ISSN 00231584, Kinetics and Catalysis, 2016, Vol. 57, No. 2, pp. 224–228. © Pleiades Publishing, Ltd., 2016. Original Russian Text © T.P. Minyukova, N.A. Baronskaya, M.P. Demeshkina, L.M. Plyasova, T.M. Yurieva, 2016, published in Kinetika i Kataliz, 2016, Vol. 57, No. 2, pp. 218–222.

Catalytic Properties of Copper Chromite Ferrites in Water Gas Shift Reaction and Hydrogen Oxidation T. P. Minyukova*, N. A. Baronskaya, M. P. Demeshkina, L. M. Plyasova, and T. M. Yurieva Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia *email: [email protected] Received April 2, 2015

Abstract—The catalytic properties of a series of copper chromite ferrite samples with the composition CuCr2 – xFexO4 (where x = 0–2) and a spineltype structure in reactions with reducing (water gas shift reaction, WGSR) and oxidizing (the oxidation of hydrogen) reaction atmospheres were studied. The sam ples were obtained by the thermal decomposition of mixed hydroxo compounds. The distribution of Cu2+ ions in the tetrahedral and octahedral crystallographic positions of spinel, which depends on the Cr3+/Fe3+ ratio, affects the apparent activation energy (Ea) in both of the reactions. In WGSR, Ea is ~33 kJ/mol for CuCr2O4, in which Cu2+ ions mainly occupy tetrahedral positions, whereas Ea ≈ 100 kJ/mol for CuFe2O4, in which Cu2+ ions mainly occupy octahedral positions. In the reaction of hydrogen oxidation, Ea is ~71 kJ/mol for CuCr2O4 or ~42 kJ/mol for CuFe2O4. The value of Ea for the mixed chromite ferrites changes with the replacement of chromium ions by iron ions and, hence, with a ratio between the amounts of copper ions in the tetrahedral and octahedral oxygen positions of spinel. Keywords: CuCr2 – xFexO4, WGSR, oxidation of hydrogen DOI: 10.1134/S0023158416020051

INTRODUCTION The coppercontaining oxide catalysts with a spineltype structure are widely used in industrially important processes, and their areas of application have been extended in recent years. Copper chromite is a wellknown catalyst for the hydrogenation/dehy drogenation of organic compounds and oxidation reactions, such as a model reaction of hydrogen oxida tion. The high activity of coppercontaining com pounds with the spinel structure in the lowtempera ture water gas shift reaction (WGSR) is known. Cop per ferrite is a component of an industrially important catalyst for WGSR. It is used as a catalyst for the oxi dation reactions of organic compounds. Recent stud ies demonstrated that the catalysts of WGSR based on spinelstructure oxides are not only highly active but also highly resistant to the unfavorable factors of a reaction atmosphere—high temperature and high water content [1–6]. The catalytic properties of mixed copper chromite ferrites have not been adequately investigated. This work is dedicated to a study of the catalytic properties of copper ferrite, copper chromite, and the mixed copper chromite ferrites CuCr2 – xFexO4 (where х = 0–2) in reactions with reducing and oxidiz ing reaction atmospheres (WGSR and the oxidation of hydrogen, respectively). Recently, Plyasova et al. [7] described the formation of these catalysts from mixed hydroxo compounds.

EXPERIMENTAL Sample Preparation The samples were obtained by the heat treatment of mixed hydroxo compounds, which were prepared by the coprecipitation of components from 10% aqueous solutions of nitrate salts at a constant acidity of pH 6.6–6.8 and a temperature of 70–75°С. The fol lowing nitrates were used: Fe(NO3)3 · 6Н2О (98%) and Cu(NO3)2 · 6Н2О (98.1%) of analytical grade (Pan reac, Spain) and Cr(NO3)3 · 9Н2О (99%; Ural Plant of Chemical Reagents, Russia). The heat treatment of hydroxo compounds was performed at 600°С in an atmosphere of air for copper ferrite or in a flow of argon for Crcontaining samples in order to prevent the possible oxidation of Cr3+ into Cr6+. Catalytic Properties Catalytic properties in WGSR were studied in a laboratory flow setup with the gaschromatographic analysis of a working mixture. Activity was measured at a pressure of 1 atm for a working mixture of CO : Н2О : Н2 = 8 : 42 : 50 at a steam/gas ratio of 0.6–0.7. The water gas shift reaction was carried out on a cata lyst (fraction, 0.14–0.25 mm) mixed with quartz sand of the same particle size. The sample weight was 1 g; the catalyst bed height adjusted for dilution with quartz was 3 cm, and the reactor diameter was 20 mm. The temperature was continuously controlled with the

