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Role of model heterogeneous gas-solid phase reversible reactions in heterogeneous catalysis
496
Journal of Molecular
Catalysis,
54 (1989) 496 - 500
ROLE OF MODEL HETEROGENEOUS GAS-SOLID PHASE REVERSIBLE REACTIONS IN HETEROGENEOUS CATALYSIS HANNA REMBERTOWICZ Institute of Znorganic Technology, Warsaw University ul. Noakowskiego 3, 00-662 Warsaw (Poland)
of Technology,
Introduction
In heterogeneous gas-solid phase reversible reactions, two types of reaction take place simultaneously on the surface of the catalyst. One of these types involves reactions between the gaseous components and the catalyst, and results directly from the contact of solid catalyst with gas components at high temperatures. Sometimes these reactions can affect the phase transitions of the bulk.of the solid catalyst. There is no equilibrium between the gaseous reagents during the catalytic process. Therefore, thermodynamic equilibrium between gaseous reagents and the solid catalyst is not attained either. However, if the gas phase composition and temperature remain unchanged over each element of the solid catalyst for a long time, the element should adjust its composition to the local reagent’s concentration and temperature. This composition is known as a stationary state, and the entire system reaches a stationary equilibrium, which is different from a thermodynamic one.
Stationary state of oxide catalysts
Studies of the mechanism of catalyst stationary state formation were carried out on the systems VzOs, VOS04, SOz, SO3 and 02, i.e. catalystreagents of the catalytic oxidation of SOz, as an example. A thermodynamic analysis of the system and the conditions of the partial reactions between gaseous reagents and the solid catalyst have been presented earlier [ 1,2]. It was believed that the stationary equilibrium between Vz05 and VOS04 resulted from kinetic competition of the opposing reactions: Vz05 + SO2 + SO3 -+ 2VOS04 and 2VOS04 + i02 --f Vz05 + 250s. Depending on the ratio of the rate of VOS04 synthesis to VOS04 decomposition, only one of these phases, Vz05 or VOS04, is found in the stationary state. The stationary equilibrium between VOS04 and Vz05 was estimated, and can be expressed by a surface in a space diagram using coordinates pso2, pso, and PO, for each temperature. An example of such a diagram is presented in Fig. 1. The diagram shows the stationary stability field of Vz05 and VOS04 as a function of the parameters pi and T. 0304-5102/89/$3.60
@ ElsevierSequoia/Printedin The Netherlands
497
(4 Fig. 1. (a) Stationary equilibrium between VzO5 2 VOSOJ phases at 807 K; (b) thermodynamic equilibrium of partial reactions: (1) VzOs + 2SO3-* VOSO4 + *,; (2) VzO5 + SO3 + SO2 + 2VOS04; (3) V205 + 2S02 + $2 + 2VOSO4 at 807 K.
1. The dynamic character of the oxide catalyst surface in stationary state conditions During the course of catalytic oxidation of SO2 on a pure Vz05 catalyst, a restructuring of the catalyst surface was observed, even if the process conditions corresponded to its stationary field. This observation was confirmed for different shapes of the catalyst. The most pronounced changes were observed on well-formed Vz05 polycrystals [ 31. The initially flat, shiny crystal faces of VzOs underwent a dramatic restructuring in the course of the catalytic oxidation of S02. Small crystals and quasi-amorphous forms were formed. The intensity of the restructuring of different crystal faces varied, and even the same face was rebuilt differently. Generally, the nature and scope of this restructuring depended on the sample heating conditions. An example of the polycrystalline Vz05 sample scanning image before and after SO2 oxidation in given conditions are shown in Figs. 2 and 3. Heating of the well-formed V,O, polycrystals under stationary stability field conditions should lead only to crystal growth, and should not result in its destruction and the formation of new small crystals, as was observed. On examination of the chemical composition of the rebuilt V,O, surface by X-ray microprobe analysis, sulphur was found in some places. Since the amount of sulphur was very small, it could not be detected by chemical analysis. The sulphur distribution on the polycrystalline surface is shown in Fig. 4. A more complete analysis of this phenomenon has been presented earlier [3]. Generally, the distribution of sulphur on the rebuilt surface was not uniform, and there was no distinct correlation between the extent of surface restructuring and the sulphur content. No sulphur was found on the rebuilt surface when the rate of catalytic oxidation of SO2 (far from the gas
498
Fig. 2. Polycrystalline Vz05 surface before reaction. Fig. 3. Polycrystalline VzO5 surface after catalytic oxidation of SOz; reaction conditions: = 0.008 atm;pSO, = 0.092 atm;po, = 0.134 atm; T = 803 K.
