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Section 4. Chemistry
https://doi.org/10.29013/AJT-20-1.2-40-46
Aronbaev Dmitry M., Ph.D., professor of the Russian Academy of Natural Sciences, Associate Professor of Samarkand State University
E-mail: diron51@mail.ru
Aronbaev Sergey D., Doctor of Chemistry, Corresponding Member of the Russian Academy of Natural Sciences, Professor of Samarkand State University Narmaeva Gavkhar Z., PhD student, Samarkand State University
Isakova Dilnoza T.,
undergraduate II year of study, Samarkand State University
Samarkand, University Boulevard
SYNTHESIS AND STUDY OF THE NANOCOMPOSITE MNO2/C
Abstract. A composite material was synthesized based on coal-graphite powder and manganese dioxide nanoparticles deposited on its surface, obtained during the reduction reaction of potassium permanganate by manganese (II) ions from aqueous solutions. The success of modifying coal-graphite powder by nanoparticles of manganese dioxide with sizes of20-55 nm has been proved by scanning microscopy and X-ray diffraction analysis. Using the method of cyclic voltammetry, was studied the behavior of the electrode from the MnO2/C composite and established its electrocatalytic activity in the anode region of potentials in the presence of H2O2 and ascorbic acid in a phosphate buffer with pH 7.4 with maximum anode peaks at + 0.72 V and + 0.255 V, respectively. The linear response of the electrode signal is observed in the concentration range of hydrogen peroxide 0.1-3.0 mm and ascorbic acid 0.25-3.0 mm.
Keywords: composite materials, coal-graphite, manganese dioxide nanoparticles, modification, carbon paste electrodes, cyclic voltammetry, hydrogen peroxide, ascorbic acid, determination.
1. Introduction of effective current sources [1; 2], capacitors [3-5],
Nanocomposites based on carbon-containing heterogeneous catalysts [6; 7], selective adsorbents
materials with the inclusion of micro / nanoparticles [8; 9] and so on. The preparation of composite car-
of transition metals and their oxides, in particular, bon-containing materials with the aim of forming
manganese dioxide, are widely used in the creation electrodes with a catalytic response on their basis
is of particular interest, since such electrodes can expand the possibilities of voltammetry in organic analysis [10-12].
Over a period of different time, the researchers performed modifications of the surface or volume of carbon-containing materials with a film or MnO2 micro / nanoparticles. At the same time, each of the authors applied their modification techniques, which were not simple and were not achieved positive reproducible results.
It should be noted here that the electrocatalytic properties of carbon-modified electrodes depend on the physical and chemical state of their surface. Therefore, almost any modification of the electrode surface requires a careful choice ofthe method ofpreparation of the film [14; 15]. Various approaches are usually used for this, including physical sorption of manganese-containing electrolytes followed by hardening of the film on the surface of the modified electrode by electrochemical methods, in particular, cyclic voltammetry [16]. However, such modification methods are considered rather complex, time-consuming and do not guarantee prolonged operation of the modified electrode, since the manganese dioxide film in contact with the electrolyte undergoes cracks and thinning during electrochemical processes.
This problem was partially solved by bulk modification of the carbon paste electrode with micro / nano particles of manganese dioxide.
Was described the manufacture of carbon paste electrodes, volumetric modified with 5-10% mic-roparticles (5 ^m) of manganese dioxide, which were successfully used in the voltammetric determination of hydrogen peroxide, ascorbic acid, riboflavin in some model and real objects [17-19].
We cannot ignore the work [20], which describes the procedure for volumetric modification of the carbon paste electrode MnO2. This modification consisted in impregnating the coal powder with a 5 M solution of manganese nitrate, followed by prolonged (for at least 8-10 hours) calcining the composite mass at a temperature of 520 ° C. During this, the
thermal decomposition of manganese nitrate to its dioxide was expected. Then, an indicator electrode was formed from the obtained composite mass using epoxy resin, which was used in the voltammetric determination of hydrogen peroxide, ascorbic acid and nitrite ions. The disadvantages of manufacturing a MnO2/C electrode by this method include the complexity and duration of the preparation of a composite electrode suitable for voltammetric and chrono-amperometric measurements.
