Basic features of Pt/C catalysts synthesised via the non-isothermal decomposition of platinum acetylacetonate

Alexandra Kuriganova*^a, Nina Smirnova^a, Alexey Yatsenko^a, Mikhail Gorshenkov^b, Nikolay Leontyev^c, Mathieu Allix^d, Aydar Rakhmatullin^d and Igor Leontyev^e
aTechnological Department, Platov South Russian State Polytechnic University (NPI), 346428 Novocherkassk, Russia. E-mail: kuriganova_@mail.ru
^bDepartment of Physical Materials Science, National University of Science and Technology MISIS, 119049 Moscow, Russia
^cDepartment of Physics, Azov-Black Sea Engineering Institute of Don State Agrarian University, 347740 Zernograd, Russia
^dConditions Extrêmes et Matériaux: Haute Température et Irradiation, CEMHTI, UPR 3079–CNRS Univ. Orléans, 45071 Orléans, France
^eDepartment of Physics, Southern Federal University, 344090 Rostov-on-Don, Russia

First published on 21st July 2025

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Introduction

Platinum nanoparticles (NPs) supported on carbon materials have been demonstrated to be effective catalysts in proton exchange polymer membrane fuel cell (PEMFC) technology.^1,2 To date, numerous methodologies have been developed for synthesizing Pt NPs and Pt/C catalysts. These developments have enabled the modulation of their quantitative and qualitative characteristics.^3,4 However, it remains imperative to establish the basic features of the synthesised Pt NPs and Pt/C catalysts, irrespective of the chosen approach, in order to achieve a comprehensive understanding of the physicochemical processes. This would enable the effective variation of the important parameters of Pt/C catalysts, which have a significant impact on their activity and stability in the corresponding electrochemical processes. These parameters include the Pt loading in Pt/C catalysts, in addition to the microstructural and crystallographic properties of Pt NPs.

It is evident that the Pt loading in the catalyst exerts a significant influence on various macro- and micro-parameters. These include the structure of the catalytic layer,⁵ gas transport within the catalytic layer,⁶ and rate of its degradation.⁷ In addition, the size of the Pt nanoparticles, value of the electrochemically active surface area (ECSA) of Pt/C,^8,9 are affected by the degree of Pt nanoparticles agglomeration on the surface of the carbon support.^10,11

The choice of the Pt precursor is pivotal for the synthesis of free Pt NPs and Pt/C, with the success of the synthesis being contingent with this choice. Although platinum acetylacetonates (Pt(acac)₂) are widely available commercially and are comparable in cost to conventional precursors for the synthesis of Pt-based materials, they are not as common as, for example, Pt halides and hexachloroplatinic acid (H₂PtCl₆) and their salts for the synthesis of Pt NPs. Consequently, metal acetylacetonates can be regarded as precursors for the synthesis of metal NPs owing to their high solubility in various organic solvents, the possibility of varying their concentration in solution and the ability to undergo thermal reduction.¹² As we have previously demonstrated, Pt(acac)₂ can be effectively employed as a precursor for synthesizing both free Pt NPs and Pt/C via non-isothermal thermal decomposition of Pt(acac)₂.^13,14 It is noteworthy that one of the important results of our work^13,14 is the fact that the synthesis method is extremely simple: it is performed in only two stages and allows easy control of the size of the Pt NPs by varying the heating rate. The present study aims to identify the features of Pt/C obtained under conditions of non-isothermal decomposition of Pt(acac)₂ with varying Pt loadings.

Materials and methods

Procedure for the synthesis of Pt/C catalysts

The method for synthesizing Pt/C catalysts included the following steps. Initially, Pt(acac)₂ (Sigma-Aldrich 97%) was dissolved in tetrahydrofuran under continuous stirring using a magnetic stirrer. The volume of solvent was chosen such that the Pt acetylacetonate was completely dissolved. The mass of Pt acetylacetonate was calculated based on the fact that the Pt loadings in the synthesized Pt/C (Pt_theor) material were 10 wt%, 15 wt%, 20 wt%, 25 wt%, and 30 wt%. The Pt catalysts corresponding to these theoretical metal loadings were designated from Pt/C_10 to Pt/C_30. After complete dissolution of the Pt acetylacetonate, a Vulcan XC-72 carbon support was added to the solution with continuous stirring. The solution was then evaporated in an ultrasonic bath at room temperature until the solvent was completely removed. The carbon support impregnated with Pt acetylacetonate obtained after evaporation was loaded into a porcelain crucible and placed in a programmable oven. Then, the required heating rate (2 K min⁻¹) was set, and the crucible with the sample was heated in the furnace. After reaching the final synthesis temperature, the sample was cooled in a furnace with its door open. The real Pt loading in the synthesized catalysts (Pt_obs) was determined by thermogravimetry.

