The Mechanism of Joint Reduction of MoO₃ and CuO by Combined Mg/C Reducer at High Heating Rates

Abstract

Understanding of the decisive role of non-isothermal treatment on the interaction mechanism and kinetics of the MoO₃-CuO-Mg-C system is highly relevant for the elaboration of optimal conditions at obtaining Mo-Cu composite powder in the combustion processes. The reduction pathway of copper and molybdenum oxides with combined Mg + C reducing agents at high heating rates from 100 to 5200 K min⁻¹ was delivered. In particular the sequence of the reactions in all the studied binary, ternary and quaternary systems contemporaneously demonstrating the effect of the heating rate on products’ phase composition and microstructure was elucidated. The combination of two highly exothermic and speedy reactions (MoO₃ + 3Mg and CuO + Mg vs. MoO₃ + CuO + 4Mg) led to a slow interaction with weak self-heating (dysynergistic effect) due to a change in the reaction mechanism. Furthermore, it has been shown that upon the simultaneous utilization of the Mg and C reducing agents, the process initiates exclusively with carbothermic reduction, and at relatively high temperatures it continues with magnesiothermic reaction. The effective activation energy values of the magnesiothermic stages of the studied reactions were determined by Kissinger isoconversional method.

1. Introduction

W, Mo and their multicomponent alloys are nominated as potential candidates for plasma facing materials (PFMs) simultaneously possessing high melting temperature, high thermal conductivity, thermal fatigue, low vapor pressure and low sputtering erosion yield [1,2,3]. Copper is considered the heat-sink component for nuclear fusion devices due to its good thermal conductivity [3,4,5]. The conventional welding of refractory metal (such as W, Mo) and Cu is an extremely difficult task caused by their immiscibility in both solid and liquid states [6,7,8,9], which can be alleviated through the utilization of well-mixed and dispersed components. The simultaneous synthesis of components of Mo-Cu composite alloy may guarantee the connection of Mo/Cu and promote high-tech development of thermonuclear fusion energy applications. Self-propagating exothermic reaction (SHS) for synthesizing composite materials and intermetallic compounds has already shown its effectiveness [10,11,12] in an in situ preparation of Mo-Cu composite powder by coreduction of metal oxide precursors via reaction’s coupling approach [13,14]. Detailed knowledge on the high speed and high-temperature kinetics of MoO₃-CuO-Mg-C system during the SHS reaction is of high importance not only for the elaboration of optimal conditions at obtaining Mo-Cu composite powder, but also extending the obtained models and interpretation on the similar systems. However, the intensive nature of exothermic SHS reactions, with the heating rate of over 10² K s⁻¹ make it difficult to explore the interaction mechanism of condensed constituents (solid, liquid) in the combustion wave. The modeling of the combustion process at controllable conditions (e.g., with programmed heating rates and tuning the process within the time at conditions closer to heating rates and temperatures in the combustion wave) will tackle the examination of the reaction pathway. The latter was studied so far by means of X-ray diffraction, electron microscopy and thermal analysis techniques without consideration the influence of high heating rates on the interaction dynamics. Motivated by the open questions and technical difficulties of combustion kinetics exploration, our current study is aimed to disclose the interaction pathway at high heating rates in the MoO₃-CuO-Mg-C system by a novel thermal analysis technique, called high-speed temperature scanner (HSTS) [15,16,17,18].

HSTS studies are suitable for the solid–solid and solid–liquid intercollisions that are unquestionably the most abundant among nature’s phase transformations [19,20]. It has already proved its ability to find out transformations and transitions in alloys, ceramic and composite materials. Our recent findings via HSTS technique [21] provided valuable knowledge about intermediate states of solid and liquid phases, their nucleation processes, phase and structure formation pathways in condensed phase at heating rates from 100 to 2500 K s⁻¹ region. In [22] the mechanism and kinetics of reactions in the MoO₃-CuO-Mg-C system has been investigated at non-isothermal conditions by DTA/TG method at low heating rates (5–20 K min⁻¹) up to 1273 K. The current work is the consequential extension of the investigation of the reaction mechanism in CuO-MoO₃-Mg-C powder mixtures from 100 up to 5200 K min⁻¹ high heating conditions up to 1573 K. The technique of local non-isothermal high heating enables us to study the sequence of concurrent reduction processes of strictly different oxides, reveal the not obvious reduction mechanism with an Mg/C reductant pair, the effect of heating rate on the phase and structure formation patterns and conversion degree. The multistep transformations in the CuO-MoO₃-Mg-C powder mixtures were demonstrated to be significantly influenced by the formation of intermediate molybdates, even up to the drastic change in reaction pathway. Furthermore, the comparative overview of reactions’ sequences in a wide range of heating rates and Arrhenius kinetic parameters allowed us to directly identify beneficial conditions, to study the main facets of the phases involved in each stage, and indicate a favorable direction in the design of the reduction process.