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aid of a thermocouple placed in a pocket of the reactor at the center of the catalyst bed. Catalyst activity in WGSR was characterized by the apparent rate constant of the forward reaction, which was determined taking into account the reversibility of the reaction in the firstorder approximation with respect to CO: C CO, m – C CO, eq ⎞ 1 k → = ln ⎛  (1)  , ⎝ C CO, frm – C CO, eq⎠ τ where CCO,m is the mole fraction of CO in the moist initial reaction mixture, CCO,frm is the mole fraction of CO in the final reaction mixture at the reactor outlet, CCO,eq is the equilibrium mole fraction of CO at the reaction temperature, and τ is the residence time. The apparent activation energies of the reaction were evaluated from the temperature dependence of the reaction rate constant in a temperature range of 150–210°С or 150–240°С for copper ferrite. All of the catalytic data are given for the catalysts that reached a steadystate activity (after continuous oper ation for no shorter than 25 h). Catalytic properties in the reaction of hydrogen oxidation were investigated by a gradientless method in a circulationflow setup followed by the gaschro matographic analysis of the working mixture after freezing the reaction product. Activity was measured at a pressure of 1 atm for the working mixture contain ing 99 vol % О2 + 1 vol % Н2. A catalyst fraction of 0.5–1.0 mm was used, and the catalyst weight varied from 0.3 to 1.9 g. The temperature was measured with the aid of a thermocouple placed in a thermocouple pocket of the reactor, which was located in the catalyst bed. Activity was characterized by the reaction rate constant under the assumption of the first order with respect to hydrogen and the zero order with respect to oxygen [8–11]. The apparent activation energy of the reaction was evaluated from the temperature depen dence of the reaction rate constant in a range of 180– 370°С for the Crcontaining samples or 420–500°С for copper ferrite. RESULTS AND DISCUSSION In this work, we studied the catalytic properties of copper chromite ferrites with different Cr3+/Fe3+ ratios obtained by the thermal decomposition of corre sponding hydroxo compounds. The chemical compo sition of the resulting samples was completely consis tent with the specified one. As found by Xray diffrac tion (XRD), the hydroxo compounds obtained were Xray amorphous. Their heat treatment at 600–650°С led to the crystallization of spineltype oxide phases. (Wellcrystallized spinels are formed at 900°С.) Previ ously, Plyasova et al. [7] found that, depending on the Cr3+/Fe3+ ratio, the resulting phases with a spinel structure occur in two modifications: cubic Fd3m and tetragonally distorted cubic (nonstandard space group F41/ddm [12], in which the spinel lattice parameters KINETICS AND CATALYSIS

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lnk [s–1] 4 3 2

1 2 3

1 4

0

5

–1 –2

6 –3 0.23

0.24

0.25

0.26

0.27

0.28 0.29 1000/RT

Fig. 1. The temperature dependences of the WGSR rate constant on different catalysts: (1) CuCr1.0Fe1.0O4, (2) CuCr1.25Fe0.75O4, (3) CuCr0.75Fe1.25O4, (4) CuCr2O4, (5) CuCr0.25Fe1.75O4, and (6) CuFe2O4.

are designated as a* and c*). The character and degree of the tetragonal distortion of Cu–Cr–Fe spinels depend on the Cr3+/Fe3+ ratio: at the ratio Cr3+/Fe3+ > 1, the ratio of the unit cell parameters is c*/a* < 1; at Cr3+/Fe3+ < 1, the ratio is c*/a* > 1; and spinel is cubic at Cr3+/Fe3+ = 1. The possible distribu tion of cations in the mixed spinels was calculated tak ing into account the preference energy of cations and the structure of mixed spinels [7]. Catalytic properties were investigated with respect to two reactions, which are fundamentally different in the redox properties of reaction atmospheres: CO + Н2О = Н2 + СО2, water gas shift in a reduc ing reaction atmosphere, and Н2 + ½О2 = Н2О, the oxidation of hydrogen in an oxidizing reaction atmosphere. The catalyst samples were preheated at 600°С. The chosen heat treatment temperature ensured the for mation of phases with the spinel structure and a suffi ciently high specific surface area of the catalysts. Catalytic Properties in Water Gas Shift Reaction Figure 1 shows the temperature dependence of the reaction rate constants in Arrhenius coordinates for copper chromite ferrites of different composition. It is evident that, in the test temperature range, the loga rithm of rate constant is a linear function of the recip rocal of temperature for all of the samples. Note that each of the determined rate constant was confirmed by the triple reproduction of samples and measurements. Two points at the same temperature in Fig. 1 show the reproduction of an activity measurement at the initial temperature after conducting all of the measurements.

