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Article

Solvent-Free Synthesis of Nickel Nanoparticles as Catalysts for CO2 Hydrogenation to Methane

by
Olga V. Netskina
1,2,*,
Kirill A. Dmitruk
1,2,
Alexander A. Paletsky
1,3,
Svetlana A. Mukha
1,
Alena A. Pochtar
1,2,
Olga A. Bulavchenko
1,
Igor P. Prosvirin
1,
Andrey G. Shmakov
3,
Anna M. Ozerova
1,4,
Janna V. Veselovskaya
1,2,
Olga I. Mazina
1 and
Oxana V. Komova
1,4
1
Boreskov Institute of Catalysis SB RAS, Pr. Akademika Lavrentieva 5, 630090 Novosibirsk, Russia
2
Department of Natural Sciences, Novosibirsk State University, 1 Pirogova Str., 630090 Novosibirsk, Russia
3
Voevodsky Institute of Chemical Kinetics and Combustion SB RAS, 3 Institutskaya Str., 630090 Novosibirsk, Russia
4
Siberian Branch of Russian Academy of Sciences, Pr. Akademika Lavrentieva 17, 630090 Novosibirsk, Russia
*
Author to whom correspondence should be addressed.
Catalysts 2022, 12(10), 1274; https://doi.org/10.3390/catal12101274
Submission received: 29 September 2022 / Revised: 14 October 2022 / Accepted: 18 October 2022 / Published: 19 October 2022
(This article belongs to the Special Issue Catalytic Purification of Pollutants and Catalytic Conversion of Solid Wastes)
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Abstract

:
The solid-state combustion method was used to prepare nickel-based catalysts for CO2 hydrogenation from [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2. These complexes were synthesized by adding nickel nitrate and perchlorate to melted imidazole. The composition and structure of the obtained complexes was confirmed by ATR FTIR, powder XRD, and elemental analysis. The stages of thermal decomposition of the complexes and their kinetic parameters were established. It was found that incomplete gasification of more thermostable Ni(C3H4N2)6](ClO4)2 led to the formation of carbon, nitrogen, and chlorine impurities. According to powder XRD and XPS, the solid products of gasification of both complexes consist of NiO and Ni0 covered with nickel hydroxide and/or a carbonate layer. In the case of the sample prepared from [Ni(C3H4N2)6](ClO4)2, this layer was pronounced. Therefore, it limits the nickel reduction in the reaction medium of CO2 hydrogenation, even at 450 °C. The surface of the sample prepared from [Ni(C3H4N2)6](NO3)2 contains nickel oxide, which is easily reduced. So, the catalyst active phase is already formed at 250 °C in the presence of CO2 and efficiently catalyzes CO2 hydrogenation as the temperature increases. Therefore, [Ni(C3H4N2)6](NO3)2 is a promising precursor for the CO2 hydrogenation catalyst, and its solvent-free synthesis follows Green Chemistry principles.

1. Introduction

Global warming on Earth accelerates with the growth of the concentration of greenhouse gases in the atmosphere, such as carbon dioxide (CO2). Adsorption of CO2 and its subsequent utilization is one of the solutions to this issue [1,2]. One of the promising ways of recycling CO2 is its hydrogenation to methane, also known as CO2 methanation:
CO2 + 4H2 ↔ CH4 + 2H2O ΔH= −165 kJ/mol
This approach simultaneously solves the task of chemical hydrogen storage [3,4,5], allows the production of biogas with a higher methane content [6,7], and can be used to remove carbon oxides (CO and CO2) from hydrogen-rich gas mixtures [8].
According to the thermodynamic calculations [9], the hydrogenation of CO2 to methane with a high selectivity at atmospheric pressure in a stoichiometric reaction mixture is possible only in the temperature range below 400 °C. At higher temperatures, the reduction of CO2 to CO is observed.
CO2 + H2 ↔ CO + H2O ΔH = 42 kJ/mol
At temperatures below 400 °C, this process cannot be carried out without catalysts due to the high energy barrier of carbon reduction [10]. Platinum group metals, Ru, Rh, and Pd, show the greatest catalytic activity in the hydrogenation of CO2 due to their excellent ability to dissociatively adsorb hydrogen, but other transition metals can be used as catalysts as well [3,11,12,13]. Nickel exhibits a lesser catalytic activity in this process, but due to its low cost, nickel catalysts are widely used in the industry [14,15]. Traditionally, their preparation involves the energy-consuming calcination of the nickel precursor [16]. As a result, nickel oxide is formed, which is reduced to metal in a stream of pure hydrogen immediately before the hydrogenation of CO2. It is possible to combine the stages of formation of nickel oxide and its reduction to metal in the self-propagating high-temperature synthesis (SHS) using nickel complex compounds as precursors of the active component [17,18,19,20,21]. In the literature [22], this type of SHS is called the solid-state combustion (SSC) method because it involves the high-temperature gasification of solid energy-rich composition. Most often, it is a mixture of nickel nitrate with organic compounds. Since Ni is the active phase of the catalysts in the CO2 hydrogenation, it is important to use the combustion preparation methods by which the Ni/NiO ratio in the condensed products may be varied. It was noted that nickel oxide is formed during the thermolysis of nickel nitrate [23], but the addition of organic compound (glycine) leads to the reduction of part of nickel ions to the metallic state [24]. In excess of the organic component, the predominant formation of the metal phase is observed [25,26,27,28,29,30]. It was shown that the thermolysis of nickel complexes with aminoguanidine and semicarbazide also leads to the formation of metallic nickel with an insignificant impurity of nickel oxide [31]. According to [32], the reduction of nickel oxide occurs due to its interaction with ammonia released during the decomposition of nitrogen-containing organic compounds, as, e.g., in the case of glycine [33].
4C2H5NO2 → 6H2O + 2NH3 + 6C + 2(CHNO) ΔH = −288 kJ/mol
2NH3 + 5NiO → 5Ni + 3H2O + 2NO
Therefore, it is preferable to use of nitrogen-containing molecules as ligands [29,32,34,35]. If the oxygen- or sulfur-containing compounds are used as organic ligands, then nickel oxide [36,37] or nickel sulfide [38] are formed, respectively. Therefore, the use of these compounds as CO2 hydrogenation catalysts requires additional reduction with hydrogen.
It should be noted that the aforementioned nickel-containing complexes were obtained using solvents that should be disposed of. It is possible to minimize the ecological impact on the environment through the use of solvent-free methods for the preparation of complexes, which corresponds to the modern concepts of the development of the chemical industry. In this work, we propose a new approach to the synthesis of nickel nitrogen-rich complexes by reacting a nickel salt with melted imidazole, which is thermally stable upon melting [39] and easily forms complex compounds with transition metals. In addition, imidazole is a nitrogen-containing compound, and its decomposition products (ammonia) can reduce nickel cations to a metallic state. Nickel-containing catalysts were obtained from the prepared complexes, which were tested in CO2 hydrogenation after activation directly in the reaction mixture.
The goals of this work were:
✔
To investigate the structure of nickel-containing complexes obtained in the reaction of nickel nitrate and perchlorate with melted imidazole;
✔
To study the thermochemical properties of the synthesized nickel-containing complexes;
✔
To test the solid products obtained by the SSC method from the synthesized nickel-containing complexes in CO2 hydrogenation to methane.

