Chemistry

V. M. ZHUKOVSKII, V. G. VLASOV

INTERACTION OF URANIUM TRIOXIDE WITH CRACKED AMMONIA IN THE PRESENCE OF ADDITIONS OF CERTAIN FOREIGN OXIDES

(Presented by Academician A. A. Balandin, 22 VIII 1963)

It is known that the presence of small amounts of foreign impurities during the reduction of metal oxides can sharply alter the course of the reduction process \((^1)\). The starting \(\mathrm{UO_3}\) was prepared by decomposition of uranium peroxide \((^2)\); it had a density of \(6.5\ \mathrm{g/cm^3}\) and a specific surface area of \(15\ \mathrm{m^2/g}\). The reducing agent was a nitrogen–hydrogen mixture \((3H_2 + N_2)\), obtained by cracking ammonia over a nickel catalyst. The investigation was carried out in a high-vacuum circulation-type apparatus by the method of continuous weighing. The additions used were chemically pure-grade ZnO, \(\mathrm{TiO_2}\) with a content of foreign impurities of not more than 0.5%, and \(\mathrm{UO_{2.04}}\), obtained by reduction of chemically pure uranium sesquioxide—oxide. In all experiments, additions of oxides to \(\mathrm{UO_3}\) were introduced in an amount of 5 mol. %. The additions were introduced by careful grinding and mixing with the starting uranium trioxide. The pure \(\mathrm{UO_3}\) used in the control experiments was prepared in the same way. Before each experiment, weighed portions of the starting materials were dried in vacuum at \(380^\circ\). The effect of temperature on the rate of reduction of pure \(\mathrm{UO_3}\) was studied in the range \(350–500^\circ\) at \(P_{3H_2+N_2} = 200\) mm Hg (Fig. 1). Under analogous conditions, the effect of the ZnO addition was studied (Fig. 2). The remaining experiments were carried out at \(400^\circ\). As is seen from Figs. 1–3, additions of ZnO and \(\mathrm{TiO_2}\) accelerated the reduction of uranium trioxide. The greatest increase in rate was caused by the ZnO addition. The presence of \(\mathrm{UO_{2.04}}\), however, somewhat slowed the process. Values of the apparent activation energies of the reduction process of \(\mathrm{UO_3}\) by cracked ammonia for different degrees of reduction are given in Table 1. They were determined from the Arrhenius equation by a graphical method.

Fig. 1. Reduction of \(\mathrm{UO_3}\) at various temperatures \(P_{3H_2+N_2}=200\) mm Hg (in %/min). \(1—350^\circ\), \(2—400^\circ\), \(3—425^\circ\), \(4—450^\circ\), \(5—500^\circ\mathrm{C}\)

Table 1

Note. Complete conversion of \(\mathrm{UO_3}\) to \(\mathrm{UO_2}\) was taken as 100% reduction.

Every chemical reaction ultimately has an electronic mechanism. Of considerable interest are attempts to apply modern concepts of solid-state physics and the electron theory of adsorption and catalysis to the study of the mechanism of topochemical reactions \(^{(3-6)}\). Although the cited works concern processes of thermal decomposition of various ionic compounds, S. Z. Roginskii believes \(^{(3)}\) that a similar electronic mechanism is naturally to be expected also for reduction reactions of metal oxides and sulfides. The physical essence of this mechanism is that, all other conditions being equal, the rate of reduction of metal oxides will be governed by the concentration of free electrons on the surface of the oxide being reduced. From this point of view, it was of interest to investigate the influence of such additives which themselves do not interact chemically either with hydrogen or with uranium trioxide, but at the same time, possessing a definite type of conductivity, may affect the concentration of free electrons in the conduction band of the lattice of the oxide being reduced.