Pso,
Fig. 4. Sulphur content on same crystal face as in Fig. 3.
phase equilibrium) was at a maximum. V4+ ions were also found in the sulphurcontaining samples. The coordination of that ion was different from the coordination of V4+ in the samples without sulphur. It was found that heating V205 in oxygen and SO2 did not cause surface restructuring. The restructuring and incorporation of sulphur into the sur-
499
face occurred only when catalytic oxidation of SO2 or SO3 decomposition could take place. To summarize, restructuring of some parts of the surface took place. However, other parts of the polycrystallites remained unchanged. Sulphur was found only on some parts of the rebuilt surface. Nevertheless, the range of parameters @,T) for surface restructuring was wider than for the appearance of sulphur. Discussion The restructuring of an oxide catalyst surface under stationary conditions was previously unknown. A similar restructuring of metal catalysts in oxidation reactions is well known [4]. It is assumed that the restructuring results from the formation and decomposition of surface intermediates in the catalytic reaction. Considering the proposed mechanism of oxidation, the two-step mechanism of Mars and van Krevelen [5] could explain the observed restructuring if reduction of the oxide catalyst leads to structures much larger than single point defects. The reoxidation of those structures might lead to the weakening of bonds in the oxide structures, easier migration of ions and consequently to restructuring. Similarly, the shear plane formation mechanism, together with rapid transport of oxygen ions in the lattice during the reduction stages [6] and subsequent reoxidation, could explain the texture destruction and reconstruction in other places in the same plane. The most important question is whether the reaction between gas components and catalyst, parallel to the catalytic reaction, plays a role in the surface restructuring. Under conditions of stationary stability of VzOs, the synthesis of VOS04 from V,O, is impossible. Taking into consideration the reversible mechanism of heterogeneous reactions, the sorption penetration of gaseous reactants into the lattice and stages of nucleus precursor formation are possible. The reverse of these reactions could cause surface restructuring. Surface restructuring was observed over all the ranges of VzOs stability. Moreover, the catalytic reaction and gas-solid reactions took place within the same range. This indicates that both reactions were responsible for surface restructuring. Nevertheless, the occurrence of both reactions was not sufficient for the observed restructuring. Different polycrystal faces were rebuilt in different manners, and one part of the same crystal face was rebuilt, while the other remained smooth. Therefore the crystal structure, as well as its defect distribution, could be an additional condition for restructuring. Another condition could be the ratio of the rate of catalytic reaction to gas-solid reaction, because the restructuring image depends on the temperature and partial pressures of gaseous reagents. The analysis of the phenomenon of sulphur incorporation into the surface of Vz05 indicates that surface restructuring is a necessary condition. However, there was no sulphur on some rebuilt surface parts. This means
500
that surface restructuring was not a sufficient condition for sulphur incorporation. It is possible that an appropriate ratio of the rate of catalytic reaction to the rate of solid-gas reaction is an additional condition. It is difficult to explain why there was no sulphur on V20, samples heated under conditions far from equilibrium of catalytic reactions, i.e. under conditions of maximum oxidation rate. One explanation for these phenomena is that both reactions have a common step in their mechanisms. As a result, they are competitive. Then the maximum oxidation rate made the gas-solid rate very small. In conclusion, the permanent restructuring of Vz05 surfaces and sulphur incorporation was observed in the course of catalytic oxidation of S02. This phenomenon took place in the stability field of V,O, when phase change was impossible. In effect, the surface has a dynamic character resulting from the catalytic oxidation of SO2 and the reversible initial steps of the gas-solid reactions.
References 1 H. Kofakowska-Rembertowicz, in Proc. 4th Znt. Symp. Metal Catalysis, Varna, 1979, p. 193. 2 H. Kohxkowska-Rembertowicz, in Proc. Conf. Solid State Chem., Acad. Minn. Met., Math. Physics, Chem. No. 611, Cracow 1981, p. 91. 3 H. Rembertowicz, in Proc. 6th Znt. Symp. Metal Catalysis, Sofia, 1987, p. 400. 4 J. Zawadzki, Roczn. Chem., 7 (1927) 369. 5 P. Mars and D. W. van Krevelen, Chem. Eng. Sci., 3 (1954) 41. 6 J. Haber, J. Less-Common Met., 54 (1977) 243.
