EFFECT OF POTASSIUM, DEPOSITION TEMPERATURE, AND PRESSING PRESSURE ON THE CATALYTIC PROPERTIES OF COBALT OXIDE–COBALTIC OXIDE IN THE OXIDATION OF AMMONIA TO NITRIC OXIDE UNDER PRESSURE

N. P. Kurin, R. K. Taranenko

(Presented by the scientific seminar of the Department of Silicate Technology)

The development of active and sufficiently stable non-platinum catalysts for the oxidation of ammonia under pressure is of substantial industrial and scientific interest.

The catalytic properties of cobalt oxide–cobaltic oxide in the reaction of ammonia oxidation to nitric oxide at atmospheric pressure have been studied by a number of authors [1, 2, 3]. According to M. I. Temkin and co-workers [3], the degree of oxidation over $\mathrm{Co_3O_4}$ when operating with a pure ammonia–air mixture under laboratory conditions is close to 97%.

One of the authors of the present communication, together with co-workers [4], showed that the catalytic properties of $\mathrm{Co_3O_4}$ in the oxidation of ammonia under pressure depend on the method of preparation. Thus, $\mathrm{Co_3O_4}$ obtained by thermal decomposition of salts is lower in activity than $\mathrm{Co_3O_4}$ obtained by precipitation with caustic potash. In this connection, it was of considerable interest to determine the effect of potassium content on the catalytic properties of this oxide. The present communication gives the results of a study of the effect of potassium content, pressing pressure, and deposition temperature on the catalytic properties of $\mathrm{Co_3O_4}$ in the reaction of ammonia oxidation under a pressure of $8\ \mathrm{kg/cm^2}$.

Table 1

Physicochemical characteristics of the catalysts

To study the effect of potassium content, six catalysts (obtained by precipitation at 95°C) were prepared according to [4]. The catalyst was studied in a flow apparatus. The degree of oxidation was determined by the evacuated-flask method [6]. The potassium content of the catalysts was analyzed by flame photometry on a UM-2 universal monochromator. The specific surface area of the catalysts was determined from air adsorption [7]. The physicochemical characteristics of the catalysts are given in Table 1.

Results Obtained and Their Discussion

The catalytic properties of Co₃O₄ as a function of potassium content were studied in the temperature range 650–800°C, with an ammonia content in the ammonia–air mixture of 9.4–10.3 vol.%, at space velocities ranging from 10,000 to 80,000 h⁻¹. Effect of space velocity and temperature. In the presence of the catalysts studied, the degree of oxidation of ammonia to nitric oxide ($\alpha$), as a function of space velocity ($V$) at constant temperature ($T$), pressure ($P$), ammonia concentration ($C_{\mathrm{NH_3}}$), and potassium content ($C_{\text{potassium}}$), passes through a maximum. Such a dependence can be traced using the example of a catalyst containing 0.046% potassium (Fig. 1). From the figure it follows that, for each temperature, there is its own optimum space velocity. Raising the temperature from 650 to 800°C leads to a decrease in the optimum space velocities from 50,000 to 20,000 h⁻¹ and in the degree of ammonia oxidation from 76.3 to 60.7%.

Fig. 1. Dependence of the degree of oxidation of NH₃ to NO ($\alpha$) on space velocity in the presence of Co₃O₄ + 0.046% K. $P = 8$ kg/cm²; temperature: 1 — 650, 2 — 700, 3 — 750, 4 — 800°C.

The dependence of the degree of ammonia oxidation on temperature for a catalyst containing 0.046% potassium is shown in Fig. 2. As is seen from the figure, for low space velocities (10,000–30,000 h⁻¹) the optimum temperature is 700°C. Increasing the space velocity from 40,000 to 80,000 h⁻¹ lowers the optimum temperature to the region of temperatures below 650°C. Raising the temperature from 750 to 800°C leads to a decrease in the degree of ammonia oxidation for all the space velocities studied.

Effect of potassium on the activity of Co₃O₄. The degree of oxidation of ammonia in the presence of Co₃O₄ as a function of potassium content and tempe-

Fig. 2. Dependence of the degree of oxidation of \(\mathrm{NH_3}\) to NO \((\alpha)\) on temperature in the presence of \(\mathrm{Co_3O_4} + 0.046\%\ \mathrm{K}\).
\(P = 8\ \mathrm{kg/cm^2}\); space velocities: curve 1—10000, 2—20000, 3—30000, 4—40000, 5—50000, 6—60000, 7—70000, and 8—80000 h\(^{-1}\).

