PROBABLE MECHANISM OF AMMONIA OXIDATION UNDER PRESSURE IN THE PRESENCE OF IRON–BISMUTH–MANGANESE OXIDE AND COBALT CATALYSTS

N. P. KURIN, P. E. BOGDANOV

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

A number of researchers who have studied oxidative gas reactions in the presence of platinum indicate that the oxygen adsorbed on its surface is found chiefly in the atomic state [1]. This led to the idea that, during the oxidation of ammonia, ammonia molecules striking such a surface react with atomic [2] or even with molecular [3] oxygen, with the formation of various intermediate compounds and final oxidation products (nitrogen, nitric oxide, nitrous oxide). Because of the complexity of the ammonia oxidation reaction, various schemes have been proposed to explain its mechanism: nitroxyl [4], imide [5], hydroxylamine [6, 7]. In addition, there are indications of the formation, in the course of ammonia oxidation, of an activated complex: catalyst—oxygen—ammonia [8]. Recently, the oxidation mechanism has been considered from the standpoint of electronic phenomena in catalysis [9] and from the standpoint of N. N. Semenov’s radical-chain theory [10].

In our opinion, preference should for the present be given to the hydroxylamine scheme. Hydroxylamine has been isolated from the products of the ammonia oxidation reaction [11]; the duration of its “life” has been calculated [2]; it is indicated that at high temperatures the complex (catalyst—oxygen—ammonia) gives hydroxylamine.

On the basis of the hydroxylamine hypothesis [6], we proposed a mechanism for the oxidation of ammonia on oxide catalysts [12, 13] according to the scheme:

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

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

\[ \mathrm{NH_2OH_{ads} \longrightarrow NO + 3H \ldots} \tag{3} \]

\[ \begin{aligned} &\mathrm{NH_2OH \longrightarrow H_2O + NH \longrightarrow H_2O + N + H \longrightarrow} \\ &\qquad \mathrm{+\,O \longrightarrow H_2O + N_2 \ldots} \end{aligned} \tag{4} \]

\[ \mathrm{4FeO + O_2 \longrightarrow 2Fe_2O_3 \ldots} \tag{5} \]

\[ \left. \begin{aligned} \mathrm{NH_3 + O} &\longrightarrow \mathrm{NH_2OH} \\ \mathrm{NH_2OH} &\longrightarrow \mathrm{H_2O + N + H} \\ \mathrm{O_2 + H} &\longrightarrow \mathrm{OH + O} \\ \mathrm{O + NH_3} &\longrightarrow \mathrm{NH_2OH,\ etc.} \end{aligned} \right\} \tag{6} \]

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

In the light of this mechanism, let us try to explain the nature of the dependences obtained by us in works [12, 13] for the degree of oxidation of ammonia as a function of various factors in the presence of oxide catalysts.

A general regularity of the dependence “degree of oxidation of ammonia—space velocity” (see the figures in works [12, 13]) is that the curves of this function pass through a maximum. The ascending part of these curves, at constant pressure, temperature, and ammonia concentration, can be explained by the fact that initially, at low space velocities, i.e., at a longer residence time of the gas mixture on the catalyst, diffusion of nitrogen oxides formed as a result of the decomposition of hydroxylamine on the catalyst surface is possible counter to the stream of the ammonia–air mixture. In this case, apparently, interaction occurs between the ammonia arriving at the catalyst and the counter-diffusing nitrogen oxide, leading to the formation of elemental nitrogen. In the high-temperature region, the process of nitrogen defixation is also promoted by partial oxidation of ammonia in the gas phase and on the walls of the contact apparatus [14].

With further increase in the space velocity (and decrease in the reaction time), counter diffusion of nitrogen oxide decreases; the amount of ammonia that can be oxidized in the gas phase and on the walls before the catalyst bed also decreases, and thus the yield of nitrogen oxide will increase. However, this increase is observed only up to a certain limit, after which the degree of oxidation of ammonia falls. This decrease is due to the fact that, with an increase in the velocity of the gas stream and a decrease in the reaction time, ammonia and oxygen will not have time to be adsorbed on the surface of the catalyst and will pass through its layer and, reacting in the gas phase, will give nitrogen and water. Along with this, the rapid gas stream increasingly carries hydroxylamine away from the layer adsorbed on the catalyst, owing to a decrease in the partial pressure of this substance in the gas phase and an increase in the driving force of desorption. Hydroxylamine, entering the gas phase, decomposes with the formation of elemental nitrogen and water.

Taken together, all these processes lead to a decrease in the degree of oxidation of ammonia; the oxidation process is disrupted, and in the cooled nitrous gases, at the outlet from the contact apparatus, a fog of finely dispersed ammonium nitrate appears.

The ascending part of the curves of the function “degree of oxidation of ammonia—temperature” can be explained by an increase in the rate of the process of ammonia oxidation with increasing temperature, which is usually observed for the overwhelming majority of reactions. In our case, the rate of the reaction of ammonia oxidation to nitrogen oxide is determined by the rate of decomposition of hydroxylamine formed on the catalyst surface. Owing to the relatively slow decomposition of hydroxylamine at low temperatures, the yield of nitrogen oxide is also low, since access of fresh portions of oxygen and ammonia to the surface occupied by hydroxylamine is hindered, and they may react together with nitrogen oxide in the gas phase with formation of elemental nitrogen. As the temperature is raised to certain limits, the rate of decomposition of hydroxylamine on the catalyst increases; a certain fraction of the surface previously occupied by hydroxylamine is freed, and the possibility is created for the adsorption of new quantities of oxygen and ammonia, and the yield of nitrogen oxide increases.

