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ISSN 2522-1841 (Online) AZERBAIJAN CHEMICAL JOURNAL № 2 2023 ISSN 0005-2531 (Print)
UDC 541.13.544.65
ELECTRODEPOSITION OF THIN MOLYBDENUM COATINGS FROM SULPHATE
SOLUTIONS
Y.E.Alizada, E.A.Salakhova, P.E.Kalantarova, A.F.Heybatova, K.I.Hajiyeva,
N.N.Khankishiyeva, D.B.Tagiev
M.Nagiyev Institute of Catalysis and Inorganic Chemistry, Ministry of Science and Education
of the Republic of Azerbaijan
elza_salahova@mail.ru
Received 02.02.2023 Accepted 14.03.2023
Based on the study of current-voitage dependencies during eiectroreduction of moiybdenum ions from sulfate electrolytes on Pt electrode, the conditions of deposition of Mo-nanocoatings were established. The influence of various factors such as temperature, content of components in the electrolyte, current density, acidity of solutions, etc., on the composition and quality of coatings was studied. It was found that, with an increase in the concentration of molybdenum and current density, the molybdenum content increases and the quaiity of coatings improves. Based on the determination of the nature of the cathodic polarization, it was found that the process of electrodeposition of molybdenum from sulfate electrolyte is mainly accompanied by chemicai poiarization. According to the experimentai data, the foUowing eiectroiyte composition (moi/i) is recommended for obtaining moiybdenum nanocoatings: 2.0 10-21.510-3 Na2Mo04 +2.0 H2SO4; pH=0.4; EV=0,005 Vs-1; t=750C, eiectrod - Pt.
Keywords: molybdenum, thin coatings, electrochemical deposition, alloys, current density.
doi.org/10.32737/0005-2531-2023-2-47-53
Introduction
Moiybdenum aiioys, which are used in aviation and space technoiogy, have shown great interest in recent years [1-4]. Aii known methods of production are mainiy thermai and are carried out at high temperatures. Gaivanic aiioys, despite some difficuities in the technoio-gy of their production are increasingiy used in industry. The appiication of wear-resistant coatings is an effective way to improve the efficiency of materiais. Researchers understand "the prospects for the use of gaivanic aiioys, as weii as the possibiiity of obtaining them with various combinations of individuai components, are iimitiess and have not yet been fuiiy reaiized". However, there are aiso works on eiectrodeposi-tion of rhenium aiioys from aqueous soiutions of eiectroiytes. It is known that, eiectroiytic aiioys have more pronounced vaiuabie technicai characteristics compared with simiiar aiioys. Deposition from aqueous soiutions of eiectroiytic aiioys based on rhenium with chaicogenides [710] wiii make it possibie to obtain materiais with high physicochemicai and structurai characteristics. The study of the kinetics of the pro-
cesses of cathodic reduction of rhenium and chaicogen ions, both separateiy and jointiy from eiectroiytes of various compositions, wiii aiiow to choose the most optimai conditions for the deposition of eiectroiytic aiioys of rhenium with chaicogenides. The eiectrochemistry of moiybdenum compounds is stiii a quite reievant area for scientific research [11-15]. The compiexity of eiectrochemicai processes invoiving moiyb-denum compounds is manifested in the existence of a iarge number of intermediate oxidation states, as weii as in the significant adsor-babiiity of moiybdenum compounds on eiec-trodes. Despite the iarge amount of experimentai materiai, the kinetics of eiectrode reactions in the presence of moiybdenum compounds is stiii uncertain [16-20]. In principie, the adsorption of these compounds is possibie in a wide range of potentiais, which certainiy, affects the kinetics of reactions occurring on eiectrodes. Moiybdenum compounds may be inciuded in the composition of eiectrode deposits by adsorption, which affects the properties of the resuiting materiais. The practicai appiication of the materiais obtained in this way can be
very multifaceted. These are electrodes modified with molybdenum (VI) compounds, which are characterized by selective oxygen evolution in chloride-containing media, are electrocatalysts for the oxygen reduction reaction for fuel cells, and are electrolytic coatings by molybdenum alloys with iron group metals, the introduction of molybdenum into which increases their corrosion resistance, microhardness. In addition, electrolytic alloys of molybdenum with iron group metals are of interest as promising catalysts in hydrogenation processes. The adsorption of molybdate ions should significantly depend on pH of the solution, as the ionic composition of the solution changes depending on the acidity of the medium. In addition, some dependence of the adsorption of mo-lybdate ions on the charge of electrode surface is possible. On the other hand, by changing the pH of the solution and electrode potential, the properties of the resulting electrode material can be significantly changed, which may find application in many areas of applied electrochemistry. Therefore, this work is devoted to studying the kinetic regularities of the processes of cathodic reduction of molybdenum ions from sulfuric acid electrolytes depending on the electrolyte composition, cathodic current density, and temperature. The purpose of this work is electrochemical synthesis of new Mo-based nanomaterials on various substrates. For this purpose, studies of cathodic processes during the reduction of molybdenum from sulfate electrolyte on Pt and Ni electrodes were conducted. The studies were carried out in solutions of the following composition (mol/l): Electrolyte composition (mol/l): 2.010"2-1.5 10"3 Na2Mo04 +2.0 H2SO4; pH=0.4; Ev=0,005 Vs-1; t=750C.
