Magnetic, oxidation and reduction behavior of spinel Ni–Cu manganite NixCu1-xMn2O4 powders

Materials Science and Engineering B 178 (2013) 1076–1080

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Magnetic, oxidation and reduction behavior of spinel Ni–Cu manganite Nix Cu1-x Mn2 O4 powders M.M. Rashad a,∗ , M. Bahgat a , M. Rasly a , S.I. Ahmed b a b

Central Metallurgical Research & Development Institute (CMRDI), P.O. Box: 87, Helwan 11421, Egypt Department of Physics, Faculty of Science, Ain Shams University, Cairo, Egypt

a r t i c l e

i n f o

Article history: Received 19 February 2013 Received in revised form 24 June 2013 Accepted 27 June 2013 Available online 9 July 2013 Keywords: Spinel manganite CO oxidation Reduction behavior Magnetic properties

a b s t r a c t Nickel copper manganites, Nix Cu1-x Mn2 O4 (where 0 ≤ X ≤ 1), powders have been synthesized via coprecipitation method. The formed particles were obtained from the precipitate precursors annealed at 800 ◦ C for 2 h. The prepared powders were tested for the catalytic oxidation of carbon monoxide (CO) into carbon dioxide (CO2 ). The results indicated that the intermediate compositions displayed better catalytic activity than the end compositions for CO oxidation due to the phenomena of synergism. The composition of x = 0.3 and 0.5 showed rapid rise in CO conversion with temperature; about 40% conversion was achieved at 200 ◦ C for the produced manganite sample of chemical composition Cu0 .7 Ni0 .3 Mn2 O4 . A correlation between magnetic properties and hydrogen reduction of manganites was demonstrated. Magnetic properties of reduced manganite were found to be duplicated. The saturation magnetization of reduced NiMn2 O4 into Ni/MnO was duplicated of order 15 times from initial value. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Emission of carbon monoxide gas in motor engines, industrial process, and even by a burning cigarette is nowadays one of the most significant environmental problems. A facile way to solve such series environmental problem is to convert CO to CO2 , which is less harmful than carbon monoxide. Spinel manganites are widely used as a catalyst for the oxidation of CO at ambient temperature, combustion of many organic compounds including hydrocarbons, as well as halide and nitrogen containing compounds [1–3]. They are also important in respiratory protection, particularly in the mining industry [4]. The oxidation process includes many chemical and physical changes which take place simultaneously and have a profound influence on the other. However, many investigators studied kinetics and mechanism of CO oxidation to CO2 [5–8], many of the related phenomena are not yet fully understood. Therefore, ways that can raise and monitor the catalytic activity of manganites even at low temperature is considered to be a topic of interest. Comparatively, few works deal with magnetic properties of spinel manganites, despite the fact that these materials are expected to be rich in interesting phenomena. Their research has been focused mainly on structural and electrical aspects [9]. Consequently, in the present investigation an attempt has been made to study the catalytic activity of introducing nickel into copper manganites for

∗ Corresponding author. Tel.: +202 5010642; fax: +202 25010639. E-mail address: [email protected] (M.M. Rashad). 0921-5107/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2013.06.022

CO oxidation and pollution control. Moreover, the effect of hydrogen reduction on the microstructures and magnetic properties of prepared manganites was also studied. 2. Experimental 2.1. Material synthesis The co-precipitation method was employed to prepare system of Nix Cu1-x Mn2 O4 (X = 0, 0.3, 0.5, 0.7, and 1.0, respectively). Appropriate amounts of Cu(NO3 )2 .6H2 O, Ni(NO3 )2 .6H2 O, and Mn(NO3 )2 .4H2 O were dissolved in deionized water. The manganite precursors were precipitated using (5 M) sodium hydroxide solution as a base at pH value 10. The aqueous suspensions were stirred at constant 500 rpm for 15 min to achieve good homogeneity and attain a stable pH conditions. The co-precipitates were filtered off, washed with deionized water and then dried in an oven at 100 ◦ C overnight. In order to form the manganite phase, the dry precursors were annealed at 800 ◦ C for 2 h. 2.2. Material characterization The phases of the prepared and reduced powders were identified from the XRD patterns collected using a Brucker axis D8 ˚ radiation in 2␪ range from diffractometer with Cu K␣ ( = 1.5406 A) 10 to 80◦ . The particle morphology was investigated by scanning electron microscope (SEM JSM-5400). Specific surface area of the produced samples was determined by BET surface area analyzer

