ISSN 0023-1584, Kinetics and Catalysis, 2018, Vol. 59, No. 1, pp. 104–111. © Pleiades Publishing, Ltd., 2018. Original Russian Text © T.N. Afonasenko, O.A. Bulavchenko, T.I. Gulyaeva, S.V. Tsybulya, P.G. Tsyrul’nikov, 2018, published in Kinetika i Kataliz, 2018, Vol. 59, No. 1, pp. 127–135.

Effect of the Calcination Temperature and Composition of the MnOx–ZrO2 System on Its Structure and Catalytic Properties in a Reaction of Carbon Monoxide Oxidation T. N. Afonasenkoa, *, O. A. Bulavchenkob, c, T. I. Gulyaevaa, S. V. Tsybulyab, c , **, and P. G. Tsyrul’nikova a

Institute of Hydrocarbon Processing, Siberian Branch, Russian Academy of Sciences, Omsk, 644040 Russia Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia cNovosibirsk State University, Novosibirsk, 630090 Russia *e-mail: [email protected] ** e-mail: [email protected]

b

Received June 22, 2017

Abstract—The effect of the calcination temperature and composition of the MnOx–ZrO2 system on its structural characteristics and catalytic properties in the reaction of CO oxidation was studied. According to X-ray diffraction analysis and H2 thermo-programmed reduction data, an increase in the calcination temperature of Mn0.12Zr0.88O2 from 450 to 900°C caused a structural transformation of the system accompanied by the disintegration of solid solution with the release of manganese ions from the structure of ZrO2 and the formation of, initially, highly dispersed MnOx particles and then a crystallized phase of Mn3O4. The dependence of the catalytic activity of MnOx–ZrO2 in the reaction of CO oxidation on the calcination temperature takes an extreme form. A maximum activity was observed after heat treatment at 650–700°C, i.e., at limiting temperatures for the occurrence of a solid solution of manganese ions in the cubic modification of ZrO2. If the manganese content was higher than that in the sample of Mn0.4Zr0.6O2, the phase composition of the system changed: the solid solution phase was supplemented with Mn2O3 and β-Mn3O4 phases. The samples of Mn0.4Zr0.6O2–Mn0.6Zr0.4O2 exhibited a maximum catalytic activity; this was likely due to the presence of the highly dispersed MnOx particles, which were not the solid solution constituents, on their surface in addition to an increase in the dispersity of the solid solution. Keywords: MnOx–ZrO2 catalysts, CO oxidation DOI: 10.1134/S0023158418010019

INTRODUCTION It is well known [1–4] that the oxides of transition metals (Cr, Co, Cu, Ni, Mn, etc.) find use as catalysts intended for the removal of volatile organic matter and CO from industrial wastes and exhaust gases from engines. They are capable of substituting supported catalysts based on noble metals, which manifest high activity, but they are expensive and sensitive to aging and poisoning with chlorine- and sulfur-containing compounds. Among the transition metal oxides, the oxides of manganese, which are inexpensive, very thermally stable, and resistant to poisons, are of the greatest interest as catalysts for the oxidation of hydrocarbons and CO [5–10]. Their catalytic activity is caused by the ability of manganese to form oxides with different degrees of oxidation of the metal (MnO2, Mn2O3, Mn3O4, and MnO) and a high capacity of the crystal lattice with respect to oxygen. The above oxides are the active catalysts of oxidation–reduction reactions because the ions of manganese readily pass from

one degree of oxidation to another. Both adsorbed and structural (lattice) oxygen participates in the oxidation of hydrocarbons and CO on the Mn-containing catalysts, as evidenced by the property of MnOx to oxidize hydrocarbons in the absence of oxygen from a gas phase [8, 9]. As is known [11, 12], the dispersity of MnOx particles in the supported manganese oxide catalysts is higher than that in the massive samples. This facilitates their increased catalytic activity because surface oxygen is more active (less strongly bound) in comparison with the bulk oxygen. In this case, the dispersity of MnOx depends on the nature of its precursor, its concentration, sample preparation method, catalyst calcination temperature, and the selection of a support. The comparison of the properties of Mn catalysts supported onto Al2O3, TiO2, and ZrO2 showed [13] that the structure and dispersity of an active phase is substantially influenced by the acid–base properties of a support, which are responsible for the strength of