Physico-chemical analysis

X-ray diffraction (XRD) measurements in this work were performed using a Panalytical X'pert Pro diffractometer in the Bragg–Brentano θ–θ geometry using X-ray radiation from a Cu anode (45 kV, 40 mA, 1.54187 Å). XRD patterns with 2θ ranging from 20° to 120° were collected in continuous-scanning mode with an interval of 0.02 s at a speed of 3° min⁻¹. The diffractometer resolution function was determined using NIST powder LaB₆ (NISTSRM660a).

Transmission electron microscopy (TEM) was performed using a 200 kV JEOL 2100 microscope with a LaB₆ cathode. The synthesized powder was mixed with ethanol; then, the mixture was ultrasonified for 10 minutes to form a slurry. A single drop of the slurry was placed on an ultrathin carbon-coated copper grid (400 mesh) using a micropipette. The grid was then dried under vacuum and placed in a TEM sample holder. TEM observations were conducted in the bright-field, high resolution and selected area electron diffraction modes.

Thermogravimetric analysis (TG) was carried out using Mettler Toledo TGA/DSC 1 in the range of 25–1000 °C at a heating rate of 10 K min⁻¹ under an air atmosphere.

Electrochemical measurements

For electrochemical studies of Pt/C catalysts with varying platinum loading, a “catalytic ink” was prepared according to the following composition: 14 mg Pt/C, 1 mL isopropyl alcohol, and a 10% aqueous dispersion of proton-conducting polymer Nafion, representing a weight ratio of 25% of the total weight of carbon black in the Pt/C material. The “catalytic ink” was stirred using a magnetic stirrer, and 12 μL of the “catalytic ink” was subsequently applied to a glass carbon electrode (working electrode). The electrode was then dried in air. Electrochemical studies were conducted in a standard three-electrode cell, wherein a platinum wire was employed as the counter electrode and a silver chloride electrode (Ag/AgCl) was utilized as the reference electrode. All potentials presented in this study are referenced to the reversible hydrogen electrode (RHE).

Prior to electrochemical studies, the working electrode was stabilised in N₂-deaerified 0.1 M HClO₄ for 40 cycles at a potential sweep rate of 50 mV s⁻¹. Subsequently, the working electrode was polarised in 0.1 M HClO₄ by deairing it with N₂ at 100 mV (RHE) for 20 minutes. Then, the electrolyte was bubbled with CO for 20 min, continuing the polarisation of the working electrode at 100 mV (RHE). Furthermore, upon disengaging the CO supply, N₂ was introduced into the electrolyte solution. Polarization of the working electrode at 100 mV was then conducted, with the process continuing for 20 minutes (RHE). Next, the electrode polarisation was deactivated, and the cyclic voltammetry (CV) curve was recorded from 100 mV over a potential range of 50–1300 mV (RHE).

The electrochemically active surface area of the Pt/C catalysts was determined based on the charge spent on CO oxidation, with the exception of the charge spent on double layer charging:

where Q_CO represents the charge passed during CO oxidation, g_Pt is the mass of Pt on the working electrode, and Q_m = 420 (μC cm⁻²)¹⁵ is the charge required to desorb a monolayer of CO_ads from 1 cm² of platinum.

Results and discussion

The analysis of the XRD powder patterns of the synthesized Pt/C catalysts (Fig. 1) shows that all diffraction peaks correspond to the face-centred cubic cell of Pt (space group Fm3m, JCPDS 00-004-0802). In addition, in all X-ray diffraction patterns in the region of 2θ angles of 20–30°, there is a highly broadened asymmetric diffraction peak characteristic of the Vulcan XC-72 carbon support.