Experimental part
Platinum and nickel electrodes with a visible surface of 0.07 cm2 were used as the working electrode. The three-electrode cell contained the electrode under study, an auxiliary platinum electrode with the surface area of 4 cm2, and silver chloride reference electrode. To study the structure and composition, the films were deposited on Pt and Ni substrates with an area of 2.0 cm2. The working temperature for
electrodeposition was 75°C, deposition time 60 min. The kinetics of the processes was controlled using measurements by the method of cyclic voltammetry on IVIUMSTAT. X-ray diffraction analysis of the obtained films was carried out using DRON-5 at CuKa radiation. The films were obtained in the galvanostatic mode without electrolyte stirring. For analysis, the cathode deposit was dissolved under heating in concentrated HNO3 acid. The amount of molybdenum was also determined separately by thiocyanate complex using colorimetric method on SPECORD 50 PLUS.
The aim of the present work was to study the kinetics of the processes accompanied by adsorption of molybdate ions on electrode. We found it interesting to study the patterns of inclusion of molybdenum in the composition of electrode deposits, which occurs in a wide range of electrode potentials. The present work was carried out to determain the possibility of obtaining Mo-nanocoatings from a sulfate electrolyte. The po-tentiostatic polarization curve presenting the reduction of molybdate ions from the investigated solution is shown in Figure 1. In the potential range of -0.28 + 0.70 V, with the increasing negative value of electrode potential, there was an increase in the cathode current density of the reaction of incomplete reduction of molybdate ions.
As is known from the literature, molybdenum is deposited from both acidic and alkaline electrolytes [9-14]. In this work, the choice of a sulfate electrolyte was made on the basis of the fact that it is possible to obtain high-quality molybdenum deposits from this electrolyte, while it is not always possible to obtain high-quality films from an alkaline electrolyte. Also, it was established by preliminary experiments that high-quality films can be obtained from sulfate electrolyte even at very low concentrations of molybdenum in the electrolyte. For this purpose, cathode processes were studied at different concentrations of molybdenum in sulfate electrolyte on Pt electrode. Figure 2 shows the cyclic polarization curves presenting the reduction of molybdenum from sulfate electrolytes.
pA
T-1-1-1-1-1-1-'-1-1-1—
0.0 OS 1.0
Potential v
Fig.1. Polarization curves of molybdenum from sulfate electrolytes. Electrolyte composition (mol./l): 1.5-103 Na2MoÔ4 + 2.0 H2S04; pH=0.4; Ev=0.005 Vs"1, t=75"C.
Potential
Fig.2. Polarization curves of molybdenum at different electrolyte concentrations. Electrolyte composition (mol./l) 1 - 2.010-2 Na2MoO4+2.0 H2SO4; 2 - 4.01.510-2 Na2MoO4+2.0 H2SO4; 3 - 1.5 10-3 Na2MoO4 +2.0 H2SO4; pH=0.4; Ev=0.005 Vs-1 t=750C.
mA
-1-1-"-1-1-1-1-1—
■0.2 0.0 0.2 0.4 0.6 V
Potential
Fig.3. Polarization curves of molybdenum from sulfate electrolytes at different potential sweep rates. Electrolyte composition (mol./l): 1.5 10-3 Na2MoO4 + 2.0 H2SO4; pH=0.4; 1 - 0.005 Vs-1, 2 - 0.02Vs-1, 3 - 0.06 Vs-1, 4 - 0.1Vs-1, T=750C.
Usuaiiy, after immersing the Pt-eiectrode in suifate soiution for 30 minutes, the eiectrode potentiai takes on a constant vaiue equai to +0.5 ± 0.05 V, which does not change with change in the concentration of Mo in the soiution. However, in the case of piatinum eiectrode, the stationary potentiai has a vaiue of +0.36 V and within 30 min takes the vaiue +0.56 V. The main roie when es-tabiishing the stationary potentiai is piayed not by the equaiization of the concentration ratios in the cathode iayer, but by the baiance between the eiectrode surface and soiution, which are due to the presence of a fiim of oxide and sparingiy soi-ubie compounds on the cathode surface.