2. Results and Discussion

2.1. Synthesis of Nickel Complexes by Melting

Nickel-containing imidazole complexes hexa(imidazole)nickel(II) nitrate [Ni(C3H4N2)6](NO3)2, and hexa(imidazole)nickel(II) perchlorate [Ni(C3H4N2)6](ClO4)2 were synthesized as precursors of nickel CO2 hydrogenation catalysts. They were prepared without the use of solvents, which significantly reduces the preparation time due to the absence of the stage of separating the complexes from solutions and also does not require the further disposal of the solvents. The complexes were synthesized in melted imidazole with nickel nitrate and perchlorate hydrates (Figure 1). Elemental analysis of the samples (Table 1) showed that the resulting compounds contained six imidazole molecules each.
The synthesized complexes were studied by the ATR FTIR spectroscopy method (Figure 2). It can be seen from the ATR FTIR spectra that no structural rearrangements of the imidazole heterocyclic ring occur when imidazole and nickel salts react, but the absorption band of stretching vibrations ν(N-H) increases in intensity and slightly shifts to the high-frequency region (from 3126 to 3140 cm−1 and 3156 cm−1 for [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2, respectively). The metal cation is coordinated by the lone pair of electrons of the pyridine nitrogen atom, breaking the hydrogen bonds between imidazole molecules. In this case, the N-H group becomes more isolated and the distance between atoms shortens. As a result, the absorption bands of stretching vibrations ν(N-H) are shifted to the high-frequency region. In addition, the obtained compounds do not contain water (Figure 2), since there are no absorption bands near 1600 cm−1, related to the bending vibrations of the water molecules δ(H2O).
The more conclusive evidence of the formation of nickel-containing imidazole complexes during the reaction of nickel salts with melted imidazole was obtained by powder XRD. Thus, the diffraction patterns of the synthesized compounds correspond to the database cards of complexes [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2 (Figure 3).
Thus, a simple energy- and resource-saving solvent-free approach was proposed for the synthesis of nickel-containing imidazole complexes [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2, which were used as catalyst precursors for CO2 hydrogenation.

2.2. Thermochemical Properties of the Synthesized Nickel Complexes

The synthesis of catalysts by the SSC method involves the high-temperature decomposition of precursors with the release of a large volume of gases, which ensures a highly dispersed state of the resulting solid product. The thermal analysis data (Figure 4) showed that the thermal decomposition of the synthesized complexes in air begins at 200 °C and has three main stages. The first stage (200–250 °C) is characterized by an endothermic effect and a weight loss of 20 and 18 % for [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2, respectively. During the second stage (250–300 °C), releasing a large amount of heat and the mass changes by 25% for [Ni(C3H4N2)6](NO3)2 and 27 % for [Ni(C3H4N2)6](ClO4)2 are observed. An exothermic effect is also observed in the third stage at temperatures above 300 °C, but for [Ni(C3H4N2)6](ClO4)2, it is low-temperature and not a pronounced stage. The total mass loss of [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2 in air was 86 % and 56 %, respectively.
According to [42], the first stage of the thermal decomposition of nickel-containing imidazole complexes is an endothermic partial loss of imidazole with its subsequent sublimation. At the second stage, an exothermic redox interaction occurs between imidazole as organic ligand and anions. The third stage can be assigned to the decomposition of the products of incomplete thermolysis of the organic ligand.
The thermal decomposition kinetics of [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2 complexes were analyzed by the Coats–Redfern method using both traditional linear fitting and the genetic algorithm to find kinetic parameters [43,44,45]. Figure 5 shows that the second approach more accurately describes the first two stages of decomposition of complexes based on the first-order kinetics, since the obtained coefficients of determination R2 are higher than 0.999.
As a result of the analysis of the thermal decomposition kinetics by the Coats–Redfern method, the activation energies of the first two stages of decomposition of [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2 complexes were determined (Table 2).
The resulting data shows that 117 kJ/mol is required to overcome the energy barrier for the elimination of two imidazole molecules from the complex in the case of the nitrate complex, and 177 kJ/mol in the case of the perchlorate complex. This suggests that imidazole–metal bond in the [Ni(C3H4N2)6](ClO4)2 complex is stronger.
The second stage of decomposition of the [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2 complexes is characterized by a significantly higher energy barrier. Thus, the activation energy of the oxidation of imidazole by the nitrate anion is 354 kJ/mol, but the interaction of the perchlorate anion with imidazole has a lower value of 253 kJ/mol and occurs at a high rate almost simultaneously with the first stage. In addition, for the perchlorate-containing complex, significantly more heat is released in the second stage of decomposition than in the case of [Ni(C3H4N2)6](NO3)2 complex (Figure 4).
The activation energy values (E) obtained by the Coats–Redfern method (genetic algorithm) were used to estimate the critical temperature of thermal explosion for [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2 complexes, which characterized their thermal stability and safety. For these calculations, DSC data from different heating rates of the complexes in air was obtained (Figure S1). As known, the critical temperature of thermal explosion (Tb) may be calculated using the Tonglai formula [46]:
T b = E − E 2 − 4 E R T 0 2 R
where R is the molar gas constant and T0 is the onset temperature at the heating rate of 0 K/min obtained by linear regression.
As the activation energy values depend on the applied method of kinetic analysis of thermogravimetric data the other calculation was also carried out using the Xue formula [47], in which there is no dependence on the activation energy:
T b = T 0 + ∂ T e ∂ ( ln β ) ′
where T0 is the is the onset temperature at the heating rate of 0 K/min, Te is the onset temperature, and β is the heating rate. The obtained results are summarized in Table 3.
The critical temperatures of thermal explosion for Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2 complexes obtained by the two methods are close. Their differences do not exceed 10 °C, which indicates that the used method of kinetic analysis was correct. As a result, it was shown that the self-propagating process of thermal decomposition of [Ni(C3H4N2)6](NO3)2 starts at 242 ± 3 °C and [Ni(C3H4N2)6](ClO4)2 starts at 254 ± 5 °C. Thus, the synthesized complexes are thermally stable in air, and the thermal decomposition with the release of gaseous products occurs only after reaching a temperature above 230 °C (Figure 4).

