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Article

Structural, Optical, and Catalytic Properties of MgCr2O4 Spinel-Type Nanostructures Synthesized by Sol–Gel Auto-Combustion Method

1
Research Center MANSiD, Department of Electrical Engineering and Computer Science, Stefan cel Mare University of Suceava, Strada Universității 13, 720229 Suceava, Romania
2
Physical, Technical and Computer Science Institute, Yuriy Fedkovych Chernivtsi National University, 2, Kotsiubynskyi St., 58002 Chernivtsi, Ukraine
3
Department of General Chemistry and Chemistry of Materials, Yuriy Fedkovych Chernivtsi National University, 2, Kotsiubynskyi St., 58002 Chernivtsi, Ukraine
4
Faculty of Geography and Geology, Alexandru Ioan Cuza University of Iaşi, 11, Carol I Blvd., 700506 Iaşi, Romania
5
Faculty of Chemistry, Alexandru Ioan Cuza University of Iaşi, 11, Carol I Blvd., 700506 Iaşi, Romania
6
Department of Chemistry and Biochemistry, Central Michigan University, 1200 S. Franklin St., Mount Pleasant, MI 48859, USA
7
Science of Advanced Materials Program, Central Michigan University, 1200 S. Franklin St., Mount Pleasant, MI 48859, USA
8
Department of Biochemistry and Biotechnology, Vasyl Stefanyk Precarpathian National University, 57 Shevchenko St., 76018 Ivano-Frankivsk, Ukraine
*
Authors to whom correspondence should be addressed.
Catalysts 2021, 11(12), 1476; https://doi.org/10.3390/catal11121476
Submission received: 6 October 2021 / Revised: 25 November 2021 / Accepted: 26 November 2021 / Published: 1 December 2021
(This article belongs to the Special Issue Oxidation Catalysis under Unconventional Methods)

Abstract

:
Spinel chromite nanoparticles are prospective candidates for a variety of applications from catalysis to depollution. In this work, we used a sol–gel auto-combustion method to synthesize spinel-type MgCr2O4 nanoparticles by using fructose (FS), tartaric acid (TA), and hexamethylenetetramine (HMTA) as chelating/fuel agents. The optimal temperature treatment for the formation of impurity-free MgCr2O4 nanostructures was found to range from 500 to 750 °C. Fourier transform infrared (FTIR) spectroscopy was used to determine the lattice vibrations of the corresponding chemical bonds from octahedral and tetrahedral positions, and the optical band gap was calculated from UV–VIS spectrophotometry. The stabilization of the spinel phase was proved by X-ray diffraction (XRD) and energy-dispersive X-ray (EDX) analysis. From field-emission scanning electron microscopy (FE-SEM), we found that the size of the constituent particles ranged from 10 to 40 nm. The catalytic activity of the as-prepared MgCr2O4 nanocrystals synthesized by using tartaric acid as a chelating/fuel agent was tested on the decomposition of hydrogen peroxide. In particular, we found that the nature of the chelating/fuel agent as well as the energy released during the auto-combustion played an important role on the structural, optical, and catalytic properties of MgCr2O4 nanoparticles obtained by this synthetic route.

