STUDY OF THE CONTACT PROPERTIES OF CHROMIUM–TIN CATALYSTS FOR THE OXIDATION OF SO₂ TO SO₃

N. P. Kurin and N. P. Figurovskaya

Introduction

The modern sulfuric-acid industry is characterized by a continuous increase in the relative importance of the contact method for producing sulfuric acid. This is explained by its higher qualities in comparison with acid obtained by the nitrous method (high concentrations, including oleum and pure sulfuric anhydride), and by the fact that improvements made in technological schemes and apparatus designs have significantly reduced the cost of contact acid.

In the contact method, the factor determining the technological scheme, the design of the contact apparatus, the possible yields, and, consequently, the cost of the acid obtained is the quality of the catalyst used.

For a long time platinum was used as the catalyst. However, platinum catalysts, along with high activity, have two serious disadvantages, namely: high sensitivity to contact poisons and high cost. This circumstance, from the very beginning of the development of contact production of sulfuric acid, determined the attempt to replace them with cheaper oxide catalysts.

As a result of the efforts of Soviet chemists (Shokin, Adadurov, Boreskov [1, 2, 3], and others), highly active vanadium contact masses were developed and introduced into production. They are less sensitive to poisons, but, like platinum catalysts, are fairly expensive. In recent times vanadium has also been finding an ever wider range of very important uses, which, along with its high cost, narrows the possibilities for its application in the sulfuric-acid industry. Therefore, the problem of finding cheaper and sufficiently active non-platinum and non-vanadium catalysts still remains of substantial interest both for the practice of sulfuric-acid production and for the theory of heterogeneous catalysis.

The works of a number of Soviet scientists (I. E. Adadurov and A. V. Gernet with co-workers [4, 5, 6, 7, 8, 9, 10, 11, 12], V. F. Postnikov, G. I. Kunin, and A. A. Astasheva [13], and others) established that one of the substances possessing rather high catalytic properties for the reaction of oxidation of sulfur dioxide to sulfuric anhydride is chromium oxide. It was also found that the catalytic properties of this substance can be strengthened and stabilized by introducing activating additives, in particular tin dioxide.

However, the above-mentioned investigators worked with chromium–tin catalysts of the composition Cr₂O₃·SnO₂, and only in one paper [4] are data given concerning a catalyst of the composition 3Cr₂O₃·SnO₂.

N. P. Kurin and N. T. Rudok [14] studied in greater detail the catalytic system Cr₂O₃ + SnO₂ over a wide range of ratios between its two initial components. In that work the catalysts were prepared by the method

Fig. 1. Diagram of the apparatus for studying the contact properties of non-platinum catalysts in the oxidation of SO₂ to SO₃. 1—cylinder with liquid SO₂; 2—glass gas holder with a layer of oil to prevent dissolution of SO₂ in water; 3—gas holders for air; 4—cylinder with nitrogen; 5—gas holder for nitrogen; 6—water-vapor bottle with a constant water level; 7—alkaline absorber for purifying nitrogen; 8 and 9—absorbers with concentrated sulfuric acid for drying nitrogen and sulfur dioxide; 10—alkaline absorber for purifying air; 11—absorber with concentrated sulfuric acid for drying air; 12—distribution panel; 13—rheometer for SO₂; 14—rheometer for air; 15—rheometer for nitrogen; 16—rheometer for nitrogen; 17—thermostat for the rheometers; 18—fan; 19—gas analyzers; 20—aspirators; 21—thermostat of the contact apparatus; 22—contact apparatus; 23—electromotor; 24—stirrer; 25—platinum–platinum-rhodium thermocouple; 26—burettes for measuring the amount of water; 27—vessel with H₂SO₄ for pressure equalization when the gas stream is switched from the gas analyzers to the atmosphere; 28—settling flask with oleum; 29—thermostat for sulfuric anhydride; 30—desorber of sulfuric anhydride; 31—gas analyzers; 32—absorber for SO₃; 33—electric heater; 34—vessel with P₂O₅; 35—mechanism for raising and lowering the thermostat; 36—heater; 37—rheostats; 38—ammeter and 39—mercury-toluene thermoregulator.

by mechanical mixing of commercial, highly calcined \(Cr_2O_3\) and \(SnO_2\), and therefore, although the general dependence of activity on the \(SnO_2\) content in the catalyst was established, the contact masses obtained did not possess high contact properties.

In this connection, the aim of the present investigation is to study the contact properties of the catalytic system \(Cr_2O_3 + SnO_2\) over a wide range of \(SnO_2\) contents when preparing contact masses in the most active form by the method of coprecipitation.

Experimental Part

Preparation of Catalysts

As indicated above, for preparing the catalysts we used the method of coprecipitation of chromium and tin hydroxides, which ensures the closest mixing and the mutual activating effect of the constituent components of the mixed catalyst.

The hydroxides were coprecipitated with a 25-percent aqueous ammonia solution from seminormal aqueous solutions of chromium nitrate and stannic chloride. Catalysts with a high content of \(Cr_2O_3\) were prepared from a solution of \(Cr(NO_3)_3\) and \(SnCl_4\) seminormal with respect to \(Cr(NO_3)_3\); catalysts in which the \(SnO_2\) content exceeded 60% (by weight) were precipitated from a solution seminormal with respect to \(SnCl_4\). To prevent hydrolysis of stannic chloride and premature precipitation of its basic salts, the solution was slightly acidified with hydrochloric acid. Precipitation was carried out at a temperature of \(80^\circ\). Before precipitation of the chromium and tin hydroxides, the solution was neutralized with a small amount of ammonia water until the first flocs of precipitate appeared, after which the required amount of 25-percent aqueous ammonia solution was added rapidly, with vigorous stirring of the entire solution by means of a mechanical stirrer. To complete the precipitation, a slight excess of precipitant was added. The resulting hydroxide precipitate was allowed to stand for 24 hours, washed with distilled water until a negative reaction for \(Cl'\) and \(NO_3'\) ions was obtained, filtered off, dried in air, and pressed into tablets under a pressure of \(200\ \text{kg}/\text{cm}^2\).

The tablets obtained were dried in a drying oven at \(40^\circ\) for 0.5 h, \(60^\circ\)—0.5 h, \(80^\circ\)—0.5 h, \(100^\circ\)—1.5 h, \(110^\circ\)—1.5 h, \(150^\circ\)—1 h, and \(200^\circ\)—1 h.

The dried catalysts made from pure \(Cr_2O_3\) and from \(Cr_2O_3 + SnO_2\) were greenish-black, while those made from \(SnO_2\) were white. The catalysts had sufficient mechanical strength and were crushed only under a load of \(20\ \text{kg}/\text{cm}^2\).

The tablets were crushed; the 1.5–2 mm fraction selected by sieving was charged into the contact apparatus in an amount of \(5\ \text{cm}^3\) and, to remove residual moisture, was dried for 3 hours in a stream of air (space velocity 600) at a temperature of \(350^\circ\).

Experimental Setup

For studying the contact properties we chose a dynamic method, which, unlike the static method, makes it possible to study the activity of the catalyst under an established steady state, when the concentrations of the initial, intermediate, and final reaction products are established as constant both in the transverse and in the longitudinal sections of the catalytic zone under conditions close to isothermal. All this is extremely necessary for correct conclusions regarding the kinetics of the process of oxidation of \(SO_2\) to \(SO_3\).

Somewhat earlier, on the basis of the indicated method, N. P. Kurin, S. A. Sitov, and others [15] developed and described an experimental setup for studying the process of oxidation of SO₂ to SO₃ in the presence of iron–copper catalysts. We introduced a number of additions into this setup that made it possible to carry out experiments with a gas mixture of constant and variable composition.