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interactions in the oxide–support system. The catalytic activity increases in the order Mn/TiO2 < Mn/Al2O3 < Mn/ZrO2. The high activity of the mixed MnOx–ZrO2 system was related to the mutual actions of components on each other [14, 15] because ZrO2 stabilizes the manganese oxide phase in the catalytically active state of MnO2, and manganese ions enhance the formation of a metastable tetragonal modification of ZrO2 with a developed specific surface area. It is well known that the cations of Mn are capable of entering into the lattice of ZrO2 to form the solid solution Zr1 – yMnyO2 [14, 16], in which lattice oxygen acquires large mobility and, as believed, high reactivity. It was hypothesized [17, 18] that the oxygen of dispersed MnOx, which not entered into the composition of a solid solution, is active in oxidation reactions. Earlier, we investigated the stages of the reduction of mixed manganese and zirconium oxides with the use of in situ X-ray diffraction (XRD) analysis, thermo-programmed reduction, and in situ X-ray photoelectron spectroscopy [19]. At the concentrations of manganese ions to 30 at %, they completely entered into the structure of the solid solution MnyZr1 – yO2 – δ. These solutions were reduced in two stages over a wide temperature range of 100–650°C. At the first stage from 100 to 500°C, the partial reduction of manganese cations in the solid solution occurred as follows: 4 + /3 +

EXPERIMENTAL Sample Preparation The samples were prepared using the precipitation of hydroxides from a mixed solution of ZrO(NO3)2 and Mn(NO3)2 salts by gradually adding a solution of NH4OH to it with continuous stirring to reach pH 10. The resulting precipitate was filtered off, washed with water to pH 6, and dried at 120°C; then, it was calcined at a specified temperature for 4 h. The samples obtained were designated as MnyZr1 – yO2, where y is the mole fraction of manganese. In the preparation of the first series of catalysts (Mn0.12Zr0.88O2 samples), their heat treatment temperature was varied in a range of 450–900°C. In the second series, the composition of catalysts was changed from Mn0.12Zr0.88O2 to Mn0.9Zr0.1O2, and they were calcined at 650°C. The individual oxides ZrO2 and MnOx were also synthesized by the above precipitation method, and they were also calcined at 650°C for 4 h. Determination of Specific Surface Areas The specific surface areas (Ssp) of the samples was measured by the BET method on a Sorpty-1750 instrument (Carlo Erba, Italy) using the single-point adsorption of nitrogen at the pressure P = 135 Torr and the temperature T = 77 K. The relative error of the measurements was ±4%.

3 + /2 +

Mn 0.3 Zr0.7O 2 − δ → Mn 0.3 Zr0.7O 2 − δ . At the second stage at 500–650°C, the irreversible release of manganese cations from the structure of the solid solution and their localization on the surface of particles were observed; this was accompanied by the reduction of manganese to the state Mn2+: 3 + /2 +

105

2 + /3 +

Mn 0.3 Zr0.7O2 − δ1 → Mn 0.3 − a Zr0.7 + aO2 − δ2 + aMnO. In the samples containing more than 30 at % Mn, a portion of manganese did not enter into the structure of the solid solution, but it formed oxide particles as the crystallized phases of Mn2O3 and Mn3O4 and amorphized MnOx. In these samples, in addition to the reduction of the solid solution MnyZr1 – yO2 – δ, the degree of oxidation of the cations of manganese in the composition of oxides reversibly consecutively changed as follows: Mn2O3 → Mn3O4 → MnO. The aim of this work was to study structural transformations in the MnOx–ZrO2 system with variations in the calcination temperature and the composition of samples and the effects of the above factors on the catalytic activity in the reaction of CO oxidation. KINETICS AND CATALYSIS