Graphical Abstract

1. Introduction

Spinel-type oxides, with the general formula AB2O4 (where A is divalent metal and B is trivalent metal), are some of the most technologically important materials, having played a key role in the development of modern electronics [1], in nuclear technology as a radiation-resistant materials [2], etc. In particular, the magnesiochromite MgCr2O4 possesses magnetic, electrical, and optical properties that make it the leading candidate for numerous technological applications in catalysis [3,4,5], high-temperature ceramics [6], interconnecting materials for solid oxide fuel cells [7], multifunctional ceramic sensor [8], and sensing [9,10,11].
As such, the development of reproducible synthetic methodologies for the reliable fabrication of spinel-type nanomaterials with controllable physical and chemical properties is of prime scientific and technological importance. Thus, various methods have been proposed to synthesize MgCr2O4 nanostructures, including the hydrothermal route [12], microwave [13], mechanical activation [14], high-temperature solid-state reaction [6], co-precipitation [3], and spray pyrolysis [15]. These methods have some drawbacks, such as high cost, lower specific surface area, large size distribution, and/or lower catalytic activity of the resulting nanoparticles. A valuable alternative is the sol–gel auto-combustion synthesis [16,17,18] as an easy and cheap method for forming high-quality nanoparticles. According to this method, MgCr2O4 nanoparticles are synthesized via a multi-step procedure including the dissolution of stoichiometric amounts of metal precursors and the chelating/fuel agent into an appropriate solvent, the formation of a gel, followed by the gradual increase of the reaction temperature and initiation of auto-combustion, during which the main chemical reactions occur, followed by the final calcination step. This synthetic method for the fabrication of spinel nanostructures allows for the growth of the nanoparticles with predictable properties. However, a proper selection of the chelating/fuel agent, metal precursors, and reaction conditions are oftentimes crucial to the purity and morphological characteristics of the final product that, in turn, will strongly influence the electrical, catalytic, magnetic, and optical properties of these spinel nanoparticles.
The main focus of the present paper was the exploration of the optimum parameters of the chelating/fuel agents in order to obtain MgCr2O4 nanoparticles with a pure spinel structure. In particular, we investigated the influence of the combustion agent on the structural formation, nanoparticle morphology, and catalytic characteristics. Moreover, it was found that the morphology of MgCr2O4 nanoparticles can be predicted from the combustion agent properties.
Several studies have been devoted to the investigation of the influence of the chelating/fuel agent on the properties of spinel-type compounds synthesized by a sol–gel auto-combustion approach. In particular, Slatineanu et al. [19] have shown that citric acid, tartaric acid, glycine, and egg white could be considered good candidates for chelating/fuel agents yielding single phase Zn ferrite nanoparticles at a calcination temperature of 973 K. The authors concluded that the nature of the fuel affects the duration of the auto-combustion reaction, yielding nanoparticles with an average size of about 40–50 nm [19].
Another interesting study was reported by Hu et al. [20], who performed a comparative analysis of MgCr2O4 and CoCr2O4 chrome-containing spinels. They used glucose as a complexing agent in the sol–gel method, followed by a post-synthesis calcination of the samples in air at 1400 °C. This procedure yielded MCr2O4 (M = Mg, Co) particles with a polyhedron morphology and average sizes ranging from around 0.4 to 3 µm. The auto combustion approach has been also used for the synthesis of the well-studied MgAl2O4 spinel nanoparticles. For example, Saberi et al. [21] used citric acid and ammonia solution in order to produce magnesium aluminate nanoparticles with a size between 20 and 100 nm by thermal treatment under argon atmosphere for 1 h at 700 °C and one hour more at 900 °C, respectively. In another paper, Habibi et al. [22] studied the influence of the calcination temperature on the sample morphology obtained within the sol–gel route by using propylene oxide as a fuel agent. Interestingly, the obtained MgAl2O4 nanoparticles has flake-like shapes, which meant that the combustion agent and the condition of synthesis allowed for variation in the particle morphology and properties. These preliminary studies shed some light on the formation of spinel nanoparticles by the combustion sol–gel method. However, the influence of the fuel/combustion agent on the morphology of MgCr2O4 nanoparticles synthesized by using hexamethylenetetramine, fructose, and tartaric acid is still not fully understood.

2. Results and Discussion

In order for the variation in size of the synthesized spinel nanoparticles to be explained, the enthalpy of combustion of the solid chelating/fuel agents should be considered. Under standard conditions, the values of the enthalpy of combustion ΔcH° of the combustion agents in solid state are −1159 ± 0.3 kJ/mol for tartaric acid [23], −2810.4 ± 0.3 kJ/mol for fructose [24], and −4200.11 ± 0.6 kJ/mol for hexamethylenetetramine [25]. Interestingly, the electron microscopy data showed that the average size of the nanoparticles increased with increasing enthalpy value of combustion of the fuel agent (see ESI). On the basis of these experimental observations, we hypothesized that chelating/fuel agents with a combustion temperature under 500 °C, short burning time of the gel, and a value of the enthalpy of combustion of about −1000 kJ/mol promote the formation of cuboidal nanoparticles with an average size of 10–16 nm similar to those of the TA chromite sample. Likewise, fuel agents with combustion temperatures above 1000 °C, longer burning time of the gel, and with values of the enthalpy of combustion around the −4000 kJ/mol will yield nanoparticles larger than 20 nm in size, whereas fuel agents with intermediate values of the combustion time, burning time, and enthalpy of combustion yield spinel nanoparticles with an average size ranging between 15 and 20 nm.