The scheme of the setup is shown in Fig. 1.

Sulfurous gas and nitrogen from cylinders (1) and (4), and air from the atmosphere, were passed into gas holders (2), (5), and (3), whence, under constant pressure created by water from the Mariotte bottle (6), they were directed into absorbers (7–11), where they were purified from impurities by washing with a solution of caustic soda and concentrated sulfuric acid. Then, by means of rheometers (13–16) placed in an air thermostat, the composition of the gas mixture was dosed, after which it passed into the coil of the contact apparatus, where it was heated to the temperature of the thermostat (21); it passed through the catalyst layer in the contact apparatus (22) and, through the equalizing vessel (27), was discharged into the atmosphere or directed to the gas analyzers (19) and aspirator (20).

Part of the gas mixture for determining the content of sulfurous gas in the initial gas was diverted to the gas analyzers (19) and aspirator (20), located before the contact apparatus. Dosing of the initial gas with sulfuric anhydride was carried out by passing part of the nitrogen through the rheometer (16) and the desorber (30) containing pure liquid SO₃. The constancy of the temperature in the automatically regulated water thermostat (29) ensured constancy of the vapor pressure of SO₃ and of the gas composition. To check the SO₃ content in the nitrogen, part of it could be directed into gas analyzers (31) filled with a titrated solution of caustic soda.

CONTACT UNIT

Labels in the figure: steel crucible; metal grating; coil; stirrer; through mercury vapor; electric furnace; contact apparatus.

Fig. 2. Diagram of the contact apparatus and thermostat.

The pure SO₃ required for dosing was obtained from oleum by distillation from flask (28) into desorber (30).

To remove the heat of the reaction of oxidation of SO₂ to SO₃ from the catalyst zone and to create isothermal operating conditions, it is necessary to develop as much as possible

increase the heat-transfer surface of the contact apparatus and to increase the rate of heat transfer. For this purpose, as shown in Fig. 2, the contact apparatus was made of a thin-walled glass tube 5 mm in diameter, and the catalyst in it was arranged in two layers. In the empty part of the tube between the two layers of catalyst, the partially reacted gas mixture, before entering the second catalyst layer, gave up heat more completely to the surrounding space, which increased the isothermicity of the process.

To increase the heat-transfer coefficient from the outer walls of the contact apparatus, the thermostat was filled with a mixture of molten nitrate salts of sodium and potassium, which was set into vigorous motion by means of a mechanical stirrer.

Before entering the contact zone, the gas mixture passed through a glass coil, the length of which was selected so that the mixture had time to heat up to the temperature of the thermostat liquid.

By regulating the strength of the heating electric current in the thermostat, the contact temperature was maintained constant with an accuracy of ±1° and was recorded by means of a platinum–platinum-rhodium thermocouple placed in the thermostat liquid.

To prevent the coil and the contact apparatus from being crushed by the solidifying melt of saltpeter when operation of the thermostat was stopped (21), the thermostat (35) was lowered by means of a lifting mechanism (35) (Fig. 1). Cracking of the glass parts when operation was resumed was prevented by slowly raising (over ~30 minutes) the thermostat (35) with molten, non-overheated saltpeter.

To calculate the degree of oxidation of SO₂ to SO₃ in the presence of the catalysts under study, the content of sulfur dioxide in the gas mixture before and after the contact apparatus was determined. Analysis for SO₂ was carried out by the iodometric method.

Results Obtained and Their Discussion

Using the methods described above, 13 catalysts were prepared and tested; their compositions are given in Table 1.

Table 1

Composition of chromium–tin catalysts

The catalysts were studied in the temperature range 350–550° on a gas mixture used in the industrial production of contact sulfuric acid, of composition SO₂ — 7.0; O₂ — 11.0 and N₂ — 82.0% (by volume), at space velocities of 600–2500. By space velocity is meant-

is understood as the number of volumes of the gas mixture, under normal conditions, that passes through one bulk volume of catalyst per hour.

In addition, the spent catalyst containing 33.13% SnO₂ was additionally tested on a gas mixture of variable composition, with the initial concentration of sulfur dioxide varied from 2.0 to 14.0, oxygen from 2.0 to 19.0, and sulfur trioxide from 1 to 10.0% by volume.

The study of each catalyst began with the establishment of a stationary state. During this period, at a given temperature, the gas mixture was passed through the contact apparatus at a specified space velocity until the degree of oxidation of SO₂ to SO₃ became constant. The time required for this process, depending on the experimental conditions, ranged from 2 to 8 hours. Usually the longest time corresponded to the initial test conditions (space velocity \(V = 600\) and \(t = 350^\circ\)), and it decreased as the space velocity and temperature were increased.

Fig. 3. Dependence of the degree of oxidation of SO₂ to SO₃ on the space velocity for a catalyst of pure Cr₂O₃.

The expenditure of time for establishing the stationary state can be explained by a change in the fine structure of the surface of the chromium–tin catalysts, with their partial sulfation as a result of interaction with SO₂, O₂, and SO₃, as well as by a certain duration of adsorption–desorption processes between the catalyst surface and the gas phase, leading to the establishment of stationary-constant concentrations of the reacting substances and reaction products in each section of the contact apparatus.

The obtained values of the degree of oxidation of SO₂ to SO₃ in the established stationary state are given in Table 2 and shown in Figs. 3–14 and 15–26.

Influence of Space Velocity

From Table 2 and Figs. 3–14 it is evident that, at constant temperature, for all the catalysts studied the degree of oxidation of SO₂ to SO₃ decreases with increasing space velocity. However, the decrease in the degree of oxidation proceeds more slowly than the space velocity increases.

Thus, for pure chromium oxide (Fig. 3), at a temperature of 475°, when the space velocity is increased 3.33-fold (from 600 to 2000), the degree of oxidation decreases only by a factor of 1.37 (from 84.4 to 61.4%).

Table 2

Degree of oxidation of SO₂ to SO₃ in the presence of chromium–tin catalysts.
Initial composition of the gas mixture: SO₂—7.0; O₂—11.0 and N₂—82.0% (by volume).
Total pressure 1 atm.

A similar dependence is observed for temperatures of 350–475° and in the presence of mixed chromium–tin catalysts.

In the region of higher temperatures (500–550°), the decrease in the degree of oxidation with increasing space velocity is expressed still less sharply. For an entire series of catalysts (Figs. 7, 8, 9, 10, 11), the isotherms at 500 and 550° are almost straight lines parallel to the abscissa axis.

The decrease in the degree of oxidation of SO₂ to SO₃ in the presence of the catalysts studied upon increasing the space velocity can be explained by a decrease in the reaction time, since

\[ \tau \text{ is proportional to } \frac{1}{v \cdot \frac{T}{273}} \quad [16]. \]

Fig. 4. Dependence of the degree of oxidation of SO₂ to SO₃ on space velocity for a catalyst containing 0.616% SnO₂.

Fig. 5. Dependence of the degree of oxidation of SO₂ to SO₃ on space velocity for a catalyst containing 1.224% SnO₂.

Fig. 6. Dependence of the degree of oxidation of SO₂ to SO₃ on space velocity for a catalyst containing 2.418% SnO₂.

Fig. 7. Dependence of the degree of oxidation of SO₂ to SO₃ on space velocity for a catalyst with a content of 6.199%.

Fig. 8. Dependence of the degree of oxidation of SO₂ to SO₃ on space velocity for a catalyst with a content of 14.179% SnO₂.