2. Materials and Methods

MoO₃ (High grade, Pobedit Company, Vladikavkaz, Russia, particle size < 15 μm), CuO (High grade, STANCHEM, Niemce, Poland, particle size < 40 μm), carbon black (P-803, Ivanovo carbon black and rubber JSC, Ivanovo, Russia, particle size < 1 μm) and magnesium (MPF-3, Ruskhim, Moscow, Russia, particle size 150–300 μm) powders were used as precursors for HSTS experiments (Figure S1).

The mechanism of reduction of copper and molybdenum oxides mixture by Mg/C combined reducers under conditions of high heating rates was investigated by HSTS technique. It allows to carry out experiments in a wide range of heating rates (from 10 to 10,000 K min⁻¹) up to 1573 K. The HSTS operation is based on the direct electric heating of thin metallic foil (e.g., Ni foil with 0.05–0.1 mm thickness). The small amount of powder (about 50 mg) was placed inside the middle part of thin metallic heater and then was heated by direct electrical current in the HSTS reaction chamber (Figure 1). Before heating, the chamber was sealed, evacuated and filled with argon up to 0.1 MPa pressure (Ar, 99.98% purity, oxygen < 0.01%). Preprogrammed heating mode was used to heat the sample. The temperature change (up to 1573 K) was recorded using a K type chromel-alumel thermocouple fixed in the central area of the foil and using a photocell fixed to the reactor wall. The inert experiment was characterized by linear temperature-time history defined by the given heating rate and coinciding with the reactive temperature profile where interactions are absent. The reaction onset temperature (T_o), the maximum peak temperature due to self-heating (T_max) and the temperature prescribed by linear heating (where the maximum exothermic effect is observed (T*)) were determined from the heating thermograms [21].

During the heating, the process was interrupted at characteristic temperatures and the quenched samples were subjected to XRD analysis (diffractometer DRON-3.0, IC Burevestnik, St. Petersburg, Russia, CuKα radiation, 25 kV, 10 mA). Not that, the samples were cooled with high cooling rate (up to 12,000 K min⁻¹) to exclude further interactions. XRD patterns of the quenched samples can be viewed in Supplementary Figures. Besides, for the characterization of the samples cooled at characteristic temperatures, the IR spectral analysis (PerkinElmer Spectrum Two FT-IR, TL8000) was utilized. The effective activation energy values of magnesiothermic reduction stage in a heating rate region of 100–1200 K min⁻¹, were determined by the Kissinger isoconversional method for the studied binary (MoO₃-Mg), ternary (MoO₃-Mg-C) and quaternary (MoO₃-CuO-Mg-C) mixtures. For microstructural analysis, a field-emission scanning electron microscope (FE-SEM, Zeiss Evo MA15, Germany) equipped with an EDS (energy dispersive spectroscopy) detector was used.

3. Results and Discussion

The joint reduction of MoO₃ and CuO utilizing Mg + C combined reducer via energy-saving combustion synthesis aimed at the preparation of composite materials with fine and well-dispersed constituents was documented in [13,23,24], The advantage of such a reducing mixture is to govern the reaction thermal regime in a wide range of temperature and to prepare Mo-Cu composite powders in controllable combustion conditions. As it is difficult to disclose the interaction mechanism at extreme conditions of the combustion wave, the process was preliminarily modeled at “soft” conditions (e.g., low heating rates) using the DTA/TG technique [22] under programmed heating rates from 5 to 20 K min⁻¹ up to 1273 K. Furthermore, to monitor and reveal mechanism of the combustion reaction due to its high velocity is still a challenge. Here we report the results of study of the reaction mechanism in CuO-MoO₃-Mg-C powder mixtures under conditions of high heating rates (from 100 to 5600 K min⁻¹) up to temperature 1573 K through revealing reactions’ sequences and the phase- and microstructure formation patterns by HSTS-1 setup. Firstly, the reduction pathways were studied in binary (CuO-MoO₃, MoO₃-Mg, MoO₃-C, CuO-C) and ternary (MoO₃-Mg-C, MoO₃-CuO-Mg, and MoO₃-CuO-C) systems, then the quaternary CuO-MoO₃-Mg-C system under the same heating conditions. The mechanism of copper oxide reduction under fast heating conditions both with Mg and (Mg + C) reducers is detailed in [21] and here that results are presented for the purpose of comparative discussion.

3.1. CuO-MoO₃, CuO-C, MoO₃-C, MoO₃-Mg Binary Systems

3.1.1. CuO-MoO₃ System

The interaction of copper(II) and molybdenum(VI) oxides is known to be accompanied by the formation of molybdates of different composition, depending on the processing/synthesis conditions [22,24,25]. In particular, when an equimolar mixture of copper and molybdenum oxides was heated at 20 K min⁻¹ heating rate, a weak endothermic reaction resulted in the formation of the CuMoO₄ salt in a quite wide temperature range (743–923 K). When the same mixture was heated at 300 K min⁻¹ heating rate (Figure 2) a weak endothermic interaction was observed at higher temperature range (823–903 K) which corresponds to the formation of copper molybdates of different composition (α-CuMoO₄, Cu₃Mo₂O₉) resolved by XRD analysis (Supplementary Figure S2). In the sample quenched at the mentioned temperature unreacted MoO₃ was observed along with molybdates. Later, at temperatures above 973 K, a wide temperature range of endothermic interactions was observed, which is the sum of several transformations, in particular, corresponding to the continuation of the salt formation process (the amount of molybdenum oxide decreases in the product) and to the modification change of the produced salt (α-CuMoO₄ to CuMoO4 III conversion) [26,27,28]. The complete formation of pure CuMoO₄ III was possible to achieve only by non-isothermal heat treatment at high temperature of 1073 K followed by isothermal heat treatment reactive by 10 min (Figure S2E). Meanwhile, under slow heating rate by DTA/TG method (5–20 K min⁻¹) the salt formation ends relatively earlier, at 923 K.