2.3. Thermal Decomposition Mechanism of the Synthesized Nickel Complexes at the High Heating Rates

One of the modern methods for studying the kinetics of thermal decomposition of high-energy material is the analysis of evolved gases by dynamic mass spectrometry [48]. The high sensitivity and instantaneous response of the mass spectrometer make it possible to describe the kinetics of thermal decomposition under conditions of high heating rate (experiment duration is 2 s), which is close to real SSC conditions. In this work, the main gaseous products of the thermal decomposition of [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2 complexes were identified by this method (Figure 6). It was noted that the composition of the gas and the dynamics of the release of individual products depended on the composition of the complex. Thus, the decomposition products of [Ni(C3H4N2)6](NO3)2 complex contain imidazole (m/z = 68), oxygen (m/z = 32), nitrogen oxides—NO (m/z = 30) and NO2 (m/z = 46), hydrogen cyanide (m/z = 27), carbon dioxide (m/z = 44), and nitrogen and/or carbon monoxide (m/z = 28). In the gaseous decomposition products of [Ni(C3H4N2)6](ClO4)2, hydrogen chloride (m/z = 36) was identified instead of nitrogen oxides due to the different nature of the anion.
The experimental results were processed using a genetic algorithm to find the kinetic parameters. Since the heating rate could not be maintained constant under the conditions of the highly exothermal decomposition, the comparison function was based on the Arrhenius equation. In this case, the integration was performed numerically under the assumption that the temperature changed uniformly between the experimental points. The activation energy values obtained for some gaseous products of thermal decomposition of [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2 complexes are given in Table 4.
Compared with the results of dynamic mass spectral analysis and thermal analysis, we can conclude that the mass loss at the first stage of decomposition of the studied complexes is associated with the successive release of two imidazole molecules. It was found that the activation energy for the removal of the first molecule (Stage 1) is much lower than that of the second (Stage 2), regardless of the composition of the complex. However, imidazole is more easily removed from the nitrate complex than from the perchlorate complex.
From Figure 6, it can be seen that simultaneously with the removal of imidazole molecules from the complex, the following processes are observed: (I) the thermolysis of imidazole with the formation of hydrogen cyanide and (II) the redox interaction of imidazole with anions, which results in the formation of carbon and nitrogen oxides. In addition, the presence of oxygen in the gaseous reaction products confirms the decomposition of anions, accompanied by the release of nitrogen oxides and hydrogen chloride in the case of [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2 complexes, respectively. All this significantly complicates the description of the kinetics of the decomposition of the complexes, especially in the case of the second stage. Moreover, the rate of these processes depends on the temperature, and on the curves for the release of gaseous products of the thermal decomposition of the complexes, low-temperature (Stage I) and high-temperature (Stage II) regions can be identified. The activation energies determined for these steps (Table 4) indicate higher thermal stability of the [Ni(C3H4N2)6](ClO4)2 complex in comparison to the [Ni(C3H4N2)6](NO3)2 complex. This is also consistent with the stability of the perchlorate anion, since its decomposition is observed only in the high-temperature region (Figure 6), and the activation energy value of oxygen evolution is two times higher than that of the nitrate anion. Analyzing the kinetics of oxygen evolution, it should be noted that the obtained values of activation energy are close to the activation energy for the decomposition of nickel nitrate (84 ± 3 kJ/mol) and perchlorate (210 ± 5 kJ/mol) given in [49,50].
Thus, using complementary methods of dynamic mass spectral analysis and thermal analysis, the thermal decomposition of [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2 complexes was studied. It was shown that the complexes decomposition begins with the successive removal of two imidazole molecules. Simultaneously, the second stage proceeds, which includes the redox interaction of imidazole with anions, the thermolysis of imidazole, and the decomposition of anions with the release of oxygen.

