Introduction

In recent years, the CRM has attracted considerable interest in the production of H2 and CO combination known as synthesis gas. This gas, with its component gases in a mole ratio close to one, is appropriate for the Fischer–Tropsch method and the creation of methanol. In addition, CRM offers a great chance for a clean environment by utilizing the greenhouse gases (GHGs): carbon dioxide and methane [1,2,3,4,5,6,7]. Because it allows chemical reactions to be carried out at low temperatures without increasing or causing the production of byproducts, heterogeneous catalysis is a crucial pillar for research on energy and its sustainability [8,9,10,11,12,13]. It has been demonstrated that heterogeneous catalysts based on transition metals are effective for producing fine and bulk compounds as well as hydrogenating petroleum. Click or tap here to enter text [14]. To find affordable, widely available hydrogenation catalysts, research has been focused on non-precious metals. Transitional metals like iron (Fe), cobalt (Co), and nickel (Ni) have drawn a lot of interest due to their unique characteristics, which include their enormous availability, low cost, lack of toxicity, and remarkable catalytic activities [14,15,16,17]. CRM is an environmentally friendly, economically feasible reaction for the conversion of CH4 and CO2 into syngas. Despite spending less time in the atmosphere than carbon dioxide, methane has a stronger warming effect. Since hydrogen is a necessary component of all petrochemical industries and renewable energy sources, the usage of GHGs in the course of the reforming tasks results in their conversion to H2, which lessens the environmental impact of emitted GHGs on the atmosphere. CRM, steam reforming of methane, and partial oxidation are well-researched methods for the creation of syngas:

$${\text{CH}}_{{4\left( {{\text{gas}}} \right)}} + {\text{CO}}_{{2\left( {{\text{gas}}} \right)}} = 2{\text{H}}_{{2\left( {{\text{gas}}} \right)}} + 2{\text{CO}}_{{\left( {{\text{gas}}} \right)}} { }\Delta H_{298} = 261{\text{ kJ mol}}^{ - 1} {\text{ CRM}}$$
(1)
$${\text{CH}}_{{4\left( {{\text{gas}}} \right)}} + {\text{H}}_{2} {\text{O}}_{{\left( {{\text{gas}}} \right)}} = 3{\text{H}}_{{2\left( {{\text{gas}}} \right)}} + {\text{CO}}_{{\left( {{\text{gas}}} \right)}} { }\Delta H_{298} = 206{\text{ kJ mol}}^{ - 1} {\text{ Steam reforming}}$$
(2)
$${\text{CH}}_{{4\left( {{\text{gas}}} \right)}} + \frac{1}{2}{\text{O}}_{{2\left( {{\text{gas}}} \right)}} = 2{\text{H}}_{{2\left( {{\text{gas}}} \right)}} + {\text{CO}}_{{\left( {{\text{gas}}} \right)}} { }\Delta H_{298} = - 36{\text{ kJ mol}}^{ - 1} {\text{ Partial oxidation}}$$
(3)

The H2/CO mole ratio from CRM is superior to traditional steam reforming for the Fischer–Tropsch process and methanol synthesis [18]. In addition to having the lowest running costs of these processes, CRM also has the added advantage of converting CO2 into useful chemicals [19].

Among the side reactions, the reverse water–gas shift (RWGS) is crucial for decreasing the H2/CO mole ratio:

$${\text{H}}_{2} + {\text{CO}}_{2} \leftrightarrow {\text{H}}_{2} {\text{O}} + {\text{CO}}\quad \Delta H^{{\text{o}}} = 42.1\;{\text{kJ}}\;{\text{mol}}^{ - 1}$$
(4)

