INVESTIGATION OF THE PROCESS OF AMMONIA OXIDATION UNDER PRESSURE IN THE PRESENCE OF IRON–BISMUTH–MANGANESE CATALYSTS

N. P. Kurin, P. E. Bogdanov

(Presented by Prof. L. P. Kulev, Doctor of Chemical Sciences)

Modern production of nitric acid is based on the method of ammonia oxidation in the presence of expensive platinum catalysts. It is known that certain oxides and their mixtures, such as $\mathrm{Fe_2O_3}$, $\mathrm{Bi_2O_3}$, $\mathrm{MnO_2}$, $\mathrm{Co_3O_4}$, and others, have been tested as catalysts at atmospheric pressure on an industrial scale [1, 2].

In view of the fact that a significant part of modern nitric-acid plants operates at elevated pressures on the order of 6–8 kg/cm², and that there is a tendency toward the construction of new plants for processing ammonia under pressure [3, 4], the study of the contact properties of non-platinum catalysts for ammonia oxidation at elevated pressure is of special industrial and scientific interest. In the literature [1, 2] there are only indications that these catalysts are unsuitable for operation under pressure, since they become extinguished. Systematic studies of the activity of various catalytic systems undertaken by the staff of the Department of Technology of Inorganic Substances of the Tomsk Polytechnic Institute show that non-platinum oxide catalysts operate stably under pressure, and that comparatively high yields of nitric oxide can be achieved under laboratory conditions.

The aim of the present investigation is the further study of the contact properties of the catalytic system $\mathrm{Fe_2O_3 + Bi_2O_3 + MnO_2}$ for the oxidation of ammonia at elevated pressures.

Preparation of catalysts

According to D. A. Epshtein and N. M. Tkachenko [5], iron oxide with additions of 20–40% $\mathrm{Bi_2O_3}$ possesses the greatest activity in the oxidation of ammonia under atmospheric pressure.

N. P. Kurin, P. E. Bogdanov, and R. K. Taranenko found that the best yields of nitric oxide at a pressure of 8 kg/cm² are obtained in the presence of a contact containing 80% $\mathrm{Fe_2O_3}$ and 20% $\mathrm{Bi_2O_3}$. The composition of this catalyst was taken as the basis by N. P. Kurin, N. I. Sus’kin, and I. I. Yakovlev in studying the system of iron–bismuth–manganese catalysts. However, the latter investigations were not carried through to completion and were limited only to the study of the indicated catalyst system with an $\mathrm{MnO_2}$ content of up to 10%.

by weight. In this connection, our investigations are a further study of the system of iron–bismuth–manganese catalysts at higher contents of MnO₂. The iron–bismuth portion of the catalysts¹ was prepared by the method of coprecipitation of hydroxides from a one-normal solution of nitrate salts, with respect to iron, with 24% ammonia water at a temperature of 75–80°. The precipitate was filtered off and washed with hot distilled water until a negative reaction for the NO₃ ion with diphenylamine was obtained.

Active MnO₂ was prepared by the method of decomposition of potassium permanganate with nitric acid [6].

The required weight quantities of iron–bismuth paste and manganese dioxide paste were thoroughly mixed and subjected to natural drying for 24 hours. The catalyst was then dried at 40, 60, and 80°C for 2 hours at each temperature and for 3 hours at 105°C. The samples were then heated at 200 and 400° for two hours each and at 550–600° for three hours.

A fraction of 1.5–2 mm was charged into the contact apparatus. Some of the catalysts were tested in pressed form. The mechanical strength of the prepared catalysts (Table 1) was characterized by a crushing load of 6–8 kg/cm².

Results obtained and their discussion

The study of the catalysts was carried out by a dynamic method [7] at a pressure of 8 kg/cm². The degree of oxidation of ammonia to NO was determined with the aid of evacuated bulbs [1].

Effect of space velocity

In the presence of the investigated iron–bismuth–manganese catalysts, the degree of oxidation of ammonia (α), as a function of the space velocity (V) at constant temperature (t), pressure (P), and ammonia concentration (\(C_{\mathrm{NH_3}}\)), passes through a maximum. Let us trace this dependence using as an example a catalyst containing 12% MnO₂ (Fig. 1, в). In the interval 600–760°C, at a pressure of 8 kg/cm² and \(C_{\mathrm{NH_3}} = 10.6\%\) (by volume), the optimal space velocities lie within the range 18000–22000 h⁻¹. The degree of oxidation of ammonia at these space velocities reached 80–84%. With a further increase in V, (α) begins to decrease and, finally, at V = 30000, slip of ammonia through the catalyst layer is observed.

