Study of the Activity of Chromium–Tin Catalysts for the Oxidation of SO₂ to SO₃

N. P. Kurin and N. T. Rudyuk

Studies by a number of Soviet researchers (I. E. Adadurov and D. V. Gernet with co-workers [1, 2, 3, 4, 5, 6, 7, 8, 9], V. F. Postnikov, G. I. Kunin, A. A. Astashev [10], and others) and foreign scientists [11] have established that chromium oxide is an active catalyst for the reaction

\[ 2\mathrm{SO}_{2} + \mathrm{O}_{2} = 2\mathrm{SO}_{3}, \tag{1} \]

which underlies the contact method for producing sulfuric acid. The possibility was also shown of activating and stabilizing the contact properties of this substance by introducing various additives, in particular tin dioxide. However, the cited studies were carried out with a chromium–tin catalyst of composition $\mathrm{Cr}_{2}\mathrm{O}_{3}\cdot\mathrm{SnO}_{2}$, and only in one article [1] are data given for a catalyst of composition $3\mathrm{Cr}_{2}\mathrm{O}_{3}\cdot\mathrm{SnO}_{2}$.

In this connection, the aim of the present work is to study the activity of the catalytic system $\mathrm{Cr}_{2}\mathrm{O}_{3} + \mathrm{SnO}_{2}$ over wide ranges of ratios between its two initial components.

Preparation of Catalysts

Catalysts of various compositions were prepared by mixing chemically pure chromium oxide that had been calcined and specially prepared $\mathrm{SnO}_{2}$. The starting substance for obtaining $\mathrm{SnO}_{2}$ was $\mathrm{SnCl}_{2}\cdot 2\mathrm{H}_{2}\mathrm{O}$. The latter, in an amount of 450 g, with acidification by HCl to prevent hydrolysis, was dissolved in 6 liters of distilled water. The solution was heated to 80° and then, by rapidly adding the calculated amount of a 25-percent aqueous ammonia solution until a weakly alkaline reaction was reached, $\mathrm{Sn(OH)}_{2}$ was precipitated. The resulting white precipitate, after threefold washing by decantation, was filtered on a vacuum filter and treated with nitric acid of specific gravity 1.4 while heating to 80°. In this process $\mathrm{Sn(OH)}_{2}$ was converted into $\mathrm{Sn(OH)}_{4}$. After thorough washing by decantation and on the filter until the $\mathrm{Cl}'$ ion was completely removed, the $\mathrm{Sn(OH)}_{4}$ precipitate was dried at room temperature, at 110°, and finally at 300°. As a result of 3-hour heating at 300°, $\mathrm{Sn(OH)}_{4}$ was converted into white $\mathrm{SnO}_{2}$. This substance was thoroughly ground in an agate mortar and sieved through a sieve with 6000 openings per $1\ \mathrm{cm}^{2}$. To remove moisture, chromium oxide was likewise heated for 3 hours at 300°, ground in an agate mortar, and sieved through the above-mentioned sieve.

Chromium oxide and tin dioxide were mixed in the required proportions and, for uniform mixing, were again ground in a mortar, moistened with distilled water, and pressed on a hydraulic press in a die with a plunger under a pressure of 5000 kg/cm² until shrinkage ceased at the given pressure. Pressing was used to provide closer contact between the particles of $\mathrm{Cr}_{2}\mathrm{O}_{3}$ and $\mathrm{SnO}_{2}$ and to create high mechanical strength of the cat-

catalysts. The resulting catalyst tablets, in order to remove water, were gradually heated to 350° and held at this temperature for 2 hours. The tablets were then crushed, and for studies of catalytic activity a fraction of 1.5–2 mm was selected with sieves; 5 ml of this fraction was charged into the contact apparatus. By the method described above, 12 catalysts were prepared, the compositions of which are given in Table 1.