2.4. Study of Solid Products Obtained by SSC from Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2 Complexes

The aim of gasification of imidazole-containing nickel complexes by the SSC method is to produce solid products. Their highly dispersed state is ensured due to intensive gas evolution. In our work, Ni/NiO-NO3-SSC and Ni/NiO-ClO4-SSC samples were obtained from [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2 complexes, respectively. The phase composition of solid products of the combustion of complexes was studied by powder XRD. It was shown that the Ni/NiO-NO3-SSC and Ni/NiO-ClO4-SSC samples contain two phases (Figure 7): metallic nickel—Ni0 (PDF 04-0850) and its oxide—NiO (PDF 47-1049). Nickel oxide is the main phase (Table 5), but its content is lower in the sample obtained from the nitrate complex.
The CSR width was used to determine the size of crystallites for each phase (Table 5) contained in the decomposition products of [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2 complexes. The size of nickel oxide crystallites is the same for Ni/NiO-NO3-SSC and Ni/NiO-ClO4-SSC. Metallic nickel obtained from the perchlorate complex is more dispersed, since its CSR is two times lower than that of Ni/NiO-NO3-SSC.
It should be noted that the reduced form of nickel is absent on the surface of Ni/NiO-NO3-SSC and Ni/NiO-ClO4-SSC according to the XPS data (Figure 8a). So, the peak at 853 eV in the Ni2p3/2 XPS spectra corresponding to Ni0 [51] was not found. Under SSC conditions, oxygen in the air probably interacts with the surface of the formed metal particles, oxidizing them to NiO [52] and Ni(OH)2 [53]. Moreover, the Ni2p3/2 peak with a binding energy of 855.8 eV can be attributed not only to nickel hydroxide, but also to nickel carbonate [54] formed due to the reaction between the generated gases and the obtained nickel-containing phases. Apparently, these compounds are amorphous, since they were not detected by powder XRD (Figure 7).
In addition to this, the analysis of the XPS survey spectra (Figure S2) shows that not only nickel and oxygen, but also impurities of carbon and nitrogen are present on the surface of Ni/NiO-NO3-SSC and Ni/NiO-ClO4-SSC (Table 6). Despite the greater value of the oxygen balance (Table 1), their content is higher in the sample obtained from the perchlorate complex. Furthermore, it contains more than 8 at.% chlorine.
The higher content of impurities in Ni/NiO-ClO4-SSC is also confirmed by ATR FTIR spectroscopy, since it is characterized by intense structureless absorption, reaching the highest values in the regions of 1100–1600 cm−1 and 600–800 cm−1 (Figure 8b). Taking into account the elemental composition of impurities (Table 6) and thermal stability of the [Ni(C3H4N2)6](ClO4)2 complex (Table 2 and Table 3), absorption in the region of 1100–1600 cm−1 can be attributed to the vibrations of -C-C- and -C-N- bonds with carbon atoms in sp3 hybridization and the stretching vibrations of double bonds -C=C- and -C=N- with carbon atoms in sp2 hybridization, according to [55,56]. Structureless absorption in the region of 600–800 cm−1 corresponds to bending vibrations of C-H and N-H groups, including those in heterocyclic compounds [57]. Thus, on the surface of Ni/NiO-ClO4-SSC there are products of incomplete decomposition of imidazole. These include amorphous carbon or carbon nitride, which are characterized by structureless absorption in the regions of 1100–1600 cm−1 and 600–800 cm−1 [58]. However, the presence of nickel carbonate in the sample is not excluded, since the absorption bands of stretching vibrations of the CO3 group are also in the region of 1400–1500 cm−1 [59,60].
Summing up the study of the solid products obtained by the combustion of [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2 complexes, it can be concluded that the [Ni(C3H4N2)6](NO3)2 complex provides samples with a lower content of impurities. Its surface contains three times more nickel in oxidized form compared to Ni/NiO-ClO4-SSC (Table 6). The probable cause may be coverage of the nickel-containing phase by the products of incomplete decomposition of the [Ni(C3H4N2)6](ClO4)2 complex, which is more thermally stable than the [Ni(C3H4N2)6](NO3)2 complex (Figure 4).

2.5. Testing the Solid Products of Nickel Complexes Decomposition in the CO2 Hydrogenation

The catalytic properties of the Ni/NiO-NO3-SSC and Ni/NiO-ClO4-SSC samples prepared by SSC from the [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2 complexes were investigated in the CO2 hydrogenation to methane. Traditionally, the catalysts are activated at 400–500 °C in a pure hydrogen flow to reduce the active component to the metallic state Ni0 [61,62,63], which catalyzes the carbon dioxide–hydrogen interaction [64]. The absence of this standard step is a distinctive feature of our work. The Ni/NiO-NO3-SSC and Ni/NiO-ClO4-SSC was activated in the flow of the reaction mixture (H2:CO2:Ar = 16:4:80), which is more efficient from a practical point of view. The reduction of the samples was monitored by FTIR spectroscopy of the gas phase at the outlet of the reactor. The appearance of the absorption bands in the 2900–3000 cm−1 range, corresponding to the stretching vibrations of the C-H bonds in the methane molecule [65] indicates the beginning of catalyst activation.
When testing the Ni/NiO-NO3-SSC and Ni/NiO-ClO4-SSC samples, the temperature in the reactor was increased from 25 to 450 °C. It was noted that in the presence of Ni/NiO-NO3-SSC methane began to form at 250 °C, and its contribution to gaseous products increased significantly at higher temperatures (Figure 9a). In the case of Ni-ClO4-SSC, the absorption bands of methane were detected only at 350 °C (Figure 9b), and their amount changed little with increasing the temperature up to 450 °C. This is due to significantly lower CO2 conversion compared to the Ni/NiO-NO3-SSC sample (Figure 10).
To determine the reason for such a large difference in activation temperatures, the activated Ni/NiO-NO3-SSC and Ni/NiO-ClO4-SSC samples were studied by the XPS method. From Figure 11, it can be seen that both catalysts contain the reduced form of nickel on the surface after activation at 450 °C. Deconvolution of the Ni2p3/2 peak revealed that the Ni0/Ni2+ molar ratio for Ni/NiO-NO3-SSC is 0.65, but it is only 0.2 for Ni/NiO-ClO4-SSC.
Such an insignificant reduction of nickel at 450 °C in the case of Ni/NiO-ClO4-SSC can be attributed to two reasons. First, impurities on the Ni/NiO-ClO4-SSC surface can prevent the access of hydrogen to the nickel-containing phase. Secondly, the reduction of nickel hydroxide occurs in a higher temperature range [66]:
NiO + H2 → Ni + H2O     250–400 °C
Ni(OH)2 + H2 → Ni + 2H2O    350–600 °C
Thus, it is more preferable to use the [Ni(C3H4N2)6](NO3)2 complex as the precursor of the active phase of the CO2 hydrogenation catalyst. In addition, the Ni/NiO-NO3-SSC catalyst from this complex is activated at a lower temperature in the presence of CO2 than the industrial NIAP-07-01 catalyst [67].