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Apparent activation energies of the water gas shift and oxidation of hydrogen reactions and the distributions of Cu2+ cations in the samples of copper chromite ferrites CuCr2–xFexO4 samples CuCr2O4 CuCr1.75Fe0.25O4 CuCr1Fe1O4 CuCr0.75Fe1.25O4 CuCr0.25Fe1.75O4 CuFe2O4

Distribution of cations over the structural positions of spinel [7]

Phase composition and the c*/a* ratio between the lattice parameter Spinel, 0.913 Spinel, 0.926 Spinel, 1.000 Spinel, 1.025 Spinel, 1.039 Spinel, 1.040

tetrahedrons (8b) 2 1.75 1.0 0.75 0.25 0

8Cu 7Cu + 1Fe 4Cu + 4Fe 3Cu + 5Fe 1Cu + 7Fe 0Cu + 8Fe

As follows from Fig. 1, the activity remained unchanged in the course of experiments. The table summarizes the values of the apparent activation energy Еа evaluated from the temperature dependence of the reaction rate constant in a temper ature range of 150–240°С and the structural charac teristics of copper chromite ferrites of different com position. It can be seen that Еа is ~33 kJ/mol for cop per chromite or ~100 kJ/mol for copper ferrite; for the mixed chromite ferrites, the value of Еа weakly increased from 33 to 43 kJ/mol as chromium ions were replaced by iron ions to the ratio Cr3+/Fe3+ = 1, whereas it considerably increased from 43 to 100 kJ/mol with the subsequent replacement of chromium ions. The apparent activation energy substantially changed depending on the composition of a sample; therefore, it is difficult to compare reaction rate constants for the samples of different composition. It should be noted that the influence of the composition of lnk [s–1] 7 6 5

5

4 3 2 2

1 0 0.15

4

1 3

0.17

0.19

0.21

0.23

Ea, kJ/mol

x

0.25 0.27 1000/RT

Fig. 2. The temperature dependence of the rate constant of the oxidation of hydrogen on different catalysts: (1) CuFe1.5Cr0.5O4, (2) CuFe1Cr1O4, (3) CuFe0.5Cr1.5O4, (4) CuCr2O4, and (5) CuFe2O4.

octahedrons (16d)

WGSR

oxidation of hydrogen

16Cr + 0Cu 14Cr + 1Fe + 1Cu 8Cr + 4Fe + 4Cu 6Cr + 5Fe + 5Cu 2Cr + 7Fe + 7Cu 8Fe + 8Cu

33 36 43 50 77 100

71 70 67 50 – 42

chromite ferrites on the activation energy of WGSR was also observed earlier [13]. Catalytic Properties in Hydrogen Oxidation Figure 2 shows the temperature dependence of the reaction rate constants of hydrogen oxidation in Arrhenius coordinates for copper chromite, copper ferrite, and chromite ferrites with different Cr3+/Fe3+ ratios. As is evident from Fig. 2, in the test temperature range, the logarithm of the rate constant is a linear function of the reciprocal of temperature for all of the samples (each of the determined rate constants was confirmed by the triple reproduction of samples and measurements). The table summarizes the apparent activation ener gies Еа of the reaction of hydrogen oxidation for the test samples evaluated from the temperature dependence of the logarithm of the reaction rate of the hydrogen oxi dation in a temperature range of 180–370°C (420– 500°C for copper ferrite) and the structural characteris tics of copper chromite ferrites of different composi tion. It can be seen that Еа is ~71 kJ/mol for copper chromite or ~42 kJ/mol for copper ferrite, whereas the value of Еа for the mixed chromite ferrites changed as chromium ions were replaced by iron ions depending on the Cr3+/Fe3+ ratio, namely, it relatively slightly decreased (67–71 kJ/mol) with the replacement of chromium ions by iron ions to the ratio Cr3+/Fe3+ = 1 and considerably decreased (67–42 kJ/mol) with the further substitution for Fe3+. The apparent activation energy substantially changed depending on the compo sition of the sample; therefore, it is difficult to compare the reaction rate constants for the samples of different composition. Figure 3 shows the dependence of the apparent activation energies of the two test reactions on the composition of samples. It is evident that the depen dences of Еа on the composition for WGSR and the oxidation of hydrogen are nearly mirror reflections of each other. KINETICS AND CATALYSIS