Table 1

Composition of chromium–tin catalysts

Method of Investigation

To study the properties of the chromium–tin catalysts described above, we used a dynamic method that makes it possible to determine the degree of oxidation of SO₂ to SO₃ under an established steady-state condition, when the concentrations of the initial components and reaction products both in the gas phase and on the catalyst surface become constant for any cross section of the contact apparatus. The latter is important for the kinetic treatment of experimental results. The scheme of the apparatus is described in the work of N. P. Kuryina, S. A. Sinova, et al. [12]. For rapid removal of the heat liberated as a result of the oxidation of SO₂ to SO₃ from the reaction zone, and for investigation of the process under conditions close to isothermal, the U-shaped contact apparatus was made of thin-walled glass tubing with an internal diameter of 5 mm and was immersed in a molten and vigorously stirred mixture consisting of 55% sodium nitrate and 45% potassium nitrate. The initial gas mixture containing 7% SO₂ + 93.0% (by volume) air was prepared by mixing the necessary quantities of previously purified sulfur dioxide and air.

The degree of oxidation of sulfur dioxide to sulfuric anhydride was determined by an iodometric method.

Our earlier studies [12] indicate that, in order to obtain stable, constant degrees of oxidation of SO₂ to SO₃ at specified temperatures, space velocity, and gas-mixture composition, a certain time must be spent establishing the steady state. Therefore, in the present work, at each temperature and space velocity the gas mixture was passed for some time (2.5–3 hours), until the degree of oxidation of SO₂ to SO₃ became constant under the specified conditions.

Results Obtained and Their Discussion

All the catalysts were investigated by the method described above. Testing of the catalysts was carried out at a space velocity of 1000 (under conditions [[unclear: continuation on next page]]

By space velocity we mean the number of volumes of the gas mixture (under normal conditions) passed per hour through one bulk volume of catalyst, and temperatures of 350, 400, 450, 500, 550, and 600°. In addition, in order to determine the effect of space velocity on the degree of oxidation, catalysts containing 0.619 and 1.224% \( \mathrm{SnO_2} \) were studied at space velocities of 200, 400, 600, 800, 1000, and 1200.

The results obtained are given in Table 2 and shown in Figs. 1, 2, 3, and 4.

Table 2

Dependence of the degree of oxidation of \( \mathrm{SO_2} \) to \( \mathrm{SO_3} \) on space velocity and temperature in the presence of chromium–tin catalysts containing 0.619 and 1.224% (by weight) \( \mathrm{SnO_2} \)

Pressure = 1 atm. Initial composition of the gas mixture: \( \mathrm{SO_2} \)—7.0%, air—93.0% by volume.

Fig. 1. Dependence of the degree of oxidation of \( \mathrm{SO_2} \) to \( \mathrm{SO_3} \) on space velocity in the presence of a catalyst containing 0.619% \( \mathrm{SnO_2} \).

As can be seen from Table 2 and Figs. 1 and 2, in the presence of catalysts containing 0.619 and 1.224% SnO₂, at all the temperatures investigated by us the degree of oxidation of SO₂ to SO₃ decreases with increasing space velocity. However, this decrease in the degree of oxidation occurs much more slowly than the space velocity increases. Thus, for example, at 550° and with a catalyst containing 1.224% SnO₂, an increase in space velocity from 200 to 1200, i.e., by a factor of 6, causes a decrease in the degree of oxidation of SO₂ to SO₃ from 87.0 to 58.8%, i.e., by ~1.5 times. It should be noted that the most pronounced lag in the decrease of the degree of oxidation of SO₂ to SO₃ behind the increase in space velocity is observed in the region of elevated temperatures, 550–600°.

Fig. 2. Dependence of the degree of oxidation of SO₂ to SO₃ on space velocity in the presence of a catalyst containing 1.224% SnO₂.

The influence of temperature on the degree of oxidation of SO₂ to SO₃ in the presence of the two indicated catalysts is clearly shown in Figs. 3 and 4.

Fig. 3. Influence of temperature on the degree of oxidation of SO₂ to SO₃ in the presence of a catalyst containing 0.619% SnO₂.