2.1. Infrared Spectroscopy

The three series of nanoparticles were analyzed using Fourier transform infrared (FTIR) spectroscopy in order to confirm the formation of the metal–oxygen bonds and ensure the elimination of the organic and nitrogen phases during the post-synthesis annealing process. Figure 1 shows the IR spectra recorded on the three series of samples synthesized by using HMTA, FS, and TA as chelating/fuel agents after thermal treatment at various temperatures: 500 °C, 700 °C, 750 °C, and 900 °C. The vibrational bands recorded at 638 cm−1, 640 cm−1, and 640 cm−1 on the three samples correspond to the lattice vibrations of the Cr3+–O2− from the tetrahedral positions. The markers recorded at 492 cm−1, 498 cm−1, 496 cm−1, 418 cm−1, 410 cm−1, and 415 cm−1 were attributed to the lattice vibrations of Mg2+, Cr3+–O2− from tetrahedral and octahedral positions [26]. The vibration bands recorded at about 940–950 cm−1 were attributed to the presence of Cr(VI)-O bond vibrations [26]. The absorption bands recorded around 1630 cm−1 (scissors-bending of H2O) indicate adsorption of water during the cooling of the samples and hydrophilic nature of the obtained particles [27].
The absence of the bands associated with the Cr(VI)-O bond vibrations suggests that the spinel form of MgCr2O4_HMTA nanoparticles were already formed after the thermal treatment at 500 °C, which confirmed the reduction of Cr (VI) to Cr (III) [26]. However, for the other two samples, the Cr(VI)-O bands disappeared only after the thermal treatment at 750 °C. Figure 1a–d shows the FTIR spectra recorded at various annealing temperatures on the three samples.

2.2. Phase Analysis

Powder X-ray diffraction analysis was carried out for samples that had been annealed at 750 °C. Figure 2 shows the XRD patterns collected for the spinel chromite samples synthesized in the presence of hexamethylenetetramine, fructose, and tartaric acid. The diffractograms clearly indicate the formation of pure MgCr2O4 spinel structures, all the peaks being successfully assigned to reflections of the MgCr2O4 reference patterns [28]. Moreover, no other reflections ascribable to crystalline MgO, Cr2O3, and CrO3 phases were detected.
Determination of the crystallite size for the three MgCr2O4 samples obtained from different precursors was performed by using the Williamson−Hall method [29]. As such, the crystallite size and microstrain are related to the size broadening through the following relationship:
β h k l c o s θ = K λ D + 4 ε s i n θ
where βhkl represents the full width at half maximum (FWHM) value of the diffraction peak, K is the shape factor (0.94), λ is the wavelength of Cu Kα (1.5406 Å) radiation, D is the crystalline size, and ε is the integral breadth related to the microstrain. Therefore, by plotting the variation of the βhkl cosθ vs. 4sinθ, we were able to calculate the crystalline size and the lattice microstrain of the samples from the intercept with the y-axis and slope of the line, respectively. Figure 3 shows the peak fitting of the five most intense peaks and the Williamson−Hall plots of the MgCr2O4_HMTA sample. The peak fitting was performed with Origin Pro software by using a Gaussian function.
The calculated values of the crystallite size (K) and microstrain (ε) for the MgCr2O4_HMTA, MgCr2O4_TA, and MgCr2O4_FS samples were found to be K = 13.5 ± 0.1; 18.7 ± 0.3, 20.1 ± 0.3 nm; and ε = −2.15%, 3.27%, and 0.65%, respectively. Interestingly, the lattice microstrain in the MgCr2O4_HMTA sample was found to be negative, indicating that the crystal was under a slight compressive stress, whereas for the MgCr2O4_TA and MgCr2O4_FS samples, the crystal was under a slight tensile stress. The values of the crystallite size in all three samples were consistently smaller than the values of the particle size determined by SEM analysis, thereby indicating that the MgCr2O4 nanoparticles presented a multidomain structure with respect to the scattering of X-rays.

2.3. UV–VIS Analysis

The UV–VIS absorption spectra were collected to determine the optical band gap values of the samples. In order to do this, the Tauc method was applied [30]:
(αhν) = A(Eg)n,
where α is the absorption coefficient; h is the Planck’s constant; ν is the frequency of incident lights; A is an arbitrary constant; Eg is the optical energy band gap; and n is the exponent that characterizes the type of optical transition, i.e., n = ½ for direct transition and n = 2 for indirect transition. Figure 4 presents (αhν)2 (where n = 1/2) as a function of hυ. The energy gap values were obtained at the intersection of the fitted linear part above the sharp absorptions around 3 eV with the horizontal axis of energy. The Eg values obtained for MgCr2O4_HMTA, MgCr2O4_FS, and MgCr2O4_TA were around 2.80 eV, 2.87 eV, and 2.98 eV, respectively. The acquired optical spectra presented a weak absorption with a shoulder of around 2.5 eV. This absorption peak might be attributed to the oxygen vacancies defects, similarly to that reported on the well-studied MgAl2O4 spinel [31,32].

