Direct Reduction in Greenhouse Gases by Continuous Dry (CO2) Reforming of Methane over Ni-Containing SHS Catalysts

The world of energy is on the cusp of profound transformation. Hydrogen or hydrogen-containing fuel mixtures in the form of synthesis gas, as carriers of clean energy, will be in the short term among the most efficient solutions to pressing environmental problems, reducing the amount of greenhouse gases as well as pollution of cities and dependence on oil-based fuels. Carbon dioxide conversion of methane is the most promising method for the production of synthesis gas due to the simultaneous consumption of two greenhouse gases and, accordingly, a successful solution to environmental problems. Ni/Mn-based catalysts have been prepared by self-propagating high-temperature synthesis (SHS) for this process. The samples were characterized by X-ray diffraction, scanning electron microscopy, and nitrogen porosimetry. The effects of the catalysts’ composition on activity, selectivity, and product yield were investigated. The influence of the content of Ni, Mn on the behavior of catalysts has been established. Comparison of spinels with different component ratios showed that they have a defective structure. Non-stoichiometric spinels with highly defective catalyst structures were obtained due to very high heating and cooling rates during SHS. They work as active sites, which underlies the high activity of the catalysts.


Introduction
The catalytic reforming of methane with carbon dioxide CH 4 + CO 2 → 2CO + 2H 2 has attracted increasing interest as an alternative for the production of hydrogen-containing fuel mixtures and synthesis gas over the past two decades. This process is industrially attractive. The International Energy Agency (IEA) World energy outlook clearly states that "natural gas is certainly set to play a central role in meeting the world's energy needs for at least the next two-and-a-half decades" [1]. This means that technologies based on methane will have priority. However, the future of power engineering is associated, of course, with hydrogen produced in various ways. These are "gray" technologies for the production of hydrogen, which involve the emission of greenhouse gases into the atmosphere. It is blue hydrogen that is produced from natural gas. It is also an alternative technology for the production of green hydrogen through the decomposition of water using the energy of the sun. This is the most expensive hydrogen. Since 1750, the amount of methane has doubled and may double again by 2050. Each year, 350-500 million tons of CH 4 is added to the air through livestock farming, coal mining, oil and natural gas extraction, rice cultivation, and landfill disposal [2]. The contribution of CO 2 to global annual emissions of anthropogenic greenhouse gases is about 75%, and CH 4 is about 14% [3]. Methane is a greenhouse gas that is more than 25 times more efficient at trapping heat in the atmosphere than carbon dioxide [4]. Reducing the sources of emissions of CH 4 and other greenhouse gases can lead to a decrease in the rate of global warming, reducing the risk of abrupt climate change. Many researchers have reported that Fe-, Co-, and Ni-based catalysts are highly active. However, the deactivation of catalysts is one of the main problems that arise in this process, mainly due to the deposition of carbon. Nickel catalysts have been extensively studied in the past decades, where the CH 4 conversion reached 68% at 80-90% selectivity to H 2 [5][6][7][8][9][10][11][12][13][14]. But insufficient research has been done on Co [15][16][17], Ni-Co [18][19][20][21][22], and Mn catalysts [23], for which the H 2 selectivity was in the region of 50%. The catalysts described in those works show high activity and excellent anti-carbon deposition properties even without prior reduction. MnO x has multiple valence states and fluorite structures with excellent oxidation and reduction characteristics [23]. It can be assumed that the Mn-doped Ni-Al-Mg-O catalyst can provide good catalytic performance in the dry reforming of methane with carbon dioxide to hydrogen and carbon monoxide.
Self-propagating high-temperature synthesis (SHS) occurs within minutes as a result of low-temperature initiation, which can lead to a high-temperature reaction. Unique materials were developed by controlled high-temperature combustion as a result of synthesis by SHS [24]. The SHS method offers a unique method for the preparation of modern, highly active ceramic catalysts, as well as carriers with controlled composition and properties which satisfy the special requirements of various types of reactions [25][26][27][28][29][30][31][32][33][34][35][36][37]. The combustion temperature can reach up to 3000 • C (depending on the composition of the system) due to the exothermic reactions that occur within just a few minutes after the start of SHS synthesis. As a result of SHS, high-speed self-sustaining reactions, similar to autowave combustion, arise and develop, which lead to the formation of compositions with the required structure. They completely or partially exclude external energy supply and involve the use of internal heat, which is released as a result of chemical reactions. The control of temperature, degree of conversion, rate of the process, as well as composition of the target products is carried out by changing the rates of heat release. Such features contribute to the formation of important advantages over traditional methods of synthesis: a significantly shorter process time (only a few minutes), the synthesis of new materials, including catalysts, based on various elements of the Periodic Table, the simplicity of the equipment and used technological process, significantly lower consumption energy. The formation of defect structures with multiple deformations of the crystal lattice is caused by high rates of heating and cooling during the SHS period. The strong influence of defects on catalytic properties is a well-known fact. It has been found that many SHS catalysts are very active in various processes. As a result, SHS has become a widely studied discipline around the world.
Based on our previous research, the preparation of SHS catalysts for methane reforming with CO 2 has been very promising. Studies on catalysts synthesized by the new SHS method were carried out in [36,[38][39][40][41][42]. In this work, we intend to study different compositions of catalysts for obtaining hydrogen-containing fuel mixtures. Systems based on Ni, Mg, Mn, and their mixtures were selected according to literature sources, and therefore, catalysts based on Ni/Mn were studied in this work in an effort at obtaining catalysts with increased activity and stability. Research on hydrogen concentration is currently underway, and the results will be presented in the future.