Thus, the vaiue of the stationary potentiai is determined by the state of the eiectrode surface. It has been estabiished that the potentiai has more positive vaiue on the passive surface than on the active one. With an increase in cathodic poiarization, there is a graduai reduction of mo-iybdenum-oxide compounds. As a resuit, the discharge of moiybdenum ions at the cathode becomes more difficuit, the rate of the process drops sharpiy, and the poiarization curve shifts towards negative potentiais. In the future, to determine the reasons hindering the eiectrode process of moiybdenum deposition in eiectroiytes under study, the influence of the potentiai sweep rate and temperature on the cathode process was studied. The influence of the potential sweep rate
on the nature of the poiarization is shown in Figure 3. As can be seen from the figure an increase in the potentiai sweep rate acceierates the process of moiybdenum deposition to varying degrees. At sweep rates above 2 mV/s, the deposition rate increases, and at sweep rates beiow 2 mV/s, there is a siight increase in the previous current. At high potentiai sweep rates, the process is iimited by diffusion poiarization, since the dependence i , -Vv is iinear, and at iow
potentiai sweep rates, a deviation from the iinear dependence is observed. This type of dependence is observed in cases where the discharge process is preceded by a chemicai reaction of dissociation of the compiex anion. Determining the cause of the observed chemicai poiarization, it can be assumed that the inhibition of the eiectrode process was caused either by kinetic iimitations, due to the chemicai reaction of the dissociation of the compiex anion into discharging ions, or by the chemicai interaction of the surface with the eiec-troiysis products and the formation of a passivat-ing fiim on the eiectrode surface. We aiso recorded the poiarization curves of moiybdenum at different cycies and it was estabiished that the nature of the poiarization curves does not change, and these data were presented in Figure 4.
Fig.4. Poiarization curves of moiybdenum from suifate eiectroiytes at different cycies. Eiectro-iyte composition (moi./i) 1.5 10-3 Na2MoO4 +2.0 H2SO4; pH=0.4; Ev=0.005 Vs-1 t=750C.
Electrodeposition of molybdenum is also significantly affected by the temperature of the electrolyte (Figure 5). It is known that the nature of polarization during the discharge of any metal does not change under the same conditions of its discharge jointly with cations of another metal. In this work, the nature of polarization is determined for molybdenum. It has been shown that to determine the type of cathodic polarization and study the mechanism of the electrode process, it is necessary to study the dependence of the current on temperature. In order to study the effect of temperature on the rate of the electrode process and the electrodep-osition of molybdenum from sulfate electrolyte, the polarization curves were recorded in the temperature range of 20-850C.
As experiments have shown, the temperature has a significant effect on the rate of the process under study. With increasing temperature, the rate of the electroreduction reaction increases, which is accompanied by an increase in wave height. It has been established that with an increase in temperature, the content of molybdenum in the deposit also increases and fine-crystalline deposits are obtained on cathode. To establish the nature of the cathodic polarization during the deposition of molybdenum on the basis of the data in Figure 4 graphs lgi-1/T were plotted for various constant values of the cathode potential. From the slope of these curves, we find the effective activation energy. Based on the re-
sults, it can be concluded that at cathode potentials up to 0.2 V, the process of molybdenum electrodeposition from sulfate electrolyte is mainly accompanied by chemical polarization.
The small value of the effective activation energy (12 KJ/mol) in the range of cathode potentials (+0.2) - (+0.1) V and its insignificant independence from the potential show that in this case the rate of the cathodic process is limited only by the diffusion of the discharged ions to cathode surface.
Thus, the totality of the results obtained gives reason to believe that the rate of the ca-thodic process at the initial stages is limited by difficulties of a chemical nature and is determined by diffusion only in the zone of the limiting current.
The effect of the substrate on the deposition of molybdenum from sulfate electrolytes was studied. Figure 6 shows cyclic polarization curves of molybdenum on nickel electrode from sulfate electrolyte. As seen from the Figure 6, the polarization curves of molybdenum recorded on nickel electrode are very different from polarization curves of molybdenum on platinum electrode and here the peak potential is 0.2V.
Thus, based on the experimental data, the following electrolyte composition (mol/l) is recommended for obtaining thin molybdenum coatings: 1.5 10-3 Na2Mo04+2.0 H2SO4; pH=0.4; Ev = 0.005 Vs-1 t=750C.
Potential
Fig 5. Polarization curves of molybdenum from sulfate electrolytes at different temperatures. 1 - 250C; 2 - 450C; 3 - 750C; 4 - 850C. Electrolyte composition (mol./l) 1.5 10-3 Na2MoO4 +2.0 H2SO4; pH=0.4; Ev=0.005 Vs-1.