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X-ray Diffraction Analysis The X-ray diffraction (XRD) analysis of the samples was carried out on a D-500 diffractometer (Siemens, Germany) with a step of 0.05° and an exposure time of 3 s at each point. For the precision measurement of parameters, the survey of some samples was performed on a D8 Advanced diffractometer (Bruker, Germany) with a step of 0.05° and an exposure time of 5 s. Thermo-Programmed Reduction The study of the samples by thermo-programmed reduction with H2 (TPR-H2) was performed on an AutoChem II 2920 precision chemisorption analyzer (Micromeritics, the United States) with a thermal conductivity detector using a mixture of 10 vol % H2 in argon at a flow rate of 25 mL/min. The purity of gases was 99.999 vol %. The experiments were carried out in a temperature range from 50 to 600°C, and the measuring cell with a sample was heated at a rate of 10 K/min. Before conducting an experiment, the sample was treated in a flow of argon at 35°C for 30 min. Catalytic Tests The catalysts were tested in the reaction of CO oxidation using a flow system with a glass reactor 170 mm

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in length and 10 mm in diameter. The initial gas mixture contained 1% CO + 99% air, and its total flow rate was 365 mL/min. The analysis of the reaction mixture at the reactor inlet and outlet was performed by chromatography; the mixtures were separated on a column 3 m in length packed with zeolite CaA. The amount of unreacted CO was determined with the aid of a thermal conductivity detector. Before conducting the catalytic tests, we mixed the samples with a binding agent (γ-Al2O3 powder) in a ratio of 1 : 1 and pelletized them with the subsequent grinding and fractionation. Under the test conditions, γ-Al2O3 did not manifest catalytic activity. We used a catalyst fraction of 0.8–1.4 mm. To avoid overheating in the course of the exothermic reaction of CO oxidation, the catalyst was mixed with quartz of the same particle size. The contact time of the sample with the reaction mixture was 0.2 s. The temperature in the catalyst bed was monitored and regulated using a Chromel–Alumel thermocouple connected with a thermoregulator (Varta, Russia). The experiment time at each temperature was 30–40 min. For checking the reproducibility of the results, four or five samples were taken in this time for chromatographic analysis. The degree of CO conversion was calculated from the formula

x CO =

(PCO PN 2 ) before − (PCO PN 2 ) after , (PCO PN 2 ) before

where PCO and PN 2 are the peak areas of CO and nitrogen, respectively, before and after the reaction (the peak area PN 2 was used as an internal standard). RESULTS AND DISCUSSION Variation of the Temperature of Catalyst Calcination The effect of the calcination temperature (Tc) of the samples of MnOx–ZrO2 on their structural properties and catalytic activity was studied using the catalyst Mn0.12Zr0.88O2 as an example. X-ray diffraction analysis. Figure 1 shows the diffractograms of the sample of Mn0.12Zr0.88O2 calcined in air at different temperatures. The phase composition of the sample changed with temperature. Thus, the catalyst calcined at 450°C was amorphous (a halo was observed in the diffraction pattern). After heat treatment at 500°C, the peaks of zirconium oxide appeared and reflections from manganese containingphases were absent. In the samples calcined at 500– 700°C, ZrO2 was present in the form of a cubic modification. After 750°C, the tetragonal splitting of peaks was observed, and shoulders appeared at 2θ = 34° and 60°, whereas the diffractogram clearly exhibited the peaks of a monoclinic modification of ZrO2 at 800°C. The quantitative phase analysis performed by the Rietveld method showed that the oxide of zirconium contained 80% cubic and 20% monoclinic modifications after heat treatment at 800°C. At 850°C, the ratio