The average nanoparticle size, unit cell parameter, and shape of Pt nanoparticles of the freshly prepared Pt/C catalysts were determined via refinement of the X-ray diffraction patterns using the Rietveld method with FullProf software.¹⁶ In this case, the spherical harmonics method^17,18 was used to model the broadening of the diffraction peaks caused by both the small Pt nanoparticle size and macrostrain.¹⁹ We have repeatedly used this technique for the treatment of the X-ray diffraction patterns of Pt/C and Pd/C catalysts.^13,20–22

It is well known that if a material is polymorphic, then all the components included in its composition contribute to the scattering of X-rays. The X-ray diffraction pattern of the Vulcan XC-72 carbon support is shown in Fig. 2. In this X-ray diffraction pattern, two broadened asymmetric diffraction peaks in the region of 2θ angles of 20–30° and 40–50° are clearly distinguishable. The intensity of the second peak is approximately 4 times less than that of the first. Precisely, the second peak influences the accuracy of processing the X-ray diffraction pattern of Pt nanoparticles since it is in the region of 2θ angles of 30–50° where the most intense Pt peaks are located, peaks (111) and (200).

However, it is noteworthy that^13,20–22 we previously dealt with both fairly high loadings of the metal component in the Pt/C catalyst (>20%) and quite large Pt NPs (>5 nm). The analysis of the X-ray diffraction pattern of Pt/C presented in²³ shows the intensity of the peak of the carbon support in the 2θ region of 20–30°, and the intensity of the peak (111) of Pt differs by at least an order of magnitude. Accordingly, the difference in the intensities of the second peak of the carbon support and the peak (111) of Pt was approximately 50 times.

In this case, the contribution of the carbon support to the scattering of X-ray radiation is small (owing to the low scattering ability of carbon compared to Pt), and ignoring the contribution of the carbon support does not introduce a significant error in determining the characteristics of Pt NPs. It would be more accurate to say that the error introduced by not considering the carbon support when refining the XRD patterns is less than the error in determining the average size of the Pt NPs and their unit cell parameters. Simultaneously, the analysis of the XRD pattern of Pt/C_10 indicates that the intensities of the (111) diffraction peak of Pt and the carbon support are the same. For the Pt/C materials synthesized in this work, the contribution of the Vulcan XC-72 carbon support to the diffraction pattern can no longer be neglected.

As noted above, in the X-ray diffraction pattern of the Vulcan XC-72 carbon support, there is an asymmetric diffraction peak in the region of 2θ angles of 20–30°. Such asymmetry is characteristic of two-phase samples. The carbon family with planar sp2 bonding is represented by graphite, where the layers of carbon hexagons are stacked in parallel; the stacking regularity of ABAB… sequence belongs to the hexagonal crystallographic system (hexagonal graphite) and that of ABCABC… to the rhombohedral system (rhombohedral graphite).^24–26

Consequently, in the first stage, we carried out refinements by applying the Rietveld method to the XRD pattern of the carbon support using these two space groups. The spherical harmonic method for both phases was used for refinement. The results of the refinement are presented in Table 1. The small size of the nanocrystallites of both the rhombohedral phase R3m and the hexagonal P6₃/mmc is noteworthy. As evidenced by the results of transmission electron microscopy (Fig. 3), the particle size of the carbon support Vulcan XC-72 is approximately 20–50 nm, each consisting of individual crystallites. The concentration of the hexagonal phase P6₃/mmc is approximately two times less than that of the rhombohedral phase (Table 1), which indicates a high defectiveness of the carbon support. These results are in good agreement with Raman spectroscopy data²⁷ according to which the I_D/I_G ratio is equal to 1.87, which indicates a high degree of structural imperfection.

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In the second stage of refining the X-ray diffraction patterns of Pt/C catalysts, only the parameters responsible for broadening the platinum peaks, the background of the XRD pattern, and the scale factors of both phases of the carbon support were refined. The results of the refinement are presented in Table 2 and Fig. 4.

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It is noteworthy that the presented method for determining the microstructural characteristics of Pt/C, considering the contribution of the carbon support, has not been introduced in the literature before. In most cases, the size of platinum nanoparticles was determined either by the (111) or (220) peak.^28–30 Indeed, in the region of 2θ angles of the diffraction peak (220) Pt, there are no peaks of the carbon support. However, this reflection is quite a high-angle; therefore, macrostrain contributes to its broadening, which leads to systematic errors in the determination of the average size of the nanoparticles.