An analogous dependence is also observed in the presence of the other iron–bismuth–manganese catalysts (Fig. 1, а, б, г, д, е, ж, з, и, к, л). The values of the optimal space velocities for the catalysts studied do not differ very greatly from one another. One may note only the appearance of a certain maximum in the optimal space velocity as the manganese dioxide content in the catalyst increases. This maximum, at all the temperatures studied, is observed at a content of 12% MnO₂ in the catalyst.

The maximum degree of oxidation of ammonia to nitric oxide was attained in the presence of this catalyst at V = 20000, \(C_{\mathrm{NH_3}} = 10.6\%\) in the initial mixture, and amounted to 84.0%.

Under conditions of optimal space velocities, the catalysts operated steadily for a long time (see Table 1).

¹ According to chemical analyses, the content of Bi₂O₃ proved to be about 22%.

Fig. 1. Dependence of the degree of oxidation of NH₃ to NO on space velocity at a pressure of 8 kg/cm². Oxide iron–bismuth–manganese catalysts with MnO₂ content, wt. %: a — 5%; b — 9%; c — 12.0%; d, e — 13.8%; f — 17.2%; g, h, i, k — 18.3%; l — 19.8%.

Vertical axis: degree of oxidation of NH₃, %.

Horizontal axis: space velocity · 1000.

Effect of Temperature

For the iron–bismuth–manganese catalysts studied, the dependence of the degree of ammonia oxidation \((\alpha)\) on temperature \((t)\) has a complex character (Fig. 2). For most catalysts and space velocities, the degree of oxidation of \(\mathrm{NH_3}\) to \(\mathrm{NO}\) at first increases with rising temperature, and then decreases; thus, the curve

Fig. 2. Dependence of the degree of oxidation of \(\mathrm{NH_3}\) to \(\mathrm{NO}\) on temperature at a pressure of \(8\ \mathrm{kg/cm^2}\). Iron–bismuth–manganese oxide catalysts with \(\mathrm{MnO_2}\) content, wt. %: \(a\)—9%; \(b\)—12.0%; \(v\)—13.8%; \(g\)—17.2%; \(d\)—19.8%.

\[ \alpha=f(t)_{v,p,C_{\mathrm{NH}_3}} \]

passes through a maximum. Each space velocity has its own optimum temperature (Fig. 2, \(a\), \(b\), \(g\), \(d\)). In the presence of catalysts containing 9, 13.8, and 19.8% \(\mathrm{MnO_2}\), at reduced space velocities

\[ \alpha=f(t)_{v,p,C_{\mathrm{NH}_3}} \]

has a minimum (the curves for space velocities 5000 and 7000 in Fig. 2, \(v\), and the curves for 10000 and 15000 in Fig. 2, \(d\)), and under some conditions a maximum

and a minimum (for example, curve 1 in Fig. 2, a). With an increase in space velocity, the optimum temperatures, other conditions being equal, change little and, in the presence of catalysts containing 9, 12, 13.8, 17.2, and 19.8% \(MnO_2\), for most space velocities are, respectively, 780, 680, 760, 680, and 720°C. For the most active catalyst with 12% \(MnO_2\), the range of optimum temperatures extends within 620–740°C.

Effect of Pressure

As can be seen from Fig. 3, the degree of oxidation of \(NH_3\) to \(NO\) decreases with increasing pressure. Thus, in the presence of a catalyst with 13.8% \(MnO_2\) (Fig. 3, b) at \(2\ \mathrm{kg/cm^2}\), 720°C, and \(V=8400\), the degree of oxidation of ammonia was 94%, whereas at \(8\ \mathrm{kg/cm^2}\) the degree of oxidation of \(NH_3\) decreased to 63.5% (Fig. 3, b, curve 4). This decrease in the degree of oxidation in many

Fig. 3. Dependence of the degree of oxidation of \(NH_3\) to \(NO\) on pressure. Oxide iron–bismuth–manganese catalysts with \(MnO_2\) content, wt.%: a — 9%; b — 13.8%; v — 17.2%; g, d — 18.3%; e — 19.8%.

cases occurs approximately inversely proportionally to the increase in pressure, which is confirmed by the rectilinear, or nearly rectilinear, character of the curves \(\alpha=f(P)_{V,t,C_{NH_3}}\) in Fig. 3. Further, in the presence of the same catalyst at 720°C, \(C_{NH_3}=10.4\%\) (vol.) and \(V=7300,\ 8400,\) and 12400, the degree of oxidation of \(NH_3\) proved to be, respectively, 62.5, 64.0, 68.0, and 72.0%. Thus, increasing the space velocity at constant pressure and with the other conditions constant may serve as an important factor in increasing the yield of nitrogen oxides during ammonia oxidation on non-platinum oxide catalysts.