The fast catalyst deactivation by carbon deposition and the high-temperature sintering of both the support and the active metal particles are disadvantages of dry reforming processes [20]. (Alumina (Al2O3) has frequently been used as catalyst support. However, a number of other support materials, including MgO, ZrO2, La2O3, TiO2, SiO2, and ZrO2-Al2O3, have been tested. There is consensus in the literature about the critical role of support materials in the CRM process [21,22,23,24]. Because of its mechanical strength, stability at high temperatures, and advantageous textural properties, alumina is frequently chosen for use as a support [25]. However, alumina-supported nickel-based catalysts are vulnerable to carbon deposition. [26]. Recent research has found that NiO/MgO or NiO/Al2O3 catalysts, in which NiO creates a solid solution with the support (NiO–MgO or NiAl2O4), inhibit carbon deposition [27,28,29,30]. In addition, it was noted that the basicity of MgO with other oxides, when used as a support, reduced carbon deposition and increased catalyst stability. The dry reforming of biogas on a Ni catalyst supported on ZrO2 and its modification with La2O3 and CeO2 were both studied by Charisiou et al. [31]. According to their findings, modified support catalysts had better conversion rates and less performance degradation during prolonged reactions. In their study, Siahvashi and Adesina [32] investigated the carbon buildup that occurs during the CO2 reforming reaction of C3H8 over alumina-supported bimetallic Mo/Co–Ni catalysts. The results showed that the reaction rate profiles of the Mo–Ni catalyst had higher H2/CO formation rates with a lower CO2 consumption rate compared to the Co–Ni catalyst. Additionally, the conversion-time analysis indicated that the Mo–Ni catalyst was more stable and active during a 72-h run, while the Co–Ni catalyst experienced noticeable deactivation after only 30 h on-stream. Cui et al. [33] investigated the Ni3Co (1 1) coke behavior and reaction process using density functional theory. It was discovered that Ni and Co have an electrical synergistic effect that keeps Ni active even after carbon buildup, and the Ni3Co (1 1) catalyst enhances O* transport properties and lowers the energy barrier for C–O bonding. As a result, carbon deposition can be prevented.

Foo and colleagues [34] looked into the kinetics of carbon deposition and removal on lanthanide-promoted Co–Ni/Al2O3 dry reforming catalysts. As a result of the rare-earth oxide interacting with the more reactive surface carbonaceous materials in redox reactions decreased coke accumulation. Using Ni catalysts supported on La2O3-Sm2O3-CeO2, La2O3-Pr2O3-CeO2, and La2O3-MgO-CeO2, Siakavelas et al. [35] examined the CO2 methanation process. It was established that a greater population of oxygen vacant sites was produced when La3+, Pr3+ and La3+, Sm3+ were simultaneously incorporated into the crystal structure of cerium oxide. Additionally, a large number of moderate basic sites was raised by the coexistence of La3+, Mg2+ and La3+, Pr3+ in CeO2. The pace of the CO2 methanation reaction was accelerated by these physicochemical characteristics at very low temperatures. Therefore, we impregnated NiO into commercially available combinations of MgO-modified Al2O3 of different MgO loadings. We denoted them by the letters 5Ni-MGx, where x (x = 20, 30, 63, and 70%) denotes the weight loading of MgO in the support. These 5Ni-MGx catalysts were characterized by N2-physisorption, XRD, TPR, and TEM, and were tested in CRM at 700 °C. The effect of the loading of the MgO modifier on the catalyst activity and stability was studied.

Experimental

Catalyst preparation

To prepare 5Ni-MGx, x = (20, 30, 63, and 70%), 20 mL of distilled water, and the appropriate amount of nickel nitrate hexahydrate [Ni (NO3)2.6H2O, 98%, Alfa Aesar] to get 5.0 wt.% loading of nickel oxide, and the required amount of support (the supports with different alumina and magnesium oxide ratios were obtained as a gift from SASOL Anckelmannsplatz 1, 20537, Hamburg, Germany) were mixed and were stirred at room temperature in a crucible. For 30 min, the solution was dried while being stirred at 80 °C. The catalysts were calcined at 700 °C for 3 h.

Catalytic testing

An amount of 0.1 g of catalyst was used to evaluate the activity in a fixed-bed, continuous-flow reactor (9.0 mm i.d. and 300 mm in length) at 1.0 atm. To show the reaction temperature, a thermocouple was attached in the center of the catalyst bed. The catalysts were pretreated by adding H2 at a flow rate of 30 mL/min for one hour at 800 °C. To remove any remaining H2 in the reactor, it was then purged with flowing N2 at a rate of 20 mL/min for 20 min at 700 °C. During the reaction, the reactant feed gas CH4/CO2/N2 with a volume ratio of 30/30/10 was injected and kept flowing at a rate of 70 mL/min, at a space velocity of 42,000 mL/(hgcat), and at a temperature of 700 °C. A thermal conductivity detector (TCD)-equipped online GC (GC-Shimadzu 2014) was used to quantitatively assess the reaction products and unconverted feed gases from the reactor.