between them changed to 15 and 85%, respectively. At the same time, weak peaks of β-Mn3O4 with the average size of coherent scattering regions (CSRs) of 190 Å appeared in the X-ray diffraction pattern of the catalyst calcined at 900°C together with a large number of narrow peaks due to the monoclinic modification of ZrO2. In order to determine in which form manganese occurs and whether its ions can be introduced into the support structure, we calculated the lattice parameter of ZrO2 in a cubic approximation (based on the reflection 111) and the reduced volume (to 750°C, the structure was considered as cubic and as tetragonal at 750°C). From the data given in Table 1, it follows that, in the range of heat treatment temperatures of 500– 650°C, the lattice parameter and the reduced volume of ZrO2 of 5.083 Å and 65.7 Å3, respectively, remained unchanged, whereas they were 5.113 Å and 66.7 Å3, respectively, in the individual oxide ZrO2. Thus, these indices in the catalyst are lower than those in the support. This can be indicative of the incorporation of manganese ions into the structure of the zirconium oxide with the formation of the solid solution MnyZr1 – yO2. Indeed, the replacement of Zr4+ cations (ionic radius, 0.79 Å) by the smaller cations Mn3+ (ionic radius, 0.66 Å) leads a decrease in the lattice parameter of the zirconium oxide [14]. At heat treatment temperatures of >650°C, the lattice parameter and the reduced volume increased to be indicative of the release of manganese ions from the structure of MnyZr1 – yO2. At Tc to 700°C, structural changes were not so significant; however, an increase in Tc to 750°C led to an increase in the lattice parameter from 5.086 to 5.099 Å, and the CSR size of ZrO2 particles increased from 150 to 210 Å. If we consider that a phase of Mn3O4 appears at 900°C and ZrO2 is converted into a monoclinic modification, the phase transformations in the MnOx– ZrO2 system can be presented as follows: at 500°C, a solid solution based on cubic ZrO2 is formed; as the temperature is increased, this solid solution is decomposed with the release of manganese oxide in the form of an X-ray amorphous phase (MnOx). In this case, ZrO2 passes into a tetragonal form, and the amorphous phase containing manganese crystallizes in the form of β-Mn3O4. As follows from Table 1, the specific surface area of the sample of Mn0.12Zr0.88O2 consecutively decreased as the heat treatment temperature was increased from 450 to 900°C. At 450°C, Ssp was 152 m2/g, whereas it was only 1 m2/g at 900°C. As the temperature was increased to 450–500°C, a dramatic decrease in Ssp from 152 to 107 m2/g was noted due to the crystallization of an amorphous phase of zirconium oxide coprecipitated with MnOx ⋅ H2O. In a range of 700–750°C, Ssp decreased from 56 to 17 m2/g, which can be explained by the degradation of the solid solution KINETICS AND CATALYSIS

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*

* m

m

m m

m m

с с

20

30

40

с

с

50

60

с

107

900°С 850°С 800°С 750°С 700°С 650°С 600°С 500°С 450°С 70

2θ, deg Fig. 1. Diffraction patterns of the sample of Mn0.12Zr0.88O2 calcined in air at different temperatures. Phases: (*) Mn3O4 and (c and m) the cubic and monoclinic modifications of ZrO2, respectively.

MnyZr1 – yO2 and the onset of a transition of the cubic modification of ZrO2 into the monoclinic one. TPR-H2. Figure 2 shows the TPR-H2 profiles of the catalyst Mn0.12Zr0.88O2. The profile of the sample calcined at 650°C includes two broad peaks with maximums at 270 and 530°C. As established previously [19], the first peak corresponds to the partial reduction of manganese cations, which are the constituents of a solid solution:

4 + /3 +

3 + /2 +

Mn 0.3 Zr1 − yO 2 − δ → Mn 0.3 Zr1 − yO 2 − δ1. The second peak characterizes the reduction of manganese to Mn2+:

Mn 3y+ /2 +Zr1− yO 2 −δ 2 → Mn 2y+−aZr1− y + aO 2 −δ3 (δ1 > δ2 > δ3), which is accompanied by the release of manganese cations from the volume and by their localization on the surface in the form of MnOx.

