Introduction

Metal powders and their production, especially ultrafine dispersed particles, are highly regarded as essential materials for different industries, such as nanotechnology, metallurgy, nuclear energy, space and mechanical engineering. Ultrafine CP is used in the chemical industry, and for powder metallurgy, because of its dispersity. Ultrafine dispersed CP is added to lubricant oils, for improving the wear resistance effect. It is also used to produce antifriction materials with metal and polymer matrix composites. CP can be used for the production of porous and permeable membranes, in chemical and engineering industries, and also as a catalyst in various oxidation reactions ^1)-(7. In addition, ultrafine CP has bactericide properties ⁸, and there is currently a focus on it, due to its dispersity.

There are a variety of methods to obtain CPs with the desired characteristics. Electrolysis is the most widely used method, but has significant drawbacks, such as, when dissolved metal anodes are used, a noticeable imbalance between the anodic and cathodic reactions CEs occurs, whereby the former exceed 100%, while the latter are much lower. Accordingly, a constant accumulation of metal ions occurs in the electrolyte volume.

Traditionally, CP is produced by the electrolysis of salts aqueous solutions, at a current density (J) around 2000 А/m², with a Cu low concentration in the electrolyte (10-13 g/L). Under these conditions, ions discharge rate onto the cathode is much higher than the rate of Cu ions transportation to the near-cathode space. In this case, the formation of a closed packed precipitate becomes impossible, and reduced Cu is precipitated in the form of powder.

The research of many scientists is devoted to the processes of obtaining CP by electrolysis, mainly focusing on the nature of Cu reduction at the cathode, under various electrolysis conditions. The influence of J, electrolyte temperature and various additives, has been established. The researchers explain the mechanism of formation of finely dispersed CP, and identify the conditions that increase its CE.

This method effectiveness is very high, allowing the metal refinement with waste. It can be managed by changing the electrolysis schemes. However, non-oxidized metal powders with sufficiently high dispersion and electrical conductivity are not always obtained.

These powders can be produced, due to the sharp decrease in the discharged ions concentration at the near-cathode space, with J values above the limiting one. Under such conditions, a rapid and uneven growth of the cathode deposit is observed, with the accumulation of disparate small crystals aggregates, and the formation of a metal powder with different dispersion particles.

However, if J at the cathode is at its limit, almost all of the discharged ions disappear. In this case, even a small increase in J results in a significant potential shift to the negative side, which facilitates the H ions discharge. The corresponding chemical reactions at the cathode are described as follows:

Меⁿ⁺ + ne^-- → Me⁰

(1)

where Меⁿ is a metal ion that can be reduced at the cathode under certain conditions (for example: Cu²⁺, Cu⁺, etc.), n⁺ is the ion charge or the metal oxidation degree, (for example, ²⁺, ³⁺, ⁴⁺, etc.), ne^- is the number of electrons that a metal ion accepts, during the reduction at the cathode, under certain conditions, and Me⁰ is a metal in its reduced form (for example, Cu⁰).

2H⁺ + 2e^- → H₂

(2)

where Н⁺ is a hydrogen ion, 2e^- is the number of electrons that it accepts during the reduction to its elemental state, and H₂ is a hydrogen molecule that was formed during the H⁺ reduction.

The occurrence of a H evolution reaction (HER) naturally reduces the CE below 100%. In actual practice, J is reduced to 70-80%, which indicates that approxim. 20-30% of the current is lost by HER ^9)-(13.

Based on the above-mentioned investigations, many opportunities exist today to improve existing methods and develop new approaches to CP production.

The aim of our work is to develop new electrochemical methods for obtaining dispersed CP in H₂SO₄ aqueous solutions with Ti⁺, and to elucidate the mechanism of powder formation, under various electrolysis conditions.

Experimental details

Material and methods

A Matrix MPS-305 D rectifying device was used as the current source. The current was measured by an E538 laboratory ammeter, and thermostating control was provided by a TW-2.02 thermostat. The CP size and form were identified by a JEOL JSM-6610LV electron microscope.

Sample preparation

The electrolysis was conducted in a 150 mL electrochemical cell, at 25 ºC, and the following reagents were used: a H₂SO₄ aqueous solution; Ti³⁺ and Ti⁴⁺ (in the form of corresponding sulphates); Cu²⁺ (in the form of chemically pure Cu(II) sulfate pentahydrate: CuSO₄∙5H₂O); Cu (99.99%) as anode; and Ti (99.70%) as cathode.

Before starting the experiments, the electrodes were cleaned by abrasives, and carefully washed by distilled water. Then, they were dried using filter paper and alcohol. The CP obtained from the experiment was washed with water, stabilized using a 0,05% solution of Na soap, to prevent oxidation, dried and weighed. Three electrolysis types with different composites were studied: 10 g/L Cu²⁺, 0-5.0 g/L Ti⁴⁺ and 100 g/L H₂SO₄, at J from 1000 to 3000 A/m², for 60 min; 50-200 g/L Ti⁴⁺ and 1-10 g/L H₂SO₄, at J from 50 to 300 А/m², for 30 min; and 50-150 g/L Ti⁴⁺, 4.0 g/L Ti³⁺ and 10-20 g/L H₂SO₄, at J from 150 to 350 A/m², for 30 min, at 25 ºС.