Catalyst characterization

Nitrogen physisorption at − 196 °C was used to measure the specific surface area of catalysts. The specific surface area was measured using a Micromeritics Tristar II 3020 device using the Brunauer–Emmett–Teller (BET) method. With the aid of a Shimadzu TGA-51 and thermal gravimetric analysis performed in the air, the amount of carbon deposition on the used catalysts was measured. The type of carbon deposited over the employed catalysts, along with the degree of graphitization, was determined using a laser Raman spectrometer (JASCO NRS-4500, Tokyo, Japan). The excitation beam employed had a wavelength of 532 nm. Using a transmission electron microscope (JEM-2100 F JEOL, Akishimashi, Tokyo, Japan), the morphology of the used catalysts was studied. An X-ray diffractometer (Miniflex Rigaku diffractometer), outfitted with Cu K X-ray radiation and operating at 40 kV and 40 mA, was used to investigate the crystalline phases of the fresh catalysts. Data collection took place over a 2-h period with a step magnitude of 0.01° and a 2- angle range of 10–85°. A Micromeritics Auto-Chem II 2920 device was used to perform H2 temperature-programmed reduction (H2-TPR). The catalyst sample was heated for one hour at 200 °C in an argon environment before to the measurements, and it was then cooled to room temperature. Using a gas mixture of H2/Ar (v/v, 10/90) at a flow rate of 40 mL/min, 0.07 g of the sample was heated at a rate of 10 °C/min to 1000 °C. Using a TCD, the H2 consumption signal was captured.

Results and discussion

Figure 1A shows the N2 adsorption–desorption isotherms of the fresh catalysts. According to IUPAC classification [36], the isotherms have H3 hysteresis loops and are of type-IV. These patterns are traits of mesoporous materials and often apply to solid particles with wedge, plate slit, and crack structures [36]. Figure 1 demonstrates, however, that N2 uptake increased in the 0.5–1 relative pressure range (P/P0). The 5Ni-MG20 catalyst had the maximum surface area (260.2 m2/g), whereas the 5Ni-MG70 catalyst had the lowest surface area (16.8 m2/g) (Table 1). Results from tests of surface area and porosity revealed that the catalysts were mesopores. The reduction in the specific surface area and pore volume with increasing the weight percentage loading of the MgO modifier might be due to the blockage of pores by the modifier. In addition, all the modified catalysts, except 5Ni-Mg70, have higher surface area than the unmodified catalysts.

Fig. 1
figure 1

N2 adsorption–desorption isotherms of the fresh catalysts and their BJH pore size distribution, based on adsorption isotherms

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Table 1 N2-physisorption results of the fresh catalysts
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Figure 1B displays the BJH pore size distributions, based on the adsorption isotherms, of the fresh catalysts. The 5Ni-Al2O3 catalyst showed monomodal pore size distribution within the micro-mesoporous range with the highest pore volume of 0.52 cm3/g. When MgO modifier was incorporated, multimodal distribution with spanning the micro-meso-macroporous range, associated with the reduction in pore volume from 0.512 to 0.105 cm3/g with increasing MgO wt.% content from 20 to 70. This reduction in pore volume could be attributed to the blockage of pores and to the increase of pore width with increasing MgO content. The 5Ni-MgO catalysts showed also multimodal pore size distribution over the micro-meso-macroporous range, associated with pore volume of 0.35 cm3/g.

