INVESTIGATION OF THE PROCESS OF AMMONIA OXIDATION UNDER PRESSURE IN THE PRESENCE OF COBALT–ALUMINUM OXIDE CATALYSTS

N. P. Kurin, M. S. Zakharov

(Presented by Prof. A. G. Stromberg, Doctor of Chemical Sciences)

As N. P. Kurin and P. E. Bogdanov have shown, pure cobalt oxide–cobaltous oxide possesses significant catalytic properties with respect to the reaction of ammonia oxidation under pressure. According to our investigations, aluminum oxide under these conditions catalyzes the process of ammonia oxidation only in the direction of the formation of elemental nitrogen. According to Scott’s experiments [1], carried out at atmospheric pressure, the introduction of aluminum oxide into a cobalt catalyst increases the activity of the latter. In this connection, it was of considerable interest to determine how the catalytic properties of cobalt oxide–cobaltous oxide are changed by the addition of aluminum oxide at elevated pressure \((8 \text{ kg/cm}^2)\), especially in view of the formation of a mixed catalyst from semiconductors of hole conductivity (cobalt oxide–cobaltous oxide) and electronic conductivity (aluminum oxide). For this purpose, cobalt–aluminum catalysts containing 5, 10, and 15% aluminum oxide were investigated.

Since the literature contains reports that in some cases pressing pressure has no effect on the catalytic properties of substances [2], in others it has a positive effect [3, 4, 5], and in still others a negative effect [6], alongside studies of the catalytic properties of unpressed cobalt–aluminum oxide catalysts, the activity was also investigated of catalysts of this system pressed under a pressure of \(800 \text{ kg/cm}^2\), containing 5 and 10% aluminum oxide.

The catalysts were prepared by coprecipitation with a 25-percent ammonia solution from semi-normal (with respect to cobalt) chemically pure nitrates of cobalt and aluminum in the presence of hydrogen peroxide. Before precipitation, the solutions were heated to a temperature of \(70–80^\circ\). During precipitation, the solution with the precipitate was vigorously stirred. The precipitate obtained was filtered and washed at room temperature until the nitrate-ion reaction was negative. The filtered precipitate was transferred to glass and dried at room temperature to a solid state (3 days), and then, over the course of 9 hours, calcined at a gradually increasing temperature up to \(600^\circ\). The calcined substance was crushed, and for investigation a fraction of 1.5–2.0 mm was selected. To prepare pressed catalysts, the mass obtained after calcination was thoroughly ground and pressed in the form of briquettes under a pressure of \(800 \text{ kg/cm}^2\). The briquettes were crushed, and for investigation a fraction of 1.5–2.0 mm was selected. The volume of catalyst in the contact tube was \(23 \text{ cm}^3\), and the bed height was 5.2–5.4 cm. The investigations were carried out with complete oxidation of ammonia in the zone of the contact apparatus. The duration of testing of each of the unpressed catalysts was 200–230 hours, and of the pressed catalysts, 100–150 hours.

Fig. 1. Dependence of the degree of oxidation of ammonia to nitric oxide on the content of aluminum oxides in the catalyst. Pressure of the gas mixture 8 kg/cm². Gas space velocity 22,000 h⁻¹. 1—640°; 2—660°; 3—700°; 4—720°C.

Investigations showed that increasing the content of aluminum oxides in the cobalt catalyst leads to a decrease in the yield of nitric oxide (Fig. 1).

The degree of oxidation of ammonia to nitric oxide in the presence of all catalysts passes through a maximum with increasing ammonia concentration, gas space velocity, and contact temperature.

The optimum contact temperatures in the presence of unpressed catalysts, with an increase in their aluminum oxide content from 5 to 10 and 15%, decrease and reach the following values: 800, 780, and 680°C; while in the presence of pressed catalysts of the first two compositions they are 780 and 740°C. The maximum yields of nitric oxide decrease successively in the presence of unpressed catalysts from 83 to 73.5 and 59.8%, and in the presence of pressed catalysts—from 75 to 55.8%. With an increase in the percentage of aluminum oxide, the optimum space velocities of the gas mixture being fed shift toward lower values. Thus, at a temperature of 760°, with an increase in the aluminum oxide content successively from 5 to 10 and 15% in the unpressed catalyst, the optimum space velocities respectively reach 73,000; 50,000–60,000; and 25,700. In the presence of pressed catalysts of the first two compositions at 760°, the optimum space velocities reached only 16,000 and 12,000.

All the catalysts had practically the same specific surface area (about 14 m²/g), despite the different content of aluminum oxide in them. However, the degree of oxidation of ammonia to nitric oxide decreased rather sharply with increasing aluminum oxide content in the latter. One possible reason for this decrease is the oxidation of ammonia at elevated pressure on the aluminum oxide contained in the surface layer of the catalyst; moreover, the more aluminum oxide is contained—

... in the catalyst, the more of it there will also be in the surface layer of the latter, and, consequently, the greater the fraction of ammonia that will be oxidized to nitrogen.

From these experiments it is clearly seen that introducing into cobalt oxide—cobaltous oxide, which possesses hole conductivity, an additive with electronic conductivity (aluminum oxide) leads to a decrease in the yield of nitrogen oxide; it may be assumed that a decrease in the hole conductivity of the mixed catalyst occurs.

From the data presented above it is seen that unpressed catalysts are considerably more active than catalysts pressed under a pressure of 800 kg/cm², although both had the same specific surface areas. In all probability, in the present case pressing leads to the formation in the catalyst of fine pores, through which the reacting substances cannot penetrate to the entire contact surface, which, naturally, leads to a decrease in the yield of nitrogen oxide. Of course, the decrease in the yield of nitrogen oxide on pressed catalysts may also occur partly as a result of structural deformation of the active centers.

Conclusions

1. The catalytic properties for the reaction of ammonia oxidation under pressure were investigated by the dynamic method for unpressed cobalt–aluminum catalysts containing 5, 10, and 15% aluminum oxide, and for pressed catalysts containing 5 and 10% aluminum oxide.

2. With changes in temperature, space velocity, and ammonia concentration, the degree of oxidation of ammonia to nitrogen oxide passes through maxima and decreases with increasing pressure of the gas mixture.

3. The introduction of aluminum oxide into the cobalt catalyst within the limits studied lowers the degree of oxidation of ammonia at a pressure of 8 kg/cm², which apparently is associated with an increase in the oxidation of ammonia to elemental nitrogen on aluminum oxide.

4. The addition of an electronic semiconductor (aluminum oxide) to a hole semiconductor (cobalt oxide—cobaltous oxide) leads to a decrease in the yield of nitrogen oxide, which is probably due to a decrease in the hole conductivity of the mixed catalyst.

5. Pressing the investigated catalysts under a pressure of 800 kg/cm² lowers their catalytic activity, which apparently is associated with a decrease in the reaction-accessible surface.

LITERATURE

1. Scott, J. Ind. Eng. Chem., 16, 81, 1924.
2. Kurin N. P., Perminov D. F., Izvestiya TPI, 61, 122, 1948.
3. Plotnikov V. A., Ivanov K. N., Pospelov D. A., ZhKhP, 8, 119, 1931.
4. Ivannikov P., ZhOKh, 9, 176, 1939.
5. Bereshchagin L. F., Freidlin L. Kh., Rubinitshtein A. M., Numanov P. U., Izv. AN SSSR, 6, 809, 1951.
6. Strizhkov O. D., Gonkberg M. G., Rubinitshtein A. M., Kazanskii B. A., Izv. AN SSSR, OKhN, 1, 35, 1953.