For the region of low space velocities (200–400), as the temperature is raised to a certain limit, the degree of oxidation of SO₂ to SO₃ increases, and then, with a further rise in temperature, begins to fall; thus the curves \(\%\) oxidation \(= f(T)_v\) pass through a maximum. At higher space velocities (600–1200), the degree of oxidation of SO₂ to SO₃ increases continuously within the temperature range we investigated. However, there is no doubt that at temperatures above 600° and increased space velocities, maxima will also appear on the curves \(\%\) oxidation \(= f(T)_v\). Moreover, these maxima and the corresponding optimum temperatures shift toward higher temperatures as the space velocity increases, as was previously observed for other catalysts [13].

Fig. 4. Effect of temperature on the degree of oxidation of SO₂ to SO₃ in the presence of a catalyst containing 1.224% SnO₂.

A similar character of the temperature dependence of the degree of oxidation of SO₂ to SO₃ (Table 3, Fig. 5) is also observed for all the other catalysts we studied.

Table 3

Dependence of the degree of oxidation of SO₂ to SO₃ on catalyst composition and temperature at a pressure of 1 atm, a space velocity of 1000, and an initial gas-mixture composition of SO₂—7.0 and air 93.0% (by volume)

The degree of oxidation marked with an asterisk was obtained at 570°.

As can be seen from Table 3 and Fig. 6, the activity of the catalyst consisting of highly calcined commercial \(Cr_2O_3\) is low. The degree of oxidation of \(SO_2\) to \(SO_3\) in the presence of the indicated catalyst at a space velocity of 1000 is, at 500 and 600°, only 27.3 and 64.2%, respectively.

Fig. 5. Dependence of the degree of oxidation of \(SO_2\) to \(SO_3\) on temperature \((v = 1000)\) and on the composition of the catalyst. The numbering of the curves corresponds to the numbering of the catalysts.

The second component of the catalytic system \(Cr_2O_3 + SnO_2\), namely \(SnO_2\), in pure form has a certain activity; however, the latter is considerably lower than the activity of \(Cr_2O_3\). The degree of oxidation of \(SO_2\) to \(SO_3\) in the presence of this substance at temperatures of 500 and 600° is, respectively, only 8.7 and 27.3%.

Fig. 6. Dependence of the degree of oxidation of \(SO_2\) to \(SO_3\) on the composition of the catalyst.

The activity of chromium–tin catalysts obtained by mechanical mixing of finely dispersed $\mathrm{Cr_2O_3}$ and $\mathrm{SnO_2}$ is not an additive property of the catalyst composition and has a more complex dependence.

As is clearly seen from Fig. 6, at all temperatures investigated, initially, with an increase in the content of tin dioxide in the catalyst, the degree of oxidation of $\mathrm{SO_2}$ to $\mathrm{SO_3}$ increases and then decreases; subsequently, with a further increase in the $\mathrm{SnO_2}$ content, it again increases, reaches a maximum value, and then again decreases until it reaches values characteristic of pure tin dioxide. Thus, the isotherms of $\%$ oxidation $= f(\%\mathrm{SnO_2})$ in the catalyst have two maxima. The first maximum is observed at a low content (1.224%), the second at a high content (49.78 wt.%) of $\mathrm{SnO_2}$ in the chromium–tin catalyst.

The highest development of catalytic properties in the oxidation of $\mathrm{SO_2}$ to $\mathrm{SO_3}$ for the catalytic system $\mathrm{Cr_2O_3 + SnO_2}$ occurs when it contains 49.78% $\mathrm{SnO_2}$; the degree of oxidation of $\mathrm{SO_2}$ to $\mathrm{SO_3}$ in the presence of a chromium–tin catalyst with the indicated $\mathrm{SnO_2}$ content considerably exceeds the degree of oxidation in the presence of $\mathrm{Cr_2O_3}$ and $\mathrm{SnO_2}$.