To complement the aforementioned observations, the IR spectrum was recorded. FTIR (Figure 3) bands of the CuMoO₄ were detected at 460, 865 and 965 cm⁻¹. The peak at 865 cm⁻¹ is assigned to vibrations of the Mo–O–Mo. The featured peak at 965 cm⁻¹ is attributed to Mo = O bond frequency. Furthermore, the absorption peak at 460 cm⁻¹ is assigned to superposition of bending mode of MoO₃ lattice and stretching vibration of square planar CuO₄. IR peaks of Cu₃Mo₂O₉ and MoO₃ appeared in their fingerprint region at 717 cm⁻¹ and 560 cm⁻¹, respectively, match well with the literature data [29,30]. IR peaks on the spectrum at 810 cm⁻¹ and 1380 cm⁻¹ correspond to in-plane B-N stretching and out-plane B-N-B bending vibrations, respectively, of boron nitride. BN suspension was used to cover the thermocouple wire during HSTS experiments. Some of the bands of Cu₃Mo₂O₉ were overlapped (818 cm⁻¹, 902 cm⁻¹, 942 cm⁻¹ and 969 cm⁻¹) with the CuMoO₄ bands in the 800–1000 cm⁻¹ region and were not observed due to their lower intensity compared to CuMoO₄ peaks.

3.1.2. CuO-C System

Carbothermic reduction of copper oxide (CuO + C) is a successive two-stage weak exothermic process (Figure 4). The first stage takes place in the temperature range of 823–973 K and is accompanied by partial reduction of CuO with the formation of Cu₂O (Figure 4B,C and Figure S3B). The second stage begins at temperatures above 973 K and corresponds to the formation of copper from copper suboxide (Figure 4C,D and Figure S3C). The further increase in temperature leads to an increase in the conversion degree and pure copper formation (Figure 4D and Figure S3D).

It is worthy of remark that in the range of low heating rates (5–20 K min⁻¹) a stepwise reduction was also observed, but the interaction was started at a temperature lower by about 100 K [22].

3.1.3. MoO₃-C System

Two weak exothermic and one endothermic peaks arise during the carbothermic reduction of molybdenum (VI) oxide (MoO₃ + 2C) (Figure 5). The process begins with a weak exothermic interaction (933–1063 K) corresponding to the formation of molybdenum dioxide (Figure S4B) according to the following reaction scheme:

MoO₃ + C → MoO₂ + CO₂ (+trace amount of CO) (933–1063 K).

Under further heating, the reduction process continues with the residual carbon. In particular, a weak endothermic interaction takes place in the temperature range 1273–1373 K, and in the XRD pattern of the quenched sample MoO₂ and Mo₂C (Figure S4C) are identified as a result of the following interaction:

MoO₂ + C → Mo₂C + CO/CO₂  (1273–1373 K).

Further increase in temperature leads to the formation of metallic molybdenum (Figure S4D) according to the following scheme:

MoO₂ + Mo₂C → Mo + CO/CO₂  (1483–1573 K).

The carbothermic reduction of molybdenum trioxide at low heating rates (V_h = 20 K min⁻¹) is also a two-step process [31]. Again, the first stage (MoO₃→MoO₂) is a weak exothermic reaction (but at lower temperature of 833–953 K), and the second is an endothermic one (>1123 K) accompanied by the formation of metallic molybdenum and/or molybdenum carbide. In contrast to the low heating rates region (5–20 K min⁻¹), at high heating rates (>100 K min⁻¹) these stages are shifted to a higher temperature range, which allows the reduction of metallic molybdenum at high temperature.

3.1.4. MoO₃-Mg System

The magnesiothermic reduction mechanism of MoO₃ + 3Mg mixture was examined in the same range of high heating rates, 100–1200 K min⁻¹. Magnesiothermic reduction of molybdenum oxide over the entire heating rate region occurs with molten magnesium. The increase in the heating rate from 100 up to 1200 K min⁻¹ increases characteristic temperatures (T_o, T*, T_max). At heating rate of 100–300 K min⁻¹ (Figure 6) the interaction initiates just after the melting process of magnesium. In particular, at a heating rate of 300 K min⁻¹, the reduction begins at 943 K along with a severe exothermic peak (943–1058 K) on the heating thermogram with a maximum temperature of 1423 K (T* = 1013 K). Note that the reduction of copper oxide with magnesium began approximately at the same temperature range at 300 K min⁻¹ heating rate (953–1073 K, T* = 993 K, T_max = 1558 K) [21]. At higher heating rates (600 and 1200 K min⁻¹) the reduction process of MoO₃ begins quite late after the melting of magnesium at temperatures of 1063 and 1083 K, respectively.