Fig. 2. Reduction of a mixture of \(\mathrm{UO_3} + 5\) mol. % \(\mathrm{ZnO}\) at various temperatures and \(P_{3H_2+N_2} = 200\) mm Hg (in %/min).
\(1\)—350°, \(2\)—375°, \(3\)—400°, \(4\)—425°, \(5\)—450°, \(6\)—475°, \(7\)—500° C

Fig. 3. Reduction of \(\mathrm{UO_3}\) in the presence of additions of foreign oxides. \(P_{3H_2+N_2} = 200\) mm Hg, \(t = 400^\circ\mathrm{C}\) (in %/min). Additives: \(1\)—\(\mathrm{UO_{2.04}}\), \(2\)—without additive, \(3\)—\(\mathrm{TiO_2}\), \(4\)—\(\mathrm{ZnO}\)

\(\mathrm{ZnO}\) and \(\mathrm{TiO_2}\) were chosen—semiconductors with electronic conductivity. Their reduction begins to proceed at an appreciable rate at 700 and 1050°, respectively. As a third additive, the oxide \(\mathrm{UO_{2.04}}\) was taken—a semiconductor with hole conductivity. In studying the electrical properties of uranium oxides \(^{(7)}\), it was established that the oxides \(\mathrm{UO_3}\), \(\mathrm{U_3O_8}\), \(\mathrm{U_3O_7}\), and \(\mathrm{U_4O_9}\) are semiconductors with an excess of metal, i.e., of the \(n\)-type, whereas the oxide \(\mathrm{UO_{2+x}}\), where \(x\) varies from 0 to 0.13–0.16, is a semiconductor with a deficiency of metal, i.e., of the \(p\)-type. The electrical conductivities of systems whose total composition varies from \(\mathrm{UO_{2.67}}\) to \(\mathrm{UO_{2.25}}\) remain approximately constant throughout this entire range of compositions and are several orders of magnitude higher than the electrical conductivity of \(\mathrm{UO_3}\). Measurements carried out in our laboratory showed that at 200° the specific electrical conductivities of \(\mathrm{UO_3}\) and \(\mathrm{U_3O_8}\) are, respectively, \(1 \cdot 10^{-8}\ \Omega^{-1}\cdot\mathrm{cm}^{-1}\) and \(1 \cdot 10^{-2}\ \Omega^{-1}\cdot\mathrm{cm}^{-1}\).

Guided by the principle of the sequence of chemical transformations of Acad. A. A. Baikov \(^{(8)}\) and by the phase diagram of the uranium—oxy-

oxygen (⁹), one may expect the following scheme of phase transformations during the reduction of uranium trioxide:

\[ \mathrm{UO_3 \to U_3O_8 \to U_3O_{8-x} \to U_4O_9 \to UO_{2+x} \to UO_2.} \]

X-ray phase analysis of the reduction products was carried out by the powder method using copper \(K_\alpha\) radiation in RKD cameras with a cassette diameter of 57.3 mm. The initial trioxide was X-ray amorphous. As the degree of reduction increased, lines of the crystalline structure of orthorhombic \(\mathrm{U_3O_8}\) appeared on the radiographs and became increasingly intense. Lines of other crystalline phases up to the composition \(\mathrm{UO_{2.55}}\) were not detected. Apparently, simultaneously with the reaction of reduction of \(\mathrm{UO_3}\) to \(\mathrm{U_3O_8}\), the sesquioxide is enriched with oxygen, which ends with the formation of the oxide \(\mathrm{UO_{2.55}}\), which is the lower limit of the solid solutions \(\mathrm{U_3O_{8-x}}\). Upon further removal of oxygen from \(\mathrm{U_3O_{8-x}}\), new lines of cubic structure appeared, while the intensity of the lines of the \(\mathrm{U_3O_{8-x}}\) structure began to decrease. It should be noted that uranium oxides over the entire composition range from \(\mathrm{UO_2}\) to \(\mathrm{U_4O_9}\) possess a very similar crystalline structure, since the formation of the phases \(\mathrm{U_4O_9}\) and \(\mathrm{UO_{2+x}}\) occurs through ordered (in the first case) and disordered (in the second) incorporation of oxygen into the face-centered cubic lattice of uranium dioxide. The oxides \(\mathrm{UO_2}\), \(\mathrm{UO_{2+x}}\), and \(\mathrm{U_4O_9}\) can be distinguished on radiographs only by the last lines at large reflection angles. The observed blurring of the cubic-structure lines at large reflection angles may indicate that reduction at this stage proceeded nonuniformly, i.e., mixtures of \(\mathrm{U_4O_9}\) and \(\mathrm{UO_{2+x}}\) were formed. When the composition of the oxide phase reached \(\sim\mathrm{UO_{2.25}}\), the lines of uranium sesquioxide disappeared completely. Reduction ended with the formation of nonstoichiometric uranium dioxide \(\mathrm{UO_{2+x}}\). In radiographing samples containing additions of foreign oxides, no deviations from the picture described above were found. The formation of any chemical compounds of the main oxide with the oxide additives was not observed under the experimental conditions considered, even when the main oxide and the additive were taken in equimolar amounts.