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Table 1 Structural parameters of the prepared Ni-substituted CuMn2 O4 ; Nix Cu1-x Mn2 O4 (X = 0, 0.3, 0.5, 0.7, and 1.0, respectively) powders. Composition

Lattice parameter a (Å)

Crystallite size (nm)

Surface area (m2 g–1 )

Pore volume (10−3 A˚ 3 )

CuMn2 O4 Cu0.7 Ni0.3 Mn2 O4 Cu0.5 Ni0.5 Mn2 O4 Cu0.3 Ni0.7 Mn2 O4 NiMn2 O4

8.291 8.372 8.355 8.376 8.399

63 105 144 98 102

33.42 47.57 28.55 31.22 20.78

9.13 12.9 8.23 7.40 5.30

(Nova 2000 series, Quantachrome Instruments, UK). The magnetic properties were measured at room temperature using a vibrating sample magnetometer (VSM; 7410 Lakeshore, USA) in a maximum applied field of 20 kOe. 2.3. Catalytic oxidation tests The prepared manganite samples were tested for the catalytic oxidation of CO to CO2 . The catalytic oxidation is carried out in a fused silica tube mounted in a horizontal tube furnace operated at both room temperature and 200 ◦ C under atmospheric pressure. A gas mixture, composing of 0.3 L CO/0.2 L O2 /0.5 L Ar, was introduced to the catalytic reactor such that the total flow rate of mixture was 1 L/min. Monitoring of individual gas concentration in the gases mixture during measuring the catalytic activity of manganite samples is performed using quadrupole mass spectrometer (QMS sample analysis HPR 20, Hidden-Analytical, Warrington, UK). 2.4. Reduction procedure The produced Nix Cu1-x Mn2 O4 powders were compacted into briquettes of 1 gm weight, 7 mm diameter, and 5 mm height. The prepared briquettes were then reduced in a thermogravimetric apparatus using 1 L/min.H2 gas at 1000 ◦ C. The course of reduction was followed by measuring the weight loss over time under controlled conditions of temperature and gas composition. For each reduction experiment, the furnace was heated to the required reduction temperature, and then the briquettes were weighed and placed in a platinum wire basket. The sample was then gradually introduced into the furnace so as to avoid thermal shock cracking and positioned in the middle of the furnace constant hot zone. First, nitrogen at a flow rate of 1 L/min.was introduced then, after soaking the sample for 10 min at the reduction temperature, the H2 reducing gas at a flow rate of 1 L/min.was introduced. The weight loss resulted from oxygen removal of the briquettes was recorded with time. At the end of the experiment, the basket with the reduced briquettes was removed and dropped by releasing its suspension wire from the balance into a conical flask containing acetone to prevent pyrophority of the reduced sample. 3. Results and discussion 3.1. XRD analysis XRD patterns of the manganite Nix Cu1-x Mn2 O4 samples (x = 0, 0.3, 0.5, 0.7, 1.0) produced at 800 ◦ C for 2 h are evinced in Fig. 1. It is clear that all peaks were shifted to lower diffraction angles as the Ni content was increased. All the diffraction patterns nicely matched the reference patterns (JCPDS# 74-2422) for cubic CuMn2 O4 and (JCPDS# 71-0852) for cubic NiMn2 O4 . This confirms the solubility of both manganites for all compositions. A structural model with a cubic lattice and space group symmetry (Fd-3m) was assumed for the Rietveld refinements of the XRD patterns. The refined of the cation distribution and the lattice parameter are quoted in Table 1. The Mn-cations were located