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Thus, from the results of the study of Cu–Cr–Fe oxides with the spinel structure at a constant copper content and a sequential change in the concentrations of chromium and iron ions from copper chromite to copper ferrite, it follows that the apparent activation energies for WGSR and the oxidation of hydrogen depend on the Cr3+/Fe3+ ratio, and the characters of this dependence in reducing and oxidizing reaction atmospheres are considerably different. It is well known that copper chromite is direct spinel, which is characterized by the arrangement of copper ions in the tetrahedral crystallographic posi tions of spinel regardless of polymorphous modifica tion [2, 3]. Balagurov et al. [14] used the precision neutron diffraction analysis of copper ferrite to dem onstrate that CuFe2O4 is inverted spinel regardless of polymorphous modification. The ratio between the lattice parameters of tetrago nally distorted spinels varies from 0.92 for CuCr2O4 to 1.04 for CuFe2O4. As found by Plyasova et al. [7], as a result of the partial replacement of chromium ions by iron ions and iron ions by chromium ions, the tetrago nal distortion of mixed spinels decreases and the c*/a* ratio becomes equal to 1 and the structure becomes cubic for the composition CuFe1.0Cr1.0О4. According to published data [14, 15], the ratio c*/a* > 1 is considered as a reliable sign of the presence of copper ions predom inantly in octahedral positions, whereas the ratio c*/a* < 1 is indicative of predominantly tetrahedral positions. Plyasova et al. [7] reported calculated data on the distribution of Cu, Fe, and Cr cations over the octa hedral and tetrahedral positions for chromite ferrites with different Cr3+/Fe3+ ionic ratios. Calculations showed that the amount of copper ions in the tetrahe dral positions of mixed spinels decreases upon the replacement of chromium ions by iron ions and the amount of copper ions in the octahedral positions decreases upon the replacement of iron ions by chro mium ions, and the copper ions are evenly arranged in the tetrahedral and octahedral positions in the CuFe1.0Cr1.0О4 spinel. The table summarizes the exper imentally found values of c*/a* and the calculated dis tributions of cations for copper chromite ferrites of dif ferent composition [7]. It can be seen that the activation energies of the chromite ferrites (in the reactions of both WGSR and the oxidation of hydrogen) vary insignifi cantly with a decrease in the number of copper ions in tetrahedral positions from 8 for chromite to 4 for the chromite ferrite CuFe1.0Cr1.0О4, and they increase con siderably in the case of WGSR and decrease in the case of the oxidation reaction of hydrogen upon the further replacement from 4 to 0 for copper ferrite. This allowed us to assume that the values of Еа obtained in this work for both of the reactions are determined by the struc tural position of copper ions in spinel. The samples after the WGSR contained a spinel phase, copper metal particles (80–100 Å), and the copper oxide Cu2O. It is likely that this copper oxide was formed as a result of the interaction of Cu0 nano KINETICS AND CATALYSIS

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Ea, kJ/mol 100 1 80 60 40

2

20 0

0.5

1.0

1.5 2.0 Fe fraction x

Fig. 3. Dependence of the activation energies of the reac tions of (1) WGSR and (2) the oxidation of hydrogen on the substitution of Cr3+ ions for Fe3+ ions in the copper chromite ferrites CuCr2 – xFexO4.

particles with atmospheric oxygen. The formation of alloys was not detected. The studies of a wide range of coppercontaining catalysts, including those with a spinel structure, toward reactions in a reducing atmo sphere [16, 17] made it possible to state that the cata lytic properties of copper nanoparticles, which are formed in the course of activation with hydrogen or under reaction conditions, depend on the interaction of the particles with an oxide matrix, which is, in turn, is determined by the structure of the Cucontaining oxide compound. With respect to the oxidation reac tions, it was found that the reduction of copper ions does not occur in an oxidizing reaction medium and the activity of copper ions is determined by the struc ture of the nearest oxygen ion environment of the cop per ions [18]. In the light of these ideas, the results obtained in this work with respect to different depen dences of the apparent activation energy Еа on the Cr3+/Fe3+ ratio in copper chromite ferrites for the test reactions can be explained by a difference between the states of copper ions under the conditions of these reactions. ACKNOWLEDGMENTS This work was supported by the Russian Academy of Sciences and Federal Agency of Scientific Organi zations (project V.45.3.6). REFERENCES 1. Ratnasamy, C. and Wagner, J.P., Catal. Rev. Sci. Eng., 2009, vol. 51, p. 325. 2. Tanaka, Y., Utaka, T., Kikuchi, R., Sasaki, K., and Eguchi, K., Appl. Catal., A, 2003, vol. 242, no. 2, p. 287. 3. Hossain, M. and Ahmed, S., Can. J. Chem. Eng., 2013, vol. 91, no. 8, p. 1450.

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Translated by V. Makhlyarchuk

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