The XRD pattern of the sample quenched at a temperature of 923 K (A) contains only characteristic diffraction lines of MoO₃ and Mg (V_h = 300 K min⁻¹) (Figure S5A). Immediately after the exothermic interaction, a partial reduction of molybdenum oxide with the formation of MoO₂ (Figure S5B) is observed. Further increase in temperature leads to molybdenum formation, but, in parallel to the reduction process, the formation of MgMoO₄ salt also occurs (Figure S5C,D). However, MgMoO₄ is absent at the end of the process, but some unreduced molybdenum dioxide is registered (Figure S5E). Incomplete reduction of molybdenum oxide can be explained by the vigorous interaction and evaporation of magnesium.

3.2. MoO₃-Mg-C, MoO₃-CuO-Mg and MoO₃-CuO-C Ternary Systems

3.2.1. MoO₃-Mg-C System

In the ternary MoO₃-Mg-C system, at low heating rate of 20 K min⁻¹, the interaction began before the melting of magnesium at 833–903 K according to the results reported in [31]. The weak exothermic carbothermic reduction was attributed to the conversion of MoO₃ to MoO₂. The process was further continued with magnesiothermic reduction after the Mg melting up to the formation of metallic molybdenum at 973–1073 K. In contrast to low heating rates, the process begins with the melting of magnesium at 933 K at high heating rates, which is immediately followed by the carbothermic reduction of molybdenum (VI) oxide and lead to the formation of molybdenum dioxide at a temperature of 953–1073 K (Figure 7 and Figure S6).

In contrast to the carbothermic reduction of molybdenum (VI) oxide (see Section 3.1.3), utilizing the Mg/C reducing pair, the formation of molybdenum carbide was not observed, and the formation of molybdenum occurs due to a strong exothermic interaction of MoO₂ and magnesium (300 K min⁻¹, 1123–1163 K, T* = 1133 K). The maximum temperature of the magnesio-carbothermic interaction is registered to be 1623 K, which is by ~200 K higher than the temperature recorded during the pure magnesiothermic reduction reaction of molybdenum oxide.

For the comparative overview, one may note, that in the CuO-0.5Mg-0.5C system the interaction began immediately after the melting of magnesium with carbothermic reduction of CuO and accompanied by the formation of copper suboxide. Then magnesium continues the reduction of Cu₂O up to the metallic copper formation [21].

3.2.2. CuO-MoO₃-Mg System

According to the heating thermogram of CuO + MoO₃ + 4Mg mixture, the process of joint magnesiothermic reduction of copper and molybdenum oxides (Figure 8a) is preceded by magnesium melting. The weak endothermic effect observed after magnesium melting is accompanied by the formation of copper molybdate (Cu₃Mo₂O₉) at 1073 K, which is immediately followed by the salt reduction process and the formation of metallic copper (1143–1273 K). Metallic molybdenum is formed at a higher temperature range (1273–1373 K). The aforementioned considerations were confirmed by X-ray analysis of the samples quenched at characteristic temperatures, according to which the XRD pattern of the sample interrupted at point C contains Cu₆Mo₅O₁₈, MgMoO₄, Cu₂O, Cu, Mg (Figure 8b C). At higher temperatures, between 1373 K and 1473 K, there are characteristic peaks of molybdenum and MgO·MoO₂ mixed oxide as well (Figure 8b D). The latter occurs as a result of the interaction between MgO and MoO₂.

The data derived from the analysis of the heating thermograms revealed that, in contrast to the magnesiothermic reduction of individual oxides, which are highly exothermic interactions (i.e., CuO + Mg with T* = 993 K, T_max = 1483 K [21] and MoO₃ + 3Mg with T* = 1013 K; T_max = 1428 K), the joint magnesiothermic reduction of oxides (CuO + MoO₃ + Mg) proceeds with weak exothermic stages. As a matter of fact, when two highly exothermic reactions, MoO₃ + 3Mg and CuO + Mg, are carried out together, a slow interaction takes place. This is a typical dysynergistic phenomenon, which comprises a combination of actions, where the effect of the simultaneous action is less than the summation of effects of the individual actions. Dysynergy in the CuO-MoO₃-Mg system may be explained by a dramatic change in the interaction pathway. In particular, as follows from the XRD analysis results of the samples quenched at different stages of the interaction, the reduction process of metal oxides by magnesium is preceded by the formation of Cu₃Mo₂O₉ salt (Figure 8b). In addition, the magnesiothermic reduction of Cu₃Mo₂O₉ is significantly slower as compared to the reduction of individual oxides. Indeed, according to the XRD data obtained, after the melting of magnesium before the initiation of the reduction process of oxides, a copper molybdate Cu₃Mo₂O₉ (at 1073 K) is identified (Figure 8b B) along with copper oxide. This is further followed by the gradual/stepwise reduction of the salt and is accompanied firstly by the formation of Cu, Cu₂O, MgMoO₄, Cu₆Mo₅O₁₈.