Table 1. Structural characteristics of the samples of Mn0.12Zr0.88O2 calcined at different temperatures Calcination temperature, °C

Phase composition

Lattice parameter of ZrO2, Å

CSR of ZrO2, Å

Ssp, m2/g

450 500

c-ZrO2

–

– 5.083

– 180

152 107

600

c-ZrO2

5.081

160

95

650

c-ZrO2

5.083

150

85

700

c-ZrO2

5.086

150

56

750

c-ZrO2

5.099

210

17

800

c-ZrO2 (80%) m-ZrO2 (20%)

5.118

250

10

850

c-ZrO2 (15%) m-ZrO2 (85%)

5.140

140

6

900

m-ZrO2 β-Mn3O4

5.125

320

1

Designations: c and m refer to cubic and monoclinic modifications, respectively. KINETICS AND CATALYSIS

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AFONASENKO et al. 470°С

2

530°С 515°С

270°С

1 3

100

200

300 400 Temperature, °С

500

600

Fig. 2. TPR-H2 profiles of the catalyst Mn0.12Zr0.88O2 calcined at different temperatures: (1) 650, (2) 800, and (3) 900°C.

τ, s 0

0.1

0.2

0.3

–0.4 ln (1 – x)

Mn0.12Zr0.88 500°C

–0.8

Mn0.12Zr0.88 600°C Mn0.12Zr0.88 750°C

–1.2 Mn0.3Zr0.7 650°C

–1.6

Mn0.4Zr0.6 650°C Mn0.5Zr0.5 650°C

–2.0

Mn0.6Zr0.4 650°C

Fig. 3. Plots of ln(1 – х) = f(τ) in the reaction of CO oxidation on the MnOх–ZrO2 catalysts different in chemical composition and calcined at different temperatures; x is the degree of CO conversion, and τ is the contact time at a reaction temperature of 200°C.

The TPR-H2 profile substantially changed as Tc was increased to 800°C, when intense phase transformations occurred in the system according to the XRD analysis data. A peak with a maximum at 270°C disappeared; the intensity of the second peak decreased, and its maximum shifted from 530 to 515°C; and a peak at 470°C appeared. It is most likely that a range of 200–410°C, where a smooth lift of the curve was observed, is the region of unresolved peaks. According to Chen et al. [15], the region of unresolved peaks is related to the presence of highly dispersed Mn3O4 + x particles on the catalyst surface, where 0 < x < 0.5, i.e., particles with different degrees of oxidation of manganese. A peak at 470°C can be attributed to the reduction of large particles on the transition Mn3O4 → MnO [9, 20]. The appearance of this peak and a decrease in peak intensity at 515°C indicate that, as a result of heat

treatment at 800°C, a portion of manganese left the solid solution and occurred in the sample in the form of MnOx particles, which converted into MnO in the course of TPR at 470°C. After heat treatment at 900°C, the TPR-H2 profile of Mn0.12Zr0.88O2 exhibited only one intense peak with a maximum at 470°C, which is characteristic of the transition Mn3O4 → MnO (see above). Its asymmetry and the occurrence of the region of unresolved peaks 220–400°C, the intensity of which is much lower than that after heat treatment at 800°C, evidence that an insignificant quantity of manganese remained in the structure of solid solution after its disintegration. The major portion of manganese after heat treatment at 900°C was present in the form of MnOx. Correlating the data obtained by XRD analysis and the TPR-H2 method, we can conclude that an increase in the heat treatment temperature of Mn0.12Zr0.88O2 from 650 to 900°C caused a structural rearrangement of the test system accompanied by the gradual degradation of the solid solution based on cubic ZrO2 with the outcrop of manganese ions and the formation of initially highly dispersed MnOx particles and then a crystallized phase of Mn3O4. Catalytic activity. Before interpreting the results of the catalytic tests of the MnOx–ZrO2 samples in the reaction of CO oxidation, let us note that the chosen composition of a reaction mixture (1% CO in air) makes it possible to ensure the first order of reaction with respect to CO and the zero order with respect to oxygen. Using several samples from both series (with variations in both heat treatment temperature and catalyst composition) as examples, we found that the function ln (1 – х) = f(τ), where x is the degree of CO conversion and τ is the contact time at a reaction temperature of 200°C, was linear over a wide range of the degrees of conversion. This is confirmed by the first order of reaction with respect to CO and the independence of catalyst activity of the feed rate of a reaction mixture. Figure 4 shows the results of the catalytic tests of Mn0.12Zr0.88O2 samples in the reaction of CO oxidation with variations of the heat treatment temperature. As this temperature was increased from 450 to 700°C, the curve of CO conversion shifted to the side of lower temperatures; that is, the catalyst activity increased. The catalysts calcined at 650 and 700°C were found the most active. The curves of CO conversion on these samples were very close to each other, and the temperature of a 50% CO conversion (T50) was 203– 206°C. On the contrary, a further increase in Tc led to a sharp decrease in the catalytic activity. On the samples calcined at 750–850°C, T50 increased from 231 to 261°C, whereas it was 390°C on the sample calcined at 900°C. It is most likely that the catalytic behavior of the Mn0.12Zr0.88O2 system is related to the degradation of a solid solution based on the cubic modification of KINETICS AND CATALYSIS