Thus, if the degree of oxidation of $\mathrm{SO_2}$ to $\mathrm{SO_3}$ in the presence of $\mathrm{Cr_2O_3}$, which is more active than tin dioxide, at temperatures of 400, 500, and 600° is respectively 7.5, 27.3, and 64.2%, then in the presence of catalyst No. 8, containing 49.78% $\mathrm{SnO_2}$ (which corresponds to the formula $\mathrm{Cr_2O_3 \cdot SnO_2}$), the degree of oxidation of $\mathrm{SO_2}$ to $\mathrm{SO_3}$ under the same conditions rises to 12.15, 51.2, and 77.9%. Considering the catalytic properties of the system $\mathrm{Cr_2O_3 + SnO_2}$ as a function of its composition, attention is drawn to the circumstance that, within the range of $\mathrm{SnO_2}$ contents from 49.78 to 79.86% (this corresponds to formulas from $\mathrm{Cr_2O_3 \cdot SnO_2}$ to $\mathrm{Cr_2O_3 \cdot 4SnO_2}$), the degree of oxidation of $\mathrm{SO_2}$ to $\mathrm{SO_3}$ remains high and close to the analogous value characteristic of a catalyst of composition $\mathrm{Cr_2O_3 \cdot SnO_2}$. The latter indicates the possibility of obtaining active chromium–tin catalysts also at other ratios between $\mathrm{Cr_2O_3}$ and $\mathrm{SnO_2}$ than that corresponding to the formula $\mathrm{Cr_2O_3 \cdot SnO_2}$.

Thus, from all that has been set forth above it is clearly seen that, for the catalytic system $\mathrm{Cr_2O_3 + SnO_2}$, when its composition is changed, the phenomenon of coactivation is distinctly observed.

As is known, the kinetics of the oxidation process of $\mathrm{SO_2}$ to $\mathrm{SO_3}$ in an excess of oxygen in the gas phase for platinum and some oxide catalysts [14, 15, 16, 12] is satisfactorily expressed by the following equation

\[ \frac{dC_{\mathrm{SO_3}}}{dt} = K \frac{C_{\mathrm{SO_2}} - C_{\mathrm{SO_2,equil.}}} {\sqrt{C_{\mathrm{SO_3}}}} . \tag{2} \]

If in equation 2 the equilibrium concentration $C_{\mathrm{SO_2,equil.}}$ and the actual concentrations $C_{\mathrm{SO_2}}$ and $C_{\mathrm{SO_3}}$ are expressed through the initial amounts $\mathrm{SO_2}$—$a$, the equilibrium $x_T$ and actual $x$ degrees of oxidation, and also, instead of $t$, the quantity proportional to it [13]

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

where $v$ is the space velocity and $T$ is the temperature in °abs., then after integration we obtain

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

A criterion for the applicability of kinetic equation (3) may be the constancy of the rate constants calculated at constant temperature but at different space velocities. We carried out such a check for two chromium–tin catalysts, Nos. 2 and 3, investigated at different space velocities.

The results of calculating the rate constants are given in Table 4.

Table 4

Rate constants of the reaction \(2\mathrm{SO}_2+\mathrm{O}_2=2\mathrm{SO}_3\) in the presence of chromium–tin catalysts

Quantities marked with an asterisk were discarded in calculating the mean value of the rate constant \((K_{\mathrm{av}})\).

As can be seen from the data of Table 4, the rate constants calculated from equation (3) at constant temperatures but at different space velocities remain constant quantities, and thus equation (3) satisfactorily expresses the kinetics of the oxidation process of \(\mathrm{SO}_2\) to \(\mathrm{SO}_3\) in the presence of the chromium–tin catalysts investigated by us. An exception is a certain nonconstancy of the rate constants at low space velocities; therefore, when calculating the mean value \(K_{\mathrm{av}}\), the rate constants at low values of the space velocity were rejected.

Taking into account the satisfactory constancy of the rate constants at elevated space velocities for catalysts Nos. 2 and 3, by equation (3) the rate constants at a space velocity of 1000 were calculated for all the other catalysts investigated by us. The results of the calculations are given in Table 5.