To explain the difference between the combined and separate reduction processes of molybdenum and copper oxides, the magnesiothermic reduction of CuMoO₄ salt was studied at similar conditions (300 K min⁻¹) (Figure 9). From the heating curve of the reduction of CuMoO₄ by magnesium, two successive and weak exothermic stages were revealed. Similar to the combined magnesiothermic reduction of oxides (after the salt formation process), the melting of magnesium at 923 K is immediately followed by the weak exothermic reduction of copper (943–1153 K), and later the weak exothermic reduction of molybdenum in the temperature range of 1333–1343 K occurs (Figure 9 and Figure S7).

3.2.3. CuO-MoO₃-C System

In the ternary CuO + MoO₃ + 3C system (Figure 10) the reduction process begins with the weak endothermic reaction of copper molybdate formation (Figure S8B), followed by a weak exothermic interaction. According to XRD analysis results, the complete reduction of copper and partial reduction of molybdenum (Figure S8C) occur during the weak exothermic reaction. Further increase in temperature leads to the formation of molybdenum carbide (Figure S8D, 1273 K). At higher temperatures (T ≥ 1423 K), metallic molybdenum is formed due to the interaction of molybdenum carbide and molybdenum dioxide. The sequence of processes in CuO + MoO₃ + 3C system can be represented by the following scheme:

MoO₃ + CuO → Cu₆Mo₅O₁₈ + Cu + MoO₂    T = 823–913 K

Cu₆Mo₅O₁₈ + C → Cu + MoO₂ + CO/CO₂    T = 923–1023 K

MoO₂ + C → Mo₂C + CO/CO₂    T = 1023–1393 K

MoO₂ + Mo₂C → Mo + CO/CO₂    T = 1393–1573 K

The experimental observations interpreted above imply that the joint reduction of CuO and MoO₃ oxides with separate reducers, such as magnesium or carbon, does not lead to the complete reduction of metals (Mo and Cu).

3.3. CuO-MoO₃-Mg-C Quaternary System

After the detailed consideration of the behavior of the binary and ternary systems, a quaternary system with a composition of CuO + MoO₃ + 1.2Mg + 2.15C was selected. The ratio of reducers was received from the optimum area of thermodynamic calculations for the joint and complete reduction of Mo and Cu metals [13]. In the selection of the mentioned composition, the combustion synthesis behavior of individual oxides with Mg/C combined reducer was also considered.

In the CuO + MoO₃ + 1.2Mg + 2.15C mixture, the process begins before the magnesium melting, with the carbothermic reduction of copper oxide at temperature up to 873 K (Figure 11a). This is evidenced by the presence of Cu₂O and the absence of MgO in the sample quenched at 873 K (Figure 11b B). During further heating, magnesium melts, and molybdenum oxide is reduced by carbon (MgO is still absent in the product) up to molybdenum dioxide. At a temperature range of 1213–1263 K, the magnesiothermic reduction of MoO₂ occurs with the formation of metallic molybdenum with maximum temperature of 1523 K. The further increase in temperature contributes to the increase in conversion degree (Figure 11b F). The formation of molybdenum carbide by an interaction of the obtained molybdenum and/or molybdenum dioxide with carbon is not excluded.

According to DTA/TG studies conducted at [22] at a heating rate of 20 K min⁻¹ in the CuO-MoO₃-Mg-C system, the interaction also begins before the magnesium melting. At 873 K, a parallel reduction of the oxides takes place: copper oxide is reduced to copper via two-step mechanism, and molybdenum oxide to MoO₂. In addition, at low heating rates, the magnesiothermic reduction process of molybdenum dioxide occurs at a lower temperature range: it begins immediately after the melting of magnesium (923 K) and molybdenum formation is observed at 993 K. At the end of the process, the product also contains a complex oxide of MoO₂ and magnesia.

The microstructural analysis of the quaternary CuO + MoO₃ + 1.2Mg + 2.15C mixture (Figure S1), and of the quenched samples at different characteristic temperatures, T = 873 K (Figure 12), T = 1023 K, 1163 K (Figure S9), T = 1263 K, T = 1573 K (Figure 13, Figure 14 and Figure S10) are presented below to demonstrate the microstructure evolution during the high heating conditions. Initial CuO + MoO₃ + 1.2Mg + 2.15C mixture contains monoclinic CuO (outlined orange), orthorhombic MoO₃ (outlined blue), carbon (outlined yellow), magnesium (outlined green).

During the heating up to 873 K (before Mg melting) coarse Mg particles are still separately present (Figure 12a, green outlined platelets) and didn’t participate in the interaction yet (Figure 12b,c). Another three components appear together in prealloyed state in the form of two different microstructural entities—ribbed laminates (spectrum 433 and points marked 1) and nanospherical agglomerates (spectrum 434 and points marked 2) (Figure 12d). More specifically, according to EDS spectrum (Figure 12e), the alloyed regions (Figure 12c,d spectrum 433) comprise more Mo and O (according to XRD analysis results it should correspond to MoO₃, according to EDS analysis atomic ratio is Mo:O = 1:3.5), and agglomerated particulates (spectrum 434, Figure 12c,d) have more copper and carbon content, and some oxygen is also present (in all probability there are carbon and Cu₂O, which was established also from the XRD patterns). The close contact of copper to carbon is also complementing to XRD analysis and postulating that CuO reduction up to Cu₂O occurred by carbon.