Fig. 9. Dependence of the degree of oxidation of SO₂ to SO₃ on space velocity for a catalyst with a content of 24.849% SnO₂.

Fig. 10. Dependence of the degree of oxidation of SO₂ to SO₃ on space velocity for a catalyst with a content of 33.13% SnO₂.

Fig. 11. Dependence of the degree of oxidation of SO₂ to SO₃ on the volumetric velocity for a catalyst containing 49.78% SnO₂.

Axis labels in the figure: degree of oxidation, %; volumetric velocity.
Catalyst: 0.22Cr₂O₃ · 49.78SnO₂.
Curves are labeled by temperature: 350, 400, 425, 450, 475, 500, 525, 550, 575.

Fig. 12. Dependence of the degree of oxidation of SO₂ to SO₃ on the volumetric velocity for a catalyst containing 66.47% SnO₂.

Axis labels in the figure: degree of oxidation, %; volumetric velocity.
Catalyst: 3.53Cr₂O₃ · 66.47SnO₂.
Curves are labeled by temperature: 350, 400, 425, 450, 500, 525, 550, 575.

Fig. 13. Dependence of the degree of oxidation of SO₂ to SO₃ on the volumetric velocity for a catalyst containing 79.66% SnO₂.

Axis labels in the figure: degree of oxidation, %; volumetric velocity.
Catalyst: 20.34Cr₂O₃ · 79.66SnO₂.
Curves are labeled by temperature: 350, 400, 425, 450, 475, 500, 525, 550, 575.

Fig. 14. Dependence of the degree of oxidation of SO₂ to SO₃ on the volumetric velocity for a catalyst containing 89.84% SnO₂.

Axis labels in the figure: degree of oxidation, %; volumetric velocity.
Catalyst: 10.16Cr₂O₃ · 89.84SnO₂.
Curves are labeled by temperature: 350, 400, 425, 450, 475, 500, 525.

The more gradual decrease in the degree of oxidation ($x$) of SO$_2$ to SO$_3$ with increasing space velocity is connected with the fact that, on the one hand, as the space velocity increases, the residence time of the gas on the catalyst decreases, causing a decrease in $x$; on the other hand, the effective concentrations of the reacting substances in each transverse section of the contact apparatus increase, which, taking into account the form of the kinetic equations (1) and (4), has the opposite effect on the value of $x$.

The combined influence of both factors leads to the result that the degree of oxidation decreases to a lesser extent than the space velocity increases.

The more gradual decrease in the degree of oxidation with increasing space velocity, especially at elevated temperatures (475–500°), can be used to increase the operating intensity of contact apparatuses.

Influence of Temperature

Another important index determining the value of a substance as a catalyst for the oxidation reaction of SO$_2$ to SO$_3$ is the dependence of the degree of oxidation on temperature.

As is known, the reaction $2\mathrm{SO}_2 + \mathrm{O}_2 = 2\mathrm{SO}_3$ is accompanied by the evolution of heat, and its equilibrium shifts with increasing temperature toward the decomposition of SO$_3$, while the equilibrium degree of oxidation decreases. Therefore, good catalysts must be capable of ensuring a high rate of oxidation of SO$_2$ to SO$_3$ in the temperature range 450–550°. At higher temperatures (above 600°), attainment of a high degree of oxidation of SO$_2$ to SO$_3$, even in the presence of good catalysts, is impossible because of the shift of the equilibrium of the indicated reaction in the reverse direction. Thus, at temperatures of 600 and 700° and a total pressure of 1 atm, the equilibrium degree of oxidation of SO$_2$ to SO$_3$ is, respectively, 73.8 and 44.20%.

As can be seen from Table 2 and Figs. 15–26, for all the chromium–tin catalysts we studied the following temperature dependence of the degree of oxidation of SO$_2$ to SO$_3$ was observed. If the gas is passed at constant space velocity, then initially the degree of oxidation increases with increasing temperature, reaches the equilibrium value or one close to it, and then begins to decrease, following the equilibrium curve. Thus the curves $x = f(t)$ pass through a maximum, and for each space velocity there is its own optimum temperature.

With increasing space velocity, the optimum temperatures shift toward higher values.

For pure chromium oxide and for most chromium–tin catalysts, in the range of space velocities investigated, the optimum temperatures lie within 450–500°, and only for catalysts with a high content of tin dioxide (79.86 and 90.84% SnO$_2$) do the optimum temperatures shift into the range 500–550°. As for tin dioxide, it has low activity, and the optimum conditions for the oxidation of SO$_2$ to SO$_3$ in the presence of this substance apparently shift toward still higher temperatures.

The ascending part of the curves $x = f(t)$ can be explained by the usual increase in the rate of the oxidation reaction of SO$_2$ to SO$_3$ with increasing temperature; the descending part of these curves is determined not by a decrease in the reaction rate, but by the shift of its equilibrium toward smaller yields of SO$_3$ with further increase in temperature, i.e., here qualitatively the same picture is observed as for other equilibrium exothermic reactions (for example, for the ammonia synthesis reaction) [16].

In conclusion, it should be noted that the optimum temperatures of the chromium–tin catalysts we studied are close to the temperature range in which industrial vanadium contact masses operate.

Fig. 15. Dependence of the degree of oxidation of SO₂ to SO₃ on temperature for a catalyst made of pure Cr₂O₃.

Labels in the graph: “Degree of oxidation, in %”; “Temperature in °C”; “Catalyst: 100% Cr₂O₃.” Curves are labeled: 500, 1000, 1500, 2000.

Fig. 16. Dependence of the degree of oxidation of SO₂ to SO₃ on temperature for a catalyst containing 0.615% SnO₂.

Labels in the graph: “Degree of oxidation, in %”; “Temperature in °C”; “Catalyst: 99.385% Cr₂O₃ + 0.615% SnO₂.” Curves are labeled: 500, 1000, 1500, 2000, 2500.

Fig. 17. Dependence of the degree of oxidation of SO₂ to SO₃ on temperature for a catalyst containing 1.92% SnO₂.

Labels in the graph: “Degree of oxidation, in %”; “Temperature in °C”; “Catalyst: 98.076% Cr₂O₃ + 1.924% SnO₂.” Curves are labeled: 500, 1000, 1500, 2000, 2500.

Fig. 18. Dependence of the degree of oxidation of SO₂ to SO₃ on temperature for a catalyst containing 2.4% SnO₂.

Labels in the graph: “Degree of oxidation, in %”; “Temperature in °C”; “Catalyst: 97.58% Cr₂O₃ + 2.42% SnO₂.” Curves are labeled: 500, 1000, 1500, 2000, 2500.

Fig. 19. Dependence of the degree of oxidation of \(\mathrm{SO_2}\) to \(\mathrm{SO_3}\) on temperature for a catalyst with a content of \(6.199\%\ \mathrm{SnO_2}\).

Axis labels: degree of oxidation, in %; temperature, in °C.

Inset label: catalyst \(93.80\%\ \mathrm{Cr_2O_3} + 6.199\%\ \mathrm{SnO_2}\).

Curve labels visible on the graph: 2500, 2000, 1500, 1000, 600.

Fig. 20. Dependence of the degree of oxidation of \(\mathrm{SO_2}\) to \(\mathrm{SO_3}\) on temperature for a catalyst with a content of \(14.179\%\ \mathrm{SnO_2}\).

Axis labels: degree of oxidation, in %; temperature, in °C.

Inset label: catalyst \(85.821\%\ \mathrm{Cr_2O_3} + 14.179\%\ \mathrm{SnO_2}\).

