oxygen at temperatures below 200°C. One of these powders, designated as powder A, was annealed in vacuo at 800°C for 1 week. The second powder, designated as powder B, was annealed in vacuo at 200°C for 3 weeks. A third U409 powder, designated as powder C, was prepared from uranium dioxide that was obtained from the Matlinckrodt Chemical Works. The MCW UO2 was oxidized to UO~.~z in the manner indicated above and was annealed at 800°C for 1 week. The surface areas of powders A and B were 0"73 m~/g and 1"75 m~/g, respectively, whereas that of the uranium dioxide powder from which they were prepared was 2.4 mZ/g. The surface areas of powder C and the MCW UO2 powder were 0.45 m~/g and 0"54 m~/g respectively. The annealing process thus caused a reduction in surface area, especially in the case of powder A. X-ray diffraction patterns and chemical analyses of the three powders showed that they were U409. The compositions of the oxide samples calculated from the weight changes during reduction were checked on some samples by chemical analyses. The O/U ratio calculated by the two methods agreed to -¢-0.01. RESULTS

AND

DISCUSSION

Typical rate curves for the reduction of the three U409 powders in hydrogen at constant pressure and temperature are shown in Fig. 1. The forms of the rate curves obtained for the three powders were fairly similar over the range of temperatures and pressures studied. However, the rate curves obtained with powder B were generally more nearly linear than those obtained with the other two powders. Weight changes on the samples indicated that the reduction product was UO~.02±o.0~. The limits of :]:0.02 on the composition of the reduced samples refer to the possible experimental error in determining the composition from the weight change. No consistent variation of composition with temperature or pressure was observed. Surface area measurements were made on samples of powders A and C reduced at 410°C. The values obtained on the two reduced samples were 0.80 m~/g and 0.45 mS/g, respectively. Thus, little particle breakdown occurred during reduction. 6

386

S. ARONSONand J. C. CLAYTON

X-ray diffraction powder patterns were taken at room temperatore of samples of powders A and C that were partially reduced at 470°C. Similar results were obtained on both powders. A mixture of U40 a and UOz+= was observed on partially reduced samples of compositions above UOz.z0. The subscript x, the value of which is less than 0.10, is used to indicate the possibility that the uranium dioxide phase is not 500co- 500ram Hg

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FIo. 1.--Typicalrate curvesfor the reductionof U~O9in hydrogen. stoicheiometric. The relative amount of the UO~_, phase increased with the degree of reduction. The U409 phase was not detected in samples of composition below UOz.zo. It thus appears that the reduction process initially converts UaO9 into nonstoicheiometric UO2+~ (0"10 > x > 0.06). The oxygen content of the latter phase is then reduced until a composition of UO2.0z±0.02 is attained. The wide limits for the value of x in the initial reduction product are given to take into account the possibility that the presence of 10--15 per cent of the U409 phase may be required for its appearance in the X-ray patterns. The X-ray data are in qualitative agreement with the limit of about UOz.0e for the boundary of the UOz+~ phase at 470°C given by GRONVOLD.(1) For the kinetic interpretation of the data, it will be assumed that penetration of the UO z lattice by hydrogen is not an important consideration. The work of ROBERTSt4) on the chemisorption of hydrogen on UO z indicates that some absorption into the lattice may occur; the rates of absorption appear to be low however. A reasonable reduction mechanism comprising three processes can be postulated for the reduction of U4Oo; (1) the reaction between hydrogen and lattice oxygen at the particle surface with the formation of water; (2) the transformation of U409 to UO2+= at the interface between the phases; and (3) the diffusion of oxygen, released at.this interface, through the UO~+~ phase to the solid-gas interface, i.e. the particle surface. For simplicity, the possible non-stoicheiometry of the product phase is temporarily neglected. The fonrt of the rate curves indicates that the diffusion process is not rate controlling. The rate curves are not parabolic in form. In agreement with this conclusion ~a~L. E. J. ROBERTS,J. Chem. Soc. 3939(1955).

Kinetics of the reduction of U,O0 in hydrogen

387

are the data from the study of the oxidation of UO2t2,a~ that predict a very rapid rate of diffusion of oxygen in the UO~+x phase at temperatures above 400°C. The equation that would be applicable if the interface reaction, the conversion of U40 9 to UO~+x were rate controlling is, in the case of uniform spherical particles, t~ (1 -- c) 1/3 = 1 - - ( k i t / r e )