To understand the interaction between the active catalyst and its support, H2-TPR was carried out (Fig. 2). All the catalyst samples showed broad negative peaks in the temperature range of 50–600 °C, which could be attributed to the hydrogen spillover into the mesopores [31]. Furthermore, 5Ni-MG20 and 5Ni-MG30 catalysts retained some hydrogen in their pores until they reached 700 °C, while 5Ni-MG63 and 5Ni-MG70 catalysts had complete removal of hydrogen from their pores at 600 °C and showed onsets of positive peaks for the beginning of Ni2+ ions reduction. This observation indicated that increasing MgO content strengthened the interaction of NiO with the support, as reported previously in the literature [37]. All of the catalysts showed a broad peak of reduction in the 700–1000 °C region, indicating a strong interaction of NiO with the support. The peak's maximum, intensity, and broadness were all dependent on the amount of MgO present in the catalyst. The reduction peaks of 5Ni-MG20 and 5Ni-MG30 catalysts had similar maximums (~ 830 °C) and broadness, but the peak of 5Ni-MG30 had higher intensity than that of 5Ni-MG20, reflecting the effect of the higher MgO content. When MgO content increased to 63 wt.% (5Ni-MG63), the reduction peak maximum shifted to ~ 860 °C, its broadness increased, and its intensity reduced a little bit in comparison to that of 5Ni-MG30. Further increase of MgO content to 70 wt.% (5Ni-MG70) resulted in shifting the reduction peak maximum to ~ 930 °C, broadening the peak, and decreasing its intensity (the lowest peak intensity). Therefore, the successive increase in MgO content developed stronger interaction between NiO with the support. Such interaction could be attributed to the formation of NiAl2O4 spinel [32], Mg2NiO3 [33], MgNi2O3 [33], and NixMg(1−x) O [34], where the letter three phases formed owing to the solid solution between NiO and MgO. Table 2 displays the amount of hydrogen consumption during the TPR analysis of the catalysts. The 5Ni-Al2O3 catalyst showed a broad reduction peak between 700 and 1000 °C, centered at ~ 830 °C. The development of the NiAl2O4 spinel phase and the strong interaction of NiO with the Al2O3 support may be responsible for the high temperature of reduction. On the other hand, 5Ni-MgO catalyst did not show any reduction peak within the examined temperature range, implying the very strong interaction between NiO and MgO and the formation of solid solution and mixed metal oxides. 5Ni-MG63 showed the highest amount of hydrogen consumption, while 5Ni-MgO catalyst exhibited the lowest amount of hydrogen consumption. These results were in parallel with the observed catalytic performance.

Fig. 2
figure 2

H2-TPR curves of the fresh catalysts

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Table 2 The Analysis of H2-consumption during H2-TPR
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The population of the basic sites of the reduced samples obtained from CO2-TPD is inserted in the support information as Table S1.

Figure 3 displays the X-ray diffraction patterns of the fresh 5Ni-MGx (x = 20, 30, 63, or 70 wt.%) catalysts. The 5Ni-MGx catalysts' XRD patterns revealed diffraction peaks at 2θ of 35°, 46°, and 67°, which could be attributed, respectively, to (110), (113), and (214) crystallographic planes of the cubic γ-alumina phase (JCPDS No. 42-1468). In addition, the cubic phase of magnesium oxide was detected at 2θ of ~ 37°, 74°, and 78°, corresponding, respectively, to the crystallographic planes of (111), (311), and (222) (JCPDS card number 89-7746). The diffraction of MgO became obvious with the increase of its content in the catalyst. The diffraction peaks of MgO could also be attributed to NiO (JCPDS Card No. 00-004-0835) because both of these oxides have the same crystal structure and form solid solutions [38].

Fig. 3
figure 3

XRD patterns of the fresh catalysts

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In the course of catalytic performance, the same input ratio and reaction temperature of 700 °C were used in a stainless-steel reactor without catalysts, as part of a blank experiment before the evaluation of the synthesized catalysts. The blank's CH4 and CO2 conversions were 1.55 and 0.25%, respectively, the smallest influence of the metallic reactor is indicated by an H2/CO mole ratio that is close to 0.11. Figure 4 shows the profiles of the CH4 and CO2 conversions as well as the H2/CO ratio. Each 5Ni-MGx catalyst's performance (x = 20, 30, 63, or 70%) was evaluated at 700 °C. The 5Ni-MG30 catalyst gave the CH4 conversion higher than the CO2 conversion. This result might be attributed to the occurrence of methane cracking. The 5Ni-MG63 catalyst exhibited the optimal CRM performance, with CH4 and CO2 conversions of 72.9 and 72.3%, respectively, and an H2/CO ratio of 0.82. Generally, the modification of alumina support with MgO reduces the generation of carbon deposit and improves the activity. The activity of 5Ni-MG63 was better than both 5Ni-MgO and 5Ni-Al2O3. The superb activity of 5Ni-MG63 could be related to its highest value of hydrogen consumption during the TPR. The higher the hydrogen consumption is, the higher the reduction of the active metal is.

Fig. 4
figure 4

Conversions of CH4 and CO2, and H2/CO mole ratio as a function of TOS (reaction conditions: CH4/CO2/N2 = 3/3/1 (v/v/v); GHSV = 42,000 ml/gcat/h; Mcat = 0.1 g; t = 700 °C)

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Additional proof on the level of graphitization and the type of carbon on the used catalysts is provided by Raman spectroscopy. Comparative catalytic performances in relation to CH4 and CO2 conversions of other nickel-based catalysts with the current study are displayed in the support information (Table S2). A satisfactory resemblance is achieved.