After magnesium melting, at 750 °C, microstructural entities were in the form of more homogeneous mixture of molten magnesium and nanoparticles of copper and MoO₂ (Figure S9a,c). At higher temperature (890 °C), two types of MoO₂ are observed (well defined grains and nanospheres), thus mixture undergoes microstructural change without changing phase composition (Figure S9b,d,e). Well-defined grains and nanospheres of MoO₂ are present along with metallic copper and molten magnesium platelets.

The SEM images of samples quenched at 1263 K contain spherical droplets of submicron size (Cu, orange arrows), spherical nanoparticles (Mo-Cu composite with higher molybdenum content), thin nanorods (width up to 100 nm) of light color (MoO₂—spectrum 461 (atomic ratio according to EDS 1:1.8), dark rods of different proportions (Mo from submicron to micron size—spectrum 457), and nanoparticles (spectrum 456) that comprise both Cu and Mo but predominantly molybdenum owing to segregation of molten copper droplets (Figure 13 and Figure S10). At higher temperature, 1573K, microstructure refinement takes place due to the continuation of reduction process from MoO₂ nanorods also by carbon, which leads to the formation of not only molybdenum, but also molybdemum carbide nanoparticles according to MoO₂ + C → Mo₂C + CO/CO₂ reaction (Figure S10).

In some regions of the sample quenched at 1573 K, Mo and Cu nanoparticles were homogeneously distributed in the nanometer range (Figure 14).

The influence of heating rate was further investigated to understand the formation conditions of byproducts. It was established that regardless the heating rate, the complete reduction of copper oxide and the first stage of reduction of molybdenum trioxide proceed with carbon followed by magnesiothermic or Mg + C combined reduction of MoO₂ (

Table 1. The influence of heating rate on the characteristic temperatures and phase composition of the products of the quaternary CuO-MoO₃-Mg-C system.

Vh, K min−1	The System under Study	T, K	XRD	T, K	XRD	T, K	XRD
20	CuO + MoO3 + + 1.1Mg + 2C	753–793	Cu2O, MoO3, Mg	793–873	MoO2, Cu, Mg	1013–1043 Tmax = 760	Mo, Cu, MgO, MgO·MoO2
100	CuO + MoO3 + + 1.2Mg + 2.15C	773–823	Cu2O, MoO3, Mg	923–1003	MoO2, Cu, Mg	1143–1163 Tmax = 940	Mo, Cu, MoO2 (main product), Mo2C, MgO
300	CuO + MoO3 + + 1.2Mg + 2.15C	833–873	Cu2O, MoO3, Mg	933–1013	MoO2, Cu, Mg	1213–1263 Tmax = 1523	Mo, Cu, MgO, MoO2(trace), Mo2C
2600	CuO + MoO3 + + 1.2Mg + 2.15C	843–883	Cu2O, MoO3, Mg	1033–1233	MoO2, Cu, Mg	1393–1583 Tmax = 1563	Mo, Cu, MgO, Mo2C
5200	CuO + MoO3 + + 1.2Mg + 2.15C	853–893	Cu2O, MoO3, Mg	1053–1263	MoO2, Cu, Mg	1403–1633 Tmax = 1583	Mo, Cu, MgO, Mo2C(trace)

). In the case of slow heating the process was completed at 1153 K and carbidization was not observed [22], but MoO₂ was present in the quenched product along with target compounds. At high heating rates the process was completed at 1574 K and may be accompanied by the formation of molybdenum carbide.

This difference in the phase composition of the products is conditioned by the difference in heating rates (Table 1), as well as by the difference in maximum temperatures (1153 K vs. 1573 K). In particular, at low heating rates (V_h = 20 K min⁻¹), the product contains molybdenum dioxide together with the main product. At relatively high heating rates (100–1200 K min⁻¹) molybdenum dioxide and molybdenum carbide were observed, and in the heating rate region from 2600–5200 K min⁻¹ molybdenum carbide is present in addition to the target materials. In the combustion synthesis, where the heating rate reaches up to 60,000 K min⁻¹ [13], the reduction was accomplished with the formation of target substances Mo, Cu, MgO due to utilization of combined reducers in the mixture CuO + MoO₃ + 1.2Mg + 2.15C. Considering that combustion synthesis of molybdenum carbide in the solid phase proceeds by the reaction diffusion mechanism, diffusion of carbon through the Mo carbide phase was believed to be the limiting stage of the carbidization process. Thus, it was possible to avoid carbidization reaction during extremely high heating conditions of the combustion wave.