Preparation of SHS Catalysts
Ni-Mn-based oxides catalysts were produced on the base of the initial batch contained Mg, MgO, Mg (NO 3 ) 2, Al, Al 2 O 3 , NiO, and KMnO 4 . The initial batch composition for the 5 materials produced is presented in Table 1. The required amount of metal nitrate, oxide, metal in the different ratios was mixed, formed in cylindrical molding forms of diameter 1 or 2 cm with the same length under a pressure of about 15 MPa, and placed in a furnace preheated at temperatures of 600-900 • C.
All samples were self-ignited, and their rate of speed of the combustion front was measured. After combustion in the SHS regime, the catalysts were immediately taken out of the furnace.

Characterisation
The synthesized catalysts were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM) with EDS analysis immediately after combustion. The specific surface area of the samples was also determined. The formation and identification of the phases were confirmed by powder X-ray powder diffraction in a SIEMENS SPELLMAN DF3 diffractometer using Cu Ka 1 with wavelength λ = 1.5406 Å. The step-scans were taken over the range of 2θ from 5 to 100 • in steps of 0.03 • /s. Crystal lattice plane parameters were calculated according to Bragg's law nλ = 2dsin θ. The SEM observations were carried out in a Quanta Inspect FEI scanning electron microscope, and the EDS patterns were carried out by an EDS analyzer on samples sputter-coated with gold with a coating thickness of 5-10 nm. The compressive strength of cylindrical samples with a diameter of 1 cm and a height of 2 cm was measured in a universal tester at an adjustable voltage of 100 kN and at a displacement rate of 100 µM/min. The specific surface area of the SHS catalysts was determined by the Brunauer-Emmett-Teller (BET) method using nitrogen adsorption on either a Micrometrics ASAP 2010 instrument or a GAPP V-Sorb 2800 Analyzer. The sample was degassed at 200 • C in high vacuum before measurement.