Curve labels visible on the graph: 2000, 1500, 1000, 600, 2500.

Fig. 21. Dependence of the degree of oxidation of \(\mathrm{SO_2}\) to \(\mathrm{SO_3}\) on temperature for a catalyst with a content of \(24.84\%\ \mathrm{SnO_2}\).

Axis labels: degree of oxidation, in %; temperature, in °C.

Inset label: catalyst \(75.168\%\ \mathrm{Cr_2O_3} + 24.832\%\ \mathrm{SnO_2}\).

Curve labels visible on the graph: 1500, 1000, 2000, 2500, 600.

Fig. 22. Dependence of the degree of oxidation of \(\mathrm{SO_2}\) to \(\mathrm{SO_3}\) on temperature for a catalyst with a content of \(33.15\%\ \mathrm{SnO_2}\).

Axis labels: degree of oxidation, in %; temperature, in °C.

Inset label: catalyst \(66.817\%\ \mathrm{Cr_2O_3} + 33.183\%\ \mathrm{SnO_2}\).

Curve labels visible on the graph: 2500, 2000, 1500, 1000, 600.

Fig. 23. Dependence of the degree of oxidation of SO₂ to SO₃ on temperature for a catalyst containing 49.76% SnO₂.

Graph labels: degree of oxidation, %; temperature, °C. Catalyst: 50.24% Cr₂O₃ + 49.76% SnO₂.

Fig. 24. Dependence of the degree of oxidation of SO₂ to SO₃ on temperature for a catalyst containing 66.47% SnO₂.

Graph labels: degree of oxidation, %; temperature, °C. Catalyst: 33.53% Cr₂O₃ + 66.47% SnO₂.

Fig. 25. Dependence of the degree of oxidation of SO₂ to SO₃ on temperature for a catalyst containing 79.86% SnO₂.

Graph labels: degree of oxidation, %; temperature, °C. Catalyst: 20.14% Cr₂O₃ + 79.86% SnO₂.

Fig. 26. Dependence of the degree of oxidation of SO₂ to SO₃ on temperature for a catalyst containing 90.54% SnO₂.

Graph labels: degree of oxidation, %; temperature, °C. Catalyst: 9.46% Cr₂O₃ + 90.54% SnO₂.

Effect of Catalyst Composition

Before turning to consideration of the effect of the composition of chromium–tin catalysts, it should first of all be noted that pure chromium oxide has a comparatively high catalytic activity, while pure tin dioxide has a low activity. Thus, each of the indicated substances is, to one degree or another, capable of accelerating the process of oxidation of SO₂ to SO₃ by molecular oxygen, i.e., it is a catalyst. This is also confirmed by the work of earlier investigators [17, 14].

As our investigations have shown (Table 2, Fig. 27), the dependence of the degree of oxidation on the SnO₂ content in chromium–tin catalysts obtained by the method of coprecipitation is complex in character, being determined by the presence of two maxima and one minimum; moreover, the position of the latter depends not only on the SnO₂ content but also partly on the temperature and space velocity.

Thus, at a space velocity of 600, for the isotherms 350, 400, 425, and 450°, the maxima correspond to catalysts 2 and 8, containing, respectively, 2.418 and 33.13% (by weight) SnO₂; under the same conditions, the minimum on the curves for the degree of oxidation—\(f(\% \mathrm{SnO_2})\)—is observed for catalyst 6, containing 14.25% SnO₂.

At the indicated space velocity, in the region of higher temperatures (475, 500, and 550°), the first maximum shifts toward a lower tin dioxide content (0.6158–1.224%), and it is less sharply expressed.

It should also be noted that the second maximum on the curves for the degree of oxidation—\(f(\% \mathrm{SnO_2})\)—is expressed as a broad plateau, lying, for the region of lower temperatures (350–450°), within the limits 24.84–66.47, and, for the region of higher temperatures (475–550°), within the limits 6–90% SnO₂.

Over the extent of the plateau, the degree of oxidation of SO₂ to SO₃ changes little with increasing SnO₂ content in the catalyst.

At other space velocities, the effect of SnO₂ content is similar to that described above, with slight deviations.

Comparing the degrees of oxidation obtained in the presence of pure Cr₂O₃, SnO₂, and mixed chromium–tin catalysts obtained by coprecipitation, it can be seen that the introduction of SnO₂ favorably affects the catalytic properties of the Cr₂O₃ + SnO₂ system, and for catalysts 4 and 8 the coefficient is expressed most strongly.

A similar character of the change in the catalytic properties of the Cr₂O₃ + SnO₂ system as a function of SnO₂ content was previously observed by Kurin and Rudok [14] for catalysts obtained by mechanical mixing of the components.

Effect of the Composition of the Gas Mixture

As is known [2,3], the composition of the initial gas mixture has a significant effect on the process of oxidation of SO₂ to SO₃ in the presence of platinum and vanadium catalysts. In this connection, we investigated the effect of the concentration of oxygen, sulfurous gas, and sulfur trioxide on the degree of oxidation of SO₂ to SO₃ in the presence of a spent catalyst containing 31.16% SnO₂, at temperatures of 400–500° and a space velocity of 1000.

In these experiments, the concentration of sulfurous gas was varied from 2.0 to 14.0 \((C_{\mathrm{O_2}} = 11.0 - \mathrm{const.},\ C_{\mathrm{SO_3}} = 0.0)\), oxygen—from 2.0 to 19.0 \((C_{\mathrm{SO_2}} = 7.0 - \mathrm{const.},\ C_{\mathrm{SO_3}} = 0)\), and sulfur trioxide—from 1.0 to 10.0% by volume \((C_{\mathrm{SO_2}} = 7.0 - \mathrm{const.},\ C_{\mathrm{O_2}} = 11.0 - \mathrm{const.})\).

The results of the experiments are given in Tables 3, 4, and 5, and are also shown in Figs. 28, 29, and 30.

Table 3

Dependence of the degree of oxidation of SO₂ to SO₃ on the oxygen concentration in the initial gas mixture
\(C_{\mathrm{SO_2}} = 7.0\% = \mathrm{const};\quad C_{\mathrm{SO_3}} = 0;\quad C_{\mathrm{N_2}} =\) balance to 100% (vol.)

Table 4

Dependence of the degree of oxidation of SO₂ to SO₃ on the concentration of sulfur dioxide in the initial gas mixture.
\(C_{\mathrm{O_2}} = 11.0;\quad C_{\mathrm{SO_3}} = 0.0;\quad C_{\mathrm{N_2}} =\) balance to 100 (vol.)

Table 5

Dependence of the degree of oxidation of SO₂ to SO₃ on the concentration of SO₃ in the initial gas mixture
\(C_{\mathrm{SO_2}} = 7.0;\quad C_{\mathrm{O_2}} = 11.0;\quad C_{\mathrm{N_2}} =\) balance to 100% (vol.)

As is evident from Table 3 and Fig. 28, the degree of oxidation of SO₂ to SO₃ increases with increasing oxygen concentration up to a certain limit, and then remains constant. At the temperatures studied, this limit is reached at oxygen concentrations of 8–15% (vol.).

An increase in the concentration of SO₂ in the initial gas in the temperature range 400–475° causes a decrease in the degree of oxidation of SO₂ to SO₃ (Table 4 and Fig. 29). It should be noted that, with increasing temperature, the decrease in the degree of oxidation of SO₂ to SO₃ as the concentration of SO₂ in the initial gas increases becomes slower, and finally, at a temperature of 500°, the degree of oxidation begins to increase slowly with increasing SO₂ concentration. As for the influence of SO₃, as is evident from Table 5 and Fig. 30, at all the temperatures studied the degree of oxidation of SO₂ to SO₃ decreases with increasing SO₃ concentration in the initial gas.

