Effect of Palladium Addition on the Rate of Interaction of Solid Acetates with Hydrogen Sulfide

A. P. Buntin, V. N. Neustroeva

Reactions involving solid substances, including topochemical reactions, have recently been receiving increasing attention in the specialized literature. This is due to the fact that they find broad application in the practice of the national economy. However, up to the present time the practical use of topochemical transformations in industry has been limited because much remains unstudied with respect to the mechanism and kinetics of this type of reaction. Consequently, expanding the field of investigation of reactions involving solid substances and identifying the laws governing the course of such systems remains one of the important problems to this day. Many scientists are working on its solution: S. Z. Roginskii, N. N. Semenov, B. V. Erofeev, G. I. Chufarov, A. P. Buntin, Garner, Yander, Fischbeck, and others.

The principal difficulty in the mechanism and kinetics of topochemical reactions lies in the fact that the rate of their occurrence depends on many factors. Among the little-studied factors may be included the influence of additives on the rate of topochemical reactions. We have carried out a study of the influence of palladium addition on the rate of interaction of solid acetates of mercury, copper, cadmium, chromium, barium, iron(III), and iron(II). Solid acetates were chosen as systems on which to study the influence of additives because, in the interaction of metal acetates with hydrogen sulfide, a volatile substance—acetic acid—is formed as a by-product of the reaction and can readily be removed from the reaction sphere.

The study was carried out under dynamic conditions (Fig. 1). The reaction proceeded in the reaction vessel (7), placed in a thermostat, at a constant temperature of \(25^\circ\mathrm{C} \pm 0.2^\circ\mathrm{C}\) and a gas-flow rate of 12 l/h. The rate of interaction of acetates with hydrogen sulfide was judged from the change in weight of the solid phase using spring quartz balances, whose sensitivity was \(4.6 \cdot 10^{-4}\) g. The by-product of the reaction—acetic acid—and unreacted hydrogen sulfide were periodically removed from the reaction vessel by evacuation with a fore-vacuum pump. The introduction of palladium additive into the solid acetates was carried out by simple mixing and thorough grinding of the solid acetate with palladium powder. The crystals of the corresponding metal acetates were crushed. For the experiments, ...

fraction 0.5–0.25 mm. Palladium powder of the grade “Palladin metall X. 4” was sieved through a sieve with 3600 openings per 1 cm². Below are presented the results of the investigation for each of the acetates separately.

Fig. 1. Schematic of the apparatus.

Mercury acetate. Mercury acetate was prepared according to the recipe of Gmelin–Kraut [4]. Analysis of the curves (Figs. 2 and 3) shows that additions of palladium considerably accelerate the process of conversion of mercury acetate into sulfide.

1.15% addition of palladium practically does not change the rate of the process.

3.21% addition increases the reaction rate by 3.23 times.

5.36% addition increases the reaction rate by 4.65 times.

10.31% addition increases the reaction rate by 6.53 times.

24.95% addition increases the reaction rate by 6.93 times.

As can be seen from Fig. 2, the greatest catalytic activity of palladium is manifested when the amount of the introduced addition is increased from 1 to 3% of the weight of mercury acetate. With a further increase in the amount of the introduced addition, the reaction rate increases not in proportion to the amount of the introduced addition. The larger the amount of the addition, the less it affects the change in the rate of interaction of mercury acetate with hydrogen sulfide. It can be seen that, in this case, a palladium addition in an amount of 10.31% is the limit of its effect on the reaction rate. Further increase in the amount of palladium added above 10% has practically no effect on the change in the rate of the process. It should be noted that the influence of palla-

Fig. 2. Change in the rate constant of the reaction of mercury acetate with hydrogen sulfide upon introduction of different amounts of palladium additive.

…is manifested mainly at the initial stage of the process. In addition, in the presence of the additive the rate maximum shifts toward the ordinate axis (Fig. 3).

Copper acetate. The copper acetate used for the investigation was recrystallized according to the procedure of Singalovskii [1]. Dehydration of the preparation was carried out first in a vacuum desiccator over sulfuric acid, then in a drying oven at a temperature of 105–110° to constant weight.