In [21; 22], a method was described for producing manganese dioxide nanoparticles by direct reduction of potassium permanganate in a neutral medium on the surface of a carbon material: 4KMnO4 + 3C + H2O = 4MnO2 + K2CO3 + 2KHCO3, while the reaction time is 8 hours or more. Such a slow reaction course can become a serious obstacle to the scaling and commercialization of this technology, which is noted in an article by Gromadsky D. G. [23], who improved the hydrothermal synthesis technology by accelerating the formation of manganese dioxide by introducing an additional reagent, a reducing agent, into the reaction mixture, which would it was easily removed at the end of the synthesis without contaminating the finished composite. Isopropyl alcohol could play the role of such a reducing agent.
Despite the fact that this method is also not without drawbacks, it raises the idea that it is possible to obtain the MnO2/C composite by chemical methods using the appropriate redox reactions, which are so diverse for manganese compounds in various oxidation states.
The goal of this work is to obtain and study the characteristics of a composite MnO2/C material based on coal-graphite powder and manganese dioxide nanoparticles deposited on its surface and obtained during the reduction of potassium permanganate by manganese (II) ions from aqueous solutions.
2. Experimental part
As precursors for the production of MnO2 nanoparticles, the reaction was used 3MnCl2 + 2KMnO4 + 2H2O = 5MnO2 + 2KCl + 4HCl
In order to modify the coal-graphite powder by manganese dioxide, solutions were prepared containing 0.1 M potassium permanganate and manganese chloride each. Coal-graphite powder obtained by grinding carbon electrodes from spectral coal in a mill and sieved through a nylon sieve with a mesh diameter of0.08-0.05 mm. 4.570 ± 0.010 g of powder was placed in a 150 ml glass, where 20 ml of the prepared KMnO4 solution and 30 ml ofMnCl2 were simultaneously poured (this ratio of ingredients theoretically allows to obtain 0.435 g of manganese dioxide, which is ~ 10% of the total mass of the MnO2/C composite The resulting mixture was stirred at a speed of > 600 rpm for 1.5 hours, and then the precipitate was separated by filtration under vacuum, washing the precipitate with filtrate and then bidistilled water.
At all stages of the production of manganese dioxide, the catalytic activity was controlled by the rate of decomposition of hydrogen peroxide.
In all cases: in the colloidal solution of MnO2, in the sediment MnO2/C, a high catalytic activity was
S834 2G.0kV 10.3mm X 50.0 SE 4/22 2019 15:23
Figure 1. SEM image of the MnO2/C composite
Manganese dioxide nanoparticles with a size of 20 to 55 nm are concentrated on the surface of the coal. Following the practice of obtaining nanoparticles by precipitation from solutions, it can be assumed that the nanoparticle sizes will be smaller when using more dilute solutions and high mixing speeds [24].
observed on the filter. The washed precipitate together with the filter was placed in an oven and dried at a temperature of110 °C for 2 hours. The dried MnO2/C precipitate was transferred to a porcelain dish and ground using a glass rod. The precipitate was rather crumbly, and this procedure was not difficult.
Micrographs ofthe modified carbon powder were obtained on a scanning electron microscope of the brand Shimadzu SSX-550 (Japan). X-ray diffraction patterns were recorded on a Shimadzu XRD-7000 diffractometer (Japan), CuKa radiation, a graphite monochromator. The analysis was carried out using the computer database ICDD (International Center for Diffraction Data) PDF-2.
The electrochemical behavior of the electrodes formed from the obtained carbon-containing composite was studied by cyclic voltammetry. 3. Results and discussion Figure 1 shows a micrograph of the MnO2/C composite obtained using a scanning electron microscope.
MnOj/C- composite
/ =
u
= 1
T
1-1
70
60
Coal powder
-I-I-I-1
ID 20 30 40 50 60 70 2 Thetta, deg.
Figure 2. X - ray diffraction patterns of coal powder and MnO2/C composite
Elemental analysis showed that the MnO2/C nanocomposite consists of 91.5% C, 5.4% Mn, and 3.1% O, which is close to the calculated data. This also indicates the stoichiometric occurrence of the redox reaction to obtain synthetic manganese dioxide.