3. Experimental Details

3.1. Materials

For the synthesis of nickel complexes, the following reagents were used: Ni(NO3)2·6H2O (GOST 4055-70, 98%); Ni(ClO4)2·6H2O (TU 6-09-02-118-86, 98%); imidazole—C3H4N2 (CAS 288-32-4, 99%).

3.2. Complex Synthesis

[Ni(C3H4N2)6](NO3)2 In order to synthesize the complex compound, nickel (II) nitrate (0.01 mol) was added to melted imidazole (0.06 mol) at 90 °C and stirred. The reaction mixture became purple and then quickly crystallized as a powder with a yield of 97%. After that, the sample was stored in a desiccator over P2O5.
[Ni(C3H4N2)6](ClO4)2 In order to synthesize the complex compound, nickel (II) perchlorate (0.01 mol) was added to melted imidazole (0.06 mol) at 90 °C and stirred. The reaction mixture became cornflower-colored and then quickly crystallized as a powder with a yield of 95%. After that, the sample was stored in a desiccator over P2O5.

3.3. Synthesis of Nickel Catalysts by Solid-State Combustion Method

In order to synthesize nickel catalysts, powders of complexes were placed in a corundum crucible and preheated on the IKA-C-Mag HS4 hot plate (IKA, Königswinter, Germany) with the heating temperature set to 500 °C. The samples melted followed by spontaneous gasification within a few minutes, leaving black powder in the crucible. The powder was granulated, and a fraction with the particle size of 0.25–0.5 mm was prepared for testing in catalytic experiments and stored in a desiccator over P2O5.
The solid products of combustion of the [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2 complexes are denoted by Ni/NiO-NO3-SSC and Ni/NiO-ClO4-SSC, respectively.

3.4. Characterization of Complexes and Solid Products of Combustion

3.4.1. Composition of Complexes and Products of Their Thermolysis

The Ni content was found by inductively coupled plasma atomic emission spectrometry on an Optima 4300 DV (PerkinElmer, Waltham, MA, USA). The carbon, hydrogen, and nitrogen contents were determined using an automatic CHNS analyzer EURO EA 3000 (Euro Vector S.p.A., Castellanza, Italy). In the analyzer, the complexes (0.5–2 mg) were combusted in a vertical reactor at 1050 °C in a flow of He with added O2. The measurement error was in the range of ±3%.
Oxygen balance was calculated based on the determined C, H, N, and Ni contents using the following equation:
O x y g e n   b a l a n c e   ( % ) = − 1600 M o l a r   m a s s ⋅ ( 2 C + H 2 + M − O )
where C is the quantity of carbon atoms, H is the quantity of hydrogen atoms, O is the quantity of oxygen atoms, and M is the quantity of metal atoms.
The X-ray powder diffraction (XRD) patterns of the synthesized nickel complexes and solid products of their combustion were acquired using a D8 Advance diffractometer equipped with a Lynxeye linear detector (Bruker AXS GmbH, Karlsruhe, Germany). The powder XRD patterns were obtained in the 2θ range from 5 to 70° with a step of 0.05° and accumulation time of 2 s using CuKα radiation (λ = 1.5418 Å). The composition of the solid-state combustion products was determined by the Rietveld method [68] using the TOPAS program (TOPAS—Total Pattern Analysis System, Version 4.2, Bruker AXS). The reference material for describing the instrumental broadening was metallic silicon. The size of the coherent scattering region (CSR) was calculated using LVol-IB (LVol-IB—the volume-weighted mean column height based on integral breadth) and LVol-FWHM (the volume-weighted mean column height based on full width at half maximum, k = 0.89) values. Crystallographic data for the complexes have been deposited with the Cambridge Crystallographic Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies of the data can be obtained free of charge by quoting the depository numbers: CCDC-1180140 for [Ni(C3H4N2)6](NO3)2; CCDC-1176420 for [Ni(C3H4N2)6](ClO4)2 (http://www.ccdc.cam.ac.uk, accessed on 1 September 2022). Theoretical powder pattern for [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2 plotted from the CIF available in [40] and [41], respectively.
Infrared spectra of the complexes and solid products of combustion were obtained by attenuated total reflection infrared spectroscopy (ATR FTIR) using an Agilent Cary 630 spectrometer (Agilent Technologies, Santa Clara, CA, USA) equipped with a Gladi ATR attachment (PIKE Technologies, Madison, WI, USA).
A SPECS photoelectron spectrometer with a PHOIBOS-150-MCD-9 hemispheric analyzer and a FOCUS-500 monochromator (SPECS Surface Nano Analysis GmbH, Berlin, Germany) (AlKα radiation, 150 W, hν = 1486.74 eV) was used to record X-ray photoelectron spectra (XPS) of the solid products of combustion. The samples were transported and supported on a conducting scotch in an inert atmosphere (argon) to prevent oxidation of the particle surface in air during storage. The binding energy (BE) scale of the spectrometer was precalibrated using Au4f7/2 (84.0 eV) and Cu2p3/2 (932.6 eV) core-level peaks. The measurement error was ±0.1 eV. The sample charge was taken into account using C1s lines (284.8 eV). Individual spectra of elements were used to calculate the composition of the surface while accounting for element sensitivity coefficients [69].