Fig. 1. XRD patterns for Nix Cu1-x Mn2 O4 sintered samples at 800 ◦ C for 2 h (X = 0, 0.3, 0.5, 0.7, and 1.0, respectively) from top to bottom, respectively.

at positions (3/4 3/4 3/4) while the metal cations (Cu2+ ) were located at positions (0 0 0). Therefore, CuMn2 O4 could be considered as a normal spinel, where the divalent metal ions are in tetrahedral site (A) while trivalent metal ions are in octahedral site (B). As increasing X-concentration, X ≥ 0.7, manganite tends to be intermediate spinels, where both divalent and trivalent metal ions were distributed in octahedral and tetrahedral sites. Consequently, at the initial increase of Ni2+ content, the number of octahedral occupations was increased which, in turn, led to increase lattice parameter. In particular, at X = 0.5, an equal distribution of divalent and trivalent ions across octahedral and tetrahedral sites. As a consequence, lattice parameter was decreased in comparison to that of X = 0.3. Continue increasing X ≥ 0.7, manganite converted to inverse spinel, where the divalent metal ion and half of trivalent ions were in octahedral sites B. The other half of trivalent ions occupied the tetrahedral sites A. Therefore, number of octahedral site position increased gradually with increasing X-content, and so lattice parameter (a) increased. Thus, the general tend of lattice parameter (a) were increased as the increasing of Ni-concentration. This goes against the Vigard’s low [10], but this behavior was according well to the distribution of cations across the tetrahedral and octahedral sites as it was discussed. The average size of the crystallites was obtained from the Rietveld refinement as well. 3.2. CO catalytic oxidation The temperature dependence of CO conversion was studied for different compositions of Nix Cu1-x Mn2 O4 . The results are given in Fig. 2 and they are listed in Table 2. Incorporation of nickel in the lattice of CuMn2 O4 showed a pronounced change in the catalytic activity for CO oxidation. CuMn2 O4 and NiMn2 O4 showed low conversion rate than the intermediate compositions. The composition with X = 0.3 and 0.5 showed rapid rise in CO conversion Table 2 Efficiency of CO oxidation as a function of catalyst composition. Chemical composition

CuMn2 O4 Ni0.3 Cu0.7 Mn2 O4 Ni0.5 Cu0.5 Mn2 O4 Ni0.7 Cu0.3 Mn2 O4 NiMn2 O4

Approx. efficiency (%) Room temperature

200 ◦ C

10 24 18 23 8

32 40 31 34 40

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Fig. 2. Conversion, % as a function of catalyst composition at room temperature and 200 ◦ C for the system Nix Cu1-x Mn2 O4 (X = 0, 0.3, 0.5, 0.7, and 1.0, respectively).

with temperature and more than 40% conversion was observed at 200 ◦ C for the prepared manganite sample: Cu0.7 Ni0.3 Mn2 O4 . Data percentage of CO conversion at various temperatures with corresponding rate of the reaction was calculated using the following relation [11,12]: Efficiency(%) =

COi − COf × 100 COi

Fig. 3. Typical isothermal reduction curves of manganites Cu0.5 Ni0.5 Mn2 O4 , and NiMn2 O4 , respectively) at 1000 ◦ C.

(CuMn2 O4 ,

for CO gas adsorption, while fast photoelectron-transfer channel can enhance the photogenerated electron transfer to complete the circuit [17]. Accordingly, the materials can be recommended for catalytic oxidation of carbon monoxide gas [14,18,15]. 3.3. Reduction behavior