Fig. 27. Dependence of the degree of oxidation of \(SO_2\) to \(SO_3\) on the \(SnO_2\) content in chromium–tin catalysts at a space velocity of 600.

Vertical axis: degree of oxidation, \(\%\).

Horizontal axis: \(Cr_2O_3 \rightarrow \%\,SnO_2\) in the catalyst.

Legend: space velocity 600; isotherms: \(400^\circ\), \(425^\circ\), \(450^\circ\), \(475^\circ\), \(500^\circ\), \(550^\circ\).

Fig. 28. Dependence of the degree of oxidation of SO₂ to SO₃ on the concentration of oxygen in the initial gas mixture.

Fig. 29. Dependence of the degree of oxidation of SO₂ to SO₃ on the concentration of SO₂ in the initial gas mixture.

Fig. 30. Dependence of the degree of oxidation of SO₂ to SO₃ on the concentration of SO₃ in the initial gas mixture.

Kinetics and Mechanism of the Reaction

A number of investigators have shown that the kinetics of the reaction

\[ 2\mathrm{SO}_2 + \mathrm{O}_2 = 2\mathrm{SO}_3 \]

in the presence of platinum [9] and a number of oxide [18, 19, 20, 15] catalysts is satisfactorily expressed by the equation

\[ \frac{d C_{\mathrm{SO}_3}}{d \tau} = K \frac{C_{\mathrm{SO}_2}-C_{\mathrm{SO}_2\mathrm{\,eq.}}}{\sqrt{C_{\mathrm{SO}_3}}}. \tag{1} \]

According to the data of N. P. Kurina and N. T. Rudok [14], this equation satisfactorily expresses the kinetics of the process of oxidation of \(\mathrm{SO}_2\) to \(\mathrm{SO}_3\) in the presence of chromium–tin catalysts obtained by the method of mechanical mixing of \(\mathrm{Cr}_2\mathrm{O}_3\) and \(\mathrm{SnO}_2\).

In the present work we have attempted to apply equation (1) to the results obtained by us for the oxidation of \(\mathrm{SO}_2\) to \(\mathrm{SO}_3\) in the presence of chromium–tin catalysts obtained by the method of coprecipitation.

If in equation (1) the concentrations of \(\mathrm{SO}_2\) and \(\mathrm{SO}_3\) at time \(\tau\) and at equilibrium are expressed through the initial concentration of \(\mathrm{SO}_2\) and the degrees of oxidation \(x\) and \(x_T\), then after integration we obtain

\[ K= \frac{\sqrt{a}}{\tau} \left[ \sqrt{x_T}\,2.303\log \frac{\sqrt{x_T}+\sqrt{x}}{\sqrt{x_T}-\sqrt{x}} - 2\sqrt{x} \right]. \tag{2} \]

The criterion for the applicability of equation (2) should be the constancy of the rate constants calculated at different space velocities and at constant temperature. From our experiments the residence time of the gas on the catalyst, \(\tau\), cannot be determined exactly, and instead of this quantity, as was shown by one of us [16], we substituted into equation (2) the quantity proportional to it,

\[ \frac{1}{v \dfrac{T}{273}}, \]

where \(v\) is the space velocity, and \(T\) is the absolute reaction temperature. The final form of the kinetic equation was:

\[ K= v\frac{T}{273}\sqrt{a} \left[ \sqrt{x_T}\,2.303\cdot \log \frac{\sqrt{x_T}+\sqrt{x}}{\sqrt{x_T}-\sqrt{x}} - 2\sqrt{x} \right]. \tag{3} \]

In calculating the rate constants \(K\) in equation (3), the following were substituted:

\(a\) — initial concentration of \(\mathrm{SO}_2\), in fractions of unity;
\(x_T\) — equilibrium degree of oxidation, in fractions of unity;
\(x\) — degree of oxidation at the given space velocity, in fractions of unity;
\(v\) — space velocity in volumes (n.c.) of the gas mixture per 1 bulk volume of catalyst per hour.

As can be seen from Table 6, in the overwhelming majority of cases the rate constant at constant temperatures but different space velocities remains constant. The observed deviations in the constancy of the values of \(K\) can partly be explained by experimental errors, connected primarily with the difficulty of establishing the steady state. The satisfactory constancy of the rate constants at different space velocities indicates that equation (3) correctly expresses the kinetic relations of the reaction of oxidation of \(\mathrm{SO}_2\) to \(\mathrm{SO}_3\) by molecular oxygen in the presence of chromium–tin catalysts obtained by the method of coprecipitation, provided that the initial composition of the gas mixture is constant \((7\%\,\mathrm{SO}_2,\ 11\%\mathrm{O}_2 + \text{nitrogen})\).

Table 6

Values of the rate constants of the reaction \(2\mathrm{SO}_2 + \mathrm{O}_2 = 2\mathrm{SO}_3\) in the presence of chromium–tin catalysts, calculated from equation (3). Initial composition of the gas mixture: \(\mathrm{SO}_2—7.0\), \(\mathrm{O}_2—11.0\), and \(\mathrm{N}_2—82.0\%\) (by volume).

Continuation of Table 6

) The figures marked with an asterisk were discarded in calculating the mean value of K.
*) Indicates that it is impossible to calculate the rate constant owing to the attainment of chemical equilibrium, or a state close to it, in the oxidation of SO₂ to SO₃.

However, as Tables 7, 8, and 9 show, calculation by this equation with a variable initial composition of the gas mixture did not give the required constancy of the rate constants. Specifically, with an increase in the content of \(O_2\) in the initial gas mixture (all other conditions being constant), the rate constants increase, while with an increase in the concentrations of \(SO_2\) and \(SO_3\) they decrease.

Table 7

Rate constants of the reaction of oxidation of \(SO_2\) to \(SO_3\) as a function of the initial oxygen concentration. Space velocity—1000, initial concentrations \(SO_2\)—7.0%, \(SO_3\)—0.0% (vol.).

Table 8

Rate constants of the reaction of oxidation of \(SO_2\) to \(SO_3\) as a function of the initial sulfur dioxide concentration. Space velocity—1000, initial concentrations \(O_2\)—11.0, \(SO_3\)—0.0% (vol.).

Table 9

Rate constants of the reaction of oxidation of \(SO_2\) to \(SO_3\) as a function of the initial \(SO_3\) concentration. Space velocity—100; initial concentration \(SO_2\)—7.0, \(O_2\)—11.0% (vol.).

Thus, the kinetics of \(SO_2\) oxidation in the presence of the chromium–tin catalysts investigated by us, just as G. K. Boreskov and T. I. Sokolova [21] showed for vanadium catalysts, proves to be different from the kinetics of \(SO_2\) oxidation on platinum catalysts with respect to the influence of the initial gas mixture.

Table 10

Rate constants of the reaction \(2\mathrm{SO}_2+\mathrm{O}_2=2\mathrm{SO}_3\) in the presence of chromium–tin catalysts, calculated by equation (5).
Initial composition of the gas mixture: \(\mathrm{SO}_2\)—7.00; \(\mathrm{O}_2\)—11.0 and \(\mathrm{N}_2\)—82.0% (by volume)

Continuation of Table 10.

Degree of oxidation of SO$_2$ to SO$_3$ in the presence of a catalyst with SnO$_2$ content, % (wt.).

1) Figures marked with one asterisk were discarded when calculating the average value $K$ avg.
2) — indicates the impossibility of calculating $K$ owing to the establishment of an equilibrium state or a state close to it.