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ZrO2 and the formation of the independent phases of monoclinic ZrO2 and Mn3O4. The estimation of the apparent activation energy (Ea) of CO oxidation in a low-temperature region at the degrees of CO conversion lower than 20% under the assumption of the first order of reaction with respect to CO showed that Ea also depends on the Tc of the catalyst. Thus, on the X-ray amorphous sample calcined at 450°C, Ea = 54 kJ/mol, whereas Ea = 41– 46 kJ/mol on the samples calcined at 500–750°C, where the MnOx–ZrO2 system is a solid solution or only begins to decompose. With a further increase in the heat treatment temperature, when ZrO2 converted into a monoclinic modification and the crystallization of a manganese-containing phase occurred, the activation energy increased: it was 54, 68, or 83 kJ/mol at 800, 850, or 900°C, respectively. In this case, Ea on the sample of Mn0.12Zr0.88O2 calcined at 900°C is comparable with the activation energy of the reaction on the massive oxide Mn2O3 (89 kJ/mol). Obviously, a change in Ea with heat treatment temperature was related to the fact that structural transformations in the MnOx–ZrO2 system are accompanied by a decrease in the degree of oxidation of manganese ions from Mn4+ in the structure of the solid solution MnyZr1 – yO2 to Mn3+ in the oxide Mn3O4 + x released from the solid solution as Tc was increased. Thus, the dependence of the catalytic activity of the MnOx–ZrO2 system in the reaction of CO oxidation on the catalyst calcination temperature passed through a maximum as a result of changes in the structure of the catalyst. In this case, the samples calcined at 650– 700°C, when the limit of occurrence of a solid solution of manganese ions in the cubic modification of zirconium oxide was reached, exhibited the highest activity (a minimum value of T50). Variation of the Catalyst Composition For optimizing the composition of the MnOx–ZrO2 catalyst, we studied a series of samples with different component contents (from Mn0.07Zr0.93 to Mn0.9 Zr0.1) and the samples of ZrO2 and Mn2O3 calcined at 650°C. X-ray diffraction analysis. Figure 5 shows the diffractograms of the MnOx–ZrO2 catalysts with different concentrations of manganese and the individual oxides ZrO2 and Mn2O3. All of the diffractograms of MnOx–ZrO2 exhibit reflections corresponding to the cubic modification of ZrO2. Table 2 summarizes the cell parameters and the average values of CSR for zirconium oxide particles calculated from the 111 reflection. The unit cell parameter and the average CSR size of ZrO2 gradually decreased as the quantity of manganese was increased up to the composition Mn0.7Zr0.3O2. The gradual decrease of the cell parameter of ZrO2 indicates the incorporation of manganese into the KINETICS AND CATALYSIS

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XCO, %

EFFECT OF THE CALCINATION TEMPERATURE AND COMPOSITION

100 90 80 70 60 50 40 30 20 10 0 80

109

450°C 500°C 600°C 650°C 700°C 750°C 800°C 850°C 900°C

120 160 200 240 280 320 360 400 Tr, °C

Fig. 4. Dependences of the degree of CO conversion on reaction temperature on the Mn0.12Zr0.88O2 catalyst calcined at different temperatures.