(1)

where COi and COf are the CO initial and final concentrations, respectively. Several physical phenomena could be involved to explain this behavior. Salker and Gurav [13] inspected a similar behavior for CO sensing using cobalt spinel manganite. As shown in the Fig. 2, the composition of X = 0.5 showed lower activity than the other intermediates but comparatively higher than the end compositions. Intermediate compositions of X = 0.3 and 0.7 gave higher activity on account of cooperative effect of Ni and Cu ions in the manganite spinels. Moreover, the composition of Ni0.3 Cu0.7 Mn2 O4 exhibited the highest catalytic activity followed by composition of X = 0.5 > 0.0 > 1.0, respectively. The improvement in the catalytic activity in the region of intermediate compositions might be attributed to the synergistic effect [14] created by breaking the activation energy barrier of the limiting steps. According to Brabers and Setten [15], the octahedral sublattice is mainly occupied by Mn3+ (Jahn–Teller ion) which accounts for the tetrahedral deformation of the spinel CuMn2 O4 . According to Goodenough and Loeb [16], if cation at the A-site forms a strong covalent bond, the corresponding octahedrally coordinated bonds at the B-site becomes weak. As a consequence, the transformation from symmetry elements of CuMn2 O4 to symmetry of NiMn2 O4 was occurred through intermediate compositions and so, catalytic oxidation of CO could be monitored by activation energy barrier accompanying with each phase transformation as shown in Fig. 2. Aside from that, it was found that the ionic replacement had a great influence in crystal size, surface area and pore volume of prepared samples which results in a clear effect on catalytic activity of prepared manganites. Therefore, it is worth to say the lower crystallite size presented, the higher surface area and pore volume occurred that enhanced CO adsorption and consequently its oxidation [11]. Table 1 lists the change of physical properties with various with x values which can be evaluated from the SBET measurements. It was found that the highest specific surface area SBET was 47.57 m2 g−1 for Cu0.7 Ni0.3 Mn2 O4 sample while the lowest value was 20.78 m2 g−1 for NiMn2 O4 particles. The high surface area can provide more sites

Selected three samples (CuMn2 O4 , Cu0.5 Ni0.5 Mn2 O4 , and NiMn2 O4 ) were isothermally reduced with hydrogen gas H2 at annealing temperature of 1000 ◦ C. The typical isothermal reduction curves obtained are shown in Fig. 3. For each reduction curve; the rate of reduction was high at the early stages and decreased as reduction proceeds till end of reduction. Moreover, for all samples, the reduction rate increases with the increase in the reduction temperature. Furthermore, complete reduction was achieved for all samples. Relatively the presence of Cu ions increases the reduction rate while the presence of Ni ions decreases it. Namely, CuMn2 O4 , Cu0.5 Ni0.5 Mn2 O4 , and NiMn2 O4 samples are completely reduced after 50, 60, and 75 min reduction time, respectively. Fig. 4 depicts XRD analysis of the reduced manganite samples. Final products that were obtained as a result of reduction process included metallic copper powders (Cu; JCPDS# 85-1326), metallic nickel powders (Ni; JCPDS# 87-0712), metallic Cu–Ni powders related to (Cu0.81 Ni0.19 ; JCPDS# 47-1406), and manganese oxide (MnO; JCPDS# 75-0626). The formed Cu/MnO catalysts prepared

Fig. 4. XRD patterns for (a) NiMn2 O4 , (b) Ni0.5 Cu0.5 Mn2 O4 , and (c) CuMn2 O4 after reduction by H2 gas at 1000 ◦ C.

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Fig. 5. SEM images (a) and (b) are represented to NiMn2 O4 and CuMn2 O4 , respectively, before reduction and SEM images (c) and (d) are represented to their products after reduction process.

via reduction of Cu–Mn spinel oxide were investigated for development of active Cu catalysts for the water gas shift reaction (WGSR) in reformed fuels [19]. 3.4. Morphologies Fig. 5 illustrates a representative scanning electron micrographs (SEM) of Ni substituted CuMn2 O4 powders; where SEM images (a) and (b) are represented to NiMn2 O4 and CuMn2 O4 , respectively, before reduction whereas SEM images (c) and (d) are related to their products after reduction process. For all samples, the particles surface morphologies of polycrystalline powders were homogeneous and appeared as a spherical like structure before reduction. However, a relatively homogeneous dense structure was observed after manganite reduction. In comparison, Sheibani et [20] synthesized heterogeneous mixtures of relatively coarse agglomerated particles Cu ∼50 wt %MnO nanocomposite powder by combustion reaction between CuO and Mn induced by high energy ball milling.