In this connection, we attempted to apply the kinetic equation derived by Boreskov for vanadium contact masses, taking into account the rate of the reverse reaction:

\[ \frac{dC_{\mathrm{SO}_3}}{d\tau} = K_1 C_{\mathrm{O}_2} \left(\frac{C_{\mathrm{SO}_2}}{C_{\mathrm{SO}_3}}\right)^{0.8} - K_2 \left(\frac{C_{\mathrm{SO}_3}}{C_{\mathrm{SO}_2}}\right)^{1.2}. \tag{4} \]

Equation (4), after the appropriate transformation and solution with respect to \(K_1\), had the form:

\[ K_1 = \frac{a \cdot v \cdot T}{273} \int_{0}^{x} \frac{x^{0.8}\,dx} {(1-x)^{0.8}\left(b-\frac{ax}{2}\right) \left[ 1-\frac{1}{K_p^{2}}\cdot \frac{x^2}{(1-x)\left(b-\frac{ax}{2}\right)} \right]}, \tag{5} \]

where, as before, \(a\) is the initial concentration of \(\mathrm{SO}_2\), \(b\) is the initial concentration of oxygen, \(x\) is the degree of oxidation of \(\mathrm{SO}_2\) to \(\mathrm{SO}_3\) at time \(\tau\), and

\[ K_p^{2}=\frac{K_1}{K_2}. \]

Unfortunately, the integral on the right-hand side of equation (5) cannot be solved in general form, and therefore, to solve it, it was necessary to resort to laborious graphical integration. In equation (5), for definite values of the reaction temperature and space velocity, the quantity

\[ \frac{a \cdot vT}{273}=\mathrm{const}=B. \]

Assigning various values to the variable \(x\) (0.1; 0.2, etc., up to 0.95), we find \(f(x)\), after which curves were constructed in the coordinates \(x\) and \(f(x)\). The area bounded by one of the curves (representing an isotherm), by ordinates, and by the abscissa axis, multiplied by the scale of the diagram, was equal to the required integral. After multiplying the integral found by the constant \(B\), we obtained the value of the rate constant \(K_1\) for the corresponding temperature and space velocity. The results of calculations by equation (5) are given in Tables 10, 11, and 12.

Table 11

Rate constants as a function of the oxygen content in the initial gas mixture, calculated by equation (5). Space velocity—1000; initial concentration \(\mathrm{SO}_2\)—7.0, \(\mathrm{SO}_3\)—0.0% (vol.)

Table 12

Rate constants calculated from equation (5) as a function of the initial concentration of SO₂ in the initial gas mixture.
Space velocity—1000, O₂—11.0; SO₃—0.0% (vol.)

The rate constants \(K_1\) at variable initial SO₃ concentration (Table 13) were calculated from equation (6)

\[ K_1 = \frac{a^{0.2}\omega_0 T}{273} \int_0^x \frac{(c+ax)^{0.8}\,dx} {(1-x)^{0.8}\left(b-\frac{ax}{2}\right) \left[ 1-\frac{1}{K_p^2}\cdot \frac{(c+ax)^2} {a^2(1-x)^2\left(b-\frac{ax}{2}\right)} \right]} , \tag{6} \]

where \(c\) is the initial concentration of SO₃.

As can be seen from Table 10, there is a more or less satisfactory constancy of the rate constants calculated from equation (5) for temperatures of 425–550°, if the gas mixture had the initial composition 7% SO₂, 11% O₂, and 0.0% SO₃ + nitrogen. At 400° and partly at 350° the rate constants change systematically, which indicates the lesser suitability of equation (5) for these conditions. However, a greater convergence of the rate constants calculated from equation (5), in comparison with the analogous values calculated from equation (3), is observed with variable oxygen content (Table 11) and sulfur trioxide content (Table 13). Equation (5), like equation (3), gives a poorly satisfactory constancy of the rate constants as a function of the initial SO₂ concentration.

Table 13

Rate constants \(K_1\), calculated from equation (6), as a function of the initial concentration of SO₃.
Space velocity—1000; SO₂—7.0; O₂—11.0% (vol.)

From all the above it is evident that equation (5), more correctly than equation 3, represents the kinetic relationships in the oxidation of SO₂ to SO₃ in the presence of the chromium–tin catalysts studied by us, with a variable initial composition of the gas mixture.

However, for practical purposes, when working with a gas mixture of constant composition (7.0% \(SO_2\); 11% \(O_2 + N_2\)), the simpler equation (3) can be successfully used to calculate the process of oxidation of \(SO_2\) to \(SO_3\) in the presence of the chromium–tin catalysts studied by us when designing contact apparatus.

A graphical treatment of the logarithms of the rate constants calculated from equation (5) is presented in Fig. 31. It was found that, in the presence of the majority of the catalysts studied, the straight lines

\[ \log K_{\mathrm{cp}} = f\!\left(\frac{1}{T}\right) \]

have a break in the temperature region 400–425°. The apparent activation energies (\(E_{\mathrm{app}}\)) of the oxidation of \(SO_2\) to \(SO_3\), calculated from the tangents of the slopes of the straight lines

\[ \log K_{\mathrm{cp}} = f\!\left(\frac{1}{T}\right), \]

are given in Table (14). In the presence of pure chromium oxide, the activation energy proved to be 67300 cal/mole. When up to 1.224% \(SnO_2\) is introduced into the catalyst, one value of \(E_{\mathrm{app}}\) is retained in the temperature range indicated in the table, although it decreases to 24600 cal/mole. For catalysts with a higher \(SnO_2\) content, two values of \(E_{\mathrm{app}}\) were found: one (12800–26000) in the region of lower temperatures (350–425°), and the other (33900–57300) in the region of higher temperatures (400–500°). It was not possible to calculate the activation energy for pure \(SnO_2\) because of the absence of a sufficient amount of experimental data.

Fig. 31. Dependence of \(\log K_{\mathrm{cp}}\) on \((1/T)\)

The data presented indicate a decrease in the activation energy when \(SnO_2\) is introduced into the catalyst.

Turning to consideration of the mechanism of the oxidation process of \(SO_2\) to \(SO_3\) in the presence of chromium–tin catalysts, it should first of all be noted that \(SO_2\) molecules in the gas phase do not react with oxygen molecules, at least in the temperature region in which the industrial synthesis of \(SO_3\) is carried out. This can be explained by the presence of large affinity forces between the atoms in the molecules of oxygen and sulfur dioxide.