The dependence of the average size of Pt nanocrystallites (NCs) on the Pt loading (Fig. 5) is nonlinear: D^S_av of Pt NCs increases with increasing Pt loading. This nature of observed dependence can be explained as follows. In the first stage of synthesis, Pt acetylacetonate dissolves. Then, the resulting solution is impregnated into a carbon support, after which the resulting material is dried. It can be assumed that after evaporation of the solvent, platinum acetylacetonate uniformly covers the entire surface of the carbon support (Fig. 6a). Accordingly, at a low concentration of Pt acetylacetonate (in fact, a low loading of the metal component in the synthesized sample), the thickness of this layer is less than the thickness of the layer at a high concentration of acetylacetonate.

Further, as we showed [14] based on in situ X-ray studies of the thermal decomposition of Pt acetylacetonate, at a temperature of approximately 440 K, Pt acetylacetonate melts (Fig. 6b) and subsequently decomposes and grows Pt nanoparticles (Fig. 6c). It is reasonable to assume that after melting, Pt acetylacetonate is in the form of melt beads distributed over the surface of the carbon support (Fig. 6b).

In this case, apparently, an increase in the thickness of the acetylacetonate layer on the support surface leads to an increase in the size of the melt droplets. Next, acetylacetonate decomposes, and the growth of Pt nanoparticles begins (Fig. 6c), with each nanoparticle growing from a drop of molten Pt acetylacetonate formed during the melting stage. The above-described scheme of Pt nanoparticle growth is indirectly confirmed by the uniform distribution of Pt nanoparticles over the surface of the carbon support (Fig. 3). Another interesting feature of the synthesized Pt/C catalysts is the fact that for them, the anisotropy factor R = D₂₀₀/D₁₁₁, which characterizes the shape of the synthesized nanoparticles and is equal to the ratio of the nanoparticle's sizes along the directions (200) and (111), is practically the same and lies in the range from 0.88 to 0.92. This corresponds to the cuboctahedral shape of Pt NPs.³¹ This shape is the most stable form and corresponds to the shape of Pt NP nuclei. Previously, we also observed a similar picture,³¹ that is, the smallest Pt NPs corresponded to the shape of a cuboctahedron. The cuboctahedral shape of the Pt NPs synthesized here indicates that using this synthesis method, the process of nanoparticle nucleation is predominant compared to the growth process.

Table 2 also presents the unit cell parameters of the Pt nanoparticles synthesized in this study. For the Pt/C_30, Pt/C_25, and Pt/C_20 catalysts, the unit cell parameter is smaller than the unit cell parameter of bulk Pt (a = 3.9231 Å). This pattern corresponds to the size effect described in our previous study.³² This effect consists of a decrease in the unit cell parameter with a decrease in the average size of Pt nanoparticles and is caused by surface relaxation. However, unlike the dependence of a (D_av) reported in ref. 32, the unit cell parameter a in the Pt/C catalysts synthesized in this study starts to increase as the nanoparticle size decreases. For Pt/C_15 and Pt/C_10 catalysts, a sharp increase in the parameter is observed. For nanoparticles with an average size of 1.45 nm, the unit cell parameter becomes larger than that for bulk Pt (Fig. 7). To the best of our knowledge, such dependence behavior has not been reported for Pt to date. This behavior can be explained by applying two competing mechanisms. One of them is surface relaxation, which leads to a decrease in the unit cell parameter. The second mechanism is an increase in the number of vacancies in nanoparticles with a decrease in their size. This increase in the vacancy number is a consequence of a significant decrease in the formation energy value of one vacancy with a decrease in the average size of nanoparticles.

The emergence of a vacancy in a nanoparticle actually means the emergence of an atom on the surface of the nanoparticle. The mass of the nanoparticle does not change, but the volume increases although the volume occupied by one vacancy is less than the volume occupied by one atom.

Thus, the appearance of a vacancy leads to an increase in the unit cell parameters. In the size range of 2.5–30 nm, the surface relaxation mechanism prevails over the vacancy mechanism, leading to a decrease in the parameter. At nanoparticle sizes less than 2.5 nm, the vacancy mechanism of increasing the parameter begins to prevail over surface relaxation.