Potential
Fig. 6. Polarization curves of moiybdenum from sulfate eiectroiytes on nickei eiectrode. Elec-troiyte composition (moi./i) i.5^10-3 Na2Mo04+2.0 H2SO4; pH=0.4; Ev=0.005 Vs-1 t=750C.
Conclusion
1. The infiuence of the potentiai sweep rate on the cathode process was studied. The influence of the potentiai sweep rate on the nature of the poiarization shows that an increase in the potentiai sweep rate acceierates the process of moiyb-denum deposition to varying degrees.
2. As experiments have shown, temperature has a significant effect on the rate of the process under study. With an increase in temperature, the rate of the eiectroreduction reaction increases, which is accompanied by an increase in wave height. It has been estabiished that with an increase in temperature, the content of moiybdenum aiso increases and fine-crystaiiine deposits are obtained on the cathode.
3. Based on the resuits, it can be conciud-ed that at cathode potentiais up to 0.2 V, the process of moiybdenum eiectrodeposition from suifate eiectroiyte is accompanied mainiy by chemicai poiarization.
4. Thus, based on experimentai data, the foiiowing eiectroiyte composition (moi/i) is recommended for obtaining thin moiybdenum coatings: 1.5-10-3 Na2Mo04+2.0 H2SO4; pH= 0.4; Ev=0.005 Vs-1 t=750C.
References
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MOLiBDENiN NAZiK TOBOQOLORiNiN SULFAT MOHLULLARINDAN ELEKTROLiTiK
CÖKDÜRÜLMOSi
Y.E.Olizada, E.O.Salahova, P.O.Kabntarova, O.F.Heybatova, K.i.Haciyeva , N.N.Xanki^iyeva, D.B.Tagiyev
Sulfat elektrolitlarindan Pt elektrodu üzürinda molibden ionlarinin elektroreduksiyasi zamani voltamper asililiqlannin tadqiqi asasinda Mo-nanoörtüklarin Qökma §araiti müayyan edilmi§dir. Müxtalif amilarin: carayan sixligimn, komponentlirin qatiligimn , temperaturun,mahlulun pH-nin va s. alinan tadaqanin keyfiyyatina va tarkibina tasiri tadqiq edilmi§dir. Müayyan edlmi§dir ki, mahlulda molibdenin qatiligimn , carayan sixligimn va temperaturun atmasi naticasinda molibdenin miqdan artir va tabaqanin keyfiyyati yax§ila§irr. Katodik qütbla§manin müayyan edilmasi asasinda müayyan edilmi§dir ki, sulfat elektrolitindan molibden elektrolitinin gökmasi prosesi asasan kimyavi qütbla§ma ila mü§ayiat olunur. Katodik qütbla§manin tabiatinin müayyan edilmasi asasinda müayyan edilmi§dir ki, molibdenin sulfat elektrolitindan elektrogökma prosesi asasan kimyavi qütbla§ma ila mü§ayiat olunur. Eksperimental malumatlara asasan, molibden nanoörtüklarinin alinmasi ügün a§agidaki elektrolit tarkibi (mol/l) tövsiya olunur: 2,0 10-2 - 1,5 10-3 Na2MoO4 + 2,0 H2SO4 ; pH=0,4; Ev=0,005 Vs-1 t=750C, elektrod - Pt.
Agar sözlzr: molibden, nazik örtükbr, elektrokimysvi gökms, 3rintil3r, сзгзуап sixligi.
ЭЛЕКТРООСАЖДЕНИЕ ТОНКИХ ПОКРЫТИЙ МОЛИБДЕНА ИЗ СУЛЬФАТНЫХ РАСТВОРОВ
Й.Э.Ализаде, Э.А.Салахова, П.Э.Калантарова, А.Ф.Гейбатова, К.И.Гаджиева, Н.Н.Ханкишиева, Д.Б.Тагиев
На основании исследования вольт-амперных зависимостей при электровосстановлении ионов молибдена из сульфатных электролитов на Pt-электроде установлены условия осаждения Mo-нанопокрытий. Исследовано влияние различных факторов, таких как температура, содержание компонентов в электролите, плотность тока, кислотность растворов и др., на состав и качество покрытий. Установлено, что с увеличением концентрации молибдена и плотности тока увеличивается содержание молибдена и улучшается качество покрытий. На основании определения природы катодной поляризации установлено, что процесс электроосаждения молибдена из сульфатного электролита сопровождается, в основном, химической поляризацией. По экспериментальным данным для получения молибденовых нанопокрытий рекомендуется следующий состав электролита (моль/л): 2,010-2 - 1,510-3 Na2Mo04 +2,0 H2SO4 ; рН=0,4 ; Ev=0,005 Вс-1 t=750C, электрод - Pt.
Ключевые слова: молибден, тонкие покрытия, электрохимическое осаждение, сплавы, плотность тока.