paramagnetic regime could be obtained, representing to the M-H hysteresis loops of manganites at room temperature [21,22]. On the other hand, Fig. 7 displays the change in magnetic properties of manganite as a result of H2 gas reduction at 1000 ◦ C. It can be seen that the paramagnetic behaviors of manganite were partially changed, depending on the output composite as a result of reduction process. Firstly, the saturation magnetization of produced composite Ni/MnO was 16.97 emu/g and coercivity was 27.33 Oe. This might be attributed to the change of the internal magnetic energy of produced Ni/MnO composite. After reduction, the surface magnetic anisotropy energy between two magnetic phases (Ferromagnetic Ni powders and antiferromagnetic

3.5. Magnetic properties Fig. 6 depicts the effect of Ni2+ ion substitution on the M-H hysteresis loops of the obtained CuMn2 O4 annealed at 800 ◦ C for 2 h using co-precipitation method. It is clear that the M-H hysteresis loops were so weak; there was no saturation magnetization even at 20,000 Oe. This is could be predicated from the magnetic nature of manganite at room temperature, spinel manganite belongs to antiferromagnetic family. Moreover, the exchange interaction between spins in magnetic materials is the main origin of spontaneous magnetization. So, owing to equal numbers of spin up (A-sublattice) and spin down (B-sublattice) magnetic moments, there is no net magnetization if no additional interaction exists. Therefore, the

Fig. 6. M-H hysteresis loops of the obtained Nix Cu1-x Mn2 O4 (X = 0, 0.3, 0.5, 0.7, and 1.0, respectively) sintered samples at 800 ◦ C for 2 h.

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´˚ and the crystallite size of produced the lattice parameter a (A) powders. 3. The microstructure of the produced powders is homogenously distributed and appeared as spherical like structure. 4. Catalytic activity of manganites showed that composition of Ni0.3 Cu0.7 Mn2 O4 exhibited the highest efficiency for CO oxidation followed by composition of X = 0.7 > 0.5 > 0.0 > 1.0, respectively, as the result of high surface area and pore size. 5. A facile route to synthesize magnetic metal/MnO composite is developed via H2 gas reduction of manganite. The magnetic properties of reduced manganite were duplicated and saturation magnetization of reduced NiMn2 O4 into Ni/MnO was duplicated of order 15 times from initial value. References

Fig. 7. M-H hysteresis loops of NiMn2 O4 , Ni0.5 Cu0.5 Mn2 O4 , and CuMn2 O4 powders after reduction by H2 gas at 1000 ◦ C.

MnO powders) was raised. As a consequence, the interaction at the interface between two phases could be increased. This led to induce easy direction of magnetization along interface. Hence, with increasing field strength, the bending of the two sublattice moments into the field direction becomes easier until both sublattice moments are aligned parallel to the field direction (spin flip phenomena) and further increase applied field, the saturation magnetization was achieved. Thus, a useful transformation for the magnetic structure was occurred. At the same time, the magnetic properties were duplicated of order 15 times from initial state. Secondly, the reduction process of other composites led to generate strong antiferromagnetic interaction. For instance, it was found for Cu/MnO composite that no net magnetization was detected at room temperature. 4. Conclusion The results can be summarized as follows: 1. Ultrafine polycrystalline manganite powders, Nix Cu1-x Mn2 O4 where (0 ≤ X ≤ 1), have been successfully synthesized using coprecipitation method at 800 ◦ C for 2 h. 2. As Ni2+ ion substitution increases, diffraction patterns sweep to the low-angle side appreciably which result in an increase of

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