The Raman spectra of the used catalysts are shown in Fig. 5. There were two zones in the spectra. Zone I ranged from 1250 to 1650 cm1, and Zone II ranged from 2400 to 3000 cm1. The D- and G-band characteristic peaks were located at 1343 and 1591 cm−1, respectively, in zone I. The D-band is associated with defective, disorganized carbon deposits (amorphous), whereas the G-band is associated with graphite. The 2D band was seen at 2669 cm−1 as a result of the band of zone I's unification and ramifications. The relative intensity sizes of the G and D bands (ID/IG) allow for an accurate assessment of the degree of carbon crystallinity that was created during the reaction. Minor ratios indicate that crystallinity is predominant due to the graphitized carbon. For our spent catalysts, the values of ID/IG were 1.15, 1.04, 1.01, and 1.12 for the 5Ni-MG20, 5Ni-MG30, 5Ni-MG63, and 5Ni-MG70, respectively. The highest (ID/IG) ratios were seen in the 5Ni-MG20 and 5Ni-MG70 catalysts, which indicate low crystallinity as a result of a lack of graphitized carbon. As a result, compared to the other catalysts, the graphitization degree of the carbon over the 5Ni-MG20 and 5Ni-MG70 catalysts was evidently lower. The deposits of poor crystalline or amorphous carbon caused the early deactivation of the catalyst, while the crystalline graphitic carbon led to enhanced catalytic performance.

Fig. 5
figure 5

Raman spectra of the spent catalysts

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Figure 6 shows the weight loss of used catalysts as a function of temperature. The weight loss below 200 °C was primarily due to the elimination of H2O and other chemisorbed species, whereas the weight loss between 200 and 500 °C was primarily brought about by the oxidation of amorphous carbon. The combustion of graphite carbon over 600 °C was the key factor, contributing to the weight loss. As shown in Fig. 6, the 5Ni-MG20 catalyst had a weight loss of 13%, while the 5Ni-MG63 had a high weight loss of 39%. This profile could be attributed to the reactivity of the catalysts. The 5Ni-MG20 exhibited low performance and thus generated low carbon deposition. On the other hand, 5Ni-MG63 catalyst showed high activity and hence generated a substantial amount of carbon deposition. The catalyst supported by unmodified Al2O3 with MgO gave the highest weight loss of 77%, which denoted that the modification of the support with MgO addition diminished the carbon formation.

Fig. 6
figure 6

TGA curves for the spent catalysts after 7 h time-on-stream

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The TEM images of 5Ni-MGx catalysts (x = 30 and 63%) at a 100 nm scale are shown in Fig. 7. Fresh and used 5Ni-MG30 catalysts are shown in Fig. 7A and B, respectively. Filamentous carbon nanotube production was visible in the catalytic waste. Figure 7C and D is for the fresh and spent 5Ni-MG63 catalysts, respectively, where the agglomerated catalyst particles were observed for the fresh 5Ni-MG63 catalyst and numerous filamentous carbon nanotubes for the spent one, respectively.

Fig. 7
figure 7

TEM images of 5Ni-MGx (x = 30 or 63%) spent catalysts

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Conclusions

Ni-based catalysts, supported on alumina-modified with MgO, generated syngas through CO2 reforming of methane (CRM). The catalytic activity for CRM increased with the increasing MgO modifier loading. 72% CH4 conversion and 73% CO2 conversion over 7-h time-on-stream were obtained when 63% MgO was used. Increasing the weight load percent of MgO to 70 wt.% decreased the catalyst surface area drastically to 17 m2/g and therefore decreased the CH4 and CO2 conversions owing to the hindrance of reducing NiO by the robust interaction with the support, as shown by the H2-TPR analysis. The 5Ni-MgO exhibited the lowest total hydrogen consumptions due to the formation of solid solution between NiO and MgO. The catalyst with 20wt.%-MgO displayed low weight loss of 11% during TGA analysis due to its low reactivity. While the optimum catalyst 5Ni-MG63 gave significant weight loss of 39% as a result of its high activity. The catalyst supported on unmodified Al2O3 gave the highest weight loss of 77%, which indicated the modification of the support with MgO addition assisted in reducing the carbon formation.