It is known that manganese dioxide, depending on the method of its preparation, can have different morphology corresponding to crystallographic forms, such as a-, ft-, j- and 8-, e -type [25]. Obviously, each of these forms may have different catalytic activity. Thus, the authors of [26], studying the reaction of oxygen reduction on epsilon-manganese dioxide on a carbon substrate (e-MnO2/C), established its greater catalytic activity compared to a-MnO2.
It may follow from this that the electrochemical characteristics of the electrode modified with manganese dioxide should be associated both with crystallographic forms and with morphologies. Most researchers involved in the problem of chemical modification of carbon paste electrodes note that this modification is mainly carried out with S-MnO2, which has hexagonal syngony. Thus, the authors performed sonochemi-cal synthesis of MnO2 / C nanocomposites, which consisted in the reduction of potassium permanganate on charcoal under the action of ultrasonic treatment of the suspension [27]. The resulting nanocompos-
■io H-1-1
0 0,2 0,4 0,6 0.8 1,0 I'ott'iuial, V (vs. Aa/'AftCI)
Figure 3. Cyclic voltammograms on a MnO2/C composite electrode in a phosphate buffer with a pH of 7.4 (dashed curve) and in a 0.5 mM hydrogen peroxide solution. Scanning speed 50 mV/s Inset: calibration dependence of the analytical signal of the electrode on the concentration of hydrogen peroxide
ite contained manganese (IV) oxide corresponding to the S modification having hexagonal syngony. The particle sizes for the MnO2 phase was 4-5 nm.
Figure 2 shows X-ray diffraction patterns of a coal powder and a MnO2 / C composite.
The X-ray diffraction pattern (Fig. 2) shows that the diffraction peaks for the MnO2 / C composite are different from those for spectral coal. The manifestation of peaks at 20 = 37.50; 43.20; 66.40, may indicate a modification.
On the composite MnO2 / C fabricated electrode, its electrochemical behavior was studied in the presence of hydrogen peroxide and ascorbic acid. Cyclic voltammograms were recorded in the range of -0.1 ^ -1.0 V in buffer solutions with a pH of 7.4 at a scanning speed of 50 mV/s.
The corresponding cyclic voltammograms are shown in (Figures 3 and 4).
The peak on the anode branch of the CVA for hydrogen peroxide was manifested at +0.72 V vs. Ag/AgCl electrode. Cathode peak at + 0.68 V.
o ■
-D 4 -0,2 0 0.2 C.I 0 K 0.3
Potential, V (vs. Ag/AgCl)
Figure 3. Cyclic voltammograms on MnO2/C - composite electrode in phosphate buffer with pH 7.4 with increasing concentrations of ascorbic
acid. Scanning speed 50 mV/s Inset: calibration dependence of the analytical signal of the electrode on the concentration of ascorbic acid
A possible electrocatalytic mechanism of this process can be described by the following steps: adsorption of hydrogen peroxide on a composite electrode; catalytic decomposition ofhydrogen peroxide on the surface ofMnO2, leading to a decrease in the oxidation state of Mn (IV) to Mn (II) or Mn (III); subsequent oxidation of Mn (II) or Mn (III) to MnO2:
MnO2 + H2O2 ■ MnO (Mn2O3) + H2O + O2 MnO (Mn2O3) + 2OH- ■ Mn02 + H2O + 2e-Since the above reactions are fast, the parallel current is much higher than the oxidation current MnO (Mn2O3) on the electrode surface in the absence of H2O2. Therefore, manganese dioxide nanoparticles act as mediators of electron transfer between hydrogen peroxide and the working electrode [28; 29].
The electrochemical behavior of this electrode was studied in the presence of ascorbic acid. The peak of anodic oxidation of ascorbic acid in a phosphate buffer with p H 7.4 is clearly observed at a potential of + 0.255 V at a scanning speed of 50 mV/s. The linear dependence of the analytical signal on the concentration of ascorbic acid in solution is observed in the range of 0.25-3.0 mm, which is comparable with studies by other authors [20; 30; 31].
Thus, the obtained composite material based on carbon graphite modified with manganese dioxide nanoparticles can be successfully used to create solid modified electrodes with a catalytic response for voltammetric analysis.
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