USING THERMIONIC EMISSION TO STUDY REDUCTION REACTIONS OF METAL OXIDES

V. S. Arkhipov, S. I. Smolyaninov

(Presented by the scientific seminar of the organic departments of the Faculty of Chemical Technology)

Among modern methods for studying chemical reactions in the solid phase, increasing attention is being devoted to the use of various electrophysical effects. For example, for reactions of thermal decomposition of solid inorganic compounds, the correlation of electrophysical characteristics with decomposition kinetics is no longer in doubt [1].

Alongside thermal decomposition reactions, electrophysical effects are used to study reduction reactions of metal oxides. Thus, Yu. S. Yusfin and A. A. Yusupkhodzhaev applied measurements of electrical conductivity during heating and reduction of iron oxides [2]. In the works of V. P. Elyutin et al. [3], a correlation was established between the electrical conductivity of an oxide and the onset of the reduction reaction of oxides by solid carbon.

Since practically important reduction reactions proceed at fairly high temperatures, it is expedient to use the phenomenon of thermionic emission for their study. The ability of a solid body to emit electrons at high temperatures is characterized by the electron work function. The work function is an electronic surface characteristic of solid substances and depends both on the chemical structure of the substance and on the state of its surface [4]. Measurement of the work function is widely used to study the properties of the surfaces of emitters, catalysts, and semiconductors [4, 5, 6]. The high reduction temperatures and the semiconductor character of oxides create the prerequisites for reliable measurement of the electron emission current. Obviously, the flux of thermoelectrons will be closely connected both with adsorption processes on the oxide surface and with the crystallochemical stage of the reaction; therefore, measurement of the emission current can characterize the kinetics of the reduction process.

Measurement of thermionic emission is especially effective for studying the mechanism of solid-phase reduction processes if the method of separate placement of the components is used [6]. In this case, it becomes possible to study dynamically the stage of mass transfer between the reacting substances, as well as processes on the surface of the reagents. The advisability of carrying out such experiments is determined by the shortcomings of existing research methods, which, as a rule, are based on recording the overall result of a multitude of elementary reactions occurring between particles of solid carbon and the oxide; at the same time, the most important processes of mass transfer between the reac-

remain inaccessible for study, which does not allow the results of the investigation to be interpreted unambiguously.

The methodology and technique for recording thermionic-emission currents have been developed sufficiently well. Usually, an extracting electric field is applied between the electron emitter and the collector, and the resulting current is measured with a galvanometer [4]. However, in the study of chemical reactions, an electric field can introduce distortions into the course of the reaction [6]; therefore it is expedient to use the initial-current region of the current–voltage characteristic, when no external voltage is present [3]. On the basis of the considerations presented, the measurement scheme may be represented as follows: the reacting substance is enclosed in a shielded reaction volume; in accordance with the chemical changes of the reagent, a thermionic-emission current flows between the shield and the reagent, which is measured with a sensitive electrometer; knowing the emission properties of the initial substance and of the reaction products, the data obtained can be used to describe the kinetics of the process. For experimental development of the proposed method, it is intended to be used to study the reaction of solid-phase reduction of iron oxides.

REFERENCES

1. Kabanov A. A. “Uspekhi khimii,” 1971, No. 11, pp. 2029–2046.
2. Yusfin Yu. S., Yusupkhodzhaev A. A. In: “Moscow Institute of Steel and Alloys.” Moscow, “Metallurgiya,” 1969, No. 50, pp. 71–79.
3. Vinitsyn V. P., Pavlov Yu. A., Shchedrin S. B. In: “High-Temperature Materials.” Moscow, “Metallurgiya,” 1968, pp. 23–45.
4. Dobretsov L. N., Gomoyunova M. V. Emission Electronics. Moscow, “Nauka,” 1966.
5. Rudnitskii L. A., Ivanov M. M. Kinetics and Catalysis. Vol. II, No. 2, 1970, pp. 403–412.
6. Enikeev E. Kh., Margolis L. Ya., Roginskii S. Z. DAN SSSR, Vol. 124, 1959, No. 3, pp. 606–608.