Fig. 3. Effect of palladium additives on the rate of interaction of mercury acetate with hydrogen sulfide.

Fig. 4. Effect of palladium additives on the rate of interaction of copper acetate with hydrogen sulfide.

Palladium additives considerably accelerate the process of conversion of copper acetate into sulfide (Fig. 4). As the amount of additive introduced is increased, the reaction rate also increases, although, as in the case of mercury acetate, there is no proportional dependence here.

A 1.06% additive increases the reaction rate insignificantly.
A 3.38% additive increases the reaction rate by a factor of 1.6.
A 4.98% additive increases the reaction rate by a factor of 2.16.
A 9.98% additive increases the reaction rate by a factor of 2.7.

Cadmium acetate. The anhydrous preparation was obtained by removing the water of crystallization by drying the crystalline hydrate first in a vacuum desiccator over sulfuric acid, then in a drying oven at a temperature of 75°C to constant weight. Cadmium acetate without additives reacts extremely rapidly with hydrogen sulfide. During the first 3 minutes of interaction, cadmium acetate is converted into sulfide by 70.78%. The experimental data show that, at such a high reaction rate, additives have practically no effect on it.

Chromium acetate. Anhydrous chromium acetate was prepared according to the procedure of Beilstein [5]. Chromium acetate reacts slowly with hydrogen sulfide, and the degree of its conversion into sulfide is 7.65%. Evidently, the reaction product formed—chromium sulfide in the form of a dense crust—envelops all the chromium acetate, creating an obstacle to further conversion of the substance. Palladium additives accelerate the process only slightly. A 10.67% additive increases the reaction rate by a factor of 1.2. The degree of conversion also does not change upon introduction of the additive.

Barium acetate. The anhydrous preparation was obtained by heating the crystalline salt at 70°C to constant weight. Under our conditions, barium acetate practically does not react with hydrogen sulfide. The degree of its conversion into sulfide is 1.67%. Palladium additions change neither the reaction rate nor the degree of conversion.

Iron(III) acetate. Anhydrous iron(III) acetate was prepared according to Belyshtein’s recipe [5]. Ferric acetate without additions reacts rather rapidly with hydrogen sulfide. The degree of conversion of the acetate into sulfide at the 3rd minute of interaction is 41.2%. In this case, what takes place is not an exchange topochemical reaction, but an oxidation–reduction reaction, and the process is accompanied by the formation of iron(II) sulfide and free sulfur. The oxidation–reduction character of the reaction was established on the basis of analysis of the reaction products. The nature of the influence of additions in this case differs from that for the exchange reactions obtained by us. Palladium does not accelerate the process, but slows it down.

Iron(II) acetate. Iron(II) acetate was obtained by interaction of barium acetate with ferrous sulfate. Owing to the rapid oxidizability of solutions of divalent iron, all operations (crystallization, dehydration of the crystals, etc.) were carried out in a nitrogen atmosphere. Nevertheless, the divalent iron preparation obtained contained impurities of trivalent iron in the amount of 12.34%. Thus, the experiments were carried out not with a pure preparation. Therefore the influence of the palladium addition tested by us obviously does not reflect the true picture of the dependence of the reaction rate of iron(II) acetate with hydrogen sulfide on palladium additions.

As a result of the investigation it was established that the rate of interaction of iron(II) acetate with hydrogen sulfide is 1.5 times lower than that for iron(III) acetate.

Discussion of Results and Conclusions

Comparing the experimental data obtained, one may conclude that, depending on the nature of the salt, palladium exerts different effects on the rate of interaction of solid acetates with hydrogen sulfide. In addition, the reaction rate of solid acetates with hydrogen sulfide changes significantly when different amounts of palladium are introduced.