Catalytic Activity Studies
The catalytic activity for CH 4 reforming with CO 2 was measured for crushed SHS catalyst with an average granule size of 3 mm. Catalysts were placed in a quartz tube and tested in a fixed bed flow reactor Carbolite STF 16/75 with controller Eurotherm 902 without prior reduction in a standard flow installation. The studies were carried out in the reaction mixture CH 4 : CO 2 : N 2 = 1:1:1 at a temperature of 700-900 • C and atmospheric pressure. The volumetric flow rate of the total flow was maintained at 860, 1720, 2580, and 3300 h −1 . The effluents were analyzed in a gas chromatograph Dani GC 86.10 HT equipped with a 30 m steel column filled with HayeSep Q 80/100 Mesh, on which gases O 2 , N 2 , CO, CO 2 , H 2 , CH 4 were analyzed at 125 • C with a thermal conductivity detector (TCD) with high purity He as the gas carrier. of the spinels' XRD peaks at different ratios of initial components showed that they had slightly shifted 2θ, which indicates a defective structure of SHS products. The presence of all the above products was also confirmed by EDS analysis on the SEM, for example, as shown in Figure 2 of Ni-Mn oxides-based SHS catalysts. The EDS spectra showed the existence of oxide compounds containing Ni, Mn, Mg, and Al, for example, spinels and mixed oxides: MgAl2O4, MnAl2O4, NiAl2O4, 3MgO NiO, and MgO 3NiO. The origination of these compounds can be from sintering of oxides, but also by Mg ions replacement by Ni and Mn, first originated MgAl2O4 spinel as shown in Figure 3. In Figure 3a, the EDS spectrum indicated MgAl2O4, where Mg was replaced by Ni and partially replaced by Mn. In the XRD spectra, these were presented as MnAl2O4, NiAl2O4, 3MgO NiO, and MgO 3NiO. In Figure 3b, the EDS spectrum indicated MgAl2O4, where Mg was partially replaced by Ni and Mn. In the XRD spectra, these were presented as MnAl2O4, NiAl2O4, MgAl2O4, 3MgO . NiO, and MgO 3NiO. In Figure 3c, the EDS spectrum also indicated MgAl2O4, where Mg was nearly completely replaced by Mn and the corresponding XRD spectra showed MgAl2O4 and MnAl2O4. The degree of replacement of ions in the spinel structure depended on the concentration of Ni or Mn ( Figure 4) in the initial SHS mixture and on the conditions of synthesis and cooling. These non-stoichiometric spinels originated during SHS due to the very high rate of synthesis heating and cooling which resulted in a high concentration of defects in SHS catalysts which were active centers of catalysis. The composition of catalysts' structures, as shown in Figure 1, can be explained by the following cascade reactions which take place during SHS process:

Characterisation of Catalysts
MgO The presence of all the above products was also confirmed by EDS analysis on the SEM, for example, as shown in Figure 2 of Ni-Mn oxides-based SHS catalysts. The EDS spectra showed the existence of oxide compounds containing Ni, Mn, Mg, and Al, for  Increasing spinel formation with increasing KMnO4 in the initial batch is shown in Figure 4 and can be explained by increasing the input of the (9)-(13) reactions. Increasing spinel formation with increasing KMnO 4 in the initial batch is shown in Figure 4 and can be explained by increasing the input of the (9)-(13) reactions.
Increasing KMnO 4 in the initial batch did not seem to affect the parameters of Mg 6 MnO 8 crystal lattice because it was produced by the reaction of two oxides, as shown in Figure 5a: In the case of MnAl 2 O 4 , increasing KMnO 4 led to a decrease in the crystal lattice parameters because MnAl 2 O 4 was produced mainly by Mg ion replacement by manganese in the MgAl 2 O 4 spinel. Since the ionic radius of Mn 2+ is 0.66 Å, and that of Mg 2+ is 0.8 Å, such substitutions lead to a decrease in the parameters of crystal lattice planes (Figure 5b).  Increasing KMnO4 in the initial batch did not seem to affect the parameters of Mg6MnO8 crystal lattice because it was produced by the reaction of two oxides, as shown in Figure 5a: In the case of MnAl2O4, increasing KMnO4 led to a decrease in the crystal lattice parameters because MnAl2O4 was produced mainly by Mg ion replacement by manganese in the MgAl2O4 spinel. Since the ionic radius of Mn 2+ is 0.66 Å, and that of Mg 2+ is 0.8 Å, such substitutions lead to a decrease in the parameters of crystal lattice planes ( Figure  5b). Concentrations of KMnO4 and NiO in the initial batch as oxidizers influenced the velocity of SHS reactions (Figure 6a). Maximum velocity was found at 5% NiO and 13% KMnO4, while it was moderate in the sample 2 Ni-Mn and 3 Ni-Mn, which were close to the stoichiometric ratio (Table 1). At these conditions, the highest combustion temperature was measured with sintering processes became more active, and as a consequence, the maximum strength of those catalysts (Figure 6b) was observed.  Increasing KMnO4 in the initial batch did not seem to affect the parameters of Mg6MnO8 crystal lattice because it was produced by the reaction of two oxides, as shown in Figure 5a: In the case of MnAl2O4, increasing KMnO4 led to a decrease in the crystal lattice parameters because MnAl2O4 was produced mainly by Mg ion replacement by manganese in the MgAl2O4 spinel. Since the ionic radius of Mn 2+ is 0.66 Å, and that of Mg 2+ is 0.8 Å, such substitutions lead to a decrease in the parameters of crystal lattice planes ( Figure  5b). Concentrations of KMnO4 and NiO in the initial batch as oxidizers influenced the velocity of SHS reactions (Figure 6a). Maximum velocity was found at 5% NiO and 13% KMnO4, while it was moderate in the sample 2 Ni-Mn and 3 Ni-Mn, which were close to the stoichiometric ratio (Table 1). At these conditions, the highest combustion temperature was measured with sintering processes became more active, and as a consequence, the maximum strength of those catalysts (Figure 6b) was observed.  (Figure 6a). Maximum velocity was found at 5% NiO and 13% KMnO 4 , while it was moderate in the sample 2 Ni-Mn and 3 Ni-Mn, which were close to the stoichiometric ratio (Table 1). At these conditions, the highest combustion temperature was measured with sintering processes became more active, and as a consequence, the maximum strength of those catalysts (Figure 6b) was observed. The specific surface area of the SHS catalysts is shown in Figure 7 and was very low (0.82-1.26 m 2 /g) because the combustion temperature for all compositions exceeded 1200 °C, and in those conditions, sintering processes take place. The increase in surface area with an increase in KMnO4 connected with the gas formation reaction KMnO4 → KMnO2 + O2, which increased porosity slightly. The specific surface area of the SHS catalysts is shown in Figure 7 and was very low (0.82-1.26 m 2 /g) because the combustion temperature for all compositions exceeded 1200 • C, and in those conditions, sintering processes take place. The increase in surface area with an increase in KMnO 4 connected with the gas formation reaction KMnO 4 → KMnO 2 + O 2 , which increased porosity slightly. The specific surface area of the SHS catalysts is shown in Figure 7 and was very low (0.82-1.26 m 2 /g) because the combustion temperature for all compositions exceeded 1200 °C, and in those conditions, sintering processes take place. The increase in surface area with an increase in KMnO4 connected with the gas formation reaction KMnO4 → KMnO2 + O2, which increased porosity slightly.

Catalytic Studies
The first experiments for conversion of CO2 and CH4 were carried out without catalysts and it was found that even at a temperature as high as 900 °C and low gas flow rate, the conversion of CH4 was just 12% and that of CO2, just 11.4%. Thereafter, experiments were carried out in the presence of catalysts at different flow rates. Figure 8 shows the influence of reaction temperature and catalyst composition on the methane and carbon dioxide conversion and product H2/CO ratio using a flow rate of 860 h −1 . An increase in the catalysis temperature for all studied Ni-Mn SHS catalysts led to an increase in CH4 and CO2 conversion, which is expected for catalysts since, at high temperatures, processes of adsorption and desorption are more intensive. However, increasing catalysis temperature did not affect products' H2/CO ratio, as shown in Figure 8.