Table 5

Rate constants of the reaction \(2\mathrm{SO}_2+\mathrm{O}_2=2\mathrm{SO}_3\) in the presence of chromium–tin catalysts

Using the Arrhenius equation,

\[ \log K=-\frac{E}{2.303\,R}\left(\frac{1}{T}\right)+\mathrm{const}, \tag{4} \]

where \(K\) is the rate constant,
\(E\) is the apparent activation energy,
\(R\) is the gas constant,

straight lines \(\log K=f\left(\frac{1}{T}\right)\) were constructed (Fig. 7), and from them the apparent activation energies were calculated (Table 6).

Table 6

Apparent activation energies of the process of oxidation of SO\(_2\) to SO\(_3\) in the presence of chromium–tin catalysts.

It is seen from Fig. 7 that in the temperature region 400–500° the straight lines

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

have a downward-directed break. In accordance with this, both for pure Cr\(_2\)O\(_3\) and SnO\(_2\), and for the mixed chromium–tin catalysts, two values of the apparent activation energy are observed: one, lower value, referring to the region of lower temperatures (below 400–500°), is 9380–23900, while the other—higher for the region of elevated temperatures (above 400–500°)—lies within 29500–39700 cal/mol. The apparent activation energy for the mixed chromium–tin catalysts is not an additive property. For the region of elevated temperatures, as the SnO\(_2\) content in the catalyst is first increased to 1.224%, it decreases, then increases and, at an SnO\(_2\) percentage equal to 33.13, reaches a maximum value, after which it again decreases until it reaches the value characteristic of pure SnO\(_2\). Mixed catalysts with an SnO\(_2\) content within the range 2.418–79.86% are characterized by a higher

activation energies than pure \(Cr_2O_3\) and \(SnO_2\). In the region of lower temperatures (curve 1, Fig. 8), the dependence of \(E_{\text{exp}}\) on the \(SnO_2\) content in the catalyst is of a more complex character.

At atmospheric pressure the reaction

\[ 2SO_2 + O_2 = 2SO_3 \]

does not proceed in the gas phase in that temperature region in which the equilibrium is favorably shifted toward the formation of sulfuric anhydride, provided that the reacting molecules \(SO_2\) and \(O_2\) are not activated by any other forms of energy besides thermal energy. The reasons for this lie in the fact that the atoms in the molecules \(O_2\) and \(SO_2\) are bound by relatively large affinity forces. In the heterogeneous oxidation process on the catalyst surface, physical and chemical affinity forces arise between the reacting gaseous components and the substance of the catalyst. The action of these forces loosens the bonds in the molecules \(SO_2\) and \(O_2\) and makes them capable of reacting.

Fig. 7. Dependence of \(\log K\) on \(\left(\dfrac{1}{T}\right)\) for chromium-tin catalysts.

The straight lines are numbered in accordance with the numbering of the catalysts.

If, for the heterogeneous process of oxidation of \(SO_2\) by molecular oxygen, only the physical forces manifested in simple physical adsorption were sufficient, then the activation energy of the process in the presence of chromium-tin catalysts would amount to only several thousand cal/mol. However, in the presence of the chromium-tin catalysts we studied, this quantity has higher values (9380–39700) and, in its order of magnitude, is close to the heats of chemical reactions.

In this connection, when considering the mechanism of the process of oxidation of \(SO_2\) to \(SO_3\) in the presence of chromium-tin catalysts, apparently the principal role should be ascribed to forces of chemical affinity, the action of which causes the formation and subsequent decomposition, on the catalyst surface, of unstable intermediate compounds with the direct participation in the process of the oxygen of the catalyst. The existence of significant affinity forces between the atoms of chromium, oxygen, and sulfur for an ordinary macro-

the phase state of the solid substance is demonstrated by the formation of numerous oxides [17] and chromium sulfates [18]. The chromium oxides CrO₃, CrO₂, Cr₅O₁₃, Cr₅O₁₂ in the macrophase state, depending on the oxygen content, may exist at temperatures below 310–410°; as for the basic chromium sulfates Cr₂O₃·2SO₃ and Cr₂O₃·3SO₃, they decompose at higher temperatures, 450–460° and above. The conditions of existence of the surface intermediate compounds of the oxide and sulfate types that are formed may differ substantially from those characteristic of macrophases. However, data on this question are lacking, and as a first approximation, when considering the mechanism of oxidation of SO₂ to SO₃, one may proceed from the usual properties of the indicated substances.