2.4. Morphological Analysis

Field-emission scanning electron microscopy (FE-SEM) analysis indicates that, regardless of the precursor used as a fuel, the resulting spinel-type nanopowders are relatively homogeneous and consist of uniformly sized nanoparticles (Figure 5). The average size of the nanoparticles were around 22 nm, 19 nm, and 16 nm for MgCr2O4_HMTA, MgCr2O4_FS, and MgCr2O4_TA, respectively (see Figures S1 and S2 from ESI).
The EDX analysis of MgCr2O4_HMTA nanoparticles further confirmed the formation of MgCr2O4 (Figure S3a). The amount of carbon detected was around 2% and most likely corresponded to the carbon-coated copper grid on which the spinel chromite nanoparticles were deposited prior to the elemental analysis.
These differences in size were presumably related to the thermal energy released during the auto-combustion process. Since the flame temperature in the case of HMTA was about 1100 °C [16], it led to the release of a larger amount of energy compared with the synthesis of MgCr2O4_FS and MgCr2O4_TA, whereby the flame temperatures were about 425 °C and 505 °C, respectively, and therefore less thermal energy was released for the respective equimolar quantity of chelating/fuel agents. The larger size of MgCr2O4_FS nanoparticles than that of MgCr2O4_TA could be explained by taking into account the burning time of the fuel agent, i.e., fructose had a burning time of about 3 min since the second one had a burning time of only 5 s. The size effect was also observed in both electrical and dielectric properties (see Figure S4 from ESI). As expected, the MgCr2O4_HMTA nanoparticles, characterized by a higher diameter, were the most conductive and had the highest value of the dielectric permittivity among the three samples (around 21 at 1 Hz). On the other hand, the MgCr2O4_TA particles, which had the smallest diameter, were less conductive and had a dielectric constant of around 10 at 1Hz.

2.5. Catalytic Activity

Figure 6 shows the catalytic activity of the three series of MgCr2O4 nanoparticles, whereby it can be seen that the catalytic activity of MgCr2O4_TA was substantially higher than that of the MgCr2O4_HMTA and MgCr2O4_FS, whose catalytic activity towards the decomposition of H2O2 was almost negligible.
As the catalytic activity is usually affected by the specific surface (size and shape) as well as by the crystalline structure of the catalyst, the higher activity of the MgCr2O4_TA nanoparticles could be tentatively attributed to the higher specific surface of the nanoparticles since the three series had the same crystal structure, according to the XRD analysis. These experimental results strongly suggest that the catalytic activity of the MgCr2O4 nanoparticles on the decomposition of hydrogen peroxide sharply increased when their average size decreased below 16 nm. Additionally, as revealed by EDX analysis, the MgCr2O4_TA nanoparticles presented a higher amount of chromium on their surface, which can be also considered responsible for their increased catalytic activity (Figure S2c). Taking this into consideration, we can tentatively explain the mechanism of catalytic action by a radicalic mechanism:
Cr3+ + 3HO-OH = Cr6+ + 3HO + 3HO⋅ + 3e
Cr6+ + 3e + 3HO-OH = Cr3+ + 3H+ + 3HOO⋅
HOO⋅ + HO* = O2 + H2O
HO⋅ + *OH = ½ O2 + H2O
H+ + HO = H2O
Such a mechanism involved the oxidation of the Cr3+ ions and the generation of HO radicals followed by its subsequent reduction to Cr3+ and the combination of the radicalic species with formation of H2O and molecular oxygen, respectively.

3. Materials and Methods

3.1. Synthesis of MgCr2O4 Nanoparticles

Analytical-grade Mg(NO3)2 ⋅ 6H2O and Cr(NO3)3 ⋅ 9H2O were used as metal precursors, whereas hexamethylenetetramine (HMTA), fructose (FS), and tartaric acid (TA) were used as chelating and combustion agents, respectively. The molar ratio of the metal cations was Mg2+:Cr3+ 1:2, whereas the molar ratio of chromite: chelating/fuel agents was 1:3.
Each synthesis was performed by mixing the reagents into the respective stoichiometric ratio followed by their dissolution in distilled water. The resulting dark violet reaction mixture was kept under air and stirred at 75 °C until a gel was formed. The gel mixture was subsequently placed in a sand bath for thermal treatment. The heating step consisted of bringing the reaction mixture to 100 °C, and then it was heated up to 350 °C with an increased temperature step of 50 °C. For each temperature steps, the gel mixture was annealed for 1 h. In order to obtain a single-phase spinel-type structure, all the as-prepared samples were subjected to an additional heat treatment at 500 °C, 700 °C, 750 °C, and 900 °C for 5, 7, 8, and 9 h, respectively. The optimization of the reaction conditions was performed by analyzing the samples after each synthesis. A schematic diagram of MgCr2O4 nanoparticle synthesis is presented in Figure 7.