(1)

where c is the fractional weight loss at time t with values from 0 to l, kl is the rate constant, and re is the particle radius. If reaction at the particle surface is rate controlling, since the surface area and gas pressure remain constant, the simplest mechanism would give a constant reaction rate in the case of a powder of uniform particle size. A simple rate equation can be written in this case: c = k2t/r o (2) The forms of the rate curves obtained with the three powders were intermediate between the theoretical curves predicted by equations (1) and (2). It is difficult to determine from the form of the experimental rate curves which mechanism is more applicable. Other evidence, however, indicates that the surface reaction is of primary importance in the reduction kinetics. The reaction rate was found to vary with the 0.5-0.9 power of the hydrogen pressure, as is discussed below. It is difficult to reconcile this relatively strong pressure dependence with a mechanism in which the rate controlling step is the conversion of U40 a to UOz+~ at the phase interface. Also, the relatively large degree of non-stoicheiometry of the initial reduction product, indicated by the X-ray measurements, can probably best be explained on the assumption that the surface reaction is slow compared with the kinetic processes in the solid. The process of homogenization within the solid would result in a high concentration of non-stoicheiometrie oxygen in the uranium dioxide phase. The uranium dioxide fluorite structure is capable of absorbing a relatively large concentration of oxygen presumably into interstitial sites in the lattice, cl,2~ For the quantitative treatment of the data, it was assumed that the mechanism corresponding to equation (2) is applicable. Possible reasons for the deviations from linearity of the rate curves are discussed below. Rate constants were determined for the initial slopes of the plots of weight loss versus time. The plots were fairly linear up to c values of 0.7-0.8 for powder B and up to c values of 0-4--0.6 for powders A and C. Rate constants were calculated in terms of the number of moles of oxygen released per square centimetre of surface per second. The surface area values given previously were used. Plots of log k versus 1 / T at hydrogen pressures of 20, 100 and 500 mm are given in Figs. 2-4 and the activation energies calculated by least-squares analyses are listed in Table 1. Figs. 2-4 also give some indication of the dependence of reaction rate on hydrogen pressure. Reaction rates at two additional pressures were determined for powders A and C at temperatures of 470°C and 500°C, respectively. The variation of the rate constant with pressure is shown in Fig. 5. The dependence of reaction rate on hydrogen pressure was calculated from least-squares plots to be approximately 0"7. However, the pressure dependence of the reaction rate decreases with increasing pressure. ~s~ p. W. M. JACOBSand F. C. TO~n'KINS, Chemistry of the Solid State (Edited by W. E. GARNER)Chap. 7. Butterworths, London (1955).

388

S. ARONSON and J. C. CLAYTON

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TABLE 1.~CALCULATEDACTIVATIONENERGIESFOlKTHE REDUCTIONOF U40 9 TO UO2

Powder typ~

Hydrogen pressure (mm Hg)

Activation energy (kcal/mole)

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The symbol a represents the adsorbed state, O(l)'represents lattice oxygen ions in excess of the composition UO~ at the particle surface and O-H* represents an oxygenhydrogen complex of unknown stability.

39O

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The assumption that hydrogen decomposes into atoms on the surface of the solid, expressed in equation (3), is suggested by the fact that the dependence of the reduction rate on pressure is a fractional power less than one and that the dependence approaches 0.5 at the higher pressures. This fractional dependence is most easily accounted for by assuming hydrogen decomposition. Using the proper assumptions and including at least one of the inverse reaction steps in the mechanism, it is possible to derive rate equations which are in reasonable agreement with the data. Quantitative interpretation will not be attempted, however, since the proposed mechanism requires further substantiation. The above discussion does point up the fact that the reduction of U409, like many surface reactions, is complex and the rate is probably controlled by more than one elementary process. The experimental activation energy is probably a composite activation energy involving more than one reaction step. Considering the probable complexity of the reaction, it is interesting that the temperature dependence of the reduction rate is experimentally independent of pressure. It has been assumed in the discussion that the reaction rate is controlled by a surface reaction occurring at a constant rate. The experimental rate curves, however, deviate considerably from linearity in the latter stages of the reaction. A reason for this deviation is that the real powders are not composed of uniform spherical particles as assumed but are distributed in size and shape. The smaller particles are consumed earlier in the reaction causing a decrease in the surface area effective in the reduction. The gradually decreasing surface area leads to a gradually decreasing reaction rate. Another possible reason for the decreasing reaction rate is the fact that a non-stoicheiometric product is initially formed on reduction. The rate of reduction may be controlled at least partially by the rate of abstraction of oxygen from the solid. After the U409 phase is entirely consumed, the concentration of oxygen in UO~+~ would decrease gradually, possibly causing a decrease in reaction rate. Another

Kinetics of the reduction of U~O~ in hydrogen

391

possibility, which appears less likely however, is that an accumulation of adsorbed water formed on reduction impedes the reaction. I f the reduction rates were purely characteristic of the chemical nature of the solid, the calculated rate constants and activation energies would be independent of the type of U409 powder used. The differences in these values among the three powders probably are due to differences in the nature of the solid surfaces. The variability of solid surfaces is a well known phenomenon.

Acknowledgements--Thiswork was performed under contract AT-11-1-GEN-14. The authors are grateful to the U.S. Atomic Energy Commission and the Westinghouse Electric Corporation for permission to publish this paper. The authors wish to acknowledge the very able assistance of R. T. PARKSin carrying out the experimental work and wish to thank Dr. R. B. Root, JR. for his help in interpreting the X-ray patterns. The authors also wish to acknowledge the helpful discussion of this work with Dr. J. BELLE.