If one takes into account the presence of a downward break in the straight line

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

which, as is known from kinetic analysis [19], indicates the parallel course of two processes, as well as the properties and regions of existence of chromium oxides and sulfates, then it may be assumed that in the region of lower temperatures, in the presence of Cr₂O₃, the oxidation of SO₂ proceeds through its interaction with CrO₂; the Cr₂O₃ formed in this process is again oxidized to CrO₂ by adsorbed oxygen. An analogous process under these conditions may proceed with the participation of Cr₅O₁₃ and Cr₅O₁₂. In the region of higher temperatures, where the existence of CrO₃ and Cr₅O₁₃ becomes less probable, the dominant role is acquired by the reactions of oxidation of SO₂ to SO₃ through the formation (from 2Cr₂O₃·3SO₃, SO₂ and O₂) and subsequent decomposition on the catalyst surface, with elimination of SO₃, of the basic sulfate Cr₂O₃·2SO₃. This process is accompanied by the expenditure of higher activation energy. As for the mechanism of coactivation, then, in accordance with work by one of us jointly with N. P. Figurovskaya [20], carried out experimentally later, we associate it with a deeper chemical interaction of Cr₂O₃ and SnO₂ than occurs in structural activation.

It may be assumed that the following reactions proceed on the surface of the chromium–tin catalysts studied by us:

\[ \left. \begin{aligned} \mathrm{Cr_2O_3} + \mathrm{SnO_2} &\rightleftarrows 2\mathrm{CrO_2} + \mathrm{SnO} \\ 2\mathrm{CrO_2} + \mathrm{SO_2} &\rightleftarrows \mathrm{Cr_2O_3} + \mathrm{SO_3} \\ 2\mathrm{SnO} + \mathrm{O_2} &\rightleftarrows 2\mathrm{SnO_2} \end{aligned} \right\} \tag{5} \]

\[ \left. \begin{aligned} 2(\mathrm{Cr_2O_3 \cdot 2SO_3}) &\rightleftarrows 2\mathrm{Cr_2O_3 \cdot 3SO_3} + \mathrm{SO_3} \\ 2\mathrm{Cr_2O_3 \cdot 3SO_3} + \mathrm{SnO_2} &\rightleftarrows 2(\mathrm{Cr_2O_3 \cdot 2SO_3}) + \mathrm{SnO} \\ 2\mathrm{SnO} + \mathrm{O_2} &\rightleftarrows 2\mathrm{SnO_2} \end{aligned} \right\} \tag{6} \]

Taking into account the character of the break in the straight lines

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

it should be considered that reactions (5) proceed predominantly at lower temperatures, and reactions (6) at elevated temperatures. These reactions are monomolecular with respect to SO₂ and O₂, which agrees with kinetic equation (2).

The foregoing does not deny the presence of structural activation for the mixed chromium–tin catalysts studied by us; on the contrary, its presence and its superposition on the phenomenon of coactivation apparently lead to the appearance of a complex dependence of the catalytic properties of the Cr₂O₃ + SnO₂ system as the SnO₂ content in the mixed catalyst increases.

Conclusions

1. The catalytic properties of the system \(\mathrm{Cr_2O_3} + \mathrm{SnO_2}\) in the oxidation of \(\mathrm{SO_2}\) by molecular oxygen were studied dynamically and by the method under isothermal conditions, as a function of the \(\mathrm{SnO_2}\) content in the catalyst (from 0 to 100 wt.%) and of temperature (350–600°C), at a space velocity of 1000 and an initial composition of the gas mixture \(\mathrm{SO_2}\)—7.0 (vol.%) + air. Catalysts with 0.619 and 1.224 wt.% \(\mathrm{SnO_2}\) were also studied at different space velocities (200 to 1200) of the gas mixture. The catalysts were prepared by mechanical mixing of commercial chemically pure \(\mathrm{Cr_2O_3}\) with \(\mathrm{SnO_2}\).