In our studies, the process of reduction of \(\mathrm{UO_3}\) to \(\mathrm{U_3O_8}\) proceeded autocatalytically, which is associated with a change in the reaction surface. In our view, the following should additionally be borne in mind. The \(\mathrm{U_3O_8}\) arising during the reduction process is in close contact with the oxide being reduced. The electronic conductivity of uranium sesquioxide is several orders of magnitude higher than that of the trioxide. At the contact between them, a difference in the electrochemical potentials of the charge carriers arises. This leads to the transfer of some electrons from the \(\mathrm{U_3O_8}\) lattice into the \(\mathrm{UO_3}\) lattice. To check the correctness of this assumption, an additional experiment was carried out. A briquette was prepared consisting of two separate layers of trioxide and uranium sesquioxide, tightly pressed under a pressure of 2700 kg/cm², and the e.m.f. of the \(\mathrm{UO_3}\)—\(\mathrm{U_3O_8}\) pair was measured. At 400° it proved to be equal to 53 mV. The sign of the e.m.f. corresponded to the transfer of electrons from \(\mathrm{U_3O_8}\) to \(\mathrm{UO_3}\). An increase in the concentration of free electrons in the conduction band of the oxide being reduced \((\mathrm{UO_3})\) could lead to an increase in the rate of the crystallochemical transformation.

It was indicated above that the introduction of additions of \(\mathrm{ZnO}\) and \(\mathrm{TiO_2}\) noticeably accelerated, while the presence of \(\mathrm{UO_{2+x}}\) slowed, the reduction reaction of \(\mathrm{UO_3}\) to \(\mathrm{U_3O_8}\). At the same time, the presence of \(\mathrm{ZnO}\) had practically no effect on the rate of reduction of \(\mathrm{UO_{2.56}}\) to \(\mathrm{UO_{2.25}}\). The apparent activation energy also did not depend on the presence of the \(\mathrm{ZnO}\) additive. The following explanation of the observed facts appears possible. The oxides \(\mathrm{ZnO}\) and \(\mathrm{TiO_2}\) are \(n\)-type semiconductors, and their electrical conductivity is considerably higher than that of uranium trioxide. In places of close contact between the oxide being reduced and the additive, there may ...

electron transfer can occur from the additive ($\mathrm{ZnO}$ or $\mathrm{TiO_2}$) to $\mathrm{UO_3}$, which leads to an increase in the rate of reduction of uranium trioxide. To test the validity of this assumption, we measured the e.m.f. of the $\mathrm{UO_3}$—$\mathrm{ZnO}$ pair. At 400° it proved to be equal to 240 mV. On the other hand, the oxide $\mathrm{UO_{2.55}}$ has sufficiently high electronic conductivity; therefore it is difficult to expect that the introduction of small amounts of $n$-type semiconductors will substantially affect the overall rate of its reduction. An admixture of the oxide $\mathrm{UO_{(2.04)}}$, which has hole conductivity, can increase the probability of recombination of free electrons with positive holes. As a result, the concentration of free electrons decreases, and the reduction of $\mathrm{UO_3}$ to $\mathrm{U_3O_8}$ is slowed.

Ural Polytechnic Institute
named after S. M. Kirov

Received
21 V 1963

CITED LITERATURE

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