Fig. 3. Dependence of the degree of oxidation of \(\mathrm{NH_3}\) to NO \((\alpha)\) on the potassium content in \(\mathrm{Co_3O_4}\).
\(P = 8\ \mathrm{kg/cm^2}\), \(T = 700^\circ\mathrm{C}\); space velocities: curve 1—10000, 2—20000, 3—30000, 4—40000, 5—50000, 6—60000, 7—70000, and 8—80000 h\(^{-1}\).

temperature is characterized by a complex dependence. At constant space velocities and temperatures of 650–700°C, the degree of oxidation of ammonia to nitric oxide decreases as the potassium content in Co₃O₄ increases from 0.046 to 0.15%, then increases when the potassium content is raised to 1.76%, and, with a further increase of this additive to 5.74%, decreases again (Fig. 3). A similar picture is also observed at a temperature of 800°C, with the only difference that, in the potassium-content range 0.046–0.106%, there is a certain increase in the degree of oxidation of ammonia to nitric oxide and thus two maxima appear on the curves \(\alpha = f(C_{\text{potassium}})\) (at 0.106 and 1.76% potassium). At 750°C the first maximum on the curves \(\alpha = f(C_{\text{potassium}})\) shifts toward a higher potassium content (0.15%). Consequently, one and the same low potassium content, when the temperature is changed, affects the catalytic properties of Co₃O₄ differently. Thus, if for a catalyst containing 0.046% potassium the maximum degree of oxidation of ammonia to nitric oxide is observed at 650–700°C, then at 800°C the maximum shifts to a catalyst containing 0.106% potassium. Such a dual character of the action of small additives was observed in [8–11] in the study of catalysts in various reactions.

An increase in the potassium content in Co₃O₄ from 1.76 to 5.74% gradually lowers the degree of ammonia oxidation at all space velocities and temperatures to the limits characteristic of Co₃O₄ obtained by thermal decomposition of cobalt nitrate, on which the degree of ammonia oxidation did not exceed 50% [4].

An increase in the potassium content leads to an increase in losses in catalyst weight during the study (Table 1). Thus, while the fresh catalysts differ in potassium content by 2 orders of magnitude, the spent catalysts contain it within the limits of one order of magnitude (Table 1). The removal of potassium can be explained by interaction of the nitric oxide formed during oxidation with potassium hydroxide according to the reaction [12]:

\[ 4\mathrm{KOH} + 6\mathrm{NO} = 4\mathrm{KNO}_2 + \mathrm{N}_2 + 2\mathrm{H}_2\mathrm{O} \]

and by subsequent evaporation of the resulting \(\mathrm{KNO}_2\), since the latter, at the temperatures of the experiments, has a certain vapor pressure.

As follows from [13], the amount of potassium remaining in the catalysts probably enters into chemical interaction with Co₃O₄, with formation of a potassium cobaltite of the spinel type, \(\mathrm{K}_2\mathrm{Co}_2\mathrm{O}_4\).

An increase in the potassium content from 0.046 to 5.74% leads to an increase in the specific surface area of fresh catalysts calcined at 600°C from 31.4 to 47.5 m²/g. The specific surface area of most catalysts spent at a temperature of 800°C decreases to 2.6–4.6 m²/g.

The productivity of a unit surface area with respect to NO \((\text{cm}^3/\text{m}^2\cdot\text{h})\) was calculated from the formula:

\[ A_{\mathrm{NO}}=\frac{V_k V C_{\mathrm{NH}_3}\alpha}{S_k}, \]

where \(V_k\) is the catalyst volume, cm³;
\(V\) is the space velocity, h⁻¹;
\(C_{\mathrm{NH}_3}\) is the content of NH₃ in the ammonia–air mixture, vol. %;
\(\alpha\) is the degree of oxidation of ammonia to nitric oxide, %;
\(S_k\) is the total surface area of the catalyst, m².

Calculations show that the productivity of a unit catalyst surface area with respect to NO at 650°C increases with increasing space velocity, but as a function of potassium content it has a minimum (at 0.15% potassium) and two maxima (at 0.046 and 1.76% potassium), with an overall tendency to decrease as potassium is increased. However, if the productivity maximum at 650°C corresponds to the catalyst containing

0.046% potassium, then at 800°C the maximum is observed for the catalyst initially containing 1.76% potassium (0.0402% in the spent catalyst).

The productivity of a unit surface area with respect to NO at 800°C is considerably higher (by a factor of 3.25–9.2) than the analogous value characteristic of 650°C, which is explained by the increase in the reaction rate with temperature.