structure of zirconium oxide with the formation of the solid solution MnyZr1 – yO2. At the same time, starting with Mn0.5Zr0.5O2, low-intensity peaks characteristic of a Mn2O3 phase appeared in the diffractograms and those corresponding to a β-Mn3O4 phase appeared starting with Mn0.7Zr0.3O2. Their intensity increased with the manganese content. As can be seen in Table 2, the specific surface area substantially depends on the composition of the catalyst. As the manganese content was increased from 7 to 40 mol %, the specific surface area increased from 64 to 103 m2/g and then insignificantly changed up to a Mn content of 60 mol %. The introduction of a larger quantity of manganese led to a decrease in Ssp, which was 80 or 44 m2/g in the samples of Mn0.7Zr0.3O2 and Mn0.9Zr0.1O2, respectively. According to the XRD analysis data, this is related to the degradation of solid solution and the formation of the independent phases of Mn2O3, β-Mn3O4, and ZrO2. Note that the specific surface area of the majority of the MnOx–ZrO2 samples was higher than the Ssp of individual manganese and zirconium oxides (36 and 43 m2/g, respectively). This is evidence of the formation of solid solution, in which the phase of ZrO2 as a catalyst constituent is characterized by a smaller CSR size than that of the individual oxide ZrO2 in accordance with the XRD analysis data. Catalytic activity. Figure 6 illustrates the results of the catalytic tests of the MnOx–ZrO2 samples with different ratios between the components. As the manganese content was increased, the curves of CO conversion shifted toward low temperatures; that is, the catalytic activity of the samples substantially increased. Thus, T50 on Mn0.12Zr0.88O2 and Mn0.3Zr0.7O2 was reached at 203 and 161°C, respectively. The Mn0.4Zr0.6O2, Mn0.5Zr0.5O2, and Mn0.6Zr0.4O2 catalysts were found most active; the curves of the degrees of CO conversion in the presence of these catalysts were very close to each other, and T50 was 149–153°C.

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*

! *

* * ! * *

* *

* *

*

*

*

*

! !

Mn2O3

* * * * *

Mn0.9Zr0.1O2 Mn0.7Zr0.3O2 Mn0.6Zr0.4O2 Mn0.5Zr0.5O2 Mn0.4Zr0.6O2 Mn0.3Zr0.7O2 Mn0.12Zr0.88O2 ZrO2

20

30

40

50

60

70

2θ, deg Fig. 5. Diffraction patterns of the MnOх–ZrO2 samples different in chemical composition and calcined at 650°C. The signals of (*) Mn2O3 and (!) Mn3O4 phases.

A further increase in the manganese content caused a decrease in the catalytic activity of the system: the curve of CO conversion shifted toward high temperatures. Thus, T50 = 168 and 180°C on Mn0.7Zr0.3O2 and Mn0.9Zr0.1O2, respectively. The massive oxide Mn2O3 possessed the smallest catalytic activity, and T50 = 223°C on it. Thus, there was a synergistic effect caused by the interaction of MnOx and ZrO2 oxides. A maximum activity was achieved at an equimolar or nearly equimolar ratio between the components in MnOx–ZrO2.

ZrO2 system was caused by the formation of a solid solution of the crystallized phases of Mn2O3 and Mn3O4 on the surface.