In the CuO + MoO₃ + 1.2Mg + 2.15C mixture at the heating rate of 300 K min⁻¹ the following sequential processes take place:

CuO + C → Cu₂O + CO/CO₂, T = 833–943 K

Cu₂O + C → Cu + CO/CO₂, T = 943–1023 K

MoO₃+ C → MoO₂ + CO/CO₂, T = 923–1023 K

MoO₂+ Mg → Mo + MgO, T = 1213–1263 K

MoO₂ + C → Mo₂C + CO/CO₂, T > 1173 K

Mo + C → Mo₂C, T > 1173 K

The experimental studies demonstrated that using a combined Mg/C reducer under fast heating conditions confer the possibility to carbon to interact at a lower temperature than magnesium, prevent the salt formation process, which outstrips the reduction process when only the magnesium is used as a reducer. The temperature shift of the reduction process facilitates the joint and complete reduction of the both metals without the noticeable presence of secondary products.

3.4. The Effective Activation Energy of Magnesiothermic Reactions

Based on the results of experiments performed in a wide range of high heating rates (100–1200 K min⁻¹), the effective values of the activation energy for the magnesiothermic reduction stages were calculated by the isoconversional method of Kissinger [32].

The derived expression for determination of activation energy by Kissinger has the following form:

$$\ln \left(\frac{{\mathrm{V}}_{\mathrm{h}}}{{\left({\mathrm{T}}_{\mathrm{m}\mathrm{a}\mathrm{x}}^{\mathrm{D}\mathrm{T}\mathrm{A}}\right)}^2}\right)=\ln \mathrm{A}-\frac{\mathrm{E}}{\mathrm{R}}\left(\frac{1}{{\mathrm{T}}_{\mathrm{m}\mathrm{a}\mathrm{x}}^{\mathrm{D}\mathrm{T}\mathrm{A}}}\right)$$

(1)

where, V_h is the heating rate (K min⁻¹), ${\mathrm{T}}_{\mathrm{m}\mathrm{a}\mathrm{x}}^{\mathrm{D}\mathrm{T}\mathrm{A}}\equiv {\mathrm{T}}^{*}$ is the temperature corresponding to the maximum advance in the DTA curve (K), A is the pre-exponential factor, E is the effective activation energy of the process, (kJ mol⁻¹), R is universal gas constant (8.31 J K⁻¹ mol⁻¹).

In particular, the activation energy of the magnesiothermic reduction reaction of MoO₃ (Figure 15 (1)) was calculated to be 110 ± 6.9 kJ mol⁻¹, which is about three times less than the E_a value of magnesiothermic reduction of copper oxide (E_{a(CuO + Mg)} = 324 kJ mol⁻¹) [21]. Both values of the activation energy for the MoO₃ + 3Mg and CuO + Mg reactions refer to the reduction process with molten Mg, where T* > T_Mgmelt in the whole range of surveyed heating rates.

The effective activation energy value of the magnesiothermic reduction stage in the MoO₃ + 1.5Mg + C mixture is 102 ± 5.7 kJ mol⁻¹ (Figure 15 (2)), which is commensurate with the activation energy value of magnesiothermic reaction in the MoO₃ + 3Mg mixture stated above (110 ± 6.9 kJ mol⁻¹).

As it was previously delivered in [21], in the CuO + 0.5Mg + 0.5C mixture, the interaction began with carbothermic reduction immediately after melting of magnesium and led to the formation of copper suboxide (

Table 2. Comparative overview of the CuO-C, MoO₃-C, CuO-Mg, MoO₃-Mg, CuO-MoO-C, CuO-MoO₃-Mg, CuO-Mg-C, MoO₃-Mg-C, CuO-MoO₃-Mg-C systems at low (20 K min⁻¹) and high (300 K min⁻¹) heating rates.