Catalytic Studies
The first experiments for conversion of CO 2 and CH 4 were carried out without catalysts and it was found that even at a temperature as high as 900 • C and low gas flow rate, the conversion of CH 4 was just 12% and that of CO 2 , just 11.4%. Thereafter, experiments were carried out in the presence of catalysts at different flow rates. Figure 8 shows the influence of reaction temperature and catalyst composition on the methane and carbon dioxide conversion and product H 2 /CO ratio using a flow rate of 860 h −1 . An increase in the catalysis temperature for all studied Ni-Mn SHS catalysts led to an increase in CH 4 and CO 2 conversion, which is expected for catalysts since, at high temperatures, processes of adsorption and desorption are more intensive. However, increasing catalysis temperature did not affect products' H 2 /CO ratio, as shown in Figure 8. . Figure 8. Influence of reaction temperature and catalyst composition on the methane and carbon dioxide conversion and product H2/CO ratio (flow rate 860 h −1 ). Figure 9 shows that the selectivity of these catalysts was very high: between 72% and 98%. The high selectivity can also be verified by the small number of byproducts identified, including hydrocarbons (C2H6, C2H4, C) and H2O. Specifically, the catalyst made with 16.7% KMnO4 in the initial SHS batch displayed the maximum selectivity at about 98%.  Figure 9 shows that the selectivity of these catalysts was very high: between 72% and 98%. The high selectivity can also be verified by the small number of byproducts identified, including hydrocarbons (C 2 H 6 , C 2 H 4 , C) and H 2 O. Specifically, the catalyst made with 16.7% KMnO 4 in the initial SHS batch displayed the maximum selectivity at about 98%.
Energies 2021, 14, 6078 9 of 13 dioxide conversion and product H2/CO ratio (flow rate 860 h −1 ). Figure 9 shows that the selectivity of these catalysts was very high: between 72% and 98%. The high selectivity can also be verified by the small number of byproducts identified, including hydrocarbons (C2H6, C2H4, C) and H2O. Specifically, the catalyst made with 16.7% KMnO4 in the initial SHS batch displayed the maximum selectivity at about 98%. As expected, increasing the flow rate resulted in decreasing conversion. As can be seen in Figure 10a, conversion of CH4 and CO2 decreased, which indicated that the limiting stage, in this case, was a very low surface area (0.2-1.3 m 2 /g). Increasing the flow rate also led to a decrease in the dehydrogenation process and the ratio H2/CO decreased at a flow rate of more than 2500 h −1 (Figure 10b). This is also shown by the increase in byproduct formation at those flow speeds. At a very high flow speed (3300 h −1 ), the product yield was low, and byproducts, such as H2O, C, and C2H6, were observed ( Figure  11a). All in all, at high flow rates, the activity of those catalysts was low (Figure 11b). As expected, increasing the flow rate resulted in decreasing conversion. As can be seen in Figure 10a, conversion of CH 4 and CO 2 decreased, which indicated that the limiting stage, in this case, was a very low surface area (0.2-1.3 m 2 /g). Increasing the flow rate also led to a decrease in the dehydrogenation process and the ratio H 2 /CO decreased at a flow rate of more than 2500 h −1 (Figure 10b). This is also shown by the increase in byproduct formation at those flow speeds. At a very high flow speed (3300 h −1 ), the product yield was low, and byproducts, such as H 2 O, C, and C 2 H 6 , were observed ( Figure 11a). All in all, at high flow rates, the activity of those catalysts was low (Figure 11b). (a) (b) Figure 11. Influence of KMnO4 concentration in the initial batch on the products yields (a) and catalytic activity per product (b).
In Figure 11a,b, it can be seen that the catalysts with 13% KMnO4 in the initial batch displayed high performance regarding the yield of the products and the activity per product.
The influence of the catalyst preparation method was investigated on 5 Ni-Mn catalysts. Figure 12 shows data on conversion, selectivity, and H2/CO ratio depending on temperature on catalysts prepared by the SHS method compared with the traditional incipient wetness impregnation method. It was shown that the conversion of the initial (a) (b) Figure 11. Influence of KMnO4 concentration in the initial batch on the products yields (a) and catalytic activity per product (b).
In Figure 11a,b, it can be seen that the catalysts with 13% KMnO4 in the initial batch displayed high performance regarding the yield of the products and the activity per product.
The influence of the catalyst preparation method was investigated on 5 Ni-Mn catalysts. Figure 12 shows data on conversion, selectivity, and H2/CO ratio depending on temperature on catalysts prepared by the SHS method compared with the traditional incipient wetness impregnation method. It was shown that the conversion of the initial Figure 11. Influence of KMnO 4 concentration in the initial batch on the products yields (a) and catalytic activity per product (b).
In Figure 11a,b, it can be seen that the catalysts with 13% KMnO 4 in the initial batch displayed high performance regarding the yield of the products and the activity per product. The influence of the catalyst preparation method was investigated on 5 Ni-Mn catalysts. Figure 12 shows data on conversion, selectivity, and H 2 /CO ratio depending on temperature on catalysts prepared by the SHS method compared with the traditional incipient wetness impregnation method. It was shown that the conversion of the initial materials, selectivity, and the ratio of reaction products were higher for SHS samples. Taking into account that the SHS method leads to the formation of catalytically active composites upon initiating heating for several minutes in comparison with many hours of heat treatment by the impregnation method, the production of synthesis gas has distinct advantages and is more profitable under the above-described conditions.
(a) (b) Figure 11. Influence of KMnO4 concentration in the initial batch on the products yields (a) and catalytic activity per product (b).
In Figure 11a,b, it can be seen that the catalysts with 13% KMnO4 in the initial batch displayed high performance regarding the yield of the products and the activity per product.
The influence of the catalyst preparation method was investigated on 5 Ni-Mn catalysts. Figure 12 shows data on conversion, selectivity, and H2/CO ratio depending on temperature on catalysts prepared by the SHS method compared with the traditional incipient wetness impregnation method. It was shown that the conversion of the initial materials, selectivity, and the ratio of reaction products were higher for SHS samples. Taking into account that the SHS method leads to the formation of catalytically active composites upon initiating heating for several minutes in comparison with many hours of heat treatment by the impregnation method, the production of synthesis gas has distinct advantages and is more profitable under the above-described conditions.