Influence of the Composition of the Gas Mixture

In the literature [1, 8] it is indicated that, in the oxidation of ammonia on non-platinum catalysts, the content of NH\(_3\) in the initial mixture should be lower than when this reaction is carried out on platinum catalysts. However, in a later work [9] the need for higher ammonia concentrations is noted in order to obtain an increased degree of oxidation in the presence of a cobalt oxide catalyst. It should be noted that all these studies were carried out

Fig. 4. Dependence of the degree of ammonia oxidation to NO on the composition of the ammonia–air mixture at a pressure of 8 kg/cm\(^2\).
Iron–bismuth–manganese oxide catalysts with MnO\(_2\) content, wt. %: a—9%, b—17.2%, c—18.3%, d—19.8%.

at atmospheric pressure. As our investigations showed (Fig. 4, a, b, c, d), at a pressure of 8 kg/cm\(^2\) the degree of ammonia oxidation, as a function of its content in the initial gas mixture, passes through a maximum. For all the catalysts we studied, the maximum of this dependence is observed at a content in the initial mixture of 12–13.4% NH\(_3\) (by volume).

Influence of the Catalyst Composition

There are no data in the literature concerning the influence of the composition of oxide catalysts on the degree of NH\(_3\) oxidation at elevated pressures. As our investigations showed (Fig. 5), the dependence of the degree of oxidation on the MnO\(_2\) content in iron–bismuth–manganese catalysts obtained by the method described above is complex in character, determined by the presence of maxima and minima. Moreover, the position of the latter depends not only on the MnO\(_2\) content but also partly on temperature. For most of the conditions studied, the first maximum is observed at an iron–bismuth–manganese catalyst content of 12% MnO\(_2\), and the second at 17.2% MnO\(_2\). The greatest development of the contact properties of this catalytic system is manifested when the catalyst contains 12% MnO\(_2\). With a further increase in the content of this additive, despite the presence of a second maximum, there is observed an in—

which tendency toward a decrease in catalytic properties in the process of ammonia oxidation to nitric oxide at a pressure of \(8\ \mathrm{kg/cm^2}\).

It should be noted that introducing \(\mathrm{MnO_2}\) into the iron–bismuth catalyst within the studied limits gives ternary catalysts with reduced activity.

Fig. 5. Dependence of the degree of oxidation of \(\mathrm{NH_3}\) to NO on the content of \(\mathrm{MnO_2}\) (wt. %) in the catalyst.

Proposed mechanism of ammonia oxidation

It is believed that on platinum catalysts ammonia enters into reaction with adsorbed atomic or molecular oxygen, with the formation of various intermediate compounds and final oxidation products (NO and \(\mathrm{N_2}\)). Of the proposed schemes—nitroxyl [10], imide [11, 12, 13], and hydroxylamine [14, 15, 16, 17]—the latter has received the greatest recognition. According to this hypothesis, interaction of \(\mathrm{NH_3}\) with atomic oxygen adsorbed on the catalyst surface is assumed, according to the reaction

\[ \mathrm{NH_3 + O_{ads} \longrightarrow NH_2OH_{ads}.} \tag{1} \]

Considering that on most oxide catalysts atomic oxygen is present in the adsorbed state in small amounts [16, 18], it may be assumed that the process of ammonia oxidation proceeds at the expense of oxygen from the surface layers of the crystal lattice of the oxide. For example, the following may be represented as the initial reaction leading to the formation of hydroxylamine:

\[ \mathrm{Fe_2O_3 + NH_3 = 2FeO + NH_2OH_{ads}.} \tag{2} \]

Hydroxylamine adsorbed on the catalyst surface decomposes with the elimination of atomic hydrogen and nitric oxide according to the reaction

\[ \mathrm{NH_2OH \rightarrow NO + 3H.} \tag{3} \]

The hydrogen, in turn, combines practically instantaneously with oxygen to form water. If hydroxylamine does not have time to decompose on the catalyst, then, according to V. P. Markov [14], it desorbs into the gas volume, where it decomposes into nitrogen and water according to the equation

Table 1

Physicochemical characteristics of the catalysts

\[ \mathrm{NH_2OH \to H_2O + NH \to H_2O + N + H \to H_2O + N_2,} \tag{4} \]

which, through recombination of nitrogen atoms, leads to its fixation.