The confirmation of our conclusion is also shown in Table 2. The occupation parameter (Pt occupation) obtained during the refinement for platinum atoms decreases as the size of the nanoparticles decreases, which actually means an increase in the number of vacancies in the Pt nanoparticles.

The electrochemically active surface area (ECSA) is a fundamental characteristic of Pt/C materials. The CO-stripping method is an effective method for determining the ECSA, as it allows for indirect evaluation of the microstructural characteristics of the catalytically active phase on the electrocatalytic activity of Pt/C materials.

Furthermore, the electrochemical oxidation of CO on Pt is a structurally sensitive reaction, providing insight into the influence of the microstructural characteristics of Pt/C materials on their electrocatalytic activity. It has been demonstrated that the CO electrooxidation potential on Pt and the shape of the CO-stripping peak (peak multiplicity) are contingent on several factors.^33,34 For instance, the multiplicity of the CO-stripping peak may be attributed to various factors, including the diffusion rate of CO from the (111) facets to the (100) facets.

This process is influenced by the presence of a strongly adsorbing anion, such as sulfate, within the supporting electrolyte. It has been demonstrated that increasing both the nanoparticle size and step density of Pt NPs results in a lower CO stripping peak potential, which may be due to a higher probability of defect formation on larger Pt particles. Furthermore, the agglomeration of Pt NPs has been shown to contribute to a reduction in the electrooxidation potential of CO owing to the reaction between CO and OH species adsorbing on different (but evidently aggregated and nearby) nanoparticles.

The mechanism of CO oxidation on Pt-containing materials can be described as follows. First, CO and oxygen-containing particles are adsorbed on the Pt surface; second, they undergo a chemical interaction:

H2O + Pt ⇄ Pt–OHads + H+ + e−

Pt–COads + Pt–OHads ⇄ CO2 + H+ + e− + 2Pt

In the initial phase, O-containing species (hereafter designated as OH_ads) are generated as a consequence of the release of water molecules from unoccupied Pt sites. Subsequently, the interaction of adsorbed CO_ads species with OH_ads results in the formation of CO₂, H⁺, an electron and two unoccupied Pt sites. Given that the second reaction occurs at the surface between two adsorbed species, it can be classified as a Langmuir–Hinshelwood reaction type. It is therefore evident that the mobility of adsorbed species on the surface, in addition to the intrinsic characteristics of the surface itself, plays a pivotal role in the CO electrochemical oxidation process.

It was determined that the value of ECSA for the examined materials is approximately 54–68 m² g⁻¹, exhibiting a decline with an increase in D^S_av and Pt loading in the Pt/C catalysts (Fig. 8a). Fig. 8b′–e′ illustrates the decomposition of the CO oxidative desorption peaks from Fig. 8b–e. For all investigated Pt/C materials, the CO electrooxidation peak is characterised by the presence of three subpeaks. The multiplicity of the CO oxidation peak, which is a characteristic feature of all the materials under investigation, is sometimes explained by the “size effect”. This effect can be defined as the tendency for the overvoltage of the CO electrooxidation reaction to increase as the size of the Pt nanoparticles decreases.^35,36

However, in the present work, Pt/C materials synthesised under non-isothermal decomposition conditions of Pt(acac)₂ were characterised by the presence of Pt nanoparticles with a rather narrow size distribution, as evidenced by XRD analysis (D = 1.45–2.00 nm) (Table 2, Fig. 6). In light of the aforementioned evidence, it can be concluded that the “size effect” is not a dominant factor in this particular case. A general observation of the TEM results (Fig. 3) indicates that Pt/C catalysts with a Pt loading exceeding 20% are characterised by the presence of Pt particle agglomerates (Fig. 3d and e). The results of Pt/C materials TEM studies lead to the conclusion that an increase in Pt loading in Pt/C catalysts is accompanied by an increase in the number of Pt agglomerates (Fig. 3). This is also reflected in the decline in the ECSA values (Fig. 8a).

It can be inferred that the initial subpeak (red), which exhibits a maximum at approximately 700–730 mV (RHE), may be attributed to the interaction between CO_ads and OH_ads species adsorbed on distinct Pt particles (inter-particle effect).¹¹ It can be reasonably deduced that an increase in the degree of agglomeration of Pt nanoparticles contributes to a reduction in the overvoltage of the electrochemical CO oxidation reaction. This is expressed as an increase in the area of the first subpeak.