3.2. Nanoparticle Characterization

To monitor the formation of the spinel structure and the absence of organic phases, we used FTIR spectroscopy. The room temperature IR spectra were collected using a PerkinElmer Spectrum TwoTM spectrometer (PerkinElmer, Waltham, MA, USA) in attenuated total reflectance (ATR) mode. The X-ray powder diffractograms (XRD) were recorded using a Bruker D8 Advance diffractometer (Brucker, Billerica, MA, USA) equipped with a Cu anode (λ = 0.15406 nm). Scanning electron microscopy images and elemental analysis were carried out with a Hitachi SU-70 Field Emission Scanning Electron Microscope (Hitachi, Tokyo, Japan) equipped with an Oxford Instrument EDX-detector. The UV–VIS spectra were recorded using an OceanOptics USB2000 spectrophotometer (OceanOptics, Tampa, FL, USA) and colloidal solutions of nanoparticles suspended in water in the presence of citric acid. The dielectric analysis was performed using a CONCEPT 40 Broadband Dielectric Spectrometer (Novocontrol GmbH, Montabaur, Germany) equipped with an Alpha-A high-performance frequency analyzer in a frequency range from 0.1 Hz to 1 MHz.

3.3. Study of the Catalytic Properties of MgCr2O4 Nanopowders

The catalytic properties of the MgCr2O4 nanoparticles were studied by measuring the volume of the formed oxygen during the catalytic decomposition of hydrogen peroxide. In a typical experiment, 100 mg of catalyst was added to 15 mL of 16% NaOH solution. The analyses were conducted on the setup for gas volume measurements (see the schematic representation in the Figure S5 from ESI). After 10 min of stirring the mixture containing MgCr2O4 nanoparticles and the alkaline solution, a volume of 1.5 mL of 3% H2O2 solution was injected, and the amount of released oxygen was recorded in time.

4. Conclusions

In this study, hexamethylenetetramine, fructose, and tartaric acid were used for the first time as chelating/fuel agents for the rational synthesis of spinel MgCr2O4 nanoparticles by using a combustion sol–gel method. The reaction conditions were optimized to ensure the versatility and reliability of the proposed route, and the resulting optimal protocol for pure spinel phase formation is reported. The experimental results strongly suggest that several factors, including the burning time of gel, the combustion temperature, and the enthalpy of combustion of the solid chelating/fuel agents, influence the size, electric properties, and dielectric properties of the resulting spinel nanoparticles. Catalytic activity of these nanoparticles for the decomposition of hydrogen peroxide decomposition showed that the maximum catalytic activity occurred when the size of the nanoparticles was smaller than 16 nm, presumably the result of an increasing amount of chromium on the surface of the nanoparticles.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/catal11121476/s1, Figure S1: Measured average size of MgCr2O4_HMTA—(a), MgCr2O4_FS—(b) and, MgCr2O4_TA—(c) nanoparticles; Figure S2: Set of SEM images used to determine the size histograms; Figure S3: EDX elemental analysis of MgCr2O4 nanoparticles synthesized by hexamethylenetetramine (HMTA)—(a), fructose (FS)—(b) and tartaric acid (TA)—(c); Figure S4: Frequency dependence of the real part of the AC electrical conductivity (a) and of the real part of dielectric permittivity (b) recorded at room temperature, on the three series of spinel MgCr2O4 nanoparticles; Figure S5: Schematic representation of experimental setup used in the evaluation of the catalytic properties; Table S1: Average particle size and size distribution of MgCr2O4 nanoparticles synthesized by hexamethylenetetramine, tartaric acid and fructose.