Taking into account the available literature data on the study of the phenomena of gas uptake and adsorption by a solid surface, experimental data from the field of catalysis, and investigations currently being carried out by the schools of Semenov, Kondrat’ev, Voevodskii, Polyakov, and others on the initiation and termination of chain transformation on a solid surface, as well as factual material on the study of the mechanism of reactions involving solid substances, one may draw a number of conclusions regarding the possible mechanism of interaction of solid acetates with hydrogen sulfide and the mechanism of the influence of palladium additions. First of all, these data make it possible to conclude that, when a solid body interacts with a gas, adsorption of the gaseous reagent by the surface of the solid substance occurs, followed by loosening of the bonds in the adsorbed molecule. In some cases this may lead to very considerable deformation, up to rupture of the molecule into atoms and radicals. There is no doubt that the structure of the adsorbed molecule, the nature of the chemical bond in it, polarizability, etc., on the one hand, and the state of the surface of the solid body and its structure, on the other, will determine the state of the adsorbed molecule.

In our case, the course of the interaction of acetates with hydrogen sulfide in the absence of a medium with a high dielectric constant, the predominantly covalent character of the bonds of the atoms in the hydrogen sulfide molecule (64% according to Syrkin and Dyatkina), the weakness of the H–S bond, and the greater possibility for the gas to come into contact with the solid surface owing to the high dispersity of the palladium powder lead us to the conclusion that dissociation of hydrogen sulfide on the solid surface (acetates, sulfides, additives) into hydrogen and sulfur atoms is possible:

\[ \mathrm{H_2S \rightleftarrows H_2S \rightleftarrows H\cdot + HS\cdot \rightleftarrows 2H + S.} \]

The hydrogen and sulfur atoms formed, being exceptionally reactive particles, will interact with valence-unsaturated molecules, in our case with particles of the crystalline substance. As a first approximation, the interaction of the radicals H and S that are formed with solid acetates may be represented by the following scheme:

\[ \begin{matrix} \mathrm{CH_3COO} \\ \mathrm{CH_3COO} \end{matrix} \mathrm{Me} - 2\mathrm{H} = 2\mathrm{CH_3COOH} + \mathrm{Me} \]

\[ \mathrm{Me} + \mathrm{H_2S} = \mathrm{MeS} + 2\mathrm{H}, \text{ etc.} \]

or

\[ \begin{matrix} \mathrm{CH_3COO} \\ \mathrm{CH_3COO} \end{matrix} \mathrm{Me} + \mathrm{S} = \mathrm{MeS} + 2\mathrm{CH_3COO} \]

\[ 2\mathrm{CH_3COO} + \mathrm{H_2S} = 2\mathrm{CH_3COOH} + \mathrm{S}, \text{ etc.,} \]

i.e., the appearance of primary active particles in the form of atoms and radicals under appropriate conditions may lead subsequently to the development of a chain in the solid and gaseous phases. But, obviously, the possibility of interaction of an atom (radical) with a molecule of the crystalline substance will depend, above all, on the strength of the bond of the metal with the elements composing the crystal lattice, and on the tendency of the cation to pass into the atomic state.

As a first approximation, the stability of compounds with respect to dissociation into uncharged particles may be characterized by the heat of formation of the compound. We took into account the heats of formation of the acetates studied, which are arranged in the series:

Assuming that atoms and radicals participate in the reaction mechanism, one might expect that the heat of formation of the compound could characterize the reactivity of the solid substance. However, this is not entirely so. The reactivity of solids depends on a whole series of causes, of which, as applied to our case, the following should be singled out: preliminary heat treatment, the oxidizability of the atoms formed, the possibility of their interaction with a side product of the reaction—acetic acid, etc. Evidently, the influence of these factors leads to the fact that the rate of interaction of mercury acetate with hydrogen sulfide is lower than that for cadmium and copper acetates. But the degree

the effect of palladium additions is in accordance with the order indicated above.

The rupture of the Me—O bond in acetates, with formation of the radical CH₃COO and a metal atom, will occur the more readily, the greater the tendency of the cation to pass from the ionic state into the atomic state. This tendency can be characterized by the ionization potential of the metal. The higher the ionization potential, the more difficult it is for the metal atom to take up electrons and, consequently, the more readily the metal will pass from the ionic state into the atomic state. The ionization potentials of the metals that enter into the acetates studied as cations have the following values:

Thus, the more readily electrons are taken up by the cation, the greater the reaction rate should be and the greater should be the effect of the additions.