System [Ref]	Vh/ K min−1	Reaction, T/K												Ea/ kJ mol−1	Composition
CuO-Mg [22]	20	843–953												-	Cu, MgO
CuO-Mg [21]	300	953–1073												424	Cu, MgO
CuO-C [22]	20	CuO + C 713–803						Cu2O + C 803–923						-	Cu2O						Cu
CuO-C [this work]	300	CuO + C 823–973						Cu2O + C 973–1153						-	Cu2O						Cu
MoO3-Mg [33]	20	923–973												123	Mo, MgO
MoO3-Mg [this work]	300	943–1058												110	Mo, MoO2, MgO
MoO3-C [22,33]	20	MoO3 + C 773–873						MoO2 + C 1103–1173						-	MoO2						Mo
MoO3-C [this work]	300	MoO3 + C 833–953						MoO2 + C 1123–1373						-	MoO2						Mo and/or Mo2C
CuO-Mg-C [22]	20	CuO + C 703–813						Cu2O + Mg 923–1063						-	Cu2O, Mg						Cu, MgO
CuO-Mg-C [21]	300	CuO + C 923–1023						Cu2O + Mg 1023–1053						320	Cu2O, Mg						Cu, MgO
MoO3-Mg-C [33]	20	MoO3 + C 833–903						MoO2 + Mg 973–1073						197	MoO2, Mg						Mo, MgO
MoO3-Mg-C [this work]	300	MoO3 + C 953–1073						MoO2 + Mg 1123–1163						102	MoO2, Mg						Mo, MgO
CuO-MoO3-Mg [22]	20	CuO + MoO3 823–903						CuMoO4 + Mg 903–1073						-	CuMoO4, Mg, MgMoO4						Cu6Mo5O18, Mo, MoO2·MgO, MgO
CuO-MoO3-Mg [this work]	300	CuO + MoO3 940–1073				Cu3Mo2O9 + Mg 1143–1273				Cu6Mo5O18 + Cu2O + Mg 1273–1373				-	Cu3Mo2O9, Mg				Cu, Cu2O, Cu6Mo5O18 MgMoO4, Mg				Cu, Mo, MoO2·MgO, MgO
CuO-MoO3-C [22]	20	CuO + C 703–803						Cu2O + MoO3 + C 803–923						-	Cu2O, MoO3						Cu, MoO2
CuO-MoO3-C [this work]	300	CuO + MoO3 + C 823–913			Cu6Mo5O18 + MoO2 + C 923–1023			MoO2 + C 1023–1393			MoO2 + Mo2C 1393–1573			-	Cu6Mo5O18 Cu, MoO2			Cu, MoO2			Mo2C, MoO2, Cu			Mo, Cu
CuO-MoO3-Mg-C [22]	20	CuO + C 753–793				Cu2O + MoO3 + C 793–873				MoO2 + Mg 993–800				-	Cu2O, MoO3, Mg				Cu, MoO2, Mg				Mo, Cu, MgO, MgO·MoO2
CuO-MoO3-Mg-C [this work]	300	CuO + C 833- 943				Cu2O + MoO3 + C 943–1023				MoO2 + Mg 1213–1263				155	Cu2O, MoO3, Mg				Cu, MoO2, Mg				Mo, Cu, MgO, Mo2C, MoO2 (trace)

). The reduction of Cu₂O is then continued with magnesium. The activation energy value of Cu₂O + Mg reaction is 320 kJ mol⁻¹, which is approximately three times higher than the activation energy value of the magnesiothermic reduction reaction in the MoO₃ + 1.5Mg + C mixture (Table 2).

In the CuO + MoO₃ + 1.2Mg + 2.15C mixture, the activation energy of the magnesiothermic reduction stage (MoO₂ + Mg) is calculated to be 155 ± 10.7 kJ mol⁻¹ (Figure 15 (3)).

It is comparatively higher than the E_a value for the MoO₃ + 1.5Mg + C mixture, but about two times lower than E_a value of the CuO + 0.5Mg + 0.5C mixture (102 ± 5.7 kJ mol⁻¹ and 320 kJ mol⁻¹, respectively) [21], indicating the change in mechanism of interaction due to joint action of combined reducers.

The values of the effective activation energies calculated for the investigated reactions at high heating rates are presented in Table 2 and collated with the results obtained by DTA/TG method at low heating rates.

According to the data obtained, the magnesiothermic reduction of copper oxide is characterized by the highest value of the activation energy, which decreases when carbon is introduced into the system, thus demonstrating the expediency of utilization of combined reducers.

In MoO₃ + 3Mg and MoO₃ + 1.5Mg + C mixtures, the magnesiothermic reduction is characterized by a lower activation energy value. The energy of activation of joint reduction of oxides with combined reducers is 155 ± 10.7 kJ mol⁻¹, which can be attributed to the magnesiothermic reduction of molybdenum dioxide in the presence of already reduced copper.

4. Conclusions

The study of a separate and joint reduction mechanism of copper and molybdenum oxides with combined reducers (Mg + C) under the fast-heating conditions revealed that, in all the studied systems (CuO-Mg-C, MoO₃-Mg-C, CuO-MoO₃-Mg-C), the reduction process of copper oxide starts at a lower temperature compared to molybdenum oxide. The reduction of copper oxide and copper suboxide proceed exclusively with carbon, while the reduction of molybdenum oxide occurs with both the carbon and magnesium. Moreover, the carbothermic reduction of molybdenum trioxide is accompanied by the formation of MoO₂, which is then reduced to molybdenum by magnesium at higher temperatures. Solely carbothermic reduction of molybdenum trioxide under fast heating conditions obtained pure molybdenum at temperatures above 1473 K. In addition, it was revealed that when two strongly exothermic and violent reactions MoO₃ + 3Mg and CuO + Mg are carried out together, there is a slow interaction with weak self-heating, defined as the dysynergistic phenomenon, which is caused by a total change in the reaction mechanism: the process of Mg-thermic reduction of metals is preceded by the formation of the comparatively stable copper molybdate salt, CuMoO₄, and the reduction of the latter proceeds slowly. In the quaternary system, the activation energy value of the magnesiothermic reduction stage is significantly lower than the activation energy values of the magnesiothermic reduction of copper oxide, but only somewhat higher than the E_a value of the magnesiothermic reduction of molybdenum oxide, indicating the expediency of using Mg/C combined reducers. The synergistic influence and reciprocity of combined reducers allows the joint reduction of both metals (Mo, Cu) with trace amounts of byproduct at higher (>2600 K min⁻¹) heating rates.