Conclusions
This paper presents a study on the production and the properties of materials with catalytic activity for carbon dioxide reforming methane, produced using the self-propagating high-temperature synthesis (SHS) method. The study was focused on the systems Ni-Mn-Mg-Al-O and Mn-Mg-Al-O, which were found to be active systems for the CO2, dry CH4 reforming process. In the Ni-Mn series tested, the max load under compression was found to increase with the increasing concentration of KMnO4 in the initial batch because there was an increase in the total amount of spinel. The surface area of the catalysts was found to be very low due to the high temperature, which resulted in the sintering of the material during SHS. It was also found that non-stoichiometric spinels were formed preferentially during SHS due to the high rate of synthesis heating and cooling, which resulted in an extensively defective structure in these SHS catalysts, which acted as active centers of catalysis and were the reason for their measured high

Conclusions
This paper presents a study on the production and the properties of materials with catalytic activity for carbon dioxide reforming methane, produced using the self-propagating hightemperature synthesis (SHS) method. The study was focused on the systems Ni-Mn-Mg-Al-O and Mn-Mg-Al-O, which were found to be active systems for the CO 2 , dry CH 4 reforming process. In the Ni-Mn series tested, the max load under compression was found to increase with the increasing concentration of KMnO 4 in the initial batch because there was an increase in the total amount of spinel. The surface area of the catalysts was found to be very low due to the high temperature, which resulted in the sintering of the material during SHS. It was also found that non-stoichiometric spinels were formed preferentially during SHS due to the high rate of synthesis heating and cooling, which resulted in an extensively defective structure in these SHS catalysts, which acted as active centers of catalysis and were the reason for their measured high catalytic activity.
The Ni-Mn catalysts produced displayed good performance in the conversion of the reactant gases and in their activity. Additionally, the SHS catalysts had very high selectivity that reached 98%. The catalysts of the Ni-Mn series at a low flow rate showed the best catalytic performance, which was expected because the Ni-Mn catalysts have a low surface area. Further increasing the specific surface area is necessary to increase the activity of these catalysts at a high flow rate. This can be done by using the solution combustion synthesis method (SCS). The production of CO and hydrogen was high and the yield of the products was up to 10%. The catalyst with the most promising catalytic performance had the following composition in the initial batch: MgO: 13.04%, Al 2 O 3 : 23.48%, MgNO 3 : 30.44%, KMnO 4 : 13.04%, NiO: 16.7%, Al: 4.0%, Mg: 8.0%.
In general, SHS is a very rapid and low-cost method for catalyst production, and promising results were received for active catalysts for the carbon dioxide reforming of methane process.

Data Availability Statement:
The data presented in this study are available upon request from the corresponding author.