Effect of the deposition temperature and pressing pressure. In [14] the Co(OH)\(_3\) precipitate is assigned a crystalline structure, whereas in [15] it is stated that Co(OH)\(_3\) particles do not have a noticeably expressed crystalline lattice and should be regarded as gels. It is also noted here that lowering the deposition temperature leads to the formation of small Co(OH)\(_3\) particles, which do not coarsen during subsequent heating. In this connection we prepared three Co\(_3\)O\(_4\) samples according to [4], precipitated at temperatures of 40, 70, and 95°C.

Fig. 4. Dependence of the degree of oxidation of NH\(_3\) to NO (\(\alpha\)) on the deposition temperature of cobalt hydroxide. \(P = 8\) kg/cm\(^2\), \(T = 650^\circ\)C, space velocities: curve 1—10000, 2—20000, 3—30000, 4—40000, 5—50000, 6—60000, 7—70000, and 8—80000 h\(^{-1}\).

As our experiments showed, increasing the deposition temperature of the initial form of the catalyst (cobalt hydroxide) from 40 to 95°C substantially lowers the catalytic activity of Co\(_3\)O\(_4\) in the oxidation of ammonia under pressure (Fig. 4). The specific surface area of fresh Co\(_3\)O\(_4\) does not depend on the deposition temperature of the initial cobalt hydroxide and is equal to \(\sim 23\) m\(^2\)/g. The bulk density, both for fresh and for spent catalysts, decreases with increasing deposition temperature.

The productivity of a unit bulk volume of catalyst with respect to NO at low space velocities (10000–20000 h\(^{-1}\)) and 650°C is practically independent of the catalyst deposition temperature; at higher space velocities and temperatures of 650 and 800°C, it decreases with increasing deposition temperature, and the more so the higher the space velocity.

Table 2

Pressing catalysts in some cases does not change the catalytic properties; in others it has a positive or negative…

influence [5, 16–20]. We prepared 5 samples of Co₃O₄ pressed under different specific pressures (Table 2).

The specific surface area of freshly calcined catalysts at 600°C decreases continuously as the pressing pressure is increased from 0 to 5000 kg/cm². As for the degree of oxidation of ammonia to nitric oxide, it is a more complex function of the pressing pressure, having a minimum at 1250 and a maximum at 3750 kg/cm² (Fig. 5). With an increase in temperature from 650 to 800°C, the sharpness of the indicated minima

Fig. 5. Dependence of the degree of oxidation of NH₃ to NO (α) on the pressing pressure of Co₃O₄. \(P = 8\) kg/cm², \(T = 800^\circ\); space velocities: curve 1—10000, 2—20000, 3—30000, 4—40000, 5—50000, 6—60000, and 7—70000 h⁻¹.

and maxima increases. The maximum degree of oxidation of ammonia to nitric oxide was: 81.3% in the presence of unpressed Co₃O₄ and 82.1% for Co₃O₄ pressed at 3750 kg/cm². Thus, for the process of ammonia oxidation under a pressure of 8 kg/cm², the optimum pressing pressure of Co₃O₄ is 3750 kg/cm². At this pressure Co₃O₄ is more productive and stable than the unpressed catalyst.

It is seen from Table 3 that, with an increase in temperature from 650 to 800°C, the productivity of a unit surface of the catalyst with respect to NO increases at all the space velocities studied. The exception is the catalyst pressed at a pressure of 1250 kg/cm², for which, at space velocities of 40000–70000 h⁻¹, a decrease in the productivity of a unit surface is observed. As follows from Table 4, the catalyst pressed at a pressure of 3750 kg/cm² is the most resistant to temperature change in the range 650–800°C; for this catalyst,

the decrease in productivity per unit bulk volume does not exceed 15%.

Table 3

Ratio of the productivity of 1 m² of catalyst surface obtained at 800°C to the analogous value at 650°C ($n$)

Table 4

Ratio of the productivity of 1 cm³ of catalyst bulk volume obtained at 800°C to the analogous value at 650°C ($m$)

Conclusions

1. The introduction of potassium additive within the investigated limits does not give, in terms of activity, industrially acceptable cobalt catalysts for the oxidation of ammonia to nitric oxide at a pressure of 8 kg/cm². It is possible that sharp modification of Co₃O₄ occurs at potassium concentrations below 0.046%.

2. Raising the precipitation temperature of the initial form of the catalyst (cobalt hydroxide) from 40 to 95°C substantially lowers the catalytic activity of Co₃O₄ in the oxidation of ammonia to nitric oxide under pressure.

3. The degree of oxidation of ammonia to nitric oxide is a complex function of the pressing pressure of Co₃O₄. The optimum pressing pressure of Co₃O₄ is 3750 kg/cm².

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