Journal of Molecular
Catalysis,
54 (1989) 496 - 500
ROLE OF MODEL HETEROGENEOUS GAS-SOLID PHASE REVERSIBLE REACTIONS IN HETEROGENEOUS CATALYSIS HANNA REMBERTOWICZ Institute of Znorganic Technology, Warsaw University ul. Noakowskiego 3, 00-662 Warsaw (Poland)
of Technology,
Introduction
In heterogeneous gas-solid phase reversible reactions, two types of reaction take place simultaneously on the surface of the catalyst. One of these types involves reactions between the gaseous components and the catalyst, and results directly from the contact of solid catalyst with gas components at high temperatures. Sometimes these reactions can affect the phase transitions of the bulk.of the solid catalyst. There is no equilibrium between the gaseous reagents during the catalytic process. Therefore, thermodynamic equilibrium between gaseous reagents and the solid catalyst is not attained either. However, if the gas phase composition and temperature remain unchanged over each element of the solid catalyst for a long time, the element should adjust its composition to the local reagent’s concentration and temperature. This composition is known as a stationary state, and the entire system reaches a stationary equilibrium, which is different from a thermodynamic one.
Stationary state of oxide catalysts
Studies of the mechanism of catalyst stationary state formation were carried out on the systems VzOs, VOS04, SOz, SO3 and 02, i.e. catalystreagents of the catalytic oxidation of SOz, as an example. A thermodynamic analysis of the system and the conditions of the partial reactions between gaseous reagents and the solid catalyst have been presented earlier [ 1,2]. It was believed that the stationary equilibrium between Vz05 and VOS04 resulted from kinetic competition of the opposing reactions: Vz05 + SO2 + SO3 -+ 2VOS04 and 2VOS04 + i02 --f Vz05 + 250s. Depending on the ratio of the rate of VOS04 synthesis to VOS04 decomposition, only one of these phases, Vz05 or VOS04, is found in the stationary state. The stationary equilibrium between VOS04 and Vz05 was estimated, and can be expressed by a surface in a space diagram using coordinates pso2, pso, and PO, for each temperature. An example of such a diagram is presented in Fig. 1. The diagram shows the stationary stability field of Vz05 and VOS04 as a function of the parameters pi and T. 0304-5102/89/$3.60
@ ElsevierSequoia/Printedin The Netherlands
497
(4 Fig. 1. (a) Stationary equilibrium between VzO5 2 VOSOJ phases at 807 K; (b) thermodynamic equilibrium of partial reactions: (1) VzOs + 2SO3-* VOSO4 + *,; (2) VzO5 + SO3 + SO2 + 2VOS04; (3) V205 + 2S02 + $2 + 2VOSO4 at 807 K.
1. The dynamic character of the oxide catalyst surface in stationary state conditions During the course of catalytic oxidation of SO2 on a pure Vz05 catalyst, a restructuring of the catalyst surface was observed, even if the process conditions corresponded to its stationary field. This observation was confirmed for different shapes of the catalyst. The most pronounced changes were observed on well-formed Vz05 polycrystals [ 31. The initially flat, shiny crystal faces of VzOs underwent a dramatic restructuring in the course of the catalytic oxidation of S02. Small crystals and quasi-amorphous forms were formed. The intensity of the restructuring of different crystal faces varied, and even the same face was rebuilt differently. Generally, the nature and scope of this restructuring depended on the sample heating conditions. An example of the polycrystalline Vz05 sample scanning image before and after SO2 oxidation in given conditions are shown in Figs. 2 and 3. Heating of the well-formed V,O, polycrystals under stationary stability field conditions should lead only to crystal growth, and should not result in its destruction and the formation of new small crystals, as was observed. On examination of the chemical composition of the rebuilt V,O, surface by X-ray microprobe analysis, sulphur was found in some places. Since the amount of sulphur was very small, it could not be detected by chemical analysis. The sulphur distribution on the polycrystalline surface is shown in Fig. 4. A more complete analysis of this phenomenon has been presented earlier [3]. Generally, the distribution of sulphur on the rebuilt surface was not uniform, and there was no distinct correlation between the extent of surface restructuring and the sulphur content. No sulphur was found on the rebuilt surface when the rate of catalytic oxidation of SO2 (far from the gas
498
Fig. 2. Polycrystalline Vz05 surface before reaction. Fig. 3. Polycrystalline VzO5 surface after catalytic oxidation of SOz; reaction conditions: = 0.008 atm;pSO, = 0.092 atm;po, = 0.134 atm; T = 803 K.