As for the descending part of the curves “degree of oxidation—temperature,” it should be explained not by a fall in the rate of the hydroxylamine decomposition reaction, but by processes leading to its desorption into the gas volume. Apparently, with further increase of temperature above some limit, the rate of desorption of hydroxylamine begins—

will increasingly exceed the rate of the decomposition reaction of this substance to nitrogen oxide on the catalyst surface, and therefore ever larger amounts of it will pass into the gas volume, which leads to the formation of nitrogen.

As was shown in [12], the dependence “degree of ammonia oxidation—temperature” in some cases has a complex character; in the curves of Figs. 2a, b, c, d maxima and minima are observed. This indicates that, in the course of ammonia oxidation, the catalyst undergoes changes, passing from one active form into another. Thus, in the case of a catalyst containing 13.8% manganese dioxide (Fig. 2b in [12]), the first active form is observed at temperatures below 600°C; then there is a transition region, where the degree of ammonia oxidation decreases, reaches a minimum, and again begins to increase with further temperature rise, with the participation in the process of a second active form of the catalyst, passing through a maximum at 760–800°C. These forms may appear as a result of the formation of new compounds in the ternary catalyst, possessing different structures and different ease of oxygen dissociation. Along with what has been set forth, at high temperatures (above 800°C) overheating of oxide catalysts may occur, leading to their sintering and, consequently, to a decrease in the yield of nitrogen oxide.

Turning to consideration of the degree of ammonia oxidation as a function of pressure, it should be noted that, with an increase in the latter, the yield of nitrogen oxide decreases for all the catalysts studied. However, when working at a constant rate of the ammonia–air mixture, this dependence must necessarily have a maximum.

The point is that the system ammonia–air studied by us should, as the thermodynamically most stable final products, have nitrogen and water. In the catalytic process of ammonia oxidation, a necessary condition for this is a relatively long residence of the gas in the catalysis zone, since the nitrogen oxide formed at the beginning, as an unstable product, will nevertheless be defixed into nitrogen. This will also be promoted by defixation of nitrogen due to the interaction of ammonia with nitrogen oxide entering the pre-catalyst space as a result of back diffusion.

As the reaction time is decreased, increasingly favorable conditions are created for carrying nitrogen oxide out of the catalytic zone, and in connection with this the degree of oxidation of ammonia to nitrogen oxide will increase up to a certain limit; after that, with further decrease of time, ammonia will not have time to react on the catalyst surface to nitrogen oxide and will begin to pass through the catalyst. This will lead to interaction of nitrogen oxide and ammonia with formation of elemental nitrogen; and at excessively short reaction times, the ammonia that has passed through, together with nitrogen oxides and water vapor, may even form ammonium nitrate in the region of lower temperatures.

The experimental values obtained by us for the degree of oxidation of ammonia to nitrogen oxide lie on the descending part of the curves “degree of oxidation as a function of pressure,” and the maximum must be located in the region of low pressures (below one atmosphere; see [12]).

The presence of a maximum for the dependence “degree of ammonia oxidation as a function of the concentration of ammonia in the gas mixture” is most likely due to the fact that, at comparatively large excesses of oxygen in the ammonia–air mixture (the percentage of ammonia is small), oxygen, owing to the greater driving force of adsorption, will displace ammonia and hydroxylamine from the reaction-capable surface of the catalyst; the latter, desorbing, are converted in the gas space into nitrogen and water.

A decrease in the concentration of oxygen in the gas mixture (the percentage of ammonia increases) leads to a decrease in the driving force of oxygen adsorption and to an increase, on the contact surface, in the concentration of ammonia and, consequently, also of hydroxylamine, which here will increasingly decompose to nitrogen oxide and water.

Finally, at a certain ratio of oxygen to ammonia, optimal conditions are established under which the highest degree of oxidation of ammonia to nitrogen oxide is attained, associated with the maximum concentration of hydroxylamine on the catalyst surface. A further decrease in the concentration of oxygen in the gas phase leads to an ever greater decline in the rate of regeneration of the catalyst through oxidation by oxygen, as a result of which the reactive surface of the catalyst decreases. This causes a decrease in the degree of oxidation of ammonia to nitrogen oxide, since the former will increasingly react by a chain mechanism [6] in the gas phase between pieces of the catalyst.

Conclusion

This work gives an explanation of the character of the dependences, obtained by us at elevated pressures, of the degree of ammonia oxidation as a function of various factors in the presence of oxide catalysts, from the standpoint of the hydroxylamine hypothesis. The opinion is expressed that, during adsorption of ammonia on oxide catalysts, hydroxylamine is formed as a result of the interaction of ammonia with the oxygen of the surface layers of the crystal lattice of the oxides.

REFERENCES

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CORRECTIONS AND NOTED MISPRINTS

Izv. TPI, vol. 102.