Our experimental data confirm this proposition. The oxidation–reduction character of the reaction leads to a more rapid course of the process of interaction of iron(II) acetate with hydrogen sulfide than in the case of the exchange reaction of hydrogen sulfide with iron(II) acetate. As is known, an oxidation–reduction process promotes more rapid acceptance of electrons by the cation. An indication that an oxidation–reduction process accelerates certain exchange topochemical reactions is found in Buntin [2] and Kapkova [3]. The effect of the same amount of palladium addition, in terms of the degree of its action on the acetates, is in accordance with the values of the ionization potentials of the metals acting as cations in the given salt. Unfortunately, strict possibilities for comparison are lacking in our case. This is connected above all with the fact that the acetates used in the work could not be obtained under identical conditions.

It is known from the literature that the development of chain transformations proceeds better in exothermic reactions, and is less probable for endothermic reactions. Calculation showed that the thermal effects are in complete agreement with the degree of influence of the palladium addition on the reaction rate.

The calculations performed for the thermal effects of the reactions, the heats of formation of the acetates, and the ionization potentials of the metals acting as cations in the given salt show that the stronger the bond of the metal with the elements constituting the crystal lattice, the less the additions affect the kinetics of the process.

From these data one may attempt to substantiate the mechanism of the influence of palladium additions as a factor promoting the development of a chain in the solid phase. There is every reason to suppose that adsorption and dissociation of the gaseous reagent (hydrogen sulfide) occur on the surface of the metallic powder, which increases the number of free particles—hydrogen and sulfur atoms—and thereby promotes acceleration of the reaction in its first stage.

As is known, a characteristic feature of palladium is its high adsorption capacity with respect to hydrogen. If, in the course of the interaction, the hydrogen atoms formed are absorbed by palladium, this will create the possibility of preventing recombina-

…of hydrogen and sulfur atoms, while the sulfur atoms will be consumed by the substance with the subsequent development of the chain according to the scheme:

\[ \begin{aligned} &\begin{matrix} \mathrm{CH_3COO}\\ \mathrm{CH_3COO} \end{matrix} \Bigg\}\mathrm{Me} + \mathrm{S} \rightarrow \mathrm{MeS} + 2\mathrm{CH_3COO} \\ &2\mathrm{CH_3COO} + \mathrm{H_2S} \rightarrow 2\mathrm{CH_3COOH} + \mathrm{S},\ \text{etc.} \end{aligned} \]

Conclusions

1. The effect of the addition of palladium on the rate of interaction of hydrogen sulfide with solid acetates of mercury, copper, cadmium, chromium, barium, iron(II), and iron(III) has been studied. It has been established that the addition of palladium leads to a significant change in the rate of conversion of the acetates into sulfides.

2. It has been shown that, depending on the nature of the salt, palladium has different effects on the rate of interaction of solid acetates with hydrogen sulfide. The degree of influence of the palladium additive is strongly affected by the ionization potential of the metal acting as the cation in the given salt, the strength of the chemical bond, and the thermal effect of the reaction.

3. It has been demonstrated that the rate of the process changes somewhat when the amount of additive introduced is increased.

4. It has been shown that, if an oxidation–reduction process takes place, it promotes a more rapid course of the interaction of solid acetates with hydrogen sulfide.

Literature

1. Sintalovskii N. Salts of Rare and Nonferrous Metals, Moscow, 1922.
2. Buntrin A. P. and Aivazyan N. A. Proceedings of TSU, 101, 3, 1949.
3. Kapskova E. I. “Izvestiya TPI,” 83, 197, 1956.
4. Gmelin-Krauts. Handbuch der anorganischen Chemie, 5, 824, 1908.
5. Beilstein. Handbuch der organischen Chemie, 2, 120, 1910.