Comparing the above results with structural data, we can assume that the reason for a change in the catalytic activity of MnOx–ZrO2 is a change in the phase composition of the samples, namely, the formation of a multiphase system (Mn2O3, Mny – aZr1 – y + aO2) instead of a phase of the solid solution MnyZr1 – yO2, which implied a change in the composition and structure of the active centers of the catalyst. In all likelihood, the increased catalytic activity of samples can be Mn0.12Zr0.88O2–Mn0.4Zr0.6O2 explained by an increase in the dispersity of solid solution particles with the manganese content, as evidenced by a decrease in the CSR size and an increase in the specific surface area of the catalyst. The samples of Mn0.4Zr0.6O2–Mn0.6Zr0.4O2 exhibited a maximum catalytic activity. This was most likely due to not only the dispersity of the solid solution but also the presence of highly dispersed MnOx particles, which possess high activity in oxidation reactions, on its surface. However, a decrease in the catalytic activity with a further increase in the manganese content of the MnOx–

ZrО2

Table 2. Structures and specific surface areas of the MnOx–ZrO2 samples of different composition calcined at 650°C Sample

Lattice Phase parameter composition of ZrO2, Å

Ssp,

CSR, Å

m2/g

c-ZrO2

5.115

250

43

Mn0.12Zr0.88O2 c -ZrO2

5.081

140

85

Mn0.3Zr0.7O2

c-ZrO2

5.040

120

94

Mn0.4Zr0.6O2

c-ZrO2

5.019

120

103

Mn0.5Zr0.5O2

c-ZrO2 Mn2O3

5.003

90

100

Mn0.6Zr0.4O2

c-ZrO2 Mn2O3

4.988

90

100

Mn0.7Zr0.3O2

Mn2O3 β-Mn3O4 c-ZrO2

4.987

70

80

Mn0.9Zr0.1O2

Mn2O3 β-Mn3O4 c-ZrO2

4.983

100

44

MnOx

Mn2O3

–

–

36

Designations: c refers to a cubic modification. KINETICS AND CATALYSIS

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XCO, %

EFFECT OF THE CALCINATION TEMPERATURE AND COMPOSITION

100 90 80 70 60 50 40 30 20 10 0 60

111

stituents of solid solution on the surface in addition to the dispersity of the solid solution particles. Mn0.12Zr0.88O2 Mn0.3Zr0.7O2 Mn0.4Zr0.6O2 Mn0.5Zr0.5O2 Mn0.6Zr0.4O2 Mn0.7Zr0.3O2 Mn0.9Zr0.1O2 MnOx

100

140

180 220 Tr, °C

260

300

340

Fig. 6. The temperature dependences of the degree of CO conversion on the MnOх–ZrO2 catalysts of different chemical composition calcined at 650°C.

CONCLUSIONS We found that the heat treatment temperature and the composition of the MnOx–ZrO2 system affect its structural characteristics and catalytic properties in the reaction of CO oxidation. According to XRD analysis and TPR-H2 data, an increase in the heat treatment temperature of Mn0.12Zr0.88O2 from 450 to 900°C caused structural transformations in the system, which were accompanied by the degradation of the solid solution MnyZr1 – yO2 and the release of manganese ions from its structure with the formation of initially highly dispersed X-ray amorphous MnOx particles and then a crystallized phase of Mn3O4on the surface. The dependence of the catalytic activity of the MnOx– ZrO2 samples in the reaction of CO oxidation on the calcination temperature, which was caused by changes in the structure of the catalyst, had an extreme form. The samples calcined at a temperature of 650–700°C, which is a limiting temperature for the existence of a solid solution of manganese ions in zirconium oxide in a cubic modification, exhibited a maximum activity. Obviously, a change in apparent Ea with increasing the Tc of the catalysts was related to the fact that structural transformations in the MnOx–ZrO2 system were accompanied by a change in the degree of oxidation of manganese ions from Mn4+ in the structure of the solid solution MnyZr1 – yO2 to Mn3+ in the oxide Mn3O4 + x, which was released from the solid solution as the heat treatment temperature was increased. An increase in the manganese content of the MnOx–ZrO2 system after heat treatment at 650°C led to a change in the phase composition: the resulting phases of manganese oxides supplemented the solid solution phase of MnyZr1 – yO2. The samples of Mn0.4Zr0.6O2–Mn0.6Zr0.4O2 exhibited a maximum catalytic activity; this fact may be explained by the presence of highly dispersed MnOx particles that are not the con-

KINETICS AND CATALYSIS

Vol. 59

No. 1

2018

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Translated by V. Makhlyarchuk