In the presence of an oxide catalyst, regeneration of the catalyst must be of substantial importance, associated, for example, with the oxidation of FeO by molecular oxygen according to the reaction

\[ 4\,\mathrm{FeO} + \mathrm{O_2} = 2\,\mathrm{Fe_2O_3}. \tag{5} \]

If this process proves to be slow, then the catalyst surface will not have sufficient reserves of oxygen, as a result of which the formation of hydroxylamine is slowed; in the intergranular space and in the pores an excess of ammonia may appear, and the reaction will pass into the gas volume according to the chain scheme [14]

\[ \begin{aligned} &\mathrm{NH_3 + O \to NH_2OH;} \qquad \mathrm{NH_2OH \to H_2O + N + H;}\\ &\mathrm{O_2 + H \to OH + O;} \qquad \mathrm{O + NH_3 \to NH_2OH}\ \text{etc.} \end{aligned} \tag{6} \]

Such a course of the process will lead to the formation of elemental nitrogen and to a decrease in the yield of nitric oxide. In the case of ammonia oxidation on an iron–bismuth catalyst, where \(\mathrm{Bi_2O_3}\) is an activating additive, the enhancement of catalytic activity may occur not through the interaction of FeO with free oxygen of the gas phase, but through interaction with \(\mathrm{Bi_2O_3}\) according to the reaction:

\[ 2\,\mathrm{FeO} + \mathrm{Bi_2O_3} = \mathrm{Fe_2O_3} + 2\,\mathrm{BiO}. \tag{7} \]

\(\mathrm{BiO}\) is more readily oxidized by oxygen to \(\mathrm{Bi_2O_3}\) than FeO is to \(\mathrm{Fe_2O_3}\).

In an analogous way one could explain the mechanism of action of \(\mathrm{MnO_2}\), if, upon its introduction into the catalyst, the activity of the latter increased. However, in reality, all the iron–bismuth–manganese catalysts we investigated have lower activity in comparison with the activity of the iron–bismuth catalyst. To explain this, it should be recognized that \(\mathrm{MnO_2}\), or the product of its dissociation that is more stable in the range of the temperatures studied, \(\mathrm{Mn_2O_3}\), enters into chemical compounds with \(\mathrm{Fe_2O_3 + Bi_2O_3}\), or gives solid solutions with lower elasticity of dissociation, which hinders the primary act of interaction of ammonia with the oxygen of the solid phase and thereby lowers the activity of the ternary catalyst.

In conclusion it should be noted that, because of the complexity of the processes occurring during ammonia oxidation, the complete kinetics of this process has not yet been developed. Therefore, even in the presence of the most thoroughly studied platinum catalysts, the kinetics of the process is still characterized mainly by empirical dependences [1].

Table 2 gives empirical equations that make it possible to calculate the degree of oxidation as a function of the space velocity in the presence of the iron–bismuth–manganese catalysts we studied at \(8\ \mathrm{kg/cm^2}\) and \(C_{\mathrm{NH_3}} = 10.5\%\) (vol.) at optimal, or close to optimal, temperatures.

Conclusions

1. The process of ammonia oxidation in the presence of oxide iron–bismuth–manganese catalysts, as a function of their \(\mathrm{MnO_2}\) content, was investigated by the dynamic method at a pressure of \(8\ \mathrm{kg/cm^2}\).

Table 2

) $x$ — degree of oxidation of $\mathrm{NH_3}$ to $\mathrm{NO}$; ) volumetric velocity; **) at $C_{\mathrm{NH_3}} = 11.1\%$.

within the range from 5.0 to 19.8 wt.%. The ratio between $\mathrm{Fe_2O_3}$ and $\mathrm{Bi_2O_3}$ was 4.

1. It has been established that the degree of oxidation of ammonia to nitrogen oxide when an ammonia–air mixture is passed through the indicated catalysts, with an increase in temperature, space velocity, and percentage ammonia content, passes through a maximum. With an increase in pressure from 2 to 16 atmospheres, the degree of oxidation continuously decreases.

2. At a pressure of 8 kg/cm$^2$, the degree of oxidation of ammonia as a function of the manganese dioxide content in the iron–bismuth–manganese catalyst is a complex function of composition. The maximum development of the catalytic properties of this system in the range of compositions studied is manifested at a content in the catalyst of 12.0% (by weight) $\mathrm{MnO_2}$.

3. The maximum degree of oxidation of ammonia to nitrogen oxide at a space velocity of 20,000 and an ammonia concentration of 10–11% (by volume) in the initial mixture was 84.0%; the catalysts operated stably for a long time and did not become extinguished.

4. The paper presents empirical equations that quantitatively express the influence of space velocity on the degree of oxidation, and offers some considerations concerning the probable mechanism of the process.

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