Table 14

Apparent activation energies of the reaction \(2\,\mathrm{SO_2} + \mathrm{O_2} = 2\,\mathrm{SO_3}\) in the presence of chromium–tin catalysts

Heterogeneous oxidation of \(\mathrm{SO_2}\) by oxygen in the presence of a solid catalyst becomes possible as a result of the loosening and overcoming of the affinity forces between the atoms of the reacting components under the influence of the physical and chemical affinity forces that arise on the surface of the catalyst during its interaction with \(\mathrm{SO_2}\) and \(\mathrm{O_2}\). If, in the catalytic oxidation of \(\mathrm{SO_2}\) to \(\mathrm{SO_3}\), only the physical forces manifested in simple physical adsorption were sufficient, then in this case the activation energy of the process of oxidation of \(\mathrm{SO_2}\) by oxygen in the presence of chromium–tin catalysts would amount to only 2000—8000 cal/mol. In reality, however, the activation energy of the process of oxidation of \(\mathrm{SO_2}\) to \(\mathrm{SO_3}\) in the presence of the chromium–tin catalysts studied by us is much higher than the cited value and lies within the limits of 12000—67000 cal/mol, i.e., it is of the same order as the heats of chemical reactions. Therefore, without deviating substantially from the truth, it may be assumed that in the catalytic process of oxidation of \(\mathrm{SO_2}\) by oxygen the principal role is played by forces of chemical affinity, whose action causes the formation and subsequent decomposition on the catalyst surface of unstable intermediate compounds [22]. This mechanism presupposes alternating oxidation and reduction of the contact surface with the direct participation of catalyst oxygen in the process of oxidation of \(\mathrm{SO_2}\) to \(\mathrm{SO_3}\). The possibility of this process for vanadium catalysts was experimentally proved by the method of labeled atoms [23] in the study of isotopic exchange in the system \(\mathrm{V_2O_5—O_2}\) [18]. Physical adsorption, caused by the action of physical forces, is also necessary for the given catalytic process; however, in this case it chiefly plays the role of a “reservoir” from which molecules of \(\mathrm{SO_2}\) and \(\mathrm{O_2}\) proceed to the active sites, where the chemical processes leading to the formation of the final product \(\mathrm{SO_3}\) take place.

The presence of significant chemical-affinity forces between the ordinary macrophases of chromium, oxygen, and sulfur dioxide is proved by the existence of numerous oxides (\(\mathrm{Cr_2O_3}\), \(\mathrm{CrO_3}\), \(\mathrm{CrO_2}\), \(\mathrm{Cr_5O_{13}}\), \(\mathrm{Cr_5O_{12}}\), etc.), as well as chromium sulfates (for example, \(\mathrm{Cr_2(SO_4)_3}\), \(\mathrm{Cr_2O_3 \cdot 2SO_3}\), \(\mathrm{Cr_2O_3 \cdot 3SO_3}\), etc.). The existence of the indicated compounds depends above all on the temperature range. Thus, \(\mathrm{CrO_3}\) exists [24] at temperatures below \(210^\circ\), \(\mathrm{CrO_2}\) below \(360^\circ\); at higher temperatures they decompose with formation of \(\mathrm{Cr_2O_3}\) and liberation of oxygen. The temperature interval of existence of \(\mathrm{Cr_5O_{13}}\) lies within \(265\text{–}360^\circ\); above \(360^\circ\) it converts into \(\mathrm{Cr_5O_{12}}\), which is stable up to \(410^\circ\), after which it converts into \(\mathrm{Cr_2O_3}\). As for chromium sulfates, \(\mathrm{Cr_2(SO_4)_3}\) at \(280^\circ\) decomposes with formation of the basic chromium sulfate \(\mathrm{Cr_2O_3 \cdot 2SO_3}\), which at temperatures above \(450\text{–}460^\circ\) converts into the more \(\mathrm{SO_3}\)-poor basic chromium sulfate of composition \(\mathrm{2Cr_2O_3 \cdot 3SO_3}\) [25]. Owing to the excess free energy of the catalyst substance at its surface, the conditions of existence of the surface intermediate compounds that form may differ considerably from those characteristic of the ordinary macrophases. However, owing to the absence of corresponding data on this question, when considering the probable mechanism of oxidation of \(\mathrm{SO_2}\) to \(\mathrm{SO_3}\) in the presence of chromium oxide, as a first approximation one may proceed from the properties of chromium compounds with oxygen and sulfur that are characteristic of the macrophase state of these substances. N. P. Kurin and S. A. Sigov proposed the following possible mechanism of oxidation of \(\mathrm{SO_2}\) by oxygen in the presence of pure chromium oxide:

\[ \left. \begin{aligned} \text{a)}\quad &2\,\mathrm{CrO_2}+\mathrm{SO_2}\rightleftarrows \mathrm{Cr_2O_3}+\mathrm{SO_3}\\ \text{b)}\quad &2\,\mathrm{Cr_2O_3}+\mathrm{O_2}\rightleftarrows 4\,\mathrm{CrO_2} \end{aligned} \right\} \tag{7} \]

\[ \left. \begin{aligned} \text{a)}\quad &\mathrm{Cr_5O_{13}}+\mathrm{SO_2}\rightleftarrows \mathrm{Cr_5O_{12}}+\mathrm{SO_3}\\ \text{b)}\quad &2\,\mathrm{Cr_5O_{12}}+\mathrm{O_2}\rightleftarrows 2\,\mathrm{Cr_5O_{13}} \end{aligned} \right\} \tag{8} \]

\[ \left. \begin{aligned} \text{a)}\quad &2\,(\mathrm{Cr_2O_3 \cdot 2SO_3})\rightleftarrows 2\,\mathrm{Cr_2O_3 \cdot 3SO_3}+\mathrm{SO_3}\\ \text{b)}\quad &2\,(2\,\mathrm{Cr_2O_3 \cdot 3SO_3})+2\,\mathrm{SO_2}+\mathrm{O_2}\rightleftarrows 4\,(\mathrm{Cr_2O_3 \cdot 2SO_3}) \end{aligned} \right\} \tag{9} \]

Reactions 7, 8, and partially 9 are monomolecular with respect to \(\mathrm{O_2}\) and \(\mathrm{SO_2}\), which agrees with the form of the kinetic equations (1) and (4). Taking into account the temperature limits of existence of the chromium compounds indicated above, it may be assumed that oxidation in the presence of chromium oxide proceeds predominantly by reactions (7) and (8) in the region of lower temperatures (up to \(400\text{–}425^\circ\)), and by reactions (9) in the region of elevated temperatures (above \(425^\circ\mathrm{C}\)).

The change in the mechanism of oxidation of \(\mathrm{SO_2}\) to \(\mathrm{SO_3}\) by oxygen in the presence of \(\mathrm{Cr_2O_3}\) on passing from the lower-temperature region to the higher-temperature region is indicated by the presence of a break in the straight line

\[ \log K = f\left(\frac{1}{T}\right), \]

which was observed by N. P. Kurin and N. T. Rudyuk [14] for commercial highly calcined \(\mathrm{Cr_2O_3}\). This break is directed downward and, as is known [26], kinetic analysis in this case indicates the parallel existence of two mechanisms of the process of oxidation of \(\mathrm{SO_2}\) to \(\mathrm{SO_3}\), characterized by different activation energies.

Namely, at temperatures below \(400\text{–}425^\circ\), oxidation of \(\mathrm{SO_2}\) to \(\mathrm{SO_3}\) proceeds more rapidly by reactions 7–8, and a smaller activation energy is expended in this; at temperatures above \(400\text{–}425^\circ\), reactions 9, with a higher activation energy, acquire the prevailing significance.

For pure, highly active chromium oxide obtained by the precipitation method, we determined the activation energy for the high-temperature region; apparently, for lower temperatures (below 425°) there must exist another, lower value of this quantity.

For all the chromium–tin catalysts we investigated with a tin dioxide content of 2.4% and higher, the straight lines $\log K=f\left(\dfrac{1}{T}\right)$ have a break in the region 400–425° and, correspondingly, two values of the activation energy: one, smaller, for low temperatures, and the other, larger, for elevated temperatures.

We connect the mechanism of the coactivation phenomenon of chromium–tin catalysts with a deeper chemical interaction of $\mathrm{Cr_2O_3}$ and $\mathrm{SnO_2}$ than takes place in structural activation, although the latter apparently also occurs.