Pso,
Fig. 4. Sulphur content on same crystal face as in Fig. 3.
phase equilibrium) was at a maximum. V4+ ions were also found in the sulphurcontaining samples. The coordination of that ion was different from the coordination of V4+ in the samples without sulphur. It was found that heating V205 in oxygen and SO2 did not cause surface restructuring. The restructuring and incorporation of sulphur into the sur-
499
face occurred only when catalytic oxidation of SO2 or SO3 decomposition could take place. To summarize, restructuring of some parts of the surface took place. However, other parts of the polycrystallites remained unchanged. Sulphur was found only on some parts of the rebuilt surface. Nevertheless, the range of parameters @,T) for surface restructuring was wider than for the appearance of sulphur. Discussion The restructuring of an oxide catalyst surface under stationary conditions was previously unknown. A similar restructuring of metal catalysts in oxidation reactions is well known [4]. It is assumed that the restructuring results from the formation and decomposition of surface intermediates in the catalytic reaction. Considering the proposed mechanism of oxidation, the two-step mechanism of Mars and van Krevelen [5] could explain the observed restructuring if reduction of the oxide catalyst leads to structures much larger than single point defects. The reoxidation of those structures might lead to the weakening of bonds in the oxide structures, easier migration of ions and consequently to restructuring. Similarly, the shear plane formation mechanism, together with rapid transport of oxygen ions in the lattice during the reduction stages [6] and subsequent reoxidation, could explain the texture destruction and reconstruction in other places in the same plane. The most important question is whether the reaction between gas components and catalyst, parallel to the catalytic reaction, plays a role in the surface restructuring. Under conditions of stationary stability of VzOs, the synthesis of VOS04 from V,O, is impossible. Taking into consideration the reversible mechanism of heterogeneous reactions, the sorption penetration of gaseous reactants into the lattice and stages of nucleus precursor formation are possible. The reverse of these reactions could cause surface restructuring. Surface restructuring was observed over all the ranges of VzOs stability. Moreover, the catalytic reaction and gas-solid reactions took place within the same range. This indicates that both reactions were responsible for surface restructuring. Nevertheless, the occurrence of both reactions was not sufficient for the observed restructuring. Different polycrystal faces were rebuilt in different manners, and one part of the same crystal face was rebuilt, while the other remained smooth. Therefore the crystal structure, as well as its defect distribution, could be an additional condition for restructuring. Another condition could be the ratio of the rate of catalytic reaction to gas-solid reaction, because the restructuring image depends on the temperature and partial pressures of gaseous reagents. The analysis of the phenomenon of sulphur incorporation into the surface of Vz05 indicates that surface restructuring is a necessary condition. However, there was no sulphur on some rebuilt surface parts. This means
500
that surface restructuring was not a sufficient condition for sulphur incorporation. It is possible that an appropriate ratio of the rate of catalytic reaction to the rate of solid-gas reaction is an additional condition. It is difficult to explain why there was no sulphur on V20, samples heated under conditions far from equilibrium of catalytic reactions, i.e. under conditions of maximum oxidation rate. One explanation for these phenomena is that both reactions have a common step in their mechanisms. As a result, they are competitive. Then the maximum oxidation rate made the gas-solid rate very small. In conclusion, the permanent restructuring of Vz05 surfaces and sulphur incorporation was observed in the course of catalytic oxidation of S02. This phenomenon took place in the stability field of V,O, when phase change was impossible. In effect, the surface has a dynamic character resulting from the catalytic oxidation of SO2 and the reversible initial steps of the gas-solid reactions.
References 1 H. Kofakowska-Rembertowicz, in Proc. 4th Znt. Symp. Metal Catalysis, Varna, 1979, p. 193. 2 H. Kohxkowska-Rembertowicz, in Proc. Conf. Solid State Chem., Acad. Minn. Met., Math. Physics, Chem. No. 611, Cracow 1981, p. 91. 3 H. Rembertowicz, in Proc. 6th Znt. Symp. Metal Catalysis, Sofia, 1987, p. 400. 4 J. Zawadzki, Roczn. Chem., 7 (1927) 369. 5 P. Mars and D. W. van Krevelen, Chem. Eng. Sci., 3 (1954) 41. 6 J. Haber, J. Less-Common Met., 54 (1977) 243.