Author Contributions

Conceptualization, A.R., G.C., A.-R.I., Y.K., M.N.P., P.F., V.M., O.K., A.K.; methodology, V.M., Ş.C., E.-D.C.-A., O.K.; investigation, V.M., Ş.C., A.D., A.R.; resources, M.N.P., A.R., Y.K.; writing—original draft preparation, V.M., G.C., O.K., Y.K., A.R.; writing—review and editing, G.C., Y.K., A.K., O.L., A.-R.I., A.D., V.G.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Ministry of Education and Science of Ukraine, grant number 0119U100728MESU, STCU-MESU research grant (# 6274), PN-III-P1-1.2-PCCDI-2017-0917 (contract no. 21PCCDI/2018), and PN-III-P4-ID-PCCF2016-0175 (contract no. PCCF18/2018). VGC thanks CNFIS for project number CNFIS-FDI-2021-0357.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. FTIR spectra recorded at ambient temperature after annealing the samples at 500 °C (a), 700 °C (b), 750 °C (c), and 900 °C (d).
Figure 1. FTIR spectra recorded at ambient temperature after annealing the samples at 500 °C (a), 700 °C (b), 750 °C (c), and 900 °C (d).
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Figure 2. XRD diffractograms of (a) MgCr2O4_HMTA, (b) MgCr2O4_FS, (c) MgCr2O4_TA, and (d) reference spectra of MgCr2O4 recorded with Diamond 4 software.
Figure 2. XRD diffractograms of (a) MgCr2O4_HMTA, (b) MgCr2O4_FS, (c) MgCr2O4_TA, and (d) reference spectra of MgCr2O4 recorded with Diamond 4 software.
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Figure 3. Peak fitting by using a Gaussian function (a) and Williamson–Hall plot (b) for the MgCr2O4_HMTA sample.
Figure 3. Peak fitting by using a Gaussian function (a) and Williamson–Hall plot (b) for the MgCr2O4_HMTA sample.
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Figure 4. Direct Tauc plot transition for MgCr2O4 nanoparticles synthesized by hexamethylenetetramine (HMTA) (1), fructose (FS) (2), and tartaric acid (TA) (3).
Figure 4. Direct Tauc plot transition for MgCr2O4 nanoparticles synthesized by hexamethylenetetramine (HMTA) (1), fructose (FS) (2), and tartaric acid (TA) (3).
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Figure 5. FE-SEM micrographs recorded on (a) MgCr2O4_HMTA, (b) MgCr2O4_FS, and (c) MgCr2O4_TA.
Figure 5. FE-SEM micrographs recorded on (a) MgCr2O4_HMTA, (b) MgCr2O4_FS, and (c) MgCr2O4_TA.
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Figure 6. Catalytic activity of MgCr2O4 nanoparticles on the reaction of hydrogen peroxide decomposition.
Figure 6. Catalytic activity of MgCr2O4 nanoparticles on the reaction of hydrogen peroxide decomposition.
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Figure 7. Schematic diagram of the synthetic route for the fabrication of MgCr2O4 nanoparticles.
Figure 7. Schematic diagram of the synthetic route for the fabrication of MgCr2O4 nanoparticles.
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Mykhailovych, V.; Kanak, A.; Cojocaru, Ş.; Chitoiu-Arsene, E.-D.; Palamaru, M.N.; Iordan, A.-R.; Korovyanko, O.; Diaconu, A.; Ciobanu, V.G.; Caruntu, G.; et al. Structural, Optical, and Catalytic Properties of MgCr2O4 Spinel-Type Nanostructures Synthesized by Sol–Gel Auto-Combustion Method. Catalysts 2021, 11, 1476. https://doi.org/10.3390/catal11121476

AMA Style

Mykhailovych V, Kanak A, Cojocaru Ş, Chitoiu-Arsene E-D, Palamaru MN, Iordan A-R, Korovyanko O, Diaconu A, Ciobanu VG, Caruntu G, et al. Structural, Optical, and Catalytic Properties of MgCr2O4 Spinel-Type Nanostructures Synthesized by Sol–Gel Auto-Combustion Method. Catalysts. 2021; 11(12):1476. https://doi.org/10.3390/catal11121476

Chicago/Turabian Style

Mykhailovych, Vasyl, Andrii Kanak, Ştefana Cojocaru, Elena-Daniela Chitoiu-Arsene, Mircea Nicolae Palamaru, Alexandra-Raluca Iordan, Oleksandra Korovyanko, Andrei Diaconu, Viorela Gabriela Ciobanu, Gabriel Caruntu, and et al. 2021. "Structural, Optical, and Catalytic Properties of MgCr2O4 Spinel-Type Nanostructures Synthesized by Sol–Gel Auto-Combustion Method" Catalysts 11, no. 12: 1476. https://doi.org/10.3390/catal11121476

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