2. With an increase in the space velocity, the degree of oxidation of \(\mathrm{SO_2}\) to \(\mathrm{SO_3}\) decreases more slowly than the space velocity increases.

3. With increasing temperature, for the most active chromium–tin catalysts the degree of oxidation of \(\mathrm{SO_2}\) to \(\mathrm{SO_3}\) first increases, reaches a maximum value, and then decreases, following the equilibrium curve; the optimum temperatures lie within the range 550–600°. For less active catalysts, the degrees of oxidation in the temperature range investigated increase continuously, and the optimum temperatures lie above 600°.

4. The change in the catalytic properties of the \(\mathrm{Cr_2O_3} + \mathrm{SnO_2}\) system with increasing \(\mathrm{SnO_2}\) content does not obey the rule of additivity and is expressed by a curve having two maxima, lying at \(\mathrm{SnO_2}\) contents in the catalyst of 1.224 and 49.78 wt.%. For the system, the phenomenon of coactivation is distinctly observed, with the highest development of catalytic properties at a content of 49.78 wt.% \(\mathrm{SnO_2}\).

5. The kinetics of the oxidation process of \(\mathrm{SO_2}\) to \(\mathrm{SO_3}\) under the conditions studied is satisfactorily expressed by the equation

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

1. In the presence of \(\mathrm{Cr_2O_3}\), \(\mathrm{SnO_2}\), and also all mixed chromium–tin catalysts, the straight lines

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

have a downward-directed break in the temperature region 400–500°.

The apparent activation energies for the regions of lower and higher temperatures are respectively 9380–23900 and 29500–39700 cal/mol.

1. The paper considers a probable mechanism of the oxidation process of \(\mathrm{SO_2}\).

Literature

1. Аdadurov I. E., Gernet D. V., and Khitun A. M. Zhurn. prikl. khim., 7, 6, 875, 1934.
2. Аdadurov I. E., Gernet D. V. Zhurn. prikl. khim., 8, 4, 606, 1935.
3. Аdadurov I. E., Afanasenko M. F., et al. Zhurn. prikl. khim., 7, 8, 1934.
4. Аdadurov I. E. and Gernet D. V. Zhurn. pr. kl. khim., 8, 4, 612, 1935.
5. Аdadurov I. E. and Fonicheva T. A. Zhurn. prikl. khim., vol. 9, 9, 158, 1936.
6. Аdadurov I. E., Gernet D. V. Zhurn. prikl. khim., 9, 4, 603, 1936.
7. Аdadurov I. E., Gernet D. V. Zhurn. prikl. khim., 10, 245, 1937.
8. Аdadurov I. E. Zhurn. prikl. khim., 11, 3, 470, 1937.
9. Gernet D. V. and Khitun A. M. Zhurn. prikl. khim., 8, 4, 598, 1935.
10. Postnikov V. F., Kunin T. I., and Astasheva A. A. Zhurn. prikl. khim., 9, 8, 1873, 1936.
11. Neuman B. Z. Elektrochem. u. ang. phys. Chem., 1, 42, 1929.
12. Kurin N. P., Sitov S. A., Semenova G. I., and Vorob’eva M. N. Izv. Tomsk. industrial institute named after S. M. Kirov, vol. 60, issue 3, 79, 1940.
13. Kurin N. P. Zhurn. khim. prom., 7, 475, 1937.
14. Taylor and Lenher. Z. Phys. Chem., 30, 1931.
15. Boreskov G. K. and Plygunov. Zhurn. prikl. khim., 6, 5, 785, 1933.
16. Kuz’minykh I. N. and Turkhan. Zhurn. khim. prom., 6, 22, 1931.
17. Simon und Schmidt. Z. anorg. u. algem. Chem., 153, 1926.
18. Wöhler Z., Pluddermann und P. Wöhler. Z. Phys. Chem., 62, 641, 1908.
19. Hinshelwood. Kinetics of Gas Reactions, ONTI, 1932.