The second subpeak, which we have designated as the “main peak” (yellow), exhibits a maximum at a potential of 800–830 mV (RHE). This may be attributed to the oxidation of CO on the Pt(100) faces,³⁷ which are the dominant surfaces based on the value of the anisotropy factor R (Table 2).

The third subcurrent (green), with a maximum potential of approximately 900–950 mV (RHE), can be attributed to the oxidation of CO on Pt(111) faces. The quantity of this subcurrent increases with elevated Pt content in the material, as evidenced by a reduction in the anisotropy factor R (Table 2).

Fig. 8b′–e′ clearly demonstrates the trend observed, whereby a reduction in the R factor results in an appreciable enhancement of CO oxidation at the Pt(111) facets, as evidenced by an expansion in the third subpeak area. However, the Pt/C_30 with the highest Pt loading, but with a similar R value to that of the Pt/C_25, exhibits a markedly different pattern (Fig. 8f′). The area of the third subpeak decreases significantly, while the area of the first subpeak increases.

This outcome may be attributed to the fact that when the Pt loading in the material exceeds 24 wt%, the agglomeration effect of Pt nanoparticles contributes more to the CO electrooxidation process compared to the crystallographic properties of Pt particles.

Conclusions

In the course of the non-isothermal decomposition of platinum acetylacetonate, Pt/C catalysts with varying platinum loading (ranging from 10% to 25%) were obtained. This study presents a method for processing the X-ray diffraction patterns of Pt/C materials with a low loading of metallic components, which allows consideration of the contribution of the carbon support. Based on the data from X-ray studies of the synthesized materials, it was shown that the synthesized carbon-supported Pt NPs exhibit a cuboctahedral shape. In addition, the dependence of the average size of Pt nanocrystallites D^S_av on the Pt loading is nonlinear: D^S_av of Pt NCs increases with increasing Pt loading. The mechanism of the formation of Pt nanoparticles is presented, describing the observed dependence. In addition, an interesting size effect was found in Pt/C nanoparticles. For ultra-small Pt/C nanoparticles (<2 nm), an increase in the unit cell parameter is observed with a decrease in their average size. Moreover, for nanoparticles with a size of 1.4 nm, the unit cell parameter exceeds the Pt counterpart. This is due to an increase in the number of vacancies in Pt nanoparticles with a decrease in their size.

It was found that in the Pt/C with the lowest Pt content (9.7 wt%), the unit cell parameter becomes larger than for the bulk Pt. The appearance of these vacancies leads to an increase in the unit cell parameter of Pt. It was determined that the microstructural characteristics of Pt nanoparticles and their degree of agglomeration, as influenced by platinum loading in Pt/C, have a differential impact on CO-stripping processes. An increase in the Pt loading in the Pt/C catalysts from approximately 10 to 25 wt% results in a concomitant decrease in the electrochemically active surface area of platinum from approximately 68 ± 2 m² g⁻¹ to 55 ± 2 m² g⁻¹. This decline is attributable to two factors: an augmentation in the mean size of Pt nanoparticles from 1.45 to 2.00 nm, and the presence of platinum agglomerates in Pt/C catalysts with platinum content exceeding 20%.

Author contributions

A. Kuriganova: investigation, data curation, and writing – original draft; N. Smirnova: funding acquisition, project administration, and writing review & editing; A. Yatsenko: investigation and data curation; M. Gorshenkov: investigation and data curation; N. Leontyev: investigation and data curation; M. Allix: conceptualization and writing – original draft; Aydar Rakhmatullin: visualization and writing – review & editing; and I. Leontyev: conceptualization, supervision, data curation, and writing–review & editing.

Data availability

All data used to arrive at the conclusions of this study are available in the main text. The data supporting the findings of this study are available from the corresponding authors upon reasonable request.

Correspondence and requests for materials should be addressed to Igor Leontyev, e-mail: https://inleontev@sfedu.ru.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Ministry of Science and Higher Education of the Russian Federation, project FENN-2024-0002. Thermogravimetric analysis measurements were conducted using the equipment of the Shared Research Center “Nanotechnologies” of Platov South-Russian State Polytechnic University (NPI).

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