3.4.2. Study of Thermochemical Properties of Nickel Complexes

The thermal decomposition of nickel complexes (5 mg) was studied using a Netzsch STA 449 C Jupiter instrument equipped with a DSC/TG holder (NETZSCH, Selb, Germany) in the temperature range of 30–500 °C under a flow of air (30 mL·min−1) with the heating rate of 5 °C·min−1. Based on the TG-DSC data, the kinetic parameters of the main two stages of the thermal decomposition of the complexes were determined using the traditional Coats–Redfern method with linear fitting [70] and the Coats–Redfern method with the genetic algorithm to find the pre-exponential factor and activation energy. Within the framework of this approach, the rate of the thermal decomposition reaction is written [71] as
d α d t = A e − E R T ( 1 − α ) n
where t is time, α is the conversion degree (or portion of decomposed compound), d α d t is the reaction rate, A is the pre-exponential factor, E is the activation energy, R is the gas constant, T is temperature, and n is the reaction order.
Taking into account the heating rate β = d T d t , Equation (10) was integrated:
∫ 0 α d α ( 1 − α ) n = A β ∫ T 0 T e − E R T d T   or − ln ( 1 − α ) = A R T 2 β E 1 − 2 R T E e − E R T   ( n = 1 )
Using this equation, the kinetic parameters were calculated. For the estimation of the efficiency of modeling the experimental data, it is necessary to introduce a fitness function (F) [72]. In this study the fitness function (12) was applied.
F = 1 N ∑ i N ( 1 2 ( α i t h e o r − α i exp e r ) 2 + ( 1 2 ( α t h e o r d t i − α exp e r d t i ) 2
where N represents the number of experimental points, i is the value of the experimental point at some temperature α i t h e o r and α i exp e r are theoretical and experimental values of the portion of decomposed compound at some temperature, α t h e o r d t i and − α exp e r d t i are theoretical and experimental values of reaction rate at some temperature. The search for activation energy and the pre-exponential factor of each stage was performed in ranges of 50–550 kJ/mol and 104–1040 s−1, respectively.
The TG-DSC data at a different heating rate (2.5, 5, 10, 20 °C/min) and the activation energy obtained by the genetic algorithm were used to estimate the critical temperature of thermal explosion for the complexes by methods described by Tonglai et al. [46] and Xue et al. [47].
The composition of gaseous products of combustion was analyzed by the dynamic mass-spectral thermal analysis (DMSTA) method, using a time-of-flight mass spectrometer with a molecular beam sampling system MSCh-4 (Plant Of Scientific Instrumentation, Sumy, USSR) under a flow of Ar (5 mL·min−1). Average heating rate ranged from 100 to 245 °C·s−1. The sample weight was 1–5 mg. The delay between measurements was 0.04 s. The identification of mass spectral signals was carried out using the mass spectra of individual substances from the NIST database.
Kinetic parameters of separated stages of the thermal decomposition of the complexes under high heating rate were found by the genetic algorithm based on mass-spectral signals with m/z of 68 and 32. The search for activation energy and the pre-exponential factor of each stage was performed in ranges of 50–550 kJ/mol and 104–1040 s−1, respectively. However, since the heating rate could not be maintained consistent during the combustion, the Coats–Redfern method for integrating the reaction rate equation could not be used. Thus, Equation (10) was integrated numerically to acquire α i t h e o r for the fitting function.

3.5. Experimental Setup for Studying CO2 Hydrogenation

Experimental investigation of the catalytic properties of nickel complexes and solid products of combustion was carried out in a fixed-bed reactor (inner diameter—12.8 mm) at atmospheric pressure. The catalyst load was 318 mg. The reactor was heated in a tubular furnace, and the temperature was controlled using a thermocouple installed in the catalytic bed. The temperature varied from 150 to 450 °C with a step of 50 °C. The temperature was increased every 20 min until catalyst activation was observed, and afterwards the duration of each step varied.
The flow of the reaction mixture (H2:CO2:Ar = 16:4:80) passed through the catalytic bed with an hourly mass feed flow rate of 19.000 mL·gcat−1·h−1. The flow rate was controlled by an RRG-12-36 flow meter (“Eltochpribor”, Moscow, Russia) with an accuracy of 1%. The composition of the gas at the reactor’s outlet was determined using an Agilent Cary 630 FTIR-spectrometer (Agilent Technologies Australia, Melbourne, Australia) with the gas cell’s internal volume of 100 cm3 and the optical path length of 10 cm.
The amount of CO2 in the reaction medium before and after the reactor was determined using nondispersive infrared sensors (Dynament Limited, Mansfield, UK). The obtained data were used to calculate CO2 (XCO2) conversion as follows:
X C O 2 = 1 − C o u t C i n ⋅ U o u t U i n ⋅ 100 %
where Cin and Cout are CO2 concentrations at the inlet and outlet of the reactor, and Uin and Uout are gas flow rates at the inlet and outlet of the reactor. Taking into account reaction stoichiometry (the reaction proceeds with a decrease in volume) and the high content of argon in the reaction medium, the Uin/Uout ratio was in the range from 0.92 to 1.

4. Conclusions

Using a solvent-free melting-assisted method, [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2 complexes were prepared as precursors of CO2 hydrogenation catalysts. Their composition and structure were confirmed by elemental analysis, ATR FTIR, and powder XRD. The thermochemical properties of the complexes were studied, and it was shown that [Ni(C3H4N2)6](ClO4)2 is more thermally stable due to the stronger imidazole–metal interactions and the thermal stability of the perchlorate anion. That is one of the reasons for the lower degree of its gasification in the SSC conditions, because the resulting solid product contains carbon, nitrogen, and chlorine impurities. There are far fewer impurities in the solid product of [Ni(C3H4N2)6](NO3)2 thermal decomposition. It was found by powder XRD that nickel oxide and reduced nickel are the main components of the solid products of gasification of the [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2 complexes. However, XPS showed that the particles’ surface is covered by an amorphous layer of hydroxide and/or carbonate nickel, which is pronounced in the case of the sample prepared from [Ni(C3H4N2)6](ClO4)2. This led to a lower degree of nickel reduction (Ni0/Ni2+ = 0.2) during the catalysts activation in the CO2 hydrogenation reaction medium, even at 450 °C. Nickel oxide on the surface of the sample prepared from [Ni(C3H4N2)6](NO3)2 ensures the formation of metallic nickel (Ni0/Ni2+ = 0.65) already at 250 °C in the presence of CO2, which efficiently catalyzes the interaction of CO2 and hydrogen with further temperature increases.
As a result, [Ni(C3H4N2)6](NO3)2 is a more promising precursor for CO2 hydrogenation catalyst. It provides a lower impurity level in solid products of gasification, a large fraction of reduced metal, and a formation of nickel oxide on the particle surface, which reduces to the catalytically active phase even at 250 °C in the CO2 presence. It should be noted that the activation of industrial catalysts for CO2 hydrogenation to methane begins only at temperatures over 350 °C under similar conditions, as established in our previous studies [67].