In this connection, we propose the following mechanism for the reaction of oxidation of $\mathrm{SO_2}$ on the surface of chromium–tin catalysts:

\[ \begin{aligned} \text{a) }& \mathrm{Cr_2O_3 + SnO_2 \rightleftarrows 2CrO_2 + SnO} \\ \text{b) }& \mathrm{2CrO_2 + SO_2 \rightleftarrows Cr_2O_3 + SO_3} \\ \text{c) }& \mathrm{2SnO + O_2 \rightleftarrows 2SnO_2} \end{aligned} \tag{10} \]

\[ \begin{aligned} \text{a) }& \mathrm{2(Cr_2O_3\cdot 2SO_3) \rightleftarrows 2Cr_2O_3\cdot 3SO_3 + SO_3} \\ \text{b) }& \mathrm{2Cr_2O_3\cdot 3SO_3 + SnO_2 + SO_2 \rightleftarrows 2(Cr_2O_3\cdot 2SO_3) + SnO} \\ \text{c) }& \mathrm{2SnO + O_2 \rightleftarrows 2SnO_2.} \end{aligned} \tag{11} \]

Reactions 10 and 11 are monomolecular with respect to $\mathrm{SO_2}$ and $\mathrm{O_2}$, which is consistent with kinetic equations 1 and 4. Taking into account the presence of a break in the straight lines $\log K=f\left(\dfrac{1}{T}\right)$ and its downward direction, it should be considered that reactions 10 and 11 proceed in parallel. However, in the temperature region 350–425° the total rate of the process of oxidation of $\mathrm{SO_2}$ to $\mathrm{SO_3}$ is determined predominantly by reactions 10. At higher temperatures $\mathrm{CrO_2}$, even if it can exist on the active surface, is present in very small concentrations, and therefore the predominant oxidation of $\mathrm{SO_2}$ to $\mathrm{SO_3}$ proceeds through the basic, more stable chromium sulfates according to reactions 11, with the expenditure of more substantial activation energies.

The mechanism of oxidation of $\mathrm{SO_2}$ to $\mathrm{SO_3}$ in the presence of chromium–tin catalysts obtained by the method of coprecipitation and by mechanical mixing of highly calcined $\mathrm{Cr_2O_3}$ with $\mathrm{SnO_2}$ is, in the main, one and the same. Evidence for this is the identical character of the dependence of the degree of oxidation of $\mathrm{SO_2}$ to $\mathrm{SO_3}$ on the concentration of $\mathrm{SnO_2}$ in the catalyst, and the existence of a break of the same direction in the straight lines $\log K=f\left(\dfrac{1}{T}\right)$ and, correspondingly, the presence of two values of the activation energy, one of which (the lower) pertains to reduced temperatures and the other to elevated temperatures.

As our experiments have shown, the method of coprecipitation of $\mathrm{Cr_2O_3}$ and $\mathrm{SnO_2}$ from aqueous salt solutions gives more active catalysts. This is connected with the fact that the oxides of chromium and tin precipitate in a finely dispersed state and are very uniformly mixed with one another, with a high development of the contact surface between the grains of $\mathrm{Cr_2O_3}$ and $\mathrm{SnO_2}$.

As a result, a thermodynamically nonequilibrium catalytic system [27] is created, with a considerable excess of free energy. The latter, as well as the high development of the contact surface between elementary grains of \(Cr_2O_3\) and \(SnO_2\), accelerates, in particular, reactions 10a and 11b and other reactions of our scheme to a higher degree than can occur for coarser contact masses obtained by mechanical mixing of the ingredients.

The increase in catalytic activity could also have been promoted by the rapid addition of the precipitant to the solution of chromium and tin salts, since in this case the capture and incorporation into the crystal lattice of elementary grains of \(Cr_2O_3\) and \(SnO_2\), respectively, of tin and chromium ions becomes more probable.

In conclusion it should be noted that the ideas developed above concerning the mechanism of the process of oxidation of \(SO_2\) to \(SO_3\) by molecular oxygen in the presence of chromium–tin catalysts should be regarded only as one approximation.

Therefore we do not deny other interpretations of possible reactions on the surface of the catalysts investigated by us, with the formation, in particular, of highly active complex compounds [8].

In this connection, it should be considered expedient to carry out experimental investigations in order to study more precisely the surface structure of chromium–tin catalysts, the thermal effects of topochemical processes, and their chemistry, using more advanced methods of investigation, in particular the tracer-atom method.

Conclusions

1. The catalytic properties of the \(Cr_2O_3 + SnO_2\) system in the oxidation of \(SO_2\) to \(SO_3\) by molecular oxygen were investigated as a function of space velocity (600–2500), temperature (350–550°C), and the composition of the gas mixture, with broad variation in the \(SnO_2\) content of the catalysts (from 0 to 100%). The catalysts were prepared by the method of coprecipitation followed by pressing. The degree of oxidation of \(SO_2\) to \(SO_3\) in the presence of the indicated catalysts was studied by a dynamic method under isothermal conditions.

2. The time required to establish the stationary state for the catalysts studied lay within the range of 2–8 hours.

3. With an increase in space velocity, all other conditions being equal, the degree of oxidation of \(SO_2\) to \(SO_3\) decreases; however, this decrease occurs more slowly than the space velocity increases.

4. With an increase in temperature, at constant space velocity and gas-mixture composition, the degree of oxidation of \(SO_2\) to \(SO_3\) initially increases and, after reaching a maximum value, decreases, following the equilibrium curve or lying somewhat below it. The optimum temperatures for most of the catalysts investigated lie within the range of 450–500°.

5. Increasing the oxygen concentration in the initial gas mixture to 8–11% in the presence of a catalyst containing 33.13% \(SnO_2\) causes an increase in the degree of oxidation; with a further increase in \(SO_2\), the degree of oxidation does not change. Increasing the initial concentrations of \(SO_2\) and \(SO_3\) lowers the degree of oxidation.

6. The change in the catalytic properties of the \(Cr_2O_3 + SnO_2\) system with increasing \(SnO_2\) content is expressed by a complex curve having 2 maxima. Of all those investigated, the most active catalysts proved to be those containing 2.418 and 33.13% \(SnO_2\). The method of coprecipitation in the preparation of catalysts ensures a higher development of the cataly—

... the catalytic properties of the system \(Cr_2O_3 + SnO_2\) in comparison with the method of mechanical mixing of the ingredients.

1. The kinetics of the oxidation of \(SO_2\) to \(SO_3\) in the presence of the catalysts studied and at a constant initial composition of the gas mixture \((7.0\%\, SO_2,\ 11\%\, O_2,\ \text{and }82\%\, N_2)\) is satisfactorily expressed by the equation

\[ \frac{dC_{SO_3}}{dt} = K\,\frac{C_{SO_2}-C_{SO_2\mathrm{\,eq}}}{\sqrt{C_{SO_3}}}, \]

and for a gas mixture with a varying initial composition, by the equation of G. K. Boreskov

\[ \frac{dC_{SO_3}}{dt} = K_1 C_{O_2}\left(\frac{C_{SO_2}}{C_{SO_3}}\right)^{0.8} - K_2\left(\frac{C_{SO_3}}{C_{SO_2}}\right)^{1.2}. \]

1. In the presence of the majority of the catalysts investigated, the straight line

\[ \log K_1=f\left(\frac{1}{T}\right) \]

has a break at temperatures of \(400\text{--}425^\circ\). In accordance with this, the apparent activation energies for the temperature ranges \(350\text{--}425^\circ\) and \(425\text{--}550^\circ\) are, respectively, \(11000\text{--}26000\) and \(34000\text{--}67000\) cal/mol.

1. In the work, assumptions are made concerning the probable mechanism of the process of oxidation of \(SO_2\) to \(SO_3\) in the presence of chromium–tin catalysts.

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