Results and discussion

Three new electrochemical mechanisms for producing CP, in the presence of Ti⁺, were investigated in this study. According to them, Cu reduction and further CP formation occurs in different ways, under diverse electrolysis conditions or electrolytes compositions.

CP production at the near-cathode space (mechanism 1)

During the Cu²⁺ and Ti⁴⁺ solution electrolysis, the following reactions take place on the Ti cathode:

a) Cu²⁺ reduction to Сu⁰, formed as a powder.

Сu²⁺+ 2е^- → Сu⁰ (Е⁰ = 0.34 V)

(3)

where Сu²⁺ is a Cu ion, in the oxidation state of 2⁺, 2e^- is the gain of 2 electrons of Сu²⁺ during Cu²⁺ reduction, Сu⁰ is the elemental Cu obtained by Сu²⁺ reduction and Е⁰ is the standard potential of the Сu²⁺ reduction reaction; the value (0.34 V) was taken from the reference book.

b) at the same time, Ti⁴⁺ reduction to Ti³⁺:

2Ti⁴⁺+ 2e^- → 2Ti³⁺(Е⁰ = -0.04 V)

(4)

Both Ti⁴⁺ and Cu²⁺ were reduced on the cathode surface. In this case, the formed Ti³⁺ were the reducing agents. The generated Ti³⁺ reacted with Cu²⁺ at the near-cathode space, according to the reaction:

2Ti³⁺+ Сu²⁺ → Сu⁰ + 2Ti⁴⁺

(5)

Cu²⁺ were furthermore reduced and Ti³⁺ were oxidized. Due to reaction (5), Ti⁴⁺ formation on the cathode and CP production near the cathode were observed.

(E = Е⁰(Сu²⁺/ Сu⁰) - Е⁰(Ti³⁺/2Ti⁴⁺) = 0.34 - (-0.04) = 0.38 V

(6)

Equation (6) shows that the redox potential difference (∆E⁰) (Cu²⁺/Cu⁰ systems and Ti³⁺/Ti⁴⁺ systems) has a positive value (+0.38 V), which indicates that Ti³⁺ can reduce Cu²⁺ to C⁰ state by reaction (5).

The possibility of the reaction (5) is also indicated by the equilibrium constant (K_equil) value equal to 10¹³. Thus, Cu⁰ powder is formed by reaction (5). As a result of this reaction, Ti⁴⁺ are formed, which are again reduced at the cathode by reaction (4). Ti³⁺ are again formed, which one more time reduce Cu²⁺ to Cu⁰ and produce Ti⁴⁺ (5). All this shows that Ti³⁺ cyclically participate in Cu²⁺ reduction reaction, and are constantly regenerated.

Reaction (5) results in the production of an additional amount of CP. During the electrolysis, HER takes place, according to reaction (2).

It is important to note that reaction (2) is possible in the presence of Ti³⁺, but the HER amount is not significant. Part of the current, which must be consumed in HER, is spent on Ti⁴⁺ reduction by reaction (4). As a result, an increase in the CE of Cu⁰ powder production is observed, the magnitude of which is significantly affected by the Ti⁴⁺ concentration (Table 1).

Thus, there is CP production both on the cathode surface (reaction (3)) and at the near-cathode space (reaction (5)). The increase in CE (Table 1) results from reaction (5). As reaction (5) occurs at the near-cathode space, not on the cathode surface, the further growth of particles does not take place, and the process results in the production of ultrafine dispersed CP.

The schematic diagram of CP production (Fig. 1), according to the mechanism 1, can be explained as follows.

At the cathode, the reaction (3) proceeds, whereby Cu²⁺ are reduced to Cu⁰ powder. The reaction (4) reduces Ti⁴⁺ to Ti³⁺. According to the reaction (5), the formed Ti³⁺ are reducing agents, so, they interact with Cu²⁺, and an additional amount of CP is formed. In this reaction, Ti⁴⁺ are simultaneously regenerated. Then, they diffuse and are again reduced at the cathode, and Ti³⁺ are formed. Ti³⁺ react again with Cu²⁺ and reduce them. This process is cyclically repeated. Thus, Ti⁴⁺ act as a catalyst. The course of these reactions is shown in Fig. 3, in the form of a diagram.

CP obtained by the mechanism 1 was studied by electron microscopy. Microscopic images (Fig. 2 (a, b)) show that ultrafine CP, with an average particle size in the range from 0.1 to 3.0 µm, was produced. In Ti⁴⁺ absence, the electrolyte particle size ranged from 10 to 80 μm (Fig. 2 (c, d)).

The experiment results allow us to infer that the current, which is completely spent on HER, in the traditional method, reduced Ti⁴⁺ and increased CP production, with Ti⁴⁺ regeneration. Thus, CP CE is significantly increased, and the particles size is decreased over 100 times.

CP production at the space between electrodes (mechanism 2)

The second type of electrolysis was performed in a H₂SO₄ solution containing Ti⁴⁺ without Cu²⁺. Mechanism 2 is as follows: reactions (3, 5) take place as in the first mechanism; the Cu anode dissolution forms Cu²⁺; Ti⁴⁺ are reduced to Ti³⁺, at the cathode (reaction (4)); and the inter-electrode space chemical interaction generated between Cu²⁺ and Ti³⁺ produces Cu⁰ powder, according to reaction (5).

This means that Ti³⁺ reduce Cu²⁺. It should be noted that, at first, ultrafine Cu particles were produced in the form of a colloid. Then, after 30-60 min, Cu particles were precipitated in the form of powder.

Ti⁴⁺ are reduced to Ti³⁺, according to reaction (4), and then regenerated, according to reaction (5). Hence, these reactions (4, 5) are cyclically repeated. It can be observed that Ti⁴⁺ are not consumed during the electrolysis, acting as a catalyst for Cu²⁺ reduction. According to reaction (5), CP is produced at the inter-electrode space.

The particles aggregation does not occur, because there is no possibility of Cu²⁺ direct reduction at the cathode surface.

The influence of Ti⁴⁺, H₂SO₄ concentration and J, on CE, was investigated during the experiments. An increase in Ti⁴⁺ concentration, in the range from 1 to 10 g/L, results in an increase in CP CE, similar to mechanism 1. The highest CE value was close to 100%, obtained at J, from 50 to 150 A/m² (Table 2).

Table 3 shows that, in favourable conditions, with H₂SO₄ concentrations varying from 50 to 200 g/L, CE was in the range from 98.9 to 99.2%. These obtained values once again prove that the current was almost completely consumed by CP production, and that H was not released during the electrolysis.

The scheme of mechanism 2 for ultrafine CP production is shown in Fig. 3.

Microscopic images of CP obtained in the inter-electrode space are shown in Fig. 4 (a, b). The average size of the spherical particles varies from 0.2 to 1.2 μm, and it becomes noticeable at a magnification of 25,000 times (Fig. 4(b)).

CP production at the near-anode space (mechanism 3)

Electrolysis was conducted in a H₂SO₄ solution, containing Ti³⁺ and Ti⁴⁺. The third mechanism is as follows: the Cu anode was dissolved forming Cu²⁺; Ti³⁺ in the solution reacted immediately with the formed Cu²⁺ (according to reaction (5)); Cu⁰ resulted, at first, in a colloid form and, later, in powder; Ti³⁺ were oxidized to Ti⁴⁺; Ti⁴⁺ diffused to the cathode surface where they were reduced to Ti³⁺; and Ti³⁺ were diffused towards the near-anode space. At the anode surface, reaction (5) occurred again to produce dispersed CP, and all of the above processes were cyclically repeated. Ti⁴⁺ were reduced at the near-cathode space, through reaction (5), where Ti³⁺ reacted with Cu²⁺. The scheme of mechanism 3 is shown in Fig. 5.

As it can be seen from Table 4, any change in J and Ti⁴⁺ concentration, in the range from 150 to 350 A/m² and from 10 to 20 g/L, respectively, did not significantly affect the CE. An average CE of 99.62% was demonstrated under experimental conditions. This indicates that the current was almost completely spent by CP production. HER was not observed here. The microscopic images of obtained CP were very close to those produced by the second mechanism. The average size of CP particles was in the range from 0.01 to 0.1 µm.

The experiments results show that Cu particles obtained by three different methods did not have the ability to grow during powder production, since the process did not occur directly at the cathode surface. In this case, the particles with only very small sizes were capable of growing, which resulted in a unification to ultrafine nanostructured forms. It is expected that CP with almost nano-sized particles is formed by mechanisms 2 and 3.

Conclusions

The research results have shown that, depending on the electrolyte compositions and electrolysis conditions, CP production can proceed by three different mechanisms.

• The main difference between the three powder production mechanisms was that it was formed in diverse parts of the electrolyte. CP can be obtained at the cathode and at the near-cathode space (mechanism 1), at the inter-electrode space, i.e. between the electrodes (mechanism 2), and at the near-anode space (mechanism 3).

• In all three cases, the powder production process proceeds in stages (electrochemical and chemical stage), and with the participation of Ti³⁺ as reducing agent. In addition, due to the cyclic nature of the reactions, Ti⁴⁺ are not consumed, and act as catalyst.

• The CP particles obtained in the presence of Ti⁺ have a spherical shape, with sizes ranging from 0.01 to 0.1 μm.

Authors’ contributions

Аbduali Bayeshov: supervised experiments; put forward the ideas of Cu²⁺ reduction in the presence of Ti⁴⁺ and explained its mechanisms; wrote the experimental part of the article. Аzhar Bayeshova: collected the literary data; wrote the literary review, introduction and conclusions; suggested an experimental method; carried out calculations; participated in the development of the idea and interpretation of the results. Umida Abduvaliyeva: carried out electrolysis and analysis of the solutions; isolated CP and prepared them for analysis. Aksulu Buketova: collected literary data; carried out part of the experiments (electrolysis, analysis of the solutions); participated in calculations.