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal12101274/s1, Figure S1: DSC data for [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2 complexes at different heating rates; Figure S2: XPS survey spectra for Ni/NiO-NO3-SSC and Ni/NiO-ClO4-SSC.

Author Contributions

Conceptualization, O.V.N. and O.V.K.; methodology, S.A.M., J.V.V. and K.A.D.; formal analysis, A.G.S. and A.A.P. (Alexander A. Paletsky); investigation, A.M.O., K.A.D., A.A.P. (Alena A. Pochtar), I.P.P., O.I.M. and O.A.B.; data curation, A.A.P. (Alexander A. Paletsky), S.A.M., I.P.P., O.A.B. and A.A.P. (Alena A. Pochtar); writing—original draft preparation, K.A.D., O.I.M. and A.M.O.; writing—review and editing, O.V.K. and A.G.S.; supervision, J.V.V.; project administration, O.V.N. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Ministry of Science and Higher Education of the Russian Federation (project no. 075-15-2020-781).

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 12 01274 g001 550
Figure 1. Synthesis of hexa(imidazole)nickel(II) nitrate by adding nickel nitrate to melted imidazole: (a) initial mixture; (b) 1 min; (c) 3 min.
Figure 1. Synthesis of hexa(imidazole)nickel(II) nitrate by adding nickel nitrate to melted imidazole: (a) initial mixture; (b) 1 min; (c) 3 min.
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Catalysts 12 01274 g002 550
Figure 2. ATR FTIR spectra for synthesized (a) [Ni(C3H4N2)6](NO3)2 and (b) [Ni(C3H4N2)6](ClO4)2 complexes (*—deformation vibration of the ring).
Figure 2. ATR FTIR spectra for synthesized (a) [Ni(C3H4N2)6](NO3)2 and (b) [Ni(C3H4N2)6](ClO4)2 complexes (*—deformation vibration of the ring).
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Catalysts 12 01274 g003 550
Figure 3. Powder XRD data for synthesized (a) [Ni(C3H4N2)6](NO3)2 [40] and (b) [Ni(C3H4N2)6](ClO4)2 [41] complexes.
Figure 3. Powder XRD data for synthesized (a) [Ni(C3H4N2)6](NO3)2 [40] and (b) [Ni(C3H4N2)6](ClO4)2 [41] complexes.
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Catalysts 12 01274 g004 550
Figure 4. Thermal analysis data for (a) [Ni(C3H4N2)6](NO3)2 and (b) [Ni(C3H4N2)6](ClO4)2 complexes prepared by melting-assisted solvent-free synthesis (I, II, III—thermolysis stages). The heating rate in air is 5 °C/min.
Figure 4. Thermal analysis data for (a) [Ni(C3H4N2)6](NO3)2 and (b) [Ni(C3H4N2)6](ClO4)2 complexes prepared by melting-assisted solvent-free synthesis (I, II, III—thermolysis stages). The heating rate in air is 5 °C/min.
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Catalysts 12 01274 g005 550
Figure 5. Modelling the thermal decomposition of (a) [Ni(C3H4N2)6](NO3)2 and (b) [Ni(C3H4N2)6](ClO4)2 complexes using the Coats–Redfern method. The heating rate in air is 5 °C/min.
Figure 5. Modelling the thermal decomposition of (a) [Ni(C3H4N2)6](NO3)2 and (b) [Ni(C3H4N2)6](ClO4)2 complexes using the Coats–Redfern method. The heating rate in air is 5 °C/min.
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Figure 6. Mass spectrometry of gas released during the thermal decomposition of (a) [Ni(C3H4N2)6](NO3)2 and (b) [Ni(C3H4N2)6](ClO4)2 complexes. The heating rate in air is above 100 °C/s (*0.05 and *0.25—correlation coefficients).
Figure 6. Mass spectrometry of gas released during the thermal decomposition of (a) [Ni(C3H4N2)6](NO3)2 and (b) [Ni(C3H4N2)6](ClO4)2 complexes. The heating rate in air is above 100 °C/s (*0.05 and *0.25—correlation coefficients).
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Figure 7. X-ray powder diffraction for Ni/NiO-NO3-SSC and Ni/NiO-ClO4-SSC.
Figure 7. X-ray powder diffraction for Ni/NiO-NO3-SSC and Ni/NiO-ClO4-SSC.
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Figure 8. (a) XPS and (b) ATR FTIR spectra for Ni/NiO-NO3-SSC and Ni/NiO-ClO4-SSC samples.
Figure 8. (a) XPS and (b) ATR FTIR spectra for Ni/NiO-NO3-SSC and Ni/NiO-ClO4-SSC samples.
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Figure 9. FTIR spectra of gases at the outlet of the reactor during the activation of (a) Ni/NiO-NO3-SSC and (b) Ni/NiO-ClO4-SSC at different temperatures in the reaction medium. Activation conditions: volume ratio is H2:CO2:Ar = 16:4:80, hourly mass feed flow rate is 19,000 cm3·gcat−1·h−1.
Figure 9. FTIR spectra of gases at the outlet of the reactor during the activation of (a) Ni/NiO-NO3-SSC and (b) Ni/NiO-ClO4-SSC at different temperatures in the reaction medium. Activation conditions: volume ratio is H2:CO2:Ar = 16:4:80, hourly mass feed flow rate is 19,000 cm3·gcat−1·h−1.
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Figure 10. CO2 conversion during the activation of Ni/NiO-NO3-SSC and Ni/NiO-ClO4-SSC at different temperatures in the reaction medium (H2:CO2:Ar = 16:4:80).
Figure 10. CO2 conversion during the activation of Ni/NiO-NO3-SSC and Ni/NiO-ClO4-SSC at different temperatures in the reaction medium (H2:CO2:Ar = 16:4:80).
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Figure 11. Deconvoluted XPS spectra of Ni/NiO-NO3-SSC and Ni/NiO-ClO4-SSC samples after activation at 450 °C in the reaction medium (H2:CO2:Ar = 16:4:80).
Figure 11. Deconvoluted XPS spectra of Ni/NiO-NO3-SSC and Ni/NiO-ClO4-SSC samples after activation at 450 °C in the reaction medium (H2:CO2:Ar = 16:4:80).
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Table
Table 1. Characteristics of the synthesized nickel complexes.
Table 1. Characteristics of the synthesized nickel complexes.
Complex,
Molar Mass		Composition, %Gross FormulaOxygen Balance, %
CalculatedFound
Ni(C3H4N2)6(NO3)2
NiC18H24N14O6
591 g/mol		Ni—9.9
C—36.6
H—4.1
N—33.2
O—16.2		Ni—9.6
C—35.7
H—4.0
N—32.4
Other («O») 1—18.3		NiC18.3H24.6N14.2«O»7−116
Ni(C3H4N2)6(ClO4)2
NiC18H24N12Cl2O8
666 g/mol		Ni—8.8
C—32.5
H—3.6
N—25.2
Cl—10.7
O—19.2		Ni—8.7
C—32.3
H—3.5
N—25.6
Cl—10.5
Other («O») 1—19.4		NiC18.3H23.7N12.4Cl2«O»8.2−96
1 Supposedly oxygen.
Table
Table 2. Kinetic parameters of thermal decomposition of [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2 complexes acquired using the Coats–Redfern method.
Table 2. Kinetic parameters of thermal decomposition of [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2 complexes acquired using the Coats–Redfern method.
ComplexParameterLinear FittingGenetic Algorithm
Stage IStage IIStage IStage II
Ni(C3H4N2)6(NO3)2E, kJ/mol124284117354
R20.99230.99120.9995
Ni(C3H4N2)6(ClO4)2E, kJ/mol170255177253
R20.99790.98720.9999
Table
Table 3. The critical temperature of thermal explosion for [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2 complexes.
Table 3. The critical temperature of thermal explosion for [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2 complexes.
Complexβi, K/minTe, °CT0, °C ∂ l n β ∂ T ,   1 / ° C E, kJ/molTb 1, °CTb 2, °C
[Ni(C3H4N2)6](NO3)22.5238.523924354245239
5244.6
10266.4
20287.4
[Ni(C3H4N2)6](ClO4)22.5259.224921250259249
5268.3
10283.6
20302.7
1 Tonglai formula; 2 Xue formula.
Table
Table 4. The kinetic parameters of decomposition of the synthesized nickel complexes at the high heating rates.
Table 4. The kinetic parameters of decomposition of the synthesized nickel complexes at the high heating rates.
ComplexAverage Heating Rate, °C/sGas
(Mass Spectrometer Signal, m/z)		ParameterObtained Value
Low-Temperature Stage (1)High-Temperature Stage (2)
[Ni(C3H4N2)6](NO3)2245Imidazole (68)E, kJ/mol84119
R20.9996
HCN (27)E, kJ/mol221221
R20.9992
N2 + CO (28)E, kJ/mol13663
R20.9999
O2 (32)E, kJ/mol99-
R20.9930
[Ni(C3H4N2)6](ClO4)2120Imidazole (68)E, kJ/mol104264
R20.9951
HCN (27)E, kJ/mol244323
R20.9973
N2 + CO (28)E, kJ/mol246339
R20.9938
O2 (32)E, kJ/mol-235
R20.9912
Table
Table 5. Powder XRD data for the solid products of [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2 combustion.
Table 5. Powder XRD data for the solid products of [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2 combustion.
SamplePhaseContent, wt%SCR Size from Lvol-IB/Lvol-FWHM, nm
Ni/NiO-NO3-SSCNiO8020 ± 1/26 ± 1
Ni2059 ± 5/74 ± 7
Ni/NiO-ClO4-SSCNiO8520 ± 1/28 ± 2
Ni1524 ± 6/33 ± 8
Table
Table 6. XPS data for the solid products of [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2 combustion.
Table 6. XPS data for the solid products of [Ni(C3H4N2)6](NO3)2 and [Ni(C3H4N2)6](ClO4)2 combustion.
SampleContent, at.% 1
NiOCNCl
Ni/NiO-NO3-SSC24.637.135.82.5-
Ni/NiO-ClO4-SSC7.514.356.613.28.3
1 Atomic percentages.
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Netskina, O.V.; Dmitruk, K.A.; Paletsky, A.A.; Mukha, S.A.; Pochtar, A.A.; Bulavchenko, O.A.; Prosvirin, I.P.; Shmakov, A.G.; Ozerova, A.M.; Veselovskaya, J.V.; et al. Solvent-Free Synthesis of Nickel Nanoparticles as Catalysts for CO2 Hydrogenation to Methane. Catalysts 2022, 12, 1274. https://doi.org/10.3390/catal12101274

AMA Style

Netskina OV, Dmitruk KA, Paletsky AA, Mukha SA, Pochtar AA, Bulavchenko OA, Prosvirin IP, Shmakov AG, Ozerova AM, Veselovskaya JV, et al. Solvent-Free Synthesis of Nickel Nanoparticles as Catalysts for CO2 Hydrogenation to Methane. Catalysts. 2022; 12(10):1274. https://doi.org/10.3390/catal12101274

Chicago/Turabian Style

Netskina, Olga V., Kirill A. Dmitruk, Alexander A. Paletsky, Svetlana A. Mukha, Alena A. Pochtar, Olga A. Bulavchenko, Igor P. Prosvirin, Andrey G. Shmakov, Anna M. Ozerova, Janna V. Veselovskaya, and et al. 2022. "Solvent-Free Synthesis of Nickel Nanoparticles as Catalysts for CO2 Hydrogenation to Methane" Catalysts 12, no. 10: 1274